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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. Interactions with soil fungi alter density dependence and neighborhood effects in a locally abundant dipterocarp species

   https://doi.org/10.1002/ece3.8478  

   Segnitz, R. Max ; Russo, Sabrina E. ; Peay, Kabir G. ( January 2022 , Ecology and Evolution)

   Seedling recruitment can be strongly affected by the composition of nearby plant species. At the neighborhood scale (on the order of tens of meters), adult conspecifics can modify soil chemistry and the presence of host microbes (pathogens and mutualists) across their combined canopy area or rooting zones. At local or small spatial scales (on the order of one to few meters), conspecific seed or seedling density can influence the strength of intraspecific light and resource competition and also modify the density‐dependent spread of natural enemies such as pathogens or invertebrate predators. Intrinsic correlation between proximity to adult conspecifics (i.e., recruitment neighborhood) and local seedling density, arising from dispersal, makes it difficult to separate the independent and interactive factors that contribute to recruitment success. Here, we present a field experiment in which we manipulated both the recruitment neighborhood and seedling density to explore how they interact to influence the growth and survival ofDryobalanops aromatica, a dominant ectomycorrhizal tree species in a Bornean tropical rainforest. First, we found that both local seedling density and recruitment neighborhood had effects on performance ofD. aromaticaseedlings, though the nature of these impacts varied between growth and survival. Second, we did not find strong evidence that the effect of density on seedling survival is dependent on the presence of conspecific adult trees. However, accumulation of mutualistic fungi beneath conspecifics adults does facilitate establishment ofD. aromaticaseedlings. In total, our results suggest that recruitment near adult conspecifics was not associated with a performance cost and may have weakly benefitted recruiting seedlings. Positive effects of conspecifics may be a factor facilitating the regional hyperabundance of this species. Synthesis: Our results provide support for the idea that dominant species in diverse forests may escape the localized recruitment suppression that limits abundance in rarer species.

    

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3. Ectomycorrhizal fungal community assembly on seedlings of a Neotropical monodominant tree

   https://doi.org/10.1111/btp.12989  

   Delevich, Carolyn A. ; Koch, Rachel A. ; Aime, M. Catherine ; Henkel, Terry W. ( July 2021 , Biotropica)

   Ectomycorrhizal tree species may benefit from positive plant–soil feedbacks, where soil environments near adult trees enhance conspecific seedling growth and survival. In tropical monodominant forests, seedling survival is particularly important, as seedling banks help maintain stand‐level dominance over generations. Positive plant–soil feedbacks may be mediated by diverse ectomycorrhizal fungal communities, which improve nutrient acquisition of heavily shaded seedlings. Despite the potential importance of these fungi, little is known about ectomycorrhizal fungal community development on seedlings of tropical monodominant trees. In Guyana, we sequentially monitored percent colonization and species composition of ectomycorrhizal fungi on an even‐age cohort of seedlings of the tropical monodominant treeDicymbe corymbosa(Fabaceae subfamily Detarioideae). Ectomycorrhizal fungi found onD. corymbosaseedlings over a 12‐month period of early development were compared to those of conspecific adults and four other ectomycorrhizal tree species in the region. Species turnover was high (80%) between 6‐ and 12‐month‐old seedlings, though the /russula‐lactarius, /clavulina, and /tomentella‐thelephora lineages were species‐rich on seedlings at all ages. The number of ectomycorrhizal morphotypes per seedling increased with age, but extent of fungal colonization did not. Seedling ectomycorrhizal fungi were shared with sympatric conspecific adults (55%) and, to a lesser extent, regional heterospecific adults (27%), but numerous species were previously unrecorded for Guyana. Over their developmentD. corymbosaseedlings did not rely strictly on adult trees for their mycobionts but appeared to foster unique assemblages of ectomycorrhizal fungi.

    

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4. 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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5. Biogeographic variation of distance‐dependent effects in an invasive tree species

   https://doi.org/10.1111/1365-2435.13306  

   Yang, Qiang ; Ding, Jianqing ; Siemann, Evan ; Godoy, ed., Oscar ( March 2019 , Functional Ecology)

   Plant pathogens and herbivores can maintain forest diversity by reducing survival of tree seedlings close to conspecifics. However, how biogeographic variation in these natural enemies affects such distance‐dependent processes is unknown. Because invasive plants escape ecologically important enemies when introduced to a new range, distance‐dependent mortality may differ between their native and introduced ranges.

   Here, we test whether the invasive treeTriadica sebiferaescaped distance‐dependent mortality when introduced to the United States from China, and examine the roles of natural enemies in native and introduced ranges. In both the United States and China, we performed field surveys along with field and greenhouse experiments with field‐collected soils and soil sterilization treatments.

   In field surveys and the field experiment, insect damage onT. sebiferaseedlings decreased with distance to conspecific trees in the native range (China), but damage was low at all distances in the introduced range (United States). In the greenhouse experiment testing the effects of soil pathogens,T. sebiferaseedling mortality decreased with soil distance from conspecific trees in both ranges but distance‐independent mortality was higher in native range soils.

   Our findings indicate that both insect herbivores and the soil biota contribute to distance‐dependent effects onT. sebiferain its native range. They suggest, however, that plants may more readily escape herbivore than soil biota distance‐dependent effects when introduced to a new range and so herbivores, rather than soil pathogens, contribute more strongly to biogeographic variation in distance‐dependent effects. These results highlight the importance of considering species biogeographic variation in distance‐dependent effects and teasing apart the roles that different natural enemies play when studying species coexistence, community diversity and biological invasions.

    

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