Evaluating pollen flow indicators for an insect-pollinated plant species

For insect-pollinated plant species, reproductive success and genetic exchange via the transfer of pollen between flowers depends (i.a.) on the efficiency, abundance and behaviour of floral visitors. These in turn are expected to respond to plant population size and flower density. High floral densities for example usually attract large numbers of pollinators that visit more flowers per plant or patch, which increases pollen deposition at short distances. Thus, population characteristics might serve as indicators for pollen dispersal patterns and help to identify suitable habitat size and quality for conservation measures. To test this hypothesis, we observed floral visitors of a generalist, entomophilous species, Comarum palustre, and compared their abundance and visitation rates in populations of different sizes and flower densities. At the same time, we mimicked pollen flow using fluorescent dye. In the large and dense populations, pollinator abundance and visitation rates were high and dye was dispersed to the edges of the populations (up to 200 m). In the medium-sized population with high flower density, insect abundance and visitation rates were unexpectedly low and dye dispersal declined very quickly. On the contrary, in the smallest population with scattered flowers, especially bumble bee abundance was similar to the large populations and dye dispersal mirrored this high bumble bee activity. Thus, our results indicate that in smaller habitat fragments, the mere size of a population might be insufficient to suggest pollen flow for a plant species. Instead, the abundance of its major pollinators should be considered.     

The population genomic signature of environmental selection in the widespread insect-pollinated tree species Frangula alnus at different geographical scales

The evaluation of the molecular signatures of selection in species lacking an available closely related reference genome remains challenging, yet it may provide valuable fundamental insights into the capacity of populations to respond to environmental cues. We screened 25 native populations of the tree species Frangula alnus subsp. alnus (Rhamnaceae), covering three different geographical scales, for 183 annotated single-nucleotide polymorphisms (SNPs). Standard population genomic outlier screens were combined with individual-based and multivariate landscape genomic approaches to examine the strength of selection relative to neutral processes in shaping genomic variation, and to identify the main environmental agents driving selection. Our results demonstrate a more distinct signature of selection with increasing geographical distance, as indicated by the proportion of SNPs (i) showing exceptional patterns of genetic diversity and differentiation (outliers) and (ii) associated with climate. Both temperature and precipitation have an important role as selective agents in shaping adaptive genomic differentiation in F. alnus subsp. alnus, although their relative importance differed among spatial scales. At the ‘intermediate'' and ‘regional'' scales, where limited genetic clustering and high population diversity were observed, some indications of natural selection may suggest a major role for gene flow in safeguarding adaptability. High genetic diversity at loci under selection in particular, indicated considerable adaptive potential, which may nevertheless be compromised by the combined effects of climate change and habitat fragmentation.     

Herbivorous insect species in the tree canopy of a relict South African forest

Abstract.

• 1 The herbivorous insects on twelve species of evergreen broadleafed trees were repeatedly sampled over a period of 11 months in a small relict forest on the east coast of South Africa. This extraordinarily speciose forest patch has an unusually high proportion of endemic tree species, some of which are extremely rare.
• 2 The insect herbivore fauna (number of species) seems to be markedly depauperate compared to that reported on native, broadleafed trees from other parts of the world. Some possible reasons for this are discussed.
• 3 The total number of herbivorous insect species on each tree species was strongly correlated with the local relative abundance of the host plant species.
• 4 There was no relationship between the total number of insect herbivore species on each tree species and the relative taxonomic isolation of the trees. The proportion of seemingly unique (= specialist) herbivorous insect species (i.e. those that occurred on one tree species only) was greatest on taxonomically isolated trees.
• 5 A fundamental deficiency in the interpretation of the data in this study, and of many other similar studies that report on the number of insect species on plants, is discussed, namely the lack of clarity on the closeness of the association between individual insect herbivore species and their respective host plants.

