Influence of substrate type and microsite availability on the persistence of foxtail pine (Pinus balfouriana, Pinaceae) in the Klamath Mountains, California

The persistence of poor competitors within species-rich assemblages is often tied to habitat heterogeneity. Here, the persistence of foxtail pine (Pinus balfouriana) in the Klamath Mountains of northern California was addressed using a two-step approach. First, the response of foxtail pine to shading from six co-occurring conifers was examined using two morphological indices. Foxtail pine increased the height to the first branch that supported foliage, and this branch was shorter when compared with those on all other sampled conifers, suggesting that foxtail pine is a poor competitor for light. Second, three hypotheses to explain foxtail pine persistence were tested: habitat heterogeneity at large spatial scales (substrate hypothesis), habitat heterogeneity at small spatial scales (microsite hypothesis), and the long lifespan of foxtail pine (successional hypothesis). Habitat heterogeneity at multiple spatial scales favored the persistence of foxtail pine. At large spatial scales, ultramafic substrates affected the importance and competitive abilities of shade-tolerant conifers. At small spatial scales, species richness, species diversity (H'), and stand density were positively correlated with microsite availability. No support was found for the successional hypothesis. Results are subsequently linked with general hypotheses of species coexistence in species-rich assemblages.     

Spatio-temporal dynamics of alternative male phenotypes in coho salmon populations in response to ocean environment

1. The coexistence of alternative reproductive phenotypes will probably be shaped by spatial and temporal variability in the environment. However, the effects of such variability on coexistence and the scale at which it operates are seldom understood. 2. To quantify such effects, we examined spatial and temporal dynamics in the abundance and frequency of alternative phenotypes of male coho salmon, Oncorhynchus kisutch Walbaum, which mature as either large fighters (age-3 'hooknoses') or small sneakers (age-2 'jacks'). Using over 20 years of data on coded-wire tagged fish released from nine Oregon hatcheries, we tested for the effects of ocean environment independent of those due to freshwater rearing. 3. Annual fluctuations of the abundance of jack and hooknose males within populations were correlated strongly by brood year (cohort) but not by return year (breeding group). This occurred independently of significant effects of release practice (i.e. the number of fish released, body size at release and date of release), indicating that a synchronized fluctuation in mortality during the first year at sea was the predominant cause. As a result, the annual frequency of the alternative phenotypes at breeding varied considerably within populations. 4. Spatial patterns in the annual fluctuations of the two phenotypes were similar (i.e. synchronous among populations), except that jacks showed local spatial structure (decreased synchrony with distance) not evident among hooknoses. This suggests that oceanic processes affecting the two phenotypes operate at different spatial scales. Despite effects on salmon abundance, the ocean environment had little influence through its effects on salmon growth on the relative frequencies of the alternative phenotypes within and among populations. 5. The results provide insight into the evolutionary dynamics of alternative phenotypes, including an intragenerational time lag that increases annual variability in phenotype frequencies at breeding (return years) and the significance of local freshwater processes, rather than oceanic processes, on phenotype expression. Freshwater processes, such as juvenile growth, timing of migration and breeding competition, operating at evolutionary and intragenerational time-scales, are probably the predominant forces affecting phenotype frequency.     

Microsite and elevation zone effects on seed pilferage,germination, and seedling survival during early whitebark pine recruitment

Tree recruitment is a spatially structured process that may undergo change over time because of variation in postdispersal processes. We examined seed pilferage, seed germination, and seedling survival in whitebark pine to determine whether 1) microsite type alters the initial spatial pattern of seed caches, 2) higher abiotic stress (i.e. higher elevations) exacerbates spatial distribution changes, and 3) these postdispersal processes are spatially clustered. At two study areas, we created a seed distribution pattern by burying seed caches in microsite types frequently used by whitebark pine's avian seed disperser (Clark's nutcracker) in upper subalpine forest and at treeline, the latter characterized by high abiotic environmental stress. We monitored caches for two years for pilferage, germination, and seedling survival. Odds of pilferage (both study areas), germination (northern study area), and survival (southern study area) were higher at treeline relative to subalpine forest. At the southern study area, we found higher odds of 1) pilferage near rocks and trees relative to no object in subalpine forest, 2) germination near rocks relative to trees within both elevation zones, and 3) seedling survival near rocks and trees relative to no object at treeline. No microsite effects were detected at the northern study area. Findings indicated that the microsite distribution of seed caches changes with seed/seedling stage. Higher odds of seedling survival near rocks and trees were observed at treeline, suggesting abiotic stress may limit safe site availability, thereby shifting the spatial distribution toward protective microsites. Higher odds of pilferage at treeline, however, suggest rodents may limit treeline recruitment. Further, odds of pilferage were higher near rocks and trees relative to no object in subalpine forest but did not differ among microsites at treeline, suggesting pilferage can modulate the spatial structure of regeneration, a finding supported by limited clustering of postdispersal processes.     

