As early as 1866, Ernst Haeckel defined “ecology” as the science addressing the interactions of organisms with their organic and inorganic environment. Many different terms have been used since then to describe and define systems where individual species interact with the environment, where soil chemical and physical properties affect plant and animal communities, where plants compete and facilitate each other, and where vegetation can create microclimatic conditions very different from the large-scale climatic conditions. In 1935, Tansley proposed his definition of such a system and argued:

I have already given my reasons for rejecting the terms “complex organism” and “biotic community”. Clements’ earlier term “biome” for the whole complex of organisms inhabiting a given region is unobjectable, and for some purposes convenient. But the more fundamental conception is, as it seems to me, the whole system (in the sense of physics), including not only the organism-complex, but also the whole complex of physical factors forming what we call the environment of the biome—the habitat factors in the widest sense. Though the organisms may claim our primary interest, when we are trying to think fundamentally we cannot separate them from their special environment, with which they form one physical system.

It is the systems so formed which, from the point of view of the ecologist, are the basic units of nature on the face of the earth. Our natural human prejudices force us to consider organisms (in the sense of the biologist) as the most important parts of these systems, but certainly the inorganic “factors” are also parts—there could be no systems without them, and there is constant interchange of the most various kinds within each system, not only between the organisms but between the organic and the inorganic. These ecosystems, as we may call them, are of the most various kinds and sizes. They form one category of the multitudinous physical systems of the universe, which range from the universe as a whole down to the atom. (Tansley 1935)

Thus, Tansley defined the ecosystem not just as a symbiosis or as part of a superorganism but as a system that is characterised by the interaction of plants and animals with their physico-chemical environment. This scientific concept has its origin in thermodynamics and systems theory. Today, we use the term “ecosystem” when we consider the entire system of soil, microorganisms, vegetation and animals, as well as the lower level of the atmosphere, with all components interacting with each other. Thus, an ecosystem is the functional unit where biogeochemical processes happen, such as the decomposition of organic matter, providing the necessary nutrients to ensure plant life, and where plant, animal and microbial species interact with each other, changing and shaping their environment. An ecosystem is also a thermodynamically open system, where energy and matter also get lost to the atmosphere or the hydrosphere, such as N losses during denitrification and nitrification, or nutrient losses via leaching and run-off. Through such processes, but also through their biospheric–atmospheric exchange and surface albedo, ecosystems feed back on their environment, affecting, for example, the local to regional climate (Chap. 9, Sect. 9.2; Chap. 16). But ecosystems are also the organisational unit where ecosystem processes, such as primary productivity and evapotranspiration, translate into ecosystem services, such as food and timber production, when used by humankind (Part V: Global Ecology). Thus, it becomes clear that ecosystems are more than an assembly of species and that ecosystems have a range of spatial, temporal and functional characteristics that are system specific, showing system-level phenomena—so-called emergent properties. Emergent properties develop only in the system as a whole and are not present with/in any of its components; examples are the ecosystem structure, biogeochemical cycling within ecosystems, species and process interactions, and time lags in ecosystem responses. Some of these ecosystem characteristics are well defined—for example, the reference system (the unit of ground area), the presence of interactions among organisms, the existence of biogeochemical cycles (although not the exact nature of these interactions, their dynamics and magnitudes) and the irreversibility of some ecosystem processes. Other characteristics are not clearly defined and depend on the research questions asked, such as the boundaries or size of ecosystems, compartmentalisation of ecosystems, complexity of species and process interactions, ecosystem structure and temporal time lags after certain impacts on ecosystems (such as drought, fire, harvest or insect outbreaks).

13.1 Boundaries and Size of Ecosystems

The boundaries and the size of an ecosystem are not well defined. Some researchers even consider ecosystems as “large and individualistic” (Currie 2011). In contrast to a catchment, which is defined by the hydrological flow of water above the bedrock into the same downstream creek or river, the horizontal boundaries of an ecosystem are generally set by another adjacent ecosystem type to start with—for example, a boreal wetland neighbouring a boreal coniferous forest or a managed grassland adjacent to a temperate deciduous forest. The upper vertical boundary of an ecosystem is typically restricted to the height of the vegetation or to the canopy height interacting with the atmosphere (generally up to about one third of the total canopy height above the canopy). The lower vertical boundary can be set by the rooting depth, although it is most often set by experimental constraints—often a 30 cm or 1 m soil depth—and not by the actual deepest root present.

The area and thus the spatial extent of an ecosystem are typically defined by the similarity of species composition and/or biogeochemical processes, but are also often a subset of another larger area—for example, foraging areas of animals covering more than one ecosystem type. The spatial extent should contain all fluxes above and below the ground area under consideration. Bormann and Likens (1967) considered river catchment areas as basic units for ecosystems within landscapes, as the element or substance budgets may be completely quantified only within this range. However, a river catchment is a mosaic of different ecosystem types. The riparian zone near creeks and ecosystems relying on rain and groundwater have very different process rates per ground area from those of peaty river valleys or dry woodlands, for example. Thus, river catchments integrate over very heterogeneous components in a given region and would thus be too large to be valid as a basic unit of an ecosystem. The opposite extreme, a rotting tree in a mixed forest, would be considered too small to define the forest ecosystem. Overall, the limits of an ecosystem must extend so far that the relevant fractions of all substance flux rates per ground area (e.g. carbon assimilation, nitrogen mineralisation and formation of groundwater) of this particular ecosystem are taken into account. From this point of view, the rotting tree trunk is considered only a partial system or a component of a forest. Thus, if one studies any given forest ecosystem, only the total flux rate per ground area describes the characteristics and limits of this ecosystem, requiring a scientific approach and methodology adequate to represent the area under study (Chap. 14). For example, in ecosystem process studies, the fetch (i.e. the upwind area until another ecosystem is present) of a flux tower to measure biosphere–atmosphere exchange of greenhouse gases is often considered the appropriate area of an ecosystem (Chap. 14, Sect. 14.1). In the field of landscape ecology, larger and more heterogeneous areas or regions are studied to understand large-scale patterns and spatial variability, which is due to smaller-scale interactions. The term biome is used for even larger scales and for subcontinental systems characterised by one major vegetation type, such as the grassland biome in North America or the temperate deciduous forest across Europe. The definition of an ecosystem is independent of the facts that some organisms (e.g. migrating birds) also have an influence on the ecosystem under study beyond its limits and that components of the ecosystem possess their own dynamics (such as a rotting tree trunk in a forest ecosystem).

