Synergistic variation of rhizosphere soil phosphorus availability and microbial diversity with stand age in plantations of the endangered tree species Parashorea chinensis

Introduction

Soil physicochemical properties and nutrient composition play a significant role in shaping microbial communities, and facilitating soil phosphorus (P) transformation. However, studies on the mechanisms of interactions between P transformation characteristics and rhizosphere microbial diversity in P-deficient soils on longer time scales are still limited.

Methods

In this study, rhizosphere soils were collected from a pure plantation of Parashorea chinensis (P. chinensis) at six stand ages in the subtropical China, and the dynamic transformation characteristics of microbial diversity and P fractions were analyzed to reveal the variation of their interactions with age.

Results

Our findings revealed that the rhizosphere soils across stand ages were in a strongly acidic and P-deficient state, with pH values ranging from 3.4 to 4.6, and available P contents ranging from 2.6 to 7.9 mg·kg-1. The adsorption of P by Fe3+ and presence of high levels of steady-state organic P highly restricted the availability of P in soil. On long time scales, acid phosphatase activity and microbial biomass P were the main drivers of P activation. Moreover, pH, available P, and ammonium nitrogen were identified as key factors driving microbial community diversity. As stand age increased, most of the nutrient content indicators firstly increased and then decreased, the conversion of other forms of P to bio-available P became difficult, P availability and soil fertility began to decline. However, bacteria were still able to maintain stable species abundance and diversity. In contrast, stand age had a greater effect on the diversity of the fungal community than on the bacteria. The Shannon and Simpson indices varied by 4.81 and 0.70 for the fungi, respectively, compared to only 1.91 and 0.06 for the bacteria. Microorganisms play a dominant role in the development of their relationship with soil P.

Discussion

In conclusion, rhizosphere microorganisms in P. chinensis plantations gradually adapt to the acidic, low P environment over time. This adaptation is conducive to maintaining P bioeffectiveness and alleviating P limitation.

Summer solstice orchestrates the subcontinental-scale synchrony of mast seeding

High interannual variation in seed production in perennial plants can be synchronized at subcontinental scales with wide consequences for ecosystem functioning, but how such synchrony is generated is unclear1-3. We investigated the factors contributing to masting synchrony in European beech (Fagus sylvatica), which extends to a geographic range of 2,000 km. Maximizing masting synchrony via spatial weather coordination, known as the Moran effect, requires a simultaneous response to weather conditions across distant populations. A celestial cue that occurs simultaneously across the entire hemisphere is the longest day (the summer solstice). We show that European beech abruptly opens its temperature-sensing window on the solstice, and hence widely separated populations all start responding to weather signals in the same week. This celestial 'starting gun' generates ecological events with high spatial synchrony across the continent.

Bornean tropical forests recovering from logging at risk of regeneration failure

Active restoration through silvicultural treatments (enrichment planting, cutting climbers and liberation thinning) is considered an important intervention in logged forests. However, its ability to enhance regeneration is key for long-term recovery of logged forests, which remains poorly understood, particularly for the production and survival of seedlings in subsequent generations. To understand the long-term impacts of logging and restoration we tracked the diversity, survival and traits of seedlings that germinated immediately after a mast fruiting in North Borneo in unlogged and logged forests 30-35 years after logging. We monitored 5119 seedlings from germination for ~1.5 years across a mixed landscape of unlogged forests (ULs), naturally regenerating logged forests (NR) and actively restored logged forests via rehabilitative silvicultural treatments (AR), 15-27 years after restoration. We measured 14 leaf, root and biomass allocation traits on 399 seedlings from 15 species. Soon after fruiting, UL and AR forests had higher seedling densities than NR forest, but survival was the lowest in AR forests in the first 6 months. Community composition differed among forest types; AR and NR forests had lower species richness and lower evenness than UL forests by 5-6 months post-mast but did not differ between them. Differences in community composition altered community-weighted mean trait values across forest types, with higher root biomass allocation in NR relative to UL forest. Traits influenced mortality ~3 months post-mast, with more acquisitive traits and relative aboveground investment favoured in AR forests relative to UL forests. Our findings of reduced seedling survival and diversity suggest long time lags in post-logging recruitment, particularly for some taxa. Active restoration of logged forests recovers initial seedling production, but elevated mortality in AR forests lowers the efficacy of active restoration to enhance recruitment or diversity of seedling communities. This suggests current active restoration practices may fail to overcome barriers to regeneration in logged forests, which may drive long-term changes in future forest plant communities.

Grazing hinders seed dispersal during crop failure in a declining oak woodland

Masting, the synchronized production of variable quantities of seeds, is a global phenomenon in diverse ecosystems, including treed grazing systems where trees and grazing animals coexist. This phenomenon can be interspersed with years of extreme crop failure, whose frequency and unpredictability are increasing. Yet, the combined impact of crop failure and grazing on seed dispersal and seed-to-seedling transition remains poorly understood. To address this concern, we investigated rodent-mediated cork-oak (Quercus suber) acorn predation, dispersal, and seedling emergence in cattle grazed and non-grazed areas in central Portugal during years with contrasting masting seasons. We found that the acorns supplied in the crop failure year were dispersed more rapidly and over longer distances than those supplied in the crop success year when other acorns were naturally available. The crop failure year also had 83 % more dispersal events and 84 % more predated acorns than the reproductive success year. However, the higher acorn predation was offset by a 2.4-fold higher percentage of unpredated dispersed acorns recruiting into seedlings. Both years ended up recruiting a similar number of seedlings. Acorns emerged seedlings 3.4 times farther in the crop failure year than in the crop success year. Cattle grazing was the main constraint on seed dispersal distance by rodents, reducing it by 53 %. Our study provides empirical evidence that cattle grazing modulates how an extreme crop failure year can surprisingly be an opportunity for the few existing acorns to have seedlings established farther apart than in a crop success year. If we are to better manage and preserve the high conservation and socio-economic value of Mediterranean cork oak woodlands in the face of climate change, we must prioritize fecund trees and carefully manage seed dispersal factors such as cattle grazing, particularly during years of crop failure.

