Stem Trait Spectra Underpin Multiple Functions of Temperate Tree Species

A central paradigm in comparative ecology is that species sort out along a slow-fast resource economy spectrum of plant strategies, but this has been rarely tested for a comprehensive set of stem traits and compartments. We tested how stem traits vary across wood and bark of temperate tree species, whether a slow-fast strategy spectrum exists, and what traits make up this plant strategy spectrum. For 14 temperate tree species, 20 anatomical, chemical, and morphological traits belonging to six key stem functions were measured for three stem compartments (inner wood, outer wood, and bark). The trait variation was explained by major taxa (38%), stem compartments (24%), and species within major taxa (19%). A continuous plant strategy gradient was found across and within taxa, running from hydraulic safe gymnosperms to conductive angiosperms. Both groups showed a second strategy gradient related to chemical defense. Gymnosperms strongly converged in their trait strategies because of their uniform tracheids. Angiosperms strongly diverged because of their different vessel arrangement and tissue types. The bark had higher concentrations of nutrients and phenolics whereas the wood had stronger physical defense. The gymnosperms have a conservative strategy associated with strong hydraulic safety and physical defense, and a narrow, specialized range of trait values, which allow them to grow well in drier and unproductive habitats. The angiosperm species show a wider trait variation in all stem compartments, which makes them successful in marginal- and in mesic, productive habitats. The associations between multiple wood and bark traits collectively define a slow-fast stem strategy spectrum as is seen also for each stem compartment

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

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

Contrasting altitudinal trends in leaf anatomy between three dominant species in an alpine meadow

Variation in leaf anatomical traits underpins the adaptations and phenotypic responses of plant species to their different natural environments. Temperature is a primary driver of plant trait variation along altitudinal gradients. However, other environmental drivers may also play important roles, and the interactions between drivers may have different effects on leaf anatomy for different species of the same larger clade. Such interactions might be especially important along shorter altitudinal (i.e. temperature) gradients. We predicted, therefore, that different monocot species could show idiosyncratic responses of leaf anatomical traits to a short altitudinal gradient. Moreover, for a species in which vegetative growth and reproduction are separated in time, its anatomical responses to altitude may differ and trade-offs between leaf and flowering stem anatomy may occur. To test these hypotheses, we examined leaf anatomy and δ 13 C signature (a possible indicator of anatomy-related water use efficiency or indicator of response to a decrease in CO 2 concentration with altitude) of three dominant and widely distributed monocot species (Scirpus distigmaticus, Elymus nutans, Carex moorcroftii) from seven elevations in an alpine meadow on the Qinghai-Tibetan Plateau. In addition, we examined the flowering stem anatomy of S. distigmaticus, across a short altitudinal gradient (four elevations) in the same region. Leaf anatomical traits (e.g. epidermal cell area, epidermal cell thickness, cuticular layer thickness, xylem transect area, phloem transect area) varied with altitude, but the patterns varied substantially among species and among anatomical traits within species. Additionally, for S. distigmaticus, (allometric) coordination between leaves and flowering stems was apparent for xylem transect area and phloem transect area. Trade-offs between leaf and flowering stem traits were also found for epidermal cell area, epidermal cell thickness and mesophyll cell area. Leaves were more responsive to altitude in their anatomical traits than flowering stems in S. distigmaticus, perhaps reflecting their relatively short period of stem development during a climatically relatively favourable season compared with that for leaves, which already start growing earlier in the year. Further research is needed on the interactive effects of environmental variables, as well as vegetative versus reproductive phenology both across and within suites of species to better understand and upscale plant anatomical responses to climate warming in alpine environments

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

Tree species identity in high-latitude forests determines fire spread through fuel ladders from branches to soil and vice versa

Peat fires in boreal and tundra regions can potentially cause a high CO2 release, because of their large soil carbon stocks. Under current and future climate warming the frequency and intensity of droughts are increasing and will cause the plant community and organic soil to become more susceptible to fire. The organic soil consumption by fire is commonly used as a proxy for fire severity and is a large source of carbon release. However, the role of organic soils in both above- and belowground fire behavior has only rarely been studied. In this study we collected soil and branches from Betula pubescens, Pinus sylvestris and Picea abies/obovata from the taiga/tundra ecotone across a large spatial scale. In laboratory fire experiments we burned different fuel type combinations to examine the fire spread through fuel ladders both from branches to soil and vice versa. We found that the tree species identity influences the fire spread from branches to soil and vice versa. The combination of chemical and structural plant traits could explain the stronger interaction between soil and coniferous spruce and pine fuels in a fire ladder compared to the deciduous birch. Therefore, total carbon emission from a boreal forest fire may not only depend on burned plant fuel, but also on the species-specific potential of the trees to ignite the soil. Carbon emission models and forest management could be improved if not only the aboveground plant fuel consumption is considered, but also the interaction between fuels in a fuel ladder and the probability of soil ignition by a forest crown fire and vice versa

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

Earthworms are not just “earth” worms: Multiple drivers to large diversity in deadwood

Earthworms are ecosystem engineers associated with important soil functions. Despite the large amount of literature on earthworm ecology, relatively few studies have examined earthworms in deadwood or quantified their importance in this habitat. We investigated earthworm communities in decaying deadwood and disentangled how their community dynamics are influenced by variation in tree species, wood decomposition stage, and forest environment. Decaying logs (of standardised size) of ten common, temperate tree species were laid out to decay on the soil surface for four years. The experiment was carried out in the “tree cemetery” experiment LOGLIFE, in two contrasting temperate forests in the central Netherlands. The decaying logs yielded surprisingly rich earthworm populations, with on average 19 individuals per meter of log and in total belonging to 12 different species. Our findings highlighted that earthworm communities in deadwood in terms of composition and abundance were influenced by tree species, wood decomposition stage and forest type with different soil properties, as well as their interactions. After one and two years of decay, earthworm abundance was higher in the logs of Picea abies and relatively fast decomposing Populus spp. than in other trees, while this pattern changed after four years with higher earthworm abundance observed in the other tree species. Overall, Populus spp. supported the highest earthworm abundance, followed by Picea abies and Quercus robur. The earthworm community composition in the logs in the two forest sites had broadly similar dynamic trends of first becoming very dissimilar between one and two years, then relatively more similar from two to four years of decay, although the community composition differed between sites. The interacting influences of tree species, wood decay stage and forest environment on earthworm communities strongly suggest that diversity in deadwood resources contributes to earthworm diversity in forests. Thus, by mixing different tree species and logging gradually through the years, forest managers could enhance the diversity of this abundant and understudied component of deadwood invertebrate diversity. Further research should study the feedback loop between earthworm communities and decomposition of deadwood