Bottlenecks for plant recruitment in woodland remnants: An ornithochorous shrub in a Mediterranean ‘relictual’ landscape

Habitat fragmentation may lead to declines in plant populations and ultimately to extinction through a disruption of demographic processes, which may result in bottlenecks or even a collapse in regeneration. Nevertheless, very few studies have assessed the net effects of habitat fragmentation on plant recruitment integrating its multiple demographic processes. Using data from a four-year field study, we analyze how habitat fragmentation affects particular demographic processes and the overall magnitude of plant recruitment. We use as a case study the bird-dispersed shrub Myrtus communis in Mediterranean woodland patches within an extremely fragmented landscape (～1% woodland cover). By means of observations and experiments, we quantified fecundity, fruit removal by frugivores, seed rain, post-dispersal seed predation by rodents and seedling emergence and survival. Within each patch, we quantified post-dispersal processes in different target microhabitats. We considered the life cycle to be a combination of consecutive life stages connected by transitional processes with specific probabilities. We calculated the overall probability of recruitment for each patch as the product of all of these probabilities. The demographic processes negatively affected by fragmentation were bird-generated seed rain and seedling emergence and survival, which were attributable, respectively, to lower fruit abundance and poorer habitat quality in the smaller patches. The negative effect of fragmentation on M. communis recruitment became stronger when all the demographic processes were integrated. Of all processes, seedling emergence and, above all, seedling survival were clearly bottlenecks for recruitment that were associated with habitat fragmentation. Results from our observations and experiments were consistent with natural patterns of regeneration given that we found higher seedling densities in larger patches and old population structures (with no saplings/juveniles) in some small patches. Our study shows that habitat fragmentation has serious negative effects on recruitment in M. communis due to demographic bottlenecks in seedling establishment. The available evidence (this and a companion study) suggests that the impoverishment of habitat quality associated with habitat fragmentation (edge effects, disturbances associated with management and microhabitat availability) can explain these results. Given that restoration at a landscape scale is likely to be extremely difficult, initial management actions should aim to improve habitat quality in the smallest woodland remnants.     

Recruitment limitations in Primula vulgaris in a fragmented landscape

Plant recruitment is limited by dispersal, if seeds cannot arrive at potential recruitment sites, and by establishment, due to a low availability of safe sites for recruitment. Seed-sowing experiments, scarcely applied along gradients of landscape alteration, are very useful to assess these limitations. Habitat loss and fragmentation may foster recruitment limitations by affecting all the processes from seed dispersal to seedling establishment. In this study, we perform a seed-sowing experiment to disentangle the importance of dispersal and establishment limitations in different stages of recruitment of the perennial herb Primula vulgaris in fragmented forests of the Cantabrian Range (Northwestern Spain). We evaluated the influence of ecological gradients resulting from habitat loss and fragmentation (modifications of habitat amount at the landscape and microhabitat scales, changes in the species’ population size, changes in seed predation and seedling herbivory) on seedling emergence, survival and early growth. We found strong evidence of dispersal limitation, as seedling emergence was very low in experimental replicates where no seeds were added. This limitation was independent of landscape alterations, as we found no relation with any of the ecological gradients studied. Establishment limitations at the germination phase were also unrelated to ecological gradients, probably because these limitations are more related to fine-scale environmental gradients. However, further monitoring revealed that seedling survival after summer and winter periods and seedling growth were conditioned by landscape alteration, as we found effects of habitat amount at the landscape and microhabitat scales, of presence of populations of P. vulgaris and of seedling herbivory. These effects were complex and sometimes opposite to what can be expected for adult plants, revealing the presence of different requirements between life stages.     

