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1. Short‐term plant–soil feedback experiment fails to predict outcome of competition observed in long‐term field experiment

   https://doi.org/10.1002/ecy.3883  

   Beckman, Noelle G.; Dybzinski, Ray; Tilman, David (December 2022, Ecology)

   Abstract Mounting evidence suggests that plant–soil feedbacks (PSF) may determine plant community structure. However, we still have a poor understanding of how predictions from short‐term PSF experiments compare with outcomes of long‐term field experiments involving competing plants. We conducted a reciprocal greenhouse experiment to examine how the growth of prairie grass species depended on the soil communities cultured by conspecific or heterospecific plant species in the field. The source soil came from monocultures in a long‐term competition experiment (LTCE; Cedar Creek Ecosystem Science Reserve, MN, USA). Within the LTCE, six species of perennial prairie grasses were grown in monocultures or in eight pairwise competition plots for 12 years under conditions of low or high soil nitrogen availability. In six cases, one species clearly excluded the other; in two cases, the pair appeared to coexist. In year 15, we gathered soil from all 12 soil types (monocultures of six species by two nitrogen levels) and grew seedlings of all six species in each soil type for 7 weeks. Using biomass estimates from this greenhouse experiment, we predicted coexistence or competitive exclusion using pairwise PSFs, as derived by Bever and colleagues, and compared model predictions to observed outcomes within the LTCE. Pairwise PSFs among the species pairs ranged from negative, which is predicted to promote coexistence, to positive, which is predicted to promote competitive exclusion. However, these short‐term PSF predictions bore no systematic resemblance to the actual outcomes of competition observed in the LTCE. Other forces may have more strongly influenced the competitive interactions or critical assumptions that underlie the PSF predictions may not have been met. Importantly, the pairwise PSF score derived by Bever et al. is only valid when the two species exhibit an internal equilibrium, corresponding to the Lotka–Volterra competition outcomes of stable coexistence and founder control. Predicting the other two scenarios, competitive exclusion by either species irrespective of initial conditions, requires measuring biomass in uncultured soil, which is methodologically challenging. Subject to several caveats that we discuss, our results call into question whether long‐term competitive outcomes in the field can be predicted from the results of short‐term PSF experiments. 

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2. When and where plant‐soil feedback may promote plant coexistence: a meta‐analysis

   https://doi.org/10.1111/ele.13278  

   Crawford, Kerri M.; Bauer, Jonathan T.; Comita, Liza S.; Eppinga, Maarten B.; Johnson, Daniel J.; Mangan, Scott A.; Queenborough, Simon A.; Strand, Allan E.; Suding, Katharine N.; Umbanhowar, James; et al (May 2019, Ecology Letters)

   Abstract Plant‐soil feedback (PSF) theory provides a powerful framework for understanding plant dynamics by integrating growth assays into predictions of whether soil communities stabilise plant–plant interactions. However, we lack a comprehensive view of the likelihood of feedback‐driven coexistence, partly because of a failure to analyse pairwise PSF, the metric directly linked to plant species coexistence. Here, we determine the relative importance of plant evolutionary history, traits, and environmental factors for coexistence through PSF using a meta‐analysis of 1038 pairwise PSF measures. Consistent with eco‐evolutionary predictions, feedback is more likely to mediate coexistence for pairs of plant species (1) associating with similar guilds of mycorrhizal fungi, (2) of increasing phylogenetic distance, and (3) interacting with native microbes. We also found evidence for a primary role of pathogens in feedback‐mediated coexistence. By combining results over several independent studies, our results confirm that PSF may play a key role in plant species coexistence, species invasion, and the phylogenetic diversification of plant communities. 

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3. Fire modifies plant–soil feedbacks

   https://doi.org/10.1002/ecy.3994  

   Warneke, Christopher R.; Yelenik, Stephanie G.; Brudvig, Lars A. (March 2023, Ecology)

