The tree-ring data used in these studies are from sites selected to amplify the climatic signal due to temperature, and are not necessarily representative of large components of the land biosphere nor indicators of large-scale enhanced carbon sequestration. Changes in radial growth in these trees do not provide information about possible shifts in respiration or allocation of carbon below-ground. Warming may also be causing negative feedbacks to forest productivity, which can counteract enhanced growth in other areas. For example, some temperature-limited sites may now be showing the negative effects of moisture stress (partially due to increased evapotranspiration caused by warmer temperatures) or insect infestation related to recent warmer conditions (ref. 8 and see Fig. 1 ).
Another means by which tree rings are being used to test for anthropogenic effects is by evaluating whether direct CO2 fertilization due to increasing atmospheric CO2 (ordinarily limiting to plant growth) is presently enhancing the growth of natural vegetation. The response of plant growth to a direct CO2 fertilization effect has been demonstrated in numerous laboratory experiments, usually using seedlings (e.g., ref. 20 ). Modeling suggests that this enhanced growth should result in greater carbon sequestering of land ecosystems, provided that this “beta factor” is sufficiently large (e.g., refs. 21 and 22).
Little is known, however, about whether such an effect is occurring on a large scale in natural vegetation, where environmental conditions are exceedingly complex. Here we review several tree-ring studies which evaluate the possible effects of direct CO2 fertilization on radial growth of trees growing in natural environmental settings.
LaMarche et al. ( 23 ) presented one of the first studies which purported to find evidence for a possible CO2 fertilization effect in tree rings. Their study was based on ring-width chronologies of high-elevation bristlecone and limber pines growing in the southwestern United States, which show unusual enhanced growth over the past century. One reason for their conclusion that this enhanced growth is due to CO2 fertilization is that high-elevation plants may be more CO2-limited than those at lower elevations ( 24 ). Yet no quantitative modeling was presented by LaMarche et al. ( 23 ) to rule out the possible contribution of favorable climatic change to account for the growth increases.
Graumlich ( 25 ) found no such evidence for CO2 fertilization in high-elevation foxtail pine and other species in the Sierra Nevada. She based her conclusions on the observations that (i) recent trends were not unusual relative to those in the pre-