Filtration artefacts in bacterial community composition can affect the outcome of dissolved organic matter biolability assays

Inland waters are large contributors to global carbon dioxide (CO2) emissions, in part due to the vulnerability of dissolved organic matter (DOM) to microbial decomposition and respiration to CO2 during transport through aquatic systems. To assess the degree of this vulnerability, aquatic DOM is often incubated in standardized biolability assays. These assays isolate the dissolved fraction of aquatic OM by size filtration prior to incubation. We test whether this size selection has an impact on the bacterial community composition and the consequent dynamics of DOM degradation using three different filtration strategies: 0.2 μm (filtered and inoculated), 0.7 μm (generally the most common DOM filter size) and 106 μm (unfiltered). We found that bacterial community composition, based on 16S rRNA amplicon sequencing, was significantly affected by the different filter sizes. At the same time, the filtration strategy also affected the DOM degradation dynamics, including the δ13C signature. However, the dynamics of these two responses were decoupled, suggesting that filtration primarily influences biolability assays through bacterial abundance and the presence of their associated predators. By the end of the 41-day incubations all treatments tended to converge on a common total DOM biolability level, with the 0.7 μm filtered incubations reaching this point the quickest. These results suggest that assays used to assess the total biolability of aquatic DOM should last long enough to remove filtration artefacts in the microbial population. Filtration strategy should also be taken into account when comparing results across biolability assays

Faunal community consequence of interspecific bark trait dissimilarity in early-stage decomposing logs

Dead tree trunks have significant ecosystem functions related to biodiversity and biogeochemical cycles. When lying on the soil surface, they are colonized by an array of invertebrate fauna, but what determines their community composition is still unclear. We apply community assembly theory to colonization of tree logs by invertebrates. During early decomposition, the attached bark is critically important as an environment filter for community assembly through habitat provision. Specifically, we hypothesized that the more dissimilar bark traits were between tree species, the more their faunal community compositions would differ. We tested this hypothesis by investigating the effects of bark traits on the invertebrate communities in the early-decomposing logs of 11 common, temperate tree species placed in the ‘common garden’ experiment LOGLIFE. Bark traits included bark looseness, fissure index, outer bark thickness, ratio of inner to outer bark thickness, punch resistance, water storage capacity and bark pH. The predominant faunal groups studied were Annelida, Isopoda, Chilopoda, Diplopoda, Diptera and Coleoptera. Our results showed (i) strong interspecific differences in bark traits, (ii) that bark traits related to environmental buffering had profound effects on the abundance of specific invertebrate groups, and (iii) the higher the overall bark trait dissimilarity between tree species, the more dissimilar these tree species were in faunal community composition, and the higher was the joint invertebrate family richness. A suite of bark traits together has fundamental afterlife effects on invertebrate community assembly, strongly filtering the colonizing invertebrates in early-decomposing logs, driving variation in their community composition and diversity. Our findings indicate that bark trait dissimilarity among tree species in forest stands is likely a better indicator of early-phase dead trunk fauna diversity than tree species diversity per se. A lay summary is available for this article.</p

Stem traits, compartments and tree species affect fungal communities on decaying wood

Dead wood quantity and quality is important for forest biodiversity, by determining wood-inhabiting fungal assemblages. We therefore evaluated how fungal communities were regulated by stem traits and compartments (i.e. bark, outer- and inner wood) of 14 common temperate tree species. Fresh logs were incubated in a common garden experiment in a forest site in the Netherlands. After 1 and 4 years of decay, the fungal composition of different compartments was assessed using Internal Transcribed Spacer amplicon sequencing. We found that fungal alpha diversity differed significantly across tree species and stem compartments, with bark showing significantly higher fungal diversity than wood. Gymnosperms and Angiosperms hold different fungal communities, and distinct fungi were found between inner wood and other compartments. Stem traits showed significant afterlife effects on fungal communities; traits associated with accessibility (e.g. conduit diameter), stem chemistry (e.g. C, N, lignin) and physical defence (e.g. density) were important factors shaping fungal community structure in decaying stems. Overall, stem traits vary substantially across stem compartments and tree species, thus regulating fungal communities and the long-term carbon dynamics of dead trees

exp_2a_13C_15N_harvest_and_t0_macroinvertebrates

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Downed deadwood habitat heterogeneity drives trophic niche diversity of soil-dwelling animals