Given these considerations regarding the spatial characteristics of an ecosystem, the reference system of any ecosystem study is the unit of ground area, typically measured in square metres or hectares. Element or substance flux rates or turnover are no longer expressed as concentrations (in grams per gram or in moles per gram) of the element or substance in certain organs or individual plants; processes are expressed per unit of ground or surface area (in grams per square metre or in moles per square metre) above or below which these fluxes occur. Similarly, substance pools are no longer constrained to single plants or tissues but also relate to the unit of ground—for example, carbon pools in soils and vegetation. For soil pools, information on the soil depth that is considered might also be important and is therefore often given as well (in grams per square metre per 30 cm depth). At the same time, stand or stocking densities (i.e. the number of trees per hectare or the number of grass tillers per square metre) and individual plant-specific ecophysiology (e.g. the sap flow rate) relate to the element or substance flux rates per unit of ground but can be highly variable within the ecosystem under study. In addition, partial systems might have flux rates different from those of the system as a whole. These partial systems can include living plants, different species, standing dead trees, leftover stubble on a field, litter layer, below-ground, etc. If the partitioning into the origin of these component fluxes is of interest, an appropriate sampling design (e.g. stratified sampling, transects, grid points; Chap. 14) or the application of new techniques (e.g. stable isotopes, eddy covariance, remote sensing) needs to be implemented.

13.2 Components of an Ecosystem

Different actors, components and compartments make up an ecosystem. These partial systems or system components interact with each other, with strong or weak interactions, with linear or nonlinear interactions, with direct or indirect interactions, and with positive or negative interactions. Depending on the research question asked, the following criteria to separate different components of an ecosystem are the ones most frequently used:

Above-ground versus below-ground: This approach roughly separates autotrophic from heterotrophic processes (exception: below-ground root (i.e. autotrophic) respiration). In addition, it is possible to separate the ecosystem into soil versus vegetation compartments
Trophic levels: Separation of organisms present in an ecosystem into producers, consumers and decomposers describes the energy (and matter) flux within an ecosystem
Functional groups: Many species that have similar characteristics are often considered a functional group or functional type (e.g. nitrogen-fixing plants, trees, invasive species, phloem-sucking insects and parasites; Chap. 20, Sect. 20.2). Species can belong to many different functional groups—for example, Robinia pseudoacacia, a nitrogen-fixing tree species, is also considered invasive in Europe
Overstorey and understorey: The vertical structure of vegetation is important for the coupling of ecosystems to the atmosphere, particularly for energy, trace gas fluxes, atmospheric deposition or rain interception. In forests, the separation into trees, shrubs, lianas/vines and herbaceous plants (understorey vegetation) is often used. For grasslands, but also forests and woodlands, separation into the upper and lower canopy, as well as in understorey or suppressed plant species is frequent

Often, multiple components and actors, as well as their interactions, are investigated in the same study; then a so-called systems approach is used, which also provides the conceptual framework to connect all components and actors mechanistically—for example, in a mathematical model (Chap. 15). In contrast, studies focusing on single species—for example, dominant species (dominating in abundance) or keystone species (being a major driver of change or of certain processes)—or on species interactions within a plant community, are often carried out within the context of evolutionary biology, community ecology and biodiversity research (Part IV, particularly Chaps. 19 and 20).

These different components of an ecosystem are not equally distributed in space. They often form a mosaic of different components (e.g. in a riparian zone); they might create patterns and thus patchiness (e.g. the hummocks and hollows in a bog). These patterns might be vertical (facilitated by different height growth, resulting in a canopy structure) and/or horizontal (facilitated by plant density and species composition). The origins of these patterns are manifold. Patterns might be the consequence of soil conditions, topography, disturbance, competition or management. In any case, research in such a patchy ecosystem must pay special care to capture this spatial variability in order to represent the entire ecosystem and not only one or several components (Chap. 14).

13.3 Ecosystem Complexity and Interactions of Processes and Drivers

Ecosystems are, by definition, a complex network of relationships and processes—that is, they are determined by a multitude of factors and interactions. The complexity of an ecosystem develops very slowly over time, starting from a few actors—often mosses and lichens, such as on bare rock in glacier fore-fields when the ice retreats or after major disturbances such as eruptions of volcanoes—and eventually reaching the highly diverse ecosystems, such as temperate grasslands or tropical forests, that we know and use. Thus, the age of ecosystems is difficult to determine, since many generations of plants, animals and microorganisms have shaped the ecosystems that we study. Furthermore, ecosystem development over time is not linear, but rates can change at varying stages over time, triggered both internally and externally. Sudden events or disturbances, occurring in minutes and hours, change ecosystems as fundamentally as slow continuous changes in environmental conditions, occurring over decades. Internal drivers of ecosystem change can be species competition and thus vegetation succession, but also accumulation and/or consumption of resources, which change basic life conditions. External drivers can be natural disturbances such as landslides or insect outbreaks, or anthropogenic impacts such as land use, climate change or introduction of invasive species. Moreover, the components of an ecosystem are able to adapt, self-organise and interact with each other, but they also show emergent properties, making ecosystems prime examples of complex, adaptive systems, according to complexity theory (Currie 2011). Here, we will give an overview of ecosystem complexity. More process-oriented examples can be found in Chaps. 14 and 16.

13.3.1 Unpredicted Existence of Neighbours

One of the most important emergent properties, from the point of view of an individual plant, is the unpredictable existence of neighbours, which can possess a multitude of different competitive traits. Thus, the success or failure of an individual plant or a species in an ecosystem is no longer solely determined by the physiological characteristics (as described in Part I) and responses of the species to the environment (as described in Part II), but is suddenly co-determined by random effects such as the presence or absence of competitive neighbours (Chap. 19, Sect. 19.3). Particularly at the early stage of stand development, the composition of vegetation is determined by germination rates, establishment and survival during the seedling stage. In the medium term, interactions among plant species become decisive (Chap. 20). In the longer term, mainly abiotic disturbance processes, sometimes also called interferences, become more important; plant responses to processes such as invasion, extreme events or management practices may drastically change ecosystems within a very short time, sometimes as short as minutes or hours (e.g. hailstorm, flooding). Element or substance pools that have accumulated over centuries and millennia may disappear or degrade within a very short period, in turn also affecting the presence or absence of certain neighbours.

13.3.2 Scaling up of Processes

Processes within ecosystems occur at different spatio-temporal levels (sometimes also named scales), i.e. from foliage to landscapes or from seconds to millennia from small to large scales, respectively, also differing among components of the ecosystem. Often processes differ profoundly on different scales. For example, while small and fast scales are often dominated by chemical and biophysical processes, larger and slower scales are rather controlled by interspecies competition, species composition and community ecology. On very large scales, environmental boundary conditions affect ecosystem structures and dynamics over centuries and millennia. Nevertheless, even if these processes are identical (for example, photosynthetic CO₂ fixation) the response of the process to the same driver (in this case, light intensity) will differ with the organisational level or scale—that is, from leaves to fully leaved branches to the entire ecosystem. Thus, processes cannot simply be scaled from the leaf to the ecosystem level because additional constraints, such as spatial structure—an emergent property of an ecosystem—come into play. One classical example of such an upscaling issue is net assimilation. While leaf photosynthesis follows a saturation function driven by the availability of light and CO₂, as well as vapour pressure deficit (Sect. 12.4 in Chap. 12), photosynthesis at the ecosystem level is rather linearly related to incoming light. Moreover, it reaches much higher assimilation rates, in comparison with the leaf or branch levels, at higher light intensities—not reaching saturation—because more light can be used by more leaves within the entire ecosystem. Moreover, the light compensation point is higher at the ecosystem level than at the leaf or branch levels because more light is needed to overcome auto- and heterotrophic respiratory losses of CO₂ in low-light conditions within the entire ecosystem, in comparison with the leaf or branch levels (Fig. 13.1). The leaf area index (LAI) and thus light attenuation (i.e. the decrease of incoming light supply deeper in the canopy), as well as the nutrient supply, determine the actual magnitude of net assimilation at the ecosystem level.

