Are climate-tree growth relationships changing in North-Central Idaho, USA?

An 861-yr Douglas-fir (Pseudotsuga menziesii [Mirb.] France) tree-ring chronology has been recently developed for the Salmon River Valley, Challis National Forest, Idaho. Its potential for climatic reconstruction is investigated using monthly instrumental records of precipitation, temperature, and Palmer Drought Severity Index from 1895 to 1995. The temporal stability of climate-tree growth relationships is analyzed by means of evolutionary (backward and forward) and moving response functions. When precipitation and temperature are used as predictors, the strongest (and temporally stable) signal is a negative response to July temperature. Another relevant signal, albeit less temporally consistent, is a positive response to May precipitation. When the Palmer Drought Severity Index is used as an integrated predictor to highlight response to summer moisture stress, the strength of the association with tree growth increases in recent decades. This information can be used to maximize the reliability of dendroclimatic reconstructions, and has important implications for expanding the range of recent studies on altered climate-tree growth relationships during the 20th century.