Master’s Thesis Abstract by David W. Peterson (1993)

Dendroecological techniques were used to study the influence of climate on the growth of subalpine fir (Abies lasiocarpa), Engelmann spruce (Picea engelmannii), subalpine larch (Larix lyallii), and whitebark pine (Pinus albicaulis) in the North Cascade Mountains of Washington state. Study sites were selected at three points on different topographic features along an elevational gradient (ridgetop, valley slope, valley floor) to characterize site influences on growth response to climate. Mixed species stands were sampled to identify possible difference in species response to climate on a common site. A total of l4 growth index chronologies were developed, representing four species and seven sites.

Most of the variance among chronologies is attributed to species differences, although site moisture is also important. Summer temperature and previous winter precipitation are the primary climatic influences on growth; spring and previous autumn climate are also related to growth. Variation in annual radial growth of subalpine fir is most closely associated with variations in previous winter precipitation and 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. Warm summer temperatures are negatively correlated with growth of whitebark pine but, as with Engelmann spruce, the negative effect is seen in growth the following year. Warm June temperatures and cool, wet April conditions are apparently beneficial for subalpine larch growth.

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 by limiting.