Spatial Scales of Pollen and Seed-Mediated Gene Flow in Tropical Rain Forest Trees

University of Michigan Department of Ecology and Evolutionary Biology and Herbarium 830 North University Avenue Ann Arbor MI 48109 USA
Tropical Plant Biology 04/2008; 1(1):20-33. DOI: 10.1007/s12042-007-9006-6

ABSTRACT Gene flow via seed and pollen is a primary determinant of genetic and species diversity in plant communities at different
spatial scales. This paper reviews studies of gene flow and population genetic structure in tropical rain forest trees and
places them in ecological and biogeographic context. Although much pollination is among nearest neighbors, an increasing number
of genetic studies report pollination ranging from 0.5–14km for canopy tree species, resulting in extensive breeding areas
in disturbed and undisturbed rain forest. Direct genetic measures of seed dispersal are still rare; however, studies of fine
scale spatial genetic structure (SGS) indicate that the bulk of effective seed dispersal occurs at local scales, and we found
no difference in SGS (Sp statistic) between temperate (N = 24 species) and tropical forest trees (N = 15). Our analysis did find significantly higher genetic differentiation in tropical trees (F
ST = 0.177; N = 42) than in temperate forest trees (F
ST = 0.116; N = 82). This may be due to the fact that tropical trees experience low but significant rates of self-fertilization and bi-parental
inbreeding, whereas half of the temperate tree species in our survey are wind pollinated and are more strictly allogamous.
Genetic drift may also be more pronounced in tropical trees due to the low population densities of most species.

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    ABSTRACT: It is well known that spatial genetic structure (SGS), a phenomenon that occurs to some degree in most plants, is influenced by species' characteristics such as dispersal biology. However, local conditions (e.g., spatial, environmental, demographic) may also contribute to SGS, but the influence of local factors is rarely quantified. To test the role of local conditions, we quantified SGS in 21 populations of the threa-tened forest tree Juglans cinerea (butternut). We observed a wide range across the populations in several measures of SGS (mean Sp statistic: 0.020, range: $0–0.04; mean F 1 : 0.06, range: $0–0.13). We found correlations between local conditions and several measures of SGS. Specifically, ecological zone (riparian/ upland), and degree of habitat linearity strongly correlated to the Sp statistic and the distance over which positive relatedness was observed (Dpos). In contrast, degree of aggregation was significantly associated with high kinship in the smallest distance class (F 1), and a signature of colonization. Land-use history had minor effects compared to spatial or ecological factors. Population age structure significantly influenced F 1 , but not other metrics. Our work suggests that spatial, demographic, and environmental factors inter-act to determine SGS, and emphasizes that different aspects of SGS are not necessarily influenced by the same ecological factors. An influence of local conditions on SGS has implications for population genetic response to rapidly changing habitats as well as forest restoration. Ó 2013 Elsevier B.V. All rights reserved.
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    ABSTRACT: Tropical agro-forest landscapes are global priority areas for biodiversity conservation. Little is known about the ability of these landscapes to sustain large late successional forest trees upon which much forest biodiversity depends. These landscapes are subject to fragmentation and additional habitat degradation which may limit tree recruitment and thus compromise numerous ecosystem services including carbon storage and timber production. Dysoxylum malabaricum is a large canopy tree species in the Meliaceae, a family including many important tropical timber trees. This species is found in highly fragmented forest patches within a complex agro-forest landscape of the Western Ghats biodiversity hot spot, South India. In this paper we combined a molecular assessment of inbreeding with ecological and demographic data to explore the multiple threats to recruitment of this tree species. An evaluation of inbreeding, using eleven microsatellite loci in 297 nursery-reared seedlings collected form low and high density forest patches embedded in an agro-forest matrix, shows that mating between related individuals in low density patches leads to reduced seedling performance. By quantifying habitat degradation and tree recruitment within these forest patches we show that increasing canopy openness and the increased abundance of pioneer tree species lead to a general decline in the suitability of forest patches for the recruitment of D. malabaricum. We conclude that elevated inbreeding due to reduced adult tree density coupled with increased degradation of forest patches, limit the recruitment of this rare late successional tree species. Management strategies which maintain canopy cover and enhance local densities of adult trees in agro-forest mosaics will be required to ensure D. malabaricum persists in these landscapes. Our study highlights the need for a holistic understanding of the incipient processes that threaten populations of many important and rare tropical tree species in human dominated agro-forest landscapes.
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    ABSTRACT: Conservation managers typically need to make prompt decisions based on limited information and resources. Consequently, generalisations have essential roles in guiding interventions. Here, we (i) critique information on some widely accepted generalisations and variables affecting them, (ii) assess how adequately genetic factors are currently incorporated into population viability analysis (PVA) models used to estimate minimum viable population sizes, and (iii) relate the above to population size thresholds of the IUCN Red List criteria for threatened species that were derived from genetic considerations. Evidence accumulated since 1980 shows that genetically effective population size (Ne) = 50 is inadequate for preventing inbreeding depression over five generations in the wild, with Ne > 100 being required to limit loss in total fitness to < 10%. Further, even Ne = 500 is too low for retaining evolutionary potential for fitness in perpetuity; a better approximation is Ne > 1000. Extrapolation from census population size (N) to Ne depends on knowing the ratio of Ne/N, yet this information is unavailable for most wild populations. Ratio averages (~�0.1–0.2) from meta-analyses are sufficient, provided adjustments are made for dissimilar life histories. Most PVA-based risk assessments ignore or inadequately model genetic factors. PVA should routinely include realistic inbreeding depression, and genetic impacts on evolutionary potential should be incorporated where appropriate. Genetic generalisations used in conservation, the treatment of genetics in PVAs, and sections of the IUCN Red List criteria derived from genetic considerations, all require revision to be more effective conservation tools.
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