James F. Cahill

Fungal volatile organic compounds (FVOCs) play key roles in fungal ecology, physiology, and biotechnological applications, but inconsistent sampling and an

Identifying refugia can provide important insights for conservationists planning protected areas. However, most current methods for identifying refugia emphasize short-term survivorship during a crisis rather than long-term recovery. Here, we propose a method for identifying refugia that emphasizes range re-expansion, and we present a case study using pollen data from three plant taxa over the past 20 kyr in northern Alaska, USA to evaluate the method’s validity. We identified a substantial decrease and subsequent recovery in spruce presence across the study region between 16.0–11.0 ka and tested the pattern’s significance using a Monte Carlo analysis. During this interval of relatively warm and dry regional climate, spruce persisted in one locality that acted as a source for post-crisis re-expansion, indicating that this locality was a refugium. Using this method to understand how successful refugia form in different crises would provide conservationists with more informative guidelines for planning protected areas.

The contribution of soil chemistry to plant growth and resilience, including presence of phytohormones, is increasingly recognized, yet characterization is limited by chemical complexity of soil matrices, diversity and low-abundance of metabolites. To enable further discoveries, we developed and characterized performance of a liquid chromatography-mass spectrometry method with solid phase extraction, integrating targeted and untargeted hormonomic approaches for comprehensive soil phytohormone profiling. Method performance was evaluated for fifteen plant growth–regulating compounds and precursors, including abscisic, gibberellic, jasmonic, and salicylic acids, auxins, cytokinins, karrikins, melatonin, and tryptophan, showing strong linearity (R² = 0.989–0.999), sensitivity (limits of detection and quantification 0.1–50.2 and 1.4–167.3 pg on-column, respectively), and precision (1.3–9.6% intraday; 3.4–34.8% interday). Soil composition impacted recovery; however, for most phytohormones rates were within 20% of matrix-adjusted spiked value, showing robustness across sandy, peat-rich, and clay-rich textures and suitable for use. We used the method to quantify analytes in research-relevant, active soils. Integration of untargeted analysis expanded coverage to 250 additional putative phytohormones and related metabolites, revealing chemical signatures potentially associated with plant community composition. This approach provides a versatile framework for investigating belowground phytohormone dynamics and their roles in plant physiology, resilience, and soil–plant feedbacks.

As droughts become longer and more intense, impacts on terrestrial primary productivity are expected to increase progressively. Yet, some ecosystems appear to acclimate to multiyear drought, with constant or diminishing reductions in productivity as drought duration increases. We quantified the combined effects of drought duration and intensity on aboveground productivity in 74 grasslands and shrublands distributed globally. Ecosystem acclimation with multiyear drought was observed overall, except when droughts were extreme (i.e., ≤1-in-100-year likelihood of occurrence). Productivity losses after four consecutive years of extreme drought increased by ~2.5-fold compared with those of the first year. These results portend a foundational shift in ecosystem behavior if drought duration and intensity increase, from maintenance of reduced functioning over time to progressive and profound losses of productivity when droughts are extreme.

Studies suggesting that native plant species adapt to invasive species are accumulating, but we still have little evidence of native species performance recovery over time, following phenotypic selection by invaders. The following sequential evidence is sufficient to infer selection‐mediated native species performance recovery in response to the interaction with an invader: (i) a decrease in native species' performance in recently invaded areas, (ii) phenotypic selection on native species' functional traits within invaded areas, and in time (iii) trait changes in the direction of selection and (iv) recovery of native species' performance. In a field study in a Canadian grassland, we tested for the outlined sequential evidence in three native species in response to the invader Bromus inermis, whose growth habit significantly reduces light availability. Phenotypic selection was measured on two key traits associated with shade tolerance and avoidance: leaf area and plant height, respectively. Only one species, Artemisia ludoviciana, met all four criteria, thus showing evidence consistent with an adaptive response to B. inermis: increased shade tolerance in response to selection (increased leaf area), further recovering its performance. No such evidence was found for the other two species: we observed a lack of selection on key traits upon encountering the invader in Hesperostipa curtiseta and no response to selection in Achillea millefolium. Identifying the functional strategies plants can adopt to coexist with an invader is key to inform restoration and management strategies. The present study points to moving beyond the documentation of adaptation by native species to invaders. Specifically, following the proposed framework, future research would increase our understanding of when and how native species may adapt to persist and thrive within invaded areas, further identifying potential constraints to adaptive responses. Read the free Plain Language Summary for this article on the Journal blog.

Plant–plant interactions, often studied in the context of plant traits, are considered crucial assembly mechanisms for plant communities and offer insights into how neighbours affect growth. Three key questions regarding these interactions have theoretical importance but limited empirical support: (1) does similarity in plant traits enhance competitive interactions among species, (2) is the “competitive environment” determined through plant traits of the resident species, and (3) do greater species diversity and niche use result in increased competition among co‐occurring species. Here we use a simple experimental design to directly test these foundational questions. In a mesocosm experiment using native grassland fragments, we investigated our questions simultaneously by manipulating trait relationships among resident and colonizing plants. To assess the net impact of neighbours on plant growth, we measured the degree of suppression experienced by focal plants compared to growth in the absence of neighbours. We further tested whether trait‐function relationships are context‐dependent by modifying available resources. Contrary to the limiting similarity hypothesis, we found that reduced overall dissimilarity mitigated neighbour‐induced growth suppression, with more negative effects occurring among dissimilar neighbours. The influence of trait dissimilarity depended on nutrient context and was stronger under increased resource availability. We found little support for the idea that specific plant traits are “functional” in creating competitive environments: no community traits (community weighted means) were associated with net neighbour effects. In contrast, resource supply significantly affected species interactions, with increased resources exacerbating negative neighbour effects on plant growth. We found that plant trait dissimilarity does function in competition but is contingent on resource context. However, trait values were not inherently competitive: traits may have physiological functions but did not function in focal plants' competitive response. We provide some of the limited data directly testing the core mechanisms of community assembly. We encourage such direct experimental tests as they are essential to building an understanding of ecological processes. Read the free Plain Language Summary for this article on the Journal blog.