     

Patterns of pollen and seed dispersal in a small, fragmented population of the wind-pollinated tree Araucaria angustifolia in southern Brazil

Paternity analysis based on eight microsatellite loci was used to investigate pollen and seed dispersal patterns of the dioecious wind-pollinated tree, Araucaria angustifolia. The study sites were a 5.4 ha isolated forest fragment and a small tree group situated 1.7 km away, located in Paranalpha State, Brazil. In the forest fragment, 121 males, 99 females, 66 seedlings and 92 juveniles were mapped and genotyped, together with 210 seeds. In the tree group, nine male and two female adults were mapped and genotyped, together with 20 seeds. Paternity analysis within the forest fragment indicated that at least 4% of the seeds, 3% of the seedlings and 7% of the juveniles were fertilized by pollen from trees in the adjacent group, and 6% of the seeds were fertilized by pollen from trees outside these stands. The average pollination distance within the forest fragment was 83 m; when the tree group was included the pollination distance was 2006 m. The average number of effective pollen donors was estimated as 12.6. Mother-trees within the fragment could be assigned to all seedlings and juveniles, suggesting an absence of seed immigration. The distance of seedlings and juveniles from their assigned mother-trees ranged from 0.35 to 291 m (with an average of 83 m). Significant spatial genetic structure among adult trees, seedlings, and juveniles was detected up to 50 m, indicating seed dispersal over a short distance. The effective pollination neighborhood ranged from 0.4 to 3.3 ha. The results suggest that seed dispersal is restricted but that there is long-distance pollen dispersal between the forest fragment and the tree group; thus, the two stands of trees are not isolated.     

Fat-tailed gene flow in the dioecious canopy tree species Fraxinus mandshurica var. japonica revealed by microsatellites

Pollen flow, seed dispersal and individual reproductive success can be simultaneously estimated from the genotypes of adults and offspring using stochastic models. Using four polymorphic microsatellite loci, gene flow of the wind-pollinated and wind-seed-dispersed dioecious tree species, Fraxinus mandshurica var. japonica, was quantified in a riparian forest, in northern Japan. In a 10.5-ha plot, 74 female adults, 76 male adults and 292 current-year seedlings were mapped and genotyped, together with 200 seeds. To estimate dispersal kernels of pollen and seeds, we applied normal, exponential power, Weibull, bivariate t-distribution kernels, and two-component models consisting of two normal distribution functions, one with a small and one with a large variance. A two-component pollen flow model with a small contribution (26.1%) from short-distance dispersal (sigma = 7.2 m), and the rest from long-distance flow (sigma = 209.9 m), was chosen for the best-fitting model. The average distance that integrated pollen flows inside and outside the study plot was estimated to be 196.8 m. Tree size and flowering intensity affected reproduction, and there appeared to be critical values that distinguished reproductively successful and unsuccessful adults. In contrast, the gene flow model that estimated both pollen and seed dispersal from established seedlings resulted in extensive seed dispersal, and the expected spatial genetic structures did not satisfactorily fit with the observations, even for the selected model. Our results advanced small-scale individual-based parentage analysis for quantifying fat-tailed gene flow in wind-mediated species, but also clarified its limitations and suggested future possibilities for gene flow studies.     

Fungal colonization and decomposition of Castanopsis sieboldii leaves in a subtropical forest

Fungi play a crucial role in the decomposition of lignin in fallen leaves but few studies have examined the functional roles of ligninolytic fungi associated with the decomposition of fallen leaves on tropical forest soils. This study examined fungal populations responsible for lignin decomposition in Castanopsis sieboldii leaves in a subtropical evergreen broad-leaved forest in southern Japan. Fallen leaves of C. sieboldii are characterized by the occurrence of bleached portions attributable to fungal colonization of leaf tissues and decomposition of lignin. The bleached area accounted for 29.7%, on average, of the total area of C. sieboldii fallen leaves in the study site. Leaf mass per unit area (LMA) and lignin content were lower in the bleached area than in the surrounding nonbleached area of the same leaves, indicating that removal of lignin enhanced mass loss from leaf tissues and created small-scale heterogeneity of decomposition within single leaves. An unidentified species of Lachnocladiaceae (Basidiomycetes) was isolated frequently from the bleached area and caused selective decomposition of lignin in leaves under pure culture conditions, indicating that this fungus was responsible for the bleaching. The greater hyphal length of basidiomycetes in the bleached area than in the nonbleached area supported the finding that this Lachnocladiaceae sp. was associated with the bleaching. The relatively rapid decomposition of C. sieboldii leaves on the subtropical forest soil is partly attributable to colonization of the litter by this Lachnocladiaceae sp.     

