Neighbor Removal Research Articles

Invasion away from roadsides was not driven by adaptation to grassland habitats in Dittrichia graveolens (stinkwort)

Invasive plants along transportation corridors can significantly threaten ecosystems and biodiversity if they spread beyond anthropogenic environments. Rapid evolution may increase the ability of invading plant populations to establish in resident plant communities over time, posing a challenge to invasion risk assessment. We tested for adaptive differentiation in Dittrichia graveolens (stinkwort), an invasive species of ruderal habitat in California that is increasingly spreading away from roadsides into more established vegetation. We collected seeds from eight pairs of vegetated sites and their nearest (presumed progenitor) roadside population. We assessed differentiation between populations in roadside and vegetated habitat for germination behavior and for response to competition in a greenhouse experiment. We also tested for increased performance in vegetated habitat with a grassland field experiment including a neighbor removal treatment. Germination rates were slightly reduced in seeds from vegetated sites, which may indicate lower seed viability. Otherwise, plants did not show consistent differences between the two habitat types. Competition strongly reduced performance of D. graveolens in both the greenhouse and in the field, but plants originating from vegetated sites did not show enhanced competitive ability. Our findings show no evidence of adaptive differentiation between D. graveolens populations from roadside and vegetated habitats to date, suggesting that invasiveness in grasslands has not been enhanced by rapid evolution in the 40 + years since this species was introduced to California. Evolutionary constraints or potentially high levels of gene flow at this small scale may limit adaptation to novel habitats along roadsides.

Individual vital rates respond differently to local‐scale environmental variation and neighbour removal

Abstract Understanding how plant fitness varies along natural gradients is critical for predicting responses to environmental change. However, individual vital rates are often used as fitness proxies without knowing how other vital rates vary along the same gradients. We investigated how canopy cover, plant–plant interactions, water availability and soil properties influenced the emergence, survival, seed production and population growth rates of eight annual plant species in semi‐arid Western Australia. We sowed seeds into sun‐exposed and shaded blocks across a reserve, removed all neighbouring plants from half of the interaction neighbourhoods, and used rainout shelters to reduce and increase precipitation relative to ambient plots. Canopy cover had strong negative effects on emergence, but few direct impacts on other vital rates and population growth rates. Direct competitive effects on survival and seed production were rare, although evident for population growth rate for 3/8 species. Competition was stronger in open than shaded plots for half of the species. Canopy cover also interacted with the watering treatment to influence survival of half of the species, but watering alone had few direct impacts on species' vital and population growth rates. We found only positive significant correlations between pairs of rates, and survival and seed production were far more frequently correlated with population growth rate than emergence. Synthesis. Our study illustrates that vital rates can respond to the same local‐scale environmental variation in different ways that are likely not driven by life history trade‐offs. We caution against using emergence as a proxy for population growth rate and emphasise that no single vital rate was a reliable fitness proxy overall. Interactions among abiotic and biotic factors were important drivers of vital and population growth rates for some species, highlighting the need to account for plant–plant interactions when predicting population responses to environmental change.

Large-Scale Predictions of Compound Potency with Original and Modified Activity Classes Reveal General Prediction Characteristics and Intrinsic Limitations of Conventional Benchmarking Calculations.

Predicting compound potency is a major task in computational medicinal chemistry, for which machine learning is often applied. This study systematically predicted compound potency values for 367 target-based compound activity classes from medicinal chemistry using a preferred machine learning approach and simple control methods. The predictions produced unexpectedly similar results for different classes and comparably high accuracy for machine learning and simple control models. Based on these findings, the influence of different data set modifications on relative prediction accuracies was explored, including potency range balancing, removal of nearest neighbors, and analog series-based compound partitioning. The predictions were surprisingly resistant to these modifications, leading to only small error margin increases. These findings also show that conventional benchmark settings are unsuitable for directly comparing potency prediction methods.

