Mechanisms inducing spatially extended synchrony in mast seeding: The role of pollen coupling and environmental fluctuation

Trees in mature forests often show intermittent reproduction. Intensive flowering and seed production occur only once in several years (mast seeding), often synchronized over a long distance. Recently, coupled map models for the dynamics of individual energy reserves have been adopted to explain the phenomena. Even in a constant environment, the trees show a large between-year fluctuation in seed crops and the reproduction can be synchronized over the whole forest if the fruit production is limited by the availability of outcross pollen (pollen coupling). The model with local coupling in which trees are coupled by pollen exchange only with the neighbors shows that a strong synchronization of tree reproduction can develop over the whole forest that may be orders of magnitude larger than the distance of direct pollen exchange between trees. However, their fluctuation is close to the period-two oscillation, and is unable to explain observed intermittent reproduction of a longer interval between mast years. Finally the effect of common environmental fluctuation experienced by different individuals is studied, when the annual productivity and the reproductive threshold of trees fluctuate between years. In the absence of pollen limitation, environmental fluctuation correlated strongly between individuals (Moran effect) failed to produce a high positive correlation in seed production between individuals. If both pollen limitation and correlated environmental fluctuation are at work, a significantly large correlation was maintained. Hence, both pollen coupling and common environmental fluctuation are needed to explain synchronized reproduction with intervals longer than 2years.     

Modeling spatial dynamics of episodic and synchronous reproduction by plant populations: the effect of small-scale pollen coupling and large-scale climate

Many plant species show masting, intermittent and synchronized reproduction at population level. In the present paper, we review the resource-based model providing a theoretically plausible physiological mechanism underlying masting. In the model, a non-linear allocation of energy reserves is considered: plants accumulate photosynthate every year, produce flowers when the energy reserve level exceeds a threshold, and set seeds at a rate limited by pollen availability. The model predicted that individual plants alter their reproductive dynamics from annual to intermittent depending on how heavily the plant invests resource in reproduction. When fruit production is limited by the availability of outcross pollen, a plant population showed diverse reproductive behavior such as completely synchronized or desynchronized reproduction. Spatial scale of reproductive synchrony tended to be a few times larger than the range of direct pollen exchange. Impact of climatic fluctuation correlated at a large spatial scale was also investigated as an alternative synchronizing factor. The variation in annual productivity and the reproductive threshold induced from climatic fluctuation was accounted for by incorporating an additional term in the model. When plants show a 2 year reproductive cycle, highly synchronized reproduction at a regional scale was induced due to correlated environmental forcing, but reproductive synchrony with long intermast periods was realized only when pollen coupling and environmental forcing were at work. These results suggest that distance-dependent processes, such as pollen exchange between nearby trees, induce synchrony at a local scale and external environmental forcing correlated at geographically large scales works to strengthen and maintain such a synchrony.     

Pollination and reproduction of a self-incompatible forest herb in hedgerow corridors and forest patches

Habitat-corridors are assumed to counteract the negative impacts of habitat loss and fragmentation, but their efficiency in doing so depends on the maintenance of ecological processes in corridor conditions. For plants dispersing in linear habitats, one of these critical processes is the maintenance of adequate pollen transfer to insure seed production within the corridor. This study focuses on a common, self-incompatible forest herb, Trillium grandiflorum, to assess plant–pollinator interactions and the influence of spatial processes on plant reproduction in hedgerow corridors compared to forests. First, using pollen supplementation experiments over 2 years, we quantified the extent of pollen limitation in both habitats, testing the prediction of greater limitation in small hedgerow populations than in forests. While pollen limitation of fruit and seed set was common, its magnitude did not differ between habitats. Variations among sites, however, suggested an influence of landscape context on pollination services. Second, we examined the effect of isolation on plant reproduction by monitoring fruit and seed production, as well as pollinator activity and assemblage, in small flower arrays transplanted in hedgerows at increasing distances from forest and from each other. We detected no difference in the proportion of flowers setting fruit or in pollinator activity with isolation, but we observed some differences in pollinator assemblages. Seed set, on the other hand, declined significantly with increasing isolation in the second year of the study, but not in the first year, suggesting altered pollen transfer with distance. Overall, plants in hedgerow corridors and forests benefited from similar pollination services. In this system, plant–pollinator interactions and reproduction seem to be influenced more by variations in resource distribution over years and landscapes than by local habitat conditions. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Recruitment limitation in dry sclerophyll forests: Regeneration requirements and potential density‐dependent effects in Eucalyptus tricarpa (L.A.S. Johnson) L.A.S. Johnson & K.D. Hill (Myrtaceae)

