Structural diversity and tree density drives variation in the biodiversity-ecosystem function relationship of woodlands andsavannas.

John L Godlee,Mariska Te Beest,Steve Makungwa,Mathew Williams,Edward T A Mitchard,José João Tchamba,Anderson Muchawona,Stephen Syampungani,Abel Siampale,Kyle G Dexter,Casey M Ryan,Manfred Finckh,Rasmus Revermann,Joris P G M Cromsigt,David Bauman,Dave J Druce,António Valter Chisingui ,Hemant Tripathi ,Natasha Ribeiro ,João M B Carreiras ,Francisco M P Gonçalves ,Samuel Bowers ,Ricardo M Holdø ,Iain Mcnicol

doi:10.1111/nph.17639

Abstract

Positive biodiversity-ecosystem function relationships (BEFRs) have been widely documented, but it is unclear if BEFRs should be expected in disturbance-driven systems. Disturbance may limit competition and niche differentiation, which are frequently posited to underlie BEFRs. We provide the first exploration of the relationship between tree species diversity and biomass, one measure of ecosystem function, across southern African woodlands and savannas, an ecological system rife with disturbance from fire, herbivores and humans. We used > 1000 vegetation plots distributed across 10 southern African countries and structural equationmodelling to determine the relationship between tree species diversity and above-ground woody biomass, accounting for interacting effects of resource availability, disturbance by fire, tree stem density and vegetation type. We found positive effects of tree species diversity on above-ground biomass, operating via increased structural diversity. The observed BEFR was highly dependent on organismal density, with a minimum threshold of c. 180 mature stems ha-1 . We found that water availability mainly affects biomass indirectly, via increasing species diversity. The study underlines the close association between tree diversity, ecosystem structure, environment and function in highly disturbed savannas and woodlands. We suggest that tree diversity is an under-appreciated determinant of wooded ecosystem structure and function.

Highlights

Understanding the relationship between biodiversity and ecosystem function has become a central endeavour in ecological science, as we seek to predict the consequences of global biodiversity change (Naeem et al, 2012)
Our findings contribute to our understanding of a universal biodiversity–ecosystem function relationship, one that is moderated in a predictable manner by environmental covariates and their interaction with biodiversity and ecosystem structure
We found that the multiple vegetation types that comprise southern African woodlands exhibit similarities in the relationship between species diversity and woody biomass, suggesting that similar processes operate across the region to determine ecosystem function

Summary

Introduction

Understanding the relationship between biodiversity and ecosystem function has become a central endeavour in ecological science, as we seek to predict the consequences of global biodiversity change (Naeem et al, 2012). While positive BEFRs which align with theory have been frequently reported, a complex picture has emerged in which the strength and direction of the BEFR varies depending on the ecosystem studied (Liang et al, 2016), the ecosystem function(s) of interest (Hector & Bagchi, 2007), and the inclusion of environmental covariates in statistical models (Vilaet al., 2005). The goal should be to study the BEFR in different environmental and ecological contexts, to develop an ecosystem-agnostic understanding of the complex interactions between biodiversity, abiotic environment and ecosystem function. Theory predicts that biodiversity will have a positive effect on ecosystem function via three principal mechanisms: (1) niche complementarity, whereby niche partitioning and ecosystem resource use efficiency is increased in diverse communities, minimising negative density-dependent effects such as conspecific competition and abundance of speciesspecific natural enemies (Barry et al, 2019); (2) selection effects, whereby diverse communities are more likely to include a highyield species; and (3) facilitation effects, whereby diverse communities are more likely to contain species combinations that enhance each others’ functional contribution (Wright et al, 2017)

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