Propagule pressure governs establishment of an invasive herb

The success of plant invasions may be limited by the availability of propagules and/or of suitable microsites, with microsite availability being affected by, for example, disturbance and interspecific competition. A mechanistic understanding of the contributions of propagule pressure and microsite limitation to plant invasions is therefore required to minimise future invasions. Here, we investigated the relative roles of propagule pressure, the availability of microsites, and their interaction on the establishment of an invasive herb, Lupinus polyphyllus, in two geographic regions representing different climate and growth conditions in Finland (a more productive southern region and a harsher central region). We carried out a field experiment in 14 L. polyphyllus populations, in which we manipulated both propagule pressure and disturbance. In a complementary greenhouse experiment, we manipulated propagule pressure and interspecific competition. Seedling establishment of L. polyphyllus was higher in the more productive southern region than in the harsher central region. The number of L. polyphyllus seedlings increased with increasing propagule pressure regardless of disturbance or interspecific competition. However, the number of L. polyphyllus seedlings per sown seed (relative establishment) tended to decrease with increasing propagule pressure, indicating that the positive effect of propagule pressure on early invasion is partially counteracted by density-dependent mortality at high seed densities. Our results highlight the dominant role of propagule pressure over disturbance and interspecific competition in the establishment of L. polyphyllus, suggesting that the early stage of invasion is limited by the availability of propagules rather than the availability of suitable microsites.     

Differential effect of inter- and intraspecific competition on the performance of invasive and native Taraxacum species

Inter- and intraspecific competitive abilities are significant determinants of invasive success and the ecological impact of non-native plants. We tested two major hypotheses on the competitive ability of invasive species using invasive (Taraxacum officinale) and native (T. platycarpum) dandelions: differential interspecific competitive ability between invasive and native species and the kin recognition of invasive species. We collected seeds from two field sites where the two dandelion species occurred nearby. Plants were grown alone, with kin (plants from the same maternal genotype) or strangers (plants from different populations) of the same species, or with different species in a growth chamber, and the performance at the early developmental stage between species and treatments was compared. The invasive dandelions outcompeted the native dandelions when competing against each other, although no difference between species was detected without competition or with intraspecific competition. Populations of native species responded to interspecific competition differently. The effect of kinship on plant performance differed between the tested populations in both species. A population produced more biomass than the other populations when grown with a stranger, and this trend was manifested more in native species. Our results support the hypothesis that invasive plants have better competitive ability than native plants, which potentially contributes to the establishment and the range expansion of T. officinale in the introduced range. Although kin recognition is expected to evolve in invasive species, the competitive ability of populations rather than kinship seems to affect plant growth of invasive T. officinale under intraspecific competition.     

Spatial autocorrelation and the analysis of invasion processes from distribution data: a study with the crayfish <Emphasis Type="Italic">Procambarus clarkii</Emphasis>

Complex spatial dynamics are frequent in invasive species; analyzing distribution patterns can help to understand the mechanisms driving invasions. We used different spatial regression techniques to evaluate processes determining the invasion of the red swamp crayfish Procambarus clarkii. We evaluated four a priori hypotheses on processes that may determine crayfish invasion: landscape alteration, connectivity, wetland suitability for abiotic and biotic features. We assessed the distribution of P. clarkii in 119 waterbodies in a recently invaded area. We used spatially explicit statistical techniques (spatial eigenvector mapping, generalized additive models, Bayesian intrinsic conditional autoregressive models) within an information-theoretic framework to assess the support of hypotheses; we also analyzed the pattern of spatial autocorrelation of data, model residuals, and eigenvectors. We found strong agreement between the results of spatial eigenvector mapping and Bayesian autoregressive models. Procambarus clarkii was significantly associated with the largest, permanent wetlands. Additive models suggested also association with human-dominated landscapes, but tended to overfit data. The results indicate that abiotic wetlands features and landscape alteration are major drivers of the species’ distribution. Species distribution data, residuals of ordinary least squares regression, and spatial eigenvectors all showed positive and significant spatial autocorrelation at distances up to 2,500 m; this may be caused by the dispersal ability of the species. Our analyses help to understand the processes determining the invasion and to identify the areas most at risk where screening and early management efforts can be focused. The comparison of multiple spatial techniques allows a robust assessment of factors determining complex distribution patterns.     

