anthropogenic period, and (ii) recent growth variations were largely explainable by climate–growth relationships.
A contrasting view is presented by Graybill ( 26 ) and Graybill and Idso ( 27 ), who argued that CO2 fertilization is detectable in certain pine species growing at high elevations of the southwestern United States, but only if they show a strip-bark growth form. In trees with a strip-bark morphology, any added CO2 should be allocated primarily to the active cambial region, resulting in a greater response ( 27 ). Graumlich ( 25 ) speculated that the disparate conclusions for trees in the southwest might be reconciled, since the LaMarche et al. ( 23 ) trees were also of a strip-bark morphology. By contrast, the trees in her study did not show this feature. Other studies include a paper by Kienast and Luxmoore ( 28 ), who showed negative results for a CO2 fertilization effect in trees in the Rocky Mountains of Colorado.
D’Arrigo and Jacoby ( 29 ) did not find evidence for a CO2 fertilization effect at the northern treeline of North America, based on evaluation of residual trends following modeling of climate–growth relationships. One possible explanation is that a threshold level of CO2 increase is needed before an effect can be detected. Another is that other factors, including cold temperatures, a short season of cambial cell division, and nitrogen deficiency could preclude a direct CO2 response in the extreme boreal forests. The unexplained increase in growth of lodgepole pine at a high-elevation site in the San Jacinto Mountains of California ( 30 ) did not occur in limber pine near the same site and, as noted in the study, could be related to changes in winter precipitation.
One of the most thorough analyses of representative boreal forest growth involved the measurement of ring widths and density of trees in mature, closed canopy, white spruce stands at 11 locations in western Canada ( 31 ). A limited number of trees were felled, and seven disks were cut from each to obtain data on cross-sectional area and taper to enable calculations of volumetric and biomass growth rate change. Jozsa and Powell concluded that biomass productivity and annual growth layer weights are related to long-term and yearly climatic variability with possible response to spruce budworm activity ( 31 ). They do not present any indication that there is a systematic growth trend that could be related to CO2 fertilization. This is an extremely important study of mature trees in natural forest stands. Thus the results are widely relevant to real-world situations.
We have briefly described some of the tree-ring evidence presently being used to assess whether recent growth changes are unusual relative to the past, and might be evidence for warming due to greenhouse gases and/or direct CO2 fertilization. A number of temperature-sensitive records, some of which date back for several millennia, do indicate unusual recent warming. Yet this is by no means taking place at all sites. Another caveat is that the trees studied here are from particular sites selected to amplify climatic signals, and are not necessarily representative of large components of the land biosphere nor enhanced sequestering of carbon ( 2 ). In addition, above-ground radial growth changes do not provide information about respiration or below-ground effects. Other changes (e.g., drought stress) could lead to negative feedback effects (e.g., refs. 2 and 8 ).
The evidence for CO2 fertilization is inconclusive at present for trees growing in natural settings, where there can be many other limiting and interacting factors. Controlled experiments simulating natural conditions underway at the Biosphere 2 facility will attempt to evaluate the combined effects of different environmental factors, and compare plant responses in different simulated ecosystems and between species ( 32 ). Such controlled studies may provide additional insights which can help resolve the uncertainties of the CO2 fertilization issue. Even if trees with a strip-bark growth form are most likely to show this effect, these types of trees are only a small component of the land biosphere.
The evidence described here provides only partial information regarding the behavior of the land biosphere. There are still many uncertainties, and it is unlikely that these issues will be resolved in the very near future. Additional studies and improved spatial and temporal coverage of tree-ring data are needed to decrease uncertainties about whether anthropogenic effects are presently taking place.
Research for this paper was supported by the Climate Dynamics Program of the National Science Foundation, ATM 94-06732. This is Lamont–Doherty Earth Observatory contribution no. 5594.
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