The Paradox of Forest Fragmentation Genetics

Abstract

Theory predicts widespread loss of genetic diversity from drift and inbreeding in trees subjected to habitat fragmentation, yet empirical support of this theory is scarce. We argue that population genetics theory may be misapplied in light of ecological realities that, when recognized, require scrutiny of underlying evolutionary assumptions. One ecological reality is that fragment boundaries often do not represent boundaries for mating populations of trees that benefit from long-distance pollination, sometimes abetted by long-distance seed dispersal. Where fragments do not delineate populations, genetic theory of small populations does not apply. Even in spatially isolated populations, where genetic theory may eventually apply, evolutionary arguments assume that samples from fragmented populations represent trees that have had sufficient time to experience drift, inbreeding, and ultimately inbreeding depression, an unwarranted assumption where stands in fragments are living relicts of largely unrelated predisturbance populations. Genetic degradation may not be as important as ecological degradation for many decades following habitat fragmentation.

Population at the edge: increased divergence but not inbreeding towards northern range limit in Acer campestre

“…This is because trees are often characterised by wide distribution, high potential for gene flow and strong human impact, including forest management and translocation of seed material (Petit and Hampe 2006). Interestingly, some authors propose the existence of 'the paradox of forest fragmentation genetics', suggesting that trees may not follow the standard consequences of population fragmentation (Kramer et al 2008;but see Piotti 2009). On the contrary, our study demonstrated that the significant spatial genetic patterns can be detected for a tree species even at relatively restricted spatial scale.…”

“…Interestingly, the genetic consequences of fragmentation seem to be generally weak in forest trees (Kramer et al 2008), although more recent reviews indicated that the effect size may be related with the pollination mechanism (Vranckx et al 2011). However, forest trees are often characterised by intensive gene flow and strong human impact through the forest management (including translocation of forest reproductive material), which make studies of the effects of population fragmentation upon the distribution of genetic diversity difficult.…”

Bayesian approach reveals confounding effects of population size and seasonality on outcrossing rates in a fragmented subalpine conifer

“…In particular, seed yield can be reduced (Salzer and Gugerli 2012), lowering (variance) effective population size. However, because tree species seem to be generally "resistant" to the impact of fragmentation on the genetic diversity (O'Connel et al Kramer et al 2008), the question of whether high adult genetic homogeneity in the Tatras (Dzialuk et al 2014) will be preserved in future generations remains open. To answer this, we need to know if potentially high pollen-mediated gene flow can preclude erosion of the gene pool in small populations.…”

Effects of converting natural forests to coniferous plantations on fruit and seed production and mating patterns in wild cherry trees

“…On the other hand, the most predictable scale for the effect of the natural-forest area on the biparental-inbreeding rate and correlated paternity was the 500-ha buffer area with the 1.26-km radius. Thus, long-distance pollen dispersal occurring such a large scale may affect the overall mating patterns (Kramer et al 2008;Ashley 2010). This spatial scale corresponds to the foraging ranges of some P. verecunda pollinators, such as birds and bees (Beekman and Ratnieks 2000;Lepais et al 2010;Hagen et al 2011;Aslan et al 2014).…”

“…Although the matrix is expected to impede pollen flow, substantial pollen immigration has been observed in isolated patches, particularly in those with reduced plant density (Sork and Smouse 2006;Kramer et al 2008). Pollen dispersal distance sometimes increases in patches with reduced plant density (Llorens et al 2012;Cô rtes et al 2013) and between fragmented patches because of changes in pollinator movements in the matrix (Kamm et al 2010).…”