Historic and recent fragmentation coupled with altitude affect the genetic population structure of one of the world's highest tropical tree line species

Abstract

Aim. To assess the effects of altitude and historic and recent forest fragmentation on the genetic diversity and structure of the wind-pollinated tropical tree line species Polylepis incana. Location. One of the highest mountain forest regions of the world, located in the Eastern Cordillera of the Ecuadorian Andes. Methods. We compared genetic diversity and structure of adult trees with those of seedlings (n = 118 in both cases) in nine forest stands spanning an altitudinal gradient from 3500 to 4100 m a.s.l. using amplified fragment length polymorphisms(AFLPs). Genetic diversity was calculated as percentage of polymorphic bands (P) and Nei?s expected heterozygosity (He); genetic differentiation was assessed using analysis of molecular variance, FST statistics and Bayesian cluster analysis. Results. Estimates of genetic diversity at the population level were significantly lower in seedlings than in adults. Genetic diversity (He-value) was, in both cases, negatively correlated to altitude and positively correlated to population size in the seedlings. Genetic differentiation of the seedlings was approximately as high (fST = 0.298) as that of the adults (fST = 0.307), and geographical differentiation was clearly reflected in both AFLP profiles, with mountain ridges acting as barriers to gene flow. Main conclusions Our study provides evidence of a historic upslope migration of P. incana in central Ecuador. In addition, it highlights the detrimental effects of unexpectedly strong genetic isolation, both recent and historical, particularly for our wind-pollinated species where the distance between forest stands was less than 25 km. We therefore additionally propose that in habitats with pronounced highmountain landscape structures, gene flow may be hampered to such an extent that species have a more pronounced sensitivity to habitat fragmentation, even among populations of wind-pollinated trees.