Forest structure, diversity, and primary production in relation to disturbance severity.

Lisa T Haber,Brady S Hardiman,William S Currie,Nicolás Correa Pascuas,Robert T Fahey,Christopher M Gough,Shea B Wales

doi:10.1002/ece3.6209

Abstract

Differential disturbance severity effects on forest vegetation structure, species diversity, and net primary production (NPP) have been long theorized and observed. Here, we examined these factors concurrently to explore the potential for a mechanistic pathway linking disturbance severity, changes in light environment, leaf functional response, and wood NPP in a temperate hardwood forest.Using a suite of measurements spanning an experimental gradient of tree mortality, we evaluated the direction and magnitude of change in vegetation structural and diversity indexes in relation to wood NPP. Informed by prior observations, we hypothesized that forest structural and species diversity changes and wood NPP would exhibit either a linear, unimodal, or threshold response in relation to disturbance severity. We expected increasing disturbance severity would progressively shift subcanopy light availability and leaf traits, thereby coupling structural and species diversity changes with primary production.Linear or unimodal changes in three of four vegetation structural indexes were observed across the gradient in disturbance severity. However, disturbance‐related changes in vegetation structure were not consistently correlated with shifts in light environment, leaf traits, and wood NPP. Species diversity indexes did not change in response to rising disturbance severity.We conclude that, in our study system, the sensitivity of wood NPP to rising disturbance severity is generally tied to changing vegetation structure but not species diversity. Changes in vegetation structure are inconsistently coupled with light environment and leaf traits, resulting in mixed support for our hypothesized cascade linking disturbance severity to wood NPP.

Highlights

Disturbances modify forest structure (Figure 1) and, in doing so, may alter core ecosystem functions, including net primary production (NPP)
We focus on two vegetation structure metrics: the Gini index of diameter at breast height (DBH) (ΔG), exhibiting a significant change with rising disturbance severity (p = .08, Adj. r2 = .23, AICc = −73.4), and the Clark and Evans aggregation index (ΔR), which did not change with disturbance but was significantly unimodally related to NPPdep
We found forest structural indexes describing vegetation structure but not species diversity changed across a disturbance severity gradient spanning 37%–86% tree basal area loss, but that wood NPP was not related to forest structural shifts mediated by disturbance

Summary

Introduction

Disturbances modify forest structure (Figure 1) and, in doing so, may alter core ecosystem functions, including net primary production (NPP). Joint investigation of forest structure–function relationships is timely as the range of disturbance severities present on temperate forest landscapes expands and, broadly reshapes plant species diversity and vegetation structure (Seidl et al, 2017; Turner, 2010) and NPP (Stuart-Haëntjens, Curtis, Fahey, Vogel, & Gough, 2015), sometimes in surprising ways (Curtis & Gough, 2018). This widespread broadening of disturbance severity is caused by a recent proliferation of low to medium severity disturbances—those that result in partial rather than complete tree mortality—from insect pests, pathogens, and extreme weather, which in many temperate regions are outpacing increases in severe stand-replacing disturbances (Cohen et al, 2016). Concurrent observations of changes in tree species diversity, vegetation structure, and NPP across a range of disturbance severities provide an opportunity to examine an understudied structure–function linkage (Fahey et al, 2016; Hardiman, Bohrer, Gough, Vogel, & Curtis, 2011)

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Discussion

Conclusion