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A mechanist understanding of tree responses to thinning and fertilization from stable isotopes in tree rings
Citation:
BROOKS, AND A. Mitchell. A mechanist understanding of tree responses to thinning and fertilization from stable isotopes in tree rings. Presented at American Geophysical Union annual fall meeting, San Francisco, CA, December 14 - 18, 2009.
Impact/Purpose:
Carbon sequestration and the possible associated economic credits have focused renewed interest in understanding how forest management affects forest growth over timescales of decades.
Description:
Carbon sequestration and the possible associated economic credits have focused renewed interest in understanding how forest management affects forest growth over timescales of decades. Two of the most common forest management tools are thinning and fertilization, and yet details on physiological responses to these tools are often lacking, particularly for long-term responses over decades of forest management. We used tree-ring growth patterns and stable isotopes to understand long-term physiological responses to thinning and fertilization using a controlled experiment begun in 1971 at Shawnigan Lake, Vancouver Island, B.C. While growth increased substantially for both thinning and fertilization treatments, and the response of the single treatment was doubled with the combination of those treatments, the mechanisms behind the response were quite different, and the combination of thinning and fertilization was not a simple sum of the two treatments. As indicated by an increase in carbon isotope ratio, fertilization increased leaf nitrogen for the first 4 years causing an increase in A and intrinsic water-use efficiency. The thinning treatment only showed a slight increase in water-use efficiency for one year, likely caused by increased light to the canopy. The most dramatic response was observed in the combination treatment where both leaf nitrogen and increased light cause a much larger increase in A and water-use efficiency. Using the oxygen isotope ratio to infer changes in transpiration, we found that leaf-level transpiration did not change during the early season for any treatment. However, late in the dry summer, typical of this site, the combination treatment showed signs of reduced stomatal conductance, likely caused by dryer soil moisture conditions perhaps as a result of greater total plant transpiration during the early part of the growing season. In the thinning treatment, leaf-level transpiration increased after 5 years. This may indicate a slow root response to increased moisture because of the decrease in competition. These findings indicate the complexity in both timing and magnitude of responses to management, and the usefulness of tree rings as recorders of past physiological responses. The fact that the combination treatment response did not fall neatly between the fertilization and thinning highlights this complexity.