The global spectrum of plant form and function: enhanced species-level trait dataset.

Here we provide the 'Global Spectrum of Plant Form and Function Dataset', containing species mean values for six vascular plant traits. Together, these traits -plant height, stem specific density, leaf area, leaf mass per area, leaf nitrogen content per dry mass, and diaspore (seed or spore) mass - define the primary axes of variation in plant form and function. The dataset is based on ca. 1 million trait records received via the TRY database (representing ca. 2,500 original publications) and additional unpublished data. It provides 92,159 species mean values for the six traits, covering 46,047 species. The data are complemented by higher-level taxonomic classification and six categorical traits (woodiness, growth form, succulence, adaptation to terrestrial or aquatic habitats, nutrition type and leaf type). Data quality management is based on a probabilistic approach combined with comprehensive validation against expert knowledge and external information. Intense data acquisition and thorough quality control produced the largest and, to our knowledge, most accurate compilation of empirically observed vascular plant species mean traits to date.

Exploring trait-performance relationships of tree seedlings along experimentally manipulated light and water gradients.

A foundational assumption of trait-based ecology is that individual performances should be predicted by its functional traits. However, the trait-performance relationships reported in literature were typically weak, probably due to the ignorance of interactions between traits and environments, intraspecific trait variability, and hard traits (directly linked to physiological processes of interest). We conducted an experiment of planting 900 seedlings of six tree species separately (one seedling per pot) along experimentally manipulated light and water gradients, monitored their survival and growth, and measured their morphological, photosynthetic, and hydraulic traits. Most trait-performance relationships depended on the environments, either marginally changing (weak trait × environment interaction) or even reversing (strong trait × environment interaction) along light or water gradients in our experiment. Such trait × environment interactions were more likely to be detected in growth models using individual-level traits than models using species mean traits, but seedling growth was not better modeled with individual-level traits than species mean traits. Additionally, none of the hard traits (photosynthetic and hydraulic traits) were better predictors than soft traits (morphological traits) modeling seedling growth and survival along light and water gradients. Our study highlights the necessities of considering trait × environment interactions when predicting response of plants to changing environments. The benefits of using individual-level traits or hard traits to predict plant performance might be reduced or even canceled if their measurement errors are not well controlled.

A measure of generalized soil fertility that is largely independent of species identity.

BACKGROUND AND AIMS: In 2019, Daou and Shipley produced an operational definition of 'generalized' soil fertility (FG) for plant community ecology and quantified FG using a structural equation model (SEM) invoking a single latent variable. We evaluate a critical assumption of this model: that FG is generalizable to any combination of plant species; i.e. that any combination of plant species will respond in the same direction to the soil 'fertility' gradient in terms of growth. METHODS: We grew nine widely different species singly in each of 25 soils from southern Quebec, Canada, whose FG value had been previously quantified. The original SEM was tested using every possible combination involving from four to nine species. KEY RESULTS: The assumption was rejected due to a subset of three species that responded to a second latent dimension. We then proposed an alternative model that includes FG plus a second latent variable that measures species' deviations from FG due to specific adaptations to soil pH. This alternative model was consistent with every combination of up to eight species. The predictions of FG when ignoring this second dimension and when using the new model were extremely correlated (r =0.98). CONCLUSIONS: The initial unidimensional model of Daou and Shipley was successful in non-acid soils but not in soils with extreme pH and when species specifically adapted to such extreme soils were included. The alternative two-dimensional model takes into account these exceptions and is consistent with the notion of shared physiological niche responses along a gradient of generalized soil fertility.

Quantifying the relationship linking the community-weighted means of plant traits and soil fertility.

Is it possible to generalize relationships between certain plant traits and soil fertility? In particular, are there quantitative relationships between community-weighted mean (CWM) trait values of leaf dry-matter content (LDMC), specific leaf area (SLA), plant height, and Grime's competitor-stress tolerator-ruderal (CSR) strategy scores and the generalized soil fertility, FG (i.e., the capacity of a soil to produce biomass when all nonsoil variables are held constant) that are generalizable across different species assemblages and geographical areas? We assessed FG in 21 sites in southern Quebec and 7 sites in southern France using a previously published method based on structural equation modeling. We then determined the CWM values of LDMC, SLA, plant height, and CSR scores in the 21 Quebec sites to obtain quantitative relationships between FG and these CWM traits. Using these relationships, we independently tested the generality of the trait-fertility relationships using data from French sites. The relationships between FG and the CWM traits were nonlinear, but displayed the expected qualitative trends as reported in the literature. In particular, the S score and CWM LDMC decreased with increasing soil fertility, and the R score and CWM SLA increased. CWM traits were more strongly correlated to measures of FG (r2 up to 0.48) than to measures of other soil characteristics (r2 up to 0.17 for nitrogen flux). Importantly, the independently tested French FG -trait relationships showed no significant deviations from these quantitative relationships. Further investigation is necessary to confirm if the same trend applies to other regions and or ecosystems.