植物种群更新限制——从种子生产到幼树建成

更新限制是指种子由于各种原因,不能够萌发并生长成幼树。它作为解释生物多样性的理论,一直受到国内外群落生态学家关注。从种源限制、传播限制和建成限制3个角度,对更新限制机制研究进展进行了综述。从种源限制而言,时空因素是影响植物种群更新限制的重要因素,因为植物结实量存在明显时空变化,造成植物更新个体出现明显的时空规律。从传播限制而言,传播数量、距离和食果动物行为均限制植物种群更新。数量上,缺乏有限传播者势必减少传播数量,但如果种子拥有较高质量,则能逃脱数量限制;距离上,植物更新个体显示出明显的Janzen-Connell格局,但传播距离趋向稳定,形成植物种群的进化稳定对策;食果动物行为上,不同传播者对更新贡献存在差异,捕食者直接降低更新,融入两类动物行为的模型更能反映食果动物对更新的限制。从建成限制而言,环境因子制约植物生长。小尺度下,微生境的好坏对于植物幼苗建成至关重要;大尺度下,植物提供较好的广告效应则能摆脱生境限制。将传播者行为、捕食者行为与幼苗的空间分布格局、种子传播机理模型等结合,建立植物更新限制机理模型应是更新限制未来的研究热点。选择稀有种和古老种为主题的长期更新限制研究,为种群恢复提供指导,也是未来重要研究方向。     

Avian seed dispersal and seedling distribution of the endangered tree species,Taxus chinensis,in patchy habitats

Background: There is limited understanding about bird dispersal behaviour and seedling distribution of endangered tree species in patchy environments, although these processes are important for plant species persistence.

Aims: We tested how patch features affected bird behaviour and seed dispersal, and thus seedling distribution of the endangered Chinese yew tree (Taxus chinensis).

Methods: In the present study, we combined field data of bird dispersal behaviour and GIS-based information to elucidate the influence of spatial features of habitat patches on bird dispersal behaviour, and the resulting effects on the seedling distribution of the endangered Chinese yew in two patchy habitats.

Results: Our results showed that the only seed source patch could attract eight bird species for dispersal at the two sites. Post-foraging movements of bird dispersers was strongly related to both topography and the relative locations of habitat patches. Yew seedlings aggregated only at the seed source and bamboo recruitment patches, which was affect by both the spatial distribution of recruitment patches and patch use by dispersers.

Conclusions: Our results emphasise that bamboo patches in both patchy environments provide the necessary conditions for germination of yew seeds, and the post-foraging behaviour of dispersers determines seed deposited in these patches. Our study highlights the importance of the dispersal behaviour of frugivorous birds in the successful regeneration and colonisation of yew populations in patchy habitats.     

Diplochory: are two seed dispersers better than one?

Diplochory is seed dispersal by a sequence of two or more steps or phases, each involving a different dispersal agent. Here, we describe five forms of diplochory and derive general characteristics of each phase of seed dispersal. The first and second phases of diplochory offer different benefits to plants. Phase one dispersal often results in escape from density-dependent seed and seedling mortality near the parent plant and can result in the colonization of habitat patches far from the parent. Phase two dispersal often moves seeds to discrete and predictable microsites, where the probability of seedling establishment is disproportionately high (i.e. directed dispersal). Diplochory appears to have evolved because combining two means of seed dispersal can increase the benefits of seed dispersal whilst reducing the likelihood of seed mortality.     

Seed- vs. microsite-limited recruitment in a myrmecochorous herb

Dispersal in myrmecochorous herbs is frequently short-distance and this might limit patch occupancy. Here we ask whether recruitment and patch occupancy in the myrmecochere Helleborus foetidus are seed- or microsite-limited and what role dispersal plays in recruitment limitation. In pine forests from two widely separated mountain systems in the Iberian Peninsula, we performed two seed addition experiments to explore the nature of recruitment limitation in this perennial herb. In an introduction experiment, we added seeds in patches far from reproductive individuals, which rarely receive any seed. In an augmentation experiment, we added seeds in patches beneath reproductive plants. Each experiment considered three main factors: seed addition (added or not added), rodent exclusion (excluded or not excluded), and microhabitat (scrub-covered or not scrub-covered). The augmentation experiment tests whether recruitment is limited by seed availability (exploring also the possible role of rodent granivory in such limitation) while the introduction experiment tests whether the empty patches occur either because they are unsuitable for the species (microsite limitation) or due to a low probability of seed arrival (dispersal limitation). Adding seeds almost always increased seedling emergence and recruitment, demonstrating that recruitment in this species is seed-limited and refuting the possibility of microsite limitation. The comparison of the introduction and augmentation experiments indicated that Helleborus recruitment dynamics and patch occupancy are constrained by additive effects of fecundity and dispersal limitation.     

