Wind-dispersed Tree Species Research Articles

Wind dispersed tree species have greater maximum height

Wind dispersed tree species have greater maximum height

Wind-dispersed seeds blur phylogeographic breaks: The complex evolutionary history of Populus lasiocarpa around the Sichuan Basin

The strength of phylogeographic breaks can vary among species in the same area despite being subject to the same geological and climate history due to differences in biological traits. Several important phylogeographic breaks exist around the Sichuan Basin in Southwest China but few studies have focused on wind-dispersed plants. Here, we investigated the phylogeographic patterns and the evolutionary history of Populus lasiocarpa, a wind-pollinated and wind-dispersed tree species with a circum-Sichuan Basin distribution in southwest China. We sequenced and analyzed three plastid DNA fragments (ptDNA) and eight nuclear microsatellites (nSSRs) of 265 individuals of P. lasiocarpa from 21 populations spanning the entire distribution range. Distribution patterns based on nSSR data revealed that there are three genetic groups in P. lasiocarpa. This is consistent with the three phylogeographic breaks (Sichuan Basin, the Kaiyong Line and the 105°E line), where the Sichuan basin acts as the main barrier to gene flow between western and eastern groups. However, the distribution pattern based on ptDNA haplotypes poorly matched the phylogeographic breaks, and wind-dispersed seeds may be one of the main contributing factors. Species distribution modelling suggested a larger potential distribution in the last glacial maximum with a severe bottleneck during the last interglacial. A DIYABC model also suggested a population contraction and expansion for both western and eastern lineages. These results indicate that biological traits are likely to affect the evolutionary history of plants, and that nuclear molecular markers, which experience higher levels of gene flow, might be better indicators of phylogeographic breaks.

Strong dispersal limitation in postfire regeneration of Baker cypress, a rare serotinous conifer.

Dispersal capacity primarily determines the spatial establishment patterns that drive range expansions and contractions in tree species. Seedling establishment in Baker cypress (Hesperocyparis bakeri [(Jeps.) Bartel]) relies predominantly on fire events, due to its cone serotiny, shade intolerance, and small seeds that require the optimal conditions of fire-exposed, mineral soil seedbeds. We quantified the density and spatial distribution of post-disturbance seedlings following the 2014 Eiler Fire in northern California and compared the observed recruitment to predictions from a mechanistic seed dispersal model. Postfire recruitment was dense, averaging 11 seedlings/m2 , and occurred primarily in the first year after fire. We estimated the mean descent velocity of the wingless seeds as ~4 m/s, the highest value reported for any putatively wind-dispersed tree species. The rapid seed descent contributed to markedly spatially constrained recruitment. Most seedlings (~81%) established within 5 m of the parent tree, and 94% established within 10 m. The maximum observed dispersal distance was 48.5 m; dispersal distance scaled linearly with canopy height. Distributions of modeled seed dispersal distance and observed seedling establishment in Baker cypress did not differ, demonstrating that wind disperses seeds a short distance because of the lack of a wing, and secondary dispersal appeared to be minimal at this recently burned site. If seed dispersal is solely reliant on wind, migration in response to rapid climate change will be impeded and potentially present difficulties in sustaining populations of this and other obligate seeder species with equally constrained dispersal.

Size-Dependent Patterns of Seed Rain in Gaps in Temperate Secondary Forests, Northeast China