Effect of flowering phenology on pollen flow distance and the consequences for spatial genetic structure within a population of Primula sieboldii (Primulaceae)

To evaluate the effects of flowering phenology on pollen flow distance and spatial genetic structure in a population of a bumblebee-pollinated herb, Primula sieboldii, we investigated the flowering phenology of 1712 flowers of 97 genets in a population in Nagano Prefecture, Japan, and constructed a mating model based on the observed mating pattern, which was revealed by paternity analysis using 11 microsatellite markers. The effects of flowering phenology were inferred by comparing estimated pollen flow distance and the level of heterozygosity in the next generation between two scenarios. In the first scenario, both the intergenet distance and flowering phenology influenced mating opportunity, while in the second scenario only intergenet distance influenced mating opportunity. Although the frequency distribution of pollen flow distance at the population level did not differ significantly between the two scenarios, the mean pollen flow distance of several flowers increased by more than 10 m as a result of variation in flowering phenology. Furthermore, accounting for flowering phenology predicted change in heterozygosity in the next generation from -0.04 to 0.07. The results showed that flowering phenology can affect pollen flow distance and spatial genetic structure.     

Short-distance pollen dispersal in a protogynous Annonaceae tree species from the Brazilian Cerrado

Annona crassiflora (Annonaceae) is a protogynous beetle-pollinated savannah tree species, widely distributed in the savannahs of the Cerrado biome. Studies on the mating system and pollen dispersal of protogynous species are very scarce. Here, we used six microsatellite loci to assess the mating system and pollen dispersal of A. crassiflora in a savannah remnant in Central Brazil. We mapped and sampled leaves of 112 adult trees and collected 74 fruits from 20 mother trees (1–4 fruits per plant) to obtain the seeds used (460) for mating system and parentage analyses. Annona crassiflora has predominantly allogamous mating systems, with a high multilocus outcrossing rate (t_m?=?0.974, SE?=?0.011) that did not differ among mother trees (F?=?1.32, p?=?0.165). However, t_m–t_s was variable among seed trees, indicating that some seeds were produced by mating among relatives. Our results also showed multiple paternity within fruits. Multilocus correlation of outcrossed paternity was high (r_p?=?0.302, SE?=?0.045), indicating that for each mother tree, the probability that the same pollen donor sired two random sibs was 30.2%, and the mean number of pollen donors per mother tree was high (6.3). We detected a maximum pollen dispersal distance of 360.7 m and an average of 124.3 m (SD?=?80 m), but most pollination events (73%) occurred at shorter distances (<?160 m), indicating short-distance pollen dispersal, most likely due to the pollinator behaviour.     

Comparing direct vs. indirect estimates of gene flow within a population of a scattered tree species

The comparison between historical estimates of gene flow, using variance in allelic frequencies, and contemporary estimates of gene flow, using parentage assignment, is expected to provide insights into ecological and evolutionary processes at work within and among populations. Genetic variation at six microsatellite loci was used to quantify genetic structure in the insect-pollinated, animal-dispersed, low-density tree Sorbus torminalis L. Crantz, and to derive historical estimates of gene flow. The neighbourhood size and root-mean-squared dispersal distance inferred from seedling genotypes ( N _b  = 70 individuals, σ _e  = 417 m) were similar to those inferred from adult genotypes ( N _b  = 114 individuals, σ _e  = 472 m). We also used parentage analyses and a neighbourhood model approach after an evaluation of the statistical properties of this method on simulated data. From our data, we estimated even contributions of seed- and pollen-mediated dispersal to the genetic composition of established seedlings, with both fat-tailed pollen and seed dispersal kernels, and slightly higher mean distance of pollen dispersal (248 m) as compared to seed dispersal (135 m). The resulting contemporary estimate of gene dispersal distance (σ _c  = 211 m) was ∼twofold smaller than the historical estimates. Besides different assumptions and statistical nuances of both approaches, this discrepancy is likely to reflect a recent restriction in the scale of gene flow which requires manager's attention in a context of increasing forest fragmentation.     