Timing of disturbance alters competitive outcomes and mechanisms of coexistence in an annual plant model

Ecological disturbance is inherently a multi-faceted phenomenon; disturbance events can differ in distinct quantifiable aspects, such as intensity, duration, spatial extent, and time since last disturbance. Though effects of disturbance timing (specifically, time within a season) have been investigated empirically, theoretical work is lacking, in part because effects of disturbance may depend on the timing relative to the life cycle of the species in question. To demonstrate a theoretical basis for the effects of timing, we develop a model of annual plants subject to soil disturbance. We show that timing of disturbance can have significant effects on community composition. In addition, we quantify the mechanisms of coexistence acting under different timing regimes and show that differences in timing lead to different coexistence mechanisms. Specifically, we find that early disturbance (which enhances germination from the seed bank) generates the storage effect, whereas coexistence under late disturbance (which reduces adult fecundity and contributions to the soil seed bank) depends more on relative nonlinearity of competition. We discuss these two distinct mechanisms within the context of the underlying ecological processes, and we also briefly consider the broader implications of our analyses for disturbance timing in real communities. Our findings extend ecological disturbance theory by linking timing to specific competitive outcomes and can be applied to a wide range of disturbance-prone communities. Because we identify the underlying mechanisms resulting from different disturbance timings, our results can potentially inform theory for conservation and invasive species management practice.     

子午岭三种生境下辽东栎幼苗定居限制

辽东栎(Quercus wutaishanica)是子午岭地区的乡土乔木树种,也是该地区气候顶极群落的建群种,其幼苗的补充更新影响着森林群落的结构及物种组成。在3种典型生境(辽东栎林、人工油松林、灌草丛)中,设置3因素(种子、干扰、遮荫)两水平的野外播种实验(随机区组设计),记录辽东栎幼苗出苗量,并监测幼苗同生群在3种生境中3a间的生长状况,以确定种子及微生境在辽东栎幼苗补充过程中的限制作用。播种实验样方大小30cm×30cm,共计216个样方。结果显示,在辽东栎林及油松林内,增加种子和干扰强度(去除枯落物),能引起出苗量和幼苗补充量的显著增加,且2种处理方式间存在交互作用,表明在郁闭林冠下,辽东栎幼苗的补充受到了种子和微生境的双重限制,枯落物是导致微生境限制的主要因素之一。在灌草丛生境,各种处理方式均不能增加幼苗的补充量,表明辽东栎无法在开阔生境(强光照、干旱)中完成实生幼苗的补充更新。3种生境中的幼苗同生群生存分析表明,辽东栎幼苗在森林群落中存活率显著高于灌草丛群落。根据幼苗生长指标判断,在3种生境中,人工油松林是辽东栎幼苗定居的最佳场所。与实验预期相反,灌木对辽东栎幼苗的补充无显著影响。     

A model to assess relationships between forest dynamics and spatial structure

Abstract. A spatially explicit model was developed to study the relationships between the dynamics and spatial structure of forest stands. The objective was to test whether tree spatial structure can be used as an indicator of stand dynamics. The model simulates the growth, mortality and recruitment of trees in a multi‐specific and uneven‐aged stand. It includes deterministic and stochastic processes so that repeated simulations do not lead to the same stand but provide several possible results for a given dynamic (defined by a set of parameters). Second‐order neighbourhood analyses were used to characterize the resulting spatial structures. They showed a high variability for a given set of parameters. Only the main trends in the spatial structure can be interpreted. Sensitivity analyses, concerning the influence of competition on spatial structure, showed that in heterogeneous stands confounding effects can hinder the interpretation of the spatial structure if all the trees are considered. The spatial structure of the canopy trees alone proved easier to interpret as it is directly linked to post recruitment competition. Inference on the dominant modality of competition (one‐sided or two‐sided) based on the spatial structure proved difficult.     