Does monocarpic Phyllostachys nigra var. henonis regenerate after flowering in Japan? Insights from 3 years of observation after flowering

Phyllostachys nigra var. henonis, a monocarpic bamboo with a 120-year flowering interval, is next predicted to flower in Japan in the 2020s. Because a huge area of the country is presently covered by stands of this species, post-flowering dieback of these stands and ensuing drastic changes in land cover may cause serious social and/or environmental problems. No study on the regeneration of this bamboo species was conducted during the last flowering event in the 1900s, and the regeneration process of this species is thus still unknown. In 2020, we encountered a localized flowering of P. nigra var. henonis in Japan and used this discovery as a rare opportunity to study the initial regeneration process of the species. Over 3 years, more than 80% of culms in the study site bloomed, but no seed was produced. In addition, no established seedlings were located. These facts strongly suggest that P. nigra var. henonis lacks the ability to produce seeds and cannot undergo sexual regeneration. Some bamboo culms were produced after flowering but died within 1 year of emergence. Small, weak culms (dwarf ramets) also appeared after flowering, but most died within 1 year as well. Three years after flowering, all culms had died, with no sign of regeneration detected. According to our 3 years of observation, this bamboo appears to be hard to regenerate-an idea completely contradicted by the fact that this species has long persisted in Japan. We thus considered other possible regeneration modes for P. nigra var. henonis.

Distance and density dependence in two native Bornean dipterocarp species

The Janzen-Connell hypothesis proposes that density and distance-dependent mortality generated by specialist natural enemies prevent competitive dominance. Much literature on Janzen-Connell mechanisms comes from the neotropics, and evidence of the role of distance and density-dependence is still relatively sparse. We tested the predictions of the Janzen-Connell hypothesis in a South-East Asian system dominated by mast fruiting species. We hypothesized that seedling survival would decrease with distance and density, seedling growth would increase, and herbivory would decrease, according to the predictions of the Janzen-Connell hypothesis. Experiments were conducted to determine the strength of the Janzen-Connell mechanism by manipulating the density and identity of tree species as a function of the distance from parent trees. Survival of conspecific seedlings was reduced near adult trees of one species, but not another. High densities of seedlings decreased the growth of conspecific seedlings of both species. In both species, herbivory rates decreased with distance in low-density areas. This study indicates that dipterocarp species experienced weak Janzen-Connell effects of distance and density dependence at the growth stage studied. Future studies in this system might focus on earlier life-history stages such as seeds and small seedlings, as well as studying mortality during mast-seeding events.

Mechanisms driving interspecific variation in regional synchrony of trees reproduction

Seed production in many plants is characterized by large interannual variation, which is synchronized at subcontinental scales in some species but local in others. The reproductive synchrony affects animal migrations, trophic responses to resource pulses and the planning of management and conservation. Spatial synchrony of reproduction is typically attributed to the Moran effect, but this alone is unable to explain interspecific differences in synchrony. We show that interspecific differences in the conservation of seed production-weather relationships combine with the Moran effect to explain variation in reproductive synchrony. Conservative timing of weather cues that trigger masting allows populations to be synchronized at distances >1000 km. Conversely, if populations respond to variable weather signals, synchrony cannot be achieved. Our study shows that species vary in the extent to which their weather cueing is spatiotemporally conserved, with important consequences, including an interspecific variation of masting vulnerability to climate change.

The selective advantage of mast flowering in Veratrum album subsp. oxysepalum: Implications of the predator satiation hypothesis

PREMISE

Synchronous, highly variable flower or seed production among years within a population (i.e., masting) has been reported in numerous perennial plants. Although masting provides ecological advantages such as enhancing pollination efficiency and/or escape from predator attack, little is known about the degree of these advantages and variations in masting behavior among populations of conspecific plants.

METHODS

We determined flowering ramet density and reproductive success (fruit-set success and herbivorous damage) of a perennial herb, Veratrum album subsp. oxysepalum, across six lowland and six alpine populations in northern Japan during 2-3 years. We then analyzed the relationship between floral density and reproductive success to assess the ecological significance of mast flowering. Flowering intervals of individual plants were estimated by counting annual scars on rhizomes.

RESULTS

Most populations had mast flowering, but the intervals between flowering for individual plants were shorter in the alpine populations than in the lowland populations. Floral damage by stem borers (dipteran larvae) and seed predation by lepidopteran larvae were intense in the lowland populations. Seed production of individual ramets increased with higher floral density owing to the effective avoidance of floral-stem damage and seed predation. Although stem borers were absent in the alpine habitat, seed predation decreased with higher floral density also in the alpine populations. Pollination success was independent of floral density in both of the alpine and lowland populations.

CONCLUSIONS

These results strongly support the predator satiation hypothesis for mast flowering by this species.

