Persistent changes in the horizontal and vertical canopy structure of fire-tolerant forests after severe fire as quantified using multi-temporal airborne lidar data

Abstract

Wildfires can influence the canopy structure of resprouter forests in multiple ways including crown scorching and consumption, tree damage and mortality, and changes in resources available for recovery. Few studies have quantified post-fire changes in the canopy structure of fire-tolerant eucalypt forests, which limits understanding of the extent and persistence of fire effects on ecosystem processes including interactions with environment. In this study, we assess the impacts of a landscape-scale wildfire on the canopy structure of fire-tolerant eucalypt forests in temperate Australia using metrics derived from airborne lidar data. Our assessment, made seven years after the wildfire, encompassed four fire severities (unburnt, low, moderate, high) and involved 1084 lidar plots (0.06 ha, including validation using 51 field plots) across an area of ~30,000 ha in a single forest type. We used Random Forest models to examine the relative importance of fire severity, pre-fire canopy metrics, climate and topography to the prediction of seven post-fire metrics representing the horizontal and vertical structure of the full canopy, the dominant and sub-dominant strata. Canopy cover at seven years post-fire was significantly reduced in moderate- and particularly high-severity plots (mean decrease of 30%). High-severity fire also decreased mean canopy height and led to changes in the canopy relief ratio and rumple index consistent with a dominant stratum that was more heterogeneous and fragmented. In addition to fire severity, the pre-fire values of each metric were important to their post-fire prediction, suggesting that canopy structural recovery after this fire could have been influenced by any lingering effects of previous fires. Topographic and climatic variables were of comparatively minor importance to the prediction of post-fire canopy metrics, although relationships with the forest drought stress index indicated that canopy recovery was slowest at the driest sites. Our study provides quantitative evidence of significant changes in the canopy structure of fire-tolerant eucalypt forests that persisted for at least seven years after high-severity wildfire. Recent trends in southern Australia of shortened wildfire intervals and a drier climate will likely narrow the windows for canopy recovery of fire-tolerant forests, realizing predictions of a gradual opening and fragmentation of forest canopies in temperate Australia and associated changes in multiple ecosystem processes.