Plant–plant interactions,environmental gradients and plant diversity: A global synthesis of community-level studies

Previous syntheses on the effects of environmental conditions on the outcome of plant–plant interactions summarize results from pairwise studies. However, the upscaling to the community-level of such studies is problematic because of the existence of multiple species assemblages and species-specific responses to both the environmental conditions and the presence of neighbors. We conducted the first global synthesis of community-level studies from harsh environments, which included data from 71 alpine and 137 dryland communities to: (i) test how important are facilitative interactions as a driver of community structure, (ii) evaluate whether we can predict the frequency of positive plant–plant interactions across differing environmental conditions and habitats, and (iii) assess whether thresholds in the response of plant–plant interactions to environmental gradients exists between “moderate” and “extreme” environments. We also used those community-level studies performed across gradients of at least three points to evaluate how the average environmental conditions, the length of the gradient studied, and the number of points sampled across such gradient affect the form and strength of the facilitation-environmental conditions relationship. Over 25% of the species present were more spatially associated to nurse plants than expected by chance in both alpine and dryland areas, illustrating the high importance of positive plant–plant interactions for the maintenance of plant diversity in these environments. Facilitative interactions were more frequent, and more related to environmental conditions, in alpine than in dryland areas, perhaps because drylands are generally characterized by a larger variety of environmental stress factors and plant functional traits. The frequency of facilitative interactions in alpine communities peaked at 1000 mm of annual rainfall, and globally decreased with elevation. The frequency of positive interactions in dryland communities decreased globally with water scarcity or temperature annual range. Positive facilitation-drought stress relationships are more likely in shorter regional gradients, but these relationships are obscured in regions with a greater species turnover or with complex environmental gradients. By showing the different climatic drivers and behaviors of plant–plant interactions in dryland and alpine areas, our results will improve predictions regarding the effect of facilitation on the assembly of plant communities and their response to changes in environmental conditions.     

Nurse plant effects on plant species richness in drylands: The role of grazing,rainfall and species specificity

The outcome of plant–plant interactions depends on environmental (e.g. grazing and climatic conditions) and species-specific attributes (e.g. life strategy and dispersal mode of the species involved). However, the joint effects of such factors on pairwise plant–plant interactions, and how they modulate the role of these interactions at the community level, have not been addressed before. We assessed how these species-specific (life strategy and dispersal) and environmental (grazing and rainfall) factors affected the co-occurrence of 681 plant species pairs on open woodlands in south-eastern Australia. Species-specific attributes affected the co-occurrence of most species pairs, with higher co-occurrence levels dominating for drought-intolerant species. The dispersal mechanism only affected drought-tolerant beneficiaries, with more positive co-occurrences for vertebrate-dispersed species. Conversely, the percentage of facilitated species at the community scale declined under higher rainfall availabilities. A significant grazing × rainfall interaction on the percentage of facilitated species suggests that grazing-mediated protection was important under low to moderate, but not high, rainfall availabilities. This study improves our ability to predict changes in plant–plant interactions along environmental gradients, and their effect on community species richness, by highlighting that: (1) species-specific factors were more important than environmental conditions as drivers of a large amount (～37%) of the pairwise co-occurrences evaluated; (2) grazing and rainfall interaction drive the co-occurrence among different species in the studied communities, and (3) the effect of nurse plants on plant species richness will depend on the relative dominance of particular dispersal mechanisms or life strategies prone to be facilitated.     

Microhabitat amelioration and reduced competition among understorey plants as drivers of facilitation across environmental gradients: Towards a unifying framework

