Temperate forest responses to carbon dioxide, temperature and nitrogen: a model analysis

The ITE Edinburgh Forest Model, which describes diurnal and seasonal changes in the pools and fluxes of C, N and water in a fully coupled forest–soil system, was parametrized to simulate a managed conifer plantation in upland Britain. The model was used to examine (i) the transient effects on forest growth of an IS92a scenario of increasing CO₂] and temperature over two future rotations, and (ii) the equilibrium (sustainable) effects of all combinations of increases in CO₂] from 350 to 550 and 750 μmol mol^?1, mean annual temperature from 7.5 to 8.5 and 9.5°C and annual inputs of 20 or 40 kg N ha^?1. Changes in underlying processes represented in the model were then used to explain the responses. Eight conclusions were supported by the model for this forest type and climate.

• 1 Increasing temperatures above 3°C alone may cause forest decline owing to water stress.
• 2 Elevated CO₂] can protect trees from water stress that they may otherwise suffer in response to increased temperature.
• 3 In N-limiting conditions, elevated CO₂] can increase allocation to roots with little increase in leaf area, whereas in N-rich conditions elevated CO₂] may not increase allocation to roots and generally increases leaf area.
• 4 Elevated CO₂] can decrease water use by forests in N-limited conditions and increase water use in N-rich conditions.
• 5 Elevated CO₂] can increase forest productivity even in N-limiting conditions owing to increased N acquisition and use efficiency.
• 6 Rising temperatures (along with rising CO₂]) may increase or decrease forest productivity depending on the supply of N and changes in water stress.
• 7 Gaseous losses of N from the soil can increase or decrease in response to elevated CO₂] and temperature.
• 8 Projected increases in CO₂] and temperature (IS92a) are likely to increase net ecosystem productivity and hence C sequestration in temperate forests.