Root traits and functioning: from individual plants to ecosystemsFine roots, the most distal portions of the root system, are responsible for the uptake of water and nutrients by plants, represent the main type of plant tissue contributing to soil organic matter accrual, and are key drivers of mineral weathering and soil microbial dynamics (Bardgett et al. 2014). Despite the overwhelming importance of fine root traits for plant and plant community functioning and biogeochemical cycles, basic information about their ecology is lacking, particularly compared to the wealth of information developed for leaves and stems. Testing hypotheses on how root traits underlie these ecosystem processes has been particularly hampered due to (1) a paucity of systematically collected data and (2) the complexity of the relationships between root traits and root, plant and ecosystem functioning. Nonetheless, the development of the field of root ecology in the last two decades has been outstanding, in particular in the compilation of belowground trait datasets (Iversen et al. 2017), methodological root ecological handbooks (Freschet et al. 2021b), novel conceptual frameworks to describe root trait diversity (Bergmann et al. 2020), its connection with belowground plant and community function (Bardgett et al. 2014, Freschet et al. 2021a), species’ distributions (Laughlin et al. 2021), and scaling up traits from the individual root to the ecosystem level (McCormack et al. 2017). The papers that feature in this Special Issue on Root traits and functioning: from individual plants to ecosystems cover different climate regions, taxonomic and spatial scales, and a diversity of traits (Table 1) and form perfect examples of this upward moment of the belowground component in plant ecology.

Plant monocultures growing for extended periods face severe losses of productivity. This phenomenon, known as ‘yield decline’, is often caused by the accumulation of above- and belowground plant antagonists. The effectiveness of plant defences against antagonists might help explaining differences in yield decline among species. Using a trait-based approach, we studied the role of 20 physical and chemical defence traits of leaves and fine roots on yield decline of 18-year old monocultures of 27 grassland species. We hypothesized that yield decline is lower for species with high defences, that root defences are better predictors of yield decline than leaf defences, and that in roots, physical defences better predict yield decline than chemical defences, while the reverse is true for leaves. We additionally hypothesized that species increasing the expression of defence traits after long-term monoculture growth would suffer less yield decline. We summarized leaf and fine root defence traits using principal component analysis and analysed the relationship between defence traits mean as a measure of defence strenght and defence traits temporal changes of the most informative components and monoculture yield decline. The only significant predictors of yield decline were the mean and temporal changes of the component related to specific root length and root diameter (e.g. the so called collaboration gradient of the root economics space). The principal component analysis of the remaining traits showed strong trade-offs between defences suggesting that different plant species deploy a variety of strategies to defend themselves. This diversity of strategies could preclude the detection of a generalized correlation between the strength and temporal changes of defence gradients and yield decline. Our results show that yield decline is strongly linked to belowground processes particularly to root traits. Further studies are needed to understand the mechanism driving the effect of the collaboration gradient on yield decline.

Numerous biodiversity–ecosystem functioning (BEF) experiments have shown that plant community productivity typically increases with species diversity. In these studies, diversity is generally quantified using metrics of taxonomic, phylogenetic, or functional differences among community members. Research has also shown that the relationships between species diversity and functioning depends on the spatial scale considered, primarily because larger areas may contain different ecosystem types and span gradients in environmental conditions, which result in a turnover of the species set present locally. A fact that has received little attention, however, is that ecological systems are hierarchically structured, from genes to individuals to communities to entire landscapes, and that additional biological variation occurs at levels of organization above and below those typically considered in BEF research. Here, we present cases of diversity effects at different hierarchical levels of organization and compare these to the species-diversity effects traditionally studied. We argue that when this evidence is combined across levels, a general framework emerges that allows the transfer of insights and concepts between traditionally disparate disciplines. Such a framework presents an important step towards a better understanding of the functional importance of diversity in complex, real-world systems.

