Cross-scale analyses of spatio-temporal variation in tree establishment and coexistence among alpine treelines.
Germino, Matthew*,1, Maher, Eliza1, Blanch, Kevin1, Graumlich, Lisa2, 1 Idaho State University, Pocatello, ID2 Montana State University, Bozeman, MT
ABSTRACT- The role of interspecific variation in regeneration niche in structuring tree communities is known for forests, but not for forest boundaries such as alpine-treeline ecotones (ATEs). Spatial and temporal patterns of Abies lasiocarpa, Pinus albicaulis, and Picea engelmannii establishment were determined at local and regional scales in ATEs of the Rocky Mountains, USA, using a combination of direct observation and dendrochronology. Significant variation in spatial patterns of tree establishment occurred among mountain ranges as a result of differences in microsite tree cover for species, and differences in the relative abundance of species among mountains. Microsites where P. albicaulis and A. lasiocarpa established had the least and most neighboring plant cover of all species, respectively. Seedlings that survived without tree cover had significant herbaceous cover, for all species alike. Recruitments were more consistent among years in forest than at timberline, suggesting that establishments are more episodic and dependent on favorable weather variations at timberline. However, there were few consistent differences in temporal patterns of establishment among species, despite strong spatial differentiation in regeneration niche. Our findings agree with differences among the species in photosynthetic tolerance of seedlings to the intense sunlight and frequent frost that occur at high %SKY in ATEs, and indicate the importance of neighborhood cover for establishing seedlings. Interspecific differences in microsite requirements for neighboring plant cover during initial establishment contribute to the apparent coexistence and possible interdependency of these conifers along a continuum of colonization and succession within ATEs. Positive interactions among tree species that result from differences in seedling ecophysiology and requirements for tree cover are likely to alter rates of forest development among ATEs.
Key words: alpine, treeline, seedling, establishment
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