Studies of facilitative interactions as drivers of plant richness along environmental gradients often assume the existence of an overarching stress gradient that equally affects the performance of all the species in a given community. However, co-existing species differ in their ecophysiological adaptations, and do not experience the same stress level under particular environmental conditions. Moreover, these studies assume a unimodal relationship between richness and biomass, which is not as general as previously thought. We ignored these assumptions to assess changes in plant–plant interactions and their effect on local species richness across environmental gradients in semi-arid areas of Spain and Australia. We aimed to understand the relative importance of direct (microhabitat amelioration) and indirect (changes in the competitive relationships among the understorey species: niche segregation, competitive exclusion or intransitivity) mechanisms that might underlie the effects of nurse plants on local species richness. By jointly studying these direct and indirect mechanisms using a unifying framework, we found that nurse plants (trees, shrubs and tussock grasses) increased local richness not only by expanding the niche of neighbouring species but also by increasing niche segregation among them, though the latter was not important in all cases. The outcome of the competition-facilitation continuum varied depending on the study area, likely because the different types of stress gradient considered. When driven by both rainfall and temperature, or rainfall alone, the community-wide importance of nurse plants remained constant (Spanish sites), or showed a unimodal relationship along the gradient (Australian sites). This study expands our understanding of the relative roles of plant–plant interactions and environmental conditions as drivers of local species richness in semi-arid environments. The results can also be used to refine predictions about the response of plant communities to environmental change, and to clarify the relative importance of biotic interactions as drivers of such responses.     

Evolutionary relationships can be more important than abiotic conditions in predicting the outcome of plant–plant interactions

Positive and negative plant–plant interactions are major processes shaping plant communities. They are affected by environmental conditions and evolutionary relationships among the interacting plants. However, the generality of these factors as drivers of pairwise plant interactions and their combined effects remain virtually unknown. We conducted an observational study to assess how environmental conditions (altitude, temperature, irradiance and rainfall), the dispersal mechanism of beneficiary species and evolutionary relationships affected the co‐occurrence of pairwise interactions in 11 Stipa tenacissima steppes located along an environmental gradient in Spain. We studied 197 pairwise plant–plant interactions involving the two major nurse plants (the resprouting shrub Quercus coccifera and the tussock grass S. tenacissima) found in these communities. The relative importance of the studied factors varied with the nurse species considered. None of the factors studied were good predictors of the co‐ocurrence between S. tenacissima and its neighbours. However, both the dispersal mechanism of the beneficiary species and the phylogenetic distance between interacting species were crucial factors affecting the co‐occurrence between Q. coccifera and its neighbours, while climatic conditions (irradiance) played a secondary role. Values of phylogenetic distance between 207–272.8 Myr led to competition, while values outside this range or fleshy‐fruitness in the beneficiary species led to positive interactions. The low importance of environmental conditions as a general driver of pairwise interactions was caused by the species‐specific response to changes in either rainfall or radiation. This result suggests that factors other than climatic conditions must be included in theoretical models aimed to generally predict the outcome of plant–plant interactions. Our study helps to improve current theory on plant–plant interactions and to understand how these interactions can respond to expected modifications in species composition and climate associated to ongoing global environmental change.     

The interplay of nurse and target plant traits influences magnitude and direction of facilitative interactions under different combinations of stress and disturbance intensities in Andean dry grassland

Aims Facilitation is a key process in vegetation dynamics, driving the response to natural and anthropogenic pressures. In harsh-grazed systems, palatable plants mainly survive when nested under unpalatable tussocks and shrubs. The magnitude and direction of positive interactions are driven by resource availability, extent of herbivory and type of nurse species. We hypothesized that different combinations of disturbance and environmental stress affect community composition in the dry Puna (southern Peruvian Andes) by modifying nurse types and plant interactions in magnitude and specific associations. We investigated whether different combinations of stress and disturbance influence species richness, type and frequency of occurrence of nurse and beneficiary species and magnitude and patterns of plant interactions; whether nurse species influence these interactions and target species change their interactions under different combinations of stress and disturbance and whether plant functional traits differ in the studied communities and influence the pattern of spatial interactions.Methods We selected three plant communities subject to different precipitation and management regimes: in each we laid a number of transects proportional to its extension. Data collected include species presence/absence, type of spatial interactions with nurse species and functional traits. We calculated species richness and rarefaction patterns, described the patterns of plant–plant spatial interactions and investigated the associations between nurse and other species in the three communities using indicator species analysis (ISA). We performed ISA and correlation analysis to investigate whether plant functional traits influenced facilitative interactions.Important findings We found that different combinations of stress and disturbance shaped a complex set of responses, including changes in the nurse species set. Nurse composition influenced magnitude and direction of plant interactions under different stress intensities. Heavy disturbance increased the relative importance of facilitation, even if the overall number of facilitated species decreased. Under equivalent disturbance regimes, increased abiotic stress led to a greater importance of facilitation. Different combinations of stress and disturbance affected the community assemblage also by changing the behaviour of some non-nurse species. Both heavy disturbance and strong stress led to a decrease of trait states; with certain combinations of stress and disturbance, preferential distribution of these states was observed. We also found that plant traits were of key importance in determining facilitative interactions. Some traits were mainly associated with one type of spatial interaction: plant architecture, life cycle and root type influenced the type of interaction between nurses and beneficiaries under different combinations of stress and disturbance. Our results also demonstrate that in plant interaction research the object of observations (species per se, species percentage, etc.) might influence outputs, and to effectively assess the impact of different stress and disturbance intensities on plant interactions it is necessary to work at the community level to consider the whole species pool.     

