Collaborative Research: Radial Growth Responses Among Naturally Occurring Western U.S. Conifers Under Changing Environmental Conditions

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This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5). This research examines growth changes in the Douglas-fir and ponderosa pine forests of the Northern Rockies (USA), which are growing under conditions of an increasingly CO2- rich atmosphere and a projected warmer and drier climate in the future. The researchers will investigate the growth responses of two co-occurring and economically important western USA conifers growing under natural conditions in relation to changing environmental variables. Specifically, radial growth rates of Douglas-fir and ponderosa pine trees will be compared at nine sites in the Northern Rockies where the trees are co-dominant, where confounding growth factors are minimized, and where between-site environmental variability is maximized. Matched tree-ring chronologies, which show the average rate of tree growth annually, will be developed and radial growth patterns examined prior and after atmospheric CO2 concentrations became significantly elevated. This will be done using growth/climate regression models, carbon isotope analysis, and analyses of growth rates during various levels of drought severity. The purpose of this research is to determine if 1) rates of intrinsic water-use efficiency (iWUE) of both species are trending significantly upward during the past 200 years because of decreased stomatal conductance associated with increasing levels of CO2; 2) increasing iWUE is positively impacting the radial growth rates of these two tree species in water-limited environments; 3) the influence of drought on the radial growth rates has decreased over the time period of instrumental climatic records (1895-present); 4) there are differential responses to CO2 fertilization between the species and spatial variation in these responses; and 5) radial growth rates of old-growth trees are not significantly less than younger tree growth rates and thus, old-growth forests may serve as important carbon sinks. Current climate models predict that the study area will experience more frequent and severe summertime droughts. Thus, a greater understanding of issues facing future ecosystems under increased CO2 and warmer, drier conditions is critical. Knowing the specific growth responses of these two tree species to prior climatic variations and rising atmospheric CO2 will provide a better understanding of how these species will respond in the next several decades. The potential positive impacts on growth resulting from increasing atmospheric CO2 could be economically and ecologically significant because of the great commercial value of these species, and their increased ability to serve as carbon sinks. The project results will be broadly disseminated with a goal that the study findings will assist in the development of management strategies of conifer forests in the western U.S. The project will also provide educational opportunities for undergraduate and graduate students as they will participate in both the field and laboratory components of the study.