Chloroplast diversity and population differentiation of Castanopsis fargesii (Fagaceae): a dominant tree species in evergreen broad-leaved forest of subtropical China

Subtropical forests in China constitute the major expanse of evergreen broad-leaved forest in East Asia. The significant genetic divergence of the keystone tree species should be expected due to the huge geomorphological and environmental changes from west to east in subtropical China. In this study, a total of 652 individuals from 27 populations of Castanopsis fargesii throughout its natural range in mainland China were genotyped with eight chloroplast microsatellite markers to investigate genetic diversity, population differentiation, and demographic history of C. fargesii. Phylogeographic structure among populations of C. fargesii was evidenced by the permutation test, revealing that N_ST was significantly higher than G_ST . The strong genetic differentiation found among populations was well in accordance with isolation-by-distance model. In addition, significant isolation by elevation was detected among populations. Significant genetic differentiations were revealed among the west, center, and east regions by approximate Bayesian computations (ABC). The genetic divergence might reflect the regional responses to the fast and dramatic uplift of Yunnan-Guizhou Plateau and Wuyi mountain range in the Pleistocene. In the present study, contraction-expansion process was detected in the west, center, and east regions, indicating that geomorphological remodeling together with climatic changes in the Pleistocene had strong impact on genetic structure of C. fargesii.     

Coexistence of three canopy tree species in a riparian forest in the Chichibu Mountains, central Japan

Three canopy tree species (Fraxinus platypoda, Pterocarya rhoifolia, andCercidiphyllum japonicum) coexist in riparian forests in the Chichibu Mountains of central Japan. We compared the forest structure and the reproductive characteristics of these species.F. platypoda was the dominant canopy species. It produced many saplings and grew in abandoned channels and floodplains, and was able to invade both large and small disturbance sites.P. rhoifolia was a subdominant species that occurred on the deposits of large-scale landslides and grew in patches containing even-aged trees.C. japonicum was the other subdominant species that produced few saplings and invaded large disturbance sites together withP. rhoifolia. Establishment sites ofC. japonicum were restricted to fine mineral soils and fallen logs. We found tradeoffs in reproductive characteristics (seed size, seed number, irregular seed production, and sprouting) among the three canopy species.F. platypoda andP. rhoifolia had large seeds and produced fruits irregularly.C. japonicum produced many small seeds every year and sprouted prolifically around the main stem. The causes of the coexistence mechanism of the three riparian canopy tree species may be both niche- and chance-determined to varying degrees. In riparian areas, the three canopy species were well-adapted to disturbances throughout their life-history.     

Fire frequency and tree canopy structure influence plant species diversity in a forest-grassland ecotone

Disturbances and environmental heterogeneity are two factors thought to influence plant species diversity, but their effects are still poorly understood in many ecosystems. We surveyed understory vegetation and measured tree canopy cover on permanent plots spanning an experimental fire frequency gradient to test fire frequency and tree canopy effects on plant species richness and community heterogeneity within a mosaic of grassland, oak savanna, oak woodland, and forest communities. Species richness was assessed for all vascular plant species and for three plant functional groups: grasses, forbs, and woody plants. Understory species richness and community heterogeneity were maximized at biennial fire frequencies, consistent with predictions of the intermediate disturbance hypothesis. However, overstory tree species richness was highest in unburned units and declined with increasing fire frequency. Maximum species richness was observed in unburned units for woody species, with biennial fires for forbs, and with near-annual fires for grasses. Savannas and woodlands with intermediate and spatially variable tree canopy cover had greater species richness and community heterogeneity than old-field grasslands or closed-canopy forests. Functional group species richness was positively correlated with functional group cover. Our results suggest that annual to biennial fire frequencies prevent shrubs and trees from competitively excluding grasses and prairie forbs, while spatially variable shading from overstory trees reduces grass dominance and provides a wider range of habitat conditions. Hence, high species richness in savannas is due to both high sample point species richness and high community heterogeneity among sample points, which are maintained by intermediate fire frequencies and variable tree canopy cover.     

