Invasive-dominated grasslands in Hawaiʻi are resilient to disturbance
Tanglehead grasslands on Lahaina Pali, Maui.
Humans have facilitated the establishment of invasive grasslands in Hawaiʻi through intentional introductions of ornamental plants and forage grasses for livestock, as well as through accidental species introductions. While many non-native grass species have naturalized in Hawaiʻi, a small proportion have become invasive.
Today, invasive-dominated grasslands cover about 25 percent of the state, and many former pasture areas are no longer being actively grazed which can lead to high fuel loads. Disturbances in these systems vary in scale and intensity and include livestock and feral ungulate grazing, military training operations, housing and road development, and wildland fire. Fires, which often start in these invasive grasslands, have been increasing in size and frequency over time.
Understanding post-disturbance vegetation dynamics can help land managers prioritize resources and activities when considering how to promote desired vegetation compositions across varied landscapes after such disturbances. To better understand what happens after grassland disturbance on Hawaiʻi Island, researchers used large equipment to move vegetation and a thin layer of topsoil in six types of grasslands dominated by different non-native and native grass and native shrub species.
Vegetation composition was measured every 3-4 months over the course of 21 months. Some species displayed 'colonizer' patterns, in which a species dominated soon after disturbance (attributable to traits like fast seed germination and growth) then faded out over time as competitor species established a firm, longer-term presence. Other species were characterized as 'colonizer + strong competitor' that dominated throughout the study. Native shrubs showed some 'colonizer' tendencies, and recruited into invasive grass-dominated sites, but were quickly overgrown and did not persist over time, highlighting the competitive ability of these invasive grass species.
Overall, the species compositions of sites returned to the same plant assemblages as pre-disturbance suggesting that the ecosystems are stable states which are resilient to disturbance. The rates of return to pre-disturbance conditions, however, varied in ways that likely related to climate and substrate variables. The results, which span some of the most invasive grass species in Hawaiʻi, suggests that post-fire succession in these grassland ecosystems will result in similar species compositions to pre-fire conditions. Therefore, if the desired management goal is native-dominated ecosystems, it will likely take large inputs of time and resources to alter the grasslands' stable states.