Answers to Analyzing Data 9.1 Questions

1. During the 41-year period before introduced grasses had invaded the park, the fire frequency was 0.22 fires per year with an average burn size of 0.26 ha per fire. In the 20-year period after introduced grasses had invaded the park, the fire frequency was 1.6 fires per year with an average burn size of 243.8 ha per fire.

2. The data in Table B indicate that fire reduces the abundance of native trees and shrubs, while it increases the abundance of introduced grasses.

3. If a fire occurs in a Hawaiian dry forest after introduced grasses are present, the introduced grasses should recover quickly and provide fuel for later fires. We would predict that this fuel would make it more likely that a second fire would occur; in addition, should a second fire occur, the increased fuel levels would probably cause it to burn with greater intensity than the first fire. Thus there is the potential for a “fire cycle” in which a fire causes the abundance of introduced grasses to increase, and also makes future fires both more likely and more intense, leading to further increases in introduced grasses and further declines in native trees and shrubs. Such a fire cycle is consistent with data in Table A: after introduced grasses arrived, fires occurred more often and covered larger areas. A fire cycle is also consistent with data in Table B: introduced grasses were least abundant in unburned areas and most abundant in areas burned twice.