Heterospecific Neighbors Research Articles

Forest tree neighborhoods are structured more by negative conspecific density dependence than by interactions among closely related species

Interactions among neighbors influence the structure of communities of sessile organisms. Closely related species tend to share habitat and resource requirements and to interact with the same mutualists and natural enemies so that the strength of interspecific interactions tends to decrease with evolutionary divergence time. Nevertheless, the degree to which such phylogenetically related ecological interactions structure plant communities remains unclear. Using data from five large mapped forest plots combined with a DNA barcode mega‐phylogeny, we employed an individual‐based approach to assess the collective effects of focal tree size on neighborhood phylogenetic relatedness. Abundance‐weighted average divergence time for all neighbors (ADT_all) and for heterospecific neighbors only (ADT_hetero) were calculated for each individual of canopy tree species. Within local neighborhoods, we found phylogenetic composition changed with focal tree size. Specifically, significant increases in ADT_all with focal tree size were evident at all sites. In contrast, there was no significant change in ADT_hetero with tree size in four of the five sites for both sapling‐sized and all neighbors, even at the smallest neighbourhood scale (0–5 m), suggesting a limited role for phylogeny‐dependent interactions. However, there were inverse relationships between focal tree size and the proportion of heterospecific neighbors belonging to closely related species at some sites, providing evidence for negative phylogenetic density dependence. Overall, our results indicate that negative interaction with conspecifics had a much greater impact on neighborhood assemblages than interactions among closely related species and could contribute to community structure and diversity maintenance in different forest communities.

Variation of the biotic neighbourhood and topographic effects on tree survival in an old‐growth temperate forest

AbstractAimsTree mortality can determine forest dynamics and succession, alter nutrient cycling and contribute to tree species co‐existence. Understanding the mechanisms that maintain species diversity is a central goal of community ecology. In this study, we incorporated survival data from a 10‐yr interval census to determine whether there is significant interspecies variation in the effects of biotic neighbourhood variables and whether there is significant spatial variation in the effects of topographic variables on tree survival in a broadleaved‐Korean pine temperate forest.LocationTemperate forest, northeast China.MethodsWe used GLMM and geographically weighted GLM as well as their corresponding test approaches to determine whether there is significant interspecies variation in the effects of biotic neighbourhood variables over variable neighbourhood radii and whether there is significant spatial variation in the effects of topographic variables on tree survival across multiple spatial scales in an old‐growth temperate forest dynamics plot in northeast China.ResultsOur results showed that the effects of neighbourhood variables on tree survival varied among species and effects of topography varied over space. However, the results depended on the neighbourhood radius and spatial scale. The neighbourhood radius affected detection of variation in neighbourhood effects among species, and the spatial scale similarly affected detection of spatial variation in the effects of topography. The effect of conspecific neighbour density on survival varied more widely among species than that of the average phylogenetic dissimilarity of heterospecific neighbours, regardless of the neighbourhood radius. Moreover, the effect of elevation varied more widely over space than those of convexity and hill shade at any quadrat scale.ConclusionsOur findings highlight that future studies should account for variations in the neighbourhood radius among species and/or circumstances as well as the spatial scale when assessing factors that contribute to species co‐existence in long‐lived organisms. Species leaf phenology, shade and drought tolerance and environmental filtering may explain these variations in the effects of neighbourhood and topographic variables on tree survival. Overall, our results indicate that both density dependence and resource niche partitioning are important mechanisms for maintaining biodiversity in this broad‐leaved Korean pine temperate forest.

Neighbourhood effects on plant reproduction: An experimental–analytical framework and its application to the invasive Senecio inaequidens