Secondary forests have become the major forest type worldwide, and are experiencing various disturbances and exhibiting obvious vegetation degradation (e.g., reduced biodiversity and decreased productivity) compared with primary forests. Forest gap is a common small-scale disturbance in secondary forests. Promoting natural regeneration under gap disturbance is an important approach to recover biodiversity and ecosystem services for temperate secondary forests. The gap size is the crucial characteristic controlling natural regeneration of many tree species. However, little is known about the spatiotemporal pattern of seed rain for gravity-dispersed and wind-dispersed tree species in gaps of varying sizes. The objectives of this study were to determine how seed rain of dominant tree species depend on gap size, and consequently, to explore some gap-based silviculture solutions for restoring secondary forests from the view of seed dispersal. The spatial distribution of seed rain in gaps with three sizes (large gaps of 250–350 m2, medium gaps of 150–250 m2, and small gaps of <150 m2), the temporal dynamics of seed rain over three years, and the relationship between seed rain and soil seed banks were explored in temperate secondary forests. The results showed that more than 90% of the seeds in seed rain were wind-dispersed, and their seed rain density and the contribution of seed rain to soil seed bank in medium gaps reached the highest (p = 0.03). The results suggest that establishing medium-sized gaps (i.e., gap size with 150–250 m2) in the secondary forests is more favorable for improving the natural regeneration potential (arrival of seeds and forming soil seed bank) of gap-dependent and wind-dispersed species (e.g., Acer mono) in gaps.

Significant patterns of fine-scale spatial genetic structure in a narrow endemic wind-dispersed tree species, Cedrus brevifolia Henry

Cedrus brevifolia is a narrowly distributed conifer species, currently limited to a single mountain in Cyprus, growing in restricted habitats on sites of different densities and sizes. This study assessed the influence of seed and pollen dispersal, as well as the effect of demographic and genetic features on the magnitude of fine-scale spatial genetic structure (SGS). Sampling was performed in 11 plots where 50 neighboring adult trees were sampled from each plot, while biparentally and paternally inherited genomes were used for analysis with microsatellites. Fine-scale SGS was significant but showed contrasting patterns among plots. Although the magnitude of SGS in C. brevifolia mainly results from restricted seed dispersal, short-distance pollen dispersal could also explain fine-scale SGS in some plots, which is rather uncommon in wind-pollinated conifer species. The lack of a general and consistent trend of SGS among plots and between genomes indicates that pollen and seed dispersal varies at plot level. The complex SGS patterns in C. brevifolia may result from the unequal ratio of male and female strobilies of trees within the same plots, at different reproductive periods. Demographic features such as habitat fragmentation did not influence the magnitude of SGS in C. brevifolia, whereas low tree aggregation reduced it. Further, the significant correlation observed between linkage disequilibrium (LD) and plots with significant SGS supports the assumption that under specific conditions, LD is likely to be caused by the magnitude of SGS.

Increased fire frequency promotes stronger spatial genetic structure and natural selection at regional and local scales in Pinus halepensis Mill.

The recurrence of wildfires is predicted to increase due to global climate change, resulting in severe impacts on biodiversity and ecosystem functioning. Recurrent fires can drive plant adaptation and reduce genetic diversity; however, the underlying population genetic processes have not been studied in detail. In this study, the neutral and adaptive evolutionary effects of contrasting fire regimes were examined in the keystone tree species Pinus halepensis Mill. (Aleppo pine), a fire-adapted conifer. The genetic diversity, demographic history and spatial genetic structure were assessed at local (within-population) and regional scales for populations exposed to different crown fire frequencies. Eight natural P. halepensis stands were sampled in the east of the Iberian Peninsula, five of them in a region exposed to frequent crown fires (HiFi) and three of them in an adjacent region with a low frequency of crown fires (LoFi). Samples were genotyped at nine neutral simple sequence repeats (SSRs) and at 251 single nucleotide polymorphisms (SNPs) from coding regions, some of them potentially important for fire adaptation. Fire regime had no effects on genetic diversity or demographic history. Three high-differentiation outlier SNPs were identified between HiFi and LoFi stands, suggesting fire-related selection at the regional scale. At the local scale, fine-scale spatial genetic structure (SGS) was overall weak as expected for a wind-pollinated and wind-dispersed tree species. HiFi stands displayed a stronger SGS than LoFi stands at SNPs, which probably reflected the simultaneous post-fire recruitment of co-dispersed related seeds. SNPs with exceptionally strong SGS, a proxy for microenvironmental selection, were only reliably identified under the HiFi regime. An increasing fire frequency as predicted due to global change can promote increased SGS with stronger family structures and alter natural selection in P. halepensis and in plants with similar life history traits.

