Biology subjects

Classen, A. T.

Publications and source records attributed to Classen, A. T..

8 recordsLinked to original sources

bioRxiv2026

Community and leaf-level controls of carbon fluxes in an invaded New Zealand alpine tussock grassland

Climate warming and plant invasion are reshaping alpine communities, yet their combined effects on ecosystem carbon (C) balance remain poorly understood. We studied how warming and invasion interact to influence community level C fluxes, and the mechanisms underpinning these responses. Using field experiments manipulating warming and invader removal in two New Zealand alpine grasslands dominated by functionally distinct invaders (a shrub, Calluna vulgaris, and a forb, hawkweeds), we quantified net ecosystem exchange (NEE), gross primary productivity (GPP) and ecosystem respiration (ER), alongside community, trait and abiotic factors. Warming did not enhance C uptake at either site, and we found no evidence of synergistic warming x invasion effects. Instead, responses were invader-dependent. At the shrub-invaded site, warming increased net C loss through elevated respiration without corresponding gains in GPP. In contrast, fluxes at the forb-invaded site were largely insensitive to treatments. Variation in C fluxes were driven by abiotic conditions and trait dominance at the shrub site, whereas species diversity (richness and evenness) exerted stronger control at the forb site. Our findings demonstrate that the impacts of invasion on ecosystem C balance depend on invader functional identity, with contrasting roles for trait dominance and biodiversity in regulating ecosystem function under warming.

ecology↗

bioRxiv2025

Warming winter disrupts mycorrhizal phenology and plant-fungal nutrient cycling.

Climate change is reshaping the timing of ecological processes in montane ecosystems, where short, snowpack-dependent growing seasons tightly constrain plant-microbe interactions. Arbuscular mycorrhizal (AM) fungi regulate plant acquisition of nitrogen (N) and phosphorus (P) during nutrient pulses triggered by melting snowpack. Yet the extent and consequences of warming-induced phenological asynchrony between AM fungi and host plants remains unknown. We experimentally advanced snowmelt in a subalpine meadow and monitored plant and AM fungal growth, soil nutrients, and AM fungal community composition throughout the growing season. Early snowmelt advanced plant greenness and root standing stock but suppressed AM fungal hyphal production, reducing available NH{square}{square} and PO43-. Hyphal allocation strategies shaped AM fungal temporal niches and species-specific responses to warming. Nutrient-foraging, edaphophilic AM fungi dominated early in the season, while rhizophilic AM fungi dominated later, following peak root growth. Our results reveal that fungal functional traits and nutrient dynamics govern the temporal niche partitioning of mycorrhizal fungi. By decoupling AM fungal activity from plant demand and nutrient mineralization windows, early snowmelt drives plant-fungal asynchrony. This decoupling weakens mycorrhizal symbioses and threatens nutrient retention and ecosystem stability under future warming. Such belowground temporal mismatches may determine the resilience of seasonally temperature-dependent ecosystems under warming.

ecology↗

bioRxiv2024

Variable responses of alpine-plant communities to warming and loss of dominant species

Responses of ecological communities to perturbations are inherently variable because responses of their constituent populations also vary. Species within a single community may show combinations of no response, positive responses, and negative responses to any given perturbation often canceling each other out resulting in small or no signal that the community level. Here we explore the impacts of warming and loss of the dominant species on alpine ecosystems in a global study. We investigate warming and species-loss treatments on population- and community-level dynamics across alpine-plant communities at two elevations in five globally-distributed mountain locations. Communities showed varied responses to treatments; no community showed strong responses to a single treatment. Rather, most sites were influenced by both perturbations. Populations within these communities responded idiosyncratically, suggesting that constituent species are not all equally robust to perturbations even when community-level effects appear weak. Our results highlight the challenge of making general predictions about population- and community-level responses of alpine ecosystems in the face of present and future perturbations.