Demographic bottlenecks in tropical plant regeneration: A comparative analysis of causal influences

Mortality factors that act sequentially through the demographic transitions from seed to sapling may have critical effects on recruitment success. Understanding how habitat heterogeneity influences the causal factors that limit propagule establishment in natural populations is central to assess these demographic bottlenecks and their consequences. Bamboos often influence forest structure and dynamics and are a major factor in generating landscape complexity and habitat heterogeneity in tropical forests. To understand how patch heterogeneity influences plant recruitment we studied critical establishment stages during early recruitment of Euterpe edulis, Sloanea guianensis and Virola bicuhyba in bamboo and non-bamboo stands in the Brazilian Atlantic forest. We combined observational studies of seed rain and seedling emergence with seed addition experiments to evaluate the transition probabilities among regeneration stages within bamboo and non-bamboo stands. The relative importance of each mortality factor was evaluated by determining how the loss of propagules affected stage-specific recruitment success. Our results revealed that the seed addition treatment significantly increased seedling survivorship for all three species. E. edulis seedling survival probability increased in the addition treatment in the two stand types. However, for S. guianensis and V. bicuhyba this effect depended strongly on artificially protecting the seeds, as both species experienced increased seed and seedling losses due to post-dispersal seed predators and herbivores. Propagules of all three species had a greater probability of reaching subsequent recruitment stages when protected. The recruitment of large-seeded V. bicuhyba and E. edulis appears to be much more limited by post-dispersal factors than by dispersal limitation, whereas the small-seeded S. guianensis showed an even stronger effect of post-dispersal factors causing recruitment collapse in some situations. We demonstrated that E. edulis, S. guianensis and V. bicuhyba are especially susceptible to predation during early compared with later establishment stages and this early stage mortality can be more crucial than stand differences as determinants of successful regeneration. Among-species differences in the relative importance of dispersal vs. establishment limitation are mediated by variability in species responses to patch heterogeneity. Thus, bamboo effects on the early recruitment of non-bamboo species are patchy and species-specific, with successional bamboo patches exerting a far-reaching influence on the heterogeneity of plant species composition and abundance.     

Optimum reproduction and dispersal strategies of a clonal plant in a metapopulation: a simulation study with Hieracium pilosella

Clonal spread is favoured in many plants at the expense of seed production in order to expand rapidly into open habitats or to occupy space by forming dense patches. However, for the dynamics of a population in a patchy landscape seed dispersal remains important even for clonal plants. We used a spatially explicit individual-based metapopulation model to examine the consequences of two trade-offs in Hieracium pilosella L: first, between vegetative and sexual reproduction, and second, between short and far-distance dispersal of seeds. Our main question was, what are the environmental conditions that cause a mixed strategy of vegetative and sexual reproduction to be optimal. The model was parameterised with field data on local population dynamics of H. pilosella. Patch dynamics were given firstly by disturbance events that opened patches in a matrix of a clonal grass that were colonisable for H. pilosella, and secondly by the gradual disappearance of H. pilosella patches due to the expanding grass. Simulations revealed opposing selection pressures on traits determined by the two trade-offs. Vegetative reproduction is favoured by local dynamics, i.e. the need for maintenance and expansion of established populations, whereas seed production is favoured by the necessity to colonise empty habitats. Similar pressures act on the proportion of seeds dispersed over short and far distances. Optimum reproductive and dispersal strategies depended on habitat quality (determined by seedling establishment probability), the fraction of dispersed seeds, and the fraction of seeds lost on unsuitable ground. Under habitat conditions supporting moderate to low seedling establishment, between 20% and 40% of reproductive effort in H. pilosella should be devoted to sexual reproduction with at least 10% of the seeds dispersed over distances suitable to attain empty patches. We conclude that in a spatially heterogeneous landscape sexual seed production in a clonal plant is advantageous even at the expense of local vegetative growth.     

Community-Wide Spatial and Temporal Discordances of Seed-Seedling Shadows in a Tropical Rainforest

