Modelling forest response to increasing CO₂ concentration under nutrient‐limited conditions

The growth rates of woody plants depend on both the rate of photosynthetic carbon gain and the availability of essential nutrients. Instantaneous carbon gain is known to increase in response to increasing atmospheric CO₂ concentration, but it is uncertain whether this will translate into increased growth in the longer term under nutrient‐limited conditions. An analytical model to address this question was developed by Comins & McMurtrie (1993, Ecological Applications 3, 666–681). Their model was further tested and analysed. Manipulation of various assumptions in the model revealed its key assumptions and allowed a more confident prediction of expected growth responses to CO₂ enrichment under nutrient‐limited conditions. The analysis indicated that conclusions about the CO₂ sensitivity of production were strongly influenced by assumptions about the relationship between foliar and heartwood nitrogen concentrations. With heartwood nitrogen concentration proportional to foliar nitrogen concentration, the model predicted a strong response of plant productivity to increasing CO₂ concentration, whereas with heartwood nitrogen concentration set constant, the model predicted only a very slight growth response to changing CO₂ concentration. On the other hand, predictions were only slightly affected by: (1) assumptions about the extent of nitrogen retranslocation out of senescing roots and foliage or wood during heartwood formation; (2) the effects of nitrogen status on specific leaf area or (3) leaf longevity; (4) carbon allocation between different plant parts; or (5) changes in the N:C ratio of organic matter sequestered in the passive pool of soil organic matter. Modification of the effect of foliar nitrogen concentration on the light utilization coefficient had only a small effect on the CO₂ sensitivity for pines. However, this conclusion was strongly dependent on the chosen relationship between single‐leaf photosynthesis and leaf nitrogen concentration. Overall, the analysis suggested that trees growing under nitrogen‐limited conditions can respond to increasing atmospheric CO₂ concentration with considerable increases in growth.