Comparative recruitment patterns of two non-pioneer canopy tree species in French Guiana

Summary A comparative study was conducted on the recruitment patterns of two non-pioneer tree species, one dispersed by arboreal mammals and birds (Virola michelii, Myristicaceae) and the other by rodents (Moronobea coccinea, Clusiaceae). These species differ in fruiting phenology, seed size, dispersal distance, germination time and seed nutrient exhaustion. In both species, establishment patterns were consistent with the escape hypothesis and the Janzen-Connell model. Virola seeds need not be buried to survive and germinate, and may produce a seedling carpet beneath the parent. Moronobea seedlings only establish from seeds buried by scatterhoarding rodents in the surrounding understory. One-year survival of Virola seedlings was 47.8% and was greater >10 m than < 10 m from the largest parent tree. In contrast, survival of Moronobea seedlings was 56% 3 years after seed dispersal. Survival of juveniles was greater in gaps than in the understory for Virola but not for Moronobea. Moronobea survival was greater than Virola survival in both microhabitats. Both species establish in the understory, yet both grew faster in gaps. Virola appeared to be more gap-dependent than Moronobea which may persist several years in the understory until a gap occurs. Virola and Moronobea illustrate two intermediate recruitment patterns along an hypothetical continuum of nonpioneer species replacement (Bazzaz and Pickett 1980; Swaine and Whitmore 1988).     

Drought acclimation of two deciduous tree species of different layers in a temperate forest canopy

Water-use strategies of Populus tremula and Tilia cordata, and the role of abscisic acid in these strategies, were analysed. P. tremula dominated in the overstorey and T. cordata in the lower layer of the tree canopy of the temperate deciduous forest canopy. Shoot water potential (), bulk-leaf abscisic acid concentration ([ABA]_leaf), abscisic acid concentration in xylem sap ([ABA]_xyl), and rate of stomatal closure following the supply of exogenous ABA (v) decreased acropetally through the whole tree canopy, and foliar water content per area (w), concentration of the leaf osmoticum (c), maximum leaf-specific hydraulic conductance of shoot (L), stomatal conductance (g_s), and the threshold dose per leaf area of the exogenous ABA (d_a) required to reduce stomatal conductance increased acropetally through the tree canopy (from the base of the foliage of T. cordata to the top of the foliage of P. tremula) in non-stressed trees. The threshold dose per leaf dry mass of the exogenous ABA (d_w) required to reduce stomatal conductance, was similar through the tree canopy. After a drought period (3 weeks), the , w, L, g_s, d_a and d_w had decreased, and c and v had increased in both species. Yet, the effect of the drought period was more pronounced on L, g_s, d_a, d_w and v in T. cordata, and on , w and c in P. tremula. It was concluded that the water use of the species of the lower canopy layer—T. cordata, is more conservative than that of the species of the overstorey, P. tremula. [ABA]_leaf had not been significantly changed in these trees, and [ABA]_xyl had increased during the drought period only in P. tremula. The relations between [ABA]_leaf, [ABA]_xyl and the stomatal conductance, the osmotic adjustment and the shoot hydraulic conductance are also discussed.     

Mating patterns and contemporary gene flow by pollen in a large continuous and a small isolated population of the scattered forest tree Sorbus torminalis

The influence of population size and spatial isolation on contemporary gene flow by pollen and mating patterns in temperate forest trees are not well documented, although they are crucial factors in the life history of plant species. We analysed a small, isolated population and a large, continuous population of the insect-pollinated tree species Sorbus torminalis in two consecutive years. The species recently experienced increased habitat fragmentation due to altered forest management leading to forests with closed canopies. We estimated individual plant size, percentage of flowering trees, intensity of flowering, degree of fruiting and seed set per fruit, and we determined mating patterns, pollen flow distances and external gene flow in a genetic paternity analysis based on microsatellite markers. We found clear effects of small population size and spatial isolation in S. torminalis. Compared with the large, continuous population, the small and isolated population harboured a lower percentage of flowering trees, showed less intense flowering, lower fruiting, less developed seeds per fruit, increased selfing and received less immigrant pollen. However, the negative inbreeding coefficients (F(IS)) of offspring indicated that this did not result in inbred seed at the population level. We also show that flowering, fruiting and pollen flow patterns varied among years, the latter being affected by the size of individuals. Though our study was unreplicated at the factor level (i.e. isolated vs non-isolated populations), it shows that small and spatially isolated populations of S. torminalis may also be genetically isolated, but that their progeny is not necessarily more inbred.     

