Temporal intraspecific trait variability drives responses of functional diversity to interannual aridity variation in grasslands

Interannual climate variation alters functional diversity through intraspecific trait variability and species turnover. We examined these diversity elements in three types of grasslands in northern China, including two temperate steppes and an alpine meadow. We evaluated the differences in community‐weighted means (CWM) of plant traits and functional dispersion (FDis) between 2 years with contrasting aridity in the growing season. Four traits were measured: specific leaf area (SLA), leaf dry matter content (LDMC), leaf nitrogen concentration (LNC), and the maximum plant height (H). CWM for SLA of the alpine meadow increased in the dry year while that of the temperate steppe in Qinghai showed opposing trends. CWM of LDMC in two temperate steppes became higher and CWM of LNC in all grasslands became lower in the dry year. Compared with the wet year, FDis of LDMC in the alpine meadow and FDis of LNC in the temperate steppe in Qinghai decreased in the dry year. FDis of H was higher in the dry year for two temperate steppes. Only in the temperate steppe in Qinghai did the multi‐FDis of all traits experience a significant increase in the dry year. Most of the changes in CWM and FDis between 2 years were explained by intraspecific trait variation rather than shifts in species composition. This study highlights that temporal intraspecific trait variation contributes to functional responses to environmental changes. Our results also suggest it would be necessary to consider habitat types when modeling ecosystem responses to climate changes, as different grasslands showed different response patterns.     

Leaf dry matter content as a predictor of grassland litter decomposition: a test of the ��mass ratio hypothesis��

The mass ratio hypothesis provides the key link between plant functional traits (PFTs) and ecosystem processes. Despite its centrality to the field it has had few direct tests. A litter decomposition study using grassland species, singly and in mixtures, was set up to see whether simple leaf traits could be used to predict the decomposition of leaf mixtures. Leaf Dry Matter Content (LDMC) was the trait that performed best. Mass loss in single species bags was best predicted using an exponential model. LDMC explained 48% of the variance in mixture mass loss. There was no significant impact of mixture species richness on mass loss. This study generally confirmed the predictions of the mass ratio hypothesis, but adds some support to other studies that indicate it needs broadening to take into account non-linear and threshold relationships between PFTs and ecosystem processes.     

Leaf diversity influences in-stream litter decomposition through effects on shredders

1. The functioning of many aquatic ecosystems is controlled by surrounding terrestrial ecosystems. In a view of growing interest in linking biodiversity to ecosystem‐level processes, we examined whether and how leaf diversity influences litter decomposition and consumers in streams. 2. We tested experimentally the hypothesis that the effects of leaf diversity on decomposition are determined by the responses of leaf consumers to resource–habitat heterogeneity. Leaves from three common riparian trees, beech (Fagus sylvatica), hazel (Corylus avellana) and ash (Fraxinus excelsior), were exposed alone and in all possible mixtures of two and three species in a stream. We analysed individual leaf species for decomposition rate, microbial respiration and mycelial biomass, and we determined the species composition, abundance and biomass of shredders in leaf bags. 3. We found that the decomposition of the fastest decomposing leaves (hazel and ash) was substantially stimulated (up to twofold higher than single species leaf packs) in mixtures containing beech leaves, which are refractory. In contrast, the decomposition of beech leaves was not affected by leaf mixing. Such species‐specific behaviour of leaves in species mixtures has been overlooked in previous studies that examined the overall decomposition of litter mixtures. 4. The effects of leaf diversity on decomposition varied with the abundance and biomass of shredders but not with microbial parameters. Beech leaves alone were less attractive to shredders than leaf packs made of hazel, ash or any mixture of species. Moreover, the presence of beech leaves in mixtures led to higher shredder abundance and biomass than we had expected from data from single species exposed alone. Lastly, we found that early instars of the caddisfly Potamophylax (the dominant shredder in terms of biomass) almost exclusively used the toughest material (i.e. beech leaves) to construct their cases. 5. Leaf pack heterogeneity may have altered shredder‐mediated decomposition. Shredders colonising diverse leaf packs benefited from the stable substratum provided by beech leaves, whereas ash and hazel leaves were primarily used as food. Thus, our findings provide strong evidence for an intimate linkage between the diversity of riparian vegetation and aquatic communities.     

