底栖分解类群对金佛山森林溪流凋落物混合分解的影响

凋落物分解对于维持源头溪流生态系统碳和养分平衡有重要意义。以亚热带典型源头溪流金佛山溪流为代表,选取3种河岸带常见凋落叶为分解对象,设计3个单种和4个混合物种的凋落物组合,在原位放置3种孔径的分解袋（0.05mm、0.25mm和2mm）,探讨混合凋落物的性状与底栖分解类群对叶片质量损失和混合效应的影响。结果表明：（1）微生物在凋落物分解过程中相对贡献均大于50%,小型和大型底栖动物进一步加速了凋落物的分解过程。（2）单种凋落物分解速率存在显著差异：八角枫（Alangium chinense,质量损失率为53.05%）>缺萼枫香（Liquidambar acalycina,30.00%）>薄叶润楠（Machilus leptophylla,12.63%）。（3）混合凋落物中仅微生物参与的处理均表现为负的非加和效应,其中八角枫+缺萼枫香、八角枫+薄叶润楠、八角枫+缺萼枫香+薄叶润楠三个处理的效应显著;小型底栖动物加入后均表现为正的非加和效应,但不显著;在微生物、小型和大型底栖动物的共同作用下,缺萼枫香+薄叶润楠和八角枫+缺萼枫香+薄叶润楠的两个处理的正的非加和效应显著。亚热带源头溪流中凋落物分解功能与河岸植物和分解者类群的复杂性密切相关。     

Species diversity and decomposition in laboratory aquatic systems: the role of species interactions

1. Interest in the effects of biodiversity on ecosystem processes is increasing, stimulated by the global species decline. Different hypotheses about the biodiversity‐ecosystem functioning (BEF) relationship have been put forward and various underlying mechanisms proposed for different ecosystems. 2. We investigated BEF relationships and the role of species interactions in laboratory experiments focussing on aquatic decomposition. Species richness at three different trophic levels (leaf detritus, detritus‐colonising fungi and invertebrate detritivores) was manipulated, and its effects on leaf mass loss and fungal growth were assessed in two experiments. In the first, monocultures and mixtures of reed (Phragmites australis), alder (Alnus glutinosa) and oak (Quercus cerris) leaf disks were incubated with zero, one or eight fungal species. Leaf mixtures were also incubated with combinations of three and five fungal species. In the second experiment, reed leaf disks were incubated with all eight fungal species and offered to combinations of one, two, three, four or five macroinvertebrate detritivores with different feeding modes. 3. Results from the first experiment showed that leaf mass loss was directly related to fungal mass and varied unimodally with the number of fungi, with a maximum rate attained at intermediate diversity in oak and reed and at maximum diversity in alder (the fastest decomposing leaf). 4. Mixing litter species stimulated fungal growth but interactions between species of fungi slowed down decomposition. In contrast, mixtures of macroinvertebrate detritivores reduced fungal mass and accelerated leaf decomposition. Possible explanations of the positive relationship between detritivore diversity and decomposition are a reduction in fungal dominance and a differentiation in the use of different resource patches promoted by higher fungal diversity. 5. In conclusion, the results show a general increase in decomposition rate with increasing biodiversity that is controlled by within‐ and between‐trophic level interactions, and support the hypothesis of both bottom‐up and top‐down effects of diversity on this process.     

Assessing the functional importance of large‐bodied invertebrates in experimental headwater streams

Recent theoretical advances in food web ecology emphasize the importance of body size disparities among species for the structure, stability and functions of ecosystems. Experimental confirmations of the functional importance of large species, independent of their trophic position, are scarce. We specifically examine the multiple ecological roles of large invertebrates from two distinct trophic levels in headwater streams. We experimentally manipulated the presence of large predatory invertebrates (two Perlid stoneflies) or detritivores (a limnephilid caddisfly and a Pteronarcys stonefly) in a two‐by‐two design in stream channels open to immigration/emigration of smaller biota. We assessed treatment effects on the trophic structure of the benthic invertebrate community, dynamics of basal resources (benthic algae and leaf litter of cedar and alder), and stability of litter decomposition rates against an experimental pulse perturbation (fine sediment input). The presence of the large invertebrates was associated with a ten‐fold decrease in the biomass of invertebrate filterers whereas other trophic groups were unaffected by the large species. The biomass of benthic algae was lower and the rate of mass loss of alder litter was higher in channels lacking the large predators, thus revealing trophic cascades operating along both algal‐based and detritus‐based food chains. The large predators had no detectable effect on the decomposition of cedar whereas both cedar and alder disappeared faster in the presence of the large detritivores. Furthermore, the large predators and large detritivores interactively influenced the decomposition of the cedar–alder mixture through a litter diversity effect and the variability of the rate of alder decomposition after a pulse of fine sediment. Because the large invertebrates affected multiple ecosystem properties, and as their absence was not rapidly compensated for by small immigrant species, our findings support the notion that large species could be critically important in controlling ecosystem structure and functioning.     