Causal hypotheses accounting for correlations between decomposition rates of different mass fractions of leaf litter.

Whole-leaf decomposition rates are the sum of the decomposition rates of each chemical fraction (water-soluble, cellulose, hemicellulose, lignin), but the decomposition rates of each fraction show complicated patterns of covariation. What explains these patterns of covariation? After measuring the k values of each fraction in 42 different mixtures of tree leaf litters from five species, we tested three alternative causal hypotheses that have been proposed in the literature concerning these mixture interactions using structura equations modeling. All three hypotheses were rejected by the data. We then proposed a new hypothesis, in which rapid decomposition of the labile (water-soluble) fraction stimulates the decomposition of lignin by white-rot fungi and the decomposition of hemicellulose by brown-rot fungi. A more rapid decomposition of hemicellulose then stimulates the decomposition of cellulose. This hypothesis is both consistent with known biology and with our data and is proposed as the most viable current hypothesis.

The complexity of trait-environment performance landscapes in a local subtropical forest.

That functional traits should affect individual performance and, in turn, determine fitness and population growth, is a foundational assumption of trait-based ecology. This assumption is, however, not supported by a strong empirical base. Here, we measured simultaneously two individual performance metrics (survival and growth), seven traits and 10 environmental properties for each of 3981 individuals of 205 species in a 50-ha stem-mapped subtropical forest. We then modelled survival/growth as a function of traits, environments and trait × environment interactions, and quantified their relative importance at both the species and individual levels. We found evidence of alternative functional designs and multiple performance peaks along environmental gradients, indicating the presence of complicated trait × environment interactions. However, such interactions were relatively unimportant in our site, which had relatively low environmental variations. Moreover, individual performance was not better predicted, and trait × environment interactions were not more likely detected, at the individual level than at the species level. Although the trait × environment interactions might be safely ignored in relatively homogeneous environments, we encourage future studies to test the interactive effects of traits and environments on individual performances and lifelong fitness at larger spatial scales or along experimentally manipulated environmental gradients.

Differences in elemental composition of tailings, soils, and plant tissues following five decades of native plant colonization on a gold mine site in Northwestern Québec.

Mining activities have significant environmental impacts, such as the production of acid mine drainage and the typical absence of vegetation on mine tailings whose absence can facilitate the migration of metals to adjacent ecosystems. We investigated the metal and metalloid composition of plants and substrates on, and near a former gold mine site to understand elemental dynamics in such environments. A mine tailings deposit rich in Mo and As in Northwestern Québec was studied following the natural colonization of the deposit by boreal plant species. The site and surrounding forest were categorized into 6 vegetation density classes (VDC) to determine if and how vegetation density, and plant elemental composition, and soil properties were linked. Macroelemental composition of plant tissues (P, K and Ca) was relatively stable, despite differences in macroelemental levels of substrates between different VDC (with lower macronutrient levels associated with less dense areas), indicating the adaptability of the three species studied (Alnus incana spp. rugosa, Betula papyrifera and Picea spp.). Results showed that across a wide range of substrate properties, it was plant species and density that explained metal and metalloid composition in plant tissues (leaves, stems, and roots), while the main environmental determinants for this were VDC, pH, Ca and Cu. Increasing vegetation density was associated with decreasing As and Mo concentrations in substrates. This study sheds light on the plasticity of alder, spruce and birch growing on mine sites, allowing us to better understand elemental dynamics on such sites, and ultimately improve their management.

Direct and indirect effects of regional and local climatic factors on trophic interactions in the Arctic tundra.

