PS 8-94 - Spatial patterns suggest warming-driven dominance shift reflects change in competition in Rocky Mountain meadow community

Monday, August 8, 2011
Exhibit Hall 3, Austin Convention Center
Michael E. Roswell, Department of Ecology, Evolution, and Natural Resources, Rutgers University, New Brunswick, NJ and John Harte, Energy and Resources Group, University of California, Berkeley, CA

Global climate change destabilizes plant community structure. How warming affects competitive hierarchies remains uncertain, yet models, risk assessments, and adaptation measures rarely account for novel interspecific interactions. In a 20-year long meadow warming experiment in the Rocky Mountains, Colorado, mountain big sagebrush (Artemisia tridentata var. vaseyana) replaced forbs as the dominant form of vegetation in the warmest, driest section of the warmed plots.  Two mechanisms could explain this shift. First, independent responses- warming negatively impacting forb performance and positively impacting A. tridentata recruitment and growth- could underlie this change. Second, shrub competitive ability may be enhanced by the warming treatment. In this case, shrub density would be lower near forbs and forb performance would be weaker near shrubs. To test for these patterns, we observed the effect of proximity to A. tridentata on the distribution and performance of two shallow-rooted forb species (Delphinium nuttallium and Erigeron speciousus). We recorded spatial data and floral output for over 1000 individuals of each species over a single field season, both in the experimental warming setup (2920 m elevation), and in three similar montane meadows on a 400m elevation gradient. We used a modified O-ring statistic that accounts for shrub size to identify patterns.


At low elevation (2760m), Artemisia tridentata density was lower than expected within 1 meter of forbs with robust (above average) floral output (p=0.02), and higher than expected within 75cm of forbs without robust floral output (p=0.01). At mid elevation (2940m), a similar but weaker pattern emerged. At high elevation (3150m), results are within confidence envelopes for random simulations, but shrub density was marginally higher near robust forbs, and marginally lower near non-robust forbs. While strong repulsion within 1m of shrubs at low altitude is consistent with data collected in the warming experiment, it is not detectable unless the actual size of forb patches and shrubs is accounted for. Climate and snowmelt date drive community structure in this system, and vary systematically with elevation in addition to experimental warming. Overall, the results support the hypothesis that temperature and snowmelt date can mediate a shift from A. tridentata facilitating D. nuttallianum and E. speciousus at high altitude to competitive exclusion at low altitude and under experimental warming. This result suggests that novel interspecific interactions, in addition to simply climate envelopes and migration rates, will dictate future species distribution.

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