Abstract Density dependence is of fundamental importance for population and range dynamics. Density‐dependent reproduction of plants arises from competitive and facilitative plant–plant interactions that can be pollination independent or pollination mediated. In small and sparse populations, conspecific density dependence often turns from negative to positive and causes Allee effects. Reproduction may also increase with heterospecific density (community‐level Allee effect), but the underlying mechanisms are poorly understood and the consequences for community dynamics can be complex. Allee effects have crucial consequences for the conservation of declining species, but also the dynamics of range edge populations. In invasive species, Allee effects may slow or stop range expansion. Observational studies in natural plant communities cannot distinguish whether reproduction is limited by pollination‐mediated interactions among plants or by other neighbourhood effects (e.g. competition for abiotic resources). Even experimental pollen supply cannot distinguish whether variation in reproduction is caused by direct density effects or by plant traits correlated with density. Finally, it is unknown over which spatial scales pollination‐mediated interactions occur. To circumvent these problems, we introduce a comprehensive experimental and analytical framework which simultaneously (1) manipulates pollen availability and quality by hand pollination and pollinator exclusion, (2) manipulates neighbourhoods by transplanting target plants, and (3) analyses the effects of con‐ and heterospecific neighbourhoods on reproduction with spatially explicit trait‐based neighbourhood models. Applying this framework to Senecio inaequidens, one of Europe's fastest plant invaders, we found that the seed set was strongly pollen limited. Reproduction had increased by pollinator‐mediated facilitation by both con‐ and heterospecific neighbours which may lead to (community‐level) Allee effects. Pollination‐independent interactions, such as amelioration of abiotic conditions through neighbours, contributed to additional positive neighbour effects. However, these pollination‐independent interactions were weaker than the pollination‐mediated interactions and they occurred over smaller spatial scales. Finally, the strength and direction of neighbourhood effects depended on neighbour traits and thus changed with the trait composition of the neighbourhood. Synthesis. By manipulating both pollen availability and target plant locations within neighbourhoods, we can comprehensively analyse spatially explicit density dependence of plant reproduction. This experimental approach enhances our ability to understand the dynamics of sparse populations and of species geographical ranges.

Neighbourhood effects determine plant–herbivore interactions below‐ground

Abstract Plant neighbours can strongly influence the interactions between herbivores and focal plants, for instance by providing food of different quality (consumptive effects) or by changing the behaviour and metabolism of the herbivore and the focal plant without being consumed (non‐consumptive effects). Determining the species‐specific contributions of consumptive and non‐consumptive effects is important to understand the ecophysiological mechanisms which underlie neighbourhood effects. We quantified the impact of nine different grassland plant species on the interaction between Taraxacum officinale and the root‐feeding insect Melolontha melolontha. We investigated the contribution of consumptive and non‐consumptive effects to the observed patterns, and evaluated the impact of neighbouring plants on the growth and physiology of T. officinale upon M. melolontha attack. Melolontha melolontha growth was strongly affected by the presence of different neighbouring species. The three grass species increased larval growth when growing with T. officinale, with Poa pratensis having a synergistic effect in the mixture compared to both monocultures. The forb Centaurea stoebe reduced larval growth when growing with T. officinale or alone. The other five species had no measurable impact on larval performance. Based on these results, P. pratensis and C. stoebe were selected for further experiments. Diet‐mixing experiments showed that P. pratensis increased M. melolontha growth when offered together with T. officinale, while C. stoebe suppressed it. When feeding was restricted to artificial diet, larval growth was not changed by the presence of P. pratensis or C. stoebe. However, when feeding was restricted to T. officinale, larval growth was increased by both heterospecific neighbours. Biomass and primary metabolism of T. officinale under attack by M. melolontha was also altered by the presence of C. stoebe and P. pratensis. Together, these results show that consumptive and non‐consumptive effects can explain the positive effect of P. pratensis. In contrast, the negative effect of C. stoebe is likely driven exclusively by intoxication. Synthesis. The performed experiments suggest that different combinations of consumptive and non‐consumptive effects are likely to contribute to the diversity of neighbourhood effects in nature. Furthermore, they show that neighbourhood effects are important factors in below‐ground plant–insect interactions.

Bottom-up and top-down effects of tree species diversity on leaf insect herbivory.