Emerging infectious disease triggered a trophic cascade and enhanced recruitment of a masting tree

There are several mechanisms that allow plants to temporarily escape from top-down control. One of them is trophic cascades triggered by top predators or pathogens. Another is satiation of consumers by mast seeding. These two mechanisms have traditionally been studied in separation. However, their combined action may have a greater effect on plant release than either process alone. In 2015, an outbreak of a disease (African swine fever, ASF) caused a crash in wild boar (Sus scrofa) abundance in Białowieża Primeval Forest. Wild boar are important consumers of acorns and are difficult to satiate relative to less mobile granivores. We hypothesized that the joint action of the ASF outbreak and masting would enhance regeneration of oaks (Quercus robur). Data from ungulate exclosures demonstrated that ASF led to reduction in acorn predation. Tree seedling data indicated that oak recruitment increased twofold relative to pre-epidemic period. Our results showed that perturbations caused by wildlife disease travel through food webs and influence forest dynamics. The outbreak of ASF acted synergistically with masting and removed herbivore top-down control of oaks by mobile consumers. This illustrates that the ASF epidemic that currently occurs across Europe can have broad effects on forest dynamics.

Global patterns in the predator satiation effect of masting: A meta-analysis

Masting, or synchronous production of large seed crops, is widespread among plants. The predator satiation hypothesis states that masting evolved to overwhelm seed predators with an excess of food. Yet, this popular explanation faced few rigorous tests. We conducted a meta-analysis of studies that related the magnitude of seed production to the intensity of seed predation. Our results validate certain theoretical notions (e.g., that predator satiation is more effective at higher latitudes) but challenge others (e.g., that specialist and generalist consumers differ in the type of functional response to masting). We also found that masting is losing its ability to satiate consumers, probably because global warming affected masting patterns. This shift might considerably impair the reproduction of masting plants.

Predator satiation is the most commonly tested hypothesis that explains the evolutionary advantages of masting. It proposes that masting benefits plant reproduction by reducing the proportion of seed crop that is consumed by predators. This hypothesis is widely accepted, but many theoretical notions about predator satiation have not been subjected to a robust evaluation. To address this issue, we conducted a meta-analysis of studies that quantified seed predation in relation to mast seeding. We found evidence of both numerical (starvation between mast years) and functional (satiation during mast years) response of consumers to masting. These two effects reinforced each other. Masting satiated invertebrate but not vertebrate seed predators. Satiation was more pronounced at higher, temperate, and boreal latitudes, perhaps because masting is more effective in reducing seed losses when plant communities are less diverse. The effectiveness of masting in satiating invertebrate consumers declined over time (1972 to 2018), probably reflecting the impact of climate change on the frequency and intensity of masting. If masting ceases to reduce seed losses, a crucial advantage of this reproductive strategy will be lost, and sustainability of many tree populations will decline.

Costs and benefits of masting: economies of scale are not reduced by negative density-dependence in seedling survival in Sorbus aucuparia

Masting is a widespread reproductive strategy in plants that helps to reduce seed predation and increase pollination. However, masting can involve costs, notably negative density-dependent (NDD) seedling survival caused by concentrating reproduction in intermittent events. Masting benefits have received widespread attention, but the costs are understudied, which precludes understanding why some plant species have evolved intense masting, while others reproduce regularly. We followed seed production, seed predation (both 13 yr), and seedling recruitment and survival (11 yr) in Sorbus aucuparia. We tested whether NDD in seedling survival after mast years can reduce the benefits of pulsed reproduction that come through predator satiation. Seed predation rates were extreme in our population (mean = 75%), but were reduced by masting. The commonly accepted, but untested, assertion that pulsed recruitment is associated with strong NDD was unsupported. Consequently, the proportion of seedlings that survived their first year increased with fruit production. This provides a rare test of economies of scale beyond the seed stage. Our results provide estimation of the costs of mast seeding, and indicate that these may be lower than expected. Low masting costs, if common, may help explain why masting is such a widespread reproductive strategy throughout the plant kingdom.

Modes of climate variability bridge proximate and evolutionary mechanisms of masting

There is evidence that variable and synchronous reproduction in seed plants (masting) correlates to modes of climate variability, e.g. El Niño Southern Oscillation and North Atlantic Oscillation. In this perspective, we explore the breadth of knowledge on how climate modes control reproduction in major masting species throughout Earth's biomes. We posit that intrinsic properties of climate modes (periodicity, persistence and trends) drive interannual and decadal variability of plant reproduction, as well as the spatial extent of its synchrony, aligning multiple proximate causes of masting through space and time. Moreover, climate modes force lagged but in-phase ecological processes that interact synergistically with multiple stages of plant reproductive cycles. This sets up adaptive benefits by increasing offspring fitness through either economies of scale or environmental prediction. Community-wide links between climate modes and masting across plant taxa suggest an evolutionary role of climate variability. We argue that climate modes may 'bridge' proximate and ultimate causes of masting selecting for variable and synchronous reproduction. The future of such interaction is uncertain: processes that improve reproductive fitness may remain coupled with climate modes even under changing climates, but chances are that abrupt global warming will affect Earth's climate modes so rapidly as to alter ecological and evolutionary links. This article is part of the theme issue 'The ecology and evolution of synchronized seed production in plants'.

