Shared friends counterbalance shared enemies in old forests

Abstract

Mycorrhizal mutualisms are nearly ubiquitous across plant communities. Yet, it is still unknown whether facilitation among plants arises primarily from these mycorrhizal networks or from physical and ecological attributes of plants themselves. Here, we tested the relative contributions of mycorrhizae and plants to both positive and negative biotic interactions to determine whether plant-soil feedbacks with mycorrhizae neutralize competition and enemies within multitrophic forest community networks. We used Bayesian hierarchical generalized linear modeling to examine mycorrhizal-guild-specific and mortality-cause-specific woody plant survival compiled from a spatially and temporally explicit data set comprising 101,096 woody plants from three mixed-conifer forests across western North America. We found positive plant-soil feedbacks for large-diameter trees: species-rich woody plant communities indirectly promoted large tree survival when connected via mycorrhizal networks. Shared mycorrhizae primarily counterbalanced apparent competition mediated by tree enemies (e.g., bark beetles, soil pathogens) rather than diffuse competition between plants. We did not find the same survival benefits for small trees or shrubs. Our findings suggest that lower largediameter tree mortality susceptibility in species-rich temperate forests resulted from greater access to shared mycorrhizal networks. The interrelated importance of aboveground and belowground biodiversity to large tree survival may be critical for counteracting increasing pathogen, bark beetle, and density threats.

Crowding, climate, and the case for social distancing among trees

“…The effects of drought (reduced plant‐available water) are filtered through fine‐grained ecological attributes including microtopography, soil water‐holding capacity, and forest density (affecting water demand), and these fine‐grained conditions can potentially mediate the realized microenvironment that trees experience. Tree neighborhoods capture this net drought effect along with resource competition and spatially nonrandom mortality processes, all factors that are all closely linked to tree‐to‐tree variance in mortality risk (Furniss, Larson, et al, 2020; Germain & Lutz, 2021; van Mantgem et al, 2018). Other studies have similarly shown the importance of both climate and forest structure to mortality risk (Restaino et al, 2019; Ruiz‐Benito et al, 2013; van Mantgem et al, 2018; Young et al, 2017), but few directly compare the relative magnitude of their effects.…”

Dancing with Douglas‐fir: Determinism dominates fungal community assembly processes

“…Fungal communities can influence the productivity, composition and health of forests through cycling nutrients, providing resources and pathogen introduction or, in the case of mycorrhizal fungi, protection from pathogens (Tedersoo et al, 2020). Thus, shifts in fungal communities could impact forests by altering interactions between trees and their environments (Germain & Lutz, 2021). Identifying the types of assembly and the variation in assembly processes can provide insight within the studied system and help generalize the drivers of deterministic and neutral processes across systems.…”

Climate warming may weaken stabilizing mechanisms in old forests

“…We found net negative heterospecific effects with climate warming, suggesting that any underlying positive interactions (e.g., shared mycorrhizal networks; Bennett et al, 2017) were unable to completely offset increasingly negative interactions among the tree species. Silvicultural interventions that cultivate enhanced facilitation among heterospecifics (Germain & Lutz, 2021; Jactel & Brockerhoff, 2007) may be able to counteract negative interactions and conserve forest community stability.…”