Spatial patterns and typology changes of an advancing treeline on Pikes Peak, CO

Background/Question/Methods

Alpine tree establishment is limited by an upper climatic boundary, forming a treeline.  Globally, treeline elevations are most often associated with growing season temperature. On Pikes Peak (Southern Rocky Mountains, Colorado), treeline advancement correlates strongly with regional annual temperature. Based on historical aerial photographs, treeline has changed from an abrupt to a diffuse typology since 1938. At the upper climatic boundary, trees are expected to grow better when clustered due to facilitation, but trees below the upper climatic boundary should grow better when separated from their neighbors due to competition. Therefore, as the treeline migrates, we expect clustering at higher elevations for younger generations of trees. To evaluate this hypothesis, we studied a 50m x 260m transect on the western slope of Pikes Peak in an area with known advancement (18m elevation since 1938). The transect encompassed the entire forest-tundra ecotone between the highest outpost seedling to the closed-canopy forest. We mapped, aged, and measured the height of every tree, sapling, and seedling (P. engelmannii) taller than 10cm in the study area (n=630). Using Ripley’s K statistical analysis, we evaluated tree clustering across elevational gradients and among age classes to find changes in the upper climatic boundary over time.

Results/Conclusions

Overall, clustering (Ripley’s K, p < 0.01, based on boot-strapping) was more prominent at lower elevations and for older cohorts, suggesting that the environment was historically harsher than it is today. The spatial distribution of trees at the moving edge changed from clustered to randomly dispersed: from 1868-1940 the moving edge was clearly clustered, from 1941-1976 it showed mixed results, and from 1977-2010 it displayed a random spatial pattern. This indicates the upward migration of the climatic boundary, while also potentially indicating that the boundary is advancing more quickly than treeline, suggesting that current treeline advance may be recruitment-limited. Treeline advance also demonstrated a reach-and-fill pattern, with sudden advancement of treeline, followed by a few decades of in-fill at lower elevations. “Reaching trees” recruited more heavily at certain elevational bands, creating peaks of tree density. In following decades, tree recruitment concentrated below these peaks with little or no recruitment above. This pattern repeated three times in the last 120 years. In general, tree recruitment increased significantly in the last 30-40 years. To our knowledge, this is the first study that examines in detail the process of changing treeline typology of an advancing treeline.