Canopy Structural Changes Following Widespread Mortality of Canopy Dominant Trees

Brady Hardiman,Christopher Gough,Gil Bohrer,Peter Curtis

doi:10.3390/f4030537

Abstract

Canopy structure affects forest function by determining light availability and distribution. Many forests throughout the upper Great Lakes region are dominated by mature, even-aged, early successional aspen and birch, which comprise 35%–40% of canopy leaf area, and which are senescing at accelerating rates. In 2008 at the University of Michigan Biological Station, we initiated the Forest Accelerated Succession ExperimenT (FASET) by stem girdling all aspen and birch in replicated stands to induce mortality. Our objective was to understand type and rate of canopy structural changes imposed by rapid but diffuse disturbance consisting of mortality of a single age-species cohort. We characterized changes in canopy structural features in 2008–2011 using ground-based Portable Canopy Lidar (PCL) in paired treated and control stands. As aspen and birch in treated plots died, gap fraction of the upper canopy increased, average leaf height decreased, total canopy height declined, and openness of the whole-canopy increased. All of these trends became more pronounced with time. Our findings suggest that as forests throughout the region pass through the impending successional transition prompted by widespread mortality of canopy-dominant early successional aspen and birch species, the canopy will undergo significant structural reorganization with consequences for forest carbon assimilation.

Highlights

Canopy structure influences forest carbon (C) gain by influencing the amount of light available to all but the uppermost canopy foliage, and the distribution of that light within the canopy
Partial canopy disturbances are inevitable throughout succession, and in stands with clumped age distributions of canopy trees are likely to manifest as pulsed-mortality events
Canopy structural metrics described here proved capable of detecting canopy structural shifts occurring from one year to the as a result of diffuse mortality in a mixed deciduous forest canopy

Summary

Introduction

Canopy structure influences forest carbon (C) gain by influencing the amount of light available to all but the uppermost canopy foliage, and the distribution of that light within the canopy. Partial canopy disturbances are inevitable throughout succession, and in stands with clumped age distributions of canopy trees are likely to manifest as pulsed-mortality events. These less-intense canopy disturbances can reorganize leaf area in the canopy, increasing heterogeneity of foliage distribution [8,9]. Disturbances which kill some fraction of canopy trees (via defoliation, storm damage, pathogens, or age-related senescence) alter canopy structure over time [10] in ways that influence availability and distribution of productivity-controlling resources such as water, light, and nitrogen (N) [11,12,13,14]

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