Abstract

Decades of theory and empirical studies have demonstrated links between biodiversity and ecosystem functioning, yet the putative processes that underlie these patterns remain elusive. This is especially true for forest ecosystems, where the functional traits of plant species are challenging to quantify. We analyzed 74,563 forest inventory plots that span 35 ecoregions in the contiguous USA and found that in ~77% of the ecoregions mixed mycorrhizal plots were more productive than plots where either arbuscular or ectomycorrhizal fungal-associated tree species were dominant. Moreover, the positive effects of mixing mycorrhizal strategies on forest productivity were more pronounced at low than high tree species richness. We conclude that at low richness different mycorrhizal strategies may allow tree species to partition nutrient uptake and thus can increase community productivity, whereas at high richness other dimensions of functional diversity can enhance resource partitioning and community productivity. Our findings highlight the importance of mixed mycorrhizal strategies, in addition to that of taxonomic diversity in general, for maintaining ecosystem functioning in forests.

Abstract

Invasions by multiple non-native plant species are common, but management programs often prioritize control of individual species that are expected to have the highest impacts. Multi-species invasions could have larger or smaller impacts than single-species invasions depending on how multiple co-occurring invaders interact to alter their abundance or per capita impacts. Synergistic interactions, such as facilitation, may lead to greater combined impacts. However, if management focuses on a single invader, suppressive interactions could produce unintended consequences, such as the release of a co-occurring invader with a stronger impact. The mechanisms described here highlight where better evidence is needed to predict the combined impacts of co-occurring invaders and which mitigation strategies are most effective. Focused research is required to provide such evidence, which can aid managers in prioritizing which plant invaders to target and in determining the best sequence of invader removal – one that minimizes detrimental impacts on communities and ecosystems.

In a nutshell:

• Management of non-native plant species mostly focuses on single invaders, but many sites contain multiple invaders

• Interactions between invaders may be positive or negative, potentially amplifying or dampening an invader's impacts on communities or ecosystems

• Selecting individual species for management could result in no overall improvement following invader control, or even increase adverse ecological impacts due to release of co-occurring invaders

• Understanding interactions among co-occurring invaders is essential for selecting the best management approach, such as targeting multiple species simultaneously or in a deliberate order

Aims

The Myrtaceae is a woody family that plays an important role in forest ecosystems globally. The recent spread of myrtle rust, caused by a fungal pathogen (Austropuccinia psidii), from its native South America into New Zealand (NZ) highlights the need to quantify the ecological importance of Myrtaceae in NZ woody ecosystems.

Location

New Zealand.

Methods

Using NZ nationwide forest and shrubland inventory data, collected from 2009 to 2014, we quantified the ecological importance of Myrtaceae based on its richness and abundance relative to co-occurring woody families. We then explored how climate and forest stand structure affect Myrtaceae importance in general and by tribe and growth form. Finally, we compared functional traits associated with plant growth and reproductive strategies with other dominant woody families and determined Myrtaceae’s contributions to community-weighted mean (CWM) trait values.

Results

Myrtaceae occurred in 74% of the study plots and its importance value was the second highest across the woody families. It was the only one in which climbers substantially contributed to the importance value (17%). Greater Myrtaceae importance values were associated with warmer and more mesic climates and early forest successional stages. Climate associations were similar within tribes and growth forms, whereas forest structure effects varied. Myrtaceae was functionally distinct from most co-occurring woody families. Contributions to CWM wood density, maximum height, and specific leaf area values were significantly greater than expected from its importance value.

Conclusions

Myrtaceae is the second-most ecologically important woody family in NZ woody ecosystems. The family has a distinctive functional trait spectrum associated with high wood density and tall stature, ensuring large and enduring carbon stocks. There will potentially be large and deleterious outcomes in forest ecosystems if taxon-specific pathogens, such as Austropuccinia psidii, spread and significantly reduce Myrtaceae importance.

Successful control and prevention of biological invasions depend on identifying traits of non-native species that promote fitness advantages in competition with native species. Here, we show that, among 76 native and non-native woody plants of deciduous forests of North America, invaders express a unique functional syndrome that combines high metabolic rate with robust leaves of longer lifespan and a greater duration of annual carbon gain, behaviours enabled by seasonally plastic xylem structure and rapid production of thin roots. This trait combination was absent in all native species examined and suggests the success of forest invaders is driven by a novel resource-use strategy. Furthermore, two traits alone—annual leaf duration and nuclear DNA content—separated native and invasive species with 93% accuracy, supporting the use of functional traits in invader risk assessments. A trait syndrome reflecting both fast growth capacity and understorey persistence may be a key driver of forest invasions.

