Print PageThe effects of the predicted changes in precipitation and temperature may have complicated and potentially opposing effects on plants. Simple models of plant response to warming climates predict vegetation moving to cooler and/or wetter locations (“marching upslope”). However, the mechanisms explaining species-specific responses to changes in temperature and water availability are most likely much more complex. Although there is evidence of uphill changes in plant distributions, some examples are unreplicated and show results merely consistent with climate change. In the absence of ruling out alternative explanations, the evidence for a causal link between climate change and reported changes in plant distribution is weak. 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, in order 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 post-fire from a soil stored seedbank. This life-history allowed us to calculate stand ages and a time series of past per-capita mortality rates by counting growth rings on live and dead individuals. We calculated past mortality rates for six elevations (1280-1870 m). 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.
Results/Conclusions
We found that the sites in the Santa Rosa Mountain transect 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 key 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. We argue that studies describing vegetation patterns consistent with climate change should include greater consideration of mechanism.
