SPRUCE-FIR GROWTH FORM CHANGES IN THE FOREST-TUNDRA ECOTONE OF ROCKY-MOUNTAIN NATIONAL-PARK, COLORADO, USA

Tree regeneration has traditionally been used as a measure of the resp onse of treeline to climate. Changes in growth form of krummholz trees may also indicate whether treeline is responding to changes in climat e. The purpose of this study was to determine whether krummholz trees in the forest-tundra ecotone of Rocky Mountain National Park, Colorado have experienced significant vertical stem growth, in the absence of mortality, and if this growth occurred in response to recent changes i n climate. We sampled and dated Engelmann spruce and subalpine fir kru mmholz leaders stratified by height class at three sampling locations to determine the dates leaders initiated growth above mean snow depth. At one sampling location, 215 additional leaders were sampled to cons truct an age structure of leader release dates. Dates of leader releas e taken from the age structure were compared with seasonal temperature s, seasonal precipitation, winter snow depths, and annual runoff using t-tests. Dates of leader release were also compared to proxy climate records for the southern Rocky Mountain region. Based on historical ph otos as well as the data presented here, both spruce and fir krummholz trees experienced significant height growth as early as the 1850's an d continued to grow vertically, at least through the 1970's. This vert ical stem growth occurred in the absence of significant mortality. Run ning mean annual temperature and May snow depth are both positively as sociated with years of leader release, suggesting that a warmer, wette r climate, possibly following the end of the Little Ice Age ca 1850, m ay have induced these changes in the ecotone.