Premise 

Invasive species tend to possess acquisitive plant traits that support fast growth and strong competitive ability. However, the relevance of symbioses with arbuscular mycorrhizal fungi (AMF) to the fast growing, acquisitive strategy of invasive species is still unclear.

Methods

We measured AMF colonization in roots of five congeneric pairs of nonnative invaders and native Eastern North American woody species (10 species total; 4 lianas, 6 shrubs) that were grown in a monoculture common garden experiment in Syracuse, NY. We then examined the relationships of AMF colonization to above and belowground traits of these species.

Key results

Total AMF colonization and arbuscule colonization were greater in invasive compared to native woody species, a pattern that was more distinct in congeneric shrubs than congeneric lianas. Rates of AMF colonization were also positively correlated with traits indicative of rapid plant growth and nutrient uptake.

Conclusions

The concordance of a resource-acquisitive strategy with higher AMF colonization suggests that symbioses with AMF may be part of the strategy by which invasive woody plants of Eastern North America are able to maintain fast growth rates and outcompete their native counterparts.

The oak (Quercus) species of eastern North America are declining in abundance, threatening the many socioecological benefits they provide. We discuss the mechanisms responsible for their loss, many of which are rooted in the prevailing view that oaks are drought tolerant. We then synthesize previously published data to comprehensively review the drought response strategies of eastern US oaks, concluding that whether or not eastern oaks are drought tolerant depends firmly on the metric of success. Although the anisohydric strategy of oaks sometimes confers a gas exchange and growth advantage, it exposes oaks to damaging hydraulic failure, such that oaks are just as or more likely to perish during drought than neighboring species. Consequently, drought frequency is not a strong predictor of historic patterns of oak abundance, although long-term climate and fire frequency are strongly correlated with declines in oak dominance. The oaks’ ability to survive drought may become increasingly difficult in a drier future. 

First principles predict that diversity at one trophic level often begets diversity at other levels, suggesting plant and mycorrhizal fungal diversity should be coupled. Local-scale studies have shown positive coupling between the two, but the association is less consistent when extended to larger spatial and temporal scales. These inconsistencies are likely due to divergent relationships of different mycorrhizal fungal guilds to plant diversity, scale dependency, and a lack of coordinated sampling efforts. Given that mycorrhizal fungi play a central role in plant productivity and nutrient cycling, as well as ecosystem responses to global change, an improved understanding of the coupling between plant and mycorrhizal fungal diversity across-scales will reduce uncertainties in predicting the ecosystem consequences of species gains and losses. 

Aim

Plants and their associated microbes influence nutrient cycling in terrestrial ecosystems, yet we have a limited understanding of how soil acidity mediates the process. Here, we investigate whether reported differences in nitrogen (N) cycling between forests dominated by arbuscular mycorrhizal (AM) trees and ectomycorrhizal (ECM) trees are related to changes in soil acid–base chemistry induced by mycorrhizal associations.

Location

Global.

Time period

1969–2018.

Major taxa studied

Trees.

Methods

We measured and synthesized variables of leaf litter quality, soil acid–base chemistry and N cycling from: (1) a landscape-scale study of 230 subplots varying widely in AM tree dominance in a 25 ha forest plot; (2) a regional-scale study of 40 AM- and 56 ECM-dominated plots in 10 temperate forests across the eastern USA; (3) a continental-scale study of > 3,000 forest plots from 10 ecoregions across the contiguous USA; and (4) a global meta-analysis of 105 study sites with co-occurring AM and ECM forest stands.

Results

Across all spatial scales, ECM-dominated forests were associated with greater soil acidity. In particular, ECM-dominated soils exhibited lower soil pH and base cations, although the magnitude of mycorrhizal-associated differences in soil acid–base chemistry depended on the biomes, with differences being more pronounced in temperate than in sub/tropical forests. Higher lignin and lower base cations in ECM tree leaf litter were related to greater soil acidity in ECM-dominated forests. Moreover, the lower inorganic N concentrations and slower N transformation rates in ECM-dominated forests were associated with their greater soil acidity.