/epubstore/S/E-D-Schulze/Plant-Ecology/OEBPS/images/72100_2_En_13_Chapter/72100_2_En_13_Fig1_HTML.png — Fig. 13.1
Upscaling of CO₂ exchange from leaves to the ecosystem. Differences in net CO₂ fluxes—that is, assimilation and respiration—in response to light intensity (PFD, photon flux density) between foliage (beech leaves) and the entire ecosystem. (Leaves: after Larcher (2003); ecosystem: data from E. Paul-Limoges)

Thus, the emergent property of an ecosystem (here, ecosystem structure) can change the response to the same driver (here, light intensity) of an ecosystem in comparison with its components. These new characteristics can be related to resource use at the ecosystem level but can also be linked to the interactions of different species (Part IV).

13.3.3 Response Functions to Interacting Drivers

Each process has a typical response function to its major driver—for example, net assimilation to light, or respiration to temperature. These response functions can be linear (Fig. 13.1) but also curvi-linear or even exponential. However, typically, processes are driven by multiple factors. In this case, nonlinear and irregular behaviour might set in. One example of such interacting drivers is the response of soil respiration to soil temperature and soil water availability. As long as soil moisture is abundant, soil respiration increases exponentially with temperature (Lloyd and Taylor 1994). However, as soon as soil water availability becomes limiting, soil respiration rates are much lower than those modelled by an exponential temperature response function (Ruehr et al. 2010; Fig. 13.2). Often one can determine threshold values below which soil water availability becomes the dominant driver. However, such thresholds are generally site specific; that is, they depend on soil and vegetation characteristics, and vary temporally—for example, with the time of the year and thus vegetation phenology.

/epubstore/S/E-D-Schulze/Plant-Ecology/OEBPS/images/72100_2_En_13_Chapter/72100_2_En_13_Fig2_HTML.png — Fig. 13.2
The dependence of soil respiration (daily averages) in a mixed temperate forest on soil temperature differs depending on soil water availability. Data are given for 2006. The Lloyd and Taylor model (R ² = 0.83) is given only if soil moisture ≥15%, depicted with *filled symbols*. *Open symbols* stand for soil moisture <15%. Soil respiration (SR) = 2.58 e^{419.6((1/56.02)−1/(T−46.02))}. (Data from N. Ruehr)

13.3.4 Self-Thinning

Some processes, such as the production of plant biomass in a stand, are driven not only by interacting environmental drivers but also by density-dependent effects. For example, intra- and interspecies competition (Chap. 19, Sect. 19.3), but also the occurrence and severity of pathogen and parasite damage affect a stand’s productivity. Self-thinning occurs when plants in dense plant populations or communities compete vigorously with each other for resources and some die, in turn decreasing the density of the survivors. The increasing growth rate of the survivors leads to continuous competition, increasing mortality, thus decreasing the number of survivors even further. Yoda et al. (1963) formalised this relationship and expressed the biomass of individual plants (W) as a function of the density of individual plants (n) in a stand, where c is a proportionality factor depending on light and nutrient supply:

$W=c{n}^{-3/2}$

(13.1)

This equation states that the biomass of an individual plant declines when plant density in this stand is increasing. Since this relationship holds for many different plant communities (Fig. 13.3), this relationship is also called the −3/2 power law, with −3/2 being the self-thinning constant. This constant can be explained by the spatial expansion of the biomass (three-dimensional volume, exponent 3) and the expansion of this biomass per unit of ground (two-dimensional area, exponent 2; Osawa and Allen 1993).

/epubstore/S/E-D-Schulze/Plant-Ecology/OEBPS/images/72100_2_En_13_Chapter/72100_2_En_13_Fig3_HTML.png — Fig. 13.3
Self-thinning in different communities. When the logarithm of the average plant weight is plotted as a function of the logarithm of the stand density, the data generate a line with a slope of −3/2. (After Silvertown and Lovett Doust (1993))

Expressing this self-thinning law based on plant biomass per ground area (B), thus multiplying the individual plant biomass (W) with the number of plants per unit of ground (n), results in:

$B=c{n}^{-1/2}$

(13.2)

Taking the logarithm of this equation yields the linear equation:

$\log B=\log c-\frac{1}{2}\log n$

(13.3)

This equation shows that the maximum biomass of a monospecific stand that can be achieved in an area depends on the number of individual plants. The slope (−1/2) applies to a broad range of growth forms (herbaceous plants, shrubs, trees) (Westoby 1984), while the parameter c describes the productivity, which is determined by many factors such as the site conditions and growth characteristics of the species.

Thus, in a managed forest stand, density-dependent effects mean that after an exponential growth phase up to canopy closure, a phase of self-thinning follows, during which mortality occurs. The growth rate of the stand decreases (in comparison with the exponential growth rate in the early stages) and the standing biomass reaches a certain level (Fig. 13.4a). The maximum attainable biomass then depends on the climate, vegetation structure, nutrient supply and management, as a result of plant plasticity to increased competition and of mortality. Climate conditions lead to regional differentiation—for example, the yield of forests in southern Germany is higher than that in northern Germany because of higher annual irradiance in the south. In managed systems, the biomass (and thus the competition among trees) is additionally controlled by different management practices such as thinning in forestry (Fig. 13.4b). Overall, the development over time of many processes resembles a saturation function (Schulze 1982), both for forests and for agricultural systems. However, in some systems—for example, intensively managed grasslands—the saturation plateau is rarely reached because mowing of such grasslands typically takes place during the exponential growth phase, when forage quality is very high, well before the biomass reaches the saturation plateau. Thus, the managed grassland is permanently kept at a juvenile stage, when regeneration growth of new foliage replaces the cut biomass.

