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Investigating old-growth ponderosa pine physiology using tree-rings, δ13C, δ18O, and a process-based model
Citation:
Ulrich, D., C. Still, J. Renee Brooks, Y. Kim, AND F. Meinzer. Investigating old-growth ponderosa pine physiology using tree-rings, δ13C, δ18O, and a process-based model. ECOLOGY. Ecological Society of America, Ithaca, NY, 100(6):e02656, (2019). https://doi.org/10.1002/ecy.2656
Impact/Purpose:
Land managers need tools to help them understand how variable environmental conditions influence forest productivity and resilience to stressors such as drought and wild-land fire. One set of promising tools are process-based tree -growth models because they incorporate physiological principles that enable them to be widely applied to diverse species and sites. A widely used stand-level process model is Physiological Principles in Predicting Growth (3-PG) because it has accurately predicted growth and productivity in changing environmental conditions and on diverse forested stands. However, calibrating and constraining process-based models is a challenge to make sure that the right answers are achieved for the right reasons. Stable isotopes (both carbon 13C and oxygen isotopes 18O) contained within tree-rings provide information on physiological processes that govern growth over time, and can be used to constrain process growth models. In this study, we developed a submodel in 3-PG to incorporate 18O building upon a previous effort that developed a submodel for 13C. We found the combination of both δ13Ccell and δ18Ocell provides a new and useful way to constrain 3-PG to providing more realistic mechanisms behind growth responses. Thus, future predictions of the isotope enabled 3-PG model will be more accurate in how forests respond to environmental stressors.
Description:
In dealing with predicted changes in environmental conditions outside those experienced today, forest managers and researchers rely on models that incorporate physiological principles to predict future forest-growth responses. However, the accuracy of these predictions depends upon the realistic integration of the physiological principles within the models. The carbon and oxygen isotope ratios of tree-ring cellulose (δ13Ccell, δ18Ocell) reveal long-term, integrated physiological responses to environmental conditions that can be useful for testing the physiological integration inherent in these models. We incorporated a submodel of δ18Ocell into the widely used Physiological Principles in Predicting Growth (3-PG) model for the first time, to complement a recently added δ13Ccell submodel. We parameterized the model using previously reported stand characteristics and long-term trajectories of tree-ring growth, δ13Ccell, and δ18Ocell collected from the Metolius AmeriFlux site in central Oregon (upland trees). We then applied the parameterized model to a nearby set of riparian trees to investigate the physiological drivers of differences in observed basal area increment (BAI) and δ13Ccell trajectories between upland and riparian trees. The model showed that greater available soil water and maximum canopy conductance likely explain the greater observed BAI and lower δ13Ccell of riparian trees. Unexpectedly, the observed and simulated δ18Ocell trajectories did not differ between the upland and riparian trees, likely due to similar source water. The δ18Ocell submodel with a Peclet effect improved model estimates of δ18Ocell because it incorporates 3-PG growth and allocation processes. Because simulated stand-level transpiration (E) is used in the δ18O submodel, aspects of leaf-level anatomy such as the effective path length for transport of water from the xylem to the sites of evaporation could be estimated.
URLs/Downloads:
DOI: Investigating old-growth ponderosa pine physiology using tree-rings, δ13C, δ18O, and a process-based model