Main conclusions

Our results indicate that the scale-invariant feedbacks between plant nutrient-use strategies and soil properties have the potential to impact forest community assembly and ecosystem processes, particularly in the context of global change.

1. Many invasion theories invoke resource competition as the primary mechanism of invader advantage. These include Darwin’s Naturalization Hypothesis (DNH), which treats phylogenetic similarity as a proxy for niche overlap and competitive intensity, and the Evolutionary Imbalance Hypothesis (EIH), which suggests the phylogenetic diversity (PD) of an introduced species’ native range is an indicator of its competitive ability.

2. Few tests of invasion theory, however, consider habitat characteristics associated with the role of competition in community assembly. In particular, plant invasions of habitats characterized by high environmental stress and disturbance levels should rarely be driven by competition. This suggests tests of EIH and DNH are habitat dependent, and their relative importance in invasiveness models should be predictable based on habitat qualities related to competitive intensity.

3. Using a dataset of plant invasions in New Zealand (NZ) natural areas that distinguishes naturalized species according to both habitat type and community impact, we evaluated the predictive ability of factors related to EIH, DNH, and covariates including year of introduction, introduction mode, and life history attributes, in driving species invasiveness. We hypothesized that EIH and DNH would be more important predictors of invasiveness in forested habitats and decline in importance as communities shifted towards those more dominated by herbaceous species and/or more sparsely vegetated.

4. We found mixed support for the role of competition linked to DNH and EIH as a driver of invasions in relation to habitat type. Native range PD was among the best predictors of invasiveness in forests, and declined in importance in more disturbed habitats, supporting EIH. In contrast, phylogenetic nearest neighbor distance (PNND) of invaders to native communities was more important in disturbed environments, suggesting competition does not drive DNH. Further, for most habitats and across all of NZ, neither PD nor PNND was as important as year of introduction or life history and growth form attributes in predicting invasiveness.

5. Synthesis: Although both native range PD and PNND predict the invasiveness of naturalized plants in NZ, the results of our habitat‐specific models indicate that only PD is consistent with an invasion mechanism based on competitive ability. Effects of PNND were greatest in grasslands that have been extensively modified by fire and grazing, suggesting they are more likely driven by invader pre‐adaptation to modified habitat conditions. Due to the importance of matching species’ traits to environmental context, invasiveness risk assessments perform better when applied to invaders of particular habitats.

The New Zealand flora has a high proportion of endemic species but has been invaded by almost the same number of non-native plant species. To support management of invasive plant species, we provide an updated inventory of New Zealand’s naturalised flora and compare it with the native flora to identify key taxonomic and functional distinctions. We also assess how the naturalised flora may impact ecosystem processes differently than the native flora using functional traits related to plant resource use strategy. The 1798 species in the naturalised flora currently comprise 43.9% of the total number of vascular plant species, and add 67 plant families and 649 genera to the total vascular flora. The naturalised flora has a greater proportion of herbaceous species and annual species than the native flora, which could influence ecosystem processes such as decomposition and nutrient cycling. Naturalised trees have higher leaf nitrogen concentration for a given leaf area than native trees, which could increase rates of nutrient cycling in invaded forest ecosystems. A greater number of naturalised species are present in larger, more northerly, and more populated regions of New Zealand. Our results demonstrate both taxonomic and functional differences between the native and naturalised flora of New Zealand that can be used to guide management of naturalised plants, including the 314 species currently managed as environmental weeds, from the local to national scale. 

Leaf litter decomposition rates (LDRs) of understory woody species vary substantially across species in temperate forest ecosystems. Using litter traits and LDR data for 78 shrub and liana species from a previous study, plus an additional litterbag experiment with varying mesh bag sizes for a subset of 17 species with rapid LDRs, we report that litter traits have nonlinear effects on LDR. In addition, we show that mesofauna, including nonnative earthworms, in general increase LDR and that the effects are greater for species of high LDR. Our results suggest that the acceleration of LDR by the co-invasion of plant species with high LDR and soil biota can promote nutrient cycling, potentially disrupting the stability of native forest ecosystems. 