Empirical invasion ecology is laden with high context dependency. If general mechanisms underlying invasion success exist, they should be detectable in species that share biological and ecological characteristics. We carried out a growth experiment with Agropyron cristatum , Bromus inermis and Poa pratensis (subsp. angustifolia ), to better understand the mechanisms underlying the invasion success of cool‐season grasses in northern prairie grasslands of North America. By using a home–away approach, we tested whether 1) non‐native plants have a higher performance than native plants, and whether invasiveness is 2) mediated by interactions with soils, such as a release from pathogens or enhanced mutualism, or 3) an adaptation to local soils. We compared seed size and weight of populations in Canada (non‐native range) and Eurasia (native range) and carried out an experiment, in which seeds from the non‐native and native ranges were planted into sterilized soil (control) and soil from a population in Canada or Eurasia, or local soils, respectively. We found inconsistent effects across species and response variables. Seed size and weight were not significantly different between native and non‐native populations. The experiment showed a seed origin effect in A . cristatum (root and total biomass) and B . inermis (root biomass), with non‐native populations outperforming native ones. Soil‐mediated effects were supported in A . cristatum (root biomass) and local adaptation in B . inermis (root and total biomass). Germination across all species and biomass in P . pratensis did not respond to treatments. Despite the high similarity of our study group, our results indicate that invasiveness might be driven by idiosyncratic causes at the species level. Mechanisms not considered in our study, such as high propagule pressure and preadaptation could also potentially explain the invasion success across species.

Species mixtures have been widely reported to increase aboveground productivity; however, how tree species mixtures affect root systems in natural forests remains unclear. We hypothesize that mixtures have a greater fine root length compared to single species‐dominated stands to support their greater productivity. Here, we collected monthly root images from the minirhizotrons installed in 18 stands either dominated by Populus tremuloides, Pinus banksiana, and their mixtures for three years (2015–2017) in post‐fire boreal forests of two stand ages (8 and 34 years old) to test our hypotheses. We found that the fine root length was higher in mixtures than in single species‐dominated stands, and the magnitude of mixture effects was greater in the 34‐ than in the 8‐year‐old stands in the third year. The mixture effects on fine root length revealed a positive relationship with forest net primary productivity. Root length production, which is the growth of new roots within a year, was not affected by tree species mixtures except for the 8‐year‐old stand in 2015. Tree species mixtures did not affect root length turnover. Root length and root biomass were not significantly correlated at stand level. Synthesis. Our results show that tree species mixtures positively affect fine root length, with that positive effect increasing with stand development in the studied natural forests. Our results suggest that the greater root length in species mixtures supported the greater forest productivity in species mixtures as the greater root length benefits plant uptake of nutrients and water. Therefore, conserving tree species diversity has implications for improving forest productivity and carbon sequestration in forest ecosystems.

Forest disturbances can alter mycorrhizal fungal communities with consequences for subsequent plant establishment and growth. One possible mitigation strategy to promote forest recovery following disturbances is inoculation with soil from late‐successional forests. However, whether these soil transfers change the composition of resident fungal communities and how seedling growth responds to such transfers is unknown. To test whether soil transfers can mitigate disturbance‐mediated impacts on ectomycorrhizal fungal (EMF) communities, we transferred soils containing fungal inoculum collected from conspecific late‐successional forests into stands disturbed by wildfire, mountain pine beetle outbreak, clear‐cut logging or salvage logging. Afterward, lodgepole pine seeds were sown to measure germination, establishment and growth of seedlings, and to characterize root‐associated and soil EMF communities by sequencing the ITS1 rDNA region. Despite evidence that soil inocula contained viable EMF communities, transfers did not impact the root‐associated or soil EMF community composition, or seed germination, seedling establishment and growth. Resident EMF community composition remained unchanged upon transfers of fungi from late‐stage successional forests, and disturbance type singularly explained the EMF community composition associated with roots and soils. Ectomycorrhizal fungal taxa typically associated with mature pine forests were particularly abundant on roots of seedlings growing in plots impacted by beetle outbreak, while ‘early‐stage’ EMF taxa were associated with seedlings growing in wildfire and logging disturbances. Synthesis and applications. Following disturbances, soil and root‐associated EMF exhibit high functional redundancy, as seedling performance did not vary with differences in EMF community composition across disturbance types. Our findings suggest that soil transfers are unlikely to be an effective strategy in these stands and may be better targeted to highly degraded forests. The presence and abundance of EMF in disturbed forests should be assessed before committing to soil transfers as part of restoration.