The influence of site factors and proximity of adjacent vegetation on tree regeneration into roadslopes

Many studies have shown evidence of very rapid natural colonization of trees on roadslopes. Tree colonization on roadslopes can be a useful tool in the ecological restoration of these degraded areas. However, little is known about the characteristics of the tree species capable of colonizing these novel habitats. Moreover, it is necessary to know how tree species respond to the two main limitations in these areas: microsite availability (i.e., roadslope characteristics) and seed arrival, which is related to propagule source, dispersion vector and landscape characteristics. The present study aims to investigate the natural colonization of tree species on roadslopes, as well as the factors determining their occurrence. We identified all tree individuals on 150 roadslopes, along 51km of a motorway. A total of 1143 individuals belonging to 18 species was recorded. Most individuals found resulted from a natural colonization process, although we found various fruit trees (present in 7 roadslopes) and Robinia pseudoacacia (2 roadslopes), which are probably associated with unintentional human-mediated seed dispersal. Only the six most abundant species were analysed in detail: Populus nigra, Quercus ilex, Quercus pyrenaica, Fraxinus angustifolia, Salix spp. and Ulmus pumila. Each roadslope was characterized both by site variables and by surrounding variables. We analysed the effect of the descriptive variables on the occurrence and density of each species on the roadslopes, and we subsequently used decision trees (classification and regression trees) to analyse the combined effect of the different predictors considered. Our results show successful tree colonization on the roadslopes, although they appear to indicate limitations to colonization associated with the availability of propagules. Furthermore, tree species dispersed by animals required a continuous flow of seeds favoured by an appropriate community of seed dispersers and a suitable landscape structure. By contrast, wind-dispersed tree species basically need favourable site characteristics on the roadslopes.

Pushing the pace of tree species migration.

Plants and animals have responded to past climate changes by migrating with habitable environments, sometimes shifting the boundaries of their geographic ranges by tens of kilometers per year or more. Species migrating in response to present climate conditions, however, must contend with landscapes fragmented by anthropogenic disturbance. We consider this problem in the context of wind-dispersed tree species. Mechanisms of long-distance seed dispersal make these species capable of rapid migration rates. Models of species-front migration suggest that even tree species with the capacity for long-distance dispersal will be unable to keep pace with future spatial changes in temperature gradients, exclusive of habitat fragmentation effects. Here we present a numerical model that captures the salient dynamics of migration by long-distance dispersal for a generic tree species. We then use the model to explore the possible effects of assisted colonization within a fragmented landscape under a simulated tree-planting scheme. Our results suggest that an assisted-colonization program could accelerate species-front migration rates enough to match the speed of climate change, but such a program would involve an environmental-sustainability intervention at a massive scale.

Propagule morphology and river characteristics shape secondary water dispersal in tree species

Plant migration is a multi-stage process often driven by multiple dispersal vector systems. Water-mediated dispersal (hydrochory) is known to move propagules of nonaquatic species over long distances, but whether propagule morphology affects floating processes is an open question. We used a multi-species approach to assess the role of propagule morphology in the dispersal of primarily wind-dispersed tree species in different urban rivers; the impact of hydraulic structures (locks, spillways) on floating was also considered. We released tagged propagules of eight tree species (Acer platanoides, Acer negundo, Acer saccharinum,Ailanthus altissima, Fraxinus excelsior, Robinia pseudoacacia, Tilia platyphyllos, Ulmus glabra) in the main lowland Spree River and in the small tributary Panke River (Berlin, Germany) and directly observed the fate of the floating propagules over river sections of 1,200 m. Our results demonstrate the following: (1) Water is an effective dispersal agent for wind-dispersed tree species, extending typical wind-related transport distances by several times. (2) Interspecific differences in transport distances reflect propagule characteristics (dry weight, maximum wing width) and river system. (3) Propagule morphology also affects deposition patterns as it was generally the large propagules that were trapped along semi-natural embankments in slow flow areas. (4) Hydraulic structures hampered but did not entirely stop water-mediated dispersal and diminished the effects of propagule morphology on floating processes. These results provide novel insights into the functioning of hydrochory as an important dispersal vector of tree species in river systems and as a driver of plant invasions.