Explaining the mechanisms underlying spatial and temporal variation in community composition is a major challenge. Nevertheless, the processes controlling temporal variation at a site (i.e., temporal β-diversity, including its turnover and nestedness components) are less understood than those affecting variation among sites (i.e., spatial β-diversity). Short-term temporal turnover (e.g., throughout an annual cycle) is expected to correlate positively with seasonal environmental variability and landscape connectivity, but also species pool size (γ-diversity). We use the megadiverse Amazonian freshwater ichthyofauna as a model to ask whether seasonality and landscape connectivity drive variation in temporal species turnover among geomorphological habitat types, while taking into account between-habitat variation in γ-diversity. 11,397 fish representing 260 species were collected during a year-long sampling program in an area containing the lowland Amazon’s four major geomorphological habitat types: rivers, floodplains, terra firme streams, and shield streams. River-floodplain systems exhibit strong but predictable seasonality (via a high-amplitude annual flood pulse), high connectivity, and high species richness with many rare species. Terra firme and shield streams exhibit low seasonality, low connectivity, and low species richness with proportionally fewer rare species. Based on these parameters we predicted that river-floodplain systems should have higher temporal turnover than stream systems. Using a null model approach combined with β-deviation calculations, we confirmed that rivers and floodplains do exhibit higher turnover (but not nestedness) than terra firme and shield streams, even when controlling for the potentially confounding effect of higher species richness in river-floodplain systems. All habitats exhibit low temporal nestedness, indicating that short-term changes in community composition result primarily from temporal species turnover. Our results provide a timely reminder that efforts to conserve the Amazon’s threatened aquatic biodiversity should account for the distinct temporal dynamics of habitat types and variation in hydrological seasonality.

Biological invasions have major impacts on a variety of ecosystems and threaten native biodiversity. Earthworms have been absent from northern parts of North America since the last ice age, but non-native earthworms were recently introduced there and are now being spread by human activities. While past work has shown that plant communities in earthworm-invaded areas change towards a lower diversity mainly dominated by grasses, the underlying mechanisms related to changes in the biotic interactions of the plants are not well understood. Here, we used a trait-based approach to study the effect of earthworms on interspecific plant competition and aboveground herbivory. We conducted a microcosm experiment in a growth chamber with a full-factorial design using three plant species native to northern North American deciduous forests, Poa palustris (grass), Symphyotrichum laeve (herb), and Vicia americana (legume), either growing in monoculture or in a mixture of three. These plant community treatments were crossed with earthworm (presence or absence) and herbivore (presence or absence) treatments. Eight out of the eleven above- and belowground plant functional traits studied were significantly affected by earthworms, either by a general effect or in interaction with plant species identity, plant diversity level, and/or herbivore. Earthworms increased the aboveground productivity and the number of inflorescences of the grass P. palustris. Further, earthworms countervailed the increasing effect of herbivores on root tissue density of all species, and earthworms and herbivores individually increased the average root diameter of S. laeve in monoculture, but decreased it in mixture. In this study, earthworm presence gave a competitive advantage to the grass species P. palustris by inducing changes in plant functional traits. Our results suggest that invasive earthworms can alter competitive and multitrophic interactions of plants, shedding light on some of the mechanisms behind invasive earthworm-induced plant community changes in northern North America forests.

Ecosystem management aims at providing many ecosystem services simultaneously. Such ecosystem multifunctionality can be limited by trade-offs and increased by synergies among the underlying ecosystem functions (EF), which need to be understood to develop targeted management. Previous studies found differences in the correlation between EFs. We hypothesised that correlations between EFs are variable even under the controlled conditions of a field experiment and that seasonal and annual variation, plant species richness, and plot identity (identity effects of plant communities such as the presence and absence of functional groups and species) are drivers of these correlations. We used data on 31 EFs related to plants, consumers, and physical soil properties that were measured over 5 to 19 years, up to three times per year, in a temperate grassland experiment with 80 different plots, constituting six sown plant species richness levels (1, 2, 4, 8, 16, 60 species). We found that correlations between pairs of EFs were variable, and correlations between two particular EFs could range from weak to strong correlations or from negative to positive correlations among the repeated measurements. To determine the drivers of pairwise EF correlations, the covariance between EFs was partitioned into contributions from plant species richness, plot identity, and time (including years and seasons). We found that most of the covariance for synergies was explained by species richness (26.5%), whereas for trade-offs, most covariance was explained by plot identity (29.5%). Additionally, some EF pairs were more affected by differences among years and seasons and therefore showed a higher temporal variation. Therefore, correlations between two EFs from single measurements are insufficient to draw conclusions on trade-offs and synergies. Consequently, pairs of EFs need to be measured repeatedly under different conditions to describe their relationships with more certainty and be able to derive recommendations for the management of grasslands.

