喜旱莲子草对同基因型邻体根系的表型可塑性: 入侵地和原产地的比较

植物对邻体根系的表型可塑性是指与无邻体对照相比, 即使个体平均可获取土壤资源相同, 在有邻体根系存在时植物也会改变根系生物量分配, 并影响其他功能性状和适合度。表型可塑性进化假说(evolution of plasticity hypothesis)认为外来植物在入侵地进化出了更强的表型可塑性。对该假说的验证多集中于外来植物对光照、水分、养分以及天敌等的可塑性进化, 但对邻体根系的可塑性在入侵植物中是否发生进化尚未见报道。我们采用同质园实验比较了喜旱莲子草(Alternanthera philoxeroides)入侵地(美国)和原产地(阿根廷)各5个基因型的适合度与功能性状对同基因型邻体根系的可塑性。结果表明: 喜旱莲子草的根冠比(P = 0.088)和比叶面积(P = 0.007)对同基因型邻体根系的可塑性在入侵地和原产地基因型间存在差异: 入侵地基因型在有邻体根系时根冠比和比叶面积增加, 而原产地基因型则相反。但是, 总生物量、贮藏根生物量、比茎长和分枝强度对邻体根系的可塑性在入侵地和原产地间没有显著差异。此外, 与分隔邻体根系相比, 同基因型邻体根系存在时总生物量(+9.9%)和贮藏根生物量(+13.9%)显著增加, 比茎长(-9.5%)显著降低。最后, 与原产地基因型相比, 总体上入侵地基因型的总生物量(+62.0%)和贮藏根生物量(+58.9%)增加, 比茎长(-28.5%)和分枝强度(-42.8%)降低。这些结果表明喜旱莲子草入侵地基因型与资源利用相关功能性状(如根冠比和比叶面积)对邻体根系的可塑性方向与原产地基因型相反; 但适合度和株型相关性状(如比茎长和分枝强度)对同基因型邻体根系的可塑性与原产地没有差异。     

Contrasting trait responses to latitudinal climate variation in two lineages of an invasive grass

Plants are expected to respond to global environmental change through shifts in functional traits and in their ranges. These shifts could alter productivity and interactions among species or genetic lineages, ultimately leading to changes in distributions and abundance. In particular, cosmopolitan species are predicted to increase growth with decreasing latitude due to differences in climate and temperature. The pattern of changes in growth may vary among genotypes within species, leading to different responses with latitude. To evaluate whether climate can affect geographically distinct genotypes of cosmopolitan invasive species differently, we evaluated the trait responses of two lineages of the common reed, Phragmites australis, to variation in environmental conditions spanning North America’s Atlantic coast. Using three reciprocal transplant common gardens, we tested for the effects of garden location and plant lineage on traits related to biomass production, flowering frequency, leaf morphology, and leaf-level physiology. We found that aboveground biomass, stem density, and flowering frequency responded non-linearly to increasing latitude in one or both lineages. These results suggest that measures of plant traits over narrow latitudinal ranges may not accurately reflect organismal-level responses to global change at broad spatial scales. Given the responses to latitude that we observed in P. australis, we propose that feedbacks between growth and reproductive rate will influence range shifts in these two lineages. Such range shifts could lead to genetic admixtures, subsequently yielding more productive, locally-adapted genotypes.     

EXPERIMENTAL STUDIES OF THE EVOLUTIONARY SIGNIFICANCE OF SEXUAL REPRODUCTION. IV. EFFECT OF NEIGHBOR RELATEDNESS AND APHID INFESTATION ON SEEDLING PERFORMANCE

To investigate the effect of neighbor relatedness in seedling populations, propagules of six Anthoxanthum odoratum parents, produced from a reciprocal diallel cross, were planted into the parental source population, a mown field. The propagules were either surrounded by four unrelated neighbors in a 1 cm square, four sibling neighbors, or no neighbors. About 45% of the emerging seedlings were attacked by aphids (Schizaphis graminum). Aphid infestation significantly reduced seedling survivorship and did not differ with parental genotype or neighbor relatedness; plants without neighbors had a slightly higher infestation risk. Plants without neighbors had lower survivorship than those surrounded by unrelated neighbors although this difference was only significant for plants infested by aphids. When infested by aphids, plants surrounded by siblings had lower survivorship than plants surrounded by non-relatives, suggesting the operation of minority advantage. These results are consistent with the frequency-dependent selection hypothesis for the evolution and maintenance of sexual reproduction.     

