Animal-dispersed Tree Species Research Articles

Successional trajectories of seed dispersal mode and seed size of canopy tree species in wet tropical forests

Despite the critical role of seed dispersal for tree colonization during forest succession in wet tropical forests, successional trajectories of seed dispersal modes are poorly documented at the community level. Overall successional trends in wet tropical forests indicate that, over time, animal-dispersed and large-seeded species increase in relative abundance in woody vegetation, whereas wind-dispersed and small-seeded species decline. Increased abundance of animal-dispersed trees during succession may be attributed to higher rates of seed deposition by animals and higher survival rates of animal-dispersed species with larger seeds (diaspores) compared to wind-dispersed species. We compiled categorical information on seed size (maximum seed length) and dispersal mode (anemochory, autochory, or zoochory) for 240 canopy tree species in six naturally regenerating forests (11–45 years since the abandonment of pasture) and two old-growth forests in lowland wet forests in northeastern Costa Rica. We evaluated dispersal mode and seed size among tree species classified as second growth and old growth specialists, and generalists based on relative abundance data in these plots. Further, we compared long-term trajectories in the dispersal and seed-size profile for 240 species of canopy tree species for seedlings, saplings, and trees ≥5 cm dbh using annual vegetation survey data from 1997 to 2013. Seed size was significantly associated with dispersal mode, which was driven by anemochorous species with intermediate seeds 6 mm to 15 mm and autochorous seeds with large seeds >15 mm. Dispersal modes of canopy tree species exhibited clear directional trajectories, with decreasing relative abundance of anemochory and increasing relative abundance of zoochory. Zoochorous seedlings showed higher survival rates than anemochorous seedlings. Species with seeds ≤ 6 mm decreased in relative abundance through succession, while species with seeds >15 mm increased within seedling and sapling communities. Seedlings with seeds >15 mm showed higher survival rates than smaller seeds in second-growth forests. The study further demonstrates the importance of seed traits, such as dispersal mode and seed size, for community assembly during forest regrowth, and the importance of frugivorous animals in this process. The abundance of animal-dispersed tree species in different size classes can be a useful indicator of the recovery of biodiversity and species interactions during forest succession in wet tropical regions.

Roads Disrupt Frugivory and Seed Removal in Tropical Animal-Dispersed Plants in French Guiana

Ecological interactions are being affected at unprecedented rates by human activities in tropical forests. Yet, the continuity of ecological functions provided by animals, such as seed dispersal, is crucial for forest regeneration and species resilience to anthropogenic pressures. The construction of new roads in tropical forests is one of the main boosters of habitat destruction as it facilitates human access to previously isolated areas and increases defaunation and loss of ecological functions. It, therefore, becomes increasingly urgent to rapidly assess how recently opened roads and associated anthropogenic activities affect ecological processes in natural habitats, so that appropriate management measures to conserve diversity can be taken. In this study, we aimed to evaluate the effects of anthropogenic pressures on the health status of a mature rainforest crossed by a newly opened road in French Guiana. For this, we combined different methods to conduct a rapid assessment of the forest’s health status. Firstly, we evaluated the activity of frugivores using camera traps deployed in four forest patches located near (<1 km) ecological corridors preserved as canopy bridges over the road during the fruiting periods of four animal-dispersed tree species. Secondly, we analyzed the fate of seeds enclosed in animal-dispersed tropical fruits by calculating the proportions of fruits consumed and seeds removed (either dispersed or predated) by frugivores. Results show that the proportion of fruits opened and consumed was lower in the forest areas located near the road than in the control forest, and this difference was more significant for plant species strictly dependent on large-bodied primates for seed dispersal than for species relying on both primates and birds. Camera traps showed the presence of small primates and kinkajous feeding on Virola fruits in the forest impacted by the road, where large primates were absent. It is thus likely that smaller frugivores exert a compensatory effect that maintains ecological functions near the road. Despite efforts made to preserve forest continuity through ecological corridors, anthropogenic pressures associated with road proximity are affecting wildlife and disrupting associated ecological functions crucial for plant regeneration, contributing to further forest degradation.