Insect herbivory can vary from an inconsequential biotic interaction to a factor that contributes substantially to the diversity of plants and animals and overall interaction diversity. As herbivory is the result of numerous ecological and evolutionary processes, including complex population dynamics and the evolution of plant defense, it has been difficult to predict variation in herbivory across meaningful spatial scales. In the present work, we characterize patterns of herbivory on plants in a species rich and abundant tropical understory genus (Piper) across forests spanning 44° of latitude in the Neotropics. We modeled the effects of geography, climate, resource availability, and Piper species richness on the median, dispersion, and skew of generalist and specialist herbivory. By examining these multiple components of the distribution of herbivory, we were able to determine factors that increase biologically meaningful herbivory at the upper ends of the distribution. Site level variables such as latitude, seasonality, and maximum Piper richness explained variation in herbivory at the local scale (plot level) better for assemblages of Piper congeners than for a single species. Predictors that varied between local communities, such as resource availability and diversity, best explained the distribution of herbivory within sites, dampening broad patterns across latitude and climate and demonstrating why generalizations about gradients in herbivory have been elusive. The estimated population means, skew, and dispersion of herbivory responded differently to abiotic and biotic factors, illustrating the need for careful studies to explore distributions of herbivory and their effects on forest diversity. Nevertheless, we observed a roughly two-fold increase in median herbivory in humid compared to seasonal forests, and this finding aligns with the hypothesis that precipitation seasonality plays a critical role in shaping interaction diversity within tropical ecosystems.

Predictive modelling is fundamental to ecology and essential for objective biodiversity assessment. However, while predictive biodiversity models are generally well-developed, models for predicting patterns within and among ecosystems have not been adequately operationalized. We contend the scarcity of such models marks a concerning gap in the scientific community’s ability to make ecosystem predictions across landscapes, and more broadly for supporting the conservation of biodiversity and ecosystem functions. We propose ecosystem spatial pattern models (ESPM) to fill this gap in modelling capacity. Under our approach to ESPM, spatial patterns of ecosystem properties are the basis for resolving ecosystem organization at local and landscape extents. Our integrative modelling framework differs from others in that it accords biotic and abiotic constituents equally, based on with their joint mechanistic influence on ecosystem dynamics. Development of ESPM is especially timely for ecosystem assessment is undergoing a contemporary groundswell, as scientists and conservation groups propose ambitious targets for ecosystem conservation and restoration.

Climate change is increasing the weather persistence in the mid-latitudes, prolonging both dry and wet spells compared to historic averages. These newly emerging environmental conditions destabilize plant communities, but the role of species interactions in this process is unknown. Here, we tested how direct and higher-order interactions (HOIs) between species may change in synthesized grassland communities along an experimental gradient of increasing persistence in precipitation regimes. Our results indicate that species interactions (including HOIs) are an important determinant of plant performance under increasing weather persistence. Out of the 12 most parsimonious models predicting species productivity, 75 % contained significant direct interactions and 92 % significant HOIs. Inclusion of direct interactions or HOIs respectively tripled or quadrupled the explained variance of target species biomass compared to null models only including the precipitation treatment. Drought dominated the plant responses, with longer droughts increasing direct competition but also HOI-driven facilitation. Despite these counteracting changes, drought intensified net competition. Grasses were generally more involved in competitive interactions whereas legumes had a stronger affinity for facilitative interactions. Under longer drought, species affinity for nutrient rich or wet environments resulted in more negative direct interactions or HOIs, respectively. We conclude that higher-order interactions, crucially depending on species identity, only partially stabilize community dynamics under increasing weather persistence.