Nurse species and indirect facilitation through grazing drive plant community functional traits in tropical alpine peatlands

Facilitation among plants mediated by grazers occurs when an unpalatable plant extends its protection against grazing to another plant. This type of indirect facilitation impacts species coexistence and ecosystem functioning in a large array of ecosystems worldwide. It has nonetheless generally been understudied so far in comparison with the role played by direct facilitation among plants. We aimed at providing original data on indirect facilitation at the community scale to determine the extent to which indirect facilitation mediated by grazers can shape plant communities. Such experimental data are expected to contribute to refining the conceptual framework on plant–plant–herbivore interactions in stressful environments. We set up a 2‐year grazing exclusion experiment in tropical alpine peatlands in Bolivia. Those ecosystems depend entirely on a few, structuring cushion‐forming plants (hereafter referred to as “nurse” species), in which associated plant communities develop. Fences have been set over two nurse species with different strategies to cope with grazing (direct vs. indirect defenses), which are expected to lead to different intensities of indirect facilitation for the associated communities. We collected functional traits which are known to vary according to grazing pressure (LDMC, leaf thickness, and maximum height), on both the nurse and their associated plant communities in grazed (and therefore indirect facilitation as well) and ungrazed conditions. We found that the effect of indirectly facilitated on the associated plant communities depended on the functional trait considered. Indirect facilitation decreased the effects of grazing on species relative abundance, mean LDMC, and the convergence of the maximum height distribution of the associated communities, but did not affect mean height or cover. The identity of the nurse species and grazing jointly affected the structure of the associated plant community through indirect facilitation. Our results together with the existing literature suggest that the “grazer–nurse–beneficiary” interaction module can be more complex than expected when evaluated in the field.     

Nurse‐plant effects on the seed biology and germination of desert annuals

Nurse‐plants generally have positive effects on understorey species by creating more suitable conditions for stress‐intolerant plants relative to open micro‐habitats. However, long‐term effects of this plant–plant facilitation system have been rarely examined. Seeds of five desert annual species from Atiquipa coastal desert in Southern Peru were used to examine whether different microenvironmental conditions under the nurse‐plants Caesalpinia spinosa Molina (Kuntze) lead to differences in seed biology and germinability of annual plants relative to open, canopy‐free conditions. Seeds collected from plants associated with nurse‐plants were predicted to be (i) larger due to more favourable growing conditions, (ii) more viable and with greater germination rates, (iii) less variable in size and viability due to reduced environmental heterogeneity, and (iv) to germinate faster to avoid apparent competition with other annuals. Seed attribute measurements and germination trials in growth chambers were used to test these predictions. Although the plant abundance of only 2 of 5 species was strongly facilitated by the nurse‐plant, no significant differences were found in seed mass, viability or relative variability between understorey and open micro‐habitats for any of the species. Contrary to our predictions, final seed germination rates of seeds from open micro‐habitats were higher, and the open micro‐habitat treatment was more favourable for germination of seeds from both open and understorey environments. Taken together, these results suggest that plant–plant facilitation does not necessarily affect seed biology traits. Further studies addressing larger distribution ranges and/or density gradients of understorey species will illuminate the potential evolutionary effects of nurse‐plants.     

