Physiological derivation of the observed relationship between net primary production and mean annual air temperature

Empirical observations show that when examined along a climatic gradient, the annual net primary production (NPP) of terrestrial ecosystems increases with warmer mean annual air temperature (MAAT). This relationship has been used in models to calculate annual NPP and also its seasonal cycle by assuming some prescribed seasonality. However, the physiological processes causing the relationship and its applicability to climate change studies have not been examined. A mechanistic model of daily carbon exchange in forest ecosystems was used to derive the relationship between NPP and MAAT and to identify the physiological processes represented in this relationship. The model reproduced the observed empirical relationship and showed that it reflected, at least for boreal and temperate coniferous forests growing on moist soils, the length of the growing season, nitrogen limitation for NPP, and lower maintenance respiration rates in warm climates than in cold climates. NPP and the seasonal cycle of CO₂ uptake and release were relatively insensitive to the temperature dependencies of photosynthesis except at the beginning and end of the growing season. These analyses demonstrate that simple physiological assumptions can result in reasonable predictions of NPP over a wide range of climates. The seasonal cycle of CO₂ uptake and release and the empirical relationship between NPP and MAAT are consequences of the physiological assumptions and hence are validations of the NPP calculations rather than direct parameterizations of NPP. The NPP component of this model may provide the framework to integrate terrestrial carbon fluxes with the land surface schemes used in global climate models.