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1. Whitebark pine facilitation at treeline: potential interactions for disruption by an invasive pathogen

   https://doi.org/10.1002/ece3.2198  

   Tomback, Diana F. ; Blakeslee, Sarah C. ; Wagner, Aaron C. ; Wunder, Michael B. ; Resler, Lynn M. ; Pyatt, Jill C. ; Diaz, Soledad ( June 2016 , Ecology and Evolution)

   In stressful environments, facilitation often aids plant establishment, but invasive plant pathogens may potentially disrupt these interactions. In many treeline communities in the northernRockyMountains of theU.S. andCanada,Pinus albicaulis, a stress‐tolerant pine, initiates tree islands at higher frequencies than other conifers – that is, leads to leeward tree establishment more frequently. The facilitation provided by a solitary (isolated)P. albicaulisleading to tree island initiation may be important for different life‐history stages for leeward conifers, but it is not known which life‐history stages are influenced and protection provided. However,P. albicaulismortality from the non‐native pathogenCronartium ribicolapotentially disrupts these facilitative interactions, reducing tree island initiation. In twoRockyMountain eastern slope study areas, we experimentally examined fundamental plant–plant interactions which might facilitate tree island formation: the protection offered byP. albicaulisto leeward seed and seedling life‐history stages, and to leeward krummholz conifers. In the latter case, we simulated mortality fromC. ribicolafor windwardP. albicaulisto determine whether loss ofP. albicaulisfromC. ribicolaimpacts leeward conifers. Relative to other common solitary conifers at treeline, solitaryP. albicaulishad higher abundance. More seeds germinated in leeward rock microsites than in conifer or exposed microsites, but the odds of cotyledon seedling survival during the growing season were highest inP. albicaulismicrosites. Planted seedling survival was low among all microsites examined. Simulating death of windwardP. albicaulisbyC. ribicolareduced shoot growth of leeward trees. Loss ofP. albicaulisto exotic disease may limit facilitation interactions and conifer community development at treeline and potentially impede upward movement as climate warms.

    

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2. Environmental filtering by pH and soil nutrients drives community assembly in fungi at fine spatial scales

   https://doi.org/10.1111/mec.14414  

   Glassman, Sydney I. ; Wang, Ian J. ; Bruns, Thomas D. ( November 2017 , Molecular Ecology)

   Whether niche processes, like environmental filtering, or neutral processes, like dispersal limitation, are the primary forces driving community assembly is a central question in ecology. Here, we use a natural experimental system of isolated tree “islands” to test whether environment or geography primarily structures fungal community composition at fine spatial scales. This system consists of isolated pairs of two distantly related, congeneric pine trees established at varying distances from each other and the forest edge, allowing us to disentangle the effects of geographic distance vs. host and edaphic environment on associated fungal communities. We identified fungal community composition with Illumina sequencing ofITSamplicons, measured all relevant environmental parameters for each tree—including tree age, size and soil chemistry—and calculated geographic distances from each tree to all others and to the nearest forest edge. We applied generalized dissimilarity modelling to test whether total and ectomycorrhizal fungal (EMF) communities were primarily structured by geographic or environmental filtering. Our results provide strong evidence that as in many other organisms, niche and neutral processes both contribute significantly to turnover in community composition in fungi, but environmental filtering plays the dominant role in structuring both free‐living and symbiotic fungal communities at fine spatial scales. In our study system, we foundpHand organic matter primarily drive environmental filtering in total soil fungal communities and thatpHand cation exchange capacity—and, surprisingly, not host species—were the largest factors affectingEMFcommunity composition. These findings support an emerging paradigm thatpHmay play a central role in the assembly of all soil‐mediated systems.

    

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3. Legacy effects of tree mortality mediated by ectomycorrhizal fungal communities

   https://doi.org/10.1111/nph.15993  

   Mueller, Rebecca C. ; Scudder, Crescent M. ; Whitham, Thomas G. ; Gehring, Catherine A. ( July 2019 , New Phytologist)

   Successive droughts have resulted in extensive tree mortality in the southwestern United States. Recovery of these areas is dependent on the survival and recruitment of young trees. For trees that rely on ectomycorrhizal fungi (EMF) for survival and growth, changes in soil fungal communities following tree mortality could negatively affect seedling establishment.