Climate change and plant reproduction: trends and drivers of mast seeding change

Climate change is reshaping global vegetation through its impacts on plant mortality, but recruitment creates the next generation of plants and will determine the structure and composition of future communities. Recruitment depends on mean seed production, but also on the interannual variability and among-plant synchrony in seed production, the phenomenon known as mast seeding. Thus, predicting the long-term response of global vegetation dynamics to climate change requires understanding the response of masting to changing climate. Recently, data and methods have become available allowing the first assessments of long-term changes in masting. Reviewing the literature, we evaluate evidence for a fingerprint of climate change on mast seeding and discuss the drivers and impacts of these changes. We divide our discussion into the main characteristics of mast seeding: interannual variation, synchrony, temporal autocorrelation and mast frequency. Data indicate that masting patterns are changing but the direction of that change varies, likely reflecting the diversity of proximate factors underlying masting across taxa. Experiments to understand the proximate mechanisms underlying masting, in combination with the analysis of long-term datasets, will enable us to understand this observed variability in the response of masting. This will allow us to predict future shifts in masting patterns, and consequently ecosystem impacts of climate change via its impacts on masting. This article is part of the theme issue 'The ecology and evolution of synchronized seed production in plants'.

Fungi and insects compensate for lost vertebrate seed predation in an experimentally defaunated tropical forest

Overhunting reduces important plant-animal interactions such as vertebrate seed dispersal and seed predation, thereby altering plant regeneration and even above-ground biomass. It remains unclear, however, if non-hunted species can compensate for lost vertebrates in defaunated ecosystems. We use a nested exclusion experiment to isolate the effects of different seed enemies in a Bornean rainforest. In four of five tree species, vertebrates kill many seeds (13-66%). Nonetheless, when large mammals are excluded, seed mortality from insects and fungi fully compensates for the lost vertebrate predation, such that defaunation has no effect on seedling establishment. The switch from seed predation by generalist vertebrates to specialist insects and fungi in defaunated systems may alter Janzen-Connell effects and density-dependence in plants. Previous work using simulation models to explore how lost seed dispersal will affect tree species composition and carbon storage may require reevaluation in the context of functional redundancy within complex species interactions networks.

Why aren't rabbits and hares larger?

Macroevolutionary consequences of competition among large clades have long been sought in patterns of lineage diversification. However, mechanistically clear examples of such effects remain elusive. Here, we postulated that the limited phenotypic diversity and insular gigantism in lagomorphs could be explained at least in part by an evolutionary constraint placed on them by potentially competing ungulate-type herbivores (UTHs). Our analyses yielded three independent lines of evidence supporting this hypothesis: (1) the minimum UTH body mass is the most influential predictor of the maximum lagomorph body mass in modern ecoregions; (2) the scaling patterns of local-population energy use suggest universal competitive disadvantage of lagomorphs weighing over approximately 6.3 kg against artiodactyls, closely matching their observed upper size limit in continental settings; and (3) the trajectory of maximum lagomorph body mass in North America from the late Eocene to the Pleistocene (37.5-1.5 million years ago) was best modeled by the body mass ceiling placed by the smallest contemporary perissodactyl or artiodactyl. Body size evolution in lagomorphs has likely been regulated by the forces of competition within the clade, increased predation in open habitats, and importantly, competition from other ungulate-type herbivores. Our findings suggest conditionally-coupled dynamics of phenotypic boundaries among multiple clades within an adaptive zone, and highlight the synergy of biotic and abiotic drivers of diversity.

Preparation burning may not improve short-term seed survival in an Amazonian savanna

The role of fire in the management of degraded areas remains strongly debated. Here we experimentally compare removal and infestation of popcorn kernels (Zea mays L. – Poaceae) and açaí fruits (Euterpe oleracea Mart. – Arecaceae) in one burned and two unburned savanna habitats in the eastern Brazilian Amazon. In each habitat, a total of ten experimental units (five per seed type) were installed, each with three treatments: (1) open access, (2) vertebrate access, and (3) invertebrate access. Generalized linear models showed significant differences in both seed removal (P < 0.0001) and infestation (P < 0.0001) among seed type, habitats and access treatments. Burned savanna had the highest overall seed infestation rate (24.3%) and invertebrate access increased açaí seed infestation levels to 100% in the burned savanna. Increased levels of invertebrate seed infestation in burned savanna suggest that preparation burning may be of limited use for the management and restoration of such habitats in tropical regions.

Seed predation selects for reproductive variability and synchrony in perennial plants

Annually variable and synchronous seed production by plant populations, or masting, is a widespread reproductive strategy in long-lived plants. Masting is thought to be selectively beneficial because interannual variability and synchrony increase the fitness of plants through economies of scale that decrease the cost of reproduction per surviving offspring. Predator satiation is believed to be a key economy of scale, but whether it can drive phenotypic evolution for masting in plants has been rarely explored. We used data from seven plant species (Quercus humilis, Quercus ilex, Quercus rubra, Quercus alba, Quercus montana, Sorbus aucuparia and Pinus pinea) to determine whether predispersal seed predation selects for plant phenotypes that mast. Predation selected for interannual variability in Mediterranean oaks (Q. humilis and Q. ilex), for synchrony in Q. rubra, and for both interannual variability and reproductive synchrony in S. aucuparia and P. pinea. Predation never selected for negative temporal autocorrelation of seed production. Predation by invertebrates appears to select for only some aspects of masting, most importantly high coefficient of variation, supporting individual-level benefits of the population-level phenomenon of mast seeding. Determining the selective benefits of masting is complex because of interactions with other seed predators, which may impose contradictory selective pressures.

