Disrupted connectivity within a metapopulation of a wind-pollinated declining conifer, Taxus baccata L.

Abstract

Population connectivity through seed and pollen dispersal determines the genetic diversity, adaptive potential, and demography of plant metapopulations. In wind-pollinated trees, population connectivity is typically maintained by long-distance pollen flow, counteracting the genetic differentiation generated by drift and restricted seed dispersal. Although strong population fragmentation is theoretically expected to disrupt connectivity in forest trees, empirical evidence remains scarce and inconclusive. We investigated contemporary connectivity within a network of small remnant populations of a declining conifer (Taxus baccata L.), which have been hypothesized to be largely isolated from each other. We tested this hypothesis using molecular data for adult trees and naturally recruited seedlings from all known remnants across a fragmented landscape spanning a length of 20 ​km, and a specifically designed statistical approach to quantify contemporary pollen and seed migration rates between populations. We additionally assessed dispersal potential using a spatially explicit parentage analysis to estimate seed and pollen dispersal kernels within one of the remnants. Estimated pairwise migration rates between populations were barely detectable for seeds, while they were larger (up to 1.1%) and significant for pollen. Both seed and pollen migration rates decreased with geographic distance between populations, more steeply in the case of pollen migration. According to parentage-based dispersal kernels, 51.8% of seeds and 11.4% of pollen travel less than 25 ​m, whereas 0.2% of seeds and 36.1% of pollen travel more than 250 ​m from a source tree. In addition, 1.2% of pollen can travel more than 2.5 ​km. We showed that strong present-day population fragmentation, with separation distances over a few kilometers between small fragments, can substantially limit the connectivity of a wind-pollinated declining tree, leading to low pollen-mediated contemporary gene flow and null or virtually null demographic connectivity via seed dispersal.