Assessing Legacy Effects of Wildfires on the Crown Structure of Fire-Tolerant Eucalypt Trees Using Airborne LiDAR Data

Lauren T Bennett,Cristina Aponte,Trent D Penman,Yogendra K Karna

doi:10.3390/rs11202433

Abstract

The fire-tolerant eucalypt forests of south eastern Australia are assumed to fully recover from even the most intense fires; however, surprisingly, very few studies have quantitatively assessed that recovery. The accurate assessment of horizontal and vertical attributes of tree crowns after fire is essential to understand the fire’s legacy effects on tree growth and on forest structure. In this study, we quantitatively assessed individual tree crowns 8.5 years after a 2009 wildfire that burnt extensive areas of eucalypt forest in temperate Australia. We used airborne LiDAR data validated with field measurements to estimate multiple metrics that quantified the cover, density, and vertical distribution of individual-tree crowns in 51 plots of 0.05 ha in fire-tolerant eucalypt forest across four wildfire severity types (unburnt, low, moderate, high). Significant differences in the field-assessed mean height of fire scarring as a proportion of tree height and in the proportions of trees with epicormic (stem) resprouts were consistent with the gradation in fire severity. Linear mixed-effects models indicated persistent effects of both moderate and high-severity wildfire on tree crown architecture. Trees at high-severity sites had significantly less crown projection area and live crown width as a proportion of total crown width than those at unburnt and low-severity sites. Significant differences in LiDAR -based metrics (crown cover, evenness, leaf area density profiles) indicated that tree crowns at moderate and high-severity sites were comparatively narrow and more evenly distributed down the tree stem. These conical-shaped crowns contrasted sharply with the rounded crowns of trees at unburnt and low-severity sites and likely influenced both tree productivity and the accuracy of biomass allometric equations for nearly a decade after the fire. Our data provide a clear example of the utility of airborne LiDAR data for quantifying the impacts of disturbances at the scale of individual trees. Quantified effects of contrasting fire severities on the structure of resprouter tree crowns provide a strong basis for interpreting post-fire patterns in forest canopies and vegetation profiles in Light Detection and Ranging (LiDAR) and other remotely-sensed data at larger scales.

Highlights

Wildfire impacts on forest ecosystems worldwide are predicted to change under fire and climate regimes [1,2,3,4]
Tree-based allometric equations assume a constant relationship of tree biomass or volume with measurable attributes such as diameter and height and do not account for either short-term decreases in tree foliage and branches after fire or more persistent effects on stem or branch architecture after severe fire [14]
Consistent with the definition of the fire-severity types, there was no evidence of fire scarring on trees at unburnt sites, whereas all trees at high-severity sites were burnt to their total tree height compared with the means of 64% of tree height at moderate-severity sites and 25% at low-severity sites (Table 2)

Summary

Introduction

Wildfire impacts on forest ecosystems worldwide are predicted to change under fire and climate regimes [1,2,3,4]. Emerging evidence suggests that the frequency of wildfires in temperate south-eastern Australia has increased in recent years [6,7,8]. The crown damage and loss associated with high-severity fires will impact tree carbon uptake, influencing stand productivity at least until the functional leaf area is recovered [11,12]. The impacts of high-severity fires on the height and architecture of tree can have practical implications [13], including the accurate estimation of canopy biomass and carbon stores based on tree allometry. Despite the many potential effects of fire on forest canopy structure, recent small-scale studies in Western Australia and Tasmania [14,16] have highlighted how little is known about the lasting effects of fires on eucalypt crowns. While broad-scale studies of landscapes dominated by eucalypt forests indicate the recovery of remotely-sensed spectral responses within about six years of wildfire [17], the contribution of eucalypt crowns versus understoreys to these spectral data, and in particular the time taken for fire-tolerant eucalypts to re-establish a full crown after severe fire, remain under-examined [18]

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