Long-term presence of tree species but not chemical diversity affect litter mixture effects on decomposition in a neotropical rainforest

Plant litter diversity effects on decomposition rates are frequently reported, but with a strong bias towards temperate ecosystems. Altered decomposition and nutrient recycling with changing litter diversity may be particularly important in tree species-rich tropical rainforests on nutrient-poor soils. Using 28 different mixtures of leaf litter from 16 Amazonian rainforest tree species, we tested the hypothesis that litter mixture effects on decomposition increase with increasing functional litter diversity. Litter mixtures and all single litter species were exposed in the field for 9 months using custom-made microcosms with soil fauna access. In order to test the hypothesis that the long-term presence of tree species contributing to the litter mixtures increases mixture effects on decomposition, microcosms were installed in a plantation at sites including the respective tree species composition and in a nearby natural forest where these tree species are absent. We found that mixture decomposition deviated from predictions based on single species, with predominantly synergistic effects. Functional litter diversity, defined as either richness, evenness, or divergence based on a wide range of chemical traits, did not explain the observed litter mixture effects. However, synergistic effects in litter mixtures increased with the long-term presence of tree species contributing to these mixtures as the home field advantage hypothesis assumes. Our data suggest that complementarity effects on mixed litter decomposition may emerge through long-term interactions between aboveground and belowground biota.     

Tree species richness affects litter production and decomposition rates in a tropical biodiversity experiment

We report data on leaf litter production and decomposition from a manipulative biodiversity experiment with trees in tropical Panama, which has been designed to explore the relationship between tree diversity and ecosystem functioning. A total of 24 plots (2025 m²) were established in 2001 using six native tree species, with 1‐, 3‐, and 6‐species mixtures. We estimated litter production during the dry season 2005 with litter traps; decomposition was assessed with a litter bag approach during the following wet season. Litter production during the course of the dry season was highly variable among the tree species. Tree diversity significantly affected litter production, and the majority of the intermediate diverse mixtures had higher litter yields than expected based on yields in monoculture. In contrast, high diverse mixtures did not show such overyielding in litter production. Litter decomposition rates were also highly species‐specific, and were related to various measures of litter quality (C/N, lignin/N, fibre content). We found no overall effect of litter diversity if the entire litter mixtures were analyzed, i.e. mixing species resulted in pure additive effects and observed decomposition rates were not different from expected rates. However, the individual species changed their decomposition pattern depending on the diversity of the litter mixture, i.e. there were species‐specific responses to mixing litter. The analysis of temporal C and N dynamics within litter mixtures gave only limited evidence for nutrient transfer among litters of different quality. At this early stage of our tree diversity experiment, there are no coherent and general effects of tree species richness on both litter production and decomposition. Within the scope of the biodiversity‐ecosystem functioning relationship, our results therefore highlight the process‐specific effects diversity may have. Additionally, species‐specific effects on ecosystem processes and their temporal dynamics are important, but such effects may change along the gradient of tree diversity.     

Questioning the Reliability of the Point Intercept Method for Assessing Community Functional Structure in Low-Productive and Highly Diverse Mediterranean Grasslands

Quantifying species relative abundances in plant communities remains a key issue for the assessment of community functional structure. This is particularly challenging when non-destructive estimates are required over time. We tested whether the point intercept method (PIM), originally developed for low-diverse communities, is relevant for assessing the aboveground biomass and functional structure of highly diverse, low-productive Mediterranean grasslands. We sampled 18 communities with the PIM along a gradient of soil depth and texture, twice over the growing season. After each sampling period, we harvested the aboveground biomass in order to measure species biomass and to assess species richness and community functional structure with plant height, leaf area and leaf dry matter content (LDMC). We investigated the relationship between point intercept measurements and aboveground biomass at three hierarchical levels (species, growth-form and community) to find generalizable calibration equations for estimating community biomass and tested for sensitivity of estimates to community structure. We then compared the community weighted mean (CWM) and variance (CWV) of LDMC, calculated with and without calibration. Differences in species growth strategy and phenology strongly impacted biomass estimates at both the species and the community level. These differences were, however, successfully accounted for by growth-form specific calibrations, which provided accurate estimates without any influence of community structure. Lack of calibration may have dramatic consequences on functional structure assessment by inducing errors in estimates of CWV up to 80 %, depending on growth-form proportions. This work contributes to a better understanding of the possible methodological biases induced during sampling with the PIM, when quantifying species relative abundances for functional structure assessment in complex communities.     

