Tree-Ring Evidence for [CO2] Fertilization of Forests and its Influence on the "Divergence Problem"
Abstract
The "divergence problem" refers to the observation that increasing temperatures are not matched by increasing radial growth at many northern forest sites. A related, and potentially offsetting, effect may be caused by CO2 fertilization of trees. Increased atmospheric CO2 could increase photosynthetic rates and cause trees to use water more efficiently, thereby increasing overall growth rates relative to climatic limiting factors. Accelerated tree growth in response to elevated atmospheric CO2 has been seen across a range of forest types, however these results have been inconsistent, and are generally based on short-term studies. Long-term studies based on tree-rings have generally been restricted to a few sites, and have produced conflicting results. Here we analyze the global record of tree-ring width for evidence of increasing growth relative to drought, and for changing sensitivity of radial growth to drought. This analysis shows that a small but highly significant proportion of trees exhibit increasing growth relative to drought over the past 130 years. These growth increases cannot be attributed to increasing water use efficiency or elevation effects, and no differences between species were detectable. These results suggest that while CO2 fertilization is occurring at some locations and will influence future forest dynamics, it cannot be expected to offset the effects of increasing temperatures, or to substantially slow the rate of carbon accumulation in the atmosphere. It also suggests that the "divergence problem" may be more widespread than is currently recognized, but its effects are obscured by the offsetting effect of CO2 fertilization.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- Bibcode:
- 2007AGUFMPP51C0662G
- Keywords:
-
- 1615 Biogeochemical cycles;
- processes;
- and modeling (0412;
- 0414;
- 0793;
- 4805;
- 4912);
- 1812 Drought;
- 1851 Plant ecology (0476);
- 3344 Paleoclimatology (0473;
- 4900);
- 4920 Dendrochronology