Climate change can influence vegetation in a variety of ways, including increases and decreases in established populations, extirpation of entire populations, and establishment of populations in new locations. Models of vegetation response to changing climate predict shifts in communities with resulting landscapes radically altered from their present states. Colonization of new sites is predicted to be widespread for many tree species, but the mechanisms by which this happens are poorly understood, because establishment and expansion of tree populations may span decades to centuries. To adequately assess how tree species will respond to changing environmental conditions we need to know not only the speed at which species can migrate into new territory, but also the underlying factors controlling recruitment and expansion patterns.

Using ponderosa pine (Pinus ponderosa) as a model species, this study utilizes a combination of dendroecology, dendroclimatology and molecular genetics to address colonization, recruitment, and expansion processes. We studied four small, disjunct populations of ponderosa pine in the Bighorn Basin of north-central Wyoming. These populations are located 30 to 100 kilometers from the nearest core populations of ponderosa pine in the western Bighorn Mountains. Packrat midden studies have shown that ponderosa pine colonized the western slopes of the Bighorn Range 1500 years ago, so the disjunct populations in the basin must be younger. All trees (living and dead) at each of the four disjunct populations were mapped, cored, and aged using tree-ring based techniques.

Results/Conclusions

We obtained records of hydroclimatic variability from the Bighorn Basin using four published tree-ring series from Pinus flexilis and Pseudotsuga menziesii. The four disjunct populations were all established within the past 500 years. Initially, the populations grew slowly with low recruitment rates until the early 19^th Century, when they experienced one or more large recruitment pulses. These pulses coincided with extended wet periods in the climate reconstruction. However, similar wet periods before the 19^th Century were not accompanied by recruitment pulses, indicating that other factors (e.g. population density, genetic variability) were also important in colonization and expansion. We are currently obtaining genetic data and carrying out population modeling to differentiate the effects of population dynamics, genetic variability, and climate variability on recruitment and expansion.