One shape fits all, but only in the aggregate: Diversity in sub‐stand scale diameter distributions

Abstract

Questions To what extent are the sub‐stand size class distributions at the neighbourhood patch scale masked at the stand level via spatial smoothing? What are the implications for naturalness assessment of obscuring, at the stand level, structures that arise from endogenous processes at the neighbourhood scale? Location Elborz Mountains, south of the Caspian Sea, Iran. Methods Natural uneven‐aged old‐growth Oriental beech (Fagus orientalis) forest subject to fine‐scale disturbances was stem‐mapped in a 1.2‐ha plot of live trees >7.5 cm DBH; neighbourhoods were delineated across multiple scales using the tree‐centred floating neighbourhood approach of spatial tessellation to connect trees to their natural neighbours. Cluster analysis was used to identify an optimal number of recurring diameter distribution types (DDT) based on the proportion of trees in each of the five size classes. The richness of DDTs was assessed and the extent of DDTs was modelled as a function of tree size. Results The number of member trees and spatial extent increased with neighbourhood scale. While the stand‐level tree size class distribution exhibited a rotated sigmoid shape, the nine most distinct DDTs were characterized by four different distribution shapes (negative exponential, increasing‐q, rotated sigmoid and unimodal) and varied considerably in the abundance of trees in different size classes. Each individual tree contributed to at least three of the nine DDTs, which were nonetheless spatially distinct and varied in extent with size class composition. As expected, all metrics exhibited spatial smoothing (i.e., increasing homogeneity with increasing scale) as small, heterogeneous tree neighbourhoods were expanded into larger neighbourhoods with more similar average composition, which converged on a rotated sigmoid shape at the broadest scale. Conclusions The stand‐level rotated sigmoid distribution reflected spatial smoothing across more heterogeneous fine‐scale tree neighbourhood patches. Further studies are needed to assess if a better measure of natural forest structure may be the diversity of sub‐stand structures and the contribution of individual trees to that diversity, rather than an aggregated metric that averages across the hallmark heterogeneity of neighbourhood dynamics.

Common ground among beech forest development stages: Matrix versus stage‐typical live tree structure

“…Although a rotated sigmoid diameter distribution has been shown to arise from smoothing with increasing spatial scale (Alessandrini et al., ; Janowiak et al., ) by averaging across disparate sub‐stand‐scale distributions (Zenner et al., ), this background matrix structure was observed in the current study at even the smallest spatial scale and changed little with increasing sampling extent. This finding is consistent with previous work demonstrating that some neighborhood structures (such as those reflected in diameter distributions) are persistent across spatial scale (Král et al., ; Paluch et al., ; Zenner et al., ). However, spatial smoothing was also detected in the reduction in distinction among deviation classes with increasing scale (cf.…”

“…Glatthorn et al., ), indicating potentially greater sensitivity at the small spatial scales that correspond to the fine‐scale patch dynamics of this disturbance regime (Král, Daněk, Janík, Krůček, & Vrška, ; Podlaski, ;). In Caspian Oriental beech, a small‐scale disturbance regime (Sagheb‐Talebi et al., ) involving canopy tree gaps that correspond to first‐ and second‐order neighborhood scales (Zenner et al., ), is thought to maintain patches of forest with irregular and uneven‐sized structures (Sefidi, Marvie Mohadjer, Mosandl, & Copenheaver, ).…”

“…In addition to a more natural delineation of neighborhoods that captures inter‐tree competitive interactions (Bonan, ), this sampling perspective can be used to objectively assess sub‐stand structures at multiple scales (Zenner & Peck, ). Aside from allowing evaluations of the actual scale dependency of different processes and structural metrics, this approach has also revealed a surprising degree of sub‐stand level structural heterogeneity (Zenner, Peck, & Sagheb‐Talebi, ).…”

“…However, it is possible that the potential value of sub‐stand structural heterogeneity for better understanding the ecological processes that transcend development stages has been underappreciated. Evidence exists for structures that are common across scales (Feldmann, Glatthorn, Hauck, & Leuschner, ; Král et al., ; Paluch et al., ; Zenner et al., ), including the existence of a “typical” stand‐level rotated sigmoid diameter distribution in natural temperate forests regardless of precise development phase (e.g., Westphal et al., ). What if small‐scale heterogeneity were not viewed as only an obstacle to distinguishing among development stages, but rather as a signature of a common structure of natural forests that unites all development stages?…”