MODELING FOREST RESPONSE TO INCREASING CO2 CONCENTRATION UNDER NUTRIENT-LIMITED CONDITIONS

The growth rates of woody plants depend on both the rate of photosynth etic carbon gain and the availability of essential nutrients. Instanta neous carbon gain is known to increase in response to increasing atmos pheric CO2 concentration, but it is uncertain whether this will transl ate into increased growth in the longer term under nutrient-limited co nditions. An analytical model to address this question was developed b y Comins and McMurtrie (1993, Ecological Applications 3, 666-681). The ir model was further tested and analysed. Manipulation of various assu mptions in the model revealed its key assumptions and allowed a more c onfident prediction of expected growth responses to CO2 enrichment und er nutrient-limited conditions. The analysis indicated that conclusion s about the CO2 sensitivity of production were strongly influenced by assumptions about the relationship between foliar and heartwood nitrog en concentrations. With heartwood nitrogen concentration proportional to foliar nitrogen concentration, the model predicted a strong respons e of plant productivity to increasing CO2 concentration, whereas with heartwood nitrogen concentration set constant, the model predicted onl y a very slight growth response to changing CO2 concentration. On the other hand, predictions were only slightly affected by: (1) assumption s about the extent of nitrogen retranslocation out of senescing roots and foliage or wood during heartwood formation; (2) the effects of nit rogen status on specific Leaf area or (3) leaf longevity; (4) carbon a llocation between different plant parts; or (5) changes in the N:C rat io of organic matter sequestered in the passive pool of soil organic m atter. Modification of the effect of foliar nitrogen concentration on the light utilization coefficient had only a small effect on the CO2 s ensitivity 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 gr owing under nitrogen-limited conditions can respond to increasing atmo spheric CO2 concentration with considerable increases in growth.