Does stress alleviation always intensify plant-plant competition? A case study from alpine meadows with simulation of both climate warming and nitrogen deposition

The effects of neighboring vegetation are important for plant fitness and community assembly under global change. What is less clear is how plant-plant interactions would change along the stress gradient. Our aim is to test the hypothesis that the abiotic stress alleviation induced by nitrogen (N) deposition and climate warming intensified the competitive effects of neighboring vegetation on two degradation indicator species of alpine meadows (i.e., Potentilla anserine and Ligularia virgaurea), as predicted by the popular “stress-gradient hypothesis” (SGH). A neighbor removal experiment was conducted with either N addition or artificial warming as treatments at both an alpine mesophytic meadows site (MESms) and a semi-hydric marsh meadows site (SHMms) on the southeastern Tibetan Plateau. The net neighbor effects on both P. anserine and L. virgaurea were extreme and moderate competition in MESms, respectively, but were mild facilitation in SHMms. In line with the SGH, both N addition and warming intensified the neighbor competitive effects on L. virgaurea in MESms and weakened the neighbor facilitative effects on L. virgaurea in SHMms. However, in contrast to the SGH, the neighbor competitive effects on P. anserine in MESms were weakened by both N addition and warming. Furthermore, the neighbor facilitative effects on P. anserine in SHMms were weakened by N addition but were not changed by warming. These diverse plant interaction-environmental gradient relationships were likely induced by the joint effects of stress type, plant strategy in response to stress and natural intensity of neighbor effects. It may be helpful to take these factors into consideration for further refinement of the SGH.

Dissolution of β-C2S Cement Clinker: Part 1 Molecular Dynamics (MD) Approach for Different Crystal Facets.

A major concern in the modern cement industry is considering how to minimize the CO2 footprint. Thus, cements based on belite, an impure clinker mineral (CaO)2SiO2 (C2S in cement chemistry notation), which forms at lower temperatures, is a promising solution to develop eco-efficient and sustainable cement-based materials, used in enormous quantities. The slow reactivity of belite plays a critical role, but the dissolution mechanisms and kinetic rates at the atomistic scale are not known completely yet. This work aims to understand the dissolution behavior of different facets of β-C2S providing missing input data and an upscaling modeling approach to connect the atomistic scale to the sub-micro scale. First, a combined ReaxFF and metadynamics-based molecular dynamic approach are applied to compute the atomistic forward reaction rates (RD) of calcium (Ca) and silicate species of (100) facet of β-C2S considering the influence of crystal facets and crystal defects. To minimize the huge number of atomistic events possibilities, a generalized approach is proposed, based on the systematic removal of nearest neighbors’ crystal sites. This enables us to tabulate data on the forward reaction rates of most important atomistic scenarios, which are needed as input parameters to implement the Kinetic Monte Carlo (KMC) computational upscaling approach. The reason for the higher reactivity of the (100) facet compared to the (010) is explained.

Climate change weakens the impact of disturbance interval on the growth rate of natural populations of Venus flytrap

AbstractDisturbances elicit both positive and negative effects on organisms; these effects vary in their strength and their timing. Effects of disturbance interval (i.e., the length of time between disturbances) on population growth will depend on both the timing and strength of positive and negative effects of disturbances. Climate change can modify the relative strengths of these positive and negative effects, leading to altered optimal disturbance intervals (the disturbance interval at which population growth rate is highest) and changes in the sensitivity of population growth rate to disturbance interval. While we know that climate may alter impacts of disturbance in some systems, we have a poor understanding of which effects of disturbance and which vital rates might drive an altered response to disturbance interval in a changing climate. We use demographic monitoring of natural populations of Dionaea muscipula, the Venus flytrap, that have experienced natural and managed fires, combined with realistic past and future climate projections, to construct climate‐ and fire‐driven integral projection models (IPMs). We use these IPMs to compare the effect of fire return interval (FRI) on population growth rate in past and future climates. To dissect the mechanisms driving FRI response, we then construct IPMs with demographic data from an experimental manipulation of fire effects (ash addition, neighbor removal) and an accidental fire. Our results show that an FRI of 10 years is optimal for D. muscipula in past climate conditions, but a longer FRI (12 years) is optimal in future climate conditions. Further, deviations from optimal FRI reduce population growth rate dramatically in the past climate, but this reduction is muted in a future climate (future minus past sensitivity = 0.006, 95% CI [0.002, 0.011]). Finally, our experimental work suggests that fire effects are driven in part by positive, additive effects of competitor removal and ash addition immediately following a fire; for one population, both these treatments significantly increased population growth rate. Our work suggests that climate change can alter the response of populations to disturbance, highlighting the need to consider the interacting effects of multiple abiotic drivers when projecting future population growth and geographical distributions.

