Changes in land cover and fire risk associated with nonnative grass invasion in Hawaii

Background/Question/Methods

It is generally accepted that the synergistic effects of fire and grass invasion have led to conversion from forest to grassland throughout the tropics.  However, there is little published data to support this conversion on a landscape scale in Hawaii. If true, nonnative grasslands are more flammable than forests due to changes in fuel loads and microclimate and, thus, are at increased risk of fire occurrence and spread.  The objective of this study was to: (i) use field data and modeling to examine differences in potential fire behavior in forests and grasslands, and (ii) measure the rate and extent of land cover change from 1950-2011 in and around two heavily managed areas at Schofield Barracks and Makua Military Reservation on Oahu, Hawaii.  We measured fuel loads, fuel height, and live and dead fuel moisture in forested and nonnative invasive grass (Megathyrsus maximus) plots (n=6) in the Waianae Kai Forest Reserve and Dillingham Airfield.   These field data were then used to parameterize the BehavePlus fire modeling program to predict potential fire behavior (flame length, rate of spread, fireline intensity) in forest vs. nonnative invasive grass ecosystems. We used historical imagery (1950-2011) to quantify the rates of change in land cover.

Results/Conclusions

Live and dead fuel moisture and fine fuel loads did not differ between forests and grasslands, but mean fuel height was lower in forests (72 cm) than in grasslands (105 cm; P=02).  Predicted fire behavior differed greatly in forests vs. grasslands.  Rate of spread was higher in grasslands (5.0 to 36.3 m min^-1) than forests (0 to 10.5 m min^-1) (P<0.001), and flame lengths were higher in grasslands (2.8-10.0 m) than in forests (0-4.3 m; P<0.01).  Invasive grassland cover increased in heavily managed areas at both Makua (320 ha) and Schofield (745 ha) at rates of 2.62 and 1.83 ha yr^-1, respectively, with more rapid rates of conversion before aggressive fire management practices were implemented in the early 1990’s.  At Makua, conversion from forest to grassland in the surrounding landscape (1244 ha) was slower (1.78 ha yr^-1) than in the managed area. Outside the managed area at Schofield (1576 ha), nonnative woody plants reestablished into grasslands at a rate of 4.75 ha yr^-1.    These results demonstrate that conversion from forest to nonnative grassland significantly impacts current and future fire risk.  Slower conversion rates in recent years suggest that active fire management can facilitate woody recovery, potentially reducing future fire potential.