Correlated evolution of diaspore traits and potential frugivore-mediated selection in a fleshy-fruited tropical lineage (Artabotrys, Annonaceae)

Dispersal syndromes are often defined by reference to fruit traits that are associated with distinct frugivore guilds. Studies rarely examine the relationship between seed traits and frugivores or test the alternative hypothesis that traits are shaped by climatic variables. We assess whether the evolution of seed size and physical defense are correlated with dispersal-related traits and climatic variables in Artabotrys, a fleshy-fruited tropical lineage. Diaspore traits and WorldClim bioclimatic variables were compiled for 43 species. Correlated evolution was evaluated using phylogenetic regression and model-fitting approaches. The best-fitting multioptima Ornstein-Uhlenbeck model suggests that lineages with smooth testa and thin pericarp (SP) have evolved toward smaller seeds with a thinner testa, whereas lineages with rough testa and/or thick pericarp have evolved toward larger seeds with a thicker testa. A smooth testa facilitates spitting and/or swallowing of intact seeds while fruits with thin pericarp may be preferentially consumed by frugivores with less destructive oral processing, enabling lower investment in seed physical defense in SP lineages. Moreover, small seeds are more likely to be swallowed intact with a food bolus. The effect of climate on seed size and physical defense is equivocal and warrants further investigation.

The influence of logging on vertebrate responses to mast fruiting

Periods of extreme food abundance, such as irregular masting events, can dramatically affect animal populations and communities, but the extent to which anthropogenic disturbances alter animal responses to mast events is not clear. In South-East Asia, dipterocarp trees reproduce in mast fruiting events every 2-10 years in some of the largest masting events on the planet. These trees, however, are targeted for selective logging, reducing the intensity of fruit production and potentially affecting multiple trophic levels. Moreover, animal responses to resource pulse events have largely been studied in systems where the major mast consumers have been extirpated. We sought to evaluate the influence of human-induced habitat disturbance on animal responses to masting in a system where key mast consumers remain extant. We used motion-triggered camera traps to quantify terrestrial mammal and bird occurrences in Sabah, Malaysian Borneo, relative to variation in fruit biomass from 69 plant families during a major (2014) and minor (2015) masting event and a non-mast year (2013), in both logged and unlogged forests. Bearded pigs (Sus barbatus) showed the clearest responses to masting and occurrence rates were highest in unlogged forest in the year following the major mast, suggesting that the pulse in fruit availability increased immigration or reproduction. We also detected local-scale spatial tracking of dipterocarp fruits in bearded pigs in unlogged forest, while this was equivocal in other species. In contrast, pigs and other vertebrate taxa in our study showed limited response to spatial or temporal variation in fruit availability in logged forest. Our findings suggest that vertebrates, namely bearded pigs, may respond to masting via movement and increased reproduction, but that these responses may be attenuated by habitat disturbance.

Impacts of tropical forest disturbance on species vital rates

Tropical forests are experiencing enormous threats from deforestation and habitat degradation. Much knowledge of the impacts of these land-use changes on tropical species comes from studies examining patterns of richness and abundance. Demographic vital rates (survival, reproduction, and movement) can also be affected by land-use change in a way that increases species vulnerability to extirpation, but in many cases these impacts may not be manifested in short-term changes in abundance or species richness. We conducted a literature review to assess current knowledge and research effort concerning how land-use change affects species vital rates in tropical forest vertebrates. We found a general paucity of empirical research on demography across taxa and regions, with some biases toward mammals and birds and land-use transitions, including fragmentation and agriculture. There is also considerable between-species variation in demographic responses to land-use change, which could reflect trait-based differences in species sensitivity, complex context dependencies (e.g., between-region variation), or inconsistency in methods used in studies. Efforts to improve understanding of anthropogenic impacts on species demography are underway, but there is a need for increased research effort to fill knowledge gaps in understudied tropical regions and taxa. The lack of information on demographic impacts of anthropogenic disturbance makes it difficult to draw definite conclusions about the magnitude of threats to tropical ecosystems under anthropogenic pressures. Thus, determining conservation priorities and improving conservation effectiveness remains a challenge.

Resilience of seed production to a severe El Niño-induced drought across functional groups and dispersal types

More frequent and severe El Niño Southern Oscillations (ENSO) are causing episodic periods of decreased rainfall. Although the effects of these ENSO-induced droughts on tree growth and mortality have been well studied, the impacts on other demographic rates such as reproduction are less well known. We use a four-year seed rain dataset encompassing the most severe ENSO-induced drought in more than 30 years to assess the resilience (i.e., resistance and recovery) of the seed composition and abundance of three forest types in a tropical dry forest. We found that forest types showed distinct differences in the timing, duration, and intensity of drought during the ENSO event, which likely mediated seed composition shifts and resilience. Drought-deciduous species were particularly sensitive to the drought with overall poor resilience of seed production, whereby seed abundance of this functional group failed to recover to predrought levels even two years after the drought. Liana and wind-dispersed species were able to maintain seed production both during and after drought, suggesting that ENSO events promote early successional species or species with a colonization strategy. Combined, these results suggest that ENSO-induced drought mediates the establishment of functional groups and dispersal types suited for early successional conditions with more open canopies and reduced competition among plants. The effects of the ENSO-induced drought on seed composition and abundance were still evident two years after the event suggesting the recovery of seed production requires multiple years that may lead to shifts in forest composition and structure in the long term, with potential consequences for higher trophic levels like frugivores.