/epubstore/S/E-D-Schulze/Plant-Ecology/OEBPS/images/72100_2_En_13_Chapter/72100_2_En_13_Fig4_HTML.png — Fig. 13.4
Relationships between standing biomass and stand age. a Increase in biomass with age of trees in managed spruce forests in Germany compared with unmanaged forests of spruce, pine and larch in eastern Europe and Siberia (Schulze et al. 1999). It is shown that even unmanaged forests reach maximum biomass. This is not the result of an equilibrium between assimilation and respiration; it is the result of different stress impacts (fire, wind). b Influence of forest management—that is, thinning—on the development of a spruce stand in southern Sweden. The zig-zag path in the development of the stand is due to the removal of biomass in forest management and the subsequent recovery of the stand. (After Kramer (1988))

13.4 Concepts of Equilibrium, Resistance and Resilience, Susceptibility and Vulnerability

The idea that ecosystems are in equilibrium with their environment (Gleason 1926) has been proven incorrect. Short- and long-term variability of the environment and the occurrence of disturbances not only constantly change the environment but also cause continuous adjustments of biogeochemical and ecophysiological processes and changes in species composition (Chap. 17). Thus, also steady-state conditions—that is, the state when processes or characteristics are not changing over time—are rare on longer time scales. Although an ecosystem is constantly subjected to sudden disturbances and slow-changing drivers, not all of these events trigger a response at the ecosystem level. The number of events or reoccurrence of events per time unit, i.e. the frequency, but also the strength of an event, i.e. the intensity, or the dose, i.e. intensity × time duration, are decisive and, together with certain ecosystem properties (see below), determine if an ecosystem is changing in response to an event or not. In the current literature, the terms “shock” and “extreme event” are also used to describe a (sudden) event or disturbance (Reichstein et al. 2013; Frank et al. 2015). In order to decide if an unusual or statistically rare disturbance or an extreme event has happened, it is necessary to know the typical/normal variability of the occurrence of disturbances or weather events. Sometimes also a combination of events is of interest, such as a combined heatwave and drought, as occurred in Central Europe in 2003, or a combined drought and insect outbreak. This means that long-term data sets of many variables are necessary, although sometimes not available. Furthermore, lagged versus non-lagged (also called concurrent) responses of an ecosystem to an event are differentiated, depending on when the ecosystem responds to a disturbance or change (Smith 2011). Additional terms used for lagged responses are legacy effects and memory effects, the latter of which should be avoided because of its human connotation.

The concept of stability includes a variety of aspects, all describing how an ecosystem reacts to or withstands internal or external perturbations. Although there are many definitions of stability (163 according to Grimm and Wissel (1997)), the response to and recovery after a perturbation are decisive in describing stability (Fig. 13.5). If an ecosystem is not reacting to a disturbance or environmental change and stays at its current state, this ecosystem is resistant to the disturbance or change. It retains its structure, and the current processes and species composition of the ecosystem will also continue in the future. This typically happens only when the event is not very strong in terms of either frequency, intensity or dose. If the event is stronger, more frequent or lasts longer, the resistance must increase to maintain the ecosystem’s current state. On the other hand, if the ecosystem is able to adapt to these perturbations (e.g. to changes in the environment), recovers over time and thus does not lose species or the capacity to carry out certain processes permanently, this ecosystem is resilient to change. It reacts to the disturbance but persists within its boundaries and thus is rather robust. If a perturbation event is even more severe, occurs in combination with other(s) or is of a new type that the ecosystem has not experienced yet, the ecosystem might not have the adaptive capacity (also called buffering capacity) to withstand or dampen such an event and might thus be transformed into another state. Thus, under these conditions, the ecosystem loses its capacity to function, changes its species composition or both. Nevertheless, one has to keep in mind that resistance or resilience are not inherently beneficial characteristics of an ecosystem. For example, restoration of a degraded ecosystem would likely benefit from less resistance or resilience in the system.

/epubstore/S/E-D-Schulze/Plant-Ecology/OEBPS/images/72100_2_En_13_Chapter/72100_2_En_13_Fig5_HTML.png — Fig. 13.5
Different aspects of stability. The resistance and resilience of the ecosystem to the first perturbation are higher than those to the second perturbation. In the second case, the ecosystem needs much longer to recover from the perturbation than in the first case. Resistance and resilience are two aspects of stability that are widely used today (sometimes also called resistance and resilience stability, respectively). However, many more definitions and special cases do exist (Chap. 17)

To describe the potential or the probability of this change in state, additional concepts have been developed: susceptibility and vulnerability. Both terms are often used as synonyms, although there is a difference. While susceptibility only considers the impact on (in our case) an ecosystem, vulnerability also takes into account the probability of the disturbance or change happening. Thus, an ecosystem is called susceptible or vulnerable to a disturbance or change if its resilience is small or already impaired, or if one expects this ecosystem to change soon and lose its current species composition and/or function. Climate change is often mentioned in this respect because of the projected change in the frequency of extreme weather and climate events.

13.5 Impacts of Slow Continuous Forcing and Sudden Disturbances

Because of their age and complexity, the responses of ecosystems to slow continuous forcing by the environment—such as global warming, the spread and establishment of invasive plants or the consequences of the presence or absence of ecosystem engineers such as beavers and elephants, but also moles and earthworms (Chap. 20)—are difficult to predict (Part V). Often the underlying mechanisms are not fully understood and thus their consequences at the ecosystem level are difficult to model. Similarly, although at first ecosystem responses to sudden disturbances are obvious—for example, damage after flooding or eradication of entire ecosystems after a volcanic eruption—the long-term responses and the recovery of the affected ecosystems are less clear. Both types of event—slow continuous forcing and sudden disturbances—have impacts on individuals, species, populations and ecosystems, and thus they change the boundary conditions for all actors and components present in ecosystems, as well as all processes taking place. Such impacts range from small adjustments—for example, the physiological acclimation to changing environmental conditions (Part II)—to changes in species composition and species mortality, or loss of entire ecosystems (Chap. 17), with impacts at different levels of organisation:

Individuals: transient damage (e.g. by late frosts), weakening by pests and parasites, failing reproduction because of weather events, death by windthrow
Populations: changes in the structure of populations and gene pools (e.g. by reduction of the number of individual plants or by the presence of new spatial barriers to pollination), extinction by events occurring over large areas (e.g. volcanic eruption)
Ecosystems: changes in species composition by arrival of non-native species (which might become invasive), loss of ecosystem components (e.g. by land use), changes or loss of nutrient cycling (e.g. after fire or due to leaching)

Slow continuous forcing, as well as disturbances, are thus events changing the species composition, as well as the continuous element or substance turnover, either slowly or suddenly, often in unexpected directions. At the level of the ecosystem, it is important whether the system loses resources or whether these resources are only relocated within the system but stay in the system. This can be nicely shown for carbon (C) cycling within ecosystems, where the following drivers can be distinguished (Schulze et al. 1999):

Drivers exerting a continuous forcing (e.g. temperature, precipitation, radiation, CO₂)
Drivers reallocating pools, but organic matter and therefore nutrient resources bound to it stay in the ecosystem (e.g. herbivory by insects, browsing by animals, grazing in extensive land use systems, C inputs into soils after windthrow)
Drivers removing species and pools such as organic matter and thus resources from the system (e.g. pathogens, pests, fire, harvest)

13.5.1 Slow Continuous Forcing

Factors such as temperature and precipitation directly control many processes of the C cycle in an ecosystem (as discussed for soil respiration in Sect. 13.3). Similar controls are also present for biomass production by plants, thus affecting the entire carbon budget of the ecosystem. These factors typically show a large temporal variability, resulting in years with large and small carbon gains and losses and highly variable process rates of substance turnover. Recently, new factors driving the C cycle in ecosystems have gained relevance—that is, the increase in the atmospheric CO₂ concentration, atmospheric N deposition, fertilisation and global warming (Part V). These slow but continuous changes in the growth conditions, particularly of plants as the primary producers in ecosystems, exert a strong forcing on many C-related processes, from leaf gas exchange to organic matter decomposition. In the long-term, this forcing disturbs the delicate balance of interacting ecophysiological and biogeochemical processes, often with pronounced consequences for ecosystem functions and the original species composition. For example, continuously increasing CO ₂ concentrations affect growth of plants but also increase their nutrient requirements. Since nutrient availability stays constant (or even decreases because of drier soils), these nutrient requirements, particularly for nitrogen (N), are not easily met, leading to larger C to N ratios in plant tissues, which in turn decompose much more slowly than tissues produced under lower CO₂ concentrations (Chap. 16, Sect. 16.2). Thus, the feedback to the atmosphere in terms of respired CO₂ from decomposition is also influenced, not only the assimilation of atmospheric CO₂.