Plant-fungal symbioses play critical roles in vegetation dynamics and nutrient cycling, modulating the impacts of global changes on ecosystem functioning. Here, we used forest inventory data consisting of more than 3 million trees to develop a spatially resolved “mycorrhizal tree map” of the contiguous United States. We show that abundances of the two dominant mycorrhizal tree groups—arbuscular mycorrhizal (AM) and ectomycorrhizal trees—are associated primarily with climate. Further, we show that anthropogenic influences, primarily nitrogen (N) deposition and fire suppression, in concert with climate change, have increased AM tree dominance during the past three decades in the eastern United States. Given that most AM-dominated forests in this region are underlain by soils with high N availability, our results suggest that the increasing abundance of AM trees has the potential to induce nutrient acceleration, with critical consequences for forest productivity, ecosystem carbon and nutrient retention, and feedbacks to climate change. 

Reactive nitrogen oxides (NOy; NOy = NO + NO2 + HONO) decrease air quality and impact radiative forcing, yet the factors responsible for their emission from nonpoint sources (i.e., soils) remain poorly understood. We investigated the factors that control the production of aerobic NOy in forest soils using molecular techniques, process-based assays, and inhibitor experiments. We subsequently used these data to identify hotspots for gas emissions across forests of the eastern United States. Here, we show that nitrogen oxide soil emissions are mediated by microbial community structure (e.g., ammonium oxidizer abundances), soil chemical characteristics (pH and C:N), and nitrogen (N) transformation rates (net nitrification). We find that, while nitrification rates are controlled primarily by chemoautotrophic ammonia-oxidizing archaea (AOA), the production of NOy is mediated in large part by chemoautotrophic ammonia-oxidizing bacteria (AOB). Variation in nitrification rates and nitrogen oxide emissions tracked variation in forest communities, as stands dominated by arbuscular mycorrhizal (AM) trees had greater N transformation rates and NOy fluxes than stands dominated by ectomycorrhizal (ECM) trees. Given mapped distributions of AM and ECM trees from 78,000 forest inventory plots, we estimate that broadleaf forests of the Midwest and the eastern United States as well as the Mississippi River corridor may be considered hotspots of biogenic NOy emissions. Together, our results greatly improve our understanding of NOy fluxes from forests, which should lead to improved predictions about the atmospheric consequences of tree species shifts owing to land management and climate change. 

As ecology enters a critical era, more comprehensive studies are needed to improve our understanding of the key themes, major trends, and potential gaps within the discipline. However, as the number of published scientific papers continues to grow, tracking the ever‐expanding body of work becomes increasingly challenging. To identify trends in ecological research, we used recently developed machine learning techniques to perform an automated content analysis on over 84,841 articles published in 33 top‐ranked scientific journals over the past four decades. We detected a clear decline in the relative frequency of classical theoretical research, and increases in data‐intensive research at both micro‐ and macroscales and on anthropogenic themes. Scattered around the periphery of the expanding thematic space, themes such as microbial ecology, genetics, biogeochemistry, and management and policy have all increased in relative frequency. New educational and research frameworks, as well as funding priorities, should incorporate these contemporary themes so that the field of ecology can better address societal challenges. 

Ecologists—especially those new to the field—are tasked with finding relevant literature matching their research interests and deciding upon a suitable venue for the publication of their work. To provide a roadmap for early career researchers to identify journals aligned with their interests, we analyzed major research themes found across the top 30 ecology journals and three high‐impact multi‐disciplinary journals (Nature, PNAS, and Science), utilizing an automated content analysis (ACA) of 84,841 article abstracts, titles, and author keywords published over the last four decades. Journals clustered into 10 distinct groups based on 46 research themes identified by ACA. We examined the frequency of ecological themes in each of these journal groups to identify the journals most associated with each theme. We found three themes (anthropogenic impacts, disease, and traits) that occurred at a high frequency in the high‐impact multi‐disciplinary journal group containing Nature, PNAS, and Science. Themes that increased in frequency over the last four decades, such as climate change, traits, anthropogenic, and cellular biology, were found more often in journals with higher impact factors, indicating that emerging research themes in ecology will likely become of interest to a broader readership over time. Our study provides a thematic review as a potential roadmap for junior ecologists to browse and publish journal articles. 