Using upper boundary constraints to quantify competitive response of desert annuals

Using a target-neighborhood approach with six annual dicot species in the Negev Desert of Israel, we tested whether neighborhood biomass constrained the upper limit of plant performance and if the slope of the upper boundary was correlated to species trait means, such as relative growth rate (RGR) and seed mass. Target individuals were measured in early spring and then collected at the onset of the dry season along with all naturally occurring neighbors within a 5-cm radius of the target. Using a minimum of 50 samples for each of the six target species, we found no significant relationships between target performance and either density or aboveground biomass of neighbors, when including all target individuals in the regressions. However, aboveground neighbor biomass did influence the maximum potential size of targets. When regressions were restricted to the largest target plant within classes of neighbor biomass, significant negative relationships were found for all six species, with neighbor biomass explaining 55 to 94% of the variance in maximum target biomass. The slopes of the regressions were used as an index of competitive response. The correlations between competitive response and estimates of species traits were not significant with the possible exception that species with lower RGRmax may be better response competitors (i.e. were less sensitive to competition). These findings indicate the usefulness of the boundary regression technique for describing competitive interactions among neighbors, particularly in low productivity environments.     

Genetic and Environmental Controls on the Establishment of the Invasive Grass, Phalaris arundinacea

Whether an exotic species becomes integrated into a community or aggressively takes it over depends upon many interacting factors. Using contextual analyses, we combined genetic data about an invasive plant with information about the neighboring species, the community, and the environment to determine what factors enable a genotype or species to invade. We transplanted 50 individuals of each of three clones of the invasive grass Phalaris arundinacea, reed canary grass, into 150 random locations within a Vermont pasture. For each individual, we recorded clonal identity, neighbor identity, community indices (species richness and species diversity), and an environmental variable (soil moisture). The response variables were survivorship, above-ground biomass, below-ground biomass and the ratio of above- to below-ground biomass. Clonal identity affected both survivorship and below-ground biomass. The fastest tillering clone had poor survivorship but survivors produced a large amount of below-ground biomass, making this clone more likely to successfully overwinter. Neighbor species affected above- and below-ground biomass. Reed canary grass produced more above- and below-ground biomass when Anthoxanthum odoratum, a common pasture grass species, was abundant. Community attributes also influenced growth. Although we expected diverse plots to repel the invasion, plants in the more diverse plots had higher amounts of below-ground biomass. Finally, environmental effects also influenced growth. Reed canary grass produced more above-ground biomass in wetter plots, confirming that it does well under wet conditions.     

Investigating the relationship between neighbor root biomass and belowground competition: field evidence for symmetric competition belowground

Little is known about how small-scale variation in neighbor biomass can influence the strength of root competition experienced by an individual plant. In this study, modified root exclusion tubes were used to vary the accessibility of the soil space surrounding Amaranthus retroflexus target plants to the neighboring plants. A gradient of root accessibility was created by drilling varying numbers of holes into standard root exclusion tubes, made of 15 cm diameter PVC pipe. Belowground competitive intensity, defined as biomass reduction due to root interactions alone, relative to growth in the absence of neighbors, was then measured along the resulting gradient of neighbor root densities. At low neighbor root abundances the strength of belowground competition was proportional to neighbor root biomass, consistent with prior evidence that belowground competition is symmetric. If belowground competition were asymmetric, neighbor roots should have had little effect on target plants when they are rare relative to those of the target plant. At higher neighbor root abundances, belowground competitive intensity should increase rapidly. The strong relationship found between neighbor root biomass and belowground competitive intensity suggests relatively small variations in root biomass could lead to large variations in belowground competition. Reduced belowground competition in areas with low root biomass could have important implications for the establishment and growth of poor belowground competitors, suggesting a mechanism by which species coexistence may occur despite extremely intense root competition.     

Community traits affect plant–plant interactions across climatic gradients

Plant abundances and demography often vary along gradients of environmental stress, and neighboring plants can amplify or diminish such variation. We asked to what degree the effects of neighboring plants on a focal species can be explained by the traits and abundances of species in the surrounding community. We studied a common understory herb, Trientalis latifolia, across climatic gradients created by topography in the Siskiyou Mountains, southwestern Oregon. We compared Trientalis fitness along these gradients with and without neighbor removal, and asked whether the effects of neighboring plants could be predicted by their community‐weighted trait values and abundances. Environmental conditions alone did not explain whether neighbors had competitive or facilitative effects on Trientalis. However, the environment interacted with neighbor traits and biomass to influence neighbor effects: at cool, higher elevations, high neighbor biomass was associated with stronger facilitative effects, while at warm, lower elevations, high neighbor dissimilarity from Trientalis was associated with stronger competitive effects. We suggest that covariation and interactions among environmental and community characteristics are key to understanding species performance along climatic gradients.     