Comparison of leaf decomposition and macroinvertebrate colonization between exotic and native trees in a freshwater ecosystem

One of the most important sources of energy in aquatic ecosystems is the allochthonous input of detritus. Replacement of native tree species by exotic ones affects the quality of detritus entering freshwater ecosystems. This replacement can alter nutrient cycles and community structure in aquatic ecosystems. The aims of our study were (1) to compare leaf litter decomposition of two widely distributed exotic species (Ailanthus altissima and Robinia pseudoacacia) with the native species they coexist with (Ulmus minor and Fraxinus angustifolia), and (2) to compare macroinvertebrate colonization among litters of the invasive and native species. Litter bags of the four tree species were placed in the water and collected every 2, 25, 39, 71, and 95 days in a lentic ecosystem. Additionally, the macroinvertebrate community on litter bags was monitored after 25, 39, and 95 days. Several leaf chemistry traits were measured at the beginning (% lignin; lignin:N, C:N, LMA) and during the study (leaf total nitrogen). We detected variable rates of decomposition among species (k values of 0.009, 0.008, 0.008, and 0.005 for F. angustifolia, U. minor, A. altissima and R. pseudoacacia, respectively), but we did not detect an effect of litter source (from native/exotic). In spite of its low decay, the highest leaf nitrogen was found in R. pseudoacacia litter. Macroinvertebrate communities colonizing litter bags were similar across species. Most of them were collectors (i.e., they feed on fine particulate organic matter), suggesting that leaf material of either invasive or native trees was used as substrate both for the animals and for the organic matter they feed on. Our results suggest that the replacement of the native Fraxinus by Robinia would imply a reduction in the rate of leaf processing and also a slower release of leaf nitrogen to water.     

Plant palatability to leaf‐cutter ants (Atta laevigata) and litter decomposability in a Neotropical woodland savanna

Although leaf‐cutter ants have been recognized as the dominant herbivore in many Neotropical ecosystems, their role in nutrient cycling remains poorly understood. Here we evaluated the relationship between plant palatability to leaf‐cutter ants and litter decomposability. Our rationale was that if preference and decomposability are related, and if ant consumption changes the abundance of litter with different quality, then ant herbivory could affect litter decomposition by affecting the quality of litter entering the soil. The study was conducted in a woodland savanna (cerrado denso) area in Minas Gerais, Brazil. We compared the decomposition rate of litter produced by trees whose fresh leaves have different degrees of palatability to the leaf‐cutter ant Atta laevigata. Our experiments did not indicate the existence of a significant relationship between leaf palatability to A. laevigata and leaf‐litter decomposability. Although the litter mixture composed of highly palatable plant species showed, initially, a faster decay rate than the mixture of poorly palatable species, this difference was no longer visible after about 6 months. Results were consistent regardless of whether litter invertebrates were excluded or not from litter bags. Similarly, experiments comparing the decomposition rate of litter from pairs of related plant species also showed no association between plant palatability and decomposition. Decomposition rate of the more palatable species was faster, slower or similar to that of the less palatable species depending upon the particular pair of species being compared. We suggest that the traits that mostly influence the decomposition rate of litter produced by cerrado trees may not be the same as those that influence plant palatability to leaf‐cutter ants. Atta laevigata select leaves of different species based – at least in part – on their nitrogen content, but N content was a poor predictor of the decomposition rates of the species we studied.     