Understanding biodiversity-productivity relationships (BPRs) is of theoretical importance, and has important management implications. Most work on BPRs has focused on simple and/or experimentally assembled communities, and it is unclear how these observed BPRs can be extended to complex natural forest ecosystems. Using data from over 115,000 forest plots across the contiguous United States, we show that the bivariate BPRs are positive in dry climates and hump-shaped in mesic climates. When considering other site characteristics, BPRs change to neutral in dry climates and remain hump-shaped in humid sites. Our results indicate that climatic variation is an underlying determinant of contrasting BPRs observed across a large spatial extent, while both biotic factors (e.g., stand age and density) and abiotic factors (e.g., soil properties) can impact BPRs within a given climate unit. These findings suggest that tradeoffs need be made when considering whether to maximize productivity vs. conserve biodiversity, especially in mesic climates. 

Forest mycorrhizal type mediates nutrient dynamics, which in turn can influence forest community structure and processes. Using forest inventory data, we explored how dominant forest tree mycorrhizal type affects understory plant invasions with consideration of forest structure and soil properties. We found that arbuscular mycorrhizal (AM) dominant forests, which are characterised by thin forest floors and low soil C : N ratio, were invaded to a greater extent by non‐native invasive species than ectomycorrhizal (ECM) dominant forests. Understory native species cover and richness had no strong associations with AM tree dominance. We also found no difference in the mycorrhizal type composition of understory invaders between AM and ECM dominant forests. Our results indicate that dominant forest tree mycorrhizal type is closely linked with understory invasions. The increased invader abundance in AM dominant forests can further facilitate nutrient cycling, leading to the alteration of ecosystem structure and functions. 

Ecological communities often exhibit greater resistance to biological invasions when these communities consist of species that are not closely related. The effective size of this resistance, however, varies geographically. Here we investigate the drivers of this heterogeneity in the context of known contributions of native trees to the resistance of forests in the eastern United States of America to plant invasions. Using 42,626 spatially referenced forest community observations, we quantified spatial heterogeneity in relationships between evolutionary relatedness amongst native trees and both invasive plant species richness and cover. We then modelled the variability amongst the 91 ecological sections of our study area in the slopes of these relationships in response to three factors known to affect invasion and evolutionary relationships –environmental harshness (as estimated via tree height), relative tree density and environmental variability. Invasive species richness and cover declined in plots having less evolutionarily related native trees. The degree to which they did, however, varied considerably amongst ecological sections. This variability was explained by an ecological section’s mean maximum tree height and, to a lesser degree, SD in maximum tree height (R2GLMM = 0.47 to 0.63). In general, less evolutionarily related native tree communities better resisted overall plant invasions in less harsh forests and in forests where the degree of harshness was more homogenous. These findings can guide future investigations aimed at identifying the mechanisms by which evolutionary relatedness of native species affects exotic species invasions and the environmental conditions under which these effects are most pronounced. 

1. Although it is widely believed that non‐native invasive species threaten the functional integrity of forest ecosystems, their impact on important ecosystem processes such as nitrogen (N) cycling is not well understood. 

2. To examine how invasive species alter ecosystem N dynamics, we established monocultures of five phylogenetic pairs of native and non‐native invasive understory woody species common to Eastern U.S. forests. 

3. After 3 years, we found invaders increased N cycling by enhancing the flow of N to the soil through greater litter N production and litter N content, and increased the uptake of available soil N, via greater fine root production and specific root length.

4. Synthesis. Our results highlight the importance of linking above‐ and below‐ground processes to better understand invader impacts on ecosystem nutrient processes. The rapid shifts in soil N processes as a result of invader dominance observed in our study suggest that invaders may be an important driver of forest ecosystem functioning.

Climate change can have profound impacts on biodiversity and the sustainability of many ecosystems. Various studies have investigated the impacts of climate change, but large-scale, trait-specific impacts are less understood. We analyze abundance data over time for 86 tree species/groups across the eastern United States spanning the last three decades. We show that more tree species have experienced a westward shift (73%) than a poleward shift (62%) in their abundance, a trend that is stronger for saplings than adult trees. The observed shifts are primarily due to the changes of subpopulation abundances in the leading edges and are significantly associated with changes in moisture availability and successional processes. These spatial shifts are associated with species that have similar traits (drought tolerance, wood density, and seed weight) and evolutionary histories (most angiosperms shifted westward and most gymnosperms shifted poleward). Our results indicate that changes in moisture availability have stronger near-term impacts on vegetation dynamics than changes in temperature. The divergent responses to climate change by trait- and phylogenetic-specific groups could lead to changes in composition of forest ecosystems, putting the resilience and sustainability of various forest ecosystems in question. 