   Abstract Although plant–soil feedbacks (interactions between plants and soils, often mediated by soil microbes, abbreviated as PSFs) are widely known to influence patterns of plant diversity at local and landscape scales, these interactions are rarely examined in the context of important environmental factors. Resolving the roles of environmental factors is important because the environmental context may alter PSF patterns by modifying the strength or even direction of PSFs for certain species. One important environmental factor that is increasing in scale and frequency with climate change is fire, though the influence of fire on PSFs remains essentially unexamined. By changing microbial community composition, fire may alter the microbes available to colonize the roots of plants and thus seedling growth post‐fire. This has potential to change the strength and/or direction of PSFs, depending on how such changes in microbial community composition occur and the plant species with which the microbes interact. We examined how a recent fire altered PSFs of two leguminous, nitrogen‐fixing tree species in Hawaiʻi. For both species, growing in conspecific soil resulted in higher plant performance (as measured by biomass production) than growing in heterospecific soil. This pattern was mediated by nodule formation, an important process for growth for legume species. Fire weakened PSFs for these species and therefore pairwise PSFs, which were significant in unburned soils, but were nonsignificant in burned soils. Theory suggests that positive PSFs such as those found in unburned sites would reinforce the dominance of species where they are locally dominant. The change in pairwise PSFs with burn status shows PSF‐mediated dominance might diminish after fire. Our results demonstrate that fire can modify PSFs by weakening the legume‐rhizobia symbiosis, which may alter local competitive dynamics between two canopy dominant tree species. These findings illustrate the importance of considering environmental context when evaluating the role of PSFs for plants. 

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4. Global patterns and drivers of plant–soil microbe interactions

   https://doi.org/10.1111/ele.14364  

   Jiang, Feng; Bennett, Jonathan A; Crawford, Kerri M; Heinze, Johannes; Pu, Xucai; Luo, Ao; Wang, Zhiheng (January 2024, Ecology Letters)

   Abstract Plant–soil feedback (PSF) is an important mechanism determining plant community dynamics and structure. Understanding the geographic patterns and drivers of PSF is essential for understanding the mechanisms underlying geographic plant diversity patterns. We compiled a large dataset containing 5969 observations of PSF from 202 studies to demonstrate the global patterns and drivers of PSF for woody and non‐woody species. Overall, PSF was negative on average and was influenced by plant attributes and environmental settings. Woody species PSFs did not vary with latitude, but non‐woody PSFs were more negative at higher latitudes. PSF was consistently more positive with increasing aridity for both woody and non‐woody species, likely due to increased mutualistic microbes relative to soil‐borne pathogens. These findings were consistent between field and greenhouse experiments, suggesting that PSF variation can be driven by soil legacies from climates. Our findings call for caution to use PSF as an explanation of the latitudinal diversity gradient and highlight that aridity can influence plant community dynamics and structure across broad scales through mediating plant–soil microbe interactions. 

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5. Three-way species interactions reverse the positive pairwise effects of two natives on an exotic invader

   https://doi.org/10.1007/s11258-023-01304-6  

   Schlau, Benjamin M.; Huxman, Travis E.; Mooney, Kailen A.; Pratt, Jessica D. (April 2023, Plant Ecology)

   Abstract The disruptive effects of tertiary species on otherwise positive pairwise species interactions (e.g. context-dependent parasitism in pollinator syndromes) is well-known. However, few—if any—studies have investigated how invasive plants affect interactions between facilitative plants and their native plant communities. Further, if tertiary invasive species can change interactions among native species from positive to negative, then a tertiary native should be capable of the same phenom for pairwise interactions between natives and invasives. Our previous research indicates invasive black mustard ( Brassica nigra ) changes interaction signs for otherwise positive species interactions between the dominant, native facilitator California buckwheat ( Eriogonum fasciculatum ) and its co-dominant beneficiary California sagebrush ( Artemisia californica ) in semi-arid California coastal sage scrub habitat. Here, E. fasciculatum and A. californica seedlings increased B. nigra shoot growth in pairwise species interactions in the greenhouse. However, in three-way species interactions, E. fasciculatum and A. californica together reduced B. nigra SLA, height, and reproductive potential while not increasing shoot DW. In three-way species interactions, B. nigra did not significantly reduce E. fasciculatum facilitation of A. californica . Also surprisingly, light competition with B. nigra resulted in an increase in A. californica height , which reduced the negative effects of A. californica light competition on shade-intolerant E. fasciculatum. In an additive field experiment, A. californica protected E. fasciculatum from facilitating germination and growth of B. nigra when water competition was minimized. Taken together, this study demonstrates the importance of studying species interactions between competitive, native perennials in the current ecological context of invaded ecosystems. 

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