Turnover of plant lineages shapes herbivore phylogenetic beta diversity along ecological gradients

Understanding drivers of biodiversity patterns is of prime importance in this era of severe environmental crisis. More diverse plant communities have been postulated to represent a larger functional trait‐space, more likely to sustain a diverse assembly of herbivore species. Here, we expand this hypothesis to integrate environmental, functional and phylogenetic variation of plant communities as factors explaining the diversity of lepidopteran assemblages along elevation gradients in the Swiss Western Alps. According to expectations, we found that the association between butterflies and their host plants is highly phylogenetically structured. Multiple regression analyses showed the combined effect of climate, functional traits and phylogenetic diversity in structuring butterfly communities. Furthermore, we provide the first evidence that plant phylogenetic beta diversity is the major driver explaining butterfly phylogenetic beta diversity. Along ecological gradients, the bottom up control of herbivore diversity is thus driven by phylogenetically structured turnover of plant traits as well as environmental variables.     

Plant–plant facilitation increases with reduced phylogenetic relatedness along an elevation gradient

Environmental conditions can modify the intensity and sign of ecological interactions. The stress gradient hypothesis (SGH) predicts that facilitation becomes more important than competition under stressful conditions. To properly test this hypothesis, it is necessary to account for all (not a subset of) interactions occurring in the communities and consider that species do not interact at random but following a specific pattern. We aim to assess elevational changes in facilitation, in terms of species richness, frequency and intensity of the interaction as a function of the evolutionary relatedness between nurses and their associated species. We sampled nurse and their facilitated plant species in two 1000–2000 m. elevation gradients in Mediterranean Chile where low temperature imposes a mortality filter on seedlings. We first estimated the relative importance of facilitation as a mechanism adding new species to communities distributed along these gradients. We then tested whether the frequency and intensity of facilitation increases with elevation, taking into account the evolutionary relatedness of the nurse species and the facilitated species. We found that nurses increase the species richness of the community by up to 35%. Facilitative interactions are more frequent than competitive interactions (56% versus 44%) and facilitation intensity increased with elevation for interactions involving distantly related lineages. Our results highlight the importance of including an evolutionary dimension in the study of facilitation to have a clearer picture of the mechanisms enabling species to coexist and survive under stressful conditions. This knowledge is especially relevant to conserve vulnerable and threatened communities facing new climate scenarios, such as those located in Mediterranean-type ecosystems.     

黄土丘陵区植物叶片与细根功能性状关系及其变化

通过植物叶片功能性状(比叶面积、叶组织密度、叶氮含量)和细根功能性状(比根长、根组织密度、根氮含量)间的相互关系,分析植物对环境的适应途径;然后根据性状间的差异进行了层次聚类,将物种划分为3大功能型,并分析了不同功能型对环境的适应策略.结果表明:黄土丘陵区延河流域149种植物的叶氮含量与比叶面积和根氮含量正相关、与叶组织密度负相关,比根长与根组织密度负相关,除了根氮含量,其余根性状与叶性状不相关.此外,功能性状间关系变化和适应策略在不同功能型之间也存在差异.功能型1的植物具有最强的耐旱力和防御力;功能型3的植物具有最强的养分维持能力用以对抗营养贫瘠的环境;功能型2的植物居中,生长速率最高,具有较强的竞争力、分布最广;根据C-S-R理论,功能型1和3属于“胁迫忍耐型”策略(S策略),功能型2则属于“竞争型”(C)和“干扰型”(R)策略的综合.研究结果为黄土丘陵区植被恢复规划及物种配置等提供依据.     

