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A mechanistic understanding of tree responses to hinning and fertilization from stable isotopes in tree rings
Citation:
BROOKS, AND A. K. Mitchell. A mechanistic understanding of tree responses to hinning and fertilization from stable isotopes in tree rings. Presented at 7th International Conference on Applications of Stable Isotope Techniques to Ecological Studies, Fairbanks, AK, August 08 - 13, 2010.
Impact/Purpose:
Carbon sequestration has focused renewed interest in understanding how forest management affects forest carbon gain over timescales of decades.
Description:
Carbon sequestration has focused renewed interest in understanding how forest management affects forest carbon gain 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. Growth increased substantially for both thinning and fertilization treatments, and the response of the single treatment was doubled with the combination of those treatments. As indicated by a decrease in carbon isotope discrimination (13C), fertilization increased leaf nitrogen for the first 3 years causing an increase in photosynthesis (A) and intrinsic water-use efficiency. The thinning treatment showed a slight decrease in water-use efficiency for one year, likely from reduced competition for water. 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. The oxygen isotope ratio (18O) also showed responses to treatment. Only the thinning treatments showed consistent significant increases in 18O above controls. This response was found in both late- and earlywood, with larger increases in 18O found in latewood. Treatment 18O increases over controls could be cause by three different drivers: treatment decreases in relative humidity, increases in leaf temperature, or decreases in stomatal conductance. Since differences were only found for the thinning treatments, changes in canopy microclimate are the likely drivers with decreases in relative humidity with thinning being the most likely based on modelling responses to each variable. These results indicate that stable isotopes provide long-term records of physiological and environmental responses to management treatments. However, caution must be used in interpreting changes in 18O since the possible mechanisms behind those changes lead to very different interpretations of response. Nevertheless, tree-ring isotopic records remain a viable way to construct long-term response functions of tree carbon gain to management and climate fluctuations.