3. Ecosystems, their structures, functions and management
3.4 Biogeochemical processes and ecosystems
Every ecosystem have their own typical and site specific biogeochemical processes, which altogether form the “the global ecosystem” with its global biogeochemical processes and cycles.
The earth is the only planet that supports abundant carbon-based life, because carbon is not in the atmosphere (unlike in the other planets) but bound in living things or their residues (e.g.
coal, oil, soil organic matter). Two gases, nitrogen and oxygen, make up 99 % of the earth’s atmosphere. Life alters atmospheric chemistry because organisms take up, utilize and cycle some elements more than others. Carbon dioxide occurs only in very small amounts in the atmosphere (0.03%) (Perry et al. 2008, p.
22).
The cycles of some important elements through the global ecosystems are illustrated in Fig. 3. The chemical elements required by life are not distributed evenly in the globe. Water and carbon are concentrated in the oceans, molecular nitrogen and oxygen in the atmosphere and other elements essential for life in the rocks. Hence cycling from one portion of the biosphere to another is essential to maintain life everywhere (Perry et al. 2008, p. 23-24).
Fig. 3. The global cycles of some biologically important elements: carbon (C), oxygen (O), nitrogen (N), sulfur (S) and water (H2O). Photosynthesis moves carbon from air and release oxygen to the atmosphere. Respiration (organisms liberate the energy of carbon compounds to power the life) reverses photosynthesis, removing oxygen from the air to recombine with carbon and form carbon dioxide. Fire has the same effect.
Microbes in soils, streams and oceans utilize nitrogen compounds for energy, releasing nitrogen gas to the air. Besides evaporation, land plants transpire water from leaves to atmosphere. Precipitaton brings water back to soils and waters. Sulphur (rocks) is emitted to air from fossil fuels, phytoplankton and by microbes in anaerobic conditions of bogs and sediments. The chemistry of atmosphere, in turn, profoundly influences factors that are of considerable consequence to life, such as global temperature and penetration of harmful radiation from the sun (Perry et al. 2008, p. 21-27, from NASA 1988).
Two cycles can be distinguished: (1) the gaseous cycle, in which elements move through atmosphere during at least some portion of their global cycle; and (2) the sedimentary cycle, in
which elements move from land to water and then to sediments, where they remain until moved to land by tectonic activity.
Time scale of gaseous cycles is some years but that of sedimentary cycles can be millions of years. The equivalent of the entire atmospheric content of carbon passes through the terrestrial biota every six years but a molecule of phosphorus eroded from hillside and moved by water to eventually reside in deep ocean and sediments may not move to land again for million of years (Perry et al. 2008, p. 24).
At the ecosystem level, ecological modeling has been an important instrument to analyse and predict ecosystem behavior in different contexts. For example, Waring & Running (2007) recognizes two basic classes of forest ecosystem (process) models: biogeochemistry models (computing growth from the seasonal dynamics of canopy carbon balance) and gap models of forest dynamics (emphasizing disturbance, recruitment and mortality processes that affect individual trees). Forest succession is normally a combination of autogenetic succession (due to minor disturbances like life cycle of single trees) and allogenetic succession (driven by major disturbances such as forest fires and storms) (Kellomäki 2009, Kuuluvainen & Aakala 2011). Both model types are important for the understanding of the complexity of the generation of ecosystem goods and services. The gap and succession models focus on longer term and biochemistry models to shorter term dynamics. Our example represents the latter type.
The purpose of the following comprehensive ecosystem biogeochemical model (Table 3, Waring & Running 2007, p. 6) here is to demonstrate the complexity of interdependent and interacting processes, which are found to be important for modeling the structure and functioning of forest ecosystems.
Waring & Running (2007) state that while no current model include all the processes completely, “it is essential that energy, carbon, water and elemental cycles are all be represented, even if simplistically. It is precisely the interaction among the cycles that are the core of ecosystem analysis”.
Each of the processes and subprocesses are in one way or another, sometimes “alone”, sometimes “together”, forming the functions (defined in ecosystem service literature as a sub-set of biophysical structure or process, de Groot et al. 2010) providing ecosystem services (cf. Chapters 5.1, 5.2; Fig. 5).
Table 3. Component processes of a comprehensive ecosystem biogeochemical model (Waring & Running 2007)
Canopy and litter interception and storage Soil surface infiltration
Leaves, stem/branches, roots, defensive compounds, reproduction Phenological timing
Sources (atmosphere, rock weathering biological fixation) Soil solution transformation
Waring & Running (2007, p. 7, following Rastetter 1996) underline, that “ecosystems, because of their dynamic and interconnected properties, cannot be subjects to classic experimentation where one variable at a time is modified.
Computer simulation models of ecosystem behavior offer a valuable experimental alternative because they allow multivariate interactions to be traced and analyzed. Models can predict responses to new conditions that do not yet exist. For example, computer simulation models can predict how stream discharge may respond to harvesting in a watershed and identify possible flood problems before any logging commences.
Similarly, they have been the primary means for evaluating potential responses of natural ecosystems to future climate change”. The latter type of model simulations have been done, for example, in case of boreal forest in Finland (Kellomäki &
Väisänen1997). As is concluded later (Ch. 6.3), joint biophysical and human-ecological models are needed for the improved and integrated management of ecosystem goods and services.