Douglas-fir and ponderosa pine forests of the Northern Rockies (NR) USA will grow under conditions of an increasingly CO2-rich atmosphere and a projected warmer and drier climate. This project will investigate the growth responses of two co-occurring and economically important western USA conifers growing under natural conditions to these changing environmental variables. Matched DF and PP tree-ring chronologies, which show the average rate of tree growth annually, are being developed and radial growth patterns examined prior and after atmospheric CO2 concentrations became significantly elevated 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 DF and PP are trending significantly upward through time 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 growing in water-limited environments; 2) the influence of drought on the radial growth rates of DF and PP has decreased over the time period of instrumental climatic records (1895-present); 4) there are both differential responses to CO2 fertilization between the species and spatial variation in these responses; and 5) radial growth rates of old-growth DF and PP are not significantly less than younger DF and PP growth rates and thus, old-growth forests may serve as important carbon sinks. Current climate models predict that the NR 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.

 

All photos are from fieldwork in August 2009 in Montana (vicinity of Missoula):