Competitive drivers of interspecific deviations of crown morphology from theoretical predictions measured with Terrestrial Laser Scanning

Harry J F Owen,William R M Flynn,Emily R Lines

doi:10.1111/1365-2745.13670

Harry J F Owen, William R M Flynn + Show 1 more

Open Access

https://doi.org/10.1111/1365-2745.13670

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Abstract

AbstractTree crown morphology is a key driver of forest dynamics, determining not only the competitiveness of an individual but also the competitive effect exerted on neighbouring trees. Multiple ecological theories, including metabolic scaling theory (MST), predict crown morphology from first principles, but typically lack consideration of competition. The accurate quantification of crown morphology to test theoretical predictions, and the canopy interactions that could alter them, has historically been limited by the simplicity and associated error of traditional crown measurements.In this study, we calculate high‐resolution two‐ and three‐dimensional crown metrics from Terrestrial Laser Scanning data for 1,441Pinus sylvestris,P.nigra,Quercus fagineaandQ. ilextrees from a water‐limited forest community in central Spain and test height‐crown metric scaling relationships. We demonstrate new TLS methods to define symmetric and asymmetric neighbourhood metrics based on tree height, crown size and neighbour projected crown area, and test the importance of neighbourhood genus diversity on crown morphology by separating competition from congeneric and heterogeneric neighbours.Competition negatively impacted all crown metrics except crown depth where onlyP.nigrashowed sensitivity. Asymmetric competition was the strongest driver of pine crown morphology, but oaks were more sensitive to symmetric competition, in line with shade tolerance expectations. Congeneric competition reducedQ.fagineacrown size and changed its shape, but we found no significant effects of heterogeneric neighbours. Most species and crown dimensions had height‐crown scaling exponents below those predicted by MST, which may be due to water limitation effects. Pines and oaks showed large differences in crown depth to height scaling, with the former shallower and the latter deeper, in contrast to theoretical predictions.Synthesis. Our study is the first to demonstrate the ability of TLS to characterise crown morphology from leaf‐wood separated clouds and competitive neighbour effects in a water‐limited forest community, and to use TLS metrics to test ecological crown scaling theory. Most crown metrics scaled below theoretical predictions. Pines were more sensitive to competition by larger neighbours and oaks to crowding from all neighbours, with competition from neighbours of the same genus having a consistent negative effect.

Highlights

The size and shape of tree crowns are first-order determinants of the light and microclimatic environment experienced by individuals, which drive growth, mortality and fecundity rates, and whole-forest dynamics (Kobe et al, 1995; Pacala et al, 1996; Purves & Pacala, 2008)
In this study we present the first test of metabolic scaling theory (MST) predictions of crown morphology scaling using Terrestrial Laser Scanning
This study has demonstrated that a wide range of Terrestrial laser scanning (TLS)-derived crown metrics scaled below theoretical predictions and were negatively affected by above-ground competition

Summary

| INTRODUCTION

The size and shape of tree crowns are first-order determinants of the light and microclimatic environment experienced by individuals, which drive growth, mortality and fecundity rates, and whole-forest dynamics (Kobe et al, 1995; Pacala et al, 1996; Purves & Pacala, 2008). | Journal of Ecolo gy 3 minimise hydraulic risk from embolism in long branches (Smith et al, 2014; Verbeeck et al, 2019), as well as interacting asymmetric and symmetric competitive effects (Coates et al, 2009) Within these environments, shade can exert positive effects (Valladares et al, 2016), for example self-shading through adaptive crown shape and arrangement (Domingo et al, 2019; Pearcy et al, 2005) and shade cast by neighbours (Kothari et al, 2021) can reduce abiotic stresses. We expect crowns to be smaller in volume and narrower in lateral extent for a given height than predicted by MST, to reduce hydraulic path length (Olson et al, 2018; Ryan & Yoder, 1997) and total evaporative demand (Dawson, 1996) to minimise risk of embolism. (H2) We test whether crown depth varies with species’ tolerance to abiotic stressors according to theoretical predictions, namely; whether drought intolerant species have deeper crowns to reduce direct exposure to radiation, and whether shade tolerant species have shallow crowns reflecting their conservative resource-use strategies (as hypothesised by Horn, 1971). (H3) We hypothesise that observed differences in scaling exponents between species will be explained by shade tolerance, and that asymmetric competition will drive crown morphology in less shade tolerant species, and with symmetric competition important for shade tolerant species. (H4) We hypothesise that congeneric-dominated neighbourhoods will have a negative impact on crown size and drive changes in crown shape through reduced complementarity in light use strategies (Fridley, 2012; Williams et al, 2017) and increased crown abrasion (Pretzsch, 2019)

| MATERIALS AND METHODS

| DISCUSSION

| CONCLUSIONS