LATE PLANTING SHORTENS THE FLOWERING PERIOD AND REDUCES FECUNDITY IN LASTHENIA CALIFORNICA

Modifications in the timing of life-history events can alter the biotic and abiotic environments experienced during an organism's lifetime. In plants, germination timing plays a critical role in relation to seasonal environmental conditions, pollinator availability, competitive dynamics, etc. Individuals can compensate for a change in timing of germination by modifying their growth and flowering time. However, biotic and abiotic factors may affect these compensatory responses. In this study we assess how biotic and abiotic differences due to planting date influence the timing of flowering, survival, and reproduction. To do this we manipulated germination timing of the annual California Goldfields (Lasthenia californica DC. ex Lindl., Asteraceae) in a serpentine grassland in northern California, by seeding three times during the growing season (November, January, and March). Neighbor removal plots were compared with control plots to examine influences of close neighbors and seasonal priority effects (due to early individuals pre-empting resources) on flowering time and duration, growth, and reproductive success (survival to reproduction and inflorescence production). Both planting date and neighbor removal treatments significantly impacted flowering time and duration, growth, and reproduction in this species. Later planting dates did delay flowering time, but this delay was minimal as flowering time was constrained within set biotic and abiotic boundaries. In addition, we find that a mixture of planting times and levels of neighbor removal can extend the duration of flowering on the landscape. Inflorescence production and survival declined with later planting dates, but neighbor removal counteracted this decline. We find that L. californica exhibits graded growth allocation, as well as plasticity in flowering time in response to planting date. This study has implications for the timing of restoration projects, as planting time influences both the timing of flowering as well as the overall reproductive success of planted individuals. Our results suggest that practitioners should aim to plant earlier in the season, but that neighbor removal may counteract some of the costs of late planting.

Resource availability drives plant–plant interactions of conifer seedlings across elevations under warming in Alaska

AbstractFuture warming may alter plant stress at high‐elevation treelines and forests, thereby changing plant–plant interactions. The relative importance of competition and facilitation may depend on the degree of resource or physical stress. According to the stress gradient hypothesis (SGH), physical stress on trees is more important at cold high elevations where facilitation predominates, and less important at low elevations where competition is the main interaction. Our goals were to investigate whether plant–plant interactions along elevational gradients corresponded to those predicted by the SGH, and to assess the effects of increasing temperatures on the growth of conifer seedlings in tundra–forest ecosystems in interior Alaska, USA. We established sites along two elevational gradients: one in tundra (four sites, 550–1170 m) and one in forest (three sites, 210–760 m). A field warming and neighbor removal experiment was conducted using transplanted seedlings of white spruce (Picea glauca). After three growing seasons, regardless of elevation, spruce seedling biomass was approximately 20% lower in seedlings with neighbor plants compared to those without neighbors. Therefore, there was no evidence for greater facilitation at high elevations across either tundra or forest elevational gradient. Seedlings in forest sites increased their shoot‐to‐root ratios and relative growth rates in height, suggesting competition for light, but this was not seen for seedlings growing in tundra sites. When warmed by greenhouses, seedling growth was stimulated in forests, especially at high elevations. In contrast, at tundra sites, warmed seedlings grew less than controls and the seedlings at high elevations showed water stress. Our results suggest that the resource stresses of low light and water availability are more important drivers of plant–plant interactions than the physical stress imposed by low temperature, possibly due to warming in the past 50 yr in this region. Further warming may increase growth of seedlings in forests, but is likely to decrease it in open tundra habitats, thereby slowing the upward movement of treeline in the near future.

Requirements for the spatial storage effect are weakly evident for common species in natural annual plant assemblages.

