"Conserving and Restoring the Roots of California's Richness"
California Native Plant Society 2012 Conservation Conference
January 10-14, 2012
Town & Country Resort
San Diego, CA
http://www.cnps.org/cnps/conservation/conference/2012/

USGS Western Ecological Research Center scientists, including fire ecologist Jon Keeley, will be attending the CNPS 2012 conference and presenting the following talk:

A Vegetation Shift: Climate warming, drought, or past disturbance?
Thursday, January 12, 2012 | 10:20 a.m. PST | Windsor Room
WERC Contributor: Jon Keeley

Simple models of plant response to warming climates predict vegetation moving to cooler and/or wetter locations (in mountainous regions, “marching upslope”). However, the mechanisms explaining species-specific responses to changes in temperature and water availability are most likely much more complex. For example, in species with episodic seedling recruitment, past disturbance history may interact with temperature and drought in producing patterns of establishment and mortality. We re-examined a recently reported vegetation shift in the Santa Rosa Mountains, California, to determine the mechanisms behind the reported uphill shift of a plant distribution. Our focus was on a key species in this reported pattern, Ceanothus greggii: an “obligate seeding” shrub that recruits postfire. This life-history allowed us to calculate stand ages and a time series of past percapita mortality rates by counting growth rings on live and dead individuals. For five elevations used in the prior study, we calculated time series of past per-capita mortality rates by counting growth rings on live and dead individuals. Using a model-selection framework, we tested three alternative hypotheses explaining the time-series patterns of mortality: H1) mortality increased over time consistent with climate warming, H2) mortality peaked 40-50 years post fire at each site, consistent with self-thinning, and H3) mortality was correlated with past drought. We found that the sites were different ages since the last fire, and that the reported increase in the mean elevation of C. greggii was due to higher recent mortality at the lower elevations which were younger sites. The time-series pattern of mortality was best explained by the stand age/self-thinning hypothesis (H2) and poorly explained by either gradual warming or drought. At least for this species, the reported distribution shift uphill appears to be an artifact of disturbance history and stand age and is not evidence for a climate warming effect.

Jon Keeley: http://www.werc.usgs.gov/keeley