Secondary seed removal in a degraded forest habitat in Madagascar

Forest restoration is a prime goal within the 2021–2030 UN “Decade of Ecosystem Resoration”. As part of these activities, natural regeneration has to be promoted for biological as well as for economic reasons. For this, the processes of seed dispersal, seed predation and germination have to be understood in the original as well as in degraded vegetation formations. We used seed removal experiments to assess post-dispersal processes that influence recruitment along a gradient of forest degradation in Madagascar analyzing seeds of three animal dispersed tree species. The percentage of seeds consumed or dispersed, declined from forest (28.6%) to degraded forest (17.2%) to savanna (10.8%). Only three out of 1080 seeds were cached and remained intact during the 14-day experiment. All three seeds were cached in the forest habitat and none in the degraded forest and savanna. The low percentage of seeds removed may be due to the lack of endemic rodents caching seeds, as only introduced rats were recorded in the area. The species-poor fauna of potential secondary seed dispersers of the region and especially in the degraded areas might represent an obstacle for diverse regeneration in degraded regions of Madagascar.

Landscape structure shapes the diversity of tree seedlings at multiple spatial scales in a fragmented tropical rainforest.

The maintenance of seedling diversity of animal-dispersed tree species is fundamental for the structure and function of forest patches in fragmented tropical rainforests. Nonetheless, the effects of landscape structure at different spatial scales on α- and β-diversity of tree seedling communities are recently explored. Using a multi-scale approach, we assessed the relative effect of landscape composition and configuration on α- and β-diversity of animal-dispersed seedlings within 16 forest patches in the Lacandona rainforest, Mexico. We assessed these effects at 13 spatial scales (from 300 to 1500 m radius, at 100 m intervals) for three metrics of effective number of species considering α- and β-diversity. We found that α-diversity was largely affected by landscape composition and β-diversity by landscape configuration. On the one hand, the amount of secondary forest influenced α-diversity. Additionally, species richness increased in landscapes with highly aggregated forest patches. On the other hand, β-diversity was affected positively by forest fragmentation and negatively by the edge contrast of forest patches with the surrounding matrix. Our findings indicate that landscape configuration is a strong driver of seedling diversity in highly deforested rainforests. Promoting forest patches and secondary forests through payment for ecosystem services' programs, favoring matrix quality within land-sharing schemes of smallholder agriculture and secondary forest management, and identifying restoration opportunities for assisted or unassisted natural regeneration are urgently needed for conservation of seedling diversity in human-modified tropical landscapes.

Seedling recruitment under isolated trees in a tea plantation provides a template for forest restoration in eastern Africa.

Natural regeneration is less expensive than tree planting, but determining what species will arrive and establish to serve as templates for tropical forest restoration remains poorly investigated in eastern Africa. This study summarises seedling recruitment under 29 isolated legacy trees (14 trees comprised of three exotic species and 15 trees comprised of seven native species) in tea plantations in the East Usambara Mountains, Tanzania. Among the findings were that pioneer recruits were very abundant whereas non-pioneers were disproportionately fewer. Importantly, 98% of all recruits were animal-dispersed. The size of legacy trees, driven mostly by the exotic Grevillea robusta, and to some extent, the native Milicia excelsa, explained abundance of recruits. The distribution of bird-dispersed recruits suggested that some bird species use all types of legacy trees equally in this fragmented landscape. In contrast, the distribution of bat-dispersed recruits provided strong evidence that seedling composition differed under native versus exotic legacy trees likely due to fruit bats showing more preference for native legacy trees. Native, as compared to exotic legacy trees, had almost two times more non-pioneer recruits, with Ficus and Milicia excelsa driving this trend. Implications of our findings regarding restoration in the tropics are numerous for the movement of native animal-dispersed tree species in fragmented and disturbed tropical forests surrounded by farmland. Isolated native trees that bear fleshy fruits can attract more frugivores, resulting not only in high recruitment under them, but depending on the dispersal mode of the legacy trees, also different suites of recruited species. When selecting tree species for plantings, to maximize visitation by different dispersal agents and to enhance seedling recruit diversity, bat-dispersed Milicia excelsa and Ficus species are recommended.