Limited effects of early snowmelt on plants,decomposers, and soil nutrients in Arctic tundra soils

In addition to warming temperatures, Arctic ecosystems are responding to climate change with earlier snowmelt and soil thaw. Earlier snowmelt has been examined infrequently in field experiments, and we lack a comprehensive look at belowground responses of the soil biogeochemical system that includes plant roots, decomposers, and soil nutrients. We experimentally advanced the timing of snowmelt in factorial combination with an open‐top chamber warming treatment over a 3‐year period and evaluated the responses of decomposers and nutrient cycling processes. We tested two alternative hypotheses: (a) Early snowmelt and warming advance the timing of root growth and nutrient uptake, altering the timing of microbial and invertebrate activity and key nutrient cycling events; and (b) loss of insulating snow cover damages plants, leading to reductions in root growth and altered biological activity. During the 3 years of our study (2010–2012), we advanced snowmelt by 4, 15, and 10 days, respectively. Despite advancing aboveground plant phenology, particularly in the year with the warmest early‐season temperatures (2012), belowground effects were primarily seen only on the first sampling date of the season or restricted to particular years or soil type. Overall, consistent and substantial responses to early snowmelt were not observed, counter to both of our hypotheses. The data on soil physical conditions, as well interannual comparisons of our results, suggest that this limited response was because of the earlier date of snowmelt that did not coincide with substantially warmer air and soil temperatures as they might in response to a natural climate event. We conclude that the interaction of snowmelt timing with soil temperatures is important to how the ecosystem will respond, but that 1‐ to 2‐week changes in timing of snowmelt alone are not enough to drive season‐long changes in soil microbial and nutrient cycling processes.     

Density Effects on Plant Height Growth and Inequality in Sunflower Populations

Comparisons between competing and non-competing sunflower （Helianthus annuus L.） populations demonstrate pronounced effects of density on plant height growth, height-to-crown width ratio, and s popuiaUon＇s height inequality. In the present study, non-destructive measurements of height and the prolected crown area of sunflower plants were taken at seven times from emergence to fruit maturation in even-aged monospeclflc stands with initial densities of 1, 4, 16, and 64 plants/m^2. The mean height of populations Increased and then decreased with increasing population density; the height Inequalities of uncrowded populations decreased during stand growth, whereas the height inequaiiUes of crowded popuisUons decreased first and then increased during stand development. The interindlvidual relationships between the relative height growth rate and height within uncrowded populations became significantly negative during population growth, whereas these relationships were negative first and then became positive during the development of crowded populations. In the uncrowded populations, the static Interindlvldual relationship between height-to-crown width ratio and volume was positive, whereas for the crowded population these relationships became negative with increasing competition for light. The data suggest that the plastic responses of plant height and height-to-crown width ratio to light competition will become more Intense with increasing competition Intensity. The results of the present study argue strongly for the Importance of size-dependent Individual-level plastic responses due to size-asymmetric light competition In generating the variations in population height inequality.     

水杉人工林建植50年后的分化特征

通过种群统计学、格局分析和树干解析等对50 a生水杉人工林林分的分化特征进行了研究.结果表明:小个体林木占有相对多数(>50.4%);林木分化现象显著,特别是树高与材积的分化十分突出,林分已出现一定强度的自然稀疏;基于TSTRAT程式的林木分级结果表明,亚优势木占绝对优势(45.7%),其树冠构成该林分的主林冠;竞争指数依次为优势木级<亚优势木级<中庸木级<被压木级,各级的基尼系数基本上低于全林总计;优势木绝对生长率和相对生长率在林分成熟期以前均大于标准木,且胸径速生期持续时问较标准木长;林分高度生长受压抑程度低于胸径生长;林木分布格局总体呈均匀型,与造林初始基本一致;优势木和被压木为聚集分布,中庸木和枯死木为随机散布,亚优势木的分布型则介于随机和聚集之间.林木种群分化既表现在个体大小的不均等性上,也表现在空间分布的非匀质性上.目前林分结构特征的主要成因可以归结为密度压力下的种内竞争.     

