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...

Effects of tree size and temperature on relative growth rate and its components of Fagus sylvatica seedlings exposed to two partial pressures of atmospheric [CO2]

“…This is in contrast to the positive effect of [CO # ] on coarse roots relative to fine roots found by Overdieck (1993) for F. sylvatica. With regard to water relations, a more relevant measure is perhaps the ratio of fine root (considered as functional root ; Barton, 1997) to assimilating area (Saxe et al, 1998). High [CO # ] significantly increased this ratio for F. sylvatica in the present study suggesting an increased ability to absorb water per unit leaf area.…”

A late Eocene palynological record of climate change and Tibetan Plateau uplift (Xining Basin, China)

“…Trees at high altitudes are also particularly sensitive to CO 2 enrichment, because they live in an atmosphere of lower partial pressure of CO 2 (Hattenschwiler et al, 2002). The current literature indicates a significantly larger average long-term biomass increment under elevated carbon dioxide for Pinaceae than for deciduous trees in studies not involving stress components (Saxe et al, 1998). New data on the effects of changing CO 2 on the evolution of the vegetation in Tibet are needed to evaluate the impact of decreasing atmospheric CO 2 levels on the studied flora.…”

Fingerprinting the impacts of global change on tropical forests

“…This is expected because CO 2 is a substrate for Rubisco, and higher CO 2 levels increase CO 2 : O 2 ratios, which reduces photorespiration (Saxe et al 1998;Curtis & Wang 1998). While most of these experiments have occurred in pots, and often using temperate species, increased growth has been demonstrated in situ for tropical tree seedlings (Wü rth et al 1998a).…”

“…The general response of plants to this increase may be an increase in resource-use efficiency, especially higher rates of photosynthesis, which improves light-use efficiency, and decreasing stomatal conductance and transpiration, which improves water-use efficiency. In addition, any acclimation of photosynthesis to higher CO 2 concentrations would reduce key enzyme usage, notably Rubisco, increasing whole-plant nutrient-use efficiency (Drake et al 1997;Saxe et al 1998). We discuss the potential effects of rising CO 2 concentrations on light-use efficiency, water-use efficiency and nutrient-use efficiency in turn.…”