Tree and forest functioning in an enriched CO₂ atmosphere

Abstract

Forests exchange large amounts of CO₂ with the atmosphere and can influence and be influenced by atmospheric CO₂. There has been a recent proliferation of literature on the effects of atmospheric CO₂ on forest trees. More than 300 studies of trees on five different continents have been published in the last five years. These include an increasing number of field studies with a long-term focus and involving CO₂×stress or environment interactions. The recent data on long-term effects of elevated atmospheric CO₂ on trees indicate a potential for a persistent enhancement of tree growth for several years, although the only relevant long-term datasets currently available are for juvenile trees.

The current literature indicates a significantly larger average long-term biomass increment under elevated CO₂ for conifers (130%) than for deciduous trees (49%) in studies not involving stress components. However, stimulation of photosynthesis by elevated CO₂ in long-term studies was similar for conifers (62%) and deciduous trees (53%). Recent studies indicate that elevated CO₂ causes a more persistent stimulation of biomass increment and photosynthesis than previously expected. Results of seedling studies, however, might not be applicable to other stages of tree development because of complications of age-dependent and size-dependent shifts in physiology and carbon allocation, which are accelerated by elevated CO₂. In addition, there are many possible avenues to down-regulation, making the predicted canopy CO₂ exchange and growth of mature trees and forests in a CO₂-rich atmosphere uncertain. Although, physiological down-regulation of photosynthetic rates has been documented in field situations, it is rarely large enough to offset entirely photosynthetic gains in elevated CO₂. A persistent growth stimulation of individual mature trees has been demonstrated although this effect is more uncertain in trees in natural stands.

Resource interactions can both constrain tree responses to elevated CO₂ and be altered by them. Although drought can reduce gas-exchange rates and offset the benefits of elevated CO₂, even in well watered trees, stomatal conductance is remarkably less responsive to elevated CO₂ than in herbaceous species. Stomata of a number of tree species have been demonstrated to be unresponsive to elevated CO₂. We conclude that positive effects of CO₂ on leaf area can be at least as important in determining canopy transpiration as negative, direct effects of CO₂ on stomatal aperture. With respect to nutrition, elevated CO₂ has the potential to alter tree–soil interactions that might influence future changes in ecosystem productivity. There is continued evidence that in most cases nutrient limitations diminish growth and photosynthetic responses to elevated CO₂ at least to some degree, and that elevated CO₂ can accelerate the appearance of nutrient limitations with increasing time of treatment. In many studies, tree biomass responses to CO₂ are artefacts in the sense that they are merely responses to CO₂-induced changes in internal nutritional status of the tree.

There are numerous interactions between CO₂ and factors of the biotic and abiotic environment. The importance of increasing atmospheric CO₂ concentrations for productivity is likely to be overestimated if these are not taken into account. Many interactions, however, are simply additive rather than synergistic or antagonistic. This appears to hold true for many parameters under elevated CO₂ in combination with temperature, elevated O₃, and other atmospheric pollutants. However, there is currently little evidence that elevated CO₂ will counteract O₃ damage. When the foliage content of C, mineral nutrients and secondary metabolites is altered by elevated CO₂, tree×insect interactions are modified. In most trees, mycorrhizal interactions might be less important for direct effects of CO₂ than for alleviating general nutrient deficiencies.

Since many responses to elevated CO₂ and their interactions with stress show considerable variability among species/genotypes, one principal research need is for comparative studies of a large variety of woody species and ecosystems under realistic conditions. We still need more long-term experiments on mature trees and stands to address critical scaling issues likely to advance our understanding of responses to elevated CO₂ at different stages of forest development and their interactions with climate and environment. The only tools available at present for coping with the consequences of rising CO₂ are management of resources and selection of genotypes suitable for the future climate and environment.