As global climate warms, species are generally expected to expand their ranges northward or to higher elevations. Climate envelopes have been used to predict where species will move, but these models have largely matched presence of adults to climatic parameters. Since a species’ ability to disperse to new areas is a function of young establishment, there is a strong environmental filter on the early life stages of a species that may not match those of adults. Treeline is an ideal study system to examine the mechanisms of warming-induced range shifts because of its discrete boundary, its strong association with temperature, and its limited disturbance regime. This study used a regional network of above- and below-treeline plots in five Great Basin mountain ranges to ask the questions: 1. Is treeline advancing upslope in the Great Basin and in what environmental contexts, and 2. How do the climate relationships compare between adults and treeline-advancing juveniles among the treeline species, Great Basin bristlecone pine (Pinus longaeva) and limber pine (Pinus flexilis)? From these data, we generated models using penalized maximum likelihood (LASSO) of important predictors for juvenile and adult life stages of each species.
Results/Conclusions
We found that treeline has advanced an average of 19m upslope in Great Basin mountain ranges. Treeline advance is higher in northerly ranges on west and south slopes and on granitic soil, a soil type dominated by limber pine. Models of basal area (adults) and density (juveniles) response to a host of environmental and biotic conditions indicate that species differ in their responses: bristlecone pine adults and juveniles largely respond to the same predictors in the same way, whereas while limber pine adult basal area can be modeled to coincide with a number of predictors, juveniles are not associated with those same predictors. Thus, bristlecone pine juveniles are likely to establish in similar climatic niche space as adults. Limber pine juveniles by contrast are establishing at much higher rates than bristlecone pine throughout the Great Basin, even in areas where limber pine adults are poorly represented. Limber pine appears much better able to take advantage of the current rapid pace of climatic warming likely due to a highly efficient dispersal vector. This study suggests models of range shifts for some species may be improved by considering life stage and history strategies. It also highlights concern for Great Basin bristlecone pine’s ability to respond to rapid climatic warming.