Project C.2:   Process modelling & testing

• Team Members
• Objectives
• Strategy
• Relevance
• Outputs
• Outcomes
• Sub-projects

Research objectives

• adding key aspects of ecosystem structure and physiological function to existing ecosystem models to make them suitable for typical applications in Australia;
• refining the understanding of the interactions between the response to environmental variables, such as CO₂ and temperature, and plant internal processes; and
• testing the response of these refined models against observations from sites across Australia.

Strategy

The CRC is concerned with the net carbon exchange from Australian ecosystems both in the present and in the future when these systems are subject to higher CO₂ concentration and probably to increased temperatures and changed rainfall patterns.

While carbon exchange under current conditions can be described reasonably well with the use of largely empirical models, the carbon exchange under different environmental conditions cannot yet be empirically observed. Empirical models therefore cannot be used to describe the carbon exchange under such different conditions. Hence, there is a need for the use of physiologically-based models.

• Subproject C.2.1 Improving process-based models
• Subproject C.2.2 Model Comparison
• Subproject C2.3 Process-based models to assess permanence of carbon stocks

Relevance

Physiologically-based models of net ecosystem carbon exchange should be based on the best understanding of relevant ecosystem processes. They are thus powerful tools to combine and summarise existing knowledge. When these tools are then applied to Australian ecosystems, both for continental applications (see Project C1: Model Coordination), and for detailed analyses of specific sites, they bring together the knowledge accumulated in the CRC in the application of principal interest to the CRC.

The use of physiologically based models is also indispensable for understanding the complex response of ecosystem carbon exchange to a range of perturbations, including those that cannot readily be studied experimentally, such as long-term plant responses to increasing CO₂ concentration. Only models that explicitly deal with the direct responses to external perturbations and all relevant system-internal feed-backs are in a position to meaningfully model the response of ecosystems to perturbations such as climate change.

Outputs

• Physiologically-based models implemented through COINS (see Project C1: Model Coordination).

Outcomes

• Better understanding held by the scientific and policy-making community of relevant processes that control net ecosystem carbon exchange.
• Better informed public decision-making on the role of vegetation sinks in managing atmospheric CO₂ concentrations.

Sub-project C.2.1

Improving process-based models

There is a range of modelling work being undertaken in other programs of the CRC. This project is to concentrate on further developing and strengthening physiologically based models. These models will use the latest understanding from plant physiology and soil science to develop the most complete description of whole-ecosystem carbon exchanges. Strong emphasis is to be placed on the processes of photosynthesis, including its response to CO₂ concentration, partitioning, water availability and nutrition, including the linkages between carbon and nutrient cycles.

These various processes have been combined in models such as CENTURY and CenW. However, while these models can adequately describe above- and below-ground carbon dynamics in uniform stands of vegetation, neither model has been formulated to deal with heterogeneous stands of vegetation, such as in woodlands or uneven-aged stands of native forests. Yet, such plant communities comprise the majority of Australia’s vegetation and are the most important components in the National Greenhouse Gas Inventory. Work will therefore concentrate on making use of the proven suitability of these models, but extend them in key areas so that they can become suitable for heterogeneous plant communities as well.

Sub-project C.2.2

Model Comparison

Detailed model comparisons are to be performed for forest plantation and other systems using existing models at the stand scale. This process is to partly be based on observed differences between models run at the continental scale (see Project C1: Model Co-ordination).

. If different models provide significantly different totals for carbon exchange when they are run at the national scale, it must create uncertainty as to the true biospheric carbon exchange for use in national carbon accounting. Work will then be undertaken to find the reasons for such discrepancies in simulation outputs, and recommend improved implementations of models in national applications.

Differences in model output are to be examined to determine:

• why models respond differently to various drivers;
• which models perform best against observations; and
• the sensitivities of respective models to key parameter changes.

The findings will be used to refine the models used within CRC programs in order to improve the robustness and accuracy of predictions of carbon stocks and fluxes.

Sub-project C2.3

Process-based models to assess the permanence of carbon stocks

Apart from increasing model functionality, the aim is to further test the impact of climate change, CO₂ fertilisation and nutrient cycles on carbon stocks. Initial emphasis is proposed to be on managed plantations that are now being planted for use in timber production and carbon trading. The long-term security of these carbon stocks is to be investigated. Such modelling should support carbon trading by examining future risks to plantation stocks from climate change.