Recruitment limitation in canopy trees is receiving increasing attention as restoration of tree cover for connectivity, biodiversity offsets, carbon‐trading and improved catchment health becomes more prominent. Recruitment limitation is often addressed by examining seed traits and germination requirements. Distance between trees is also often explored as a factor, particularly in agricultural landscapes where forest structure has been altered, with large distances between fragments generating Allee effects linked to pollen limitation. Fewer studies have examined how short distances between trees (high stem densities), which can characterize regenerated forests, might affect recruitment. This study examined recruitment limitation by exploring germination requirements of Eucalyptus tricarpa (red ironbark) and the effects of stem density on seed characters and germination. Eucalyptus tricarpa is a canopy species in the box‐ironbark forests of central Victoria, Australia. During the early European settlement phase in the 1800s these forests were heavily impacted by human disturbances including mining, logging and livestock grazing. In many regenerated stands, now more than 100 years old, current stem density exceeds those documented in 19th century reports and there is little regeneration of key canopy species such as E. tricarpa. In laboratory trials seed germinated at 18°C in darkness, and while viability varied among populations, overall it remained high (91–96%) and did not differ among populations with different stem densities. An in situ germination trial addressing sowing season, ground‐cover, soil‐ripping, canopy‐cover and stem density recorded no germination. High stem density in coppicing box‐ironbark forest was hypothesized to limit resources for reproduction; however, no relationship was found between nearest neighbour distance, allocation to selected reproductive structures or seed viability. The relative insensitivity of reproductive allocation to intraspecific competition (high stem density) may be interpreted as a trait linked to unpredictable environments and consistent with long‐lived species. In terms of management, recruitment limitation in E. tricarpa appears not primarily attributable to seed viability, tested seedbed treatments or stem density so further factors, including fire, now require exploration.     

Small‐scale genetic structure and mating patterns in an extensive sessile oak forest (Quercus petraea (Matt.) Liebl.)

Oaks (Quercus) are major components of temperate forest ecosystems in the Northern Hemisphere where they form intermediate or climax communities. Sessile oak (Quercus petraea) forests represent the climax vegetation in eastern Germany and western Poland. Here, sessile oak forms pure stands or occurs intermixed with Scots Pine (Pinus sylvestris). A large body of research is available on gene flow, reproduction dynamics, and genetic structure in fragmented landscapes and mixed populations. At the same time, our knowledge regarding large, contiguous, and monospecific populations is considerably less well developed. Our study is an attempt to further develop our understanding of the reproduction ecology of sessile oak as an ecologically and economically important forest tree by analyzing mating patterns and genetic structure within adult trees and seedlings originating from one or two reproduction events in an extensive, naturally regenerating sessile oak forest. We detected positive spatial genetic structure up to 30 meters between adult trees and up to 40 meters between seedlings. Seed dispersal distances averaged 8.4 meters. Pollen dispersal distances averaged 22.6 meters. In both cases, the largest proportion of the dispersal occurred over short distances. Dispersal over longer distances was more common for pollen but also appeared regularly for seeds. The reproductive success of individual trees was highly skewed. Only 41 percent of all adult trees produced any offspring while the majority did not participate in reproduction. Among those trees that contributed to the analyzed seedling sample, 80 percent contributed 1–3 gametes. Only 20 percent of all parent trees contributed four or more gametes. However, these relatively few most fertile trees contributed 51 percent of all gametes within the seedling sample. Vitality and growth differed significantly between reproducing and nonreproducing adult trees with reproducing trees being more vital and vigorous than nonreproducing individuals. Our study demonstrates that extensive, apparently homogenous oak forests are far from uniform on the genetic level. On the contrary, they form highly complex mosaics of remarkably small local neighborhoods. This counterbalances the levelling effect of long‐distance dispersal and may increase the species’ adaptive potential. Incorporating these dynamics in the management, conservation, and restoration of oak forests can support the conservation of forest genetic diversity and assist those forests in coping with environmental change.     