The diversity of plant neighbors commonly results in direct, bottom‐up effects on herbivore ability to locate their host, and in indirect effects on herbivores involving changes in plant traits and a top‐down control by their enemies. Yet, the relative contribution of bottom‐up and top‐down forces remains poorly understood. We also lack knowledge on the effect of abiotic constraints such as summer drought on the strength and direction of these effects. We measured leaf damage on pedunculate oak (Quercus robur), alone or associated with birch, pine or both in a long‐term tree diversity experiment (ORPHEE), where half of the plots were irrigated while the other half remained without irrigation and received only rainfall. We tested three mechanisms likely to explain the effects of oak neighbors on herbivory: (1) Direct bottom‐up effects of heterospecific neighbors on oak accessibility to herbivores, (2) indirect bottom‐up effects of neighbors on the expression of leaf traits, and (3) top‐down control of herbivores by predators. Insect herbivory increased during the growth season but was independent of neighbor identity and irrigation. Specific leaf area, leaf toughness, and thickness varied with neighbor identity while leaf dry matter content or C:N ratio did not. When summarized in a principal component analysis (PCA), neighbor identity explained 87% of variability in leaf traits. PCA axes partially predicted herbivory. Despite greater rates of attack on dummy caterpillars in irrigated plots, avian predation, and insect herbivory remained unrelated. Our study suggests that neighbor identity can indirectly influence insect herbivory in mixed forests by modifying leaf traits. However, we found only partial evidence for these trait‐mediated effects and suggest that more attention should be paid to some unmeasured plant traits such as secondary metabolites, including volatile organic compounds, to better anticipate the effects of climate change on plant‐insect interactions in the future.

Neighbour presence, not identity, influences root and shoot allocation in pea.

Competition is a key feature that structures the composition of plant communities. A growing body of evidence is showing that the presence of neighbours, especially belowground neighbours, induces varied morphological responses in plants. However, in many species, it is not known whether neighbour identity also influences plant morphological responses such as biomass allocation patterns. To assess plant response to above- and belowground neighbour presence and identity, we conducted a greenhouse experiment consisting of conspecific (pea; Pisum sativum L.) and heterospecific (oat; Avena sativa L.) neighbours growing with a P. sativum focal plant. Four interaction regimes were constructed including shoot, root, or ‘full’ interaction (root & shoot) treatments, as well as a control with no interactions permitted. Our results showed that pea plants responded negatively to the presence of neighbours, and in particular, the presence of belowground neighbours. Treatments where belowground interactions were permitted (full and root interactions) had lower root and shoot mass fractions (R:S ratios) than where shoot interactions were permitted. Shoot and root allocation and R:S ratios of focal pea plants were not affected by neighbour identity, suggesting that neighbour presence, but not identity, influenced allocation patterns. The impact on P. sativum of a neighbouring competitor was more prominent than neighbour identity, showing that some plants may not discriminate between the identity of neighbours even though they are capable of responding to their presence.

Temporal variation in plant neighborhood effects on the defoliation of primary and secondary hosts by an insect pest

AbstractPlant neighborhood effects on herbivore damage have been observed in many systems although few studies have assessed the different component effects of the neighborhood (conspecific neighbors vs. heterospecific neighbors) on defoliation. No earlier studies have monitored how temporal scale influences neighborhood effects. We tested hypotheses on resource concentration/dilution and associational effects in the eastern spruce budworm (Choristoneura fumiferana)–forest system over a 10‐yr period across different stands including stands dominated by highly vulnerable hosts (balsam fir), stands dominated by a species with low vulnerability (black spruce), and mixed composition stands (fir and spruce). We observed persistent resource concentration effects on the primary host (balsam fir) during the increasing phase of the outbreak in balsam fir‐dominated and mixed stands, while both resource dilution and associational susceptibility effects were observed on the secondary host (black spruce) and the strength of associational susceptibility increased with an increase in resource dilution. We did not observe associational effects on the primary host in mixed and secondary host‐dominated stands but in stands dominated by the primary host, we observed associational resistance when the resource was highly depleted. Therefore, the complexity of neighborhood effects suggests that future studies should consider the separate effects of conspecific and heterospecific neighbors, as well as changes through time in order to predict herbivore damage in different systems, and provide better preventive and reactive strategies to manage herbivore outbreaks.

Spatial vegetation patterns and neighborhood competition among woody plants in an East African savanna.

The majority of research on savanna vegetation dynamics has focused on the coexistence of woody and herbaceous vegetation. Interactions among woody plants in savannas are relatively poorly understood. We present data from a 10-yr longitudinal study of spatially explicit growth patterns of woody vegetation in an East African savanna following exclusion of large herbivores and in the absence of fire. We examined plant spatial patterns and quantified the degree of competition among woody individuals. Woody plants in this semiarid savanna exhibit strongly clumped spatial distributions at scales of 1-5m. However, analysis of woody plant growth rates relative to their conspecific and heterospecific neighbors revealed evidence for strong competitive interactions at neighborhood scales of up to 5m for most woody plant species. Thus, woody plants were aggregated in clumps despite significantly decreased growth rates in close proximity to neighbors, indicating that the spatial distribution of woody plants in this region depends on dispersal and establishment processes rather than on competitive, density-dependent mortality. However, our documentation of suppressive effects of woody plants on neighbors also suggests a potentially important role for tree-tree competition in controlling vegetation structure and indicates that the balanced-competition hypothesis may contribute to well-known patterns in maximum tree cover across rainfall gradients in Africa.

