CLIMATE, GEOGRAPHY, AND TREE ESTABLISHMENT IN SUB-ALPINE MEADOWS OF THE OLYMPIC MOUNTAINS, WASHINGTON, USA

Noticeable changes in vegetation distribution have occurred in the Pac ific Northwest during the last century as trees have established in so me subalpine meadows. To study the relationship of this process to cli mate, recently established trees were aged in six subalpine meadows in the Olympic Mountains, Washington. The sites represent three points a long a steep precipitation gradient. Subalpine fir (Abies lasiocarpa) has been establishing at the dry end of the gradient, mountain hem loc k (Tsuga mertensiana) at the wet end, and both species in the center. Establishment patterns were compared with deviations from the century- long average for these weather variables: winter precipitation, Palmer Drought Severity Index, and winter, October, and May temperatures. Re sults show that establishment occurred in dry areas when weather condi tions were wetter than average, and in wet areas under drier than aver age conditions. Establishment at central sites did not show consistent relationships with climate. If future climatic conditions continue to warm, establishment of subalpine fir in subalpine meadows in dry area s may cease and mountain hemlock may resume in wet areas.