Habitat heterogeneity is one of the important drivers of biodiversity, but the underlying mechanisms are still vague. Increasing habitat heterogeneity often coincides with greater habitat space and/or greater food availability for organisms, but it is unclear whether and how these components are related to habitat selection. Downed dead wood (i.e., dead wood laying on the ground) is an important but understudied component of dead plant matter that creates spatial habitat heterogeneity for soil fauna on the ground. To unravel the role of dead wood for providing niche dimensionality for soil fauna via variance in resource use, we sampled Collembola communities from moss growing on bark and from bark on the logs of two tree species (pedunculate oak and Norway spruce) and from soil next to the logs. We assessed habitat selection of Collembola species combining abundance data with trophic position data using stable isotope ratios (i.e., δ13C and δ15N). Based on the Bray-Curtis dissimilarity, Collembola community composition differed significantly among the three habitats (i.e., moss, bark and soil). Collembola community-weighted mean δ13C and δ15N values also differed among the three habitats (average δ13C values of moss: −25.94‰, bark: −25.30‰, soil: −25.60‰; average δ15N values of moss: −4.80‰, bark: −3.83‰, soil: −5.11‰), partly reflecting isotopic differences of the habitats themselves (δ13C of moss: −29.83‰, bark: −28.26‰, soil: −29.04‰; δ15N of moss: −7.59‰, bark: −4.18‰, soil: −7.84‰). These findings indicate that Collembola depend on food sources derived from the habitats they inhabit at least at the community level. Collembola community-weighted variance of both δ13C and δ15N values ranked as bark &gt; moss &gt; soil. Species-level data suggested that the bark community included both bark-specific food specialists as well as species which likely use bark only as a temporary habitat based on their morphological traits supporting high mobility. These findings together indicate that the trophic niche variation of the Collembola community is larger in bark than in soil. Our results stress the importance of downed dead wood as a source of habitat heterogeneity, which in turn promotes soil animal diversity, likely via heterogeneity in food sources. Furthermore, the presence of dead wood on the ground could have consequences for organic matter processing by the soil animal community through sustaining a larger trophic niche variation even at a small spatial scale.</p

Non-additive effects of leaf and twig mixtures from different tree species on experimental litter-bed flammability

Aims: Tree species can affect litter flammability through leaf size and shape. Larger, simpler-shaped leaf litters form better-ventilated, more flammable litter-beds. However, leaves are generally mixed with twigs in the forest litter layer and together they likely contribute most to surface fire behavior. Here we ask: “Do leaf-twig mixtures have non-additive effects on litter-bed flammability?” Methods: Using laboratory fires, we tested the direction and magnitude of non-additivity of inter- and intra-specific leaf-twig mixtures on litter-bed flammability for four tree species contrasted in leaf size and shape and widespread in fire-prone temperate-boreal forests. Results: Across species, small needles reduced mixture fuel-bed ignitibility through filling the space between twigs and inhibiting ventilation. Within the small broad-leaved species, the thin, frequently branched and open spaced twigs were too loosely packed to be flammable, while in mixtures the small broad leaves connected these twigs to produce flammable fuel-beds. Once ignited, across species flame spread rate in mixtures was driven by leaves, while fire sustainability was predicted by fuel mass. Fuel-bed flammability was driven more by leaves at larger leaf-to-twig ratio. Conclusions: For the first time, we demonstrated the existence and mechanisms of non-additive effects of leaf-twig mixtures on experimental litter-bed flammability

Diversity of macro-detritivores in dead wood is influenced by tree species, decay stage and environment

Diplopoda (millipedes) and Isopoda (woodlice) are among the most abundant macro-detritivores in temperate forests. These key regulators of plant litter decomposition are influenced by habitat and substrate quality, including that of dead wood. Dead wood provides shelter and resources to macro-detritivores, but the relative effects of tree species, wood decay stage, forest environment and their interactions on macro-detritivore communities are poorly known. To unravel these effects, we combined a reciprocal field incubation experiment and direct field sampling to compare the Diplopoda and Isopoda communities in logs of silver birch (Betula pendula) and Norway spruce (Picea abies) in two contrasting sites in terms of soil texture, pH, fertility and microclimate. We found: (1) a curvilinear relationship between wood decay stage and abundance of Diplopoda and Isopoda, by using wood density as a measure for the decay stage; (2) the pH of dead wood was a good predictor of wood decay stage in a site with pH close to neutrality but not in an acidic site; (3) Diplopoda and Isopoda community composition on different tree species converged during the decay process, consequently tree species are more important in the substrate selection of macro-detritivores at the beginning of their dead wood decomposition; (4) tree species, the growing environment of the trees and the decomposition environment of the logs strongly determined Diplopoda and Isopoda community composition in dead wood, these drivers of macro-detritivore communities interacted with each other and with the wood decay stage. Thus, when trying to understand and predict future patterns of macro-detritivore diversity under regimes of changing land-use and climate, these interactions should be taken into account. An important next step will be to quantify the feedback of macro-detritivore community composition to dead wood decomposition itself. This feedback may be better understood from the combination of (1) the complex interactions of tree species, wood decay stage and forest environment on the macro-detritivore community and (2) the functional traits of these macro-detritivore species. A better knowledge about these feedbacks can help in predicting carbon storage and nutrient cycling functions of dead wood in forests differing or changing in tree species composition and abiotic environment. (C) 2014 Elsevier Ltd. All rights reserved

