14.2.4.2 The terrestrial system

The metabolic processes that are responsible for plant growth and maintenance and the microbial turnover, which is associated with dead organic matter decomposition, control the cycle of carbon, nutrients, and water through plants and soil on both rapid and intermediate time-scales. Moreover, these cycles affect the energy balance and provide key controls over biogenic trace gas production. Looking at the carbon fixation-organic material decomposition as a linked process, one sees that some of the carbon fixed by photosynthesis and incorporated into plant tissue is perhaps delayed from returning to the atmosphere until it is oxidised by decomposition or fire. This slower carbon loop through the terrestrial component of the carbon cycle affects the rate of growth of atmospheric CO₂ concentration and, in its shorter term expression, imposes a seasonal cycle on that trend (Chapter 3, Figure 3.2a). The structure of terrestrial ecosystems, which respond on even longer time-scales, is determined by the integrated response to changes in climate and to the intermediate time-scale carbon-nutrient machinery. The loop is closed back to the climate system, since it is the structure of ecosystems, including species composition, that largely sets the terrestrial boundary condition of the climate in terms of surface roughness, albedo, and latent heat exchange (see Chapter 3, Section 3.2.2).

Modelling interactions between terrestrial and atmospheric systems requires coupling successional models to biogeochemical models and physiological models that describe the exchange of water and energy between vegetation and the atmosphere at fine time-scales. At each step toward longer time-scales, the climate system integrates the more fine-scaled processes and applies feedbacks onto the terrestrial biome. At the finest time-scales, the influence of temperature, radiation, humidity and winds has a dramatic effect on the ability of plants to transpire. On longer time-scales, integrated weather patterns regulate biological processes such as the timing of leaf emergence or excision, uptake of nitrogen by autotrophs, and rates of organic soil decay and turnover of inorganic nitrogen. The effect of climate at the annual or interannual scale defines the net gain or loss of carbon by the biota, its water status for the subsequent growing season, and even its ability to survive.

As the temporal scale is extended, the development of dynamic vegetation models, which respond to climate and human land use as well as other changes, is a central issue. These models must not only treat successional dynamics, but also ecosystem redistribution. The recovery of natural vegetation in abandoned areas depends upon the intensity and length of the agricultural activity and the amount of soil organic matter on the site at the time of abandonment. To simulate the biogeochemistry of secondary vegetation, models must capture patterns of plant growth during secondary succession. These patterns depend substantially on the nutrient pools inherited from the previous stage. The changes in hydrology need also to be considered, since plants that experience water stress will alter the allocation of carbon to allocate more carbon to roots. Processes such as reproduction, establishment, and light competition have been added to such models, interactively with the carbon, nitrogen, and water cycles. Disturbance regimes such as fire are also incorporated into the models, and these disturbances are essential in order to treat successfully competitive dynamics and hence future patterns of ecosystem. It should be noted also that these forcing terms themselves might be altered by the changes that result from changes in the terrestrial system.

This coupling across time-scales represents a significant challenge. Immediate challenges that confront models of the terrestrial-atmosphere system include exchanges of carbon and water between the atmosphere and land, and the terrestrial sources and sinks of trace gases.

Prognostic models of terrestrial carbon cycle and terrestrial ecosystem processes are central for any consideration of the effects of environmental change and analysis of mitigation strategies; moreover, these demands will become even more significant as countries begin to adopt carbon emission targets. At present, several rather complex models are being developed to account for the ecophysiological and biophysical processes, which determine the spatial and temporal features of primary production and respiration (see Chapter 3, Sections 3.6.2 and 3.7.1). Despite recent progress in developing and evaluating terrestrial biosphere models, several crucial questions remain open. For example, current models are highly inconsistent in the way they treat the response of Net Primary Production (NPP) to climate variability and climate change – even though this response is fundamental to predictions of the total terrestrial carbon balance in a changing climate. Models also differ significantly in the degree of CO₂ fertilisation they allow, and the extent to which CO₂ responses are constrained by nutrient availability; the extent to which CO₂ concentrations affect the global distribution of C₃ and C₄ photosynthetic pathways; and the impacts of climate, CO₂ and land management on the tree-grass balance. These are all areas where modelling capability is limited by lack of knowledge, thus making it crucially important to expand observational and experimental research. Important areas are interannual variability in terrestrial fluxes and the interplay of warming, management, and CO₂ enrichment responses at the ecosystem scale. Moreover, these issues must be far better resolved if there is to be an adequate verification scheme to confirm national performance in meeting targets for CO₂ emissions. (See Chapter 3, Sections 3.6.2 and 3.7.1.)

Finally, while progress will be made on modelling terrestrial processes, more integrative studies are also needed wherein terrestrial systems are coupled with models of the physical atmosphere and eventually with the chemical atmosphere as well.

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