Aim

Modern species invasions result in the global reshuffling of regional floras, but biogeographical biases in floristic exchanges (origin effects) are underexplored. We compared habitat‐level invasion patterns in two environmentally similar regions, and ask whether plant exchanges are also similar or if one region largely invades the other. 

Location

Eastern North America (ENA) and East Asia (EAS).

Methods

We compiled a new dataset of the 1293 naturalized (i.e. non‐native, self‐sustaining) and invasive (i.e. spreading) plant taxa in EAS, including the habitats they invade and their native distributions. We tested for biases by biogeographical origin, growth form and habitat in EAS invasions, and compared them with those for ENA. 

Results

EAS contains 51% fewer naturalizations than ENA, but with a similar biogeographical representation. However, invasions in each region show large differences in biogeographical affinity, taxonomic representation and habitat. Invasions in ENA are biased from East Asia (29% invasive), while invaders in EAS come from a fairly uniform set of major temperate regions. Taxonomically, 54% of Asteraceae naturalizations in EAS are invasive compared with only 16% in ENA. Open habitats are highly invaded in both regions (75% of invasions), but forests are significantly more invaded in ENA than EAS (29% vs. 9%). Reciprocal invasions are asymmetric: EAS contributes more woody invaders to ENA than expected (56% woody, P < 0.001), while in EAS nearly all (91%) invaders from ENA are herbaceous. 

Main conclusions

Although they represent regions of similar temperate environments, the origin, taxonomy and habitat affinities of plant invaders in EAS and ENA floristic regions are strongly contrasting. These differences are robust to differences in introduction effort when the invasiveness of species once naturalized is considered. We suggest these patterns support a historical perspective of invasions that invokes differences in regional selection pressures that pre‐adapt certain floras for invasion into particular environmental conditions.

Biotic resistance, the ability of communities to resist exotic invasions, has long attracted interest in the research and management communities. However, inconsistencies exist in various biotic resistance studies and less is known about the current status and knowledge gaps of biotic resistance in forest ecosystems. In this paper, we provide a brief review of the history and mechanisms of the biotic resistance hypothesis, and summarize the central topics and knowledge gaps related to biotic resistance with a special emphasis on forest ecosystems. Overall, although the amount of research efforts on biotic resistance in forest ecosystems has increased since the mid-2000s, aspects such as resistance to exotic pests and pathogens remain understudied. In addition, we synthesize ecological and statistical explanations of observed inconsistencies and provide suggestions for future research directions. Some of the observed inconsistencies on biotic resistance can be attributed to (1) the interactive or additive effects of other ecological processes and (2) the statistical artifacts of modifiable areal unit problem. With the advancement of new statistical knowledge and tools, along with availability of big data, biotic resistance research can be greatly improved with the simultaneous consideration of key ecological processes, the attention to various scales involved, and the addition of understudied systems. 

• Invaders often have greater rates of production and produce more labile litter than natives. The increased litter quantity and quality of invaders should increase nutrient cycling through faster litter decomposition. However, the limited number of invasive species that have been included in decomposition studies has hindered the ability to generalize their impacts on decomposition rates. Further, previous decomposition studies have neglected roots. 

• We measured litter traits and decomposition rates of leaves for 42 native and 36 nonnative woody species, and those of fine roots for 23 native and 25 nonnative species that occur in temperate deciduous forests throughout the Eastern USA. 

• Among the leaf and root traits that differed between native and invasive species, leaf nitrogen and specific leaf area were significantly associated with decomposition rate. However, native and nonnative species did not differ systematically in leaf and root decomposition rates. We found that among the parameters measured, litter decomposer activity was driven by litter chemical quality rather than tissue density and structure. 

• Our results indicate that litter decomposition rate per se is not a pathway by which forest woody invasive species affect North American temperate forest soil carbon and nutrient processes.

