The effects of grass invasion and fire severity on Acacia koa regeneration

With spreading invasive grass increasing wildfire occurrence worldwide, there is a need for a better understanding of the relationships between native trees, fire, and grass so that land managers can increase their forested landscapes’ fire resilience. One challenge to this line of inquiry is that remotely-sensed data often fails to accurately measure grass cover underneath forest canopies. The 2018 Keauhou Ranch Fire in Hawaiʻi presents an opportunity to investigate grass, fire, and native tree interactions while simultaneously testing the accuracy of a novel method of remote grass measurement. In this study, I assessed grass cover using oblique aerial photos and investigated the relationships of pre-fire grass cover with burn severity and post-fire recovery of the native tree Acacia koa . I had three questions: 1) At what level of precision can pre-fire grass cover be accurately (≥60% accuracy) estimated from oblique aerial photos? 2) How are post-fire koa regeneration densities affected by fire severity? 3) How are koa regeneration densities affected by pre-fire grass cover and its interaction with fire severity? My team collected burn severity and vegetation measurements from 30 transects stratified across three land cover types (mid-elevation woodland, montane woodland, and montane shrubland). We visually estimated pre-fire exotic grass cover from oblique aerial imagery taken in 2014 and evaluated those estimates with in situ data from 60 transects of the same cover types that did not burn in the Keauhou Ranch Fire. We measured burn severity (percent mortality, scorch height, char height) and exotic post-fire grass cover at the burned transects. I discovered that, using a four-category level of precision, pre-fire grass cover estimates from aerial photos were 67% accurate in the montane woodland cover type, 100% accurate in montane shrubland, but only 20% accurate in mid-elevation woodland. Although accuracy of pre-fire grass cover estimates in montane shrubland were high, variance in that cover type was low, with grass cover never exceeding 24%. Within montane woodland, post-fire koa regeneration densities were higher with increased fire severity, but this trend reversed when pre-fire tree densities were high. I detected no effect of pre-fire grass cover, nor its interaction with fire severity, on koa regeneration density.

Name thesis:degree_name

Master of Science

Level thesis:degree_level

Masters

Discipline thesis:degree_discipline

Wildlife, Aquatic and Wildlands Science and Management

Grantor

Texas Tech University

Year dc:date.issued

2021

Author dc:creator

• Hamilton, Natalia Piper

Chair dc:contributor.committeechair

• Gill, Nathan

Committee members dc:contributor.committeemember

• Cox, Robert
• Yelenik, Stephanie

• Hawaii
• Acacia Koa
• Acacia
• Koa
• Invasive Grass
• Wildfire
• Fire
• Grass
• Island of Hawaii
• Big Island

Language dc:language.iso

eng

Handle dc:identifier.uri

https://hdl.handle.net/2346/87507

OAI identifier oai:identifier

oai:ttu-ir.tdl.org:2346/87507