Effects of climate on radial growth of subalpine conifers in the North Cascade Mountains

Dendroecological techniques were used to study the influence of climate on the growth of subalpine fir (Abies lasiocarpa (Hook.) Nutt.), Engelmann spruce (Picea engelmannii Parry), and subalpine larch (Larix lyallii 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 species response to climate on a common site. Species differences account for most of the variability in radial growth patterns and response to climate. The greatest differences are between subalpine fir and subalpine larch, while the greatest similarities are between subalpine fir and Engelmann spruce. After species, aspect is the most significant factor affecting growth response to climate. Spring snowpack and summer temperature are the primary climatic factors influencing growth. Spring snowpack is negatively correlated with growth for all three species, but the relationship is strongest for subalpine fir and weakest for subalpine larch. Subalpine larch growth is positively correlated with June 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 temperatures are associated with reduced growth the following year. The response 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 aspect sites, but could also increase the frequency and severity of summer drought conditions on ridges and south-aspect sites, where summer soil moisture may already be limiting.