The role of seed predation in the maintenance of the Cross Timbers ecotone of Oklahoma, USA

I investigated how seed predation differed among tree species and among microhabitats across the Cross Timbers and what that variation may tell us about how this ecotone is maintained. The ecotone is located in Oklahoma, USA, between the eastern deciduous forest and tallgrass prairie where seeds of eight common tree species were placed in three microhabitats (oak forest, tallgrass prairie, and sumac shrub/small-tree/grass mix). After nine days in the field, percent seeds remaining were scored for each of the 120 (8 species×3 microhabitats×5 replicates) dishes. I found for both wind-dispersed tree species, (ash, elm) there was significantly more predation in the prairie microsite, with similar small predation levels in the shrub and forest. For two of the three bird-dispersed species (dogwood, hackberry), there was significantly more predation in the prairie and shrub microsites compared to the forest. Red cedar, however, was not taken by predators very much anywhere. Finally, all three mammal-dispersed tree species (two oaks, pecan) showed significantly more predation in the shrub and forest microsites compared to the prairie. Whereas wind- and bird-dispersed species suffered less predation as microsites became more woody and dark, the dominant oaks showed the opposite trend. Consequently, seed predators are not preventing oaks from advancing across this ecotone, but yearly fluctuations in predator population density, especially in the shrub transitional zone, could be helping to maintain it.

Phylogeographic studies of Brazilian “campo-rupestre” species: Wunderlichia mirabilis Riedel ex Baker (Asteraceae)

Wunderlichia mirabilis Riedel ex Baker, a Brazilian wind-dispersed tree species, has a disjunct pattern of geographical distribution. This species occurs in ‘campo rupestre’ locations of the Espinhaço Range, andGoiás Mountains, Brazil. The phylogeography of W. mirabilis was reconstructed on the basis of the genetic variability and phylogeny of the nuclear (ITS) and chloroplast (trnL) non-coding spacer regions. AMOVA andMantel analysis revealed for both markers that genetic variability is high within populations and moderate-tohigh among populations. Rates of seed flow are higher than those of pollen flow, and there is a non-significantgenetic x geographic correlation. The spread of Wunderlichia mirabilis probably occurred from Serra do Cabral through other serras in the Espinhaço Range of Minas Gerais and Espírito Santo, in an easterly direction, andthrough Goiás Mountains, Serra da Canastra in Minas Gerais and São Paulo in a westerly movement from the Serra do Cabral, Minas Gerais.

Urban rivers as dispersal corridors for primarily wind-dispersed invasive tree species

Urbanization may have a large affect on biodiversity patterns by enhancing biological invasions. Urban habitats harbour high numbers of introduced plant species and may function as starting points for invasions along urban–rural gradients. As information on underlying mechanisms is critical for managing biological invasions, we test the role of rivers as dispersal corridors for primarily wind-dispersed ornamentals. We released tagged fruits of three invasive tree species in the Spree River (Berlin, Germany) and directly observed the fate of the floating samaras. The number of floating samaras declined exponentially with distance from the release point. A quarter floated 1200 m within 3 h. Despite marked differences in fruit morphology, there were no interspecific differences in floating capacity. We showed hydrochory to be an effective dispersal agent in wind-dispersed tree species, extending wind-related transport distances by several times. In this way, rivers are expected to link urban propagule sources with natural habitats downstream. Our results suggest that planting native tree species along river corridors would help prevent invasion risks and contribute to implementing principles of ecological design in urban greenways and generally consider the importance of eradicating wind-dispersed invasive tree species in floodplains in early invasion stages to prevent further water-mediated dispersal.