Why do sex ratio dimorphisms exist in Quercus masting? Evolution of imperfect synchronous reproduction in Monoecious trees

Masting is synchronous intermittent production of seeds in perennial plant populations. Some self-compatible monoecious Quercus species, such as oaks, exhibit sex ratio dimorphism and produce a certain proportion of male flowers, even in a year when no seed set occurs. To investigate sex ratio dimorphism in masting trees, we introduced sexual allocation as an evolutionary trait into the Resource Budget Model and examined the evolution of the sex ratio. Analytical and numerical findings show that (1) perfectly synchronous intermittent reproduction does not evolve; (2) if the fruiting cost of female flowers R_c is sufficiently large and the pollen limitation β is intermediate, annual reproduction does not evolve; (3) under conditions (2), sex ratio dimorphism can evolve across a wide region of parameter space; (4) after dimorphism is established, individuals with a female-biased sex ratio receive much more pollen supply from male-biased individuals and tend to show intermittent reproduction with or without synchrony; and (5) dimorphism is maintained with irregular and nearly discontinuous changes of sex ratio. These results suggest that sex ratio dimorphism contributes to improving pollen availability and causes resource depletion and the occurrence of intermittent reproduction in female-biased individuals.     

How do plants know when other plants are flowering? Resource depletion,pollen limitation and mast‐seeding in a perennial wildflower

Mast-seeding is the synchronous and periodic reproduction by plant populations. This phenomenon has been widely studied from a community-level perspective, but we know extremely little about how plants are able to synchronize reproduction. Here, we present the first experimental test of proximate mechanisms of mast-seeding, by preventing reproduction in an iteroparous, mast-seeding wildflower. Through a series of experiments, we show that mobile carbohydrate stores (NSC) control alternate-year flowering by individual plants; seed set depletes NSC which prevents flowering the following year. Plants are synchronized by density-dependent pollen limitation; when plants flower asynchronously, they set fewer seeds, which prevents NSC depletion. Therefore, these individual plants flower in subsequent years and become synchronized. Because mast-seeding is a consequence of physiological controls of reproduction, differences in plant resource acquisition and allocation could dramatically change patterns of seed production, and changes in plant consumers and pollinators could change selection on physiological and developmental pathways.     

Pollen movement to flowering canopies of pistillate individuals of three rain forest tree species in tropical Australia

This study measured the quantities of effective pollen vectors and their pollen loads arriving at the canopies of dioecious tropical rain forest trees in north-east Queensland. Population flowering synchrony, effective pollinator populations and pollen loads transferred between staminate and pistillate trees were compared among three insect-pollinated tree species. All three were visited by a wide range of insects, 75% of which (mostly 3–6 mm long) carried conspecific pollen. Fewer than 8% of individual insects were found to be carrying single-species pollen exclusively and none could be described as specialist pollen foragers. The introduced honeybee carried greater quantities of pollen than any native species but was not necessarily a reliable pollinator. The brief flowering periods in Neolitsea dealbata (3–4 weeks) and Litsea leefeana (4–5 weeks) populations were synchronized among individuals. Flowering in the Diospyros pentamera population extended over 15 weeks and most individuals were in flower for most of this period. Staminate trees began flowering earlier, produced more flowers and attracted relatively more insects than did pistillate trees, suggesting a density-dependent response of pollinators to flowering performance. Pollen was trapped in greater quantities on insects at staminate trees than at pistillate trees. Insect numbers increased at peak flowering periods and Diptera were the most abundant flower visitors. Anthophilous Coleoptera were more numerous at staminate than at pistillate trees in all three tree species populations. Larger quantities of pollen were mobilized during peak flowering times although the greatest quantities were transferred to pistillate canopies towards the end of the population flowering periods. Diptera carried pollen more often to pistillate N. dealbata and L. leefeana trees than did other groups whereas Coleoptera carried pollen more often to pistillate D. pentamera trees. The two contrasting flowering performances in the three tree species are discussed with reference to mechanisms that facilitate pollen transfer between staminate and pistillate trees.     

Reproductive biology of the dry forest tree Enterolobium cyclocarpum (guanacaste) in Costa Rica: a comparison between trees left in pastures and trees in continuous forest

We compared the rate of pollen deposition, the likelihood of fruit production, the number of seeds per fruit, the outcrossing rate, and the progeny vigor of the tropical dry forest tree Enterolobium cyclocarpum for individuals in pastures vs. individuals in continuous forest. We found that flowers from trees growing in continuous forests were more likely to have pollen deposited on their stigmas than flowers from trees in pastures (52.1 vs. 32.3%, respectively). We also found that trees from continuous forests were almost six times more likely to set fruits and produce more seeds per fruit than trees in pastures. Morever, progeny from trees in continuous forests were, on average, more vigorous than the progeny from trees in pastures, as indicated by 12 of 16 indicators of plant vigor. However, there was no significant difference in the multilocus estimate of the outcrossing rate between the two groups of trees (tm = 1.00 and 0.99 for trees from continuous forest and trees from pastures, respectively). But there are differences in the correlation of paternity between the progeny of the two groups, where the progeny from trees in pastures showed a lower correlation of paternity than progeny of trees from continuous forests (r(p) = 0.104 and r(p) = 0.189, respectively). We argue that the mechanisms that regulate progeny vigor are disrupted in trees from pastures. We discuss the implications of these findings for the conservation of E. cyclocarpum.     