Oak, Quercus chenii, dispersal in fragmented urban forests: acorn horizontal and vertical dispersal, seedling establishment and growth

Habitat fragmentation results in the loss of seed dispersers and subsequently leads to low regeneration and functional decline of forests. However, very few studies on disperser decline and seedling regeneration have been conducted in urban forest systems. In three isolated forest patches in Wuhan, central China, acorn dispersers, acorn dispersal, acorn deposition patterns in soil seed banks and seedling recruitment of the dominant animal-dispersed tree species (Quercus chenii) were investigated over four consecutive years (2014 to 2017). The hypothesis tested was that the loss of acorn dispersers caused by forest fragmentation leads to low regeneration and functional degradation of the urban forest system. As predicted by the hypothesis, very few acorn dispersers (only a few Chinese white-bellied rats, Niviventer confucianus, and Eurasian jays, Garrulus glandarius, were observed) existed in the forest patches; thus, almost all of the acorns were left on the soil surface under the mother trees, where conditions are not favorable for germination and establishment. Large acorns had higher probabilities of germination and seedling establishment than did small acorns when they were buried in the soil. These results suggest that regeneration of fragmented urban forests is increasingly hampered by sharp reduction in seed disperser abundance. Planting large seeds in the soil may compensate for the low regeneration caused by disperser loss in fragmented urban forests.

How complementary are large frugivores for tree seedling recruitment? A case study in the Congo Basin

AbstractLarge frugivores provide critical seed dispersal services for many plant species and their extirpation from forested ecosystems can cause compositional shifts in regenerating plant cohorts. Yet, we still poorly understand whether large seed-dispersers have complementary or redundant roles for forest regeneration. Here, to assess the functional complementarity of large-bodied frugivores in forest regeneration, we quantified the effects of varying abundance of hornbills, primates and the forest elephant on the density, species richness and the mean weighted seed length of animal-dispersed tree species among seedlings in five sites in a forest–savanna mosaic in D. R. Congo, while accounting for percentage forest cover and the local presence of fruiting trees. We found that the abundance of primates was positively associated with species richness of seedlings, while percentage forest cover was negatively associated (R2 = 0.19). The abundance of hornbills, the presence of elephants and percentage forest cover were positively associated with mean seed length of the regenerating cohort (R2 = 0.13). Spatially explicit analysis indicated that some additional processes have an important influence on these response indices. Primates would seem to have a preponderant role for maintaining relatively high species richness, while hornbills and elephant would seem to be predominantly responsible for the recruitment of large-seeded trees. Our results could indicate that these taxa of frugivores play complementary functional roles for forest regeneration. This suggests that the extirpation of one or more of these dispersers would likely not be functionally compensated for by the remaining taxa, hence possibly cascading into compositional shifts.

Comparing seed removal rates in actively and passively restored tropical moist forests

High rates of seed removal can impede forest recovery, but tropical seed removal studies are few and mainly from the neotropics. Little is known about the comparative influences of active restoration (i.e. planting) and passive restoration (i.e. protection of natural regrowth) on seed removal. We conducted an evaluation of seed removal in grasslands, natural forests (tropical moist semideciduous forest), and actively (21-, 17-, 16-, 11-, 8-, and 6-year-old) and passively (21-year-old) restored forests in Kibale National Park, Uganda. We wanted to compare the effect of vegetation type, time since restoration and restoration actions (i.e. active vs. passive) on removal of seeds of five animal-dispersed tree species during wet and dry seasons. Seeds were either fully exposed or placed in closed mesh cages or under a mesh roof. We used differential removal rates between these treatments to attribute seed removal to different animal taxa. Seed removal rate (percentage of seed removed over a 4-day period) was highest in passively restored forests, compared with actively restored forests, grasslands, and natural forests. We detected no significant relationship between time since restoration and seed removal rates within actively restored sites. Seed removal rate from roofed treatments was not significantly different from removal from open treatments but was significantly higher than removal from closed treatments, which we interpret as reflecting the greater effect of small mammals versus insects. Smaller seeds tended to be removed at a greater rate than larger seeds. We discuss the implications of these findings for forest regeneration.