Climate change can impact ecosystems by reshaping the dynamics of resource exploitation for predators and their prey. Alterations of these pathways could be especially intense in ecosystems characterized by a simple trophic structure and rapid warming trends, such as in the Arctic. However, quantifying the multiple direct and indirect pathways through which climate change is likely to alter trophic interactions and their relative strength remains a challenge. Here, we aim to identify direct and indirect causal mechanisms driven by climate affecting predator-prey interactions of species sharing a tundra food web. We based our study on relationships between one Arctic predator (Arctic fox) and its two main prey - lemmings (preferred prey) and snow geese (alternate prey) - which are exposed to variable local and regional climatic factors across years. We used a combination of models mapping multiple causal links among key variables derived from a long-term dataset (21 years). We obtained several possible scenarios linking regional climate factors (Arctic oscillations) and local temperature and precipitation to the breeding of species. Our results suggest that both regional and local climate factors have direct and indirect impacts on the breeding of foxes and geese. Local climate showed a positive causal link with goose nesting success, while both regional and local climate displayed contrasted effects on the proportion of fox breeding. We found no impact of climate on lemming abundance. We observed positive relationships between lemming, fox and goose reproduction highlighting numerical and functional responses of fox to the variability of lemming abundance. Our study measures causal links and strength of interactions in a food web, quantifying both numerical response of a predator and apparent interactions between its two main prey. These results improve our understanding of the complex effects of climate on predator-prey interactions and our capacity to anticipate food web response to ongoing climate change.

Les changements climatiques peuvent avoir un impact sur les écosystèmes au travers des modifications de la dynamique d'exploitation des ressources par les prédateurs et leurs proies. Dans le cas de l'Arctique, caractérisée par un réseau trophique simple et une sensibilité marquée au réchauffement climatique, l'altération de ces relations trophiques pourrait être particulièrement importante. Cependant, la quantification des nombreux liens directs et indirects à travers lesquels les changements climatiques peuvent affecter les interactions trophiques demeure un défi. Notre objectif est d'identifier les mécanismes causaux directs et indirects, sous-tendus par le climat, affectant les interactions prédateur-proie au sein d'un réseau trophique au cœur de la toundra. Notre étude se base sur les relations entre un prédateur (renard arctique) et ses deux proies principales -le lemming (proie préférée) et la grande oie des neiges (proie alternative)- et qui subissent un accroissement des précipitations et des températures au travers des années. Nous avons utilisé une combinaison de modèles illustrant les liens causaux multiples entre les variables clés issues d'une base de données à long-terme (21 ans). Nous avons obtenu plusieurs scénarios possibles reliant les facteurs climatiques régionaux (Oscillation Arctique) et les températures et précipitations locales à la reproduction de nos 3 espèces. Nos résultats suggèrent que les facteurs climatiques régionaux et locaux présentent des impacts directs et indirects sur la reproduction du renard arctique et de l'oie des neiges. Le climat local présente un lien causal positif avec le succès de nidification de l'oie, alors que le climat local et régional démontrent un effet contrasté sur la proportion de renard en reproduction. Aucune relation entre les facteurs climatiques et l'abondance des lemmings n'a été trouvée. Nous avons observé des liens causaux positifs entre la reproduction du lemming, du renard et de l'oie, mettant en évidence les réponses numériques et fonctionnelles du renard arctique face à la variabilité de l'abondance de lemming. Notre étude est une des premières à mesurer les liens causaux et les forces d'interaction entre les espèces partageant un même réseau trophique, quantifiant ainsi la réponse numérique du prédateur et les interactions apparentes entre ses proies principales. Ces résultats améliorent notre compréhension des effets complexes du climat sur les interactions prédateur-proie et notre capacité à anticiper la réponse des réseaux trophiques aux changements climatiques en cours.

Generalized AIC and chi-squared statistics for path models consistent with directed acyclic graphs.

We explain how to obtain a generalized maximum-likelihood chi-square statistic, XML2 , and a full-model Akaike Information Criterion (AIC) statistic for piecewise structural equation modeling (SEM); that is, structural equations without latent variables whose causal topology can be represented as a directed acyclic graph (DAG). The full piecewise SEM is decomposed into submodels as a Markov network, each of which can have different distributional assumptions or functional links and that can be modeled by any method that produces maximum-likelihood parameter estimates. The generalized XML2 is a function of the difference in the maximum likelihoods of the model and its saturated equivalent and the full-model AIC is calculated by summing the AIC statistics of each of the submodels.

Functional niche occupation and species richness in herbaceous plant communities along experimental gradients of stress and disturbance.

