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, ΦST 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 (φST= 0.298) as that of the adults (φST= 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 high‐mountain 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.

Low genetic diversity and fragmentation effects in a wind-pollinated tree, Polylepis multijuga Plige (Rosaceae) in the high Andes

“…Average heterozygosity for P. multijuga (He = 0.12-0.14) was lower than reported for other species with similar life histories and measured using dominant markers (He = 0.25-0.27) (Nybom 2004). These values are very similar to those obtained in a study of P. incana (He = 0.13-0.17) that also has a fairly restricted range from Ecuador to Peru and is currently classified as vulnerable (Hensen et al 2011). The low genetic diversity found in these endemic species is predicted to increase inbreeding and decrease their ability to adapt to environmental change (Frankham et al 2010).…”

“…TDR was relatively protected from grazing since it was located in a deep ravine. The level of differentiation (PhiPT = 0.135) found for adults in this study is less than half that reported for adult and seedling P. incana (PhiPT = 0.307, 0.298, respectively) (Hensen et al 2011). The high level of differentiation found for P. incana, which is also a wind pollinated tree, suggests that historical fragmentation operating on the scale of hundreds of years has decreased gene flow in this species (Hensen et al 2011).…”

“…The level of differentiation (PhiPT = 0.135) found for adults in this study is less than half that reported for adult and seedling P. incana (PhiPT = 0.307, 0.298, respectively) (Hensen et al 2011). The high level of differentiation found for P. incana, which is also a wind pollinated tree, suggests that historical fragmentation operating on the scale of hundreds of years has decreased gene flow in this species (Hensen et al 2011). The moderate level of differentiation we found in this study is similar to that of P. pauta (PhiPT = 0.126) (Aragundi et al 2011) and is consistent with a pattern of more recent fragmentation (Aragundi et al 2011;Vanden-Broeck et al 2011).…”

Prediciendo la distribución de Polylepis: bosques Andinos vulnerables y cada vez más importantes

“…Como con la mayoría de las especies de árboles tropicales, la fragmentación de los bosques de Polylepis disminuyen significativamente la diversidad genética de las poblaciones a pesar de ser polinizada por el viento, que a menudo muestran una menor pérdida genética de la fragmentación (Hensen et al 2012). Además, las poblaciones en las elevaciones más altas tienen menor diversidad genética que otras poblaciones (Aragundi et al 2011, Hensen et al 2012) y se ha encontrado que la diversidad genética en P. multijuga era aproximadamente la mitad que otro especies polinizadas por el viento (Quinteros-Casaverde et al 2012). Esto es posiblemente la resulta de los cuellos de botella genéticos y efectos fundadores durante las migraciones de ladera del Holoceno o la corta duración de la temporada de cultivo y temperaturas más frías en las elevaciones más altas (Hensen et al 2012).…”

“…Además, las poblaciones en las elevaciones más altas tienen menor diversidad genética que otras poblaciones (Aragundi et al 2011, Hensen et al 2012) y se ha encontrado que la diversidad genética en P. multijuga era aproximadamente la mitad que otro especies polinizadas por el viento (Quinteros-Casaverde et al 2012). Esto es posiblemente la resulta de los cuellos de botella genéticos y efectos fundadores durante las migraciones de ladera del Holoceno o la corta duración de la temporada de cultivo y temperaturas más frías en las elevaciones más altas (Hensen et al 2012). La baja diversidad genética en las elevaciones más altas tendrá un impacto directo sobre la viabilidad de las primeras poblaciones que migran con el cambio climático.…”