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 response of treeline to climate. Changes in growth form of krummholz trees may also indicate whether treeline is responding to changes in climate. 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 in climate. We sampled and dated Engelmann spruce and subalpine fir krummholz 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 construct an age structure of leader release dates. Dates of leader release taken from the age structure were compared with seasonal temperatures, 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 photos as well as the data presented here, both spruce and fir krummholz trees experienced significant height growth as early as the 1850's and continued to grow vertically, at least through the 1970's. This vertical stem growth occurred in the absence of significant mortality. Running mean annual temperature and May snow depth are both positively associated with years of leader release, suggesting that a warmer, wetter climate, possibly following the end of the Little Ice Age ca 1850, may have induced these changes in the ecotone.