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WIREs Clim Change
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Forest productivity under climate change: a checklist for evaluating model studies

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Climate change is highly likely to impact on forest productivity over the next century. The direction and magnitude of change are uncertain because many factors are changing simultaneously, such as atmospheric composition, temperature, rainfall, and land use. Simulation models have been widely used to estimate how these interacting factors might combine to alter forest productivity. Such studies have used many different types of models with different underlying assumptions. To evaluate predictions made by such studies, it is essential to understand the type of model and the assumptions used. In this article, we provide a checklist for use when evaluating modeled estimates of climate change impacts on forest productivity. The checklist highlights the assumptions that we believe are critical in determining model outcomes. Models are classified into different general types, and assumptions relating to effects of atmospheric CO2 concentration, temperature, water availability, nutrient cycling, and disturbance are discussed. Our main aim is to provide a guide to enable correct interpretation of model projections. The article also challenges modelers to improve the quality of information provided about their model assumptions. WIREs Clim Change 2011 2 332–355 DOI: 10.1002/wcc.108

Figure 1.

Comparison of the effects of increasing atmospheric CO2 concentration on instantaneous leaf photosynthesis (filled symbols) and annual canopy photosynthesis (open symbols). Leaf photosynthesis data were measured by Jeff Warren at the Oak Ridge National Laboratories (ORNL) FACE (free air CO2 enrichment) experiment. Annual canopy photosynthesis was estimated using the MAESTRA canopy photosynthesis model (www.bio.mq.edu.au/maestra) parameterized for the ambient treatment at ORNL FACE, with only atmospheric CO2 concentration varying. The y‐axes are scaled so that points overlap at current ambient CO2 (360 µmol mol−1).

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Figure 2.

Effect of different assumptions about acclimation to temperature on predicted net leaf carbon uptake (= photosynthesis − respiration). Photosynthesis is assumed to have a peaked response to temperature, with an optimum at 25°C; respiration is assumed to increase exponentially with temperature. The thick solid line shows net leaf carbon uptake at ambient growth conditions (optimum at 19°C). The dashed line is generated by assuming that optimum and maximum temperatures for photosynthesis increase by 5°C but respiration rates do not change (acclimation of photosynthesis only; optimum at 20°C). The dotted line is generated by assuming that optimum and maximum temperatures for photosynthesis and the reference temperature for respiration all increase by 5°C (independent acclimation of photosynthesis and respiration; optimum at 22°C). The dot‐dash line is generated by assuming that optimum and maximum temperatures for photosynthesis increase by 5°C, and that respiration at the reference temperature of 20°C is a constant proportion of photosynthesis (acclimation of photosynthesis and respiration in tandem; optimum at 30°C).

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Figure 3.

Illustration of the importance of the threshold for drought effects on forest transpiration, simulated with a typical simple bucket model. (a) The ‘drought modifier’, i.e., the relative effect of water availability on transpiration for two soil types of different texture, as a function of the soil volumetric water content. Three important water contents are marked for the loam soil type: θFC is the water content at ‘field capacity’, θCRIT the ‘critical’ water content and θPWP the ‘permanent wilting point’ (PWP). The critical water content is much lower for the sand soil type, but the decline to the PWP is much steeper. (b) Example simulation of the water balance for 1 year with a simple bucket model for the two soil types. Soil moisture storage reaches lower values in the sand soil type because transpiration is not reduced until a lower soil water content. (c) Simulated drought stress for the two soil types over the year. Drought stress is shown to depend strongly on the critical water content and PWP, with a much later onset of drought in the sandy soil, but a much steeper decline.

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