Several factors decrease plant survival throughout their lifecycles. Among them, seed dispersal limitation may play a major role by resulting in highly aggregated (contagious) seed and seedling distributions entailing increased mortality. The arrival of seeds, furthermore, may not match suitable environments for seed survival and, consequently, for seedling establishment. In this study, we investigated spatio-temporal patterns of seed and seedling distribution in contrasting microhabitats (bamboo and non-bamboo stands) from the Brazilian Atlantic Forest. Spatial distribution patterns, spatial concordance between seed rain and seedling recruitment between subsequent years in two fruiting seasons (2004–2005 and 2007–2009), and the relation between seeds and seedlings with environmental factors were examined within a spatially-explicit framework. Density and species richness of both seeds and seedlings were randomly distributed in non-bamboo stands, but showed significant clustering in bamboo stands. Seed and seedling distributions showed across-year inconsistency, suggesting a marked spatial decoupling of the seed and seedling stages. Generalized linear mixed effects models indicated that only seed density and seed species richness differed between stand types while accounting for variation in soil characteristics. Our analyses provide evidence of marked recruitment limitation as a result of the interplay between biotic and abiotic factors. Because bamboo stands promote heterogeneity in the forest, they are important components of the landscape. However, at high densities, bamboos may limit recruitment for the plant community by imposing marked discordances of seed arrival and early seedling recruitment.     

Effects of habitat alteration on the effectiveness of plant-avian seed dispersal mutualisms: Consequences for plant regeneration

Anthropogenic habitat alteration may affect the dispersal service provided by avian seed dispersers, ultimately causing regeneration collapse, through a decay in both the quantitative (seed removal) and qualitative (seed arrival to safe sites) components of seed dispersal effectiveness. However, despite its implications for management in real-world landscapes, few studies have investigated the shifts in components of seed dispersal effectiveness resulting from habitat alteration. We advocate the use of stage-specific transition probabilities, combined with data on seed shadows and bird abundance and mobility, for a mechanistic inference of the consequences for recruitment of the disruption of plant-frugivore mutualism in altered habitats. Such an approach allows the identification of regeneration bottlenecks, evaluates the differential contribution to recruitment of quantitative and qualitative components of seed dispersal, and provides the means to compare seed dispersal limitation. We exemplify our conceptual approach with studies of seed dispersal and recruitment in the wild olive tree in unaltered and severely altered adjacent sites. We show that simplification of the habitat substantially affected bird abundance, diversity and mobility, which caused a reduction in fruit removal and a concomitant simplification of the seed shadows compared to the unaltered site. Linked to these shifts, postdispersal seed survival and seedling emergence and survival were affected. The final outcome of habitat alteration was the collapse of the regeneration dynamics with very few seeds escaping the influence of maternal plants and reaching the safest sites for recruitment (dispersal limitation). As predicted, the collapse in the regeneration dynamics resulted from severe decays in the quantitative but especially in the qualitative components of seed dispersal effectiveness. Management of fleshy-fruited plant populations in altered habitats should thus pay attention to landscape elements that promote frugivore abundance, diversity and mobility and that alleviate the dispersal limitation.     

Establishment and reproduction of Aeschynomene virginica (L.) (Fabaceae) a rare,annual, wetland species in relation to vegetation removal and water level

Aeschynomene virginica is a rare annual plant found in freshwater tidal wetlands of the eastern United States. We hypothesized that standing vegetation and water inundation were two important environmental factors in its population dynamics. To test these hypotheses, we sowed seeds into plots with undisturbed vegetation or plots with all aboveground vegetation removed in 1998 and 1999. Presence/absence of seedlings was noted and seedling survival to reproduction, final size, and seed set were measured throughout both growing seasons. Seedling establishment from germination to the first true leaf stage increased with decreasing water depth. Vegetation removal plots had greater seedling establishment, higher seedling survival, and higher seed set per plant than non-removal plots. In a greenhouse study designed to test the effects of water level on seed germination and seedling establishment, no seedlings established in submerged soils, and seed germination and seedling establishment were lower in waterlogged soil than in wet soil. Physical stress associated with deeper water likely limits the distribution of A. virginica to higher elevations, where seeds that colonize patches with low vegetative cover are more likely to produce reproductive adults that produce more seeds relative to patches with established vegetation. A. virginica appears to be a fugitive species specializing on open habitat patches in tidal wetlands. This species may be dependent on disturbances for population establishment and maintenance.     

