Testing a general approach to assess the degree of disturbance in tropical forests

Abstract

Questions Is there any theoretical model enabling predictions of the optimal tree size distribution in tropical communities? Can we use such a theoretical framework for quantifying the degree of disturbance? Location Reserve of Yangambi, northeast region of the Democratic Republic of Congo. Methods We applied an allometric model based on the assumption that a virtually undisturbed forest uses all available resources. In this condition, the forest structure (e.g. the tree size distribution) is theoretically predictable from the scaling of the tree crown with tree height at an individual level. The degree of disturbance can be assessed through comparing the slopes of the tree size distribution curves in the observed and predicted conditions. We tested this tool in forest stands subjected to different degrees of disturbance. We inventoried trees >1.3 m in height by measuring the DBH in three plots of 1 ha each, and measured tree height, crown radius and crown length in a sub‐sample of trees. Results All tree species, independently of the site, shared the same exponents of allometric relationships: tree height vs tree diameter, crown radius vs tree height, crown length vs tree height and consequently crown volume vs tree height, suggesting that similar trajectories of biomass allocation have evolved irrespective of species. The observed tree size distributions appeared to be power laws (excluding the finite size effect) and, as predicted, the slope was steeper in the less disturbed forest (−2.34) compared to the most disturbed (−1.99). The difference in the slope compared to the theoretical fully functional forest (−2.65) represents the metric for assessing the degree of disturbance. Conclusions We developed a simple tool for operationalizing the concept of ‘disturbance’ in tropical forests. This approach is species‐independent, needs minimal theoretical assumptions, the measurement of only a few structural traits and requires a low investment in equipment, time and computer skills. Its simple implementation opens new perspectives for effectively addressing initiatives of forest protection and/or restoration.

Constant theoretical conductance via changes in vessel diameter and number with height growth in Moringa oleifera

“…Therefore, knowing how leaf area (and thus productivity) scales as individual trees increase in size, together with the assumption that forest resource use is always maximal (meaning that a community is saturated with individuals, whether it is composed of many small or few large individuals), allows prediction of the slope of tree size distributions in any community (Simini et al , 2010; Anfodillo et al , 2012). Deviation from the predicted distribution can even be diagnostic of forest disturbance (Coomes et al , 2003; Sellan et al , 2017). This energy equivalence expectation, supported here by constant conductance per unit leaf area, represents a potential link between hydraulics, stand productivity, and the shaping of optimal canopy height and density by microsite (Eagleson, 1982; Cabon et al , 2018).…”

Pantropical variability in tree crown allometry

“…In the tropics, scaling exponents from crown allometric relationships have been shown to vary between forests and savannas, with savanna tree crowns tending to be larger for a given stem diameter than those of forest trees at the species scale (Archibald & Bond, 2003) and the site scale (Shenkin et al., 2020). However, the scaling exponents of crown allometric relationships do not change when comparing three neighbouring forest typologies (mixed forests, edge forests and regrowth forests) that have different light availability (Sellan et al., 2017).…”

Stable Allometric Trajectories in Picea abies (L.) Karst. Trees along an Elevational Gradient

“…This trend is more pronounced in conifers, therefore we could expect Norway spruce to grow comparatively fewer leaves at higher elevations. However, other studies have found little variations in the relationship between crown volume and tree height in forests with different compositions, disturbance histories, and elevations, but within limited latitudinal ranges [35][36][37]. This highlights the need for more field-based research on the drivers of tree structural balance and allocation patterns.…”

“…We also expect that allometric relationships, given their fundamental importance in determining tree fitness, are constant along the gradient. This hypothesis is based on the general quantitative theory of forest structure and dynamics [38,39] and is backed by existing studies providing evidence of the stability of allometric relationships despite environmental variations [33,[35][36][37].…”

“…At the tree level, allometric relationships define the balance that must exist between different functional parts in order to maintain functionality and increase fitness [32]. The fact that these scaling relationships are commonly observed within a very limited set of possible exponents suggests a strong driver of natural selection against individuals that deviate from "the least-bad" structure [33,34] and underlies existing models [35][36][37] and theories [38,39]. However, there is still uncertainty regarding the stability of allometric relationships between certain structural parts [30,[40][41][42].…”