Effects of population density on male and female reproductive success in the wind-pollinated, wind-dispersed tree species Betula maximowicziana

The effects of population density on male and female reproductive success of Betula maximowicziana were evaluated in two mixed and two post-fire stands, with various population densities, ranging from 1.9 to 300.0 trees per ha, in central Hokkaido, Japan. First, we investigated ecological determinants of reproductive success (seed set and germination) of both seeds collected from the trees (tree seeds) and dispersed seeds collected from seed traps (dispersed seeds). We then evaluated the effects of population density on seed set and germination of tree seeds and dispersed seeds using a generalized linear mixed model (GLMM). Subsequently, we genotyped 950 seeds collected from mother trees and 940 seeds trapped after dispersal derived from tree seeds and dispersed seeds, respectively, using eleven microsatellite loci. Using the acquired data, we then evaluated the outcrossing rate and effective number of pollen donors (Nep) of the tree seeds, and the genetic structure of both pollen pools and dispersed seed populations. The seed set and germination rate of dispersed seeds was significantly lower both in the lowest-density stand and in the highest-density stand. The GLMM revealed that seed set and germination rates of dispersed seeds may be maximal at approximately 120 trees per hectare (optimal density). Outcrossing rates were consistently high (tm = 0.995), regardless of the population density. In contrast, Nep was lower in the lowest-density stands. Significant genetic structure of the dispersed seed population was found in two low-density stands, probably due to the limitation of overlapping seed shadows.

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.

Fruiting trees as dispersal foci in a semi-deciduous tropical forest.

Quantification of seed rain patterns is an initial step toward explaining variation in plant recruitment, and consequently, organization of forest communities. Spatially contagious patterns of seed deposition, where seeds are patchily dispersed with some sites receiving relatively high densities and others receiving low densities of seeds, may be a common phenomenon for which we have very little knowledge. For example, prior feeding events by frugivores (monkeys and birds) combined with transport and dispersal of seeds to other fruiting trees may result in the contagious deposition of non-conspecific seeds below them. Here, we examined whether fruiting trees act as dispersal foci in the semi-deciduous tropical rainforest of the Dja Reserve, Cameroon. Seed rain was sampled below the canopies of nine tree species: three typically dispersed by large, frugivorous birds, three dispersed by monkeys, and three dispersed by wind. We found no evidence that monkeys generate spatially contagious patterns of seed rain under fruiting trees at which they feed. However, we found that rates of deposition of non-conspecific seeds and species richness of seeds delivered by birds (hornbills and turacos) were significantly greater during fruiting than non-fruiting periods, and significantly greater under fruiting individuals of bird-dispersed tree species than under fruiting individuals of monkey- or wind-dispersed tree species. Additionally, during fruiting periods, the composition of non-conspecific seed rain under bird-dispersed tree species was more similar to other bird-dispersed trees than to monkey- or wind-dispersed tree species. The contagious dispersal of non-conspecific seeds to fruiting, bird-dispersed trees leads to higher seed densities under fruiting trees than those caused by local seed production. Non-conspecific seeds deposited in high densities may experience increased seed mortality even far from parent trees if predators are generalists. Alternatively, in the absence of complete density-dependent mortality, contagious seed dispersal could result in associations among species dispersed by the same dispersal agent.