Seed dispersal turns an experimental plantation on degraded land into a novel forest in urban northern Puerto Rico

Planting tree species with desirable traits may catalyze forest regeneration in increasingly common degraded lands by restoring soil properties and attracting seed dispersers. We sampled forest regeneration in an experimental plantation of Albizia lebbek, an introduced N-fixing species, on a degraded pasture in northern Puerto Rico, 27years after its establishment. We hypothesized that the abundance of animal-dispersed tree species within this unmanaged plantation would be (1) greater than in adjacent pastures; (2) higher at distances closer to secondary forest; and (3) increased over the 20years since floristic surveys were last conducted in the site. We sampled forest structure and tree species composition, and compared the abundance of animal-dispersed tree species on plots located within the adjacent pasture and the A. lebbek plantation, within the plantation in plots distanced at 10, 35, and 60m from secondary forest, and within the plantation at 7 vs. 27years after plantation establishment. We found higher abundance of animal-dispersed tree species in the A. lebbek plantation than in the pasture. Distance from secondary forest had no effect on the abundance of animal-dispersed species. The abundance of animal-dispersed species increased by 14% between 7 and 27years after plantation establishment. In all, these results indicate that A. lebbek trees facilitated the establishment of native animal-dispersed species by shading out tall grasses and acting as perches for animals throughout the plantation. We found that A. lebbek lost its dominance in the site to another introduced N-fixing species, Leucaena leucocephala, possibly due to unfavorable traits such as susceptibility to pathogens. Our findings document a transition from an experimental plantation to a novel urban forest that harbors an array of native tree species, largely due to the dispersal of seeds by animals. A noteworthy example is the presence of Guaiacum officinale, an animal-dispersed and endangered tree species native to the dry forests of southern Puerto Rico. Our results highlight how monocultures of introduced tree species with desirable traits can catalyze the regeneration of native tree species in chronically degraded lands.

Reduced hornbill abundance associated with low seed arrival and altered recruitment in a hunted and logged tropical forest.

Logging and hunting are two key direct threats to the survival of wildlife in the tropics, and also disrupt important ecosystem processes. We investigated the impacts of these two factors on the different stages of the seed dispersal cycle, including abundance of plants and their dispersers and dispersal of seeds and recruitment, in a tropical forest in north-east India. We focused on hornbills, which are important seed dispersers in these forests, and their food tree species. We compared abundances of hornbill food tree species in a site with high logging and hunting pressures (heavily disturbed) with a site that had no logging and relatively low levels of hunting (less disturbed) to understand logging impacts on hornbill food tree abundance. We compared hornbill abundances across these two sites. We, then, compared the scatter-dispersed seed arrival of five large-seeded tree species and the recruitment of four of those species. Abundances of hornbill food trees that are preferentially targeted by logging were two times higher in the less disturbed site as compared to the heavily disturbed site while that of hornbills was 22 times higher. The arrival of scatter-dispersed seeds was seven times higher in the less disturbed site. Abundances of recruits of two tree species were significantly higher in the less disturbed site. For another species, abundances of younger recruits were significantly lower while that of older recruits were higher in the heavily disturbed site. Our findings suggest that logging reduces food plant abundance for an important frugivore-seed disperser group, while hunting diminishes disperser abundances, with an associated reduction in seed arrival and altered recruitment of animal-dispersed tree species in the disturbed site. Based on our results, we present a conceptual model depicting the relationships and pathways between vertebrate-dispersed trees, their dispersers, and the impacts of hunting and logging on these pathways.

Animal-dispersed pioneer trees enhance the early regeneration in Atlantic Forest restoration plantations

Planting of native trees has been adopted in many tropical regions worldwide as a central forest restoration method, but little is known concerning the role that these planted species play in catalyzing forest regeneration beneath their canopies. We investigated the role of animal-dispersed tree species in catalyzing the regeneration of woody species in the understory of restoration plantings. We assessed both the density and richness of tree seedlings within plots located beneath the canopy of both animal-dispersed and abiotic-dispersed tree species planted in three riparian forest restoration sites with ages of five, six and eight years. The proportion of animal-dispersed tree seedlings increased with plantation age. The richness of animal-dispersed tree seedlings was higher beneath animal-dispersed trees in the eight-year-old planting. The density of animal-dispersed tree seedlings was higher under animal-dispersed trees when sites were analyzed altogether. The top three species in regeneration density beneath the canopy were animal-dispersed trees, and from the top ten, seven were animal-dispersed species. We suggest that animal-dispersed pioneer trees which facilitate natural regeneration and promote a high density and richness of woody species beneath their canopies should be considered as “framework” species for tropical forest restoration.