Decomposition of spruce litter needles of different quality by Setulipes androsaceus and Thysanophora penicillioides

Different components of functional biodiversity, such as functional type richness and composition, have been reported to affect the decomposition of litter mixtures. In spite of the numerous reports of these effects, mechanisms underlying patterns of decomposition in litter mixtures are still unclear. We analyzed whether mixture decomposition was affected by: (a) the number of species in the mixture (mixture richness); and (b) the mixture’s functional composition (% of fast- vs. slow-decomposing species included in the mixture). We then tested if variation between observed and expected values of decomposition in mixtures was associated to: (c) the initial litter characteristics of the component species (initial nitrogen, lignin, cellulose and hemicellulose content of litters); and (d) the chemical heterogeneity of the mixtures (variation in the same chemical compounds between the components in each mixture). When up to 5 species representing different functional types were included, both species richness and functional composition showed statistically significant non-additive, and in general positive, effects on litter mixture decomposition. The positive effect of mixture richness on decomposition did not disappear, but was much less marked, when considering mixture with slow-decomposing species only. Although the main driver of decomposition in a mixture is still the average decomposability of the component species (itself largely determined by litter quality), the species interactions in a mixture add a consistent source of variability that is worth considering when predicting the decomposability of a given mixture. We showed that a greater positive difference between observed decomposition rates and that expected from its component species alone was found in mixtures with higher mean nitrogen content and a higher heterogeneity in non-labile compounds. Our results offer quantitative proof that litter chemical heterogeneity, as well as the mean quality of the mixture, can affect the decomposability in litter mixtures.     

Do physical plant litter traits explain non‐additivity in litter mixtures? A test of the improved microenvironmental conditions theory

The decomposition rates of plant litter mixtures often deviate from the averaged rates of monocultures of their component litter species. The mechanisms behind these non‐additive effects in decomposition of litter mixtures are lively debated. One plausible explanation for non‐additive effects is given by the improved microenvironmental condition (IMC) theory. According to this theory, plant litter species, whose physical characteristics improve the microclimatic conditions for decomposers, will promote the decomposition of their co‐occurring litter species. We tested the IMC theory in relation to leaf litter and soil moisture in two contrasting moisture conditions in a dry subarctic mountain birch forest with vascular plant leaf litters of poor and high quality. The non‐additive effects in mass loss of litter mixtures increased when moisture conditions in litter and soil became more favourable for plant litter decomposition. The sign of this increase (antagonistic or synergistic) in non‐additive effects was more predictable for litter mixtures of poor litter quality. Although the specific mechanisms underlying the IMC theory depended on the litter quality of the litter mixtures, a standardized water holding capacity (WHC) was the litter trait most closely related to the non‐additive effects in mixtures of both poor and high quality litter types. Furthermore, we found that higher dissimilarity in WHC traits between the component litter species in a mixture increased synergistic effects in litter mixtures under limiting moisture conditions. However, under improved moisture conditions, increased antagonistic effects were observed. Thus, we found clear support for the IMC theory and showed that climatic conditions and leaf litter physical traits determine whether the non‐additive effects in litter mixtures are antagonistic or synergistic. Our study emphasizes the need to include litter physical traits into predictive models of mixing effects on plant litter decomposition and in general suggests climate specificity into these models.     

Decomposition dynamics in mixed-species leaf litter

Literature on plant leaf litter decomposition is substantial, but only in recent years have potential interactions among leaves of different species during decomposition been examined. We review emerging research on patterns of mass loss, changes in nutrient concentration, and decomposer abundance and activity when leaves of different species are decaying in mixtures. Approximately 30 papers have been published that directly examine decomposition in leaf mixtures as well as in all component species decaying alone. From these litter‐mix experiments, it is clear that decomposition patterns are not always predictable from single‐species dynamics. (Characteristics of decomposition in litter‐mixes that deviate from responses predicted from decomposition of single‐species litters alone are designated "non‐additive"; "additive" responses in mixes are predictable from component species decaying alone.) Non‐additive patterns of mass loss were observed in 67% of tested mixtures; mass loss is often (though not always) increased when litters of different species are mixed. Observed mass loss in some mixtures is as much as 65% more extensive than expected from decomposition of single‐species litter, but more often mass loss in mixtures exceeds expected decay by 20% or less. Nutrient transfer among leaves of different species is striking, with 76% of the mixtures showing non‐additive dynamics of nutrient concentrations. Non‐additive patterns in the abundance and activity of decomposers were observed in 55% and 65% of leaf mixes, respectively. We discuss some methodological details that likely contribute to conflicting results among mixed‐litter studies to date. Enough information is available to begin formulating mechanistic hypotheses to explain patterns in litter‐mix experiments. Emerging patterns in the mixed‐litter decomposition literature have implications for relationships between biodiversity and ecosystem function (in this case, the function being decomposition), and for potential mechanisms through which invasive plant species could alter carbon and nutrient dynamics in ecosystems.     

