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.

中文翻译：

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了解草入侵、火灾严重程度和相思树再生以促进夏威夷火山国家公园的森林恢复

随着入侵草在全球范围内增加野火的发生，需要更好地了解本地植物、火和入侵草之间的关系，以帮助恢复计划促进生态系统恢复力。入侵草在热带地区对改变火势尤其成问题，但在夏威夷，恢复地点通常种植单一栽培的本土树金合欢树，它可以通过固氮促进草的生长。这一点，再加上在厚厚的树冠下估计火灾前的草覆盖的困难，使恢复夏威夷生态系统的尝试变得复杂。我们研究了 2018 年夏威夷 Keauhou Ranch 火灾，以调查三个问题：（1）从斜航拍照片中可以准确估计火灾前草地覆盖的精度水平是多少？(2) 火灾后A. koa 的再生密度如何受火灾严重程度的影响？和 (3) 火后A. koa怎么样受火灾前草覆盖影响的再生密度及其与火灾严重程度的相互作用？我们从 30 个横断面中收集了烧伤严重程度和火灾后再生数据，这些断面分布在中海拔林地、山地林地和山地灌木丛群落中。我们使用来自相同覆盖类型的 60 个未烧毁断面的定量原位数据评估了来自倾斜航空图像的火灾前草覆盖的视觉估计。在山地林地 ( n = 9) 和山地灌木地 ( n = 11) 中，草覆盖类别的火灾前估计准确率为 67% ，但在中海拔林地 ( n = 10) 中准确率仅为 20% 。在火灾前树木密度较低的山地林地中，A. koa随着火灾严重程度的增加，再生密度更高，但当火灾前的树木密度高时，这种趋势会逆转。我们没有检测到火灾前草覆盖对A. koa再生密度的影响，也没有检测到它与火灾严重程度的相互作用。这表明通过种植A. koa进行恢复可能会成功地促进耐火A. koa森林的生长，尽管关于A. koa的草促进可能对生态系统中其他物种的影响存在潜在问题需要考虑.