Multiple stages of tree seedling recruitment are altered in tropical forests degraded by selective logging

Tropical forest degradation is a global environmental issue. In degraded forests, seedling recruitment of canopy trees is vital for forest regeneration and recovery. We investigated how selective logging, a pervasive driver of tropical forest degradation, impacts canopy tree seedling recruitment, focusing on an endemic dipterocarp Dryobalanops lanceolata in Sabah, Borneo. During a mast-fruiting event in intensively logged and nearby unlogged forest, we examined four stages of the seedling recruitment process: seed production, seed predation, and negative density-dependent germination and seedling survival. Our results suggest that each stage of the seedling recruitment process is altered in logged forest. The seed crop of D. lanceolata trees in logged forest was one-third smaller than that produced by trees in unlogged forest. The functional role of vertebrates in seed predation increased in logged forest while that of non-vertebrates declined. Seeds in logged forest were less likely to germinate than those in unlogged forest. Germination increased with local-scale conspecific seed density in unlogged forest, but seedling survival tended to decline. However, both germination and seedling survival increased with local-scale conspecific seed density in logged forest. Notably, seed crop size, germination, and seedling survival tended to increase for larger trees in both unlogged and logged forests, suggesting that sustainable timber extraction and silvicultural practices designed to minimize damage to the residual stand are important to prevent seedling recruitment failure. Overall, these impacts sustained by several aspects of seedling recruitment in a mast-fruiting year suggest that intensive selective logging may affect long-term population dynamics of D. lanceolata. It is necessary to establish if other dipterocarp species, many of which are threatened by the timber trade, are similarly affected in tropical forests degraded by intensive selective logging.

Transient synchrony among populations of five foliage-feeding Lepidoptera

Studies of transient population dynamics have largely focused on temporal changes in dynamical behaviour, such as the transition between periods of stability and instability. This study explores a related dynamic pattern, namely transient synchrony during a 49-year period among populations of five sympatric species of forest insects that share host tree resources. The long time series allows a more comprehensive exploration of transient synchrony patterns than most previous studies. Considerable variation existed in the dynamics of individual species, ranging from periodic to aperiodic. We used time-averaged methods to investigate long-term patterns of synchrony and time-localized methods to detect transient synchrony. We investigated transient patterns of synchrony between species and related these to the species' varying density dependence structures; even species with very different density dependence exhibited at least temporary periods of synchrony. Observed periods of interspecific synchrony may arise from interactions with host trees (e.g., induced host defences), interactions with shared natural enemies or shared impacts of environmental stochasticity. The transient nature of synchrony observed here raises questions both about the identity of synchronizing mechanisms and how these mechanisms interact with the endogenous dynamics of each species. We conclude that these patterns are the result of interspecific interactions that act only temporarily to synchronize populations, after which differences in the endogenous population dynamics among the species acts to desynchronize their dynamics.

The combined impacts of experimental defaunation and logging on seedling traits and diversity

Animals can have both positive (e.g. via seed dispersal) and negative (e.g. via herbivory) impacts on plants. The net effects of these interactions remain difficult to predict and may be affected by overhunting and habitat disturbance, two widespread threats to tropical forests. Recent studies have documented their separate effects on plant recruitment but our understanding of how defaunation and logging interact to influence tropical tree communities is limited. From 2013 to 2016, we followed the fate of marked tree seedlings (n = 1489) from 81 genera in and outside experimental plots. Our plots differentially excluded small, medium and large-bodied mammal herbivores in logged and unlogged forest in Malaysian Borneo. We assessed the effects of experimental defaunation and logging on taxonomic diversity and plant trait (wood density, specific leaf area, fruit size) composition of seedling communities. Although seedling mortality was highest in the presence of all mammal herbivores (44%), defaunation alone did not alter taxonomic diversity nor plant trait composition. However, herbivores (across all body sizes) significantly reduced mean fruit size across the seedling community over time (95% confidence interval (CI): -0.09 to -0.01), particularly in logged forest (95% CI: -0.12 to -0.003). Our findings suggest that impacts of mammal herbivores on plant communities may be greater in forests with a history of disturbance and could subsequently affect plant functional traits and ecological processes associated with forest regeneration.

Effectiveness of predator satiation in masting oaks is negatively affected by conspecific density

Variation in seed availability shapes plant communities, and is strongly affected by seed predation. In some plant species, temporal variation in seed production is especially high and synchronized over large areas, which is called ‘mast seeding’. One selective advantage of this phenomenon is predator satiation which posits that masting helps plants escape seed predation through starvation of predators in lean years, and satiation in mast years. However, even though seed predation can be predicted to have a strong spatial component and depend on plant densities, whether the effectiveness of predator satiation in masting plants changes according to the Janzen-Connell effect has been barely investigated. We studied, over an 8-year period, the seed production, the spatiotemporal patters of weevil seed predation, and the abundance of adult weevils in a holm oak (Quercus ilex) population that consists of trees interspersed at patches covering a continuum of conspecific density. Isolated oaks effectively satiate predators, but this is trumped by increasing conspecific plant density. Lack of predator satiation in trees growing in dense patches was caused by re-distribution of insects among plants that likely attenuated them against food shortage in lean years, and changed the type of weevil functional response from type II in isolated trees to type III in trees growing in dense patches. This study provides the first empirical evaluation of the notion that masting and predator satiation should be more important in populations that start to dominate their communities, and is consistent with the observation that masting is less frequent and less intense in diverse forests.