13.5.2 Sudden Disturbances and Reallocation of Pools

These continuously changing drivers and their impacts are quite different from sudden disturbances which, for example, interrupt production and decomposition of organic matter. “Catastrophes” such as hurricanes, snow damage and herbivory by insects are examples of these sudden disturbances, as are smaller disturbances such as those caused by moles or wild boars, as well as grazing in extensive (land use) systems, as long as the organic matter is only relocated among different pools but not exported out of the system. Such disturbances can directly accelerate decomposition of plant and soil organic matter by increased litter production in response to the disturbance, or they can change the aggregate structure of the soil—one of the factors controlling the activities of soil biology. This type of disturbance may lead to temporary accumulation of organic matter in certain compartments (e.g. as litter or woody debris) and to reallocation of pools and thus resources. However, these disturbances generally do not change the ecosystem, since most ecosystems are quite resilient to natural disturbances—that is, they can adapt and buffer their impacts. Sometimes, combined disturbances happen—for example, drought plus the occurrence of a pathogen. Then, the second disturbance, often only occurring after the first one, can have devastating effects, even leading to species loss and ecosystem change (Box 13.1).

Box 13.1: Impacts of Combined Disturbances

A well-known example from western North America is the combination of unusually hot and dry summers and mild winters over the recent years, followed by an outbreak of the mountain pine beetle (Dendroctonus ponderosae) (Fig. 13.6a) carrying the blue stain fungus, which blocks the xylem and thus the water and nutrient transport in the tree. Millions of hectares of conifer forests have been killed over the last decade. Normally, the beetle attacks only weak and old trees. However, because of the drought, many trees were already water-stressed when they got infected with the fungus and became a breeding ground for the larvae of the beetle. Climate change also favoured the beetle populations, which typically die back in cold winters, restricting their range. Nowadays, even two life cycles of the beetle are possible within 1 year, increasing the beetle population, which in turn also infests healthy, mature trees in larger areas. These trees die within 3–4 years (Fig. 13.6b), turning the forest from a carbon sink (sequestering C in biomass and soils) into a carbon source. This is when trees can become natural hazards themselves, as fuel for fires or by damaging power lines when they fall. In addition, loss of CO₂ assimilation and loss of transpiration but also a larger snowpack with a faster snowmelt negatively affect annual carbon sequestration while increasing water run-off out of these forests in spring.

/epubstore/S/E-D-Schulze/Plant-Ecology/OEBPS/images/72100_2_En_13_Chapter/72100_2_En_13_Fig6_HTML.jpg — Fig. 13.6
a Mountain bark beetle *Dendroctonus ponderosae* (Photo courtesy of B. Wermelinger). b Landscape with infested trees in Wyoming, USA, in 2009 (Photo courtesy of J. King, enabled by Light Hawk)

13.5.3 Sudden Disturbances and Loss of Pools

Disturbances that result in removal of biomass and resources from the ecosystem (export of resources) have impacts rather different from those disturbances after which pools and thus resources remain in the ecosystem. Loss of organic matter primarily occurs through harvests and fires, but also after large pest and pathogen infestations, by animals browsing and grazing in intensive land use systems where animal faeces are not returned to the pastures. In all of these disturbances, biomass and associated nutrients are lost or exported from the ecosystem and/or decomposed somewhere else—for example, in human-dominated systems, in sewage works or by burning of waste. The heterotrophic organisms of the ecosystem in which the organic matter was once produced are no longer involved in the decomposition process.

Harvests and intensive grazing remove not only carbon as plant organic matter but also nutrients with the export of biomass. This export of nutrients can quickly lead to soil degradation if nutrients are not resupplied by fertilisation. Historically, litter raking in forests was an important forest use. Newly fallen leaves were collected and used during the winter as bedding for animals in stables and taken to the fields in spring as fertiliser. As leaves and needles have considerably higher nutrient contents than wood, this form of forest use drastically decreased forest soil fertility and led to reallocation of nutrients from forests to fields over large areas of land. The effect of this type of forest use is still visible after decades as shallow humus layers in the affected forests (Schulze (2000), a site at Aubure in France). The impacts of intensive grazing strongly depend on the stocking rate of the livestock, which must be in accordance with the vegetative growth to be sustainable. If these sustainable stocking rates are exceeded, overgrazing occurs, which results first in a change of vegetation composition, then in a decrease of vegetation cover, then open (bare) soil and, finally, soil erosion—visible, for example, as tracks of grazing on steep slopes in alpine meadows. Measurement of net ecosystem CO₂ fluxes over a pasture in Panama clearly demonstrated how overgrazing can lead to large CO₂ losses, turning the pasture into a carbon source for the 2 years of the study (Wolf et al. 2011).

Another globally very important disturbance is fire, which can destroy standing biomass or deep peat soils over large areas in a short time. The intensity of the fire depends mainly on the available biomass but also on the wind speed, the moisture content of the fuel and oxygen concentrations (relevant for below-ground smouldering peat fires). Therefore, rare fires—in which plant biomass and soil organic matter that have accumulated in the long interval between the fires are consumed (stand-replacing fires)—can have more drastic effects than frequent and thus less intense ground fires. A famous example of such a fire is the one in Yellowstone National Park in 1988, which destroyed 320,000 ha over a couple of weeks because of special weather conditions and wildfire suppression for more than 50 years. Low-intensity fires (surface fires) are frequent in savanna systems, where temperatures at ground level usually remain below 100 °C and 40 °C is rarely reached at a 10 cm soil depth (Bradstock and Auld 1995). The often very short duration (only minutes) of the fire at the same site, owing to the low fuel load (i.e. biomass), is the reason for these fire characteristics.

Fire releases large amounts of carbon and nitrogen from the system in the form of different gaseous oxidation products (Konovalov et al. 2014), while base cations mostly remain in the ash in the ecosystem. A fraction of the remaining organically bound nitrogen is readily transformed into ammonium and nitrate in the warm soils covered with dark ash. Thus, seeds, young plants and surviving rhizomes experience good growth conditions, sometimes even better than those before the fire. Therefore, fire is used worldwide as an agricultural practice to burn off plant residues that would otherwise not decompose easily (e.g. in dry and hot areas) and to support new growth by increasing nutrient availability right after the fire. However, in the long-term, fire will lead to a reduction of soil fertility, particularly for the N supply, if this resource loss is not counteracted either by legumes or by fertilisation.

