Sören Eliot Weber

Peatlands store one-third of the global soil carbon (C) on only ∼3% of Earth’s land surface but risk becoming C sources to accelerate climate change due to warming-induced C loss. Elevated CO ₂ (eCO ₂ ) may mitigate this loss by boosting plant C uptake. However, how eCO ₂ and climate warming interact to influence peatland C dynamics remains unclear. Here, we investigated peatland C fate at five warming levels (+0°, +2.25°, +4.5°, +6.75°, and + 9°C), each under ambient CO ₂ (aCO ₂ ) and eCO ₂ (+500 parts per million), by model-guided synthesis of 18 measured variables for each of the 10 treatments from an experiment. Our results showed that the peatland C loss increased with warming at a rate of 62.6 g C m ⁻² year ⁻¹ °C ⁻¹ , reaching 476.5 ± 104.7 g C m ⁻² year ⁻¹ at +9°C under aCO ₂ , primarily because of increased Sphagnum mortality and accelerated decomposition. eCO ₂ reduced the loss rate to 17.3 g C m ⁻² year ⁻¹ °C ⁻¹ , nearly halving the C loss at +9°C (269.4 ± 80.5 g C m ⁻² year ⁻¹ ), mainly due to enhanced tree and shrub growth. Methane emissions increased with warming, similarly under aCO ₂ and eCO ₂ . Our findings suggest that northern peatlands likely become major C sources under climate warming, although rising atmospheric CO ₂ concentration may partially slow down the C loss. Climate-driven shifts in plant community structure, rather than peat decomposition alone, play a critical role in regulating peatland C fate and may threaten the stability of peatland ecosystem functions and services in the future.

Diverse ecological communities can reduce pathogen transmission; a phenomenon known as the dilution effect of biodiversity. This effect, in turn, depends on variation in host species’ ability to spread disease (i.e., host competence) and which host species persist under biodiversity loss. Mutualistic symbionts can influence host competence and are sensitive to variation in host biodiversity; yet whether mutualists can modify dilution effects is unknown, partially because quantifying host competence during epidemics remains challenging. We experimentally tested whether mutualists could modify dilution effects by establishing gradients of host species richness, community competence, and access to the below-ground arbuscular mycorrhizal fungus (AMF), Rhizoglomus irregulare . Plant hosts were then exposed to the fungal root pathogen, Rhizoctonia solani , and disease spread was monitored over the course of an epidemic. Overall, host richness did not affect epidemic outcomes. Instead, AMF inoculation increased disease risk via species-specific increases in host competence. Moreover, communities containing highly competent host species experienced more disease, but only in the presence of mutualistic AMF. These results underscore the importance of plant community composition in mycorrhiza-mediated disease dynamics and suggest that predicting disease in our changing word might require a deeper understanding of host-symbiont and pathogen-symbiont interactions under global change.

The viability of peatlands as terrestrial carbon sinks is unknown as peatland carbon losses could be offset, or exacerbated, by increased plant growth. The response of fine‐roots to changing conditions in these ecosystems will be critical to understanding plant access to water, acquisition of nutrients, and ecosystem structure and function. We examined the plant functional type‐specific responses of fine‐root production to warming and elevated CO ₂ treatments, and to warming treatment‐induced deepening of water‐tables in a forested boreal bog. We hypothesized that shrubs would show the most dynamic responses of fine‐root production. We used minirhizotron cameras to estimate fine‐root production, depth and peak annual standing crop from images of individually tracked roots across 7 years (2015–2021) in the SPRUCE (Spruce and Peatland Responses Under Changing Environments) experiment. Fine‐root production increased with warming for shrubs 10.3, trees 2.2 and herbs 4.6%°C ⁻¹ . When water‐tables dropped to their lows of 78 cm deep, shrubs deepened their fine‐root production from 7 to 29 cm deep, and trees from 9 to 17 cm, while herbs produced fine‐roots around 31 cm deep independent of shifts in water‐tables. Fine‐root standing crop increased with warming most strongly for shrubs, even more so under elevated CO ₂ . Synthesis. We found that the fine‐roots of shrubs responded the most strongly to warming, elevated CO ₂ and lowered water‐tables. The greater responsiveness of shrub fine‐root production may partially explain increasing ‘shrubification’ at this site and in other peatlands with long‐term drainage.

