What follows fallow? Assessing revegetation patterns on abandoned sugarcane land in Hawaiʻi

Millions of hectares of agricultural land have been abandoned globally in recent decades, presenting opportunities for secondary vegetation growth and restoration. While abandoned fields have the potential to return to ecological communities with similar species diversity to their pre-agricultural state, they alternatively may transition to novel ecosystems or persist in degraded states that may have alternative functions that impact ecological and human communities. Yet we lack an understanding of how vegetation naturally recovers on disturbed lands. Using remote sensing and land survey data, we characterized the structure, composition, and function of secondary vegetation canopies on former sugarcane fields in Hawaiʻi that were abandoned between 4 and 117 years ago. We used a species distribution model to identify patches of uncultivated land with similar environmental conditions to abandoned sugarcane fields to serve as reference ecosystems. Using these reference ecosystems, we evaluated how secondary ecosystems at different ages since abandonment compare in terms of canopy structure, composition, and function. Grasses were prevalent in the years immediately following abandonment, but shrubs and trees dominated canopy structure on fields that had been abandoned more than 20 years. Non-native species constituted most of the secondary vegetation, but native vegetation cover increased on sugarcane fields that had been abandoned longer than 25 years. Secondary vegetation recovered canopy functional traits in ≤ 53 years since abandonment. Completely recovering the structural properties of reference ecosystems would require over a century. Abandoned sugarcane fields are unlikely to recover the native composition of reference ecosystems through unassisted vegetation recovery. Our findings contribute to a growing body of literature that characterizes whether and when the globally increasing area of abandoned agricultural land may passively recover, which can direct restoration efforts on abandoned lands to enhance ecosystem services or guide alternative management to achieve socio-cultural objectives.