In boreal coniferous forests, the fire sequence determines the amount of carbon that may be stored in the soil (Fig. 13.7). Calculated over a period of 6000 years (since the retreat of the Laurentide Ice Sheet in North America), the frequently burned Pinus banksiana stands contain only half as much C in the soil as the less frequently burned Picea mariana stands. These amounts, in turn, are significantly smaller than those in very rarely burned blanket bogs where, independently of the fire frequency, the high water table conserves the carbon in the soil, indicating this ecosystem’s potential to accumulate organic matter. Saturation occurs in all cases—that is, there is a balance between the formation of organic matter in the soil and its consumption by respiration and fires—but this balance is also affected by the high frequency of fires and the low productivity of the vegetation (Harden et al. 2000).

/epubstore/S/E-D-Schulze/Plant-Ecology/OEBPS/images/72100_2_En_13_Chapter/72100_2_En_13_Fig7_HTML.png — Fig. 13.7
Modelled changes in soil carbon in a wetland fen with a low frequency of fires and in *Picea mariana* and *Pinus banksiana* stands experiencing increased fire frequencies. The zig-zags of the curves reflect the frequency with which fires occurred. The figure illustrates the cumulative amount of C that is removed from these ecosystems directly or indirectly by fires (Harden et al. 2000)

13.5.4 Impacts on Species Composition

Disturbances not only change the element or substance pools and associated fluxes, but also affect the composition of species within the ecosystem. Areas suitable for growth are opened that were previously occupied by other species now weakened or killed by the disturbance. The remaining, surviving organisms may spread or new species may appear because seedlings can establish without competitors, for example, after a stand-replacing fire. The greatest number of species is found with average disturbance intensities (the intermediate disturbance hypothesis; Fig. 13.8) (Connell 1978) (Chap. 20, Sect. 20.2). If the frequency of disturbance increases, only a few specialists remain.

/epubstore/S/E-D-Schulze/Plant-Ecology/OEBPS/images/72100_2_En_13_Chapter/72100_2_En_13_Fig8_HTML.png — Fig. 13.8
Conceptual model of the relationship between species diversity and resource availability, and between species diversity and habitat disturbance. Species diversity reaches a maximum with average ecosystem disturbance. (After Hobbie et al. (1994))

At sites where fires occur regularly, vegetation has adapted to fires, with thick bark (e.g. Sequoia in North America), capacity to resprout (e.g. many Mediterranean shrubs), the presence of lignotubers (i.e. thick swelling of the root crown, filled with starch, lots of meristems to resprout, e.g. Eucalyptus in Australia), or cones that need high temperatures to open and release seeds (e.g. Banksia in Australia). Sometimes, only the outer layer of the woody stems is burned and turns to charcoal, so regeneration from living tissues at the base of the stem is still possible. The Australian grass tree (Xanthorrhoea) even contains a resin that is difficult to burn and protects the stem from fire. In savanna systems, the sparse trees (between 10% and 50% cover) can typically survive (Fig. 13.9a, b); the grasses present in these systems can regrow from meristems close to the ground or below the ground, and herbaceous vegetation can germinate from the seeds below the ground (seed bank) and establish very quickly. Lush growth of herbaceous species is thus often a consequence of surface fires, as many nutrients become readily available in the ash (Fig. 13.9c) and shortly afterward because of high mineralisation rates in dark soils (the albedo effect), despite large fractions of C and N being lost from the system. Some plants—the so-called pyrophytes—even need fire to stimulate flowering or open their fruits (Fig. 13.9d).

/epubstore/S/E-D-Schulze/Plant-Ecology/OEBPS/images/72100_2_En_13_Chapter/72100_2_En_13_Fig9_HTML.png — Fig. 13.9
Examples of vegetation adapted to fire. a, b Fire vegetation on Mount Kilimanjaro (Shira Plateau). a *Erica arborea* as stunted vegetation that regenerates from charred branches. On average, there is a fire every 3–5 years. b Charred *E. arborea* at a site with less frequent fires. The bifurcated twigs show that the plant had survived at least one fire. Between the last and the most recent fire a substantial trunk had developed, and the intensity of the fire was therefore high. Both pictures were taken 2 years after the fire. Rejuvenation of the bushes had occurred, with bushy growth on the frequently burned site, where little biomass had accumulated between fires. In contrast, growth from the charred thick trunk was comparatively sparse. In the foreground is *Helichrysum splendidum*, a typical fire indicator. c Vigorous germination of *Festuca obturbans* and *Kniphofia thomsonii* after an intense fire in the alpine zone of Mount Kilimanjaro. In the foreground are charred twigs of *Erica arborea* and *Erica trimera*. d Fruits of the Australian *Hakea* (Proteaceae) open only after a fire. (Photos: E. Beck)

Often, continuous slow forcing and sudden disturbances create new growth conditions that favour different plant species from those prior to these events. But the new species might also feed back on other organisms and organism groups in the ecosystem (Chap. 19, positive and negative feedbacks), as well as on the environmental growth conditions. Thus, they create an environment that favours their own species, at the expense of its native competitor. Some of the underlying mechanisms at the molecular and plant levels have been already discussed in Parts I and II (e.g. Chap. 12), but new (feedback) processes come into play when we are considering the ecosystem level (Sect. 13.3, emergent properties; Chap. 19, positive and negative feedbacks). An example from North America illustrates this situation very nicely:

The Mediterranean grasses Bromus tectorum and Bromus rubens successfully invaded the North American Artemisia tridentata shrubland since the end of the nineteenth century, which led to a marked reduction of Artemisia (West and Young 2000) and a pronounced change in the fire frequency. Bromus is a winter annual, which germinates in the autumn, establishes and grows over winter and completes its life cycle in late spring/summer, avoiding most of the hot and dry summers as seed. Artemisia is a long-lived shrub, which grows throughout the summer and flowers in late summer/early autumn. It contains terpenoid compounds against herbivory and can resprout after a fire. When Bromus was introduced accidentally to this perennial ecosystem, a plant with not only a very different life cycle but also very different ecophysiology was introduced (Germino et al. 2016). Bromus has very high water use early in the growing season before seed set, when water availability in the soil is still high. However, this means that the soil water pool is depleted faster and Artemisia, which must survive the dry summer period, does not have enough water later in the season. Thus, Artemisia dies, often without having formed seeds. In addition, the effect of differential water use is enhanced by fire. Because of the high fuel load with dead Bromus biomass during summer, the fire frequency has increased from 60–110 years to only 3–4 years (D’Antonio and Vitousek 1992). This again favours Bromus, which germinates from seeds in the autumn, when its own standing biomass has been burned and its competitor, Artemisia, has been weakened or killed. Moreover, Artemisia cannot regenerate as fast as Bromus, and Artemisia seedlings are not able to compete with Bromus when competing directly. This example demonstrates that the consequences of resource use are quite different at the plant level versus the community level—that is, when competition for the same resource sets in and “saving” water might just provide water for a competitor. Therefore, many successful invasive species have relatively high resource demands, not being very resource efficient (Fig. 13.10). This example also shows how changes in the native species composition owing to slow changes and disturbances can in turn affect the ecosystem’s environment. Fire frequencies have changed because Bromus produces a lot of standing biomass that burns easily and, as a grass, it can withstand fires easily and either germinate from the seed bank or regrow from root collar meristems. Thus, shortened fire intervals have favoured the invasive grass over the native perennial shrub.