1 Boreal bogs sequester large stores of terrestrial carbon in waterlogged peat occupied by extensive networks of vascular plant roots. Historically, cold temperatures and shallow water tables in bogs have favored the growth of Sphagnum over vascular plants. Seasonally variable water tables in bogs constrain woody shrub and tree root production to shallow, oxic acrotelm peat horizons while herbs with aerenchymous root tissue can grow below the water table. Warming and elevated CO ₂ are altering ecosystem functioning in nutrient-limited, rain-fed (ombrotrophic) bogs, via direct impacts on physiology and indirectly via altering water table levels. The environmental changes are shifting interactions between vascular plant fine-roots and the surrounding peat horizons and increasingly favoring vascular plant growth relative to Sphagnum sp. Altered functioning of fine-roots may dramatically affect the role of plants in these ecosystems because of the importance of these organs for plant resource acquisition. We examined how fine-roots across vascular PFTs differentially respond to warming & elevated CO ₂ manipulations and the consequent water table depression in a forested, boreal bog. We used minirhizotrons (cameras inserted belowground) to measure fine-root production, depth distribution, and standing crop from 2015-2021 for each PFT. We found that daily rates of fine-root production accelerated more for shrubs than for trees or herbs. Shrubs and trees grew their roots more deeply with depressed water table levels and shrub fine-roots became narrower. On an annual basis, fine-root production increased with warming, though these rates varied among PFTs and exhibited interactions with elevated CO ₂ . Standing crop of fine-roots increased with warming temperatures, most strongly for shrubs under elevated CO ₂ , because of these productivity responses. From these results, we expect boreal bogs will become increasingly dominated by shrubs under future warmer temperatures, higher atmospheric CO ₂ concentrations, and lower water table levels.

Summary: Plants may benefit from more diverse communities of arbuscular mycorrhizal fungi (AMF), as functional complementarity of AMF may allow for increased resource acquisition, and because a high AMF diversity increases the probability of plants matching with an optimal AMF symbiont. We repeatedly radiolabeled plants and AMF in the glasshouse over c. 9 months to test how AMF species richness (SR) influences the exchange of plant C (¹⁴C) for AMF P (³²P & ³³P) and resulting shoot nutrients and mass from a biodiversity–ecosystem functioning perspective. Plant P acquisition via AMF increased with sown AMF SR, as did shoot biomass, shoot P, and shoot N. The rate of plant C transferred to AMF for this P (C:P) decreased with sown AMF SR. Plants in plant communities benefit from inoculation with a variety of AMF species via more favorable resource exchange. Surprisingly, this effect did not differ among functionally distinct communities comprised entirely of either legumes, nonlegume forbs, or C3 grasses.

Biogeochemical cycling and properties of ecosystems depend, in part, on the assimilation of carbon and uptake of mineral nutrients and water by plants. Belowground, these processes can be studied with the root economic spectrum (RES) which describes plant resource acquisition strategies along a spectrum of fast (e.g., longer and thinner roots) to slow acquisition (e.g., shorter and thicker roots). However, plant-mediated controls on ecosystem functions have often been studied aboveground, leaving the belowground processes largely understudied. This is especially true for water-saturated wetlands, such as peatlands, where drivers of root resource acquisition strategies may be very different than in upland soils. In order to reliably predict peatland plant responses to environmental change and the subsequent shifts in biogeochemical cycles, we need a better understanding of plant fine root trait plasticity under climate warming. We investigated RES trait-environment linkages at a peatland whole-ecosystem climate change experiment (SPRUCE, Minnesota, USA). We collected shrub and tree (spruce and larch) fine root samples from root ingrowth cores installed in enclosures (n=10) with warming (+0 ℃ to +9 ℃) and elevated CO2 (ambient and +500 ppm above ambient) treatments, and plots without enclosure (n=2), over five years (2014-2017 and 2022-2023). We obtained root economic trait data, such as specific root length (SRL) and root tissue density (RTD), from the samples using a root scanning software, as well as root chemistry using an isotope-ratio mass spectrometer. In addition, we collected root exudation rate data from trees and shrubs in each enclosure in 2022. To estimate the contribution of increased soil temperature and elevated CO2 on RES traits, we will build linear mixed effect models for each root trait (response variables) with soil temperature, soil moisture and elevated CO2 treatment as fixed effects and year and microtopography (hummock and hollow) as random effects. Preliminary results suggest that shrubs respond to warming by shifting to a stronger resource acquisition strategy, as indicated by increasing SRL by soil temperature and slightly decreasing RTD. Tree SRL did not change along the warming and elevated CO2 treatments, but RTD seems to decrease, particularly in the 9 ℃ warming treatment, in general indicating a slow resource acquisition strategy. We also found indications of an increasing nutrient-mining strategy with warming for shrubs, where increasing root exudation rates in higher soil temperature may lead to increasing plant-available N and increased root N uptake.

