Theoretical models of migration in seed plants predict that migration rates are a result of the interaction between two forces: the ability and mode of seed dispersal, and the control that the environment exercises over seed germination and seedling establishment. This model may be adequate for some plant migrations; however, past migrations of several tree species have been much faster than either models or empirical calculations predict. For example, Douglas fir migrated out of a Pleistocene refugium at the tip of the southern Rocky Mountains into the Wah-Wah/Confusion Ranges in the Great Basin at an average rate of 286 m/yr, but empirical data based on average maximum Douglas fir seed dispersal distances and average seedling survival rates (12%) predict a maximum migration rate of only 80 m/yr.

A closer look at seed dispersal rates shows that a small proportion of seeds are dispersed 1,200m or more; long distance dispersal is correlated with tree height as well as wind velocity. Data on seedling survival shows that the survival rate is much higher (90% of 13 yr old seedlings) for seedlings that establish outside of a Douglas fir stand, in contrast to establishment in the parent stand (10% of 13 yr old seedlings). One ecological factor contributing to this difference may be a higher likelihood of mycorrhizal mats under non-Douglas fir vegetation; mycorrhizae are crucial to Douglas fir seedling establishment.

An expanded numerical model of migration in Douglas fir calibrated to the Holocene migration history incorporates long distance seed dispersal rates, wind velocity, tree height, and probability of seedling establishment in safe sites.

75/125 YEARS