Abstract
With invasive grasses increasing wildfire occurrence worldwide, a better understanding of the relationships between native plants, fire, and invasive grass is needed to help restoration plans facilitate ecosystem resilience. Invasive grasses are particularly problematic for altering fire regimes in the tropics, yet in Hawaiʻi, restoration sites are often planted with monocultures of the native tree Acacia koa, which can promote grass growth via nitrogen fixation. This, combined with the difficulty of estimating pre-fire grass cover under thick canopies, complicates attempts to restore Hawaiian ecosystems. We studied the 2018 Keauhou Ranch Fire in Hawaiʻi to investigate three questions: (1) at what level of precision can pre-fire grass cover be accurately estimated from oblique aerial photos? (2) how are post-fire A. koa regeneration densities affected by fire severity? and (3) how are post-fire A. koa regeneration densities affected by pre-fire grass cover and its interaction with fire severity? We collected burn severity and post-fire regeneration data from 30 transects stratified across mid-elevation woodland, montane woodland, and montane shrubland communities. We evaluated visual estimates of pre-fire grass cover from oblique aerial imagery with quantitative in situ data from 60 unburned transects of the same cover types. Pre-fire estimates of grass cover categories were 67% accurate in montane woodland (n = 9) and 100% accurate in montane shrubland (n = 11), but only 20% accurate in mid-elevation woodland (n = 10). In montane woodlands with low pre-fire tree densities, A. koa regeneration densities were higher with increased fire severity, but this trend reversed when pre-fire tree densities were high. We detected no effect of pre-fire grass cover, nor its interaction with fire severity, on A. koa regeneration density. This indicates that restoration through the planting of A. koa may be successful in promoting fire-resilient A. koa forest, although there are potential issues to consider regarding the effects that A. koa’s grass promotion may have on other species within the ecosystem.
Highlights
The potential for wildfire ignition “based on flammability and exposure to ignition vectors” [1] has been increasing in fire-prone regions globally throughout the past century [2], and the world is seeing larger, more frequent wildfires as a result [3], necessitating restoration plans that incorporate fire resiliency
In an effort to achieve this knowledge, we investigated a method for estimating grass cover from oblique aerial photos, and analyzed the relationships between fire severity, grass cover, and the dominant native tree, Acacia koa
If there is no relationship between A. koa regeneration and the interaction between fire severity and grass cover in montane woodland sites, as our study indicates, land managers wishing to promote A. koa can continue to plant it in sites with high grass cover without concern of A. koa regeneration being suppressed by grasses after fire
Summary
The potential for wildfire ignition “based on flammability and exposure to ignition vectors” [1] has been increasing in fire-prone regions globally throughout the past century [2], and the world is seeing larger, more frequent wildfires as a result [3], necessitating restoration plans that incorporate fire resiliency. Invasive species that are fire-tolerant can crowd out native species that are slower to establish in a post-fire landscape, creating a positive feedback loop in which an increase in the abundance of invasive species leads to more frequent wildfire, which in turn leads to a further increase in the abundance of invasive species [20]. Such positive feedback increases the resilience of the ecosystem’s invaded state [21], and makes the restoration of diverse, native landscapes without invasive grasses difficult [22,23]
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