BACKGROUND AND AIMS: The patterns of niche occupation in functional trait space have been widely studied to understand the processes of community assembly, but are rarely linked to environmental conditions (here, stress and disturbance). In this study, we investigate (1) how the pattern of functional niche occupation, incorporating intraspecific trait variation and covariation, varies along experimental gradients of stress and disturbance, (2) whether habitat filtering and/or limiting similarity modify the pattern, and (3) whether their strength varies as a function of species richness or levels of stress and disturbance. METHODS: We constructed an experimental system consisting of 24 herbaceous plant mesocosms under different levels of stress and disturbance, and measured ten traits on five individuals for each species in each mesocosm. We quantified the total functional niche volume occupied by an entire mesocosm, the functional niche overlap among species within a mesocosm and the average functional niche volume occupied per species, and investigated how these metrics varied from species-poor to species-rich mesocosms along gradients of stress and disturbance. KEY RESULTS: Species richness and functional niche overlap correlated positively to disturbance at low and medium levels of stress, but peaked at the intermediate level of disturbance when stress was high. The total functional niche volume and average functional niche volume did not change significantly along these gradients. Compared to null models, each mesocosm occupied a smaller total functional niche volume (habitat filtering) and the species within each mesocosm overlapped less and were more functionally specialized (limiting similarity). Moreover, the standardized metrics (to the null expectations) did not change significantly under different levels of stress and disturbance. CONCLUSIONS: This experimental evidence shows that both habitat filtering and limiting similarity determine the patterns of functional niche occupation and species richness, but their strength does not change along environmental gradients of stress and disturbance.

The measurement and quantification of generalized gradients of soil fertility relevant to plant community ecology.

We propose an operational definition of soil "fertility" that is applicable to plant community ecology and develop a method of measuring and quantifying it, using structural equations modeling, that is generalizable to soils in different regions whose fertility has different causes. To do this, we used structural equation modeling (SEM). The measurement submodel predicts the latent "generalized fertility," FG , of a soil using four indicator variables: the relative growth rates of Festuca rubra, Trifolium pratense, Triticum aestivum, and Arabidopsis thaliana. The direct causes of FG in this study were the supply rates of NO3- , P, and K as well as three indirect causes consisting of three physical soil properties, but these can change between studies. The model was calibrated using 76 grassland soils from southern Quebec, Canada and independently tested using aboveground net primary productivity (NPP) of the natural vegetation over two growing seasons. Both the measurement submodel and the full SEM fit the data well. The FG values predicted 51% of the variance in NPP and were a better predictor than any other single variable, including the actual nutrient flux rates. Furthermore, this model can be applied to grassland soils anywhere because of its modular nature in which the causes and effects of soil fertility are clearly separated.

Linking hard and soft traits: Physiology, morphology and anatomy interact to determine habitat affinities to soil water availability in herbaceous dicots.

BACKGROUND AND AIMS: Species' habitat affinities along environmental gradients should be determined by a combination of physiological (hard) and morpho-anatomical (soft) traits. Using a gradient of soil water availability, we address three questions: How well can we predict habitat affinities from hard traits, from soft traits, and from a combination of the two? How well can we predict species' physiological responses to drought (hard traits) from their soft traits? Can we model a causal sequence as soft traits → hard traits → species distributions? METHODS: We chose 25 species of herbaceous dicots whose affinities for soil moisture have already been linked to 5 physiological traits (stomatal conductance and net photosynthesis measured at soil field capacity, water use efficiency, stomatal conductance and soil water potential measured when leaves begin to wilt). Under controlled conditions in soils at field capacity, we measured five soft traits (leaf dry matter content, specific leaf area, leaf nitrogen content, stomatal area, specific root length). KEY RESULTS: Soft traits alone were poor predictors (R2 = 0.129) while hard traits explained 48% of species habitat affinities. Moreover, hard traits were significantly related to combinations of soft traits. From a priori biological knowledge and hypothesized ecological links we built a path model showing a sequential pattern soft traits → hard traits → species distributions and accounting for 59.6% (p = 0.782) of habitat wetness. CONCLUSIONS: Both direct and indirect causal relationships existed between soft traits, hard traits and species' habitat preferences. The poor predictive abilities of soft traits alone were due to the existence of antagonistic and synergistic direct and indirect effects of soft traits on habitat preferences mediated by the hard traits. To obtain a more realistic model applicable to a population level, it has to be tested in an experiment including species competition for water supply.

Community divergence and convergence along experimental gradients of stress and disturbance.

