Assessing the drivers of vegetation succession on abandoned sugarcane land

D. Nākoa Farrant and Ashley E. Larsen, Bren School of Environmental Science and Management, University of California Santa Barbara, Santa Barbara, CA, Dar A. Roberts, Department of Geography, University of California Santa Barbara, Santa Barbara, CA, Carla D'Antonio, Environmental Studies, University of California Santa Barbara, Santa Barbara, CA

Background/Question/Methods
Millions of hectares of agricultural land have been abandoned globally since the mid-20th century. We still lack an understanding of when and where vegetation succession on abandoned fields tends toward ecosystems resembling their pre-agricultural conditions or degraded vegetated states that typically to provide fewer ecosystem services. This study examines vegetation succession patterns on former sugarcane fields in Hawai‘i that were abandoned during the 1990s following the closure of plantations that operated for more than a century. Management practices were similar across plantations, allowing us to study the effects of environmental context on vegetation succession trajectories on abandoned sugarcane fields. Here we address the question: What abiotic, biotic, and anthropogenic factors drive native versus invasive species succession patterns on abandoned sugarcane land? Changes in vegetation cover on abandoned fields were detected across annual composite Landsat images from 1998 to 2019. Native and invasive vegetation cover are distinguished based on phenology using spectral unmixing and validated with several carbon assessments that provide reference land cover throughout the study period.
Results/Conclusions
We find that ecosystems dominated by invasive species are prevalent on abandoned sugarcane fields across the four largest Hawaiian Islands. Preliminary findings indicate that native vegetation succession tended to occur when abandoned sugarcane fields were at higher elevations, where they were also closer to intact forest. We discuss the implications of these results and ongoing analyses of these succession patterns in the context of key landscape variables which aim to characterize more nuanced threshold conditions that determine succession trajectories. Insight derived from the unique opportunities in Hawai‘i can inform global efforts to manage land more sustainably with an improved understanding of the conservation opportunities or threats that abandonment may present to ecosystem biodiversity and function.