Positive effects of shrubs on plant species diversity do not change along a gradient in grazing pressure in an arid shrubland

Facilitative or positive interactions among species are driven mainly by the environmental amelioration or protection from grazing provided by nurse plants. Some studies have suggested that protection from grazing is inconsequential in water-limited environments because of low herbivore densities and their grazing effects. Others, however, argue that herbivores have a major effect on semi-arid plant communities, and that protection from grazing is a significant factor driving positive plant–plant interactions in such environments. We identified a gradient in grazing pressure in a semi-arid shrubland in south-eastern Australia along which we compared soil condition, incident radiation and plant composition beneath two nurse shrub species with open (shrub-free) interspaces. Our aim was to assess the degree of microclimatic amelioration provided by both shrubs, and changes in the interactions (intensity, importance and frequency) between both nurse shrubs and their understorey species, and their effects on species richness at the community level. Both the relative interaction intensity (RII) and interaction importance (I_imp) indices of plant–plant interactions were generally positive and independent of grazing pressure. Soil beneath both nurse plants had significantly greater indices of nutrient cycling and infiltration, and contained more C and N than soil in the open. Almost twice as many species occurred under the canopies of both shrubs (44 species) than in the open (23 species), and the composition of species differed significantly among microsites. Fifty-four percent of all perennial plant species occurred exclusively under shrubs. Our results suggest that environmental amelioration is a stronger driver of the facilitatory effect of shrubs on their understorey species than protection from grazing. Our conclusions are based on the fact that the substantial effect of plant–plant interactions on plant species richness was largely independent of grazing pressure. Irrespective of the underlying mechanism for this effect, our study illustrates the ecological role of shrubs as refugia for understorey plants in semi-arid environments and cautions against management practices aimed at reducing shrub populations.     

Environment differentially affects the functional and phylogenetic structures of plant communities in a dry evergreen Afromontane tropical forest

Testing how local environmental conditions influence plant community assembly is important to understand the underlying mechanisms that promote and/or maintain biodiversity. Functional traits are used to find the broad spectrum of resource use strategies that plants use to respond to environmental variation. The patterns and drivers of plant community assembly through the lens of traits and phylogeny; however, remain to be studied in a uniquely biodiversity rich but poorly known fragmented dry Afromontane forest of Ethiopia. Here, we combined trait and community phylogenetic data from thirty sampling plots of 20 × 20 m size to determine the functional and phylogenetic structures and their drivers in a fragmented, human-dominated dry evergreen Afromontane forest. We found phylogenetic and functional clustering of plants in which the effect of environment was found to be trait specific. A weak phylogenetic signal for traits was detected suggesting that species resource use strategies may not be inferred using species phylogenetic distance. Additionally, we found functional traits to be weak in predicting species abundance distribution. Overall, while this study shows a non-random community assembly pattern, it also highlights the importance of deterministic processes being trait specific.     

Community-level relaxation of plant defenses against herbivores at high elevation

Ecological gradients shape the evolution of plant functional traits through variation in climate, abiotic factors, and biological processes. While recent observations highlight elevation clines in functional trait variation, several traits vary independently of others and vary in species-specific manner, thereby hindering community-level generalization. In this study, we examined whether whole plant communities at different elevations independently converge to elevation-specific trait values. Using a combination of field-derived trait quantification and a literature survey, we analyzed eight functional traits relating to plant palatability [specific leaf area (SLA), leaf dry matter content, carbon-to-nitrogen ratio], and defenses (spines, trichomes, silica, and the relative importance of phenolic- or non-phenolic-based chemical defenses in plants) of plants growing along several mountain transects in Switzerland, and computed community-level means for each trait. We observed a general decline of most defense traits in communities at high elevations. The individual traits’ patterns underlined a concerted decline of the physico-chemical defense syndrome at high elevation, and low SLA values at high elevation also indicate investment in the production of leaves with slow turnover rates. In addition, we found that high-elevation plant trait values tend to converge, while at low elevation, to diverge within communities. Our data suggest that community-level defense syndromes along elevation gradients are influenced by ecological filtering. Lower herbivore pressures select for reduced plant defenses, while harsh environmental conditions select for investment in tolerance-related traits. Secondly, since species are highly variable in their functional trait space within communities, niche-partitioning theory might explain which variabilities of the defense functional space are selected within communities, particularly at low elevation.     

