Community Structure and Functional Role of Limber Pine (Pinus flexilis) in Treeline Communities in Rocky Mountain National Park

Laurel A Sindewald,Eric R Neumeyer,Diana F Tomback

doi:10.3390/f11080838

Abstract

Research Highlights: Limber pine (Pinus flexilis) is abundant in some alpine treeline ecotone (ATE) communities east of the Continental Divide in Rocky Mountain National Park (RMNP) and the Colorado Front Range. Limber pine may be able to colonize the ATE under changing climate aided by directed seed dispersal by Clark’s nutcrackers (Nucifraga columbiana). Cronartium ribicola, white pine blister rust, is a growing threat to limber pine and may affect its functional role within the ATE. Background and Objectives: The ATE is sensitive, worldwide, to increasing temperature. However, the predicted advance of treeline under a changing climate may be modified by tree species composition and interactions. We aimed to (1) examine the conifer species composition and relative abundances in treeline communities with limber pine; (2) assess which functional roles limber pine assumes in these communities—tree island initiator, tree island component, and/or solitary tree; and (3) determine whether limber pine’s occurrence as a tree island initiator can be predicted by its relative abundance as a solitary tree. Materials and Methods: We selected four study sites in RMNP above subalpine forest limber pine stands. We sampled the nearest tree island to each of forty random points in each study site as well as solitary tree plots. Results: Across study sites, limber pine comprised, on average, 76% of solitary trees and was significantly more abundant as a solitary tree than Engelmann spruce (Picea engelmannii) or subalpine fir (Abies lasiocarpa). Limber pine was a frequent component of multi-tree islands in three study sites, the major component in one study site, and dominated single-tree islands at two study sites. At three of four study sites, no species had significantly greater odds of being a tree island initiator. Limber pine was found less often as a tree island initiator than predicted from its relative abundance as a solitary tree, given the likely role of solitary trees in tree island formation.

Highlights

The alpine treeline ecotone (ATE), the transitional zone between subalpine forest and alpine tundra, is a mosaic of alpine tundra vegetation such as cushion plants and graminoids, bare substrate, rocks, and semi-upright or krummholz trees growing individually or together in tree islands on the landscape [1,2,3,4,5,6]
Study sites differed in community structure in terms of the proportion of tree islands that were multi-tree vs. single-tree, the dimensions of tree islands, the growth form of islands, and the density of solitary trees
We suggest that the discrepancy may be explained by the conditions where limber pine grows as a solitary tree, which may be so harsh that seeds of other tree species are unlikely to germinate, and/or that seedlings have low survival probability

Summary

Introduction

The alpine treeline ecotone (ATE), the transitional zone between subalpine forest and alpine tundra, is a mosaic of alpine tundra vegetation such as cushion plants and graminoids, bare substrate, rocks, and semi-upright or krummholz trees growing individually or together in tree islands on the landscape [1,2,3,4,5,6]. The ATE, extending from the timberline (the upper limit of subalpine forest) to treeline (the upper limit of tree growth) [8], is considered highly sensitive to climate change given its close correlation. Treelines worldwide have been predicted to advance upward as global average temperatures increase [14]. Only 52% of treelines—examined between 1900 and 2006—had advanced in response to increases in average temperature [15], indicating that the temperature–treeline relationship is complex and impacted by other variables

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Conclusion