The theoretical impacts of anthropogenic habitat degradation on genetic resources have been well articulated. Here we use a simulation approach to assess the magnitude of expected genetic change, and review 31 studies of 23 neotropical tree species to assess whether empirical case studies conform to theory. Major differences in the sensitivity of measures to detect the genetic health of degraded populations were obvious. Most studies employing genetic diversity (nine out of 13) found no significant consequences, yet most that assessed progeny inbreeding (six out of eight), reproductive output (seven out of 10) and fitness (all six) highlighted significant impacts. These observations are in line with theory, where inbreeding is observed immediately following impact, but genetic diversity is lost slowly over subsequent generations, which for trees may take decades. Studies also highlight the ecological, not just genetic, consequences of habitat degradation that can cause reduced seed set and progeny fitness. Unexpectedly, two studies examining pollen flow using paternity analysis highlight an extensive network of gene flow at smaller spatial scales (less than 10 km). Gene flow can thus mitigate against loss of genetic diversity and assist in long-term population viability, even in degraded landscapes. Unfortunately, the surveyed studies were too few and heterogeneous to examine concepts of population size thresholds and genetic resilience in relation to life history. Future suggested research priorities include undertaking integrated studies on a range of species in the same landscapes; better documentation of the extent and duration of impact; and most importantly, combining neutral marker, pollination dynamics, ecological consequences, and progeny fitness assessment within single studies. Keywords: fragmentation; gene flow; genetic diversity; inbreeding; neotropical trees; selective logging Introduction Anthropogenic destruction of tropical forests has dramatically increased in recent decades, and the threat to tropical ecosystems is well articulated (eg Bawa and Seidler, 1998;White et al, 1999;Trejo and Dirzo, 2000;Ghazoul and McLeish, 2001). For biodiversity, the single most important impact of these activities is loss of primary habitat (Lira et al, 2003). However, the degradation of habitat also leaves a lasting legacy with which biota must cope if they are to continue to survive. The need for a more integrative approach to this problem, which combines ecology and population genetics, is evident (Sih et al, 2000; Young and Clarke, 2000). For trees, degradation of primary habitat results from two main processes, fragmentation of forest into patches following clearance and disturbance of habitat following extraction processes, such as selective logging.Tropical trees are thought to be particularly vulnerable to the effects of habitat degradation due to their demographic and reproductive characteristics, including low density of occurrence, complex self-incompatible breeding systems, high rates of outcrossing ...

Habitat destruction and the resultant fragmentation of the remaining forest are a common phenomenon in the tropics. Most investigations emphasize the potential dangers of fragmentation in isolating patches of forest and exposing populations to loss of species diversity through founder effects, genetic drift, inbreeding, and restricted gene flow. However, a limited number of studies have shown that gene flow may be extensive in tropical trees, suggesting that it may occur between forest fragments and also ''isolated'' remnant trees. There is an urgent need to quantify pollen flow within and between forest fragments to test the veracity of such views and determine the genetic value of such fragments for in situ conservation. Microsatellite markers are used to genotype individuals of Swietenia humilis from a highly fragmented forest mosaic to directly quantify pollen-mediated gene flow. Distances of pollen flow more than 10 times greater than previously reported were detected. Our results show that some tropical angiosperm tree species may be much more adaptable and resilient to habitat destruction and fragmentation than previously considered. The description of many remnant trees as isolated or ''living dead'' may be more a conditioning of human perception than a true reflection of their potential conservation value.

Ash trees (genus. Here we sequence the genome of a low-heterozygosity Fraxinus excelsior tree from Gloucestershire, UK, annotating 38,852 protein-coding genes of which 25% appear ash specific when compared with the genomes of ten other plant species. Analyses of paralogous genes suggest a whole-genome duplication shared with olive (Olea europaea, Oleaceae). We also re-sequence 37 F. excelsior trees from Europe, finding evidence for apparent long-term decline in effective population size. Using our reference sequence, we reanalyse association transcriptomic data 3 , yielding improved markers for reduced susceptibility to ash dieback. Surveys of these markers in British populations suggest that reduced susceptibility to ash dieback may be more widespread in Great Britain than in Denmark. We also present evidence that susceptibility of trees to H. fraxineus is associated with their iridoid glycoside levels. This rapid, integrated, multidisciplinary research response to an emerging health threat in a non-model organism opens the way for mitigation of the epidemic.

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With large tracts of once continuous forest now modified by human use to leave reduced and highly fragmented stands of trees, the determination of the genetic consequences of forest fragmentation is a priority for ascertaining the conservation value of resultant stands, and in formulating sustainable management strategies. The levels and distribution of genetic diversity over 10 microsatellite loci were investigated within a fragmented population of the neotropical tree Swietenia humilis Zucc. High levels of genetic variation, typical of a highly outcrossing species, were found in all fragments at all loci (mean HE = 0.548). The majority of the variation was within rather than between fragments (RST = 0.032), giving high indirect estimates of gene flow (Nm = 8.9), probably reflecting the genetic structure of the trees present under more continuous forest. A high proportion of loci also showed significant departures from Hardy-Weinberg equilibrium with associated significant levels of FIS. The initial effects of fragmentation were, however, seen in the fragments through the loss of low-frequency alleles present in the continuous 'control' stand. The percentage of this allelic loss increased with a decrease in fragment size.

Meeting the multimillion hectare commitments for forest and landscape restoration (FLR) will require billions of tree seed and seedlings. However, the adequacy of seed supply in terms of quantity, genetic diversity and quality has received scant attention in FLR planning. We surveyed 139 FLR projects worldwide and identified widespread problems in the availability and diversity of tree seed, with potentially deleterious consequences for the vigor, productivity and long-term persistence of restored tree populations. Large projects and those focused on climate change mitigation were particularly associated with multiple problems in seed sourcing. To avoid large-scale failure in FLR, we recommend: (1) national assessments of seed supply and demand for FLR, (2) reviewing FLR targets and funding cycles, (3) fostering sharing of knowledge and experiences regarding seed supply and selection, (4) enhancing seed exchange across landscapes, and (5) introducing regulations for seed quality and strengthening capacities for compliance.

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