Changes in spatial point patterns of pioneer woody plants across a large tropical landslide

We assessed whether the relative importance of positive and negative interactions in early successional communities varied across a large landslide on Casita Volcano (Nicaragua). We tested several hypotheses concerning the signatures of these processes in the spatial patterns of woody pioneer plants, as well as those of mortality and recruitment events, in several zones of the landslide differing in substrate stability and fertility, over a period of two years (2001 and 2002). We identified all woody individuals with a diameter >1 cm and mapped them in 28 plots measuring 10 × 10-m. On these maps, we performed a spatial point pattern analysis using univariate and bivariate pair-correlation functions; g (r) and g₁₂ (r), and pairwise differences of univariate and bivariate functions. Spatial signatures of positive and negative interactions among woody plants were more prevalent in the most and least stressful zones of the landslide, respectively. Natural and human-induced disturbances such as the occurrence of fire, removal of newly colonizing plants through erosion and clearcutting of pioneer trees were also identified as potentially important pattern-creating processes. These results are in agreement with the stress-gradient hypothesis, which states that the relative importance of facilitation and competition varies inversely across gradients of abiotic stress. Our findings also indicate that the assembly of early successional plant communities in large heterogeneous landslides might be driven by a much larger array of processes than previously thought.     

Testing Bergmann's rule and the resource seasonality hypothesis in Malagasy primates using GIS-based climate data

We tested four major hypotheses on the ecological aspects of body mass variation in extant Malagasy strepsirrhines: thermoregulation, resource seasonality/scarcity, resource quality, and primary productivity. These biogeographic hypotheses focus on the ecological aspects of body mass variation, largely ignoring the role of phylogeny for explaining body mass variation within lineages. We tested the independent effects of climate and resource-related variables on variation in body mass among Malagasy primates using recently developed comparative methods that account for phylogenetic history and spatial autocorrelation. We extracted data on lemur body mass and climate variables for a total of 43 species from 39 sites. Climatic data were obtained from the WorldClim database, which is based on climate data from weather stations compiled around the world. Using generalized linear models that incorporate parameters to account for phylogenetic and spatial autocorrelation, we found that diet and climate variables were weak predictors of lemur body mass. Moreover, there was a strong phylogenetic effect relative to the effects of space on lemur body mass in all models. Thus, we failed to find support for any of the four hypotheses on patterns of geography and body mass in extant strepsirrhines. Our results indicate that body mass has been conserved since early in the evolutionary history of each genus, while species diversified into different environmental niches. Our findings are in contrast to some previous studies that have suggested resource and climate related effects on body mass, though these studies have examined this question at different taxonomic and/or geographic scales.     

Ontogenetic shifts in plant–plant interactions in a rare cycad within angiosperm communities

Gymnosperms and angiosperms can co-occur within the same habitats but key plant traits are thought to give angiosperms an evolutionary competitive advantage in many ecological settings. We studied ontogenetic changes in competitive and facilitative interactions between a rare gymnosperm (Dioon sonorense, our target species) and different plant and abiotic neighbours (conspecific-cycads, heterospecific-angiosperms, or abiotic-rocks) from 2007 to 2010 in an arid environment of northwestern Mexico. We monitored survival and growth of seedlings, juveniles, and adults of the cycad Dioon sonorense to evaluate how cycad survival and relative height growth rate (RHGR) responded to intra- and interspecific competition, canopy openness, and nearest neighbour. We tested spatial associations among D. sonorense life stages and angiosperm species and measured ontogenetic shifts in cycad shade tolerance. Canopy openness decreased cycad survival while intraspecific competition decreased survival and RHGR during early ontogeny. Seedling survival was higher in association with rocks and heterospecific neighbours where intraspecific competition was lower. Shade tolerance decreased with cycad ontogeny reflecting the spatial association of advanced stages with more open canopies. Interspecific facilitation during early ontogeny of our target species may promote its persistence in spite of increasing interspecific competition in later stages. We provide empirical support to the long-standing assumption that marginal rocky habitats serve as refugia from angiosperm competition for slow-growing gymnosperms such as cycads. The lack of knowledge of plant–plant interactions in rare or endangered species may hinder developing efficient conservation strategies (e.g. managing for sustained canopy cover), especially under the ongoing land use and climatic changes.     

微立地土壤水分—物理性质差异及对水曲柳幼林生长的影响

采用较小立地尺度对土壤水分物理性质的变化及与水曲柳生长的关系进行研究。结果表明,除传统尺度立地上土壤水分－物理性质存在较大差异外,该尺度立地内部微立地间亦存在较大的变化,一些指标如非毛管孔隙度、土壤通气度和土壤渗透系数等在微立地间的变化甚至超出立地间的变化,土壤水分－物理性质的差异,使得不同微立地间水曲柳生长存在较大差异,同一立地内不同微立地间林木生长差异明显高于不同立地间的差异,土壤水分－物理性质及林木生长受到局部地段微立地的影响,微立地的研究对进一步实现适地适树具有重要意义。     

