Invasive grasses change landscape structure and fire behaviour in Hawaii

Questions How does potential fire behaviour differ in grass-invaded non-native 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 land-cover 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 m·min−1) than forests (0–10.5 m·min−1; 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 ha·yr−1, 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.

[1]  P. Vitousek,et al.  Biological invasions by exotic grasses, the grass/fire cycle, and global change , 1992 .

[2]  Michelle C. Mack,et al.  ALTERATION OF ECOSYSTEM NITROGEN DYNAMICS BY EXOTIC PLANTS: A CASE STUDY OF C4 GRASSES IN HAWAII , 2001 .

[3]  Eric F. Lambin,et al.  Fires and land‐cover change in the tropics:a remote sensing analysis at the landscape scale , 2000 .

[4]  K. Mckinley,et al.  Fuels or microclimate? Understanding the drivers of fire feedbacks at savanna–forest boundaries , 2012 .

[5]  P. Vitousek,et al.  Microclimate Change and Effect on Fire Following Forest‐Grass Conversion in Seasonally Dry Tropical Woodland 1 , 1998 .

[6]  R. F. Hughes,et al.  Chapter 11: Fire and nonnative invasive plants in the Hawaiian Islands bioregion , 2008 .

[7]  C. D’Antonio,et al.  Long-term impacts of invasive grasses and subsequent fire in seasonally dry Hawaiian woodlands. , 2011, Ecological applications : a publication of the Ecological Society of America.

[8]  R. Burgan,et al.  BEHAVE : Fire Behavior Prediction and Fuel Modeling System -- FUEL Subsystem , 1984 .

[9]  G. Asner,et al.  Effects of an African grass invasion on Hawaiian shrubland nitrogen biogeochemistry , 1996, Plant and Soil.

[10]  Shaun R. Levick,et al.  Context-dependent vegetation dynamics in an African savanna , 2011, Landscape Ecology.

[11]  Robert B. Jackson,et al.  Positive feedbacks of fire, climate, and vegetation and the conversion of tropical savanna , 2002 .

[12]  R. F. Hughes,et al.  INVASION BY A N2-FIXING TREE ALTERS FUNCTION AND STRUCTURE IN WET LOWLAND FORESTS OF HAWAII , 2005 .

[13]  Barry W. Brook,et al.  Postcards from the past: charting the landscape-scale conversion of tropical Australian savanna to closed forest during the 20th century , 2006, Landscape Ecology.

[14]  J. Kauffman,et al.  Biomass, Carbon, and Nitrogen Pools in Mexican Tropical Dry Forest Landscapes , 2003, Ecosystems.

[15]  Joe H. Scott,et al.  Assessing Crown Fire Potential by Linking Models of Surface and Crown Fire Behavior , 2003 .

[16]  Lindsay B. Hutley,et al.  Testing the grass‐fire cycle: alien grass invasion in the tropical savannas of northern Australia , 2003 .

[17]  P. Vitousek,et al.  Cattle Grazing, Forest Loss, and Fuel Loading in a Dry Forest Ecosystem at Pu'u Wa'aWa'a Ranch, Hawai'i1 , 2000 .

[18]  J. Fownes,et al.  Preliminary biomass equations for eight species of fast-growing tropical trees. , 1992 .

[19]  G. Moran,et al.  Genetic Differentiation in Natural Populations of Grevillea robusta , 1997 .

[20]  Michael J. Mac,et al.  Status and trends of the nation's biological resources , 2001 .

[21]  Lisa Mandle,et al.  Woody exotic plant invasions and fire: reciprocal impacts and consequences for native ecosystems , 2011, Biological Invasions.

[22]  Spatial and temporal variability of guinea grass (Megathyrsus maximus) fuel loads and moisture on Oahu, Hawaii , 2013 .

[23]  X. Lee,et al.  Introduction to wildland fire , 1997 .

[24]  Bruce D. Clarkson,et al.  A manual of the flowering plants of Hawaii , 1992 .

[25]  F. Putz,et al.  Grass-dominated vegetation, not species-diverse natural savanna, replaces degraded tropical forests on the southern edge of the Amazon Basin , 2011 .

[26]  Courtesy B Gagné.  Hawaii and the Pacific Islands , 1999 .

[27]  Ian D. Davies,et al.  Landscape‐scale positive feedbacks between fire and expansion of the large tussock grass, Ampelodesmos mauritanica in Catalan shrublands , 2005 .

[28]  Joe H. Scott,et al.  Standard Fire Behavior Fuel Models: A Comprehensive Set for Use with Rothermel?s Surface Fire Spread Model , 2015 .

[29]  D. Richardson,et al.  Effects of Invasive Alien Plants on Fire Regimes , 2004 .

[30]  R. Marrs,et al.  Variation in aboveground biomass and necromass of two invasive species in the Atlantic rainforest, southeast Brazil , 2009 .

[31]  P. Vitousek,et al.  Alien Grass Invasion and Fire In the Seasonal Submontane Zone of Hawai'i , 1991 .

[32]  S. Campbell,et al.  Mortality of Prosopis pallida (mesquite) following burning , 2002 .

[33]  Robert C. Seli,et al.  BehavePlus fire modeling system, version 4.0: User's Guide , 2005 .

[34]  J. Rowe,et al.  Concepts of fire effects on plant individuals and species. , 1983 .

[35]  Patricia L. Andrews,et al.  Evaluation of a Dynamic Load Transfer Function Using Grassland Curing Data , 2006 .

[36]  Competition between Native Hawaiian Plants and the Invasive Grass Megathyrsus maximus: Implications of Functional Diversity for Ecological Restoration , 2012 .

[37]  P. Vitousek.  EFFECTS OF ALIEN PLANTS ON NATIVE ECOSYSTEMS* , 2004 .

[38]  W. Hoffmann,et al.  Impact of the invasive alien grass Melinis minutiflora at the savanna‐forest ecotone in the Brazilian Cerrado , 2004 .

[39]  J. Eischeid,et al.  Online Rainfall Atlas of Hawai‘i , 2013 .

[40]  S. Cordell,et al.  Effects of non-native grass invasion on aboveground carbon pools and tree population structure in a tropical dry forest of Hawaii , 2006 .

[41]  D. Ward,et al.  Fire in the Brazilian Amazon 2. Biomass, nutrient pools and losses in cattle pastures , 1998, Oecologia.

[42]  Brian Beckage,et al.  Fire feedbacks facilitate invasion of pine savannas by Brazilian pepper (Schinus terebinthifolius). , 2009, The New phytologist.

[43]  Clifford W. Smith,et al.  FIRE AND ALIEN PLANTS IN Hawai`i: RESEARCH AND MANAGEMENT IMPLICATIONS FOR NATIVE ECOSYSTEMS , 2005 .

[44]  J. Funk,et al.  Altering Light Availability to Restore Invaded Forest: The Predictive Role of Plant Traits , 2010 .

[45]  J. Kauffman,et al.  Interactions of Fire and Nonnative Species Across an Elevation/Plant Community Gradient in Hawaii Volcanoes National Park , 2010 .

[46]  James K. Brown.  Handbook for inventorying downed woody material , 1974 .

[47]  Zdravko Baruch,et al.  African Grass Invasion in the Americas: Ecosystem Consequences and the Role of Ecophysiology , 2000, Biological Invasions.