Invasive Grasses in Hawaii and their Impacts
The frequency of wildfires in the Hawaiian Islands has increased by 400% over the last century. The combination of less rainfall and severe drought from climate change, along with the introduction and unmanaged growth of non-native grasses, have contributed to the increased frequency and intensity of wildfires across the State.
Negative Impacts: A subset of introduced grasses in the Hawaiian Islands has become widespread, highly invasive, and a continuous threat to the natural environment, the economy and human health. Today, many formerly native shrublands and forests are now dominated by one or a few non-native grass species that smother and exclude other plants, grow and spread quickly during wet periods, but may rapidly dry out and become extreme fire hazards during dry seasons or extended drought.
Hawaiʻi’s Most Invasive Non-Native Grasses
The following 10 species are among the most invasive1, habitat-modifying, and potentially fire-promoting2 grasses in the Hawaiian Islands. Please note that under severe conditions, any vegetation can burn, and areas under active management are less likely to burn. The inclusion of these 10 plant species in this non-regulatory list does not preclude their use, nor does it discount their utility in ranches or other industries where they play an important role.
🔥🛠️ Fountain Grass (Cenchrus setaceus) – High Fire Hazard; Ecosystem Modifier
🔥🛠️ Guinea Grass (Megathyrsus maximus) -High Fire Hazard; Ecosystem Modifier
🔥🛠️ Buffel Grass (Cenchrus ciliaris) – High Fire Hazard; Ecosystem Modifier
🔥🛠️ Molasses Grass (Melinis minutiflora) – High Fire Hazard; Ecosystem Modifier
🔥🛠️ Beardgrass (Schizachyrium microstachyum) – High Fire Hazard; Ecosystem Modifier
🔥🛠️ Broomsedge (Andropogon virginicus) – High Fire Hazard; Ecosystem Modifier
🔥🛠️ Pampas Grass (Cortaderia jubata & C. selloana) – Moderate/High Fire Hazard (drought); Ecosystem Modifier
🔥🛠️ California Grass (Urochloa mutica) – Moderate Fire Hazard (drought); Wet Ecosystem Modifier
🔥🛠️ Cane Grass (Cenchrus purpureus) – Moderate Fire Hazard (drought); Ecosystem Modifier
🔥🛠️ Kikuyu Grass (Cenchrus clandestinus) – Moderate Fire Hazard (drought); Ecosystem Modifier
1🛠️= Habitat-modifying invasive weed, as determined by HPWRA scores and published literature.
2🔥= Fire risk, as determined by the Weed Fire Risk Assessment Scores
History of Introduced Grasses
Pre-Contact Hawaiʻi: Prior to human contact, the native flora of the Hawaiian Islands had a relatively low number of grass species (ca. 51-52), and grassland ecosystems were uncommon, and largely restricted to dry coastal dunes, or the subalpine upper slopes of Mauna Kea and Haleakalā volcanoes. Fire was later used by Hawaiians to promote and expand fields of the indigenous pili grass (Heteropogon contortus), the preferred material for thatching houses in drier, leeward areas of the islands.
Post-Contact Hawaiʻi: Since the arrival of Europeans, however, land-clearing, fire, the introduction of grazing ungulates, and the intentional or accidental introduction of non-native grasses for agriculture, landscaping and horticulture, have resulted in the establishment and naturalization of over 200 grass species from around the globe. Additionally, the loss of agricultural lands is correlated with an increase in invasive grasses. Currently, an estimate of over 25% of the islands are now covered by non-native grasses. The decline of the sugar cane industry in 1990’s has had landscape level consequences for the Islands, as large tracts of land were left abandoned, allowing invasive grasses and other weedy plants to establish and dominate the landscape.
Although the negative impacts on unmanaged lands can be devastating, on managed lands, many of these grasses continue to be valued for their use as food for livestock and people, or as groundcover for lawns, landscaping, and as ornamentals. In particular, well-managed ranches and grazing lands can inhibit the establishment of other invasive weeds that threaten neighboring conservation lands and can reduce fine fuels and serve as fire breaks for bordering natural areas and population centers.
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