Neighbour effects on tree architecture: functional trade‐offs balancing crown competitiveness with wind resistance

Abstract

Summary The architecture of trees is the result of constrained, morphologically plastic growth – constrained by an underlying architectural model embedded in their genome, the structure of which can be significantly altered during growth to match the changing environmental conditions to which the tree is exposed. Here, we examined the hypothesis that crowding from neighbours should cause trees to optimize traits for light competition at the expense of wind resistance, with the reverse being true for trees lacking neighbours. Previous studies have examined the influence of light competition or wind resistance on shaping tree architecture, but few, if any, have simultaneously addressed trade‐offs for optimizing these traits in response to crowding from neighbouring trees in forests, as compared to open‐grown conditions. We studied the response of tree‐ and branch‐level architectural traits of temperate, broad‐leaved, deciduous tree species of differing shade tolerance and wood strength from multiple locations across the north‐eastern United States. Trees ranged in size (4–83 cm diameter at 1·3 m) and crowding conditions (open‐ and forest‐grown) and occupied different canopy positions. The open‐grown trees represented a null condition, where the lack of neighbouring trees to shape architectural traits could be contrasted with the influence of different levels of crowding in forests. Our results show strong evidence for a tree neighbourhood‐induced convergence of architectural traits across species and conditions, even when trees are growing in urban rather than natural forest conditions. After accounting for crowding, the effects of species and sample location contributed very little to explaining variation in architectural traits. One exception was crown dimensions, for which species‐specific differences explained about 15% of the residual variation. Under open‐grown conditions, alleviation of light competition caused trees to develop relatively large crowns and branches and a squat growth form suitable to resist greater wind exposure. By contrast, increasing shading from neighbouring trees caused forest‐grown trees to become increasingly more spindly in the main stem, with slender branches sparsely distributed over a disproportionately large crown volume – presumably to maximize light capture. Although the latter is an intrinsically less wind‐stable form, it can be adopted to increase light capture because neighbouring trees reduce exposure to the wind, which should greatly reduce the likelihood of stem breakage or uprooting under critical wind pressures. A http://onlinelibrary.wiley.com/doi/10.1111/1365-2435.12865/suppinfo is available for this article.

Corner's rules as a framework for plant morphology, architecture and functioning – issues and steps forward

“…apple, Dutra Schmitz et al ., ). Similarly, shading by neighboring trees in natural or agricultural systems increases stem slenderness and decreases branching (MacFarlane & Kane, ). Such studies document the variation spectra of morphological and architectural traits, here stem size or shape and branching frequency, and their relationships.…”

Change in drivers of mangrove crown displacement along a salinity stress gradient

“…The uniform distribution of found for canopy trees in the MFs is evidence that the location and size of neighbours are less relevant than winds to the direction of crown displacement at higher environmental stress sites, as opposed for trees growing in the IBs. Crown displacement for IB A. germinans trees growing below the canopy was best explained only by the sole effect of asymmetric neighbourhoods, suggesting that below canopy trees at lower salinities are well protected from wind dynamic loading, and that canopy trees in lower salinities are protected from winds by dense, bigger and crowded crowns, as found by MacFarlane and Kane (), who discuss that the reduced mechanical stability of spindly stems can be sustained by the protection from wind provided by neighbouring trees.…”

“…Within MFs, the observed reduced canopy cover and reduced crown overlap related to smaller trees could explain the greater effect of wind at higher salinities; smaller tree sizes and crowns could render greater canopy openness. Thus, higher wind speeds in the vertical scale of the forest would have greater influence in crown displacement, as found for edge trees and for open‐grown trees versus crowded trees in temperate forests (Brüchert & Gardiner, ; MacFarlane & Kane, ). However, the reduced crown displacements away from neighbours at higher salinities, showed by the MCMCglm, could represent an advantage in terms of wind protection.…”

“…The observed directionality of crown displacement towards SW and angles of alignment, best explained by the synthesized effect of wind and neighbours, points to a greater influence of wind directionality on crown displacement at higher salinities and suggests an increased susceptibility to wind dynamic loading, presumably as a consequence of salinity‐induced reduction in growth rate (Ball, ; Nguyen, Stanton, Schmitz, Farquhar, & Ball, ). This is ultimately expressed by smaller canopies (Vovides et al, ) with lower wind damping (James, Haritos, & Ades, ; MacFarlane & Kane, ). Within MFs, the observed reduced canopy cover and reduced crown overlap related to smaller trees could explain the greater effect of wind at higher salinities; smaller tree sizes and crowns could render greater canopy openness.…”

Trans‐species predictors of tree leaf mass

“…We used two methods to capture competition from neighboring trees, because we hypothesized that, at any given dimensions, competition would influence allocation to leaf mass. First, each study tree was assigned to a crown class (CC), based on the position of the tree in the canopy: (1) overtopped, (2) intermediate, (3) co‐dominant, or (4) dominant, to indicate shading experienced by the tree (MacFarlane and Kane ). Second, the general crowding experienced from surrounding trees was assessed by tallying trees with a dbh >10 cm around the subject tree in a 7.3 m radius plot (these specifications were chosen to match plot guidelines from the US FIA program).…”