Coexistence in spatially varying environments is theorized to be promoted by a variety of mechanisms including the spatial storage effect. The spatial storage effect promotes coexistence when (1) species have unique vital rate responses to their spatial environment and, when abundant, (2) experience stronger competition in the environmental patches where they perform better. In a naturally occurring southwest Western Australian annual plant system, we conducted a neighbor removal experiment involving eleven focal species growing in high-abundance populations. Specifically, we measured species' fecundity across a variety of environmental gradients in both the presence and absence of neighbors. For the environmental variables that we measured, there was only limited evidence for species-specific responses to the environment, with a composite variable describing overstory cover and leaf litter cover being the best predictor of fecundity for a subset of focal species. In addition, although we found strong evidence for intraspecific competition, positive environment-competition covariance was only detected for one species. Thus, positive environment-competition covariance may not be as common as expected in populations of species growing at high abundance, at least when tested in natural assemblages. Our findings highlight the inherent limitations of using natural assemblages to study spatial coexistence mechanisms, and we urge empirical ecologists to take these limitations into account when designing future experiments.

Colonisation of the alpine tundra by trees: alpine neighbours assist late-seral but not early-seral conifer seedlings

ABSTRACT Background: Climate change is projected to alter the elevation and latitude of treeline globally. Seed germination and seedling survival are critical controls on treeline expansion. Neighbouring alpine plants, either through competition for resources or through altered microclimate, also affect seedlings emerging in the alpine zone. With warming, alpine plant species may interact with each other more or less strongly. Aims: To determine whether establishing tree seedlings and an alpine herb are similarly sensitive to alpine plant neighbours under ambient and altered climate. Methods: We imposed active heating, watering, and removed all plants adjacent to emerging conifer seedlings and an alpine herb. Results: Picea engelmannii seedlings showed lower survival compared with Pinus flexilis 3 weeks following neighbour removal, and after 1 year only survived in watered plots. Pinus seedlings responded to neighbour removal by lowering the quantum yield of photosynthesis (ϕPSII). Contrary to expectations from the stress gradient hypothesis, survival was reduced without neighbours near the low-elevation range limit of Chionophila jamesii. Conclusions: Pinus flexilis has higher expansion potential into the alpine, while Picea engelmannii requires moist conditions that could be facilitated by neighbours to expand its range. This implies likely range expansion by P. flexilis with consequences for alpine plant diversity and ecosystem function.

Geographic location, local environment, and individual size mediate the effects of climate warming and neighbors on a benefactor plant.

Predictions of plant responses to global warming frequently ignore biotic interactions and intraspecific variation across geographical ranges. Benefactor species play an important role in plant communities by protecting other taxa from harsh environments, but the combined effects of warming and beneficiary species on their performance have been largely unexamined. We analyzed the joint effects of elevated temperature and neighbor removal on the benefactor plant Silene acaulis, in factorial experiments near its low- and high-latitude range limits in Europe. We recorded growth, probability of reproduction and fruit set during 3 years. The effects of enhanced temperature were positive near the northern limit and negative in the south for some performance measures. This pattern was stronger in the presence of neighbors, possibly due to differential thermal tolerances between S. acaulis and beneficiary species in each location. Neighbors generally had a negative or null impact on S. acaulis, in agreement with previous reviews of overall effects of plant-plant interactions on benefactors. However, small S. acaulis individuals in the north showed higher growth when surrounded by neighbors. Finally, the local habitat within each location influenced some effects of experimental treatments. Overall, we show that plant responses to rising temperatures may strongly depend on their position within the geographic range, and on species interactions. Our results also highlight the need to consider features of the interacting taxa, such as whether they are benefactor species, as well as local-scale environmental variation, to predict the joint effects of global warming and biotic interactions on species and communities.