The effects of seed size and pericarp on seedling recruitment and biomass in Cryptocarya alba (Lauraceae) under two contrasting moisture regimes

Establishment success of plants derived from large seeds has been proposed to be greater than that of those derived from smaller ones, particularly under unfavourable conditions of moisture. Therefore, the advantages conferred by large seeds in terms of seedling performance may be modulated by abiotic conditions. The effect of seed size on Cryptocarya alba seedling performance (as determined by seedling recruitment and seedling size) was evaluated under two contrasting rainfall regimes (wet and dry year regime), simulated in the laboratory. It was also determined whether the presence of a pericarp, which had been shown to reduce germination, decreases desiccation and if this counterbalances the greater recruitment of seeds without a pericarp, especially under unfavourable conditions of moisture. Large seeds had a greater probability of recruitment and their seedlings attained a greater biomass, independently of the amount of water applied. In the simulated wet year regime, seeds with a pericarp showed a greater probability of recruitment than those lacking a pericarp. However, seedlings derived from both seed types attained a similar biomass. Under the dry year regime, seeds with and without a pericarp showed similar recruitment probabilities and their seedlings had similar biomasses. These results do not support the assumption that under favourable conditions of moisture, individual differences in seed size would not matter in term of seedling performance. A possible explanation in this case, is the presence of recalcitrant seeds in C. alba, which determines a very short time period for germination following dispersal. Therefore, any attribute that increases germination (e.g., large seeds) would be advantageous, independently of the prevailing abiotic conditions.     

Trophic and non-trophic pathways mediate apparent competition through post-dispersal seed predation in a Patagonian mixed forest

Post‐dispersal predation can be a major source of seed loss in temperate forests. Little is known, however, about how predator‐mediated indirect interactions such as apparent competition alter survival patterns of canopy tree seeds. Understorey plants may enhance tree seed predation by providing sheltered habitat to granivores (non‐trophic pathway). In addition, occurrence of different tree seeds in mixed patches may lead to short‐term apparent competition between seed types, because of the granivores’ foraging response to changes in food patch quality (trophic pathway). We hypothesised that understorey bamboo cover and mixing of seed species in food patches would both increase tree seed predation in a Nothofagus dombeyi?Austrocedrus chilensis forest in northern Patagonia, Argentina. Seed removal experiments were conducted for three consecutive years (2000–2002) differing in overall granivory rates. Seed patch encounter and seed removal rates were consistently higher for the larger and more nutritious Austrocedrus seeds than for the smaller Nothofagus seeds. Seed removal was greater beneath bamboo than in open areas. This apparent competition pathway was stronger in a low‐predation year (2000) than in high‐predation years (2001–2002), suggesting a shift in microhabitat use by rodents. Patch composition had a significant, though weaker, impact on seed survival across study years, whereas seed density per patch enhanced encounter rates but did not influence seed removal. Removal of the less‐preferred Nothofagus seeds increased in the presence of Austrocedrus seeds, but the reciprocal indirect effect was not observed. However, this non‐reciprocal apparent competition between seed species was only significant in the high‐predation years. Our study shows that granivore‐mediated indirect effects can arise through different interaction pathways, affecting seed survival patterns according to the predator's preference for alternative seed types. Moreover, results indicate that the occurrence and relative strength of trophic vs non‐trophic pathways of apparent competition may change under contrasting predation scenarios.     

Establishment of Epiphytic Bromeliads in Successional Tropical Premontane Forests in Costa Rica

Plant community composition is the combined result of species-specific competitive abilities and the availability of propagules. For epiphytic plants, current hypotheses consider that dispersal-related factors are most important. By controlling seed dispersal constraints, we experimentally examined whether the community composition of epiphytic bromeliads in a tropical premontane area is determined during early phases of seedling recruitment. Also, we tested whether establishment success was related to eco-physiological traits of the species. A total of 7200 seeds were artificially affixed on several host trees in two secondary forest patches and in a mature forest stand. Four bromeliad species with differing physiological characteristics (CAM, C₃-CAM, and C₃) and habitat preference (secondary vs. primary forest) were selected. We found that differences in seed germination probability among habitats and species were not likely to influence community assembly. After 2 yr, seedling survival and plant development were relatively higher in the early-successional forest. Seedling establishment success was not associated with specific physiological and morphological adaptations or habitat preference of the studied species. Our results were not consistent with the described community composition and rates of population recruitment of the studied species in the same successional habitats. The results support the hypothesis that chance and historic events related to seed dispersal have an important influence on community assembly of epiphytic plants. In addition, differences in growth rates and reproductive turnover among species are expected to influence the relative abundance and recruitment rates in a particular habitat.     