Masting in ponderosa pine: comparisons of pollen and seed over space and time

Many plant species exhibit variable and synchronized reproduction, or masting, but less is known of the spatial scale of synchrony, effects of climate, or differences between patterns of pollen and seed production. We monitored pollen and seed cone production for seven Pinus ponderosa populations (607 trees) separated by up to 28?km and 1,350?m in elevation in Boulder County, Colorado, USA for periods of 4?C31?years for a mean per site of 8.7?years for pollen and 12.1 for seed cone production. We also analyzed climate data and a published dataset on 21?years of seed production for an eighth population (Manitou) 100?km away. Individual trees showed high inter-annual variation in reproduction. Synchrony was high within populations, but quickly became asynchronous among populations with a combination of increasing distance and elevational difference. Inter-annual variation in temperature and precipitation had differing influences on seed production for Boulder County and Manitou. We speculate that geographically variable effects of climate on reproduction arise from environmental heterogeneity and population genetic differentiation, which in turn result in localized synchrony. Although individual pines produce pollen and seed, only one-third of the covariation within trees was shared. As compared to seed cones, pollen had lower inter-annual variation at the level of the individual tree and was more synchronous. However, pollen and seed production were similar with respect to inter-annual variation at the population level, spatial scales of synchrony and associations with climate. Our results show that strong masting can occur at a localized scale, and that reproductive patterns can differ between pollen and seed cone production in a hermaphroditic plant.     

Pollen limitation and seed set associated with year-to-year variation in flowering of Gmelina arborea in a natural tropical forest

The year-to-year variations in flowering, pollen and fruit production in ten Gmelina arborea individuals in a natural forest were observed over a period of five years, 2009?2013, and pollinator visits were observed closely over two years, 2011 and 2012. A pollen supplementation experiment was also undertaken in two years (2011 and 2012) of contrasting flowering levels. Considerable year-to-year variations were observed in flower, pollen and fruit production. The observed variation represented a normal sequence of good-year and poor-year cycling which is most parsimoniously attributed to resource allocation. The average level of pollen production per individual tree in good flowering years oscillated between 6.6 and 9.62 × 10⁸, which in the poor flowering years was between 1.92 and 3.07 × 10⁸. The magnitude of pollen limitation across years was highly significant (p < 0.0001). Fruit set through supplemental pollination was 73% and 96% greater than that of the open-pollinated branches in the year 2011 and 2012, respectively. Pollen limitation, irrespective of the arrival of outcross pollen, can reduce annual seed set in predominantly bee-pollinated trees. However, various other factors may cause flower abortion in addition to pollen limitation. The results of this study would be very valuable to the seed orchard managers and the silviculturists to manage the seed production areas (SPAs) of G. arborea. The study ultimately recommends supplemental pollination in seed orchards of G. arborea for better genetic gain and good seed yield.     

Attaining the canopy in dry and moist tropical forests: strong differences in tree growth trajectories reflect variation in growing conditions

Availability of light and water differs between tropical moist and dry forests, with typically higher understorey light levels and lower water availability in the latter. Therefore, growth trajectories of juvenile trees—those that have not attained the canopy—are likely governed by temporal fluctuations in light availability in moist forests (suppressions and releases), and by spatial heterogeneity in water availability in dry forests. In this study, we compared juvenile growth trajectories of Cedrela odorata in a dry (Mexico) and a moist forest (Bolivia) using tree rings. We tested the following specific hypotheses: (1) moist forest juveniles show more and longer suppressions, and more and stronger releases; (2) moist forest juveniles exhibit wider variation in canopy accession pattern, i.e. the typical growth trajectory to the canopy; (3) growth variation among dry forest juveniles persists over longer time due to spatial heterogeneity in water availability. As expected, the proportion of suppressed juveniles was higher in moist than in dry forest (72 vs. 17%). Moist forest suppressions also lasted longer (9 vs. 5 years). The proportion of juveniles that experienced releases in moist forest (76%) was higher than in dry forest (41%), and releases in moist forests were much stronger. Trees in the moist forest also had a wider variation in canopy accession patterns compared to the dry forest. Our results also showed that growth variation among juvenile trees persisted over substantially longer periods of time in dry forest (>64 years) compared to moist forest (12 years), most probably because of larger persistent spatial variation in water availability. Our results suggest that periodic increases in light availability are more important for attaining the canopy in moist forests, and that spatial heterogeneity in water availability governs long-term tree growth in dry forests.     

