Tree establishment and growth drive treeline advance and change treeline form on Pikes Peak (Colorado) in response to recent anthropogenic warming

Abstract

Treeline advance is a well-established response of treeline ecotones to climate change. However, the degree to which tree growth and establishment drives treeline movement is widely debated. We used geographic information system (GIS) analysis of aerial photographs and dendrochronological analysis of tree growth and age structure to examine treeline dynamics on Pikes Peak (Colorado). The rate of treeline advance at the site (1938–2017) was 0.235 m·year–1, and it accelerated through time. Several sites have transformed from abrupt to diffuse topology. Regional temperatures significantly increased after the 1890s, particularly in the last half-century. Tree growth was inhibited by late spring snow in the 1935–1985 window and enhanced by growing season temperature in the 1965–2009 window. Tree establishment above treeline appears to have transformed treeline topology and set up the potential for further treeline advance. We conclude that if current climatic trends and system relationships continue, treeline should continue to advance because (i) there are large numbers of seedlings and saplings above the treeline due to continuous significant seedling recruitment and (ii) growth of trees in the treeline ecotone is positively related to growing season temperatures, which are increasing. Other limits to the system, such as drought or topographic barriers, may arise in the future.