It is about time: genetic variation in the timing of leaf‐litter inputs influences aquatic ecosystems

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Riparian leaf litter decomposition on pond bottom after a retention on floating vegetation

Allochthonous (e.g., riparian) plant litter is among the organic matter resources that are important for wetland ecosystems. A compact canopy of free‐floating vegetation on the water surface may allow for riparian litter to remain on it for a period of time before sinking to the bottom. Thus, we hypothesized that canopy of free‐floating vegetation may slow decomposition processes in wetlands. To test the hypothesis that the retention of riparian leaf litter on the free‐floating vegetation in wetlands affects their subsequent decomposition on the bottom of wetlands, a 50‐day in situ decomposition experiment was performed in a wetland pond in subtropical China, in which litter bags of single species with fine (0.5 mm) or coarse (2.0 mm) mesh sizes were placed on free‐floating vegetation (dominated by Eichhornia crassipes, Lemna minor, and Salvinia molesta) for 25 days and then moved to the pond bottom for another 25 days or remained on the pond bottom for 50 days. The leaf litter was collected from three riparian species, that is, Cinnamomum camphora, Diospyros kaki, and Phyllostachys propinqua. The retention of riparian leaf litter on free‐floating vegetation had significant negative effect on the carbon loss, marginal negative effects on the mass loss, and no effect on the nitrogen loss from leaf litter, partially supporting the hypothesis. Similarly, the mass and carbon losses from leaf litter decomposing on the pond bottom for the first 25 days of the experiment were greater than those from the litter decomposing on free‐floating vegetation. Our results highlight that in wetlands, free‐floating vegetation could play a vital role in litter decomposition, which is linked to the regulation of nutrient cycling in ecosystems.     

Macroinvertebrate identity mediates the effects of litter quality and microbial conditioning on leaf litter recycling in temperate streams

Plant litter decomposition is an essential ecosystem function that contributes to carbon and nutrient cycling in streams. Aquatic shredders, mainly macroinvertebrates, can affect this process in various ways; they consume leaf litter, breaking it down into fragments and creating suitable habitats or resources for other organisms through the production of fine particulate organic matter (FPOM). However, measures of litter‐feeding traits across a wide range of aquatic macroinvertebrates are still rare. Here, we assessed the contributions of 11 species of freshwater macroinvertebrates to litter decomposition, by measuring consumption rate, FPOM production, and assimilation rate of highly decomposable (Alnus glutinosa) or poorly decomposable (Quercus robur) leaf litter types. In general, an increase in the quality of litter improved the litter consumption rate, and fungal conditioning of the leaf litter increased both the litter consumption rate and FPOM production. Macroinvertebrates specializing in leaf litter consumption also appeared to be the most sensitive to shifts in litter quality and the conditioning process. Contrary to expectations, the conditioning process did not increase the assimilation of low‐quality litter. There was a strong correlation between the relative consumption rate (RCR) of the two litter types, and the relative FPOM production (RFP) was strongly correlated to the RCR. These findings suggest a consistent relationship between RCR and macroinvertebrate identity that is not affected by litter quality, and that the RFP could be inferred from the RCR. The varying responses of the macroinvertebrate feeding traits to litter quality and the conditioning process suggest that the replacement of a shredder invertebrate species by another species could have major consequences for the decomposition process and the detritus‐based food web in streams. Further studies onto the importance of invertebrate identity and the effects of litter quality in a variety of freshwater ecosystems are needed to understand the whole ecosystem functioning and to predict its response to environmental changes.     

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.     

Effects of diversity loss on ecosystem function across trophic levels and ecosystems: A test in a detritus‐based tropical food web

Abstract We investigated the effects of biodiversity loss across trophic levels and across ecosystems (terrestrial to aquatic) on ecosystem function, in a detritus‐based tropical food web. Diversities of consumers (stream shredders) and resources (leaf litter) were experimentally manipulated by varying the number of species from 3 to 1, using different species combinations, and the effects on leaf breakdown rates were examined. In single‐species shredder treatments, leaf diversity loss affected breakdown rates, but the effect depended on the identity of the leaves remaining in the system: they increased when the most preferred leaf species remained, but decreased when this species was lost (leaf preferences were the same for all shredders). In multi‐species shredder assemblages, breakdown rates were lower than expected from single‐species treatments, suggesting an important role of interspecific competition. This pattern was also evident when oneleaf species was available but not with higher leaf diversity, suggesting that lowered leaf diversity promotes competitive interactions among shredders. The influence of diversity and identity of species across trophic levels and ecosystems on stream functioning points to complex interactions that may well be reflected in other types of ecosystem.     