Connecting litter quality,microbial community and nitrogen transfer mechanisms in decomposing litter mixtures

Synergistic effects on decomposition in litter mixtures have been suggested to be due to the transfer of nitrogen from N‐rich to N‐poor species. However, the dominant pathway and the underlying mechanisms remain to be elucidated. We conducted an experiment to investigate and quantify the control mechanisms for nitrogen transfer between two litter species of contrasting nitrogen status (¹⁵N labeled and unlabeled Fagus sylvatica and Fraxinus excelsior) in presence and absence of micro‐arthropods. We found that ¹⁵N was predominantly transferred actively aboveground by saprotrophic fungi, rather than belowground or passively by leaching. However, litter decomposition remained unaffected by N‐dynamics and was poorly affected by micro‐arthropods, suggesting that synergistic effects in litter mixtures depend on complex environmental interrelationships. Remarkably, more ¹⁵N was transferred from N‐poor beech than N‐rich ash litter. Moreover, the low transfer of ¹⁵N from ash litter was insensitive to destination species whereas the transfer of ¹⁵N from labeled beech litter to unlabeled beech was significantly greater than the amount of ¹⁵N transferred to unlabeled ash suggesting that processes of nitrogen transfer fundamentally differ between litter species of different nitrogen status. Microbial analyses suggest that nitrogen of N‐rich litter is entirely controlled by bacteria that hamper nitrogen capture of microbes in the environment supporting the source‐theory. In contrast, nitrogen of N‐poor fungal dominated litter is less protected and transferable depending on the nitrogen status and the transfer capacity of the microbial community of the co‐occurring litter species supporting the gradient‐theory. Thus, our results challenge the traditional view regarding the role of N‐rich litter in decomposing litter mixtures. We rather suggest that N‐rich litter is only a poor nitrogen source, whereas N‐poor litter, can act as an important nitrogen source in litter mixtures. Consequently both absolute and relative differences in initial litter C/N ratios of co‐occurring litter species need to be considered for understanding nitrogen dynamics in decomposing litter mixtures.     

Litter quality, stream characteristics and litter diversity influence decomposition rates and macroinvertebrates

1. We examined the relative importance of litter quality and stream characteristics in determining decomposition rate and the macroinvertebrate assemblage living on autumn‐shed leaves. 2. We compared the decomposition rates of five native riparian tree species (Populus fremontii, Alnus oblongifolia, Platanus wrightii, Fraxinus velutina and Quercus gambelii) across three south‐western streams in the Verde River catchment (Arizona, U.S.A.). We also compared the decomposition of three‐ and five‐species mixtures to that of single species to test whether plant species diversity affects rate. 3. Decomposition rate was affected by both litter quality and stream. However, litter quality accounted for most of the variation in decomposition rates. The relative importance of litter quality decreased through time, explaining 97% of the variation in the first week but only 45% by week 8. We also found that leaf mixtures decomposed more quickly than expected, when all the species included were highly labile or when the stream environment led to relatively fast decomposition. 4. In contrast to decomposition rate, differences in the invertebrate assemblage were more pronounced across streams than across leaf litter species within a stream. We also found significant differences between the invertebrate assemblage colonising leaf mixtures compared with that colonising pure species litter, indicating non‐additive properties of litter diversity on stream invertebrates. 5. This study shows that leaf litter diversity has the capacity to affect in‐stream decomposition rates and stream invertebrates, but that these effects depend on both litter quality and stream characteristics.     

Does initial litter chemistry explain litter mixture effects on decomposition?