Population ecology of wild sunflowers: effects of seed density and post-dispersal vertebrate seed predators

Assessing the effects of seed density on the population dynamics of wild plant species with crop relatives will be vital in determining the potential effects of introducing traits into wild populations as a result of crop-to-wild gene flow. We examined experimental sunflower (Helianthus annuus) patches in eastern Kansas to determine the effects of seed density and predation on seedling recruitment and seed production in the next generation. High seed density treatment plots had significantly more seedlings and adult plants than did low seed density treatment plots. Overwinter vertebrate seed predator exclusion treatments resulted in increases in plant density compared to plots in which vertebrates were not excluded. Control patches (no seeds added) contained virtually no plants. Head production and estimated total seed production for a patch were not statistically different among treatments (excluding control plots). Although initial seed density and vertebrate post-dispersal seed predation do appear to have effects on seedling recruitment, neither appear to be limiting seed production of competing adult plants. Therefore, variation in seed densities (over the range examined) may have limited effects on local population dynamics. It is important to note that the choice of seed densities may affect the results obtained: the seed densities used in this study may, in retrospect, be higher than in the small roadside populations typical in eastern Kansas, yet other natural sites have much larger densities. Further, the effects of increased seed density at a local site may have other important effects such as altering metapopulation dynamics through increased long-distance dispersal or increased local seed bank size.     

Plant recruitment bottlenecks in temperate forest fragments: seed limitation and insect herbivory

Numerous studies have documented declines in plant diversity in response to habitat loss in fragmented landscapes. However, determining the mechanisms that lead to species loss is challenging using solely a correlative approach. Here we link correlative assessments of plant community composition with seed additions for a focal species to test the hypothesis that distributions of forests plants within a fragmented landscape are limited by seed dispersal. Woody plant species richness of fragments declined as fragments (n=26) became more isolated by agricultural fields. We predicted that if these isolation effects were driven by poor dispersal rather than other effects associated with habitat loss, then plants should vary in their response to isolation in relation to their seed size (i.e., stronger effects for plants with larger seeds). As predicted under this dispersal limitation hypothesis, sensitivity of bird-dispersed shrubs to isolation was related to their seed mass, with species with heavy seeds (e.g., Lindera benzoin) exhibiting stronger declines in presence across isolation gradients than species with light seeds. Seed addition experiments were performed for Lindera benzoin in two high isolation forest fragments (nearest neighbor mean distance=803 m) where Lindera was naturally absent, and two low isolation fragments (nearest neighbor mean distance=218 m) with naturally occurring Lindera populations. Seed addition and control plots (n=50 1 m² plots per fragment) were monitored for 13 censuses over 3 years. Across all four fragments, seed additions resulted in significant increases in Lindera seedling recruitment with no differences in final seedling establishment among fragments. However, insect herbivory was higher on Lindera seedlings in high isolation compared to low isolation fragments and was negatively correlated with seedling survival over some years. Consistent with prior work, our results confirm that seed dispersal plays a significant role in affecting plant diversity in fragmented landscapes. However, results also suggest the need for a better understanding of how additional processes, such as herbivory, may be altered as habitat is lost and what effects such changes have for forest plants.     

Arrival ≠ Survival

Seed dispersal is commonly a limiting process in ecosystem recovery, and several recent studies have proposed novel methods for overcoming this important biological barrier, particularly in tropical pastures. Multiple experiments in various regions have shown that bird perches attract birds and increase seed dispersal but not seedling recruitment in degraded habitats. New bat‐focused restoration applications, such as roost boxes and fruit oils, have proven capable of attracting animals and augmenting seed dispersal, but these applications have yet to be vetted by seedling establishment data. Seeds and seedlings in pastures have low probability of survival, attributable to predation, dessication, rot, and competition with ruderal vegetation. As such, these novel applications are unlikely to have the desired effect of accelerating tropical forest succession. Given that seed dispersal is meaningless if arriving seeds cannot survive, and that seedling recruitment measurements are not prohibitively difficult to take, we suggest that studies of novel seed dispersal techniques should include a measure of seedling recruitment. Without this information, it cannot be assumed that such applications accelerate forest recovery.     

