Surface sampling of wetlands in a tropical dry forest after a large fire: Wind direction dominates the transport and deposition of fire proxies
A major fire raged through the dry tropical forests of Bandipur Tiger Reserve, a ‘biodiversity hotspot’ in southern India, in February 2019. The fire occurred in patches, burning >10,000 acres of forest, becoming one of the largest forest fires in India in recent times. Very few studies have been able to capture the dynamics of fire proxies from an active surface fire, especially in the dry tropics. We opportunistically sampled two wetlands roughly sandwiched between the largest and the second-largest burnt patches a week after the fire was extinguished. We collected surface samples from each wetland and looked at popular fire proxies – macrocharcoal, microcharcoal, microcharcoal/pollen (C/P) ratio and the abundance and distribution of Polycyclic Aromatic Hydrocarbons (PAHs). Macrocharcoal counts were low (mean ~5), while the C/P ratio was ~1. Low Molecular Weight (LMW) PAH molecules Phenanthrene, Anthracene, Fluoranthene and Pyrene were found in both sites, while High Molecular Weight (HMW) PAHs were only found in one. None of the proxies is particularly indicative of the large surface fire that occurred ~15 km away. Analysis of wind speed and direction from weather station data and forward and backward HYSPLIT model trajectories tell us that both wetlands were not downstream of the smoke plume. There was also no recorded precipitation between the fire event and the sampling date. Through this opportunistic study of fire proxies, we show that wind direction and wet scavenging are essential factors determining the transport and deposition of fire proxies in this environment. Hence reconstruction of fire histories should be done using multi-site data since the absence of fire proxies does not equate to the absence of fire.