We created 24 mesocosms containing mixtures of herbaceous species arranged along experimentally maintained gradients of stress (external abiotic constraints limiting biomass production) and disturbance (events causing partial or total live biomass destruction) in order to determine the degree to which community assembly is deterministic or historically contingent during succession. In this 7-yr experiment, we found taxonomic divergence and functional convergence during the last 3 yr. Although communities became more functionally dissimilar as the difference in the level of stress increased, they were equally taxonomically different irrespective of the amount of difference between them in terms of stress and disturbance. In addition, comparing communities experiencing the same conditions, taxonomic community structure was more dissimilar as the levels of stress and disturbance decreased. Therefore, community assembly was largely deterministic from a functional perspective but more historically contingent from a taxonomic perspective, and the relative importance of taxonomic historical contingency decreased as the levels of stress and disturbance frequency increased.

An experimental test of CSR theory using a globally calibrated ordination method.

Can CSR theory, in conjunction with a recently proposed globally calibrated CSR ordination ("StrateFy"), using only three easily measured leaf traits (leaf area, specific leaf area and leaf dry matter content) predict the functional signature of herbaceous vegetation along experimentally manipulated gradients of soil fertility and disturbance? To determine this, we grew 37 herbaceous species in mixture for five years in 24 experimental mesocosms differing in factorial levels of soil resources (stress) and density-independent mortality (disturbance). We measured 16 different functional traits and then ordinated the resulting vegetation within the CSR triangle using StrateFy. We then calculated community-weighted mean (CWM) values of the competitor (CCWM), stress-tolerator (SCWM) and ruderal (RCWM) scores for each mesocosm. We found a significant increase in SCWM from low to high stress mesocosms, and an increase in RCWM from lowly to highly disturbed mesocosms. However, CCWM did not decline significantly as intensity of stress or disturbance increased, as predicted by CSR theory. This last result likely arose because our herbaceous species were relatively poor competitors in global comparisons and thus no strong competitors in our species pool were selectively favoured in low stress and low disturbed mesocosms. Variation in the 13 other traits, not used by StrateFy, largely argeed with the predictions of CSR theory. StrateFy worked surprisingly well in our experimental study except for the C-dimension. Despite loss of some precision, it has great potential applicability in future studies due to its simplicity and generality.

Predicting habitat affinities of herbaceous dicots to soil wetness based on physiological traits of drought tolerance.

Background and Aims: Soil water availability is an important mechanism filtering plant species but the functional basis of this filtering in herbaceous dicots is poorly studied. The authors address three questions: Which physiological traits best predict different levels of drought tolerance or avoidance in herbaceous dicots? To what degree can species' habitat preferences along the gradient of soil moisture availability be predicted by their physiological responses to drought? What are the direct and indirect relationships between the physiological traits and how do they interact to determine the species' habitat preferences? Methods: Twenty-five species of herbaceous dicots whose field distributions span a gradient of soil moisture from continually moist to dry were chosen. Under controlled conditions, watering was stopped in a treatment group, the plants were monitored until death of the above-ground tissues and compared with a control group watered at field capacity. Fourteen traits related to plant water economy were measured, including stomatal conductance, net photosynthesis and the visual wilting of leaves. Traits were then analysed using a cumulative link model and path analysis. Key Results: Five physiological traits (stomatal conductance and net photosynthesis measured at soil field capacity, water use efficiency, stomatal conductance and soil water potential measured when leaves begin to wilt) related to the ability to acquire resources (when water is not limiting) or conserve water (when it is limiting) best predicted different levels of drought tolerance or avoidance in herbaceous dicots. Conclusions: Species' habitat preferences can be fairly predicted by their physiological responses to drought ( R 2 = 0·48). Strong direct and indirect relationships between the five identified traits (plus net photosynthesis at wilting and the time until death) led to synergistic and antagonistic relationship in a path analysis model. To allow better prediction of species distributions along a wetness gradient, the next step would be to link these physiological traits to more accessible functional traits.

Occupancy and overlap in trait space along a successional gradient in Mediterranean old fields.

PREMISE OF THE STUDY: Secondary succession is a worldwide phenomenon affecting plant communities. Studying functional variation during succession aids in understanding the mechanisms through which environmental shifts drive succession. We investigated changes in the functional space occupied by herbaceous communities during succession. Furthermore, since different traits are differently affected by environmental conditions, we asked how considering different sets of plant traits impacts those changes. METHODS: Using a chronosequence of Mediterranean old fields (2-42 yr after abandonment), we analyzed shifts of the occupied functional space during succession, how the volume of occupied functional space varies compared with null expectations, and the functional overlap between communities of different successional status. We repeated these analyses considering (1) the leaf-height-seed functional dimensions separately and together and (2) different sets of traits representing those dimensions. KEY RESULTS: From early to late succession, a shift toward nutrient conservative-light competitive species occurred. Functional strategies of mid-successional communities appeared more diverse than expected by chance and less diverse than expected for early and late communities. Early and middle stages overlapped the most. These patterns were generally robust to the choice of functional axes, though important trait-specific exceptions occurred. CONCLUSIONS: We showed evidence for a well-defined history of successive dominance of different assembly mechanisms along succession, resulting in a generally stronger functional diversification in mid-succession. We also demonstrated that different traits typically grouped under one functional dimension can substantially affect the results, discouraging the use of surrogate traits from the same dimension.