植物竞争对3种移植树苗生长的影响

在植物幼苗生长过程中,总是受到包括地下根竞争在内的各种竞争影响。植物间的竞争主要同有效光辐射、水分和各种营养相关。当外来植物侵入森林群落时,可能受到群落中其它植物竞争的影响。该文通过移植尾叶桉(Eucalyptus urophylla)、大叶相思(Acacia auriculaeformis)两种外来种和本地种荷木(Schima superba)幼苗,挖沟排除根竞争和砍树创造林窗来排除地上竞争的野外实验,研究植物竞争对幼苗生长的影响。有根竞争时,荷木、尾叶桉和大叶相思幼苗的生物量和净初级生产力均小于没有根竞争时,可见地下根竞争对3种幼苗生长有抑制性影响。尾叶桉、大叶相思和荷木3种幼苗受到的总竞争强度分别是0.357 9、0.338 3和0.198 9,受到的地下根竞争强度则分别是0.104 3、0.053 04和0.118 8,受到的地上竞争强度则分别是0.285 1、0.277 0和0.090 85。尾叶桉和大叶相思两种幼苗受到的总竞争强度间的差异不显著,但都显著大于荷木;3种幼苗受到的地上竞争强度间的差异同总竞争强度情况相似;尾叶桉和荷木两种幼苗受到的地下根竞争强度间的差异不显著,但都显著大于大叶相思。地上竞争对阳性树种尾叶桉和大叶相思两种幼苗的生长影响大,而地下根竞争则对耐阴性强的荷木幼苗影响大。尾叶桉和大叶相思两种外来种幼苗受到的总竞争强度均大于本地种荷木幼苗,这反映了这两种外来树种侵入次生林这样的群落受到竞争影响大。     

不同性别邻体和土壤灭菌对青杨幼苗生物量的影响

尽管植物邻体关系受到广泛关注,但关于雌雄异株植物性别间邻体关系的研究还较少.本研究比较了雌、雄青杨幼苗生物量的积累对其邻株性别和土壤灭菌的响应差异,并分析了性别间的邻体效应关系及不同性别混栽下生物量的差异.结果表明:同性邻株模式降低了青杨雌株或雄株生物量的积累,而异性邻株模式在未灭菌条件下促进了雌株的生物量积累;土壤灭...     

Plasticity in above‐ and belowground resource acquisition traits in response to single and multiple environmental factors in three tree species

Functional trait plasticity is a major component of plant adjustment to environmental stresses. Here, we explore how multiple local environmental gradients in resources required by plants (light, water, and nutrients) and soil disturbance together influence the direction and amplitude of intraspecific changes in leaf and fine root traits that facilitate capture of these resources. We measured population‐level analogous above‐ and belowground traits related to resource acquisition, i.e. “specific leaf area”–“specific root length” (SLA–SRL), and leaf and root N, P, and dry matter content (DMC), on three dominant understory tree species with contrasting carbon and nutrient economics across 15 plots in a temperate forest influenced by burrowing seabirds. We observed similar responses of the three species to the same single environmental influences, but partially species‐specific responses to combinations of influences. The strength of intraspecific above‐ and belowground trait responses appeared unrelated to species resource acquisition strategy. Finally, most analogous leaf and root traits (SLA vs. SRL, and leaf versus root P and DMC) were controlled by contrasting environmental influences. The decoupled responses of above‐ and belowground traits to these multiple environmental factors together with partially species‐specific adjustments suggest complex responses of plant communities to environmental changes, and potentially contrasting feedbacks of plant traits with ecosystem properties. We demonstrate that despite the growing evidence for broadly consistent resource‐acquisition strategies at the whole plant level among species, plants also show partially decoupled, finely tuned strategies between above‐ and belowground parts at the intraspecific level in response to their environment. This decoupling within species suggests a need for many species‐centred ecological theories on how plants respond to their environments (e.g. competitive/stress‐tolerant/ruderal and response‐effect trait frameworks) to be adapted to account for distinct plant‐environment interactions among distinct individuals of the same species and parts of the same individual.     