The Interplay among Acorn Abundance and Rodent Behavior Drives the Spatial Pattern of Seedling Recruitment in Mature Mediterranean Oak Forests

The patterns of seedling recruitment in animal-dispersed plants result from the interactions among environmental and behavioral variables. However, we know little on the contribution and combined effect of both kinds of variables. We designed a field study to assess the interplay between environment (vegetation structure, seed abundance, rodent abundance) and behavior (seed dispersal and predation by rodents, and rooting by wild boars), and their contribution to the spatial patterns of seedling recruitment in a Mediterranean mixed-oak forest. In a spatially explicit design, we monitored intensively all environmental and behavioral variables in fixed points at a small spatial scale from autumn to spring, as well as seedling emergence and survival. Our results revealed that the spatial patterns of seedling emergence were strongly related to acorn availability on the ground, but not by a facilitationeffect of vegetation cover. Rodents changed seed shadows generated by mother trees by dispersing most seeds from shrubby to open areas, but the spatial patterns of acorn dispersal/predation had no direct effect on recruitment. By contrast, rodents had a strong impact on recruitment as pilferers of cached seeds. Rooting by wild boars also reduced recruitment by reducing seed abundance, but also by changing rodent’s behavior towards higher consumption of acorns in situ. Hence, seed abundance and the foraging behavior of scatter-hoarding rodents and wild boars are driving the spatial patterns of seedling recruitment in this mature oak forest, rather than vegetation features. The contribution of vegetation to seedling recruitment (e.g. facilitation by shrubs) may be context dependent, having a little role in closed forests, or being overridden by directed seed dispersal from shrubby to open areas. We warn about the need of using broad approaches that consider the combined action of environment and behavior to improve our knowledge on the dynamics of natural regeneration in forests.     

Dispersal and habitat cuing of Eurasian red squirrels in fragmented habitats

Animal dispersal and subsequent settlement is a key process in the life history of many organisms, when individuals use demographic and environmental cues to target post-dispersal habitats where fitness will be highest. To investigate the hypothesis that environmental disturbance (habitat fragmentation) may alter these cues, we compared dispersal patterns of 60 red squirrels (Sciurus vulgaris) in three study sites that differ in habitat composition and fragmentation. We determined dispersal distances, pre- and post-dispersal habitat types and survival using a combination of capture–mark–recapture, radio-tracking and genetic parentage assignment. Most (75%) squirrels emigrated from the natal home range with mean dispersal distance of 1,014 ± 925 m (range 51–4,118 m). There were no sex-related differences in dispersal patterns and no differences in average dispersal distance, and the proportion of dispersers did not differ between sites. In one of the sites, dispersers settled in patches where density was lower than in the natal patch. In the least fragmented site, 90% of animals settled in the natal habitat type (habitat cuing) against 44–54% in the more strongly fragmented sites. Overall, more squirrels settled in the natal habitat type than expected based on habitat availability, but this was mainly due to individuals remaining within the natal wood. In the highly fragmented landscape, habitat cuing among emigrants did not occur more frequently than expected. We concluded that increased habitat fragmentation seemed to reduce reliable cues for habitat choice, but that dispersing squirrels settled in patches with lower densities of same-sex animals than at the natal home range or patch, independent of degree of fragmentation.     

Seed dispersal distance,seed morphology,and recruitment in the Chilean sclerophyllous tree Quillaja saponaria: implications for passive restoration in a semiarid ecosystem

Recolonization of wind-dispersed tree species in degraded areas may decline with distance from remnant forest fragments because seed rain frequently decreases with distance from the seed source. However, regeneration of these species may be even more limited to sites close to the seed source if dispersal distance is negatively affected by seed mass, and germination probability is positively affected by seed mass. We evaluated these hypotheses in a Mediterranean-type ecosystem of central Chile, using the wind-dispersed tree species Quillaja saponaria. We assessed the seed rain curve in a degraded open area adjacent to a remnant forest fragment of this species, and related seed mass with dispersal distance from the seed source. Then, we evaluated the relationship between seed mass, germination, and seedling growth, and if seeds that fall nearer the seed source have greater germination probability. We found a decreasing seed rain with the distance from the seed source. Seed mass was not related to dispersal distance, although seeds with higher wing area dispersed further. Germination probability was significantly and positively related to the seed mass. We observed no significant relationship between distance and germination probability. We conclude that germination probability of this species does not vary along the seed rain curve, and that the recruitment density would be greater near the seed source only due to decreasing seed rain with distance. Our results suggest that this species has the potential to be passively restored in degraded areas, especially within the first 70 m from the remnant forest fragments.