Mechanisms of treeline species range shift in Basin and Range sub-alpine bristlecone pine/limber pine forests

Background/Question/Methods

Land management agencies use vulnerability assessment to predict how species distributions will be affected by abiotic changes and to inform its management decisions. With abiotic change, biotic interactions are also likely to be affected. Current vulnerability assessments lack an understanding of the mechanisms by which species distributions change as a result of biotic interactions. Treeline forests may be moving upslope in response to climate change but individual tree species may not respond equally, depending on abiotic and biotic responses to climate change. Great Basin bristlecone pine and limber pine are two species which compete for limiting resources at treeline in Basin and Range forests. Bristlecone pine is currently the dominant species at treeline but downslope limber pine may be poised to expand its range upslope into or above bristlecone pine habitat. As a first step in determining treeline dynamics, we asked whether there is evidence to support current invasion of limber pine into bristlecone pine forests. For this, we censused the sub-alpine forest on Campito Mountain in the White Mountains, CA, measuring tree ages by whorl-counting to group the trees into juvenile or adult age classes. Differential distributions between the age classes would show changing interspecific population dynamics in this forest.

Results/Conclusions

Preliminary results show a change in relative population sizes at treeline.  While the bristlecone pine adult population dwarfs the limber pine adult population, a more equitable distribution is evident in juveniles.  Using the adult species population frequencies as a comparison, observed juvenile population sizes showed a significant difference from expected values, with limber pine making up the majority of individuals (p<0.001). On the ground, it is apparent that this forest is transitioning from a bristlecone pine-dominated to a limber pine-dominated one.  Informal observations show this pattern being repeated throughout the White Mountains and possibly throughout Basin and Range forests. Future work will seek to determine whether this change is a pulse of limber pine during favorable conditions or whether this demographic change may be more permanent.  For this, we will use common garden experiments to determine interspecific tolerances along an abiotic gradient and dendrochronological dating to determine past climatic tolerance extremes for both species in several locations.