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Unthinned slow-growing ponderosa pine (Pinus ponderosa) trees contain muted isotopic signals in tree rings as compared to thinned trees
Citation:
Sohn, J., J. Renée Brooks, J. Bauhus, M. Kohler, T. Kolb, AND N. McDowell. Unthinned slow-growing ponderosa pine (Pinus ponderosa) trees contain muted isotopic signals in tree rings as compared to thinned trees. TREES. Springer, New York, NY, 28(4):1035-1051, (2014).
Impact/Purpose:
In recent years, droughts have increased in forest mortality in the semi arid regions of the USA. Most climate scenarios predict an increase of more frequent and severe drought periods in the near future at the global scale. Chronic water stress leading into and during droughts was identified as a main driver for decreased forest productivity and for increased forest mortality at the global scale including regional forest diebacks. Furthermore, large decreases of net primary production and carbon sequestration associated with increased mortality rates can transform forests into CO2 sources, which can lead to a positive feedback to climate warming. Thinning has been recommended as an effective restoration tool for P. ponderosa forests in western USA as it helps to reduce mortality and to improve tree productivity and water status. Our objective was to utilize stable isotope stored in tree rings to better understand the long-term mechanistic basis for past growth responses to thinning. We found that when trees are relieved from the stress of high densities, they respond much more readily to the current environment.
Description:
We analysed the oxygen isotopic values of wood (δ18Ow) of 12 ponderosa pine (Pinus ponderosa) trees from control, moderately, and heavily thinned stands and compared them with existing wood-based estimates of carbon isotope discrimination (∆13C), basal area increment (BAI), and gas exchange to examine if increases in water availability and thus changes in stomatal conductance (gc) were causing higher growth rates after thinning. We conducted a sensitivity analysis to identify additional factors that may influence observed differences in δ18Ow among treatments. We found that heavy thinning (HT) led to more depleted δ18Ow relative to the control throughout the first post-thinning decade, whereas moderate thinning (MT) was not significantly different from controls in δ18Ow. Both HT and MT treatments varied more in 18Ow after treatment than did control trees. Results of the sensitivity analysis suggest six potential drivers of δ18Ow, including source water, environmental drivers and physiological effects. When modelling δ18Ow time-series of the first post-thinning decade using surrogate data of multiple potential drivers at once, the best fit with measured time-series was obtained with a model including relative humidity and δ18O of source water. We conclude that direct effects of physiological parameters were only a minor determinant of δ18Ow in our trees. However, we found insufficient reasons to assume that the 18Ow signal among treatments was generated by a difference in relative humidity or source water usage between treatments. Rather, as modelled δ18Ow time series overestimated the magnitude of δ18Ow variation for all treatments but most for control trees, a higher environmental sensitivity maybe due to decreasing reliance on remobilized compounds after thinning. Consequently, the more depleted δ18Ow of HT trees was likely caused by a greater expression of the common trend towards more depleted δ18Ow apparent in all series in the post-thinning period.