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 micromoles mol-1, mean annual temperature from 7.5 to 8.5 and 9.5 degrees 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 degrees 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.