Increased root oxygen uptake in pea plants responding to non-self neighbors

Recent studies have demonstrated that plants alter root growth and decrease competition with roots of the same individual (self); however, the physiological traits accompanying this response are still widely unknown. In this study, we investigated the effect of root identity on gas exchange in the model species pea (Pisum sativum L.). Split-root plants were planted so that each pot contained either two roots of the same plant (self) or of two different plants (non-self), and the responses of biomass, photosynthesis, and respiration were measured. The photosynthetic rate was not affected by the identity of the root neighbor. We found a reduction of leaf dark respiration by half, accompanied by an increase in nocturnal root respiration by 29 % in plants neighboring with non-self. The activity of the alternative oxidase (AOX) pathway increased when plants responded to non-self neighbors. The increased activity of AOX in plants responding to non-self indicates carbon imbalances in roots, possibly as a consequence of increased root exudation and communication between individuals. If such an effect occurs more widely, it may change the assumptions made for the quantity of respiration as used in carbon budget models.     

Small scale genotypic richness stabilizes plot biomass and increases phenotypic variance in the invasive grass Phalaris arundinacea

Aims We aim to understand how small-scale genotypic richness and genotypic interactions influence the biomass and potential invasiveness of the invasive grass, Phalaris arundinacea under two different disturbance treatments: intact plots and disturbed plots, where all the native vegetation has been removed. Specifically, we address the following questions (i) Does genotypic richness increase biomass production? (ii) Do genotypic interactions promote or reduce biomass production? (iii) Does the effect of genotypic richness and genotypic interactions differ in different disturbance treatments? Finally (iv) Is phenotypic variation greater as genotypic richness increases?Methods We conducted a 2-year common garden experiment in which we manipulated genotype richness using eight genotypes planted under both intact and disturbed conditions in a wetland in Burlington, Vermont (44°27′23″N, 73°11′29″W). The experiment consisted of a randomized complete block design of three blocks, each containing 20 plots (0.5 m 2) per disturbed treatment. We calculated total plot biomass and partitioned the net biodiversity effect into three components: dominance effect, trait-dependent complementarity and trait-independent complementarity. We calculated the phenotypic variance for each different genotype richness treatment under the two disturbance treatments.Important findings Our results indicate that local genotypic richness does not increase total biomass production of the invasive grass P. arundinacea in either intact or disturbed treatments. However, genotypic interactions underlying the responses showed very different patterns in response to increasing genotypic richness. In the intact treatment, genotypic interactions resulted in the observed biomass being greater than the predicted biomass from monoculture plots (e.g., overyielding) and this was driven by facilitation. However, facilitation was reduced as genotypic richness increased. In the disturbed treatment, genotypic interactions resulted in underyielding with observed biomass being slightly less than expected from the performance of genotypes in monocultures; however, underyielding was reduced as genotypic richness increased. Thus, in both treatments, higher genotypic richness resulted in plot biomass nearing the additive biomass from individual monocultures. In general, higher genotypic richness buffered populations against interactions that would have reduced biomass and potentially spread. Phenotypic variance also had contrasting patterns in intact and disturbed treatments. In the intact treatment, phenotypic variance was low across all genotypic richness levels, while in the disturbed treatment, phenotypic variance estimates increased as genotypic richness increased. Thus, under the disturbed treatment, plots with higher genotypic richness had a greater potential response to selection. Therefore, limiting the introduction of new genotypes, even if existing genotypes of the invasive species are already present, should be considered a desirable management strategy to limit the invasive behavior of alien species.     

Field tests of density-and frequency-dependent selection in Erigeron annuus (Compositae)

Selection may maintain genetic diversity in natural populations if the physical or biotic environment is variable over space and-or time. Because density and genotype frequencies can be heterogeneous, and because genotypes may differ in competitive ability, both density-and frequency-dependent selection have been considered to be potentially important evolutionary processes. To address the possibility that intraspecific interactions among plants are a source of fitness variation in Erigeron annuus, we conducted field experiments over 2 yr that were designed to examine the potential of population density, genotype frequency, and their interaction to act as selective agents. In both experiments, apomictic genotypes of Erigeron were paired. Seedlings were planted into plots that differed in density and the identity of minority and majority genotype. There was evidence for a differential effect of density among genotypes for only one year's experiment, suggesting that density-dependent selection is either weak or temporally variable. Genotype frequency had no effect on fitness in either year, and thus there was no evidence for frequency-dependent selection. In addition, the lack of a frequency ;ts density interaction demonstrates that resource partitioning, one mechanism for frequency dependence, is not strong among Erigeron genotypes. If frequency-dependent selection does occur in this species, it is either too weak to detect even in large field experiments, or occurs only in the presence of a selective agent (e.g., pathogens) that was lacking in our experiments.     