Experimentally increased nutrient availability at the permafrost thaw front selectively enhances biomass production of deep-rooting subarctic peatland species

Climate warming increases nitrogen (N) mineralization in superficial soil layers (the dominant rooting zone) of subarctic peatlands. Thawing and subsequent mineralization of permafrost increases plant-available N around the thaw-front. Because plant production in these peatlands is N-limited, such changes may substantially affect net primary production and species composition. We aimed to identify the potential impact of increased N-availability due to permafrost thawing on subarctic peatland plant production and species performance, relative to the impact of increased N-availability in superficial organic layers. Therefore, we investigated whether plant roots are present at the thaw-front (45 cm depth) and whether N-uptake (15N-tracer) at the thaw-front occurs during maximum thaw-depth, coinciding with the end of the growing season. Moreover, we performed a unique 3-year belowground fertilization experiment with fully factorial combinations of deep- (thaw-front) and shallow-fertilization (10 cm depth) and controls. We found that certain species are present with roots at the thaw-front (Rubus chamaemorus) and have the capacity (R. chamaemorus, Eriophorum vaginatum) for N-uptake from the thaw-front between autumn and spring when aboveground tissue is largely senescent. In response to 3-year shallow-belowground fertilization (S) both shallow- (Empetrum hermaphroditum) and deep-rooting species increased aboveground biomass and N-content, but only deep-rooting species responded positively to enhanced nutrient supply at the thaw-front (D). Moreover, the effects of shallow-fertilization and thaw-front fertilization on aboveground biomass production of the deep-rooting species were similar in magnitude (S: 71%; D: 111% increase compared to control) and additive (S + D: 181% increase). Our results show that plant-available N released from thawing permafrost can form a thus far overlooked additional N-source for deep-rooting subarctic plant species and increase their biomass production beyond the already established impact of warming-driven enhanced shallow N-mineralization. This may result in shifts in plant community composition and may partially counteract the increased carbon losses from thawing permafrost

Disentangling effects of key coarse woody debris fuel properties on its combustion, consumption and carbon gas emissions during experimental laboratory fire

Coarse woody debris is a key terrestrial carbon pool, and its turnover through fire plays a fundamental role in global carbon cycling. Coarse dead wood fuel properties, which vary between tree species and wood decay stages, might affect its combustion, consumption and carbon gas emissions during fire, either directly or indirectly through interacting with moisture or ground-wood contact. Using controlled laboratory burns, we tried to disentangle the effects of multiple biotic and abiotic factors: tree species (one conifer and three hard wood species), wood decay stages, moisture content, and ground-wood contact on coarse wood combustion, consumption, and CO2 and CO emissions during fire. Wood density was measured for all samples. We found that, compared to the other tested factors, wood decay stages acted as a predominant positive driver increasing coarse wood flammability and associated CO2 and CO emissions during fire. Wood moisture content (30 versus 7%) moderately inhibited wood flammability with slight interaction with wood decay effects. Wood decay effects can be mainly attributed to the decreasing wood density as wood becomes more decomposed. Our experimental data provides useful information for how several wood properties, especially moisture content and wood decay stages, with wood density as the key underlying trait, together drive coarse wood carbon turnover through fire to the atmosphere. Our results will help to improve the predictive power of global vegetation climate models on dead wood turnover and its feedback to climate

Data from: Nonadditive effects of consumption in an intertidal macroinvertebrate community are independent of food availability but driven by complementarity effects

Suboptimal environmental conditions are ubiquitous in nature and commonly drive the outcome of biological interactions in community processes. Despite the importance of biological interactions for community processes, knowledge on how species interactions are affected by a limiting resource, e.g. low food availability, remains limited. Here, we tested whether variation in food supply causes non-additive consumption patterns, using the macroinvertebrate community of intertidal sandy beaches as a model system. We quantified isotopically labelled diatom consumption by three macroinvertebrate species (Bathyporeia pilosa, Haustorius arenarius and Scolelepis squamata) kept in mesocosms in either monoculture or a 3-species community at a range of diatom densities. Our results show that B. pilosa was the most successful competitor in terms of consumption at both high and low diatom density, while H. arenarius and especially S. squamata consumed less in a community than in their respective monocultures. Non-additive effects on consumption in this macroinvertebrate community were present and larger than mere additive effects, and similar across diatom densities. The underlying species interactions, however, did change with diatom density. Complementarity effects related to niche-partitioning were the main driver of the net diversity effect on consumption, with a slightly increasing contribution of selection effects related to competition) with decreasing diatom density. For the first time we showed that non-additive effects of consumption are independent of food availability in a macroinvertebrate community. This suggests that in communities with functionally different, and thus complementary, species, non-additive effects can arise even when food availability is low. Hence, at a range of environmental conditions, species interactions hold important potential to alter ecosystem functioning