/epubstore/S/E-D-Schulze/Plant-Ecology/OEBPS/images/72100_2_En_13_Chapter/72100_2_En_13_Fig10_HTML.png — Fig. 13.10
Invasive plant species. a Invasive species often spread along roads. *Salsola kali* (Chenopodiaceae) can be found close to roads in the semi-deserts of Utah in western North America. *Salsola* is a salt-tolerant species from Asian deserts. Away from the kerb of the road, the Mediterranean *Bromus* dominates. b Mediterranean *Bromus* in the *Artemisia* shrubland of Wyoming. *Artemisia* cover is already broken. (Photos: E.-D. Schulze)

13.6 Ecosystem Budget Approach

Biogeochemical processes that take place in terrestrial ecosystems can be described as fluxes into, out of or within ecosystem components and compartments, but also as size changes of element or water pools present in ecosystems. Often element or nutrient pools are also called stocks—for example, soil carbon stocks. Pools or stocks can be compared with the amount of money in a bank account, while fluxes in this analogy would be all transfers—that is, deposits and withdrawals from this bank account. Thus, if the withdrawals exceed the deposits, the pool is decreasing in size or even disappears; if the deposits exceed the withdrawals, the pool is increasing in size. One can also calculate the net budget of all gains (deposits) and losses (withdrawals). This net budget (the sum of all transfers) can be either positive (gains > losses) or negative (gains < losses), depending on the fluxes (transfers). If one considers the CO₂ budget of an ecosystem, a positive net budget represents the condition when assimilation exceeds respiration; a negative net budget represents the condition when assimilation is lesser than respiration. If one considers the C budget, additional fluxes need to be considered—for example, methane uptake and loss, volatile organic carbon losses, dissolved organic and inorganic losses, etc. Often these fluxes have a special sign convention, depending on the discipline in which they are used. So plant ecophysiological calculate with positive numbers when assimilation is studied, while micrometeorologists calculate with negative numbers when they consider assimilation, since CO₂ taken up by plants is lost from the atmosphere. In all disciplines, the following units are used to describe biogeochemical processes:

Pool or stock size (amount of an element or substance): grams per square metre
Unidirectional element or substance flux rate: grams per square metre per unit of time
Net element or substance budget: grams per square metre per unit of time, often partitioned into losses and gains, also called sources and sinks

Ecosystem budget studies have become increasingly frequent in recent decades, although tracing and measuring all processes within an ecosystem will hardly be possible, despite the application of sophisticated new methods. Mechanisms regulating the size of the flux include regulation by the substrate (i.e. feed-forward regulation), regulation by the products (i.e. feedback regulation), branching, modulation and co-limitation (Parts I and II). The details of selected biogeochemical fluxes are described in Chap. 16.

13.6.1 Stand Growth

Growth of plant stands depends not only on assimilation of CO₂ and the availability of water but also on the turnover and availability of nutrients (Chap. 12, Sect. 12.4). This means that to describe stand growth, the net budgets of carbon, water and nutrients need to be quantified. Water and CO₂ may be regarded in ecosystems as renewable resources from an indefinitely large atmospheric pool (with certain exceptions, such as deserts). This is different for nutrients. Generally, nutrients are released during decomposition of organic matter or during weathering of primary minerals or bedrock, and they are then reversibly absorbed to the soil ion exchange system or to soil organic matter (SOM). The ash content of biomass shows the amount of nutrients taken up by plants from the soil (neglecting above-ground uptake from deposition). These nutrients become available by decomposition of organic biomass when it is not exported (for example, by harvesting) but remains as dead biomass (litter, debris) in the system (Chap. 16). Ulrich (1987) has formalised the connection between the availability of CO₂, water and nutrients, and their incorporation into organic substances, with an equation of substance and energy balance for organic matter in ecosystems:

$\begin{array}{l}a{\mathrm{C}\mathrm{O}}_2+x{\mathrm{M}}^{+}+y{\mathrm{A}}^{-}+\left(y-x\right){\mathrm{H}}^{+}+z{\mathrm{H}}_2\mathrm{O}+\mathrm{energy}\\ {}\kern1em \leftrightarrow \left({\mathrm{C}}_a{\mathrm{H}}_{2z}{\mathrm{O}}_z{\mathrm{M}}_x{\mathrm{A}}_y\right)\mathrm{org}.\kern0.5em \mathrm{matter}+\left(a+\dots \right){\mathrm{O}}_2\end{array}$

(13.4)

where M⁺, A⁻ and H⁺ are cations, anions and hydrogen ions, and the coefficients a, x, y and z are stoichiometric coefficients in the soil solution. To maintain high production rates (of organic matter containing these substances) in managed systems, the use of elements from the environment owing to harvesting must be replaced by fertilisation.

13.6.2 Mean Residence Times

In a well-balanced system, formation of biomass and mineralisation of organic matter would occur simultaneously and be in equilibrium. However, this equilibrium does not occur in the real world, as there are considerable time lags between use (for biomass formation) and release (via mineralisation) of resources, described by the mean residence time (MRT; Eq. 13.5) in the respective compartment.

$\mathrm{MRT}=\frac{\mathrm{Pool}}{\mathrm{Flux}\ \mathrm{rate}}$

(13.5)

where Pool is given in mass per area and Flux rate is given in mass per area and time. Thus, the unit of MRT is time.

For example, leaves are synthesised within one year, but the foliage stays on a plant maybe for years and the litter is decomposed over many months to years—that is, nutrients bound in leaves will, on average, become available for further uptake and growth only after 2–8 years (Persson et al. 2000). The mean residence time of these nutrients in the foliage is thus 2–8 years. In wood, nutrients may be bound for more than 100 years. The decomposition of a tree trunk takes decades, and thus the delay between uptake of resources and return of the same resources is a lot longer for wood than for leaf litter (Chap. 16, Sect. 16.2.2).

The accumulation of litter is thus a sign that the shedding of leaves or needles exceeds the capacity of soil organisms to decompose this litter. Fast decomposition is limited either by lack of the organisms capable of mineralisation this litter, by plant compounds that are difficult to metabolise, by unfavourable climatic conditions, by stabilisation in the soil, or by a combination thereof. An open stand typically provides better conditions for mineralisation (higher soil temperature and moisture, affecting soil organisms) than a closed stand. Evergreen needles are less decomposable than deciduous leaves. Woody material decomposes more slowly than foliage. Depending on the decomposability of organic matter, different fractions of soil organic matter have very different ages, ranging from very young (recent soil carbon, up to 30 years old) to soil organic matter, which might be older than 1000 years (Townsend et al. 1995; Schulze et al. 2000; Schlesinger and Bernhardt 2013).

