Fire is an important feature of several tropical savanna and grassland ecosystems (Keeley, 2006; Bowman et al., 2009). Fires open up vegetation and initiate secondary succession pathways (D’Antonio & Vitousek, 1992; Blair, 1997; Brooks, D’Antonio, et al., 2004; Sankaran, et al., 2005; Mandle et al., 2011; Hiremath & Sundaram, 2013) and can thus alter plant community composition and structure. Conversely, plant community composition and structure also determine fuel availability, flammability and contiguity, and can thus strongly influence fire patterns and characteristics (Burton et al., 2019; Ferreira et al., 2020).
Invasions of grasslands by woody exotics, both trees and shrubs, is a widespread phenomenon and a serious threat to grassland ecosystems worldwide (Keeley, 2001; Driscoll et al., 2010; Sundaram & Hiremath, 2012; Srinivasan et al., 2012a; Boudiaf et al., 2013). Such invasions can alter fire dynamics by influencing both species composition and vegetation structure, and in turn fuel characteristics (Bond & Parr, 2010). Because woody invaders in grasslands have different physiological characteristics from the native grasses (e.g C3 woody plant invasion vs C4 grasses), they can have particularly drastic effects on fire regimes (D’Antonio, et al., 2004; Bond & Parr, 2010; Mandle et al., 2011). Woody plant invasions in grasslands can alter fuel properties such as load, packing ratios, continuity, and spatial, vertical and horizontal structure. These alterations can strongly determine fire frequency, seasonality and intensity, depending on the physiology and ecology of the invasive species (D’Antonio, et al., 2004; Driscoll et al., 2010; Mandle et al., 2011; Estes, et al., 2017). Further, differences in vertical structuring of vegetation caused by woody invasives can, in some cases, change fire typology from surface fires to crown fires (Van Wilgen et al., 1998). At present, even though the effects of grass invaders on fire regimes have been extensively studied (D’Antonio & Vitousek, 1992; D’Antonio, et al., 2004; D'Antonio & Vitousek, 2016), the effects of woody plant invasion on grassland fire regimes are not fully understood.
In this study, we characterized the effects of invasion by three woody species on fire intensities in montane grasslands of the Nilgiri Biosphere Reserve (NBR), in the Southern Western Ghats, India. The upper elevations of the NBR are characterized by a unique forest-grassland mosaic, with stretches of tropical montane grasslands interspersed with pockets of stunted tropical evergreen forests, called ‘sholas’, in the valleys. Of the many exotics introduced in these grasslands over the years, three woody species have become particularly invasive, namely Acacia mearnsii (Black wattle, or wattle), Cytisus scoparius (Scotch broom) and Ulex europaeus (common gorse, or gorse; Joshi et al. 2018). Specifically, we used the differenced Normalized Burn Ratio (dNBR), which quantifies fire intensity based on the analysis of satellite images taken before and after a fire (Key & Benson 2006), to characterize fire intensities in uninvaded grassland and patches invaded by the three woody species following a wildfire that occurred in the last week of February, 2017. The dNBR is based on Near Infrared (NIR) and the Short-Wave Infrared (SWIR) bands which have high sensitivity to changes in vegetation caused by fires. NIR is absorbed the most following fires, while SWIR is reflected the most. The dNBR can discern minor changes in vegetation characteristics after a fire, such as in the amount of green cover and charring; by measuring changes in biomass loss, carbon release and smoke production (Key & Benson, 2006; Miller & Thode, 2007; Richardson et al., 2016).
Specifically, we asked: 1) How does invasion by woody species affect fire intensities relative to uninvaded grasslands, and does the effect differ between the three woody invasive species? Because topographical parameters like slope and aspect can also affect fire intensity both directly and indirectly by influencing the rate of fire spread and relative availability of moisture in the fuel (Emery & Gross, 2005; Hamman et al., 2011), we additionally asked: 2) How does topography influence the relationship between woody plant invasion and fire intensity?