The interplay between facilitation and habitat type drives spatial vegetation patterns in global drylands

The spatial configuration of vascular vegetation has been linked to variations in land degradation and ecosystem functioning in drylands. However, most studies on spatial patterns conducted to date have focused on a single or a few study sites within a particular region, specific vegetation types, or in landscapes characterized by a certain type of spatial patterns. Therefore, little is known on the general typology and distribution of plant spatial patterns in drylands worldwide, and on the relative importance of biotic and abiotic factors as predictors of their variations across geographical regions and habitat types. We analyzed 115 dryland plant communities from all continents except Antarctica to: 1) investigate the general typology of spatial patterns, and 2) assess the relative importance of biotic (plant cover, frequency of facilitation, soil amelioration, height of the dominant species) and abiotic (aridity, rainfall seasonality and sand content) factors as predictors of spatial patterns (median patch size, shape of patch‐size distribution and regularity) across contrasting habitat types (shrublands and grasslands). Precipitation during the warmest period and sand content were particularly strong predictors of plant spatial patterns in grasslands and shrublands, respectively. Facilitation associated with power‐law like and irregular spatial patterns in both shrublands and grasslands, although it was mediated by different mechanisms (respectively soil ammelioration and percentage of facilitated species). The importance of biotic attributes as predictors of the shape of patch‐size distributions declined with aridity in both habitats, leading to the emergence of more regular patterns under the most arid conditions. Our results expand our knowledge about patch formation in drylands and the habitat‐dependency of their drivers. They also highlight different ways in which facilitation affects ecosystem structure through the formation of plant spatial patterns.     

Functional trait changes in the floras of 11 cities across the globe in response to urbanization

Urbanization causes major environmental changes globally, which can potentially homogenize biota across cities through the loss and gain of particular types of species. We examine whether urban environments consistently select for plants with particular traits and the implications of such changes on the functional composition of urban floras. We classified plant recorded in 11 cities around the globe as species that have either colonized (arrived and naturalized), persisted or been lost (local extirpation) following urbanization. We analyzed how 10 traits previously linked with plant responses to environmental conditions explained membership of these three groups, by comparing colonisers with persistent and extirpated plants through individual city‐level Bayesian models. Then, we used meta‐analysis to assess consistency of traits across urban areas. Finally, we explored several possible scenarios of functional change using these results. On average, urban colonizers had heavier seeds, unspecialised nutrient requirements, were taller and were annual species more often, especially when compared to locally extirpated plants. Common trends of functional change in urban plant communities include shifts towards taller and heavier‐seeded plants, and an increased prevalence of the short‐lived species, and plants without mutualistic nutritional strategies. Our results suggest that plant traits influence the species that succeed in urban environments worldwide. Different species use different ecological strategies to live in urban environments, as suggested by the importance of several traits that may appear as trait constellations. Plant height and seed mass were the only traits associated with both colonizer and extirpated plant status in urban environments. Based on our data, predicting colonization in urban environments may be easier than identifying extirpation‐prone plants; albeit some regional variation, colonization seems strongly driven by environmental conditions common to most cities (e.g. altered disturbance regimes), whereas extirpation may depend more on processes that vary across cities.     

Fungal community assembly in soils and roots under plant invasion and nitrogen deposition

Fungal community composition in the Anthropocene is driven by rapid changes in environmental conditions caused by human activities. This study examines the relative importance of two global change drivers – atmospheric nitrogen (N) deposition and annual grass invasion – on structuring fungal communities in a California chaparral ecosystem, with emphasis on arbuscular mycorrhizal fungi. We used molecular markers, functional groupings, generalized linear statistics and joint distribution modeling, to examine how environmental variables structure taxonomic and functional composition of fungal communities. Invasive grasses had a lower richness and relative abundance of symbiotic fungi (both AMF and other fungi) compared to native shrubs. We found a higher richness and abundance of rhizophilic (e.g. Glomeraceae) and edaphophilic (e.g. Gigasporaceae) AMF with increasing soil NO₃. Our findings suggest that invasive persistence may decrease the presence of multiple soil symbionts that native species depend on for pathogen protection and increased access to soil resources.     