Territorial battles between fiddler crab species.

Many species worldwide are impacted by habitat loss. This may result in increased competition both within species and between species. Many studies have demonstrated that when two previously non-overlapping species are forced to compete over a resource, one species is likely to become dominant over the other. This study explores the impact a larger species of fiddler crab (Tabuca elegans—previously known as Uca elegans) has when invading an area previously used solely by a smaller species (Austruca mjoebergi—previously known as Uca mjoebergi). Here we show that, while there are some detrimental effects of living next to a heterospecific, they are relatively minor. New heterospecific neighbours fight more regularly with resident crabs, but each fight is no longer or more escalated than those between the resident and a new conspecific male. The residents are not specifically targeted by intruding heterospecifics, thus, given the large advantage of having a heterospecific neighbour in terms of lowered competition for females, the overall impact of species mixing is probably not as negative as might have been predicted.

Intra- and interspecific interactions of Scots pine and European beech in mixed secondary forests

By the mid successional stages, secondary forests of Scots pine in Europe are dominated by mixed stands of pioneer Scots pine and late-successional European beech. The objective of this study was to explore the interactions of pine and beech with their conspecific and heterospecific neighbours in these forests. To accomplish the objective, pine and beech trees were stem-mapped in forty 500 m2 plots randomly located within 18 mixed stands in Milomlyn Forest District, northern Poland. The interactions within and between the species were analysed through two structurally different univariate and bivariate second-order summary statistics, i.e. pair correlation function g(r) and mark correlation function kmm(r). Field measurements showed that the overstorey was dominated by even-aged pine, whereas uneven-aged beech was the only species in the understorey. Pine trees presented an aggregation, while beech trees exhibited a dispersed structure in all stands. In addition, pine trees showed strong attraction to beech trees at small spatial scales (0–2 m). Negative correlation was found between tree height and diameter at breast height of beech, while there was no correlation between height and diameter of pine trees. We conclude that pine trees exhibit negative intraspecific interactions at small spatial scales that are mostly driven by their competitive interactions. Beech trees show strong positive intraspecific interactions and form clumps within pine canopy cover. The strong positive interspecific interactions of pine and beech are the outcome of their different shade tolerance. Our results help to explain successful coexistence of pine and beech in the study site and highlight detailed tree-tree interactions of the species in mixed stands.

Relative importance of density dependence and topography on tree mortality in a subtropical mountain forest

Tree mortality is an important process in forest dynamics, and potentially affects species coexistence and community assembly. Detecting spatial pattern of tree mortality and examining the possible driving factors are critical to understand the determinants of tree death. In 2015, we conducted a census of snags (with dbh⩾10cm) of woody plants in the 25-ha Badagongshan (BDGS) forest dynamics plot. Based on this survey data, we used the univariate, bivariate pair correlation function, g(r), to analyze the spatial distribution pattern of snags and the association between snags and living trees. In addition, we used generalized linear mixed models (GLMM) to examine the relative importance of neighbor and topographical factors on tree mortality. Analyses demonstrated two main results. First, at the community level, snags showed a consistent aggregated distribution at 0–30m scales, and snags and living trees were significantly negatively correlated at 1–12m scales. At the species level, 10 of the 18 common species (with ⩾25 snag individuals) displayed significant negative associations between living trees and snags at different scales in the 0–50m range. Second, both basal area of conspecific neighbors and basal area of heterospecific neighbors showed a significant positive relationship on the occurrence of snags at different levels (community, guild and species). Topographic factors showed limited correlations with the occurrence of snags. Additionally, tree size was significantly negatively correlated with adult and large tree mortality, but non-significantly correlated with old trees. In summary, tree mortality is nonrandom and mainly driven by interspecific competition and intraspecific negative density dependence in this subtropical mountain evergreen and deciduous broad-leaved mixed forest in China. Topographic conditions had little relation on tree mortality, but tree size was an important predictor of tree death at the community level. Our study demonstrates that interactions of species tended to be more important in affecting tree mortality than habitat variables in this forest, which can further improve our understanding of forest dynamics and provide guidance to forest management.