The Processes of Height-rank Determination Among Individuals and Neighbourhood Effects inChenopodium albumL. Stands

The height ofChenopodium albumL. plants grown in monocultureat three different densities was followed throughout the growingseason to examine size-rank determination processes with specialreference to the effects of neighbourhood conditions. Changesin height rank of plants in the stands were assessed by therank correlation between final height and the height at eachmeasurement during the growing season. The height ranks of plantswere almost fixed 1–2 weeks after canopy closure whenthe stand height was 10–20% of final stand height, andfixation occurred earlier in the denser plot. At each measurement,the effects of neighbourhood were evaluated as the partial correlationcoefficient between height growth and neighbourhood index withheight held constant (r_GN.H), in which competitive asymmetrywas incorporated. During the early period of the growing season,r_GN.Hwasnon-significant or positive (plants with taller and/or closerneighbours elongated faster), indicating no local competition.Just after canopy closure,r_GN.Hbecame negative, indicating localcompetition. A plant's rank changed only in an initial shortperiod of the competition. Plants occupying the upper canopyof stands at the end of the growing season were distinguishedby greater height growth during the initial short period ofcompetition after canopy closure, although these plants werenot necessarily taller before the onset of local competition.These results suggest that the fate of a plant in a crowdedstand is determined in the early stage of stand development.Copyright1999 Annals of Botany Company Height growth, neighbourhood competition, local competition, height-rank of plants in population, size difference, asymmetric competition,Chenopodium albumL.     

Effects of historical demography and ecological context on spatial patterns of genetic diversity within foxtail pine (Pinus balfouriana; Pinaceae) stands located in the Klamath Mountains, California

The density and dispersion of individuals, nonequilibrium demographics, and habitat fragmentation all affect the magnitude and extent of spatial genetic structure within forest tree populations. Here, we investigate the link between historical demography and spatial genetic structure within ecologically contrasting stands of foxtail pine (Pinus balfouriana) in the Klamath Mountains of northern California. We defined two stand types a priori, based largely on differences in foxtail pine density and basal area, and for each type we sampled two stands. Population expansions, likely from Pleistocene bottlenecks, were detected in three of the four stands. The magnitude and extent of spatial autocorrelation among genotypes at five nuclear microsatellites differed dramatically among stands, with those having lower foxtail pine density exhibiting strong patterns of isolation by distance. Moran's I statistics were 7-fold higher for the first distance class (<25 m) in these stands relative to those observed in stands with higher foxtail pine density (I(25) = 0.14 vs. 0.02). We conclude that differences in spatial genetic structure between stand types are due to differences in ecological attributes that affected expansion from inferred bottlenecks.     

Sexual segregation in North American elk: the role of density dependence

We investigated how density‐dependent processes and subsequent variation in nutritional condition of individuals influenced both timing and duration of sexual segregation and selection of resources. During 1999–2001, we experimentally created two population densities of North American elk (Cervus elaphus), a high‐density population at 20 elk/km², and a low‐density population at 4 elk/km² to test hypotheses relative to timing and duration of sexual segregation and variation in selection of resources. We used multi‐response permutation procedures to investigate patterns of sexual segregation, and resource selection functions to document differences in selection of resources by individuals in high‐ and low‐density populations during sexual segregation and aggregation. The duration of sexual segregation was 2 months longer in the high‐density population and likely was influenced by individuals in poorer nutritional condition, which corresponded with later conception and parturition, than at low density. Males and females in the high‐density population overlapped in selection of resources to a greater extent than in the low‐density population, probably resulting from density‐dependent effects of increased intraspecific competition and lower availability of resources.     

Plant diversity in tropical forests: a review of mechanisms of species coexistence

Evidence concerning mechanisms hypothesized to explain species coexistence in hyper-diverse communities is reviewed for tropical forest plants. Three hypotheses receive strong support. Niche differences are evident from non-random spatial distributions along micro-topographic gradients and from a survivorship-growth tradeoff during regeneration. Host-specific pests reduce recruitment near reproductive adults (the Janzen-Connell effect), and, negative density dependence occurs over larger spatial scales among the more abundant species and may regulate their populations. A fourth hypothesis, that suppressed understory plants rarely come into competition with one another, has not been considered before and has profound implications for species coexistence. These hypotheses are mutually compatible. Infrequent competition among suppressed understory plants, niche differences, and Janzen-Connell effects may facilitate the coexistence of the many rare plant species found in tropical forests while negative density dependence regulates the few most successful and abundant species.