Defaunation and habitat disturbance interact synergistically to alter seedling recruitment

Vertebrate granivores destroy plant seeds, but whether animal-induced seed mortality alters plant recruitment varies with habitat context, seed traits, and among granivore species. An incomplete understanding of seed predation makes it difficult to predict how widespread extirpations of vertebrate granivores in tropical forests might affect tree communities, especially in the face of habitat disturbance. Many tropical forests are simultaneously affected by animal loss as well as habitat disturbance, but the consequences of each for forest regeneration are often studied separately or additively, and usually on a single plant demographic stage. The combined impacts of these threats could affect plant recruitment in ways that are not apparent when studied in isolation. We used wire cages to exclude large (elephants), medium, (sambar deer, bearded pigs, muntjac deer), and small (porcupines, chevrotains) ground-dwelling mammalian granivores and herbivores in logged and unlogged forests in Malaysian Borneo. We assessed the interaction between habitat disturbance (selective logging) and experimental defaunation on seed survival, germination, and seedling establishment in five dominant dipterocarp tree species spanning a 21-fold gradient in seed size. Granivore-induced seed mortality was consistently higher in logged forest. Germination of unpredated seeds was reduced in logged forest and in the absence of small to large-bodied mammals. Experimental defaunation increased germination and reduced seed removal but had little effect on seed survival. Seedling recruitment however, was more likely where logging and animal loss occurred together. The interacting effects of logging and hunting could therefore, actually increase seedling establishment, suggesting that the loss of mammals in disturbed forest could have important consequences for forest regeneration and composition.

Bearded pig (Sus barbatus) utilisation of a fragmented forest–oil palm landscape in Sabah, Malaysian Borneo

Context Oil palm plantations have become a dominant landscape in Southeast Asia, yet we still understand relatively little about the ways wildlife are adapting to fragmented mosaics of forest and oil palm. The bearded pig is of great ecological, social and conservation importance in Borneo and is declining in many parts of its range due to deforestation, habitat fragmentation and overhunting. Aims We assessed how the bearded pig is adapting to oil palm expansion by investigating habitat utilisation, activity patterns, body condition and minimum group size in a mosaic landscape composed of forest fragments and surrounding oil palm plantations. Methods We conducted our study in Sabah, Malaysian Borneo, in and around the Lower Kinabatangan Wildlife Sanctuary, a protected area consisting of secondary forest fragments (ranging 1200–7400ha) situated within an extensive oil palm matrix. We modelled bearded pig habitat use in forest fragments and oil palm plantations using survey data from line transects. Camera traps placed throughout the forest fragments were used to assess pig activity patterns, body condition and minimum group size. Key results All forest transects and 80% of plantation transects showed pig presence, but mean pig signs per transect were much more prevalent in forest (70.00±13.00s.e.) than in plantations (0.91±0.42s.e.). Pig tracks had a positive relationship with leaf cover and a negative relationship with grass cover; pig rooting sites had a positive relationship with wet and moderate soils compared with drier soils. Ninety-five percent of pigs displayed ‘good’ or ‘very good’ body condition in forests across the study area. Pigs also aggregated in small groups (mean=2.7±0.1s.e. individuals), and showed largely diurnal activity patterns with peak activity taking place at dawn and dusk. Groups with piglets and juveniles were more active during the day and less active at night as compared to overall activity patterns for all groups. Conclusions Our findings suggest that bearded pigs in our study area regularly utilise oil palm as habitat, as indicated by their signs in most oil palm sites surveyed. However, secondary forest fragments are used much more frequently and for a wider range of behaviours (e.g. nesting, wallowing) than adjacent oil palm plantations. These forests clearly remain the most important habitat for the bearded pig in the Lower Kinabatangan Wildlife Sanctuary, and their protection is a high conservation priority for this species. Implications Consistent bearded pig presence in oil palm is potentially an indication of successful adaptation to agricultural expansion in the study area. The apparently good body condition displayed by the vast majority of pigs in our study likely results from year-round cross-border fruit subsidies from surrounding oil palm plantations. The consistent diurnal activity displayed by groups containing piglets and juveniles may indicate predator avoidance strategies, whereas the substantial nocturnal activity we observed by other groups could suggest fewer threats for larger individuals. However, the overall effects of oil palm expansion in the region on bearded pig population health, foraging ecology, and movement ecology remain unknown.A

Complex pollination of a tropical Asian rainforest canopy tree by flower-feeding thrips and thrips-feeding predators

PREMISE OF THE STUDY

In tropical rainforests of Southeast Asia, a highly fecund thrips (Thrips spp.) responds rapidly to the mass flowering at multiple-year intervals characteristic of certain species such as the canopy tree studied here, Shorea acuminata, by feeding on flower resources. However, past DNA analyses of pollen adherent to thrips bodies revealed that the thrips promoted a very high level of self-pollination. Here, we identified the pollinator that contributes to cross-pollination and discuss ways that the pollination system has adapted to mass flowering.

METHODS

By comparing the patterns of floral visitation and levels of genetic diversity in adherent pollen loads among floral visitors, we evaluated the contribution of each flower visitor to pollination.