Stereotypic behaviours are heterogeneous in their triggers and treatments in the American mink, Neovison vison, a model carnivore

Stereotypic behaviours (SBs) are common in confined animals including captive Carnivora, which display diverse forms of SB: often whole-body movements (e.g. pacing), but also head-only movements (e.g. head twirling) and ‘scrabbling’ (scratching at enclosure boundaries). Although often pooled together, emerging evidence indicates that SBs are heterogeneous, suggesting that subtypes differ in their causes, triggers, and consequently treatments. In mink, a model carnivore, scrabbling seems to be elicited by neighbouring conspecifics. We tested this hypothesis via three studies of 32 males (individually caged in rows and separated by solid partitions). Study 1 investigated whether neighbour proximity affects the location of any SBs, and Study 2, whether removing neighbours reduces any SBs. Results revealed that although mink typically avoided proximity to their neighbours, scrabbling was uniquely directed towards neighbours who were close to the shared cage partition. It was also the only SB significantly elevated by having all-male neighbours, and reduced by removing neighbours. Study 3 then investigated whether environmental enrichment, a standard SB treatment, would reduce or abolish different SBs equally, to assess whether scrabbling is simply easier to alleviate than other SBs. Enrichment reduced all SB subtypes, but logistic regressions revealed that the odds of complete abolition were higher for whole-body and head-only SBs than for scrabbling. Overall, these naturally solitary carnivores thus seem to avoid conspecific proximity, but they specifically direct their stereotypic scrabbling at neighbours; and their scrabbling is reduced by neighbour removal, while their whole-body and head-only SBs are instead better alleviated with enrichment. Understanding that carnivore SBs are heterogeneous in their triggers and most effective treatments may help zoos, breeding centres and mink farms improve the design of their enclosures and the efficacy of their enrichments.

Factors controlling shrub encroachment in subtropical montane systems

AbstractAimWe aimed to assess the effects of temperature rise and neighbour removal on growth and performance of juvenile individuals of Baccharis uncinella shrubs in a montane grassland. We hypothesized that removal of neighbouring grasses and increased air temperature would enhance B. uncinella growth.LocationHigh‐elevation forest–grassland patches, southern Brazil.MethodsAn experiment with factorial design was carried out from Aug 2014 to Sept 2015 using air temperature and neighbour removal as factors with two levels each. Air temperature was increased using acrylic open‐top chambers (OTC). For the neighbour removal treatment we removed neighbouring grasses from 30 cm around randomly selected B. uncinella juveniles. Survival, growth and plant physiological status were assessed at the end of the experiment.ResultsPlants with OTCs grew more than plants without them. Neighbour removal did not have a significant effect on plant growth. These data suggest that an increase in temperature may enhance encroachment of B. uncinella in the studied grasslands, and that neighbour grasses may act as facilitators for B. uncinella shrubs by improving water balance.ConclusionsConsidering the capacity of B. uncinella to expand from forest edges towards grasslands, it is likely that montane grasslands in southern Brazil will be subject to B. uncinella encroachment within the next decades. The potential synergistic – negative – effects of global warming and habitat loss might jeopardize the high biodiversity of this ecosystem. Specific management actions and conservation public policies are needed to protect this already threatened montane system.

The importance of facilitative interactions on the performance of Colobanthus quitensis in an Antarctic tundra

AbstractAimsThe sign of interactions among plants in very harsh environments is under debate. The Antarctic tundra is one of the harshest environments on Earth and only two vascular plants (Deschampsia antarctica and Colobanthus quitensis) have successfully established natural populations. D. antarctica mostly establishes facilitative interactions with other species (mosses), but there is no information about the inter‐specific interactions established by C. quitensis. We assessed whether C. quitensis grows frequently associated with D. antarctica, and if D. antarctica neighbours have a positive effect on the survival, growth and photochemical efficiency of C. quitensis individuals.LocationKing George Island, Antarctic Peninsula.MethodsTo assess the spatial association among the two Antarctic vascular plants fifty 50 m × 50 cm quadrats were sampled on each of four different substrates: moss carpet areas; dead moss carpet areas dominated by D. antarctica; a transition zone between dead moss carpets and fellfields; and fellfields characterized by very poor vegetation cover. Infrared thermal images were taken to estimate whether growth associated with D. antarctica affected the foliar temperature of C. quitensis. The importance of D. antarctica neighbours on the growth, survival and photochemical efficiency of C. quitensis was evaluated with a neighbour removal experiment.ResultsThe number of C. quitensis individuals associated with D. antarctica was significantly higher than when growing alone in the moss carpet and the dead moss carpet, while in the transition zone there was a trend in that direction. C. quitensis individuals growing associated with D. antarctica were bigger than those growing alone in these three substrate types. In the fellfield site there were no significant differences, neither in the number nor the size of individuals when growing alone or associated with D. antarctica. Foliar temperature of C. quitensis individuals associated with D. antarctica was slightly (1.1°C) but significantly higher than in those growing alone. The growth, survival and photochemical efficiency of C. quitensis individuals with neighbours were higher than in individuals with neighbours removed.ConclusionsOur results indicate that D. antarctica has a facilitative effect on the growth, survival and photochemical efficiency of C. quitensis. Thus, facilitative interactions are present and are important in one of the harshest environments on Earth, although results from the fellfield site indicate that further research is needed.

