EFFECTS OF CLIMATE ON RADIAL GROWTH OF SUB-ALPINE CONIFERS IN THE NORTH CASCADE MOUNTAINS

Dendroecological techniques were used to study the influence of climat e on the growth of subalpine fir (Abies lasiocarpa (Hook.) Nutt.), Eng elmann spruce (Picea engelmannii Parry), and subalpine larch (Larix ly allii Parl.) in the North Cascade Mountains of Washington state. Study sites were selected on different topographic features at three points along an elevation gradient (ridgetop, valley slope, and valley floor ) to characterize site influences on growth response to climate. Mixed species stands were sampled to identify possible differences in speci es response to climate on a common site. Species differences account f or most of the variability in radial growth patterns and response to c limate. The greatest differences are between subalpine fir and subalpi ne larch, while the greatest similarities are between subalpine fir an d Engelmann spruce. After species, aspect is the most significant fact or affecting growth response to climate. Spring snowpack and summer te mperature are the primary climatic factors influencing growth. Spring snowpack is negatively correlated with growth for all three species, b ut the relationship is strongest for subalpine fir and weakest for sub alpine larch. Subalpine larch growth is positively correlated with Jun e temperature. Subalpine fir growth is positively correlated with July -August temperature. Engelmann spruce growth is positively correlated with June-August temperature, but unusually warm July-August temperatu res are associated with reduced growth the following year. The respons e of subalpine forests in the North Cascades to future climate change will depend on winter snowpack accumulations and spring snowmelt rates . Earlier meltouts and warmer summers would benefit growth on north as pect sites, but could also increase the frequency and severity of summ er drought conditions on ridges and south-aspect sites, where summer s oil moisture may already be limiting.