Detritivore stoichiometric diversity alters litter processing efficiency in a freshwater ecosystem

Many studies have estimated relationships between biodiversity and ecosystem functioning, and observed generally positive effects. Because detritus is a major food resource in stream ecosystems, decomposition of leaf litter is an important ecosystem process and many studies report the full range of positive, negative and no effects of diversity on decomposition. However, the mechanisms underlying decomposition processes in fresh water remain poorly understood. Organism body stoichiometry relates to consumption rates and tendencies, and decomposition processes of litter may therefore be affected by diversity in detritivore body stoichiometry. We predicted that the stoichiometric diversity of detritivores (differences in C: nutrient ratios among species) would increase the litter processing efficiency (litter mass loss per total capita metabolic capacity) in fresh water through complementation regarding different nutrient requirements. To test this prediction, we conducted a microcosm experiment wherein we manipulated the stoichiometric diversity of detritivores and quantified mass loss of leaf litter mixtures. We compared litter processing efficiency among litter species in each microcosm with single species detritivores, and observed detritivores with nutrient‐rich bodies tended to prefer litter with lower C: nutrient ratios over litter with higher C: nutrient ratios. Furthermore, litter processing efficiencies were significantly higher in the microcosms containing species of detritivores with both nutrient‐rich and ‐poor bodies than microcosms containing species of detritivores including only nutrient‐rich or ‐poor bodies. This might mean a higher stoichiometric diversity of detritivores increased litter processing efficiency. Our results suggest that ecological stoichiometry may improve understanding of links between biodiversity and ecosystem function in freshwater ecosystems.     

Riparian plant species loss alters trophic dynamics in detritus-based stream ecosystems

Riparian vegetation is closely connected to stream food webs through input of leaf detritus as a primary energy supply, and therefore, any alteration of plant diversity may influence aquatic ecosystem functioning. We measured leaf litter breakdown rate and associated biological parameters in mesh bags in eight headwater streams bordered either with mixed deciduous forest or with beech forest. The variety of leaf litter types in mixed forest results in higher food quality for large-particle invertebrate detritivores (‘shredders’) than in beech forest, which is dominated by a single leaf species of low quality. Breakdown rate of low quality (oak) leaf litter in coarse mesh bags was lower in beech forest streams than in mixed forest streams, a consequence of lower shredder biomass. In contrast, high quality (alder) leaf litter broke down at similar rates in both stream categories as a result of similar shredder biomass in coarse mesh bags. Microbial breakdown rate of oak and alder leaves, determined in fine mesh bags, did not differ between the stream categories. We found however aquatic hyphomycete species richness on leaf litter to positively co-vary with riparian plant species richness. Fungal species richness may enhance leaf litter breakdown rate through positive effects on resource quality for shredders. A feeding experiment established a positive relationship between fungal species richness per se and leaf litter consumption rate by an amphipod shredder (Gammarus fossarum). Our results show therefore that plant species richness may indirectly govern ecosystem functioning through complex trophic interactions. Integrating microbial diversity and trophic dynamics would considerably improve the prediction of the consequences of species loss.     

Effects of leaf litter addition on meiofaunal colonization of azoic sediments in a subtropical mangrove in Hong Kong

The responses of major meiofaunal taxa and nematode species assemblage to the decaying leaf litter of the mangrove Kandelia candel were investigated through a field colonization experiment in subtropical Hong Kong. Sixty-four replicate azoic and organic-free sediment cores were treated with leaf litter additions of 0x, 0.5x, 1x and 2x natural sediment organic concentration, respectively, and retrieved 1, 10, 30 and 60 days post-placement. Replicate cores of ambient sediment were also taken at each sampling date to provide baseline information. Results of ANOVAs suggested that either different meiofaunal taxa responded to the leaf litter in different ways or the response of the same taxon changed over decomposition time. Multivariate ordination performed on nematodes revealed an alteration in community structure after 10, 30 and 60 days between controls and treatments. This alteration was attributed to some deposit feeding nematodes, particularly a bacterivorous species, Diplolaimella sp., which bloomed in all the cores treated with leaf litter, testifying to the important role such meiofauna plays in the process of detritus decomposition.     

