Invasive grasses change landscape structure and fire behaviour in Hawaii

Abstract

Nomenclature Wagner et al. (1999) for all native Hawaiian species; USDA ARS-GRIN (www.ars-grin.gov/ npgs/) for Megathyrsus maximus.Abstract Questions: How does potential fire behaviour differ in grass-invaded nonnative forests vs open grasslands? How has land cover changed from 1950-2011 along two grassland/forest ecotones in Hawaii with repeated fires? Location: Non-native forest with invasive grass understory and invasive grassland (Megathyrsus maximus) ecosystems on Oahu, Hawaii, USA.Methods: We quantified fuel load and moisture in non-native forest and grassland (Megathyrsus maximus) plots (n = 6) at Makua Military Reservation and Schofield Barracks, and used these field data to model potential fire behaviour using the BehavePlus fire modelling program. Actual rate and extent of landcover change were quantified for both areas from 1950-2011 with historical aerial imagery.Results: Live and dead fuel moisture content and fine fuel loads did not differ between forests and grasslands. However, mean surface fuel height was 31% lower in forests (72 cm) than grasslands (105 cm; P < 0.02), which drove large differences in predicted fire behaviour. Rates of fire spread were 3-5 times higher in grasslands (5.0-36.3 mÁmin À1 ) than forests (0-10.5 mÁmin À1 ; P < 0.001), and flame lengths were 2-3 times higher in grasslands (2.8-10.0 m) than forests (0-4.3 m; P < 0.01). Between 1950 and 2011, invasive grassland cover increased at both Makua (320 ha) and Schofield (745 ha) at rates of 2.62 and 1.83 haÁyr À1 , respectively, with more rapid rates of conversion before active fire management practices were implemented in the early 1990s.Conclusions: These results support accepted paradigms for the tropics, and demonstrate that type conversion associated with non-native grass invasion and subsequent fire has occurred on landscape scales in Hawaii. Once forests are converted to grassland there is a significant increase in fire intensity, which likely provides the positive feedback to continued grassland dominance in the absence of active fire management.

The Contemporary Scale and Context of Wildfire in Hawai‘i

“…), and buffel grass ( Cenchrus ciliaris L.). These species form con tin u ous, nearly mono typic fuel beds, ig nite eas i ly, at tain ex tremely high fi ne fuel loads, and are ca pa ble of resprouting and / or establishing from seed more vig orously in the post fi re en vi ron ment than is the ma jor ity of na tive veg e ta tion (Freifelder et al 1998, Beavers et al 1999, Castillo et al 2003, Ellsworth et al 2014. Nonnative grasses also read ily in vade existing na tive wood lands (D ' Antonio et al 2000, Litton et al 2006, D ' Antonio et al 2011, with some spe cies capa ble of col o niz ing young, bar ren lava fl ows.…”

“…Grass in va sions else where in both tem per ate and trop i cal eco sys tems have led to sim i lar eco log i cal changes in terms of in creas ing fi ne fuel loads and con ti nu ity and in creas ing re sultant fi re in ten sity (Platt and Gottschalk 2001, Rossiter et al 2003, Hoffmann et al 2004, Veldman and Putz 2011. In Hawai ' i, the exten sive area cov ered by non na tive grasses (Elmore et al 2005, Varga and Asner 2008) (Figure 1) sup ports fre quent wild fi res that kill woody plants and erode the mar gins of forested ar eas, lead ing to in creased grass cov er, for est con trac tion, and of ten for est frag menta tion (Blackmore and Vitousek 2000, Ellsworth et al 2014). This in va sive grass -wild fi re cy cle has had large im pacts on na tive dry forest and in creas ingly threat ens me sic for ests as well Vitousek 1992, Ellsworth et al 2014).…”

“…Yet the ex tent to which wild fi re has be come a per va sive feature of Hawai ' i ' s land scapes has not been well quan ti fi ed. Wildfi re in Hawai ' i dif fers greatly from that of most of the con ti nen tal United States in that fi re oc curs year -round (Chu et al 2002) and is supported by vast, un broken ex panses of highly fi re -prone, non na tive eco sys tems dom i nated by in va sive grasses and other fi re -adapted plants (Elmore et al 2005, Varga and Asner 2008, Ellsworth et al 2014 ( Figure 1 ). These non na tive spe cies reg u larly en croach into Hawai ' i ' s na tive eco sys tems, which con tain one of the world ' s highest rates…”

Management implications for the reintroduction of the endangered Hawaiian state flowerHibiscus brackenridgei

“…In particular, Megathyrsus maximus dominates most lowland areas of O'ahu and poses a major threat to native species in the Wai'anae region by affecting resource availability, such as access to nutrients and sunlight (D'Antonio & Vitousek 1992;Ammondt et al 2013). The maximum height for M. maximus has been found to be around 105 cm in open grasslands and 72 cm in forests (Ellsworth et al 2014). Previous research suggests that M. maximus grows 39-94% faster than native Hawaiian plants (Ammondt & Litton 2012) and thus must be managed so it does not outcompete natives for sunlight.…”

Effects of anthropogenic wildfire in low-elevation Pacific island vegetation communities in French Polynesia

“…Despite the potential importance of wildfires for conservation, public health, and human-environment interactions, very few studies of wildfire effects on Pacific island vegetation exist, and reliable, quantitative information on conservation threats is limited (Keppel et al 2004). Almost nothing is known about how contemporary wildfire affects plant communities in the Pacific outside of Hawai'i (Hughes et al 1991;D'Antonio and Vitousek 1992;Hughes and Vitousek 1993;Freifelder et al 1998;D'Antonio et al 2000D'Antonio et al , 2001Ainsworth and Kauffman 2009, 2013Angelo and Daehler 2013;Ellsworth et al 2014), with a few studies in New Caledonia (McCoy et al 1999), western Polynesia (Hjerpe et al 2001;Elmqvist et al 2002;Franklin 2007) and Guam (Athens and Ward 2004). Furthermore, human-caused wildfires are increasingly occurring on Pacific islands in the modern era (Trauernicht et al 2015), and islands are especially susceptible to anthropogenic disturbances (Keppel et al 2014).…”