Inter-annual higher germination from smaller than medium-sized premontane wet forest fragments for an animal-dispersed tree species in Costa Rica

Forest fragmentation is pervasive in tropical landscapes, and animal-dispersed tree species are among the most threatened. Seed source is an important factor for active conservation and restoration efforts for such species but many studies show lower germination levels for seeds collected from small fragments compared to larger forests. However, the increasing rarity of large forests makes them difficult to be used as practical seed sources. We assessed the potential of small to medium-sized fragments (19–209 ha) to serve as seed sources for the conservation and restoration of an animal-dispersed tree Lacistema aggregatum (Lacistemataceae) in Costa Rican premontane wet forests. Germination, seedling survivorship, and growth for 2 years were quantified in a screen house environment. Two years later, more rigorous germination tests were conducted. Germination levels differed substantially among source trees. Seeds from some individuals had no germination inter-annually, suggesting that fecundity alone may be an incomplete indicator of individual fitness. Furthermore, in contrast to many previous studies, germination was better for seeds from smaller fragments for both study years. Subsequent seedling survivorship did not counteract the trends generated by germination, and the better performance of seeds from smaller fragments was retained for 2 years as ex situ seedlings. Higher seed quality in smaller fragments was associated with larger seed size in trees near forest edges, which might result from higher potential outcrossing rates. Our results suggest a previously unrecognized potential of small fragments as seed sources.

Comparison of direct and indirect genetic methods for estimating seed and pollen dispersal in Fagus sylvatica and Fagus crenata

The comparison between estimates of historical gene flow, using variance in allelic frequencies, and estimates of contemporary gene flow, using parentage assignment, is expected to provide insights into ecological and evolutionary processes at work within and among populations. Genetic variation at microsatellite loci was used to quantify genetic structure in two wind pollinated, gravity and animal-dispersed tree species ( Fagus sylvatica L. and Fagus crenata Blum.) and to derive historical estimates of gene flow. The gene dispersal distances estimated assuming effective population density to be 1/4 of the observed density were ∼77 m in European beech and ∼40 m in Japanese beech. Parentage analyses and a neighbourhood model approach were used to estimate contemporary patterns of seed and pollen dispersal. Our results suggest restricted seed dispersal abilities in both European beech ( δ s = 10.5 m) and Japanese beech ( δ s = 12.4 m), with an exponential shaped seed dispersal kernel. A non-negligible rate of seed immigration ( m s = 27%) was detected in European beech sites but not in Japanese beech site. Pollen dispersal within studied sites also appeared limited ( δ p = 41.63 m in European beech and δ p = 79.4 m in Japanese beech), despite high rate of pollen immigration ( m p = 68% in European beech and m p = 40% m in Japanese beech). Interestingly, contemporary and historical estimates of gene flow were within the same order of magnitude (a few tens of meters).

Frequent long-distance gene flow in a rare temperate forest tree (Sorbus domestica) at the landscape scale

Precise empirical data on current gene flow by pollen, both with respect to distance and abundance, is crucial to understand whether habitat fragments are functionally connected. Based on a large-scale inventory ( approximately 100 km(2)) in which all individuals of a naturally scattered forest tree (Sorbus domestica) were mapped, we inferred current gene flow by pollen using genetic paternity analysis. We detected an extensive network of effective pollen transfer. Although short pollen flow distances were most abundant, 10% of the assigned pollen donors were more than 2 km away from their female mating partners, and 1.8% were even at a distance of 12-16 km. This latter pollen flow shows that current long-distance gene flow over a fragmented landscape clearly occurs. Pollen dispersal was well described by a fat-tailed inverse curve. Using parentage analysis of established trees, maternally inherited chloroplast markers and diameter at breast height measurements as an indicator of individual tree age, we were able to infer regular seed dispersal distances over several hundred metres up to more than 10 km. We conclude that in temperate, insect-pollinated and animal-dispersed tree species such as S. domestica, fragmented subpopulations are functionally connected by gene flow through both pollen and seed.