Litter decomposition in a temperate and a tropical stream: the effects of species mixing,litter quality and shredders

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Are functional traits and litter decomposability coordinated across leaves,twigs and wood? A test using temperate rainforest tree species

Synthesis This study compared the decomposability of leaf, twig and wood litter from 27 co‐occurring temperate rainforest tree species in New Zealand. We found that interspecific variation in decomposition was not coordinated across the three litter types. Analysis of the relationships between functional traits and decomposition revealed that traits predictive of wood decomposition varied among the species independently from traits predictive of the decomposition of leaf and twig litter. We conclude that efforts to understand how tree species influence C, N and P dynamics in forested ecosystems through the decomposition pathway need to consider the functional traits of multiple plant structures. Plant functional traits are increasingly used to evaluate changes in ecological and ecosystem processes. However our understanding of how functional traits coordinate across different plant structures, and the implications for trait‐driven processes such as litter decomposition, remains limited. We compared the functional traits of green leaves and leaf, twig and wood litter among 27 co‐occurring tree species from New Zealand, and quantified the loss of mass, N and P from the three litter types during decomposition. We hypothesised that: a) the functional traits of green leaves, and leaf, twig and wood litter are co‐ordinated so that species which produce high quality leaves and leaf litter will also produce high quality twig and wood litter, and b) the decomposability of leaf, twig and wood litter is coordinated because breakdown of all three litter types is driven by similar combinations of traits. Trait variation across species was co‐ordinated between leaves, twigs and wood when angiosperm and gymnosperm species were considered in combination, or when angiosperms were considered separately, but trait coordination was poor for gymnosperms. There was little coordination among the three litter types in their decomposability, especially when angiosperms and gymnosperms were considered separately; this was caused by the decomposability of each of the three litter types, at least partially, being driven by different functional traits or trait combinations. Our findings indicate that although interspecific variation in the functional traits of trees can be coordinated among leaves, twigs and wood, different or unrelated traits predict the decomposition of these different structures. Furthermore, leaf‐level analyses of functional traits are not satisfactory proxies for function of whole trees and related ecological processes. As such, efforts to understand how tree species influence C, N and P dynamics in forested ecosystems through the decomposition pathway need to consider functional traits of other plant structures.     

Comparisons of Macrophyte Breakdown,Associated Plant Chemistry,and Macroinvertebrates in a Wastewater Dominated Stream

Replacement of native macrophyte species with exotic or invasive ones affects the quality of detritus entering streams and can alter nutrient cycles and community structure in aquatic ecosystems. Decomposition of air‐dried native hardstem bulrush (Schoenoplectus acutus), invasive southern cattail (Typha domingensis), and exotic common reed (Phragmites australis) were studied in an urban stream (Las Vegas, Nevada, USA) using litter bags. Samples were analyzed for dry mass, lignin, nutrients, trace elements, and macroinvertebrates. Litter type and sediment deposited on plant material influenced material loss. Trace elements arsenic and selenium increased in plant material to concentrations considered marginal for ecosystem contamination by exposure day 76. Mercury increases were inconsistent across plant species and did not exceed limits. Bulrush decomposed faster, and tended to have higher selenium concentrations, than did invasive southern cattail and exotic common reed. Macroinvertebrate communities colonizing litter bags were similar across plant litter types, but differed from mesh‐only bags and samples collected with a kick‐net. Macroinvertebrate exclusion resulted in significantly lower loss rates, but functional feeding groups such as shredders were not associated with decomposition differences. The caddisfly, Smicridea, physically modified stem material and aided in processing, but microbes appeared most important in biological material breakdown. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Palatability, decomposition and insect herbivory: patterns in a successional old-field plant community