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.     

Copenhagen – a significant source of birch (Betula) pollen?

Current aerobiological research applies the hypothesis that the main source of atmospheric birch (Betula) pollen is forest trees. Our results indicate that the measured levels in Copenhagen are not only due to birch trees in Danish forests but that the urban areas also seem to be a significant source of birch pollen. A number of episodes in 2003 with enhanced pollen levels in Copenhagen seem to be associated with parks and gardens inside and just outside the city. Our results also indicate one long-range transport episode from remote sources in Poland and Germany. Finally, our results show that the pollen levels vary considerably over the day and geographically between Copenhagen and the city of Roskilde, 40 km away. We suggest, that these differences in time and space in the pollen levels are mapped using an integrated monitoring strategy.     

Evolution of life history in balsam fir (Abies balsamea) in subalpine forests

Subalpine forests in North America and Japan dominated by Abies spp. show an unusual pattern of regeneration in which recruitment is confined to a narrow window in time and space that just precedes the death of the largest trees. Previously, Silvertown suggested that selection in such forests should favour delayed reproduction. A graphical model supporting this prediction also suggests that trees in ''normal'' forests should benefit from precocious reproduction when they are taller than their neighbours, while trees in wave populations should not. Here, we present a field test of the two hypotheses based upon a comparison of the life history of trees in adjacent wave and non-wave populations at Whiteface Mountain, New York. The results show that reproduction commences at a similar age in both kinds of forest, and that trees in the subalpine zone reproduce later and die earlier than conspecifics in lowland forests. The failure of the first hypothesis can be explained by modifying our original assumptions about how reproductive costs and benefits vary with age in the model. In our test of the second hypothesis, we find that the reproductive behaviour of individual trees in the two kinds of forest is different and consistent with our prediction. Phenotypic plasticity for age at first reproduction appears to be present only in the population where it is adaptive.     

Flowering phenology and wind-pollination efficacy of heterodichogamous Juglans mandshurica (Juglandaceae)

BACKGROUND AND AIMS: Heterodichogamy differs from normal dichogamy, in that it involves two mating types (protogyny and protandry) that occur at a 1 : 1 ratio in a population. Flowering phases of the two mating types are synchronized and reciprocal, which was considered to ensure between-type outcrossing. This study aims to quantify the flowering pattern and pollination efficacy in Juglans mandshurica, a wind-pollinated heterodichogamous tree. METHODS: The pattern of flowering phenology was monitored within individual trees and pollen traps were used to measure air-borne pollen loads during the spring in 2003 and 2004. Pollen longevity was determined by staining technique. Also a pollen supplementation experiment was performed in 2004 to assess pollen limitation of fruit production. KEY RESULTS: There was no overlap between sexual functions within individual trees. Flowering periods of the two mating types were reciprocal and synchronous in both 2003 and 2004. Air-borne pollen loads were large, and protogynous and protandrous individuals each produced a high pollination peak, consistent with the two blooming periods. Maximum pollen longevity was about 4 h for protandrous individuals, and 3 h for protogynous individuals. Pollen supplementation did not increase fruit production in either protogynous or protandrous individuals. CONCLUSIONS: Heterodichogamous flowering in Juglans mandshurica effectively avoids selfing, promotes between-type outcrossing, and leads to efficient pollination in a natural population.     

