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1. Experimental and observational evidence of negative conspecific density dependence in temperate ectomycorrhizal trees

   https://doi.org/10.1002/ecy.3808  

   Jevon, Fiona V. ; De La Cruz, Dayna ; LaManna, Joseph A. ; Lang, Ashley K. ; Orwig, David A. ; Record, Sydne ; Kouba, Paige V. ; Ayres, Matthew P. ; Matthes, Jaclyn Hatala ( July 2022 , Ecology)

   Yavitt, Joseph B. (Ed.)

   Conspecific negative density dependence (CNDD) promotes tree species diversity by reducing recruitment near conspecific adults due to biotic feedbacks from herbivores, pathogens, or competitors. While this process is well-described in tropical forests, tests of temperate tree species range from strong positive to strong negative density dependence. To explain this, several studies have suggested that tree species traits may help predict the strength and direction of density dependence: for example, ectomycorrhizal-associated tree species typically exhibit either positive or weaker negative conspecific density dependence. More generally, the strength of density dependence may be predictably related to other species-specific ecological attributes such as shade tolerance, or the relative local abundance of a species. To test the strength of density dependence and whether it affects seedling community diversity in a temperate forest, we tracked the survival of seedlings of three ectomycorrhizal-associated species experimentally planted beneath conspecific and heterospecific adults on the Prospect Hill tract of the Harvard Forest, in Massachusetts, USA. Experimental seedling survival was always lower under conspecific adults, which increased seedling community diversity in one of six treatments. We compared these results to evidence of CNDD from observed sapling survival patterns of 28 species over approximately 8 years in an adjacent 35-ha forest plot. We tested whether species-specific estimates of CNDD were associated with mycorrhizal association, shade tolerance, and local abundance. We found evidence of significant, negative conspecific density dependence (CNDD) in 23 of 28 species, and positive conspecific density dependence in two species. Contrary to our expectations, ectomycorrhizal-associated species generally exhibited stronger (e.g., more negative) CNDD than arbuscular mycorrhizal-associated species. CNDD was also stronger in more shade-tolerant species but was not associated with local abundance. Conspecific adult trees often have a negative influence on seedling survival in temperate forests, particularly for tree species with certain traits. Here we found strong experimental and observational evidence that ectomycorrhizal-associating species consistently exhibit CNDD. Moreover, similarities in the relative strength of density dependence from experiments and observations of sapling mortality suggest a mechanistic link between negative effects of conspecific adults on seedling and sapling survival and local tree species distributions. 

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2. Shade tolerance and mycorrhizal type may influence sapling susceptibility to conspecific negative density dependence

   https://doi.org/10.1111/1365-2745.13237  

   Brown, Alissa J. ; Payne, Christopher J. ; White, Peter S. ; Peet, Robert K. ; Mariotte, ed., Pierre ( July 2019 , Journal of Ecology)

   The maintenance of tree diversity has been explained by multiple mechanisms. One of the most thoroughly studied is conspecific negative density dependence, in which specialist plant enemies reduce survivorship of seeds, seedlings or saplings located near adult conspecifics. Although there is much support that conspecific negative density dependence occurs in temperate forests, only a subset of the species investigated thus far exhibit this recruitment pattern. It remains unclear what drives differential susceptibility to conspecifics among tree species. Previous investigators have considered shade tolerance and mycorrhizal type (arbuscular mycorrhizal vs. ectomycorrhizal association) as two traits that might explain differential susceptibility to conspecific negative density dependence.

   Here, we test whether these two plant traits predict susceptibility of tree saplings to conspecific negative density dependence in a temperate hardwood forest using three responses: spatial point patterns of saplings, sapling growth and sapling survival.

   Spatial patterns of saplings indicate that shade tolerant species are less sensitive to conspecifics than shade intolerant species, but show no differences based on mycorrhizal type. Conversely, shade tolerant saplings exhibit reduced growth, but not survival, when located in areas with high conspecific density. We interpret this finding in light of the conservative functional strategies of shade tolerant species, which typically have low leaf nitrogen levels and slower growth to divert resources to tissue defence against enemies. We found an effect of mycorrhizal type interacting with adult conspecific density, where arbuscular mycorrhizal species show a greater reduction in growth than ectomycorrhizal species in areas dense with conspecifics.

   Synthesis. We conclude that the shade tolerance level and the mycorrhizal type of temperate forest saplings may influence how their growth and survival respond to the adult conspecific trees in their neighbourhoods.