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.     

Nearest-neighbor tree species combinations in tropical forest: the role of chance, and some consequences of high diversity

In three permanent inventory plots comprising 12.4 ha of undisturbed forest at La Selva, Costa Rica, all stems ≥10 cm dbh were mapped and identified to species. There were 1628, 1478 and 1954 trees in the plots, representing 168, 166 and 171 species respectively. We determined the species of each nearest-neighbor pair of trees, and asked whether the occurrence of species pairs conforms to a simple random mixing model. If trees are randomly mixed in terms of species, the expected frequency of any nearest neighbor species combination is a function of the relative abundance of the two species. Departures from random mixing could arise from species interactions, differential responses to habitat, or both. The number of possible ij species combinations increases approximately as the square of the number of species. For the 168 species in plot 1, for example, there are 14 196 possible combinations. We compared the expected frequency of each species combination in the three plots (42 736 combinations in all) with observed frequencies. Over 98% of the combinations had observed frequencies of zero and expected frequencies close to zero. A consequence of high diversity is low density of most individual species, and exceedingly low frequencies of the vast majority of species combinations. For each of the 805 combinations with observed frequencies >0, we used simulation to generate a distribution of expected frequencies. We used a t-test to compare the observed frequency with the mean of the simulated distribution for each combination. Only 40 combinations (0.09% of the possible species combinations in the plots) departed from expected frequencies; 39 combinations were more common, and one less common than expected. The overwhelming majority of nearest neighbor species combinations occur at frequencies predictable from their individual abundances.     

Competition intensity along a productivity gradient in a low-diversity grassland

Competition among plants often shifts from roots to shoots as productivity increases and species composition changes. We examined competition in an old field with low diversity to test whether this shift occurred along a productivity gradient without species turnover. Forty plots received one of four nitrogen treatments (0, 5, or 15 g added N m-2 yr-1 or 400 g m-2 yr-1 of sawdust added to immobilize N) annually for 5 yr. All N levels were dominated by the perennial grasses Agropyron cristatum and Bromus inermis. Transplants of Agropyron were grown with all neighbors, roots of neighbors, or no neighbors present to measure total, root, and shoot competition. Transplant growth was 22%-165% higher in subplots without neighbors present, which indicates that competition occurred. Competition from neighbor roots was primarily responsible for suppression of transplant growth over the entire productivity gradient. In contrast to previous field experiments that found either an increase in total competition intensity or a shift from root to shoot competition with increasing productivity, we found neither. Increases in total competition intensity or shifts from root to shoot competition found along other gradients may be caused by changes in species composition and not by increased resources or neighbor biomass. These results suggest that different competitive mechanisms may operate in low-diversity vegetation than in more diverse natural vegetation.     

Root allocation and foraging precision in heterogeneous soils

Root growth patterns respond to small-scale resource heterogeneity and the presence of roots of neighboring plants, but how a plant integrates its responses to these cues is not well understood. In the presence of neighbors, plants may shift allocation to roots as a consequence of plant size and root:shoot allometry, as a response to resource depletion by neighbors, or through a direct response to neighbor presence. The same response pathways also have the potential to alter proliferation in resource-rich patches in soil.Four species of grassland plants were grown in the greenhouse as single plants, monocultures, and mixtures. Root length allocation as a function of shoot mass was examined for background soil and fertilized patches. Plants grown with same-species neighbors followed the same allometric trajectory as single plants for root length in background soil, so any change in root allocation was due only to reduced plant size. Root proliferation in patches declined with neighbors, consistent with a response to resource depletion. Mixtures overproduced roots in both background soil and in patches, relative to plants of the same size in monocultures.     