Exploring the Ecological History of a Tropical Agroforestry Landscape Using Fossil Pollen and Charcoal Analysis from Four Sites in Western Ghats,India

Contrary to expectations, some human-modified landscapes are considered to sustain both human activities and biodiversity over the long-term. Agroforestry systems are among these landscapes where crops are planted under native shade trees. In this context, ancient agroforestry systems can provide insight into how farmers managed the landscape over time. Such insight can help to quantify the extent to which tropical forests (especially habitat-specialist trees) are responding to local and landscape-level management. Here, we extracted fossil pollen (indicator of past vegetation changes) and macroscopic charcoal (indicator of biomass burning) from four forest hollows’ sedimentary sequences in an ancient agroforestry system in Western Ghats, India. We used a mixed-modelling approach and a principal components analysis (PCA) to determine past trajectories of forest change and species composition dynamics for the last 900 years. In addition, we reconstructed the long-term forest canopy dynamics and examined the persistence of habitat-specialist trees over time. Our results show that the four sites diverged to a surprising degree in both taxa composition and dynamics. However, despite these differences, forest has persisted over 900 years under agricultural activities within agroforestry systems. This long-term analysis highlights the importance of different land-use legacies as a framework to increase the effectiveness of management across tropical agricultural lands.     

Density‐dependent pre‐dispersal seed predation and fruit set in a tropical tree

Negative density‐dependent demographic processes operating at post‐dispersal seed, seedling, and juvenile stages are the dominant explanation for the coexistence of high numbers of tree species in tropical forests. At adult stages, the effect of pollinators and pre‐dispersal fruit predators are often dependent on the density or abundance of flowers and fruit in the canopy, but each have opposite effects on individual realized reproduction. We studied the effect of density on total and mature fruit set and pre‐dispersal predation rates within individual tree canopies in a common canopy tree species, Jacaranda copaia in a 50‐ha forest census plot in central Panama. We sampled all reproductive sized trees in the plot (n = 188) across three years and estimated fruit set and predation rates. Population‐wide pre‐dispersal seed predation averaged between 6–37% across years. Using linear mixed effects models, we found that increased density and fecundity of conspecific neighbours increased focal tree fruit set, but also the rate of pre‐dispersal predation. An interaction between individual and neighbourhood fruit production predicted lower predation rates at high individual and neighbourhood fecundities, which suggests predator satiation at high fruit abundance levels. However, the rate at which fruit set increased with conspecific neighbour fruit production was greater than the rate at which fruit were lost to predation, resulting in an overall positive effect of neighbour density on mature fruit production in focal trees. Our results run counter to the expectation of a uniformly negative effect of density across all life stages in tropical trees and suggest further exploration of the role of spatial clumping, pollen dispersal limitation, and predation at pre‐dispersal adult stages in maintenance of species diversity in plant communities.     

Vegetative reproduction of trees in some European natural forests

Various means of vegetative reproduction in unexploited forests in western Europe are illustrated with examples. Root suckers are sometimes almost the only method of forest regeneration near the limits of tree growth on the Wadden islands and they can play an important role in forest gaps and riverine forests. Trunk suckers finally replacing their parent trees occur in Alnus, Tilia and Ulmus. Partial uprooting of trees, favoured by special soil conditions, was shown to be an important condition for vegetative reproduction. Temporary survival after uprooting gives opportunity for development of reiterative sprouts, that can replace the vertical axis of a fallen tree. Contact of living branches or even whole stems with soil or mouldering logs favours the growth of adventitious roots. Thus vegetatively reproduced individual trees establish before the uprooted parent tree finally dies. Examples of the clonal spreading of trees are given and a special strategy of layering its branches in pasture woods has been described for beech. Modern forestry rigorously eliminates conditions suitable for the vegetative reproduction of forest trees so their abilities in this respect are often underestimated. Vegetative reproduction seems to be particularly important under circumstances where natural growth is difficult e.g. near the limits of tree growth, on dynamic sites and under heavy shade.     

Determinants of biased sex ratios and inter-sex costs of reproduction in dioecious tropical forest trees

Estimates of the sex ratio and cost of reproduction in plant populations have implications for resource use by animals, reserve design, and mechanisms of species coexistence, but may be biased unless all potentially reproductive individuals are censused over several flowering seasons. To investigate mechanisms maintaining dioecy in tropical forest trees, we recorded the flowering activity, sexual expression, and reproductive effort of all 2209 potentially reproductive individuals within 16 species of Myristicaceae over 4 years on a large forest plot in Amazonian Ecuador. Female trees invested >10 times more biomass than males in total reproduction. Flowering sex ratios were male-biased in four species in ≥1 year, and cumulative 4-year sex ratios were male-biased in two species and for the whole family, but different mechanisms were responsible for this in different species. Annual growth rates were equivalent for both sexes, implying that females can compensate for their greater reproductive investment. There was no strict spatial segregation of the sexes, but females were more often associated with specific habitats than males. We conclude that male-biased sex ratios are not manifested uniformly even after exhaustive sampling and that the mechanisms balancing the higher cost of female reproduction are extremely variable.