Coexistence of plant species in a biodiversity hotspot is stabilized by competition but not by seed predation

Understanding the mechanisms of species coexistence is a key task for ecology. Recent theory predicts that both competition and predation (which causes apparent competition among prey) can either promote or limit species coexistence. Both mechanisms cause negative interactions between individuals, and each mechanism promotes stable coexistence if it causes negative interactions to be stronger between conspecifics than between heterospecifics. However, the relative importance of competition and predation for coexistence in natural communities is poorly known. Here, we study how competition and apparent competition via pre‐dispersal seed predators affect the long‐term fecundity of Protea shrubs in the fire‐prone Fynbos biome (South Africa). These shrubs store all viable seeds produced since the last fire in fire‐proof cones. Competitive effects on cone number and pre‐dispersal seed predation reduce their fecundity and can thus limit recruitment after the next fire. In 27 communities comprising 49 990 shrubs of 22 Protea species, we measured cone number and per‐cone seed predation rate of 2154 and 1755 focal individuals, respectively. Neighbourhood analyses related these measures to individual‐based community maps. We found that conspecific neighbours had stronger competitive effects on cone number than heterospecific neighbours. In contrast, apparent competition via seed predators was comparable between conspecifics and heterospecifics. This indicates that competition stabilizes coexistence of Protea species, whereas pre‐dispersal seed predation does not. Larger neighbours had stronger competitive effects and neighbours with large seed crops exerted stronger apparent competition. For 97% of the focal plants, competition reduced fecundity more than apparent competition. Our results show that even in communities of closely related and ecologically similar species, intraspecific competition can be stronger than interspecific competition. On the other hand, apparent competition through seed predators need not have such a stabilizing effect. These findings illustrate the potential of ‘community demography’, the demographic study of multiple interacting species, for understanding plant coexistence.

Species with greater seed mass are more tolerant of conspecific neighbours: a key driver of early survival and future abundances in a tropical forest.

Multiple niche-based processes including conspecific negative density dependence (CNDD) determine plant regeneration and community structure. We ask how interspecific and intraspecific density-dependent interactions relate to plant life histories and associated functional traits. Using hierarchical models, we analysed how such interactions affected first-year survival of seedling recruits of 175 species in a tropical forest, and how species abundances and functional traits are related to interspecific variation in density-dependent effects. Conspecific seedling neighbour effects prevailed over the effects of larger conspecific and all heterospecific neighbours. Tolerance of seedling CNDD enhanced recruit survival and subsequent abundance, all of which were greater among larger seeded, slow-growing and well-defended species. Niche differentiation along the growth-survival trade-off and tolerance of seedling CNDD strongly correlated with regeneration success, with manifest consequences for community structure. The ability of larger seeded species to better tolerate CNDD suggests a novel mechanism for CNDD to contribute to seed-size variation and promote species coexistence through a tolerance-fecundity trade-off.

Collateral damage or a shadow of safety? The effects of signalling heterospecific neighbours on the risks of parasitism and predation.

Although males often display from mixed-species aggregations, the influence of nearby heterospecifics on risks associated with sexual signalling has not been previously examined. We tested whether predation and parasitism risks depend on proximity to heterospecific signallers. Using field playback experiments with calls of two species that often display from the same ponds, túngara frogs and hourglass treefrogs, we tested two hypotheses: (1) calling near heterospecific signallers attractive to eavesdroppers results in increased attention from predatory bats and parasitic midges (collateral damage hypothesis) or (2) calling near heterospecific signallers reduces an individual's predation and parasitism risks, as eavesdroppers are drawn to the heterospecifics (shadow of safety hypothesis). Bat visitation was not affected by calling neighbours. The number of frog-biting midges attracted to hourglass treefrog calls, however, rose threefold when played near túngara calls, supporting the collateral damage hypothesis. We thus show that proximity to heterospecific signallers can drastically alter both the absolute risks of signalling and the relative strengths of pressures from predation and parasitism. Through these mechanisms, interactions between heterospecific guild members are likely to influence the evolution of signalling strategies and the distribution of species at both local and larger scales.

How does habitat filtering affect the detection of conspecific and phylogenetic density dependence?