KEY RESULTS

The big-eyed bug, Geocoris sp., a major thrips predator, was an inadvertent pollinator, and importantly contributed to cross-pollination. The total outcross pollen adhering to thrips was approximately 30% that on the big-eyed bugs. Similarly, 63% of alleles examined in S. acuminata seeds and seedlings occurred in pollen adhering to big-eyed bugs; about 30% was shared with pollen from thrips.

CONCLUSIONS

During mass flowering, big-eyed bugs likely travel among flowering S. acuminata trees, attracted by the abundant thrips. Floral visitation patterns of big-eyed bugs vs. other insects suggest that these bugs can maintain their population size between flowering by preying upon another thrips (Haplothrips sp.) that inhabits stipules of S. acuminata throughout the year and quickly respond to mass flowering. Thus, thrips and big-eyed bugs are essential components in the pollination of S. acuminata.

Spatiotemporal strategies that facilitate recruitment in a habitat specialist tree species

Our understanding of processes underlying plant recruitment emerges from species and habitats that are widely distributed at regional and global scales. However, the applicability of dispersal-recruitment models and the role of dispersal limitation versus microsite limitation have not been examined for specialized habitats. In patchy, freshwater Myristica swamp forests (Western Ghats, India), we examine the roles of primary seed dispersal, secondary seed removal and microsite suitability for the establishment of a swamp specialist tree, Myristica fatua We estimated primary seed shadows, performed secondary removal experiments and enumerated recruits in swamp sites. Steady-state fruiting was observed with the extended production (>7 months) of small numbers of fruits. Frugivores dropped most of the large and heavy seeds under parent crowns, while a few seeds were transported over short distances by hornbills. Seed placement experiments indicated that removal, germination and establishment were similar within swamp microsites, while seeds failed to survive in matrix habitats surrounding the swamp. Crabs, which were major secondary removers of M. fatua, did not alter the initial seed dispersal patterns substantially, which led to the retention of seeds within the swamp. Distribution of saplings and adults from previous seasons also suggest that dispersal-recruitment dynamics in the swamp specialist M. fatua did not strictly follow predictions of Janzen-Connell model while abiotic effects were significant. Large seeds, steady-state fruiting and small crop sizes may be significant selective forces facilitating escape from density and distance-dependent effects in space and time in specialist plant species such as M. fatua.

Extended flowering intervals of bamboos evolved by discrete multiplication

Numerous bamboo species collectively flower and seed at dramatically extended, regular intervals - some as long as 120 years. These collective seed releases, termed 'masts', are thought to be a strategy to overwhelm seed predators or to maximise pollination rates. But why are the intervals so long, and how did they evolve? We propose a simple mathematical model that supports their evolution as a two-step process: First, an initial phase in which a mostly annually flowering population synchronises onto a small multi-year interval. Second, a phase of successive small multiplications of the initial synchronisation interval, resulting in the extraordinary intervals seen today. A prediction of the hypothesis is that mast intervals observed today should factorise into small prime numbers. Using a historical data set of bamboo flowering observations, we find strong evidence in favour of this prediction. Our hypothesis provides the first theoretical explanation for the mechanism underlying this remarkable phenomenon.

Biotic and abiotic drivers of dipterocarp seedling survival following mast fruiting in Malaysian Borneo

South-East Asian tropical rain forests experience sporadic, but profuse, seed production after general flowering, leading to the synchronous emergence of various seedlings and subsequent seedling dynamics, which play a crucial role in determining species distribution and coexistence. We examined the relative importance of both biotic (initial height, conspecific seedling density) and abiotic (canopy openness, per cent sand, soil water content) drivers using survival data for 1842 seedlings of 12 dipterocarp species for 1.5 y following mast fruiting in an old-growth Bornean tropical rain forest. More than 30% of all dipterocarp seedlings survived 1.5 y after mast fruiting. When all species were analysed together, we found that initial seedling height, canopy openness and conspecific seedling density affected dipterocarp seedling survival. Negative density dependence indicated that predators were not satiated, but dipterocarp seedlings rather suffered from host-specific natural enemies or intraspecific competition. Species-level analyses of seven dipterocarp species showed large variation in response to biotic and abiotic factors. These results suggest that interspecific differences in the relative importance of biotic and abiotic effects on seedling survival might contribute to species coexistence.

Impacts of logging on density-dependent predation of dipterocarp seeds in a South East Asian rainforest

Much of the forest remaining in South East Asia has been selectively logged. The processes promoting species coexistence may be the key to the recovery and maintenance of diversity in these forests. One such process is the Janzen-Connell mechanism, where specialized natural enemies such as seed predators maintain diversity by inhibiting regeneration near conspecifics. In Neotropical forests, anthropogenic disturbance can disrupt the Janzen-Connell mechanism, but similar data are unavailable for South East Asia. We investigated the effects of conspecific density (two spatial scales) and distance from fruiting trees on seed and seedling survival of the canopy tree Parashorea malaanonan in unlogged and logged forests in Sabah, Malaysia. The production of mature seeds was higher in unlogged forest, perhaps because high adult densities facilitate pollination or satiate pre-dispersal predators. In both forest types, post-dispersal survival was reduced by small-scale (1 m(2)) conspecific density, but not by proximity to the nearest fruiting tree. Large-scale conspecific density (seeds per fruiting tree) reduced predation, probably by satiating predators. Higher seed production in unlogged forest, in combination with slightly higher survival, meant that recruitment was almost entirely limited to unlogged forest. Thus, while logging might not affect the Janzen-Connell mechanism at this site, it may influence the recruitment of particular species.