Abstract We tested the hypothesis that interactions in litter mixtures (expressed as the difference between observed and expected decomposition rates) are greater when the component species differ more in their initial litter chemistry. Thereto, we collected freshly senesced leaf litter from a wide range of species from an old field and woodland vegetation, and a fen ecosystem in The Netherlands. Litterbags with either mono-specific litter (20 and 15 species), or litter mixtures (50 and 42 species pairs) of randomly drawn combinations of two representatives from different plant functional types were incubated for 20, 35 and 54 weeks in a purpose-built decomposition bed (woodland/old field) or in the field (fen). Species showed a wide range of decomposition rates. For the woodland/old field species, initial litter C and P concentrations were significantly correlated with litter decomposition rate. For the fen species, litter phenolics concentration was correlated with decomposition rate. If the Sphagnum species were left out of the analyses, initial litter P and phenolics concentration both showed a significant relationship, albeit only with the remaining mass after 1 year. Differences between observed and expected decomposition were often considerable in individual litter mixtures. Regression analysis showed that such differences were not related to the differences in litter chemistry of the component species. Furthermore, litter mixtures containing species with very different initial litter chemistry did not show stronger interaction when tested against litter mixtures containing chemically similar litter types. From these observations we conclude that the difference in initial single litter chemistry parameters of the component is not a useful concept to explain interactions in litter mixtures.Due to an error in the citation line, this revised PDF (published in December 2003) deviates from the printed version, and is the correct and authoritative version of the paper.     

Plant functional diversity mediates the effects of vegetation and soil properties on community-level plant nitrogen use in the restoration of semiarid sandy grassland

Species-rich plant communities use nitrogen (N) more efficiently in grassland ecosystems; however, the role of plant functional diversity in affecting community level plant N-use has received little attention. We examined plant N content, stock and N-use efficiency at community-level along a restoration gradient of sandy grassland (mobile dune, semi-fixed dune, fixed dune and grassland) in Horqin Sand Land, northern China. We used the functional trait-based approach to examine how plant functional diversity, reflected by the most abundant species’ traits (community-weighted mean, CWM) and the dispersion of functional trait values (FDis), affected N-use efficiency in sandy grassland restoration. We further used the structure equation model (SEM) to evaluate the direct or indirect effects of plant species richness, biomass, functional diversity and soil properties on community-level plant N-use efficiency. We found that plant biomass and its N stock increased following sandy grassland restoration, and there were lower plant N content and higher N-use efficiency in semi-fixed dune, fixed dune and grassland as compared with mobile dune. N-use efficiency was positively associated with plant species richness, biomass, CWM plant height, CWM leaf C:N, FDis and soil gradient, but SEM results showed that species richness, CWM leaf C:N, plant biomass and FDis controlled by soil properties were the main factors exerting direct effects. CWM plant height also had a positive effect on N-use efficiency through its indirect effect on plant biomass. Soil gradient increased N-use efficiency through an indirect effect on vegetation rather than a direct effect. Final SEM models based on different plant functional diversity explained over 74% of variances in N-use efficiency. Effects of plant functional diversity on N-use efficiency supported both the mass ratio hypothesis and the complementarity hypothesis. Our results clearly highlight the important role of plant functional diversity in mediating the effects of vegetation and soil properties on community level plant N-use in sandy grassland ecosystems.     

Shifts in plant functional community composition under hydrological stress strongly decelerate litter decomposition

Litter decomposition is a key process of nutrient and carbon cycling in terrestrial ecosystems. The decomposition process will likely be altered under ongoing climate change, both through direct effects on decomposer activity and through indirect effects caused by changes in litter quality. We studied how hydrological change indirectly affects decomposition via plant functional community restructuring caused by changes in plant species’ relative abundances (community‐weighted mean (CWM) traits and functional diversity). We further assessed how those indirect litter quality effects compare to direct effects. We set up a mesocosm experiment, in which sown grassland communities and natural turf pieces were subjected to different hydrological conditions (dryness and waterlogging) for two growing seasons. Species‐level mean traits were obtained from trait databases and combined with species’ relative abundances to assess functional community restructuring. We studied decomposition of mixed litter from these communities in a common “litterbed.” These indirect effects were compared to effects of different hydrological conditions on soil respiration and on decomposition of standard litter (direct effects). Dryness reduced biomass production in sown communities and natural turf pieces, while waterlogging only reduced biomass in sown communities. Hydrological stress caused profound shifts in species’ abundances and consequently in plant functional community composition. Hydrologically stressed communities had higher CMW leaf dry matter content, lower CMW leaf nitrogen content, and lower functional diversity. Lower CWM leaf N content and functional diversity were strongly related to slower decomposition. These indirect effects paralleled direct effects, but were larger and longer‐lasting. Species mean traits from trait databases had therefore considerable predictive power for decomposition. Our results show that stressful soil moisture conditions, that are likely to occur more frequently in the future, quickly shift species’ abundances. The resulting functional community restructuring will decelerate decomposition under hydrological stress.     