ecology↗

bioRxiv2023

Gains in soil C storage under anthropogenic N deposition are rapidly lost following its cessation

Anthropogenic nitrogen (N) deposition enhanced the global terrestrial carbon (C) sink and partially offset anthropogenic CO2 emissions. This was driven by the suppression of microbial activity associated with the breakdown of soil organic matter. However, since the implementation of emission abatement policies in the 1970s, atmospheric N deposition has declined globally, and the consequences of this decline are unknown. Here, we assessed the response of soil C storage, and associated microbial activities, in a long-term field study that experimentally increased N deposition for 24-years. We measured soil C and N, parameters of microbial activity, and compared effect sizes of soil C in response to, and in recovery from, the N deposition treatment across the history of our experiment (1994-2022). Our results demonstrate that the accumulated C in the organic horizon has been lost and exhibit additional deficits 5-years post-termination of the N deposition treatment. These findings, in part, arise from mechanistic changes in microbial activity. Soil C in the mineral soil was less responsive thus far in recovery. If these organic horizon C dynamics are similar in other temperate forests, the Northern Hemisphere C sink will be reduced and climate warming will be enhanced.

ecology↗

bioRxiv2022

The overlooked dimension: soil depth drives microbial community structure and function along a salinity gradient

AimA fundamental challenge in soil macroecology is to understand how microbial community structure shapes ecosystem functions along environmental gradients of land surface (i.e., horizontal dimension). However, little is known of microbial community structure-function relationships along environmental gradients of soil depth (i.e., vertical dimension) in extreme environments. A full understanding of the consequences of environmental change for microbial communities structure and subsequent changes in microbial functions could enable more accurate predictions of extreme environmental change effects. Here, we leveraged a 200-km desert soil salinity gradient that is created by a 12-year saline-water irrigation to evaluate how soil microbial community structure-function relationships change with soil salinity in the horizontal and vertical dimensions. LocationThe Tarim basin of Taklamakan desert. TaxaSoil bacteria and fungi. MethodsWe assessed the prime ecological processes controlling the assembly of microbial communities and the activity of enzymes relevant to carbon, nitrogen, and phosphorus cycling along soil salinity gradients across study sites (horizontal dimension) and soil depths (vertical dimension) by using the general linear model, hierarchical variance partitioning, and path model. ResultsDifferences in soil depth (on the scale of meters) was as important as geographic distance (on the scale of kilometers) in shaping the structure of bacterial and fungal communities, while both the vertical and horizontal variability in enzymatic activity were largely attributed to the increase in the heterogeneity of soil properties, such as soil texture, water content, and pH. Main conclusionsOur results suggest that dispersal limitation and environmental heterogeneity, not soil salinization, along soil depth governs microbial community assembly and enzymatic activity, respectively. This work highlights that conservation efforts of soil macroecology should consider soil depth as a key attribute in the face of ongoing salinization in arid ecosystems.

ecology↗

bioRxiv2022

Globally invariant metabolism but density-diversity mismatch in springtails

Soil life supports the functioning and biodiversity of terrestrial ecosystems1, 2. Springtails (Collembola) are among the most abundant soil animals regulating soil fertility and flow of energy through above- and belowground food webs3-5. However, the global distribution of springtail diversity and density, and how these relate to energy fluxes remains unknown. Here, using a global dataset collected from 2,470 sites, we estimate total soil springtail biomass at 29 Mt carbon (threefold higher than wild terrestrial vertebrates6) and record peak densities up to 2 million individuals per m{superscript 2} in the Arctic. Despite a 20-fold biomass difference between tundra and the tropics, springtail energy use (community metabolism) remains similar across the latitudinal gradient, owing to the increase in temperature. Neither springtail density nor community metabolism were predicted by local species richness, which was highest in the tropics, but comparably high in some temperate forests and even tundra. Changes in springtail activity may emerge from latitudinal gradients in temperature, predation7, 8, and resource limitation7, 9, 10 in soil communities. Contrasting temperature responses of biomass, diversity and activity of springtail communities suggest that climate warming will alter fundamental soil biodiversity metrics in different directions, potentially restructuring terrestrial food webs and affecting major soil functions.

