Print PageBackground/Question/Methods
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 understand the factors controlling recruitment and expansion patterns. One such factor, that can play a major role in population development, is the relative contribution of long-distance dispersal, versus intrinsic growth, over the course of a newly-founded population’s development.
We studied four disjunct populations of ponderosa pine, in the Bighorn Basin of north-central Wyoming, to examine the contribution and timing of long-distance dispersal to population growth. These populations are separated from continuous ponderosa pine forest by distances ranging from 15 to over 100 km. There is strong evidence that the initial colonizers are still present, and that there has been no erasure of past trees at the sites, giving us a complete record of population history. All trees in each of the populations were aged using tree-ring based techniques and needle tissue was collected and used for molecular genetic analysis at nine microsatellite loci. Trees recruiting from long-distance dispersal events were identified as individuals containing alleles not previously present in the population.
Results/Conclusions
Results showed that the proportion of each population that was established from long-distance events decreased over time. However, the rate of long-distance events remained fairly constant, ranging from 0-6 events/decade, throughout each population’s history. Almost every individual (>80%) that established in the century following the initial colonization event, at each site, was the product of long-distance dispersal. This level of immigration may be vital in the early stages of expansion, if populations are to overcome Allee effects and exceed a threshold size, below which they will not persist. Empirical estimates of long-distance dispersal rates and their contribution to population growth are essential for understanding species responses and capabilities to respond to changing climate.