Plant lifespan has important evolutionary, physiological, and ecological implications related to population persistence, community stability, and resilience to ongoing environmental change impacts. Although biologists have long puzzled over the extraordinary variation in plant lifespan and its causes, our understanding of interspecific variability in plant lifespan and the key internal and external factors influencing longevity remains limited. Here, we demonstrate the concurrent impacts of environmental, morphological, physiological, and anatomical constraints on interspecific variation in longevity among >300 vascular dicot plant species naturally occurring at an elevation gradient (2800-6150 m) in the western Himalayas. First, we show that plant longevity is largely related to species’ habitat preferences. Ecologically stressful habitats such as alpine and subnival host long-lived species, while productive ruderal and wetland habitats contain a higher proportion of short-lived species. Second, longevity is influenced by growth form. Small-statured cushion plants with compact canopies and deep roots, most found on cold and infertile alpine and subnival soils, had a higher chance of achieving longevity. Third, plant traits reflecting plant adaptations to stress and disturbance modulate interspecific differences in plant longevity. Importantly, we show that longevity and growth are negatively correlated. Slow-growing plants are those that have a higher chance of reaching a maximum age. Finally, changes in plant carbon, nitrogen, and phosphorus content in root and leaf tissue were significantly associated with variations in longevity. We discuss the link between the longevity and productivity and stability of studied Himalayan ecosystems and the intrinsic growth dynamics and physiological constraints under increasing environmental pressure.

Snowpack dynamics have a major influence on wildlife movement ecology and predator-prey interactions. Specific snow properties such as density, hardness, and depth determine how much an animal sinks into the snowpack, which in turn drives both the energetic cost of locomotion and predation risk. Here, we quantified the relationships between 15 field-measured snow variables and snow track sink depths for widely distributed predators (bobcats [Lynx rufus], coyotes [Canis latrans], wolves [C. lupus]) and sympatric ungulate prey (caribou [Rangifer tarandus], white-tailed deer [Odocoileus virginianus], mule deer [O. hemionus], and moose [Alces alces]) in interior Alaska and northern Washington, USA. We first used generalized additive models to identify which snow metrics best predicted sink depths for each species and across all species. For species occurring in both sites, we then tested whether the snow metric-sink depth relationship differed across regions. Finally, we used breakpoint regression to identify thresholds for the best-performing predictor of sink depth for each species (i.e., values wherein tracks do not appreciably sink into the snow). Near-surface (0-10cm) snow density was the strongest predictor of sink depth across species. This relationship varied slightly by region for wolves and moose but did not differ for coyotes. Thresholds of support occurred at snow densities of 230 kg/m3 for coyotes, 280 kg/m3 for bobcats, 290 kg/m3 for wolves, 340 kg/m3 for deer, 440 kg/m3 for caribou, and 550 kg/m3 for moose. Together, these critical thresholds define the bounds of “danger zones,” the range of snow density in which carnivores have a comparative movement advantage over ungulates. These results can be used to link predator-prey relationships with spatially explicit snow modeling outputs and projected future changes in snow density. As climate change rapidly reshapes snowpack dynamics, these danger zones provide a useful framework to anticipate likely winners and losers of future winter conditions.

