Fire traps in the wet subtropics: new perspectives from Hong Kong

Abstract

AbstractFires undermine efforts to restore degraded forests in the wet tropics and subtropics. Grasslands and shrublands established after fires are more fire-susceptible than forests and tend to be set alight more often, creating a positive feedback loop that curbs succession (i.e. create a fire trap). Understanding the factors that underpin the strength of these fire traps could transform restoration programmes by identifying the steps needed to escape them.Fire traps are notoriously challenging to quantify because multiple factors influence fire occurrence and vegetation recovery. Here we used multi-decadal satellite imagery from Landsat to create a 34-year time series of burn areas and vegetation dynamics in wet subtropical Hong Kong. These dynamic maps were then used to characterise (1) the influence of successional stage on fire occurrence, having accounted for topographical and ignition source imbalances using neighbourhood analyses and entropy balancing (EBAL) weights, and (2) recovery time to the next successional stage by survival analysis with EBAL weights.Our analyses revealed that fire regimes in the wet subtropics are defined by strong fire-vegetation feedbacks. Grasslands and shrublands were 20 and 9 times more susceptible to fires than forests in similar topographic positions. Human activities compounded these differences by disproportionally introducing more ignition sources to grasslands (2.3 times) and shrublands (2.0 times) than to forests.Burnt shrublands recovered to forests faster (19 years) than grasslands (40 years). Proximity to forest patches had strong positive effects on recovery rates, highlighting the importance of seed sources. Post-fire recovery was faster on wetter northwest-facing sites and valleys. Overall, topography strongly influenced recovery processes but hardly affected fires occurrence.Synthesis and applications. Our study provides the first quantification of fire-trap processes in the wet subtropics, which provides new opportunities for evidence-based fire suppression and post-fire restoration. Our results suggest that (1) fire traps could be mitigated by fire-suppression programmes as the traps are currently exacerbated by ignition source imbalance; (2) establishing green fire breaks represent an effective fire suppression measure in the wet subtropics; and (3) active restoration could target areas where models predict sluggish post-fire natural regeneration.