Competitive ability, stress tolerance and plant interactions along stress gradients.

Exceptions to the generality of the stress-gradient hypothesis (SGH) may be reconciled by considering species-specific traits and stress tolerance strategies. Studies have tested stress tolerance and competitive ability in mediating interaction outcomes, but few have incorporated this to predict how species interactions shift between competition and facilitation along stress gradients. We used field surveys, salt tolerance and competition experiments to develop a predictive model interspecific interaction shifts across salinity stress gradients. Field survey and greenhouse tolerance tests revealed tradeoffs between stress tolerance and competitive ability. Modeling showed that along salinity gradients, (1) plant interactions shifted from competition to facilitation at high salinities within the physiological limits of salt-intolerant plants, (2) facilitation collapsed when salinity stress exceeded the physiological tolerance of salt-intolerant plants, and (3) neighbor removal experiments overestimate interspecific facilitation by including intraspecific effects. A community-level field experiment, suggested that (1) species interactions are competitive in benign and, facilitative in harsh condition, but fuzzy under medium environmental stress due to niche differences of species and weak stress amelioration, and (2) the SGH works on strong but not weak stress gradients, so SGH confusion arises when it is applied across questionable stress gradients. Our study clarifies how species interactions vary along stress gradients. Moving forward, focusing on SGH applications rather than exceptions on weak or nonexistent gradients would be most productive.

Facilitative and competitive interaction components among New England salt marsh plants.

Intra- and interspecific interactions can be broken down into facilitative and competitive components. The net interaction between two organisms is simply the sum of these counteracting elements. Disentangling the positive and negative components of species interactions is a critical step in advancing our understanding of how the interaction between organisms shift along physical and biotic gradients. We performed a manipulative field experiment to quantify the positive and negative components of the interactions between a perennial forb, Aster tenuifolius, and three dominant, matrix-forming grasses and rushes in a New England salt marsh. Specifically, we asked whether positive and negative interaction components: (1) are unique or redundant across three matrix-forming species (two grasses; Distichlis spicata and Spartina patens, and one rush; Juncus gerardi), and (2) change across Aster life stages (seedling, juvenile, and adult). For adult Aster the strength of the facilitative component of the matrix-forb interaction was stronger than the competitive component for two of the three matrix species, leading to net positive interactions. There was no statistically significant variation among matrix species in their net or component effects. We found little difference in the effects of J. gerardi on Aster at later life-history stages; interaction component strengths did not differ between juveniles and adults. However, mortality of seedlings in neighbor removal plots was 100%, indicating a particularly strong and critical facilitative effect of matrix species on this forb during the earliest life stages. Overall, our results indicate that matrix forming grasses and rushes have important, yet largely redundant, positive net effects on Aster performance across its life cycle. Studies that untangle various components of interactions and their contingencies are critical to both expanding our basic understanding of community organization, and predicting how natural communities and their component parts will respond to environmental change.

Interactions among salt marsh plants vary geographically but not latitudinally along the California coast.

The strength of species interactions often varies geographically and locally with environmental conditions. Competitive interactions are predicted to be stronger in benign environments while facilitation is expected to be stronger in harsh ones. We tested these ideas with an aboveground neighbor removal experiment at six salt marshes along the California coast. We determined the effect of removals of either the dominant species, Salicornia pacifica, or the subordinate species on plant cover, aboveground biomass and community composition, as well as soil salinity and moisture. We found that S. pacifica consistently competed with the subordinate species and that the strength of competition varied among sites. In contrast with other studies showing that dominant species facilitate subordinates by moderating physical stress, here the subordinate species facilitated S. pacifica shortly after removal treatments were imposed, but the effect disappeared over time. Contrary to expectations based on patterns observed in east coast salt marshes, we did not see patterns in species interactions in relation to latitude, climate, or soil edaphic characteristics. Our results suggest that variation in interactions among salt marsh plants may be influenced by local‐scale site differences such as nutrients more than broad latitudinal gradients.