A central goal at the interface of ecology and conservation is understanding how the relationship between biodiversity and ecosystem function (B–EF) will shift with changing climate. Despite recent theoretical advances, studies which examine temporal variation in the functional traits and mechanisms (mass ratio effects and niche complementarity effects) that underpin the B–EF relationship are lacking. Here, we use 13 years of data on plant species composition, plant traits, local‐scale abiotic variables, above‐ground net primary productivity (ANPP), and climate from the alpine tundra of Colorado (USA) to investigate temporal dynamics in the B–EF relationship. To assess how changing climatic conditions may alter the B–EF relationship, we built structural equation models (SEMs) for 11 traits across 13 years and evaluated the power of different trait SEMs to predict ANPP, as well as the relative contributions of mass ratio effects (community‐weighted mean trait values; CWM), niche complementarity effects (functional dispersion; FDis) and local abiotic variables. Additionally, we coupled linear mixed effects models with Multimodel inference methods to assess how inclusion of trait–climate interactions might improve our ability to predict ANPP through time. In every year, at least one SEM exhibited good fit, explaining between 19.6% and 57.2% of the variation in ANPP. However, the identity of the trait which best explained ANPP changed depending on winter precipitation, with leaf area, plant height and foliar nitrogen isotope content (δ¹⁵N) SEMs performing best in high, middle and low precipitation years, respectively. Regardless of trait identity, CWMs exerted a stronger influence on ANPP than FDis and total biotic effects were always greater than total abiotic effects. Multimodel inference reinforced the results of SEM analysis, with the inclusion of climate–trait interactions marginally improving our ability to predict ANPP through time. Synthesis. Our results suggest that temporal variation in climatic conditions influences which traits, mechanisms and abiotic variables were most responsible for driving the B–EF relationship. Importantly, our findings suggest that future research should consider temporal variability in the B–EF relationship, particularly how the predictive power of individual functional traits and abiotic variables may fluctuate as conditions shift due to climate change.

This article is a Commentary on Blume‐Werry et al. (2023), 240: 502–514.

Global climate and land use change are causing woody plant encroachment in arctic, alpine, and arid/semi‐arid ecosystems around the world, yet our understanding of the belowground impacts of this phenomenon is limited. We conducted a globally distributed field study of 13 alpine sites across four continents undergoing woody plant encroachment and sampled soils from both woody encroached and nearby herbaceous plant community types. We found that woody plant encroachment influenced soil microbial richness and community composition across sites based on multiple factors including woody plant traits, site level climate, and abiotic soil conditions. In particular, root symbiont type was a key determinant of belowground effects, as Nitrogen‐fixing woody plants had higher soil fungal richness, while Ecto/Ericoid mycorrhizal species had higher soil bacterial richness and symbiont types had distinct soil microbial community composition. Woody plant leaf traits indirectly influenced soil microbes through their impact on soil abiotic conditions, primarily soil pH and C:N ratios. Finally, site‐level climate affected the overall magnitude and direction of woody plant influence, as soil fungal and bacterial richness were either higher or lower in woody encroached versus herbaceous soils depending on mean annual temperature and precipitation. All together, these results document global impacts of woody plant encroachment on soil microbial communities, but highlight that multiple biotic and abiotic pathways must be considered to scale up globally from site‐ and species‐level patterns. Considering both the aboveground and belowground effects of woody encroachment will be critical to predict future changes in alpine ecosystem structure and function and subsequent feedbacks to the global climate system.