Biodiversity and litter decomposition: a case study in a Mediterranean stream

The importance of riparian diversity for the functioning of headwater streams has been demonstrated repeatedly. For example, mixing litter from different riparian tree species can influence their decomposition rates, an effect that is contingent on both the litter assemblage and the detritivore community. However, the effects of mixing litter species have been studied mostly in temperate streams, and very few studies have been done in non- temperate streams. Mediterranean streams are often subjected to recurrent flow intermittency, and their riparian tree and stream detritivore communities have unique sets of species and traits, which probably influence the effects of litter mixtures on decomposition. We hypothesized that high dissimilarity in litter traits could promote effects of litter mixtures on decomposition that would be counteracted by the low abundance and small body size of detritivores in Mediterranean streams. We manipulated litter diversity and the size-class and presence/absence of detritivores in a 2nd-order Mediterranean stream in a 46-d experiment and found substantial but contingent effects on litter mixtures. Mixture effects were not significant on average, but both negative and positive effects of litter mixtures occurred. For instance, mixing soft and nutrient-rich litter species led to up to 9.6% increase in leaf mass loss. Microbial activity accounted for 85% of total leaf mass loss, and no effect of litter mixture was observed when detritivores were excluded. In contrast, the presence of detritivores, despite their relatively low abundance and diversity, was a key factor for litter decomposition and promoted effects of litter mixture. These results suggest that the extinction of a few key taxa (riparian tree species or large detritivores) could impair nutrient and C cycling in Mediterranean streams with potential consequences for stream food webs

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

Litter identity mediates predator impacts on the functioning of an aquatic detritus-based food web

During past decades, several mechanisms such as resource quality and habitat complexity have been proposed to explain variations in the strength of trophic cascades across ecosystems. In detritus-based headwater streams, litter accumulations constitute both a habitat and a resource for detritivorous macroinvertebrates. Because litter edibility (which promotes trophic cascades) is usually inversely correlated with its structural complexity (which weakens trophic cascades), there is a great scope for stronger trophic cascades in litter accumulations that are dominated by easily degradable litter species. However, it remains unclear how mixing contrasting litter species (conferring both habitat complexity and high quality resource) may influence top–down controls on communities and processes. In enclosures exposed in a second-order stream, we manipulated litter species composition by using two contrasting litter (alder and oak), and the presence–absence of a macroinvertebrate predator (Cordulegaster boltonii larvae), enabling it to effectively exert predation pressure, or not, on detritivores (consumptive versus non-consumptive predation effects). Leaf mass loss, detritivore biomass and community structure were mostly controlled independently by litter identity and mixing and by predator consumption. However, the strength of predator control was mediated by litter quality (stronger on alder), and to a lesser extent by litter mixing (weaker on mixed litter). Refractory litter such as oak leaves may contribute to the structural complexity of the habitat for stream macroinvertebrates, allowing the maintenance of detritivore communities even when strong predation pressure occurs. We suggest that considering the interaction between top–down and bottom–up factors is important when investigating their influence on natural communities and ecosystem processes in detritus-based ecosystems

Diversity patterns of leaf-associated aquatic hyphomycetes along a broad latitudinal gradient

Information about the global distribution of aquatic hyphomycetes is scarce, despite the primary importance of these fungi in stream ecosystem functioning. In particular, the relationship between their diversity and latitude remains unclear, due to a lack of coor- dinated surveys across broad latitudinal ranges. This study is a first report on latitudinal patterns of aquatic hyphomycete diversity associated with native leaf-litter species in five streams located along a gradient extending from the subarctic to the tropics. Exposure of leaf litter in mesh bags of three different mesh sizes facilitated assessing the effects of including or excluding different size-classes of litter-consuming invertebrates. Aquatic hyphomycete evenness was notably constant across all sites, whereas species richness and diversity, expressed as the Hill number, reached a maximum at mid-latitudes (Medi- terranean and temperate streams). These latitudinal patterns were consistent across litter species, despite a notable influence of litter identity on fungal communities at the local scale. As a result, the bell-shaped distribution of species richness and Hill diversity devi- ated markedly from the latitudinal patterns of most other groups of organisms. Differences in the body-size distribution of invertebrate communities colonizing the leaves had no effect on aquatic hyphomycete species richness, Hill diversity or evenness, but inverte- brates could still influence fungal communities by depleting litter, an effect that was not captured by the design of our experiment

Seasonal variations overwhelm temperature effects on microbial processes in headwater streams: insights from a temperate thermal spring

Carbon cycling in headwater streams is mostly driven by the decomposition of allochthonous organic matter, and to a lesser extent by primary production. Quantifying the influence of temperature on these processes is therefore essential to better anticipate the consequences of global warming for stream ecological functioning. In this study, we measured alder litter microbial decomposition and associated fungal biomass and diversity, using leaf discs enclosed in fine-mesh bags along a natural geothermal temperature gradient, in both spring and winter. We monitored the chlorophyll-a accrual in biofilms growing on ceramic tiles. The temperature gradient, from upstream to downstream, ranged from 15.3 to 14.2 °C in spring and 18.2 to 13.2 °C in winter. Autotrophs and heterotrophs exhibited contrasting responses to temperature. The expected positive effect of temperature was actually observed for chlorophyll-a accrual only, while an apparent temperature-independence of litter decomposition rate was found. Moreover, temperature effects on heterotrophic and autotrophic organisms depended on the season, with higher litter decomposition rates, sporulation rates, fungal biomass and chlorophyll-a in spring, despite a lower mean water temperature than in winter. Together, these results suggest that the influence of temperature remained largely overrode by seasonal effects. This result is likely due to annual variations in light availability, and may involve indirect positive interactions between microbial primary producers and decomposers