Conspecific negative density dependence (CNDD) has been recognized as a key mechanism underlying species coexistence, especially in tropical forests. Recently, some studies have reported that seedling survival is also negatively correlated with the phylogenetic relatedness between neighbors and focal individuals, termed phylogenetic negative density dependence (PNDD). In contrast to CNDD or PNDD, shared habitat requirements between closely related individuals are thought to be a cause of observed positive effects of closely related neighbors, which may affect the strength and detectability of CNDD or PNDD. In order to investigate the relative importance of these mechanisms for tropical tree seedling survival, we used generalized linear mixed models to analyze how the survival of more than 10 000 seedlings of woody plant species related to neighborhood and habitat variables in a tropical rainforest in southwest China. By comparing models with and without habitat variables, we tested how habitat filtering affected the detection of CNDD and PNDD. The best-fitting model suggested that CNDD and habitat filtering played key roles in seedling survival; but that, contrary to our expectations, phylogenetic positive density dependence (PPDD) had a distinct and important effect. While habitat filtering affected the detection of CNDD by decreasing its apparent strength, it did not explain the positive effects of closely related neighbors. Our results demonstrate that a failure to control for habitat variables and phylogenetic relationships may obscure the importance of conspecific and heterospecific neighbor densities for seedling survival.

Plasticity in plant functional traits is shaped by variability in neighbourhood species composition.

Plant functional traits can vary widely as a result of phenotypic plasticity to abiotic conditions. Trait variation may also reflect responses to the identity of neighbours, although not all species are equally responsive to their biotic surroundings. We hypothesized that responses to neighbours are shaped by spatial community patterns and resulting variability in neighbour composition. More precisely, we tested the theoretical prediction that plasticity is most likely to evolve if alternative environments (in this case, different neighbour species) are common and encountered at similar frequencies. We estimated the frequencies of encountering different neighbour species in the field for 27 grassland species and measured the aboveground morphological responses of each species to conspecific vs heterospecific neighbours in a common garden. Responses to neighbour identity were dependent on how frequently the experimental neighbours were encountered by the focal species in their home community, with the greatest plasticity observed in species that encountered both neighbours (conspecific and heterospecific) with high and even frequency. Biotic interactions with neighbouring species can impose selection on plasticity in functional traits, which may feed back through trait divergence and niche differentiation to influence species coexistence and community structure.

Spatial patterns of sapling mortality in a moist tropical forest: consistency with total density‐dependent effects

Density‐dependent mortality (DDM) is major driver of species coexistence in tropical forests. We assessed the occurrence and strength of this mechanism among saplings (i.e. trees with a diameter of 1–4 cm) in the moist tropical forest of Barro Colorado Island (BCI), Panama, over six inter‐census periods (26 years), by using spatial point pattern analysis. We considered conspecific density‐dependent mortality, but also total DDM (i.e. the effects of conspecific and heterospecific neighbours all together) upon saplings of all species (i.e. at the community level), and on saplings of light‐demanding and shade‐tolerant ones, separately (i.e. at the light‐guild level). We also analysed the effects of conspecific DDM at the community and light‐guild levels. Conspecific density‐dependent mortality affected a lower proportion of species than was to be expected. Its strength was not significantly related with species abundance, and it had relatively weak effects at the community and light guild levels. Conversely, we detected a significant spatial signature of total DDM among all saplings, and among saplings of the light‐demanding and shade‐tolerant species, but its strength showed pronounced temporal variation. Total DDM among light‐demanding saplings was stronger than among shade‐tolerant ones and its effects were particularly marked 10–15 years after the occurrence of unusually severe droughts associated with El Niño events. Our study indicates that conspecific DDM is relatively unimportant among saplings in comparison with total DDM at community and light‐guild levels. This finding contrasts strongly with the results for seedlings, where conspecific DDM was constituted the dominant process. The pronounced temporal variations observed in the occurrence and strength of total DDM among saplings indicate that time‐varying events associated with climatic disturbances such as canopy openings or episodes of high recruit emergence might be important drivers of this process. They also emphasize the importance of considering time frames longer than one decade to study density‐dependent effects among saplings in tropical forests.

