Sapling growth rates reveal conspecific negative density dependence in a temperate forest

Abstract

Local tree species diversity is maintained in part by conspecific negative density dependence (CNDD). This pervasive mechanism occurs in a variety of forms and ecosystems, but research to date has been heavily skewed toward tree seedling survival in tropical forests. To evaluate CNDD more broadly, we investigated how sapling growth rates were affected by conspecific adult neighbors in a fully mapped 25.6 ha temperate deciduous forest. We examined growth rates as a function of the local adult tree neighborhood (via spatial autoregressive modeling) and compared the spatial positioning of faster‐growing and slower‐growing saplings with respect to adult conspecific and heterospecific trees (via bivariate point pattern analysis). In addition, to determine whether CNDD‐driven variation in growth rates leaves a corresponding spatial signal, we extended our point pattern analysis to a static, growth‐independent comparison of saplings and the next larger size class. We found that negative conspecific effects on sapling growth were most prevalent. Five of the nine species that were sufficiently abundant for analysis exhibited CNDD, while only one species showed evidence of a positive conspecific effect, and one or two species, depending on the analysis, displayed heterospecific effects. There was general agreement between the autoregressive models and the point pattern analyses based on sapling growth rates, but point pattern analyses based on single‐point‐in‐time size classes yielded results that differed markedly from the other two approaches. Our work adds to the growing body of evidence that CNDD is an important force in temperate forests, and demonstrates that this process extends to sapling growth rates. Further, our findings indicate that point pattern analyses based solely on size classes may fail to detect the process of interest (e.g., neighborhood‐driven variation in growth rates), in part due to the confounding of tree size and age.

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

“…Consistent with our results, others have found Carya ovata to exhibit patterns of CNDD (Johnson et al, ; LaManna et al, ; Ramage et al, ). Interestingly, our result that Fagus grandifolia exhibits a positive response to conspecifics is unique among other studies, which have instead found strong CNDD effects for this species (Comita et al, ; Johnson et al, ; Ramage et al, ; Zhu, Comita, et al, ). However, differences between our findings and those of others may be a result of the relatively small size of our plots and the limited geographic scope of our study area.…”

“…Cross‐study comparison of interspecific susceptibility to CNDD is made difficult by the wide variation in experimental design, statistical approach, and life stage focus taken by investigators, as well as the problem of non‐overlapping species across studies in different geographic regions. Although few patterns emerge, it does appear that the response of Acer rubrum and Fraxinus americana to conspecifics is variable and may depend on the environmental context (Comita et al, ; Johnson et al, ; LaManna et al, ; McCarthy‐Neumann & Ibáñez, ; Ramage et al, ). This observation corroborates our finding that Acer rubrum showed different spatial patterns across our study plots.…”

Interactions among trees: A key element in the stabilising effect of species diversity on forest growth

“…Complementarity occurs when species exhibit niche partitioning or when interspecific interactions enhance the capture of resources (Cardinale et al., ). Species complementarity therefore implies that interspecific interactions are less detrimental to tree growth than intraspecific interactions, consistent with coexistence theory (Chesson, ; Ramage et al., ). Furthermore, by enhancing the capture of resource by species, or at least by reducing it less than intraspecific interactions, interspecific interactions could make species less sensitive to fluctuations in resource availability (e.g., soil water availability), and thus, more generally, less sensitive to fluctuations in environmental conditions.…”

“…Furthermore, most studies consider diversity directly, for instance, by computing species richness, while here we consider the effect of the ratio of inter – to intraspecific interactions on individual growth. In doing so, we are able to directly assess predictions arising from the coexistence theory which requires stronger intraspecific competition relative to interspecific competition to maintain species richness (Chesson, ; Ramage et al., ). The major limitation, however, remains, as in other studies of the insurance hypothesis with trees, that our analyses only consider one demographic process: individual growth.…”

Physical, but not chemical, antiherbivore defense expression is related to the clustered spatial distribution of tropical trees in an Amazonian forest

“…Numerous studies have documented the effects of CNDD in species‐rich tropical forests both including seeds and seedling mortality (Chen, Umaña, Uriarte, & Yu, ; He & Duncan, ; Johnson et al, ; LaManna et al, ; Metz, Souza, & Valencia, ; Peters, ; Wright, ), and sapling survival as well (Peters, ; Ramage et al, ). However, there are some common tree species (e.g., >50 individuals/ha) that grow in large clusters of conspecific saplings and adults, defying the expectations of the CNDD.…”