Modelling forest-growth response to increasing CO2 concentration in relation to various factors affecting nutrient supply

It is well recognized that photosynthesis of C3 plants is highly responsive to CO2 concentration. However, in natural ecosystems, plants are subject to a range of feedback effects that can interact with increased photosynthetic carbon gain in different ways so that it is not clear to what extent increased photosynthesis will translate into increased growth. To assess the probable growth response of nutrient-limited forests to increasing CO2 concentration, we use a previously developed modelling framework and apply it under conditions where the supply of nutrients is affected by a range of different factors. Our analysis indicates that forest growth is likely to be highly stimulated by increasing CO2 concentration in forests with high fertility, in forests with nitrogen fixing plants, in those subject to fire or where nitrogen in wood is effectively removed from the biologically active cycle either through physical removal of stems in harvesting or through continued stem growth over long time periods. Forest growth is likely to be stimulated by CO2 concentration in both phosphorus-and sulphur-limited forests provided nutrients in heartwood of trees are removed from the active nutrient cycle. Without this removal from the cycling system, however, sulphur-limited forests should show little response to increasing CO2. In phosphorus-limited forests without phosphorus removal, the response to increasing CO2 depends further on the equilibration state of the large pool of unavailable secondary phosphorus. Considered over periods of centuries during which the secondary pool has equilibrated, growth of phosphorus-limited forests is likely to be only weakly stimulated by increasing CO2 concentration. However, over shorter periods, increasing CO2 concentration should lead to a substantial increase in productivity. In general, it can be concluded that systems that are more open with respect to nutrient gains and losses are likely to be more responsive to increasing CO2 concentration than systems where the amount of available nutrients is less variable. In more open systems, operation at a lower internal nutrient concentration as a result of increasing atmospheric CO2 concentration can lead to reduced nutrient losses per unit carbon gain. Our analysis shows that the effect of increasing CO2 on forest growth can differ substantially between forests due to interactions with a range of factors that affect nutrient supply. The response of a particular forest to increasing CO2 concentration can only be predicted if the main factors controlling nutrient supply and growth in that forest are understood and incorporated into an assessment.