Asymmetric dispersal in response to wind direction in a Ponderosa pine forest

Background/Question/Methods

A pillar of sustainable forestry is the maintenance of genetic diversity as forests with greater diversity are more resilient to external stressors. Dispersal through the movement of pollen and seeds plays a key role in maintaining forest genetic diversity, however pollen and seeds trajectories are notoriously challenging to track. In particular, wind-dispersed species, such as conifers, are subject to multiple factors at individual and population scales, including plant characteristics, patch structure, and wind behavior. In this study, we examine the connection between wind patterns and dispersal within a coniferous forest. We sequenced 182 adult Pinus ponderosa trees within an 800-meter range located in a routinely logged forest in Lassen National Forest, California. This forest is a monoculture of mature P. ponderosa and experiences a consistent, westward wind during the standard period of pine pollination, from April through July. Samples were sequenced using genotyping-by-sequencing methods, and parentage assignment was used to assess effective dispersal distance and direction of pollen and seeds. Forest genetic diversity was estimated using Gower’s dissimilarity index, where individuals that are distantly related will have a higher value.

Results/Conclusions

Of the genotyped trees, parentage was identified for 67 samples (36.8%), suggesting consistent dispersal of both pollen and seeds at a local scale. Pollen donors were identified as the farthest dispersing parent in 70% of offspring and dispersed an average of 302.3 meters, compared to seeds which dispersed an average of 223.4 meters. Asymmetric dispersal was discovered along the east to west bearing, which is congruent with wind behavior during pollination season. Trees aligned with the seasonal wind direction were more related to one another (slope = -4.35e-05, P-value = < 0.0001), compared to individuals in the opposite direction (slope = 1.65e-0.5, P-value = < 0.0001). These results suggest that patterns of genetic diversity in this population are driven by pollen dispersal along wind currents, resulting in asymmetric genetic structure.