Non-native invasive species are often more productive aboveground than co-occurring natives. Because aboveground productivity is closely tied to plant nitrogen (N) uptake and use, high invader leaf productivity should be associated with root growth and plant N use strategies. However, little is known about the above- and belowground carbon (C) and N use strategies of native and invasive plants. We measured shoot and root attributes and soil properties associated with 10 native and 14 non-native invasive forest shrubs and lianas of the Eastern U.S. in a common garden in Syracuse, New York (USA), including leaf growth and chemistry (C, N), root growth, specific root length (SRL), root tissue density, and associated soil C and N concentration, each determined at 2-month intervals (July–November). Non-native species had greater leaf and root production, leaf N concentration, and SRL, but lower leaf N resorption rates and root N concentration than natives. Soil N concentration associated with non-natives was significantly lower than that of native species. Our results suggest that greater aboveground productivity of invasive forest species is linked to greater production of fine roots that may increase the capacity of invaders to take up soil resources. In addition, our findings suggest that invaders beget more rapid plant-soil N feedbacks by promoting N cycling compared to the strategy of slow growing native species that emphasizes recycled plant N. Such differences in N use strategy between native and non-native species would significantly impact forest soil nutrient cycling. 

Questions

How does spatial scale (extent and grain) influence the relative importance of different environmental factors as determinants of plant community composition? Are there general scale thresholds that mark the transition from primarily edaphic to primarily climatic control of plant communities?

Location

Global.

Methods

We surveyed the empirical literature and identified 89 analyses from 63 published studies that analysed vegetation–environment relationships involving at least two categories of predictor variables (edaphic, climatic, topographic, biotic, spatial or disturbance‐related). For each analysis, we identified the primary predictor variable (i.e. the variable that explained the most variation in community composition) and the relative effect size of the best predictor variable from each category. We defined ‘primacy’ as the proportion of times a variable category was primary when it was measured, and analysed primacy and the relative effect size of each category as a function of spatial extent and grain. We also analysed the subset of studies that measured both edaphic and climatic variables to identify spatial extent and grain thresholds for the primacy of these factors.

We surveyed the empirical literature and identified 89 analyses from 63 published studies that analysed vegetation–environment relationships involving at least two categories of predictor variables (edaphic, climatic, topographic, biotic, spatial or disturbance‐related). For each analysis, we identified the primary predictor variable (i.e. the variable that explained the most variation in community composition) and the relative effect size of the best predictor variable from each category. We defined ‘primacy’ as the proportion of times a variable category was primary when it was measured, and analysed primacy and the relative effect size of each category as a function of spatial extent and grain. We also analysed the subset of studies that measured both edaphic and climatic variables to identify spatial extent and grain thresholds for the primacy of these factors.

Results

Edaphic variables had the highest primacy in the overall data set and at fine grain sizes (<200 m2), but there were no strong trends in primacy across studies of varying spatial extent. We detected trends of increasing relative effect size of climatic variables with increasing spatial extent, and decreasing relative effect size of edaphic variables with increasing spatial grain, although these patterns were not statistically significant. Among studies that measured both edaphic and climatic variables, the importance of climate factors relative to edaphic factors increased with increasing spatial extent and grain, with scale thresholds of 1995 km2 for extent and 295 m2 for grain.

Conclusions

Our study illustrates that vegetation–environment relationships depend on the spatial scale (extent and grain) of observation and provide empirical support for the view that there is a transition from a primarily edaphic influence to a primarily climatic influence on plant community composition with increasing spatial scale.

Ranunculus kadzusensis is an endangered aquatic plant species that commonly reproduces in the rice paddies of Korea and Japan during winter and early spring. Here, we investigated the effects of main aquatic environmental factors—light, temperature, and water depth—on its growth, with the goal of seeking information that will contribute to its in situ conservation. As the amount of shading increased, biomass, maximum shoot length, number of branches, flowers, and fruits, main stem diameter, and maximum leaf length decreased. Although seed germination occurred under a 12-h photoperiod and at either 30/20°C (day/night) or 20/15°C, most plants died at the higher temperature. Survival was 0% for surface-grown plant sets when tested in a wintertime pond experiment. The rate of maximum shoot extension was greatest for plants grown at depths of 50 and 100 cm versus those at 20 cm. Thus, we demonstrated that R. kadzusensis is intolerant of high temperatures and shade, which may explain why its growth is limited to paddies with no shading and where temperatures are low early in the year, before rice cultivation begins.