Effect of canopy closure on pollen dispersal in a wind-pollinated species (Fagus sylvatica L.)

The effect of non-reproductive trees and saplings as a physical barrier to pollen dispersal in wind-pollinated species?? forests has not received enough attention in the literature so far. The neighborhood seedling model was used to fit pollen dispersal models for beech at different stages of gap recolonization and to elucidate the effect of saplings as a physical barrier on pollen dispersal at local scale. Phenological overlap of leaf emergence, and pollen release as well as wind directionality patterns were also examined. As a case study, we used a mixed beech-oak forest that was managed as open woodland until 1974. The ban on entry of cattle has led to the recolonization of empty spaces by seedlings and saplings of beech (Fagus sylvatica L.) and two oak species (Quercus petraea (Matts.) Liebl. and Q. pyrenaica Willd.) and, at last, to canopy closure. The average pollen dispersal distance for the first plants that regenerated in the gaps was almost twice those found for recently installed seedlings and seeds collected in traps, supporting the hypothesis that the understory may act as a physical barrier to pollen dispersal. Although a substantial part of effective pollination directionality is at random, horizontal winds and vertical anabatic winds may explain some of this directionality. At the time of beech pollen release, leaves of beech and sessile oak are fully developed, enhancing pollen interception by the saplings. Explicit models of pollen dispersal for wind-pollinated trees should incorporate the effect of canopy closure caused by growth of saplings and account for leaf phenology of co-occurring species in the forest.     

Patterns of tree replacement: canopy effects on understory pattern in hemlock-northern hardwood forests

The effect of canopy trees on understory seedling and sapling distribution is examined in near-climax hemlock-northern hardwood forests in order to predict tree replacement patterns and assess compositional stability. Canopy trees and saplings were mapped in 65 0.1-ha plots in 16 tracts of old-growth forests dominated by Tsuga canadensis, Acer saccharum, Fagus grandifolia, Tilia americana, and Betula lutea in the northeastern United States. Seedlings were tallied in sub-plots. Canopy influence on individual saplings and sub-plots was calculated, using several indices for canopy species individually and in total. For each species sapling and seedling distributions were compared to those distributions expected if saplings were located independently of canopy influence. Non-random distributions indicated that sapling and seedling establishment or mortality were related to the species of nearby canopy trees. Hemlock canopy trees discriminate against beech and maple saplings while sugar maple canopy favors beech saplings relative to other species. Basswood canopy discourages growth of saplings of other species, but produces basal sprouts. Yellow birch saplings were rarely seen beneath intact canopy. Since trees in these forests are usually replaced by suppressed seedlings or saplings, canopy-understory interactions should influence replacement probabilities and, ultimately, stand composition. I suggest that hemlock and basswood tend to be self-replacing, maple and beech tend to replace each other, and birch survives as a fugitive by occupying occasional suitable gaps. This suggests that these species may co-exist within stands for long periods with little likelihood of successional elimination of any species. There is some suggestion of geographical variation in these patterns.     

Distance-dependent pollen limitation of seed set in some insect-pollinated dioecious plants

By applying hand pollination, we studied pollen limitation of seed set of female plants in four dioecious plant species with insect-pollination. The effect of hand pollination increased with distance to the nearest male plant. Distances at which seed set was 50% of its maximal value (after hand pollination) were: 2.3 m for Valeriana dioica, 5.3 m for Salix repens, 8.5 m for Asparagus officinale and 10.6 m for Bryonia dioica. We discuss to what extent the reduced seed set was caused by either fewer visits or by visits of a lower quality. We argue that quantifying distance-dependent seed set in dioecious plants may be a good way of studying effects of habitat fragmentation on the breakdown of mutualistic pollination systems.