Population dynamics of the Mexican cycadDioon eduleLindl. (Zamiaceae): life history stages and management impact

The demographic dynamics of three populations of Dioon edule Lindl. (Zamiaceae) were studied in a fragmented landscape using projection matrix modelling. Compared with other plant species, D. edule behaves like a tree life-form species. Density and spatial distribution patterns varied among populations according to models for animal-dispersed tree species. In all scenarios, l was most sensitive to changes in abundance of adult plants. The elasticity reproductive component (F) for the three populations was zero and stasis values (L) were higher, this being a function of the permanence of non-reproductive individuals. It was detected that disturbance influences the population dynamics of D. edule as a function of adult plant persistence. This observation suggests that the conservation of adult plants is critical for D. edule and perhaps for all cycads species. Adult plant decapitation should be halted at the ‘Monte Oscuro’ population, subjected to sustainable management since 1990, if higher seed production is needed in rural nurseries. © 2008 The Linnean Society of London, Botanical Journal of the Linnean Society, 2008, 157, 381–391. ADDITIONAL KEYWORDS: conservation biology – demography – elasticity analysis – leaf-crown removal – projection modelling – survival.

Improved recruitment of a lemur-dispersed tree in Malagasy dry forests after the demise of vertebrates in forest fragments

The objective of this study was to examine how the processes of seed dispersal and seed predation were altered in forest fragments of the dry forest of Madagascar, where the usual seed dispersers and vertebrate seed predators were absent, using a lemur-dispersed tree species (Strychnos madagascariensis; Loganiaceae) as an example. We then assessed how the changes in vertebrate community composition alter the regeneration pattern and establishment of this tree species and thus, ultimately, the species composition of the forest fragments. By using size-selective exclosures, data from forest fragments were compared with results from continuous forest where vertebrate dispersers and predators were abundant. Visits to the exclosures by mammalian seed predators were monitored with hair traps. In the continuous forest up to 100% of the seeds were removed within the 7 days of the experiments. A substantial proportion of them was lost to seed predation by native rodents. In contrast, practically no predation took place in the forest fragments and almost all seeds removed were dispersed into the safety of ant nests by Aphaenogaster swammerdami, which improves chances of seedling establishment. In congruence with these findings, the abundance of S. madagascariensis in the forest fragments exceeded that of the continuous forest. Thus, the lack of vertebrate seed dispersers in these forest fragments did not lead to a decline in regeneration of this animal-dispersed tree species as would have been expected, but rather was counterbalanced by the concomitant demise of vertebrate seed predators and an increased activity of ants taking over the role of seed dispersers, and possibly even out-doing the original candidates. This study provides an example of a native vertebrate-dispersed species apparently profiting from fragmentation due to flexible animal-plant interactions in different facets, possibly resulting in an impoverished tree species community.

Estimation of the seed dispersal kernel from exact identification of source plants

The exact identification of individual seed sources through genetic analysis of seed tissue of maternal origin has recently brought the full analytical potential of parentage analysis to the study of seed dispersal. No specific statistical methodology has been described so far, however, for estimation of the dispersal kernel function from categorical maternity assignment. In this study, we introduce a maximum-likelihood procedure to estimate the seed dispersal kernel from exact identification of seed sources. Using numerical simulations, we show that the proposed method, unlike other approaches, is independent of seed fecundity variation, yielding accurate estimates of the shape and range of the seed dispersal kernel under varied sampling and dispersal conditions. We also demonstrate how an obvious estimator of the dispersal kernel, the maximum-likelihood fit of the observed distribution of dispersal distances to seed traps, can be strongly biased due to the spatial arrangement of seed traps relative to source plants. Finally, we illustrate the use of the proposed method with a previously published empirical example for the animal-dispersed tree species Prunus mahaleb.

The effects of scarification and shelterwood treatments on naturally regenerated seedlings in southern Sweden