Competition and facilitation structure plant communities under nurse tree canopies in extremely stressful environments.

Nurse plant facilitation in stressful environments can produce an environment with relatively low stress under its canopy. These nurse plants may produce the conditions promoting intense competition between coexisting species under the canopy, and canopies may establish stress gradients, where stress increases toward the edge of the canopy. Competition and facilitation on these stress gradients may control species distributions in the communities under canopies. We tested the following predictions: (1) interactions between understory species shift from competition to facilitation in habitats experiencing increasing stress from the center to the edge of canopy of a nurse plant, and (2) species distributions in understory communities are controlled by competitive interactions at the center of canopy, and facilitation at the edge of the canopy. We tested these predictions using a neighbor removal experiment under nurse trees growing in arid environments. Established individuals of each of four of the most common herbaceous species in the understory were used in the experiment. Two species were more frequent in the center of the canopy, and two species were more frequent at the edge of the canopy. Established individuals of each species were subjected to neighbor removal or control treatments in both canopy center and edge habitats. We found a shift from competitive to facilitative interactions from the center to the edge of the canopy. The shift in the effect of neighbors on the target species can help to explain species distributions in these canopies. Canopy‐dominant species only perform well in the presence of neighbors in the edge microhabitat. Competition from canopy‐dominant species can also limit the performance of edge‐dominant species in the canopy microhabitat. The shift from competition to facilitation under nurse plant canopies can structure the understory communities in extremely stressful environments.

Different responses of alpine plants to nitrogen addition: effects on plant-plant interactions.

The different responses of plant species to resource stress are keys to understand the dynamics of plant community in a changing environment. To test the hypothesis that nitrogen (N) increase would benefit N competitive species, rather than N stress-tolerant species, to compete with neighbours, we conducted an experiment with neighbour removal, N addition and soil moisture as treatments in an alpine grassland on the southeastern Tibetan Plateau. Both growths and competitive-response abilities (CRA, the ability to tolerate the inhibitory effects of neighbors) of Kobresia macrantha, Polygonum viviparum and Potentilla anserine in wet site were facilitated by N addition, conversely, both growths and CRA of Taraxacum mongolicum and Ligularia virgaurea were suppressed by N addition, indicating that the responses of CRA of target species under N addition were consistent with the N utilization strategies of them. Moreover, the facilitative effects of N addition on competitive-response abilities of Kobresia macrantha and Polygonum viviparum were not found at the dry site, illustrating that soil moisture can alter the changes of neighbour effects caused by N addition. Life strategy of dominant species in plant community on the undisturbed southeastern Tibetan Plateau may shift from N stress-tolerant to N competitive, if the N increases continuously.

Competition does not explain the absence of a carnivorous pitcher plant from a nutrient-rich marsh

Why is the carnivorous pitcher plant, Sarracenia alata Wood, largely absent from eutrophic habitats? Two hypotheses are addressed: (1) Sarracenia alata is competitively excluded by non-carnivorous plants where resource availability is high and (2) S. alata cannot tolerate stressful conditions unrelated to competition in a eutrophic wetland. We tested these hypotheses using a reciprocal transplant experiment to compare the performance of Sarracenia alata in eutrophic marsh and oligotrophic bog, crossed with soil source and neighbor removal treatments. Multiple environmental co-variables were also measured to identify factors responsible for transplant performance. Survivorship was 46.5% greater in the bog than in the marsh, and a significant proportion of the variation was explained by higher redox potential in the bog. Transplants were not negatively impacted by neighbors in either community. Results lead us to reject the competitive inferiority hypothesis and provide support for a hypothesis of inadequate stress avoidance. We suggest that a lack of tolerance of abiotic stressors, as opposed to competition, be given greater consideration when explaining the distributions of terrestrial hydrophytic carnivorous plants.