Measuring pollen flow in forest trees: an exposition of alternative approaches

Steady reduction of once-extensive forested habitats into isolated fragments will have unintended consequences for genetically disrupted tree populations. Gene flow in forest trees involves both pollen and seed flow, and here we describe two alternative analytical models for pollen flow analysis, PARENTAGE and TWOGENER With PARENTAGE models, we can assess the numbers and frequency spectra of pollinating males for a single female, and the spatial distribution of those pollinating males. Parentage analysis establishes the male parentage of offspring, and uses the inter-parent distances to establish the spatial distribution of pollination. With TWOGENER analysis, we gauge the degree of pollen pool non-overlap of widely spaced females, to derive estimates of the effective number of fathers per mother and the average distance of pollination. Parentage analysis suggests large numbers of fathers per mother while TWOGENER analysis suggests smaller numbers of ʹeffective pollinatorsʹ. Because of TWOGENER’s emphasis on "effective pollen flow", it tends to yield shorter average pollination distances. The two approaches are complementary, but suited to different sampling designs and questions. Here, we attempt to reconcile sometimes-divergent findings while also discussing emerging findings on pollen movement across complex landscapes.