This study investigated the effects and interaction effects of scarification versus no scarification and shelterwood versus clearcut on natural regeneration in southern Sweden. Focus was placed on the most common tree species, i.e. Scots pine ( Pinus sylvestris L.), Norway spruce ( Picea abies (L.) Karst.) downy birch ( Betula pubescens Ehrh.) and silver birch ( Betula pendula Roth), which are intended to form future stands. In addition, less common tree species were investigated, which would provide greater biodiversity in the future stands. The study was conducted on eight sites well distributed over southern Sweden. The shelterwoods were dominated by pine. Both the scarification and shelterwood treatments favoured the establishment of pine while spruce establishment was less affected. For birch, the scarification increased the density of seedlings in the shelterwood while it decreased the density of seedlings in the clearcut. In general, the less common tree species were not affected by the treatments. Instead, seed supply appeared to be a more important factor for regeneration success. However, the total density of the animal-dispersed tree species was favoured by a shelterwood and the total density of wind-dispersed tree species was favoured by scarification. The grass cover was reduced by the shelterwoods and hence the clearcuts were more grass-dominated. Furthermore, scarification increased the occurrence of herbs. The study showed that scarification and cutting treatments are efficient tools to steer the natural regeneration.

The effects of forest type, harvesting and stand refinement on early seedling recruitment in a tropical rain forest

Maintenance of high plant diversity in tropical rain forests has been hypothesized to be related to intense seed predation, and studies have shown that the impact of seed-eaters varies according to seed size, forest type and level of disturbance. The hypothesis that seed survival and seedling recruitment depending on seed size, are related to forest type and the intensity of harvesting and stand refinement was therefore tested at Paracou, French Guiana, for three animal-dispersed tree species. Two forest areas, corresponding to two forest types, were selected based on the occurrence of a dominant species, Eperua falcata (Caesalpiniaceae). Two factors were tested: forest type (E. falcata-rich and E. falcata-poor) and level of harvesting (unlogged, low intensity of logging, low intensity of logging and thinning liberation by tree-poison girdling) within each forest type. Overall, there was no effect of species nor of logging treatment within forest type after 6 wk. There was only a significant effect of forest type on seed survival in the large-seeded Carapa procera, a rodent-dispersed species in the Meliaceae. In the two medium-sized seed species, Pradosia cochlearia and Manilkara bidentata, two frugivore-dispersed Sapotaceae, the effect of treatment was overall significant but was also highly variable between plots within treatments. After 1 y, C. procera showed little seedling recruitment due to low seed removal and dispersal, and high seed-seedling predation under all conditions. Neither forest type, nor treatment nested within forest type, affected seedling recruitment in P. cochlearia whereas M. bidentata seedlings were favoured in unlogged conditions at the E. falcata-poor forest type. These results suggest that species characteristics (such as seed size and palatability), or local tree diversity and resources for granivores select for contrasting patterns in tree recruitment between forest type in the rodent-dispersed tree species, and these interact with level of tree harvesting and stand refinement in the other frugivore-dispersed tree species.

Effects of Dispersal Pattern and Mammalian Herbivores on Seedling Recruitment for Virola michelii (Myristicaceae) in French Guiana1

ABSTRACTThe effects of dispersal pattern (seeds in small clumps vs. seeds scattered in pairs) and distance to the nearest Carapa procera (Meliaceae; a tree that produces seeds preferred by terrestrial vertebrates) on survival of seeds and seedlings were examined for the animal–dispersed tree species Virola michelii (Myristicaceae) in a mature forest‘at Paracou, French Guiana, in 1992 and 1993. We assessed the putative role of ground–dwelling mammalian herbivores, rodents, and ungulates that filter the seed shadow, acting either as dispersers or predators and thus modifying the original pattern of seed dispersal made by frugivores. We measured the effects of simulated seed burial by rodents using marked seeds and quantified the effect of protecting seeds and seedlings from ground–dwelling vertebrates on seedling germination and survival with fence exclosures in 1992. Dispersal pattern had short–term but no long–term effects on the proportion of V. michelii seeds that survived one year later as seedlings. In the short term, within six weeks, clumped seeds survived better than scattered seeds in both years. Marked seeds that were removed from their site of dispersal were eaten; rodents only rarely buried seeds of V. michelii, and seed burial reduced seed and seedling survivorship. The combined effect of the factors year and Carapa proximity significantly affected seed survival within six weeks. Although six–week seed survival was greater in 1993 than in 1992, seedling establishment was lower in 1993 than in 1992 following a lower rainfall regime during the key period of seed germination (February). One–year seed and seedling survivorship was similar between treatments and years. Seed survival and seedling establishment in V. michelii was dependent on vertebrates in the short term and on climate in the long term. Overall, seed and seedling survivorship depended on a combination of these factors.