Conspecific and phylogenetic density‐dependent survival differs across life stages in a tropical forest

Summary Ecologists have long recognized that plant performance is affected by the density and composition of neighbouring individuals. With the advent of highly resolved species‐level phylogenies, it has become possible to test whether such density‐dependent neighbourhood interactions are also phylogenetically dependent. Most studies of density dependence have focused on a single life stage; however, the relative importance of different neighbourhood interactions may shift over the lifetime of an individual. We examined effects of conspecific neighbour density, heterospecific neighbour density and average phylogenetic relatedness of heterospecific neighbours on the survival of seedlings, saplings, juveniles and adult trees of 29 focal tree species using long‐term, spatially explicit forest dynamics data and a highly resolved DNA barcode phylogeny from the tropical forest of Barro Colorado Island (BCI), Panama. Our results show a decline in the strength of conspecific negative density dependence across life stages: strong negative conspecific neighbour effects at early life stages gave way to weak positive conspecific neighbour effects for adult trees. In contrast, the effect of heterospecific neighbour density on survival showed no clear trend with life stage. We found evidence of phylogenetic density dependence in the BCI forest, with a significant negative impact of neighbourhood relatedness on focal tree survival, but only for later life stages. In contrast to studies from other tropical forests, neighbourhood relatedness had a significant positive effect on seedling survival. Furthermore, we found that focal species varied much more widely in their sensitivity to conspecific neighbour density than in their reactions to heterospecific neighbour density or phylogenetic relatedness. Synthesis. Overall, our results demonstrate that both conspecific density dependence and phylogenetic density dependence influence tropical tree survival, but that their relative importance varies with life stage and among species. Our study highlights the need to incorporate multiple life stages and multiple species when assessing the factors contributing to individual survival and species coexistence for long‐lived organisms.

Species diversity and identity effects on the water consumption of tree sapling assemblages under ample and limited water supply

Studies examining the influence of biodiversity on ecosystem functioning have rarely considered water turnover, the quantitatively most important biogeochemical flux in ecosystems and a process with high sensitivity to climate warming. With a tree sapling experiment consisting of three diversity levels (1, 3, 5 species), 11 different species combinations and two soil moisture levels (moist and dry), we examined the influence of tree species diversity and species identity on stand transpiration (T) under ample and restricted water supply. We further asked whether growth in mixture leads to adaptive responses in the hydraulic system and water loss regulation in plants with heterospecific neighbors compared to plants in monoculture. In moist soil, T was on average ∼11% higher in the mixtures than in the monocultures (significant net diversity effect), which can mostly be attributed to a selection effect. Overyielding in T was highest in mixtures when Tilia cordata and/or Fraxinus excelsior were present. Both species developed larger leaf areas (LA) and sapwood areas (SA) in monocultures than the other species and furthermore increased LA and SA from the monocultures to the mixtures. Thus, inherent species differences in LA and hydraulics, but also neighbor effects on these traits determined T to a large extend. In dry soil, the diversity effect on T was not larger but slightly smaller, which is not in agreement with other published studies. We conclude that differences between pure and mixed sapling assemblages in stand water consumption and drought response are mainly caused by species identity effects, while species diversity seems to be less influential.

Plant neighborhood effects on herbivory: damage is both density and frequency dependent.

Neighboring plants can affect the likelihood that a focal plant is attacked by herbivores. Both the density of conspecific neighbors (resource concentration or dilution effects) and the relative density of heterospecific neighbors (associational effects or effects of neighbor frequency) within the local neighborhood can affect herbivore load and plant damage. Understanding how these neighborhood effects influence processes such as plant competition or natural selection on plant resistance traits will require knowing how both plant density and frequency affect damage, but previous studies have generally confounded density and frequency effects. In this study, we independently manipulated the absolute density and frequency (i.e., relative density) of two plant species (Solanum carolinense and Solidago altissima) to characterize neighborhood composition effects on S. carolinense damage by herbivores, providing the first picture of how both density and frequency of neighbors influence damage in a single system. We found both a positive effect of S. carolinense density on S. carolinense damage (a resource concentration effect) and a nonlinear effect of S. altissima frequency on S. carolinense damage (associational susceptibility). If these types of patterns are common in nature, future studies seeking to understand neighborhood effects on damage need to incorporate both density and frequency effects and capture any nonlinear effects by selecting a range of values rather than focusing on only a pair of densities or frequencies. This type of data on neighborhood effects will allow us to understand the contribution of neighborhood effects to population-level processes such as competition, the evolution of plant resistance to herbivores, and yield gains in agricultural crop mixtures.