13.6.3 Loss of Resources

Terrestrial ecosystems are thermodynamically open systems—that is, energy and matter get lost. Some of these losses are unavoidable, since they occur naturally either as part of the energy budget (Chap. 9), during background soil biogeochemical and plant ecophysiological or defence processes (Chaps. 11 and 12), while some of these losses have natural agents such as wind, water and fire. But also anthropogenic activities play a major role—for example, human-induced fires, forest and agricultural management (Sect. 13.5.3), and environmental pollution. The resources that are lost include:

Water: owing to the energy budget, run-off and infiltration into deeper soil horizons
Mineral particulate matter: during dust storms and owing to water erosion
Organic particulate matter: owing to erosion and during fires
Carbon and nutrients (natural): via decomposition and nitrification, respiration and volatile organic carbon (VOC) production, infiltration into deeper soil horizons, run-off, erosion after natural disturbances
Carbon and nutrients (anthropogenic): during fires and owing to management of terrestrial ecosystems
Increased nutrient losses: owing to soil acidification after environmental pollution (N deposition)

Some of these resources lost from one ecosystem can be beneficial to other ecosystems. For example, mineral dust transported across the Atlantic Ocean fertilises the Amazonian forest with basic cations (Bristow et al. 2010). On the one hand, some losses are core for closing biogeochemical cycles. Denitrification (the release of N₂) is the only natural process capable of closing the global N cycle. On the other hand, some losses from terrestrial ecosystems have detrimental effects on ecosystem health—of the ecosystem losing the resource and of the ecosystem(s) receiving it. For example, accelerated nitrate and cation losses occurred because of soil acidification after environmental pollution (N deposition) and resulted in decreased forest health and tree growth across Europe in the 1980s and 1990s.

Chemical conditions in the soil are primarily dependent on the constitution of the original bedrock (Chap. 11). However, these conditions are changed as a consequence of the mobilisation and uptake of nutrients by microorganisms and plants—for example, when seasonality of plant growth, and thus demand, is decoupled from supply via microbial decomposition and nitrification processes. Since nitrate in the soil is not bound to minerals or organic matter, it can be leached into deeper soil horizons and/or the groundwater, taking along cations. For example, if nitrate is formed in autumn, when most plants stop growing, large nitrate and cation losses occur during winter. In summer, however, the nitrogen requirement of the vegetation can even exceed the supply from the soil. This asynchronous pattern of supply and demand leads, in the end, to changes in habitat conditions, with local overexploitation (loss of cations in the upper mineral layer—that is, podzolisation and acidification of soils; Chap. 11, Sect. 11.1) or accumulation of intermediary products, given that degradation is impeded by decreasing pH (e.g. raw humus).

But also anthropogenic activities, such as acid deposition with strong acids (H₂SO₄, HNO₃), affect chemical conditions in the soil. The rate of soil acidification depends on the mineral constitution of the bedrock and the cumulative acid inputs. On limestone soils with a high CaCO₃ content, incoming acid deposition is at first balanced by weathering of carbonate (Eqs. 13.6 and 13.7; Fig. 13.11a):

/epubstore/S/E-D-Schulze/Plant-Ecology/OEBPS/images/72100_2_En_13_Chapter/72100_2_En_13_Fig11_HTML.png — Fig. 13.11
Change of nutrient availability with soil pH. a Changes in the pH of the soil solution with continuing weathering as a consequence of the cumulative proton stress—that is, acid deposition (after Schulze and Ulrich (1991)). b Availability of nutrients depends on the pH of the soil solution (Larcher 2003)

${\mathrm{Ca}\mathrm{CO}}_3+{\mathrm{H}}_2{\mathrm{CO}}_3\leftrightarrow {\mathrm{Ca}}^{2+}+2{\mathrm{H}\mathrm{CO}}_3^{-}$

(13.6)

and:

${\mathrm{H}}_2{\mathrm{CO}}_3\leftrightarrow {\mathrm{H}}^{+}+{\mathrm{H}\mathrm{CO}}_3^{-}\leftrightarrow {\mathrm{CO}}_2+{\mathrm{H}}_2\mathrm{O}$

(13.7)

The Ca²⁺ ions that are released occupy the charges that are freed at the soil exchange sites (Chap. 11, Sect. 11.1). With time, CaCO₃ will be continuously consumed, the soil pH will further decrease and a reversible exchange of cations will occur with clay minerals and organic matter. Under continuing acid inputs and thus loss of cations from the exchange buffer, the H⁺ buffering will be achieved by metal oxides and hydroxides, leading to a pH-dependent increase in the availability of certain metal ions. For example, Mn²⁺ becomes mobile at a pH between 5 and 4.2. At a pH of 4.2, the soil reaches another stable buffer system, the one buffered by Al hydroxides (Chap. 7, Sect. 7.5). The iron buffer range (with Fe³⁺) is reached below pH 3.8. The availability of ions is very variable during the course of this process, and each element is specifically dependent on the pH of the soil solution (Fig. 13.11b).

The chemical changes in the soil are reversible—that is, by fertilisation or liming, provided that the clay minerals are not restructured. As soon as the crystalline structure of silicates and clay minerals is changed (e.g. by dissolving the Al lattice in a replacement of alkaline cations with protons; Chap. 11), a reversal into the original state is no longer possible, not even by abundant supply of cations. The ecosystem’s health is damaged irreversibly.

13.7 Summary

Terrestrial ecosystems are functional units in a given heterogeneous landscape and include soil, microorganisms, vegetation and animals, as well as the lower level of the atmosphere.
Ecosystems are complex networks of relationships and processes. Here, biogeochemical processes happen and plant, animal and microbial species interact with each other.
All ecosystems, managed and unmanaged, are affected by environmental and human drivers, even very remote ones. There are no “natural” ecosystems.
An ecosystem is a thermodynamically open system, where energy and matter can also get lost to the atmosphere or the hydrosphere. Thus, to study ecosystems, a systems approach is used. Pools and fluxes of energy and matter must be measured per unit of ground area.
Ecosystems exhibit emergent properties, such as ecosystem structure, biogeochemical cycling within ecosystems, species and process interactions, or time lags in ecosystem responses.
Ecosystems are characterised by nonlinear response functions for processes with interacting drivers. These include saturation in production of organic matter with increasing biomass and exponentially increasing respiration with temperature, counteracted by low soil moisture.
The availability of resources (especially water and nutrients) determines the turnover rates in an ecosystem. Internal cycling and resource losses can be major factors. Uptake of nutrients by plants and their remineralisation usually do not occur simultaneously; hence in ecosystems, carbon is accumulated and losses of nitrogen and nutrients may occur. Harvesting and grazing are important factors removing resources from an ecosystem.
Ecosystems are generally not at equilibrium or at steady-state with respect to the various biogeochemical processes or species composition. Disturbances constantly change these dynamics. Depending on the resistance and the resilience of ecosystems, slow continuous forcing, as well as sudden disturbances, affect their processes and species composition.

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