Seed and Seedling Ecology of a Monocarpic Tropical Tree, Tachigalia Versicolor

Tachigalia versicolor is a monocarpic canopy tree species on Barro Colorado Island, Panama, This 2—yr demographic and physiological study examined whether seed and seedling survival of this monocarpic species is high compared to nonmonocarpic species previously studied, as life history theory predicts. For the weighted average of two trees, the percentage of offspring estimated to survive from dispersed seed to germination, 1 yr, and 2 yr was 30, 7.2, and 4.3%, respectively. The large seeds (500—600 mg) suffered greater mortality in absolute numbers than did the seedlings. Major mortality agents of seeds were bruchid beetles (Amblycerus tachygaliae) and terrestrial vertebrates, while vertebrate herbivores and damping—off fungi killed seedlings. Seedlings survival and growth were enhanced under canopies of dying T. versicolor adults, relative to under canopies of living conspecific and nonconspecific adults. The majority of 2—yr—old seedlings occurred within 0.40 m from the parent trees, reflecting the original peak of the distribution of wind—dispersed seeds. Few seedlings persisted beyond 40 m. Seedling survival in the first 2 mo was higher in shaded understory than in light—gaps, due to higher vertebrate herbivory in sun than shade. Seedling growth and survival beyond 2 mo was higher in light—gaps than in shaded understory. T. versicolor seedlings exhibited a wide range in maximum net photosynthetic rate when grown in contrasting light conditions, and were able to acclimate to a sudden increase in light level by means of morphological and physiological adjustments of newly developed leaves. These results indicate that T. versicolor can utilize a wide range of light environments in the forest during seedling establishment. Relative to nomonocarpic wind—dispersed tree species previously studied in the same community. T. versicolor ranks near the top in its seed mass, dispersal capability, photosynthetic flexibility to contrasting light conditions, and probability of seedling survival in the shaded understory through 1 yr. These characteristics enable T. versicolor to establish large numbers of seedlings. The mean total number of offspring of two trees estimated to survive through 2 yr was 4395 out of 103 055 dispersed seeds and 30 529 germinated seeds. In comparison to nonmonocarpic species, juvenile survival of T. versicolor is high as expected for this monocarpic tree species.

On Limits to Seed Production

On Limits to Seed Production

Mass allocation, moisture content, and dispersal capacity of wind-dispersed tropical diaspores.

Among 34 wind-dispersed tree species on Barro Colorado Island, Panama, the wet mass of diaspores ranges over six orders of magnitude, The seed mass as a percentage of diaspore mass (S/D) varies greatly among species from 14 to 94% with a mean value of 61% The mean percent moisture of diaspores is 10% no consistent differences occur between seed and non-seed components of diaspores in percent moisture. Wing-loading (weight/area) and hence dispersal capacity varies over one order of magnitude among these species and is correlated more highly with S/D than with percent moisture of diaspores. Compared to fruit diaspores, seed diaspores have less mass and a greater S/D, and are slightly more dehydrated. As a result, seed diaspores have lower wing-loading and greater dispersal capacity than fruit diaspores. The morphological/aerodynamic features of a diaspore also have a significant effect on the mass variables. Overall, the type of diaspore (fruit or seed) is of major importance in explaining mass differences of diaspores among these wind-dispersed species; morphological features and dehydration ability contribute secondarily.

Pathogen mortality of tropical tree seedlings: experimental studies of the effects of dispersal distance, seedling density, and light conditions.

We present results of two experiments designed to identify the relative importance of dispersal distance, seedling density, and light conditions on pathogen-caused mortality of tropical tree seedlings. The field experiment on Barro Colorado Island, Panama, demonstrated that both an increase in dispersal distance and a decrease in seedling density reduce levels of damping-off disease among seedlings of Platypodium elegans, and that there is an interaction between the two factors. The results indicated significant variation among sites in pathogen activity and suggested that seedlings are more vulnerable to disease when establishing around their parent tree than around other conspecific trees.The second experiment in a screened enclosure used potted seedlings of 18 wind-dispersed tree species exposed to two levels of sunlight and seedling density. The results indicated that environmental conditions similar to those in light-gaps significantly reduce pathogen activity. They also confirmed that high seedling density increases disease levels, especially under shaded conditions.Seedlings of 16 of the 18 species experienced pathogencaused mortality, but in widely varying amounts. Seed weight was not a good predictor of a species' vulnerability to pathogens. Adult wood density, an indicator of growth rate and successional status, was inversely correlated with a species' vulnerability to pathogens. Fast-growing, colonizing species, whose seedlings require light-gaps, lacked strong resistance to seedling pathogens, relative to slow-growing species able to tolerate shade and escape seedling pathogens. We discuss these results in the context of seed dispersal as a means of escaping from seedling pathogens.