PS 33-153 - Comparison of lifespan and survival for 114 herbaceous perennials across five ecosystems in the western US

Tuesday, August 7, 2012
Exhibit Hall, Oregon Convention Center
Chengjin Chu1, Kris M. Havstad2, Debra Peters3 and Peter B. Adler1, (1)Department of Wildland Resources and the Ecology Center, Utah State University, Logan, UT, (2)Jornada Experimental Range, USDA Agricultural Research Service, Las Cruces, NM, (3)Jornada Basin Long Term Ecological Research Project, USDA Agricultural Research Service, Las Cruces, NM
Background/Question/Methods

Demographic characteristics such as survival and life expectancy play critical roles in population dynamics and, consequently, community structure. Though increasing attention has focused on factors underlying geographic variation in a species' demographic rates, the difficulty of collecting long-term observation data for multiple species from multiple sites has limited our understanding of the influence of life form and environmental drivers. We compiled five long-term datasets from Sonoran desert, Arizona (1915-1935), Sagebrush steppe, Idaho (1923-1957), Southern mixed prairie, Kansas (1934-1972), Northern mixed prairie, Montana (1932-1945) and Chihuahuan desert, New Mexico (1915-1960), USA, to test for site-dependent demographic responses for forbs and grasses. In these datasets, all individual plants in 1-m2 permanent quadrats were annually identified and mapped, allowing us to track the lifespan of individual genets. Based on these data, we characterized the survival curves of 114 species.

Results/Conclusions

Pooling forb and grass species together, we found significant differences in first year survival rates across the five sites, but differences in life expectancy at year 1 and the shape of the survival curves (Type III) were not significant. For forbs, first year survival and life expectancy, but not the shape of the survival curve, differed significantly among sites. Grasses showed statistically indistinguishable patterns across all sites. In addition, we found significant differences between forbs and grasses in all three demographic rates, with forbs conforming more to Type II survivorship curves than grasses. Our results emphasize that much of the variation in survival across and within species is related to life form (forbs vs. grasses). In contrast, geographic differences in demography were relatively subtle, especially for the grasses, despite dramatic variation in climate and soils across the five sites.