ecology↗

bioRxiv2019

Multi-taxon inventory reveals highly consistent biodiversity responses to ecospace variation

Amidst the global biodiversity crisis, identifying drivers of biodiversity variation remains a key challenge. Scientific consensus is limited to a few macroecological rules, such as species richness increasing with area, which provide limited guidance for conservation. In fact, few agreed ecological principles apply at the scale of sites or reserve management, partly because most community-level studies are restricted to single habitat types and species groups. We used the recently proposed ecospace framework and a comprehensive data set for aggregating environmental variation to predict multi-taxon diversity. We studied richness of plants, fungi, and arthropods in 130 sites representing the major terrestrial habitat types in Denmark. We found the abiotic environment (ecospace position) to be pivotal for the richness of primary producers (vascular plants, mosses, and lichens) and, more surprisingly, little support for ecospace continuity as a driver. A peak in richness at intermediate productivity adds new empirical evidence to a long-standing debate over biodiversity responses to productivity. Finally, we discovered a dominant and positive response of fungi and insect richness to organic matter accumulation and diversification (ecospace expansion). Two simple models of producer and consumer richness accounted for 77 % of the variation in multi-taxon species richness suggesting a significant potential for generalization beyond individual species responses. Our study widens the traditional conservation focus on vegetation and vertebrate populations unravelling the importance of diversification of carbon resources for diverse heterotrophs, such as fungi and insects.

ecology↗

bioRxiv2017

A systematic survey of regional multitaxon biodiversity: evaluating strategies and coverage

BackgroundIn light of the biodiversity crisis and our limited ability to explain variation in biodiversity, tools to quantify spatial and temporal variation in biodiversity and its underlying drivers are critically needed. Inspired by the recently published ecospace framework, we developed and tested a sampling design for environmental and biotic mapping. We selected 130 study sites (40 x 40 m) across Denmark using stratified random sampling along the major environmental gradients underlying biotic variation. Using standardized methods, we collected site species data on vascular plants, bryophytes, macrofungi, lichens, gastropods and arthropods. To evaluate sampling efficiency, we calculated regional coverage (relative to the known species number per taxonomic group), and site scale coverage (i.e., sample completeness per taxonomic group at each site). To extend taxonomic coverage to organisms that are difficult to sample by classical inventories (e.g., nematodes and non-fruiting fungi), we collected soil for metabarcoding. Finally, to assess site conditions, we mapped abiotic conditions, biotic resources and habitat continuity.\n\nResultsDespite the 130 study sites only covering a minute fraction (0.0005 %) of the total Danish terrestrial area, we found 1774 species of macrofungi (54 % of the Danish fungal species pool), 663 vascular plant species (42 %), 254 bryophyte species (41 %) and 200 lichen species (19 %). For arthropods, we observed 330 spider species (58 %), 123 carabid beetle species (37 %) and 99 hoverfly species (33 %). Correlations among species richness for taxonomic groups were predominantly positive. Overall, sample coverage was remarkably high across taxonomic groups and sufficient to capture substantial spatial variation in biodiversity across Denmark. This inventory is nationally unprecedented in detail and resulted in the discovery of 143 species with no previous record for Denmark. Comparison between plant OTUs detected in soil DNA and observed plant species confirmed the usefulness of carefully curated environmental DNA-data. Species richness did not correlate well among taxa suggesting differential and complex biotic responses to environmental variation.\n\nConclusionsWe successfully and adequately sampled a wide range of diverse taxa along key environmental gradients across Denmark using an approach that includes multi-taxon biodiversity assessment and ecospace mapping. Our approach is applicable to assessments of biodiversity in other regions and biomes where species are structured along environmental gradient.

ecology↗