Home-field advantage in decomposition of leaf litter and insect frass

Home-field advantage (HFA) hypothesis regarding litter decomposition states that litter is decomposed more rapidly in the habitat from which it is derived (i.e., home) than in other habitat (i.e., away) due to local adaptation of soil decomposers. We tested the HFA hypothesis regarding decomposition of leaf litter, insect frass, and their mixtures, using laboratory incubation of leaf litter from an evergreen (Pinus densiflora) and a deciduous (Quercus acutissima) tree species, frass excreted by two insect herbivores (Dendrolimus spectabilis and Lymantria dispar) fed on one of the two trees, and soil collected underneath the two trees. We found evidence that decomposers in each soil were specialized to decompose the litter derived from the tree species above them, indicating that the HFA occurred in litter decomposition. In contrast, the HFA was not detected in the decomposition of insect frass or litter-frass mixtures. Mixing with D. spectabilis frass non-additively decelerated, while mixing with L. dispar frass non-additively accelerated, decomposition of the mixtures, independent of soil and litter types. These indicate that the presence of insect herbivores may make it difficult to form and maintain a decomposer community specialized to a certain leaf litter, and that it may consequently cancel or weaken HFA in litter decomposition.     

Fungi are involved in the effects of litter mixtures on consumption by shredders

1. Decomposition of litter mixtures in both terrestrial and aquatic ecosystems often shows non‐additive diversity effects on decomposition rate, generally interpreted in streams as a result of the feeding activity of macroinvertebrates. The extent to which fungal assemblages on mixed litter may influence consumption by macroinvertebrates remains unknown. 2. We assessed the effect of litter mixing on all possible three‐species combinations drawn from four tree species (Alnus glutinosa, Betula pendula, Juglans regia and Quercus robur) on both fungal assemblages and the rate of litter consumption by a common shredder, Gammarus fossarum. After a 9‐week inoculation in a stream, batches of leaf discs were taken from all leaf species within litter mixture combinations. Ergosterol, an indicator of fungal biomass, and the composition of fungal assemblages, assessed from the conidia released, were determined, and incubated litter offered to G. fossarum in a laboratory‐feeding experiment. 3. Mixing leaf litter species enhanced both the Simpson’s index of the fungal assemblage and the consumption of litter by G. fossarum, but had no clear effect on mycelial biomass. Specifically, consumption rates of J. regia were consistently higher for mixed‐species litter packs than for single‐species litter. In contrast, the consumption rates of B. pendula were not affected by litter mixing, because of the occurrence of both positive and negative litter‐mixing effects in different litter species combinations that counteracted each other. 4. In some litter combinations, the greater development of some fungal species (e.g. Clavariopsis aquatica) as shown by higher sporulation rates coincided with increased leaf consumption, which may have resulted from feeding preferences by G. fossarum for these fungi. 5. Where litter mixture effects on decomposition rate are mediated via shredder feeding, this could be due to indirect effects of the fungal assemblage.     

Tree functional diversity affects litter decomposition and arthropod community composition in a tropical forest

Disturbance can alter tree species and functional diversity in tropical forests, which in turn could affect carbon and nutrient cycling via the decomposition of plant litter. However, the influence of tropical tree diversity on forest floor organisms and the processes they mediate are far from clear. We investigated the influence of different litter mixtures on arthropod communities and decomposition processes in a 60‐year‐old lowland tropical forest in Panama, Central America. We used litter mixtures representing pioneer and old growth tree species in experimental mesocosms to assess the links between litter types, decomposition rates, and litter arthropod communities. Overall, pioneer species litter decomposed most rapidly and old growth species litter decomposed the slowest but there were clear non‐additive effects of litter mixtures containing both functional groups. We observed distinct arthropod communities in different litter mixtures at 6 mo, with greater arthropod diversity and abundance in litter from old growth forest species. By comparing the decay of different litter mixtures in mesocosms and conventional litterbags, we demonstrated that our mesocosms represent an effective approach to link studies of litter decomposition and arthropod communities. Our results indicate that changes in the functional diversity of litter could have wider implications for arthropod communities and ecosystem functioning in tropical forests.     