Color polymorphism is an adaptive strategy in which a species exhibits multiple color phenotypes in a population. Often times, phenotypes are variably suited to different environmental conditions which may buffer the population against variable conditions. Modern climate change is creating novel selective pressures for many species, especially in winter habitats. Few studies have quantified the benefits of polymorphism for allowing species to cope with climate-induced environmental change. We investigated how color polymorphism mediates selective pressures in ruffed grouse Bonasa umbellus, a widespread and winter-adapted bird species of North American forests. Ruffed grouse display phenotypic variation in plumage color, ranging from red to gray. Over five winter seasons (2015-2022), we monitored weather conditions, habitat use, and weekly survival for 94 ruffed grouse to test whether individuals had lower survival when grouse were phenotypically mismatched with snow cover (e.g., a gray bird on a snowless landscape or a red bird in snow). Grouse phenotypically mismatched with snow cover had lower survival, but only when winter survival rates were lowest. During winters of lower overall survival, red grouse exhibited higher survival during snow-free periods, whereas gray grouse had higher survival when snow was present. We also found that open habitat negatively impacted survival, regardless of color. While the effect of phenotypic mismatch was variable among years, it was a stronger predictor of winter survival than land cover features, suggesting that snow is an important habitat feature mediating overwinter survival. Our work offers an advancement in understanding how environmental variability affects geographic variation in and maintenance of multiple color phenotypes in seasonally-snow covered environments. Our finding that interactions between color morph and snow cover are important for conferring winter survival provides further evidence that color polymorphism may serve as a buffer against rapidly changing conditions and a pathway for persistence of polymorphic species.

Agricultural expansion has markedly reduced forests and reconfigured landscapes. These changes incur a well-known detrimental impact on the biodiversity of local forest patches, but the effects on species persistence at entire landscapes comprised of multiple patches are debated. We investigated how regional diversity is affected by habitat loss, fragmentation, and cattle grazing, and how species respond to deforestation both locally and regionally. We also investigated how the heterogeneity in species distribution (beta-diversity) buffers landscapes against local diversity losses. The vast majority of the 251 ant species found in our study were negatively affected by both habitat loss and cattle at local forest patches, drastically reducing diversity at these patches compared to pristine forests. Despite local declines in diversity, however, heavily fragmented landscapes could still retain most species due to the high heterogeneity in species distribution. We found that beta-diversity is the main component of regional diversity. Results from several studies suggest that this component is maximized when remnant primary habitats in a landscape are spread across vast areas. Although preserving local diversity may be important for the adequate functioning of the ecosystem locally, our results indicate that the maintenance of many small forest patches in a landscape can buffer regional biodiversity against local species losses. Our results suggest that even small forest remnants in otherwise deforested landscapes can prevent most regional-scale species extirpations, and therefore also merit conservation efforts.

Weed species are ecological models that recently received considerable attention due to their particular strategies linked to their ruderal-competitive traits. They are known to have the potential to provide additional floral resources for insects in flower-poor agroecosystems. However, their floral traits are much more scarcely studied than those of plants found in other habitats, such as grasslands. The aim of this study was to describe the floral phenotype of weeds and to determine to what extent their floral traits match their ecological strategies as described on the basis of leaf traits. We therefore cultivated 19 forb weeds from perennial agroecosystems, previously identified in Mediterranean fields, in a greenhouse for seven months and collected data on 12 floral and 5 leaf traits. We tested whether these traits covaried and whether they exhibited an ecological strategy at the phenotype scale. We found that in matters of flower production, weed species face a trade-off: either numerous small, low-stature flowers with small quantities of pollen and nectar, or few, large, higher-held flowers with more pollen and nectar. The floral traits were found to reflect Grime’s CSR strategies: the weed species producing fewer but costlier flowers belonged to C-strategy species, whereas those producing more but less costly flowers belonged to species dominated by an R strategy These findings indicate that the potential of weeds as floral resources for insects is related to their ecological strategies, which are known to be affected by agricultural practices that filter species composition. This implies that, as for the provision of other ecosystem services, weed communities can be managed so as to select species with interesting floral traits for pollinators.