    

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3. Ectomycorrhizal fungi drive positive phylogenetic plant–soil feedbacks in a regionally dominant tropical plant family

   https://doi.org/10.1002/ecy.3083  

   Segnitz, R. Max ; Russo, Sabrina E. ; Davies, Stuart J. ; Peay, Kabir G. ( June 2020 , Ecology)

   While work in temperate forests suggests that there are consistent differences in plant–soil feedback (PSF) between plants with arbuscular and ectomycorrhizal associations, it is unclear whether these differences exist in tropical rainforests. We tested the effects of mycorrhizal type, phylogenetic relationships to overstory species, and soil fertility on the growth of tree seedlings in a tropical Bornean rainforest with a high diversity of both ectomycorrhizal and arbuscular mycorrhizal trees. We found that ectomycorrhizal tree seedlings had higher growth in soils conditioned by close relatives and that this was associated with higher mycorrhizal colonization. By contrast, arbuscular mycorrhizal tree seedlings generally grew more poorly in soils conditioned by close relatives. For ectomycorrhizal species, the phylogenetic trend was insensitive to soil fertility. For arbuscular mycorrhizal seedlings, however, the effect of growing in soils conditioned by close relatives became increasingly negative as soil fertility increased. Our results demonstrate consistent effects of mycorrhizal type on plant–soil feedbacks across forest biomes. The positive effects of ectomycorrhizal symbiosis may help explain biogeographic variation across tropical forests, such as familial dominance of the Dipterocarpaceae in southeast Asia. However, positive feedbacks also raise questions about the role of PSFs in maintaining tropical diversity.

    

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4. Tree species diversity increases with conspecific negative density dependence across an elevation gradient

   https://doi.org/10.1111/ele.13996  

   LaManna, Joseph A. ; Jones, F. Andrew ; Bell, David M. ; Pabst, Robert J. ; Shaw, David C. ; Gurevitch, ed., Jessica ( March 2022 , Ecology Letters)

   Elevational and latitudinal gradients in species diversity may be mediated by biotic interactions that cause density‐dependent effects of conspecifics on survival or growth to differ from effects of heterospecifics (i.e. conspecific density dependence), but limited evidence exists to support this. We tested the hypothesis that conspecific density dependence varies with elevation using over 40 years of data on tree survival and growth from 23 old‐growth temperate forest stands across a 1,000‐m elevation gradient. We found that conspecific‐density‐dependent effects on survival of small‐to‐intermediate‐sized focal trees were negative in lower elevation, higher diversity forest stands typically characterised by warmer temperatures and greater relative humidity. Conspecific‐density‐dependent effects on survival were less negative in higher elevation stands and ridges than in lower elevation stands and valley bottoms for small‐to‐intermediate‐sized trees, but were neutral for larger trees across elevations. Conspecific‐density‐dependent effects on growth were negative across all tree size classes and elevations. These findings reveal fundamental differences in biotic interactions that may contribute to relationships between species diversity, elevation and climate.

    

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5. Reduced aboveground tree growth associated with higher arbuscular mycorrhizal fungal diversity in tropical forest restoration

   https://doi.org/10.1002/ece3.2487  

   Holste, Ellen K. ; Holl, Karen D. ; Zahawi, Rakan A. ; Kobe, Richard K. ( September 2016 , Ecology and Evolution)

   Establishing diverse mycorrhizal fungal communities is considered important for forest recovery, yet mycorrhizae may have complex effects on tree growth depending on the composition of fungal species present. In an effort to understand the role of mycorrhizal fungi community in forest restoration in southern Costa Rica, we sampled the arbuscular mycorrhizal fungal (AMF) community across eight sites that were planted with the same species (Inga edulis, Erythrina poeppigiana, Terminalia amazonia,andVochysia guatemalensis) but varied twofold to fourfold in overall tree growth rates. TheAMFcommunity was measured in multiple ways: as percent colonization of host tree roots, byDNAisolation of the fungal species associated with the roots, and through spore density, volume, and identity in both the wet and dry seasons. Consistent with prior tropical restoration research, the majority of fungal species belonged to the genusGlomusand genusAcaulospora, accounting for more than half of the species and relative abundance found on trees roots and over 95% of spore density across all sites. GreaterAMFdiversity correlated with lower soil organic matter, carbon, and nitrogen concentrations and longer durations of prior pasture use across sites. Contrary to previous literature findings,AMFspecies diversity and spore densities were inversely related to tree growth, which may have arisen from trees facultatively increasing their associations withAMFin lower soil fertility sites. Changes toAMFcommunity composition also may have led to variation in disturbance susceptibility, host tree nutrient acquisition, and tree growth. These results highlight the potential importance of fungal–tree–soil interactions in forest recovery and suggest that fungal community dynamics could have important implications for tree growth in disturbed soils.

    

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