Plant functional types – a strategic perspective

Abstract. This paper discusses the use of Plant Functional Types to describe vegetation types – from plant communities to formations – in order to understand the ecological constraints of plants and plant communities on a higher level of abstraction. The terminology around plant functional types, traits and strategies is in need of further clarification and unification. The use of the term plant functional type is recommended. We are in need of a total environmental framework in which, hierarchically or reticulately, plant functional types can be compared and interpreted.     

Phylogenetic distance among beneficiary species in a cushion plant species explains interaction outcome

Determining which drivers lead to a specific species assemblage is a central issue in community ecology. Although many processes are involved, plant–plant interactions are among the most important. The phylogenetic limiting similarity hypothesis states that closely related species tend to compete stronger than distantly related species, although evidence is inconclusive. We used ecological and phylogenetic data on alpine plant communities along an environmental severity gradient to assess the importance of phylogenetic relatedness in affecting the interaction between cushion plants and the whole community, and how these interactions may affect community assemblage and diversity. We first measured species richness and individual biomass of species growing within and outside the nurse cushion species, Arenaria tetraquetra. We then assembled the phylogenetic tree of species present in both communities and calculated the phylogenetic distance between the cushion species and its beneficiary species, as well as the phylogenetic community structure. We also estimated changes in species richness at the local level due to the presence of cushions. The effects of cushions on closely related species changed from negative to positive as environmental conditions became more severe, while the interaction with distantly related species did not change along the environmental gradient. Overall, we found an environmental context‐dependence in patterns of phylogenetic similarity, as the interaction outcome between nurses and their close and distantly‐related species showed an opposite pattern with environmental severity.     

Plant–environment interactions through a functional traits perspective: a review of Italian studies

Abstract

Italy is among the European countries with the greatest plant diversity due to both a great environmental heterogeneity and a long history of man–environment interactions. Trait-based approaches to ecological studies have developed greatly over recent decades worldwide, although several issues concerning the relationships between plant functional traits and the environment still lack sufficient empirical evaluation. To draw insights on the association between plant functional traits and direct and indirect human and natural pressures on the environmental drivers, this article summarizes the existing knowledge on this topic by reviewing the results of studies performed in Italy adopting a functional trait approach on vascular plants, bryophytes and lichens. Although we recorded trait measurements for 1418 taxa, our review highlighted some major gaps in plant traits knowledge: Mediterranean ecosystems are poorly represented; traits related to belowground organs are still overlooked; traits measurements for bryophytes and lichens are lacking. Finally, intraspecific variation has been little studied at community level so far. We conclude by highlighting the need for approaches evaluating trait–environment relationship at large spatial and temporal scales and the need of a more effective contribution to online databases to tie more firmly Italian researchers to international scientific networks on plant traits.     

Negative relationship between interspecies spatial association and trait dissimilarity

Species’ response to environmental site conditions and neighborhood interactions are among the important drivers of species’ spatial distributions and the resultant interspecies spatial association. The importance of competition to interspecies spatial association can be inferred from a high degree of trait dissimilarity of the associated species, and vice versa for environmental filtering. However, because the importance of environmental filtering and competition in structuring plant communities often vary with spatial scale and with plant life stage, the species’ spatial association–trait dissimilarity relationship should vary accordingly. We tested these assumptions in a fully mapped 50‐ha subtropical evergreen forest of China, where we assessed the degrees of interspecies spatial associations between adult trees and between saplings at two different spatial scales (10 m versus 40 m) and measured the degrees of trait dissimilarity of the associated species using six traits (leaf area, specific leaf area, leaf dry‐matter content, wood density, wood dry‐matter content and maximum height). Consistent across spatial scales and plant life stages, the degree of interspecies spatial association and the degree of overall trait dissimilarity (i.e. all six traits together) were negatively correlated, suggesting that environmental filtering might help assemble functionally similar species in the forest under study. However, when we looked into the spatial association–trait dissimilarity relationship for individual traits, we found that the relationships between interspecies spatial associations and the dissimilarity of wood density and dry‐matter content were significant for adults but not for saplings, suggesting the importance of wood traits in species’ survival during ontogeny. We conclude that processes shaping interspecies spatial association are spatial scale and plant life stage dependent, and that the distributions of functional traits offer useful insights into the processes underlying community spatial structure.