Local biotic adaptation of trees and shrubs to plant neighbors

Natural selection as a result of plant–plant interactions can lead to local biotic adaptation. This may occur where species frequently interact and compete intensely for resources limiting growth, survival, and reproduction. Selection is demonstrated by comparing a genotype interacting with con‐ or hetero‐specific sympatric neighbor genotypes with a shared site‐level history (derived from the same source location), to the same genotype interacting with foreign neighbor genotypes (from different sources). Better genotype performance in sympatric than allopatric neighborhoods provides evidence of local biotic adaptation. This pattern might be explained by selection to avoid competition by shifting resource niches (differentiation) or by interactions benefitting one or more members (facilitation). We tested for local biotic adaptation among two riparian trees, Populus fremontii and Salix gooddingii, and the shrub Salix exigua by transplanting replicated genotypes from multiple source locations to a 17 000 tree common garden with sympatric and allopatric treatments along the Colorado River in California. Three major patterns were observed: 1) across species, 62 of 88 genotypes grew faster with sympatric neighbors than allopatric neighbors; 2) these growth rates, on an individual tree basis, were 44, 15 and 33% higher in sympatric than allopatric treatments for P. fremontii, S. exigua and S. gooddingii, respectively, and; 3) survivorship was higher in sympatric treatments for P. fremontii and S. exigua. These results support the view that fitness of foundation species supporting diverse communities and dominating ecosystem processes is determined by adaptive interactions among multiple plant species with the outcome that performance depends on the genetic identity of plant neighbors. The occurrence of evolution in a plant‐community context for trees and shrubs builds on ecological evolutionary research that has demonstrated co‐evolution among herbaceous taxa, and evolution of native species during exotic plants invasion, and taken together, refutes the concept that plant communities are always random associations.     

Similar non-random processes maintain diversity in two tropical rainforests.

Quadrat-based analysis of two rainforest plots of area 50 ha, one in Panama (Barro Colorado Island, BCI) and the other in Malaysia (Pasoh), shows that in both plots recruitment is in general negatively correlated with both numbers and biomass of adult trees of the same species in the same quadrat. At BCI, this effect is not significantly influenced by treefall gaps. In both plots, recruitment of individual species is negatively correlated with the numbers of trees of all species in the quadrats, but not with overall biomass. These observations suggest, but do not prove, widespread frequency-dependent effects produced by pathogens and seed-predators that act most effectively in quadrats crowded with trees. Within-species correlations of mortality with numbers or biomass are not found in either plot, indicating that most frequency-dependent mortality takes place before the trees reach 1 cm in diameter. Stochastic effects caused by BCI's more rapid tree turnover may contribute to a larger variance in diversity from quadrat to quadrat at BCI, although they are not sufficient to explain why BCI has fewer than half as many tree species as Pasoh. Finally, in both plots quadrats with low diversity show a significant increase in diversity over time, and this increase is stronger at BCI. This process, like the frequency-dependence, will tend to maintain diversity over time. In general, these non-random forces that should lead to the maintenance of diversity are slightly stronger at BCI, even though the BCI plot is less diverse than the Pasoh plot.     

Functional traits determine tree growth and ecosystem productivity of a tropical montane forest: Insights from a long‐term nutrient manipulation experiment

Trait‐response effects are critical to forecast community structure and biomass production in highly diverse tropical forests. Ecological theory and few observation studies indicate that trees with acquisitive functional traits would respond more strongly to higher resource availability than those with conservative traits. We assessed how long‐term tree growth in experimental nutrient addition plots (N, P, and N + P) varied as a function of morphological traits, tree size, and species identity. We also evaluated how trait‐based responses affected stand scale biomass production considering the community structure. We found that tree growth depended on interactions between functional traits and the type or combination of nutrients added. Common species with acquisitive functional traits responded more strongly to nutrient addition, mainly to N + P. Phosphorous enhanced the growth rates of species with acquisitive and conservative traits, had mostly positive effects on common species and neutral or negative effects in rare species. Moreover, trees receiving N + P grew faster irrespective of their initial size relative to trees in control or to trees in other treatment plots. Finally, species responses were highly idiosyncratic suggesting that community processes including competition and niche dimensionality may be altered under increased resource availability. We found no statistically significant effects of nutrient additions on aboveground biomass productivity because acquisitive species had a limited potential to increase their biomass, possibly due to their generally lower wood density. In contrast, P addition increased the growth rates of species characterized by more conservative resource strategies (with higher wood density) that were poorly represented in the plant community. We provide the first long‐term experimental evidence that trait‐based responses, community structure, and community processes modulate the effects of increased nutrient availability on biomass productivity in a tropical forest.