   We used tree‐focused and stand‐scale measurements to examine the impact of pinyon pine mortality on the performance of surviving juvenile trees and the potential for mutualism limitation of seedling establishment via alteredEMFcommunities.

   Mature pinyon mortality did not affect the survival of juvenile pinyons, but increased their growth. At both tree and stand scales, high pinyon mortality had no effect on the abundance ofEMFinocula, but led to alteredEMFcommunity composition including increased abundance ofGeoporaand reduced abundance ofTuber. Seedling biomass was strongly positively associated withTuberabundance, suggesting that reductions in this genus with pinyon mortality could have negative consequences for establishing seedlings.

   These findings suggest that whereas mature pinyon mortality led to competitive release for established juvenile pinyons, changes inEMFcommunity composition with mortality could limit successful seedling establishment and growth in high‐mortality sites.

    

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4. The potential for mycobiont sharing between shrubs and seedlings to facilitate tree establishment after wildfire at Alaska arctic treeline

   https://doi.org/10.1111/mec.14143  

   Hewitt, Rebecca E. ; Chapin, III, F. Stuart ; Hollingsworth, Teresa N. ; Taylor, D. Lee ( May 2017 , Molecular Ecology)

   Root‐associated fungi, particularly ectomycorrhizal fungi (EMF), are critical symbionts of all boreal tree species. Although climatically driven increases in wildfire frequency and extent have been hypothesized to increase vegetation transitions from tundra to boreal forest, fire reduces mycorrhizal inoculum. Therefore, changes in mycobiont inoculum may potentially limit tree‐seedling establishment beyond current treeline. We investigated whether ectomycorrhizal shrubs that resprout after fire support similar fungal taxa to those that associate with tree seedlings that establish naturally after fire. We then assessed whether mycobiont identity correlates with the biomass or nutrient status of these tree seedlings. The majority of fungal taxa observed on shrub and seedling root systems wereEMF, with some dark septate endophytes and ericoid mycorrhizal taxa. Seedlings and adjacent shrubs associated with similar arrays of fungal taxa, and there were strong correlations between the structure of seedling and shrub fungal communities. These results show that resprouting postfire shrubs support fungal taxa compatible with tree seedlings that establish after wildfire. Shrub taxon, distance to the nearest shrub and fire severity influenced the similarity between seedling and shrub fungal communities. Fungal composition was correlated with both foliar C:N ratio and seedling biomass and was one of the strongest explanatory variables predicting seedling biomass. While correlative, these results suggest that mycobionts are important to nutrient acquisition and biomass accrual of naturally establishing tree seedlings at treeline and that mycobiont taxa shared by resprouting postfire vegetation may be a significant source of inoculum for tree‐seedling establishment beyond current treeline.

    

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5. Invasive legume management strategies differentially impact mutualist abundance and benefit to native and invasive hosts

   https://doi.org/10.1111/rec.13081  

   Komatsu, Kimberly J. ; Simms, Ellen L. ( December 2019 , Restoration Ecology)

   Determining the best management practices for plant invasions is a critical, but often elusive goal. Invasive removals frequently involve complex and poorly understood biotic interactions. For example, invasive species can leave potent legacies that influence the success of native species restoration efforts, and positive plant‐microbial feedbacks may promote continued reinvasion by an exotic species following restoration. Removal methods can vary in their effects on plant–soil feedbacks, with consequences for restoration of native species. We determined the effects of invasion by a leguminous shrub (French broom;Genista monspessulana) on the density and community composition of, and benefit conferred by, its microbial mutualists in its invading range. Densities of soil‐dwelling rhizobia were much higher in areas invaded byG. monspessulanarelative to uninvaded areas, and this increased density of rhizobia fed back to increase seedling growth of both the invader and native legumes. We further compared how three techniques for removingG. monspessulanaaffected the densities of rhizobia relative to areas whereG. monspessulanawas still present. Removal by hand‐pulling reduced soil rhizobial densities, and reduced growth of one native legume, while having no effect on the growth of the invader. Overall, our results show that the consequences of restoration techniques, both above‐ and belowground, could be critical for the successful removal of an invasive legume and restoration of native species.

    

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