Reinforcing loose foundation stones in trait-based plant ecology.

The promise of "trait-based" plant ecology is one of generalized prediction across organizational and spatial scales, independent of taxonomy. This promise is a major reason for the increased popularity of this approach. Here, we argue that some important foundational assumptions of trait-based ecology have not received sufficient empirical evaluation. We identify three such assumptions and, where possible, suggest methods of improvement: (i) traits are functional to the degree that they determine individual fitness, (ii) intraspecific variation in functional traits can be largely ignored, and (iii) functional traits show general predictive relationships to measurable environmental gradients.

The global spectrum of plant form and function.

Earth is home to a remarkable diversity of plant forms and life histories, yet comparatively few essential trait combinations have proved evolutionarily viable in today's terrestrial biosphere. By analysing worldwide variation in six major traits critical to growth, survival and reproduction within the largest sample of vascular plant species ever compiled, we found that occupancy of six-dimensional trait space is strongly concentrated, indicating coordination and trade-offs. Three-quarters of trait variation is captured in a two-dimensional global spectrum of plant form and function. One major dimension within this plane reflects the size of whole plants and their parts; the other represents the leaf economics spectrum, which balances leaf construction costs against growth potential. The global plant trait spectrum provides a backdrop for elucidating constraints on evolution, for functionally qualifying species and ecosystems, and for improving models that predict future vegetation based on continuous variation in plant form and function.

A global meta-analysis of the relative extent of intraspecific trait variation in plant communities.

Recent studies have shown that accounting for intraspecific trait variation (ITV) may better address major questions in community ecology. However, a general picture of the relative extent of ITV compared to interspecific trait variation in plant communities is still missing. Here, we conducted a meta-analysis of the relative extent of ITV within and among plant communities worldwide, using a data set encompassing 629 communities (plots) and 36 functional traits. Overall, ITV accounted for 25% of the total trait variation within communities and 32% of the total trait variation among communities on average. The relative extent of ITV tended to be greater for whole-plant (e.g. plant height) vs. organ-level traits and for leaf chemical (e.g. leaf N and P concentration) vs. leaf morphological (e.g. leaf area and thickness) traits. The relative amount of ITV decreased with increasing species richness and spatial extent, but did not vary with plant growth form or climate. These results highlight global patterns in the relative importance of ITV in plant communities, providing practical guidelines for when researchers should include ITV in trait-based community and ecosystem studies.

A traits-based test of the home-field advantage in mixed-species tree litter decomposition.

BACKGROUND AND AIMS: Litter often decomposes faster in its environment of origin (at 'home') than in a foreign environment ('away'), which has become known as the home-field advantage (HFA). However, many studies have highlighted the conditional nature of the HFA, suggesting that current understanding of this phenomenon is not yet sufficient to generalize across systems. METHODS: The HFA hypothesis was tested for mono-specific and mixed-species litter using a tree-based experiment that manipulated the functional identity and diversity of the host tree community. Litter types of varying quality were transplanted between several host tree communities and decomposition rates were measured using litterbags. Since the decomposer community should respond to traits of the litter input and not their taxonomic identity, a traits-based index of litter-tree similarity was developed. KEY RESULTS: Mono-specific litter exhibited HFA, but when the same litter was decomposed in mixture, this trend was not observed. Mixed-species litter decomposed on average no faster or slower than monoculture litter and exhibited both positive and negative species interactions. These non-additive interactions of decomposition rates in mixture were influenced by the degree of similarity between litter and tree traits. Both synergistic and antagonistic interactions decreased in magnitude with increasing litter-tree similarity such that mixture rates were predictable from monocultures. CONCLUSIONS: The HFA occurred more strongly for mono-specific litter than for the litter types mixed together because interactions between species may have masked this effect. However, when expressed as a function of trait similarity between litters and tree communities, the HFA was not detected.