Higher temperature reduces the effects of litter quality on decomposition by aquatic fungi

1. We investigated the effects of riparian plant diversity (species number and identity) and temperature on microbially mediated leaf decomposition by assessing fungal biodiversity, fungal reproduction and leaf mass loss. 2. Leaves of five riparian plant species were first immersed in a stream to allow microbial colonisation and were then exposed, alone or in all possible combinations, at 16 or 24 °C in laboratory microcosms. 3. Fungal biodiversity was reduced by temperature but was not affected by litter diversity. Temperature altered fungal community composition with species of warmer climate, such as Lunulospora curvula, becoming dominant. 4. Fungal reproduction was affected by litter diversity, but not by temperature. Fungal reproduction in leaf mixtures did not differ or was lower than that expected from the weighted sum of fungal sporulation on individual leaf species. At the higher temperature, the negative effect of litter diversity on fungal reproduction decreased with the number of leaf species. 5. Leaf mass loss was affected by the identity of leaf mixtures (i.e. litter quality), but not by leaf species number. This was mainly explained by the negative correlation between leaf decomposition and initial lignin concentration of leaves. 6. At 24 °C, the negative effects of lignin on microbially mediated leaf decomposition diminished, suggesting that higher temperatures may weaken the effects of litter quality on plant litter decomposition in streams. 7. The reduction in the negative effects of lignin at the higher temperature resulted in an increased microbially mediated litter decomposition, which may favour invertebrate‐mediated litter decomposition leading to a depletion of litter stocks in streams.     

Can the biomass-ratio hypothesis predict mixed-species litter decomposition along a climatic gradient?

Background and Aims

The biomass-ratio hypothesis states that ecosystem properties are driven by the characteristics of dominant species in the community. In this study, the hypothesis was operationalized as community-weighted means (CWMs) of monoculture values and tested for predicting the decomposition of multispecies litter mixtures along an abiotic gradient in the field.

Methods

Decomposition rates (mg g⁻¹ d⁻¹) of litter from four herb species were measured using litter-bed experiments with the same soil at three sites in central France along a correlated climatic gradient of temperature and precipitation. All possible combinations from one to four species mixtures were tested over 28 weeks of incubation. Observed mixture decomposition rates were compared with those predicted by the biomass-ratio hypothesis. Variability of the prediction errors was compared with the species richness of the mixtures, across sites, and within sites over time.

Key Results

Both positive and negative prediction errors occurred. Despite this, the biomass-ratio hypothesis was true as an average claim for all sites (r = 0·91) and for each site separately, except for the climatically intermediate site, which showed mainly synergistic deviations. Variability decreased with increasing species richness and in less favourable climatic conditions for decomposition.

Conclusions

Community-weighted mean values provided good predictions of mixed-species litter decomposition, converging to the predicted values with increasing species richness and in climates less favourable to decomposition. Under a context of climate change, abiotic variability would be important to take into account when predicting ecosystem processes.     

Leaf decomposition depends on nutritional trait values but increasing trait variability does not always increase decomposition efficiency

The mass ratio (MRH) and niche complementarity (NCH) hypotheses can explain how leaf trait composition drives decomposition, an ecosystem process linked to nutrient cycling and carbon sequestration. However, few studies have used an experiment designed to disentangle the role of the mechanisms proposed by these hypotheses. This is especially true regarding the role of leaf functional traits for decomposition rates in tropical ecosystems. Here, we quantified the biomass loss of 120 leaf mixtures assembled according to four quasi-orthogonal combinations of different mean trait values (community-weighted mean; CWM) and trait variability (functional diversity; FD) of three leaf functional traits (leaf nitrogen and leaf magnesium concentrations and specific leaf area). We found that CWM values of leaf nutritional traits were positively related to greater biomass loss. This supports the hypothesis that the mean trait values of leaf mixtures can drive biomass loss (MRH). However, contrary to NCH expectations, in some circumstances, increasing trait variability of leaf nutritional traits decreased biomass loss. Our results reinforce some previous evidence that, together, CWM and FD can explain leaf decomposition and highlight that the mean resource quality of leaf mixtures is a driver of biomass loss. Also, as previously reported for temperate ecosystems, trait variability does not always increase leaf decomposition in tropical ecosystems. Therefore, there is a need to consider simultaneously both MRH and NCH in future studies, using an appropriate design, keeping in mind that both mechanisms will always be present in any species mixture or combination.