Litter Quality Modulates Effects of Dissolved Nitrogen on Leaf Decomposition by Stream Microbial Communities

Rates of leaf litter decomposition in streams are strongly influenced both by inorganic nutrients dissolved in stream water and by litter traits such as lignin, nitrogen (N) and phosphorus (P) concentrations. As a result, decomposition rates of different leaf species can show contrasting responses to stream nutrient enrichment resulting from human activities. It is unclear, however, whether the root cause of such discrepancies in field observations is the interspecific variation in either litter nutrient or litter lignin concentrations. To address this question, we conducted a controlled laboratory experiment with a known fungal community to determine decomposition rates of 38 leaf species exhibiting contrasting litter traits (N, P and lignin concentrations), which were exposed to 8 levels of dissolved N concentrations representative of field conditions across European streams (0.07 to 8.96 mg N L−1). The effect of N enrichment on decomposition rate was modelled using Monod kinetics to quantify N effects across litter species. Lignin concentration was the most important litter trait determining decomposition rates and their response to N enrichment. In particular, increasing dissolved N supply from 0.1 to 3.0 mg N L−1 accelerated the decomposition of lignin-poor litter (e.g.  15% of lignin, 1.4× increase ± 0.2 SD, n = 9). Litter nutrient concentrations were less important, with a slight positive effect of P on decomposition rates and no effect of litter N. These results indicate that shifts in riparian vegetation towards species characterized by high litter lignin concentrations could alleviate the stimulation of C turnover by stream nutrient enrichment

TerrHum: an iOS application for classifying terrestrial humipedons and some considerations about soil classification

International audienceThe name TerrHum is an abbreviation of the words “Terrestrial” (not hydromorphic, not submerged) and “Humipedon” (organic and organic-mineral humus horizons). With this application, it is possible to describe and classify terrestrial forest and grassland topsoils in a system published as a Special Issue entitled “Humusica 1– Terrestrial Natural Humipedons” in the journal Applied Soil Ecology. The iOS application TerrHum allows the storage of the main content of Humusica 1 on a cellular phone. Images, diagrams and simplified tables of classification may be recalled with a few touches on the screen. Humus forms, representing five humus systems, are classified based on the vertical arrangement of diagnostic horizons and their attributes. TerrHum allows accessing specific figures that are stored in a virtual cloud and can be downloaded the first time the user recalls them. Once all figures have been opened in the device, the application is ready to use, without any further internet connection. The application is in continuous evolution

TerrHum: an iPhone app for classifying forest humipedons.

The knowledge of a little number of specific terms is necessary to investigate and describe the forest topsoils: diagnostic components, diagnostic organic and organic-mineral horizons and the 17 series of humus horizons composing all the observed real forest not submerged topsoils. Diagnostic horizons are grouped in humus forms, which represent five humus systems. To become a good topsoil investigator is then only a question of field experience. No mean to do otherwise: you must go in the field with a blade and a good manual and put your hand in the soil. You have to make a hole and to observe on your knee a wall of the pit, from the top to the bottom, detecting all the characters that you find indicated in the manual. At the beginning you will be discouraged, things change from a site to another and never are exactly as in the manual. After few days of difficult survey, you will be able to know your soil even without doing a hole. Be patient and follow what it is indicated in the published first eight articles of Humusica (http://intra.tesaf.unipd.it/people/zanella/hmanual.html). On the poster, you find some examples of diagnostic properties of forest topsoils, and a dichotomy key of classification, you can copy paste and take with you in the field. An iPhone application (Terrhum) allows to bring in the field the necessary information for a fast classification of the topsoil

Effects of mineral soil and forest floor on the regeneration of pedunculate oak, beech and red oak

Early regeneration is a critical life stage that affects the future species composition of forests. Knowledge about regeneration success under different environmental conditions allows better understanding of forest dynamics. We studied the effects of seedbed conditions on the establishment and performance of seedlings of pedunculate oak, beech and red oak. In 50 plots of a tree-diversity oriented research platform in mature forests in northern Belgium (TREEWEB), we installed a field experiment with three treatments (potting soil, mineral soil, mineral soil + forest floor), in which we sowed seeds of each species. We monitored early establishment and survival, height, root and shoot biomass of the seedlings after two growing seasons. Mineral soil negatively affected seedling establishment and performance relative to the potting soil. The negative soil effects did not vary with measured abiotic soil properties. In general, the forest floor did not deteriorate or mitigate the soil effects, and only for root biomass did the forest floor partly compensate the negative soil effects. Forest floor effects did not vary with the measured forest floor properties. In the studied forests, creating bare soil was not enough to promote regeneration; improving soil properties might be important for the success of natural regeneration.Fil: De Groote, Stefanie R. E.. University of Ghent; BélgicaFil: Vanhellemont, Margot. University of Ghent; BélgicaFil: Baeten, Lander. University of Ghent; BélgicaFil: Carón, María Mercedes. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto Multidisciplinario de Biología Vegetal. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Instituto Multidisciplinario de Biología Vegetal; Argentina. Universidad Nacional de Salta; ArgentinaFil: Martel, An. University of Ghent; BélgicaFil: Bonte, Dries. University of Ghent; BélgicaFil: Lens, Luc. University of Ghent; BélgicaFil: Verheyen, Kris. University of Ghent; Bélgic