We tested the hypothesis that selective feeding by insect herbivores in an old‐field plant community induces a shift of community structure towards less palatable plant species with lower leaf and litter tissue quality and may therefore affect nutrient cycling. Leaf palatability of 20 herbaceous plant species which are common during the early successional stages of an old‐field plant community was assayed using the generalist herbivores Deroceras reticulatum (Mollusca: Agriolomacidae) and Acheta domesticus (Ensifera: Gryllidae). Palatability was positively correlated with nitrogen content, specific leaf area and water content of leaves and negatively correlated with leaf carbon content and leaf C/N‐ratio. Specific decomposition rates were assessed in a litter bag experiment. Decomposition was positively correlated with nitrogen content of litter, specific leaf area and water content of living leaves and negatively correlated with leaf C/N‐ratio. When using phylogentically independent contrasts the correlations between palatability and decomposition versus leaf and litter traits remained significant (except for specific leaf area) and may therefore reflect functional relationships. As palatability and decomposition show similar correlations to leaf and litter traits, the correlation between leaf palatability and litter decomposition rate was also significant, and this held even in a phylogenetically controlled analysis. This correlation highlights the possible effects of invertebrate herbivory on resource dynamics. In a two‐year experiment we reduced the density of above‐ground and below‐ground insect herbivores in an early successional old‐field community in a two‐factorial design by insecticide application. The palatability ranking of plants showed no relationship with the specific change of cover abundance of plants due to the reduction of above‐ or below‐ground herbivory. Thus, changes in the dominance structure as well as potentially associated changes in the resource dynamics are not the result of differences in palatability between plant species. This highlights fundamental differences between the effects of insect herbivory on ecosystems and published results from vertebrate‐grazing systems.     

Direct and indirect food web regulation of microbial decomposers in headwater streams

The direct and indirect regulation of primary productivity has been well established in autotrophic‐based ecosystems; however, less is known about the processes affecting decomposers in detrital‐based ecosystems. Because, small headwater, woodland streams are a dominate feature in most ecosystems and are tightly linked to terrestrial detritus, understanding decomposer‐mediated functions in these systems is critical for understanding carbon processes across the landscape. In this light, we conducted a microcosm and mesocosm experiment to test the direct and indirect food web effects on decomposers in small stream ecosystems. The results from the microcosm experiment supported an existing literature, demonstrating that nutrients directly stimulate decomposers and that microbivores directly reduce decomposers. Based on well‐founded food web theory in autotrophic systems, we predicted that fishes from different trophic‐functional guilds would indirectly stimulate decomposers by enhancing dissolved nutrients and by reducing microbivore densities. Our mesocosm experiment partially supported these predictions. Specifically, we found that fishes that consumed mostly terrestrial foods increased decomposers from the bottom–up by enhancing allochthonous nutrient loading into the stream ecosystems. Contrary to our predictions, however, predatory fishes that consume microbivores did not increase decomposers from the top–down. Rather, in streams with the predatory fish species, microbivores increased (rather than decreased) on leaf litter. This may have resulted from an experimental artifact associated with refuge provided by leaf packs. In conclusion, our data demonstrate that decomposers are regulated by similar direct and indirect processes important in autotrophic‐based ecosystems. This provides further evidence that food web processes can regulate leaf decomposition and flux of detrital carbon through ecosystems.     

Elevated UV-B radiation has no effect on litter quality and decomposition of two dune grassland species: evidence from a long-term field experiment

From studies on living plant tissues it has been inferred that elevated UV‐B radiation could negatively affect litter quality and subsequent decomposition. However, in general, the effects of UV‐B radiation on litter chemistry and decomposition reported in the literature are variable and are often only marginally (if at all) significant. This might be due to the ecologically unrealistic conditions under which these experiments were performed. We investigated the effects of elevated UV‐B radiation on litter quality and subsequent decomposition on initial litter chemistry and long‐term (2 years) decomposition of freshly senesced Carex arenaria and Calamagrostis epigejos leaf litter under ecologically realistic conditions. This material was collected from a dune grassland that had received UV‐B radiation treatments for three growing seasons. It was then used in a 2‐year decomposition study using litter bags. We found no significant effects of elevated UV‐B radiation on any of the litter chemistry parameters in either of the two species, nor did we find significant effects on litter decomposition. However, we did find significant differences in litter decomposition between the species. These differences were related to the interspecific differences in litter chemistry, particularly the litter phenolics concentration. These results show that litter quality and decomposition in dune grasslands are, also under ecologically realistic conditions, not affected by UV‐B radiation. Instead, litter decomposition is determined by constitutive interspecific differences in litter chemistry. In conclusion, with our results added to the already existing literature, the preponderance of evidence now clearly suggests that elevated UV‐B radiation has very little, if any, impact on litter quality and subsequent decomposition in real ecosystems.     

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.