Urban environments represent a theatre for life history evolution. Species able to survive in cities can adapt to the local and often divergent environmental conditions compared to rural or natural environments. Dispersal determines establishment, gene flow, and thus the potential for local adaptation. Since habitats in urban environments are highly fragmented, and showing substantial turnover, contrasting adaptive effects on dispersal are expected. Fragmentation selects against dispersal while patch turn-over is expected to promote the evolution of dispersal. We here show both processes to act in concert when different scales are considered. Dispersal behaviour of juvenile, lab-reared garden spiders from two mid-sized European cities were tested under standardized conditions. While long-distance dispersal showed to be overall rare, short-distance dispersal strategies increased with urbanization at small scales, but declined when urbanization was quantified at large scales. We discuss the putative drivers behind these differences in natal dispersal and highlight its importance for urban evolution and ecology.

Competition among pollinators for floral resources is a phenomenon of both basic and applied importance. While competition is difficult to measure directly under field conditions, it can be inferred indirectly through the measurement of floral resource depletion. In this study, we conducted a pollinator exclusion experiment to calculate nectar depletion rates in summer across 16 grassland sites in the German regions of Franconia and Saxony-Anhalt. Overall depletion rates were estimated at 95% in Franconia and 79% in Saxony-Anhalt, indicating strong nectar limitation and, by implication, competition among pollinators. Despite being ubiquitous in our study regions, honey bees were scarce at our sites at the time of nectar sampling. This demonstrates that wild pollinators alone are capable of massive nectar depletion, and the addition of managed honey bees to wild pollinator communities may intensify already competitive conditions. Nevertheless, the manifest diversity of the pollinator communities at our sites indicates that other factors, such as non-trophic constraints or temporal variation in food limitation, can mitigate competitive exclusion despite immediate conditions of acute food scarcity.

Like many top consumers, parasites can regulate feeding of their prey via trait-mediated means. If parasites modify the feeding behavior of ecologically important grazers, they may have cascading effects on the structure and functioning of whole plant communities. The extent to which parasites can influence plant communities in this way is largely dependent on the strength of their behavioral alteration, their prevalence in host grazers, and the density of those hosts. Recent experiments and comparative surveys in southeastern USA salt marshes revealed that common larval trematode parasites suppress the per capita grazing impacts of the marsh periwinkle (Littoraria irrorata), generating a trophic cascade that protects foundational marsh plants from drought-associated overgrazing. Here, we conducted a field manipulation wherein we modified grazer host density while holding infection prevalence constant at an ecologically relevant level (20%) to determine whether the indirect, facilitative effects of parasites on marsh plants varied with the density of grazers. We found that parasites had significant positive impacts on marsh net primary productivity at moderate densities of snails (≥50 snails/ 0.5 m2), but that the positive effects of parasites were negligible at lower densities. Our results confirm the findings of previous studies that parasites can protect marsh plants from overgrazing at sufficiently high prevalence, but show that their ability to do so depends on host density.

Species interactions shape native plant communities, influencing both composition and ecosystem processes, with invasion by non-native species threatening these dynamic relationships, native species, and function. The consequences of invasive plants in particular may stretch across taxa to impact plant, insect, and soil microbial communities directly and indirectly, with consequences for ecological functioning. In northern mixed-grass prairies, invasion by two annual brome grasses, Bromus arvensis and B. tectorum, negatively impacts rangeland plants; however, the simultaneous effects on insects and soil microbes (bacteria and archaea), and the implications for ecological function, have received less attention. Here, using observational field studies conducted at two mixed-grass prairie sites in Montana and Wyoming, we assessed the relationships between plants, insects, and soil microbes across gradients of invasion by B. arvensis and B. tectorum. Overall, we found differences in plant and insect communities and functional groups with increasing invasion abundance for both brome species. However, associations between invasion and the soil microbial community were species specific, as we only saw these relationships under B. tectorum invasion, implying B. tectorum may have more substantial consequences for rangeland management. While invasion by annual bromes may cause changes in certain plant and insect functional groups, such as C4 perennial grasses and certain insect herbivores, soil microbial functional groups may be less impacted, especially under B. arvensis invasion. This work sheds light on the need to explore changes in natural communities across taxa and to all invasive species, as ecosystem effects are likely to be contingent upon both.