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

Abstract

1. 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.2. 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.3. 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.

Light can modify density‐dependent seedling mortality in a temperate forest

“…Tall seedlings are probably more resistant to conspecific inhibition than small seedlings in our study, perhaps because tall seedlings can divert more resources to tissue defence (Develey-Riviere & Galiana, 2007;Barton & Koricheva, 2010) or attain greater carbon gains from neighbouring trees than small seedlings (Teste et al, 2009). However, we cannot rule out the possibility that the growth rather than the survival of tall seedlings is reduced by conspecific inhibition (Brown et al, 2020), especially over the relatively short two-year period of our study. In contrast to our initial expectations, the survival of tall seedlings appears to be inhibited by conspecific and heterospecific tree neighbours, suggesting that size-asymmetric competition with trees and/ or density-responsive generalist enemies becomes important once seedlings are taller (Dickie et al, 2005;Kupferschmid et al, 2020;Wulantuya et al, 2020).…”

“…Such unfavourable environments may, on the other hand, have the benefit of less crowding from neighbouring plants (Zambrano et al, 2017). Whether or not such interactions of density-dependent effects and abiotic conditions explain variation in the vital rates of plants and contribute to community dynamics still remains an open question, especially in temperate forests, despite considerable research efforts devoted to this field (McCarthy-Neumann & Ibañez, 2013;Brown et al, 2020;Yao et al, 2020).…”

“…The growth form of plants may also play a role, as exemplified by the possibility that tree and shrub species differ in their ability to regenerate after enemy attack or episodic disturbance (Bellingham & Sparrow, 2000;Wang et al, 2020). Further, sensitivity to density-dependent effects and abiotic habitat conditions likely varies among species based on their life history strategies (Uriarte et al, 2018;Brown et al, 2020). For instance, sensitivity to negative density-dependent effects has been shown to increase with the species' shade intolerance (McCarthy-Neumann & Ibañez, 2013;Brown et al, 2020).…”

“…Further, sensitivity to density-dependent effects and abiotic habitat conditions likely varies among species based on their life history strategies (Uriarte et al, 2018;Brown et al, 2020). For instance, sensitivity to negative density-dependent effects has been shown to increase with the species' shade intolerance (McCarthy-Neumann & Ibañez, 2013;Brown et al, 2020).…”

“…We also used statistical neighbourhood models with variable neighbourhood size (Zambrano et al, 2020) to assess density-dependent effects (Piao et al, 2013;Johnson et al, 2014;Brown et al, 2020).…”