Tropical forest regeneration following land abandonment is driven by primary rainforest distribution in an old pastoral region

Context Tropical forest regeneration is increasingly prominent as agro-pastoral lands are abandoned. Regeneration is characterised as favouring 'marginal' lands; however, observations of its drivers are often coarse or simple, leaving doubt as to spatial dynamics and causation. Objectives We quantified the spatial dynamics of forest regeneration relative to marginality and remnant forest cover in a 3000 km(2) pastoral region in northern tropical Australia. Methods Classification and regression trees related the extent and distribution of regeneration to soil agricultural potential, land-cover history, terrain slope, distance to primary forest, and primary forest fragment size, as defined by aerial photography. Results Secondary forest extent and distribution overwhelmingly reflect the proximity and size of primary forest fragments. Some 85 % of secondary forest area occurs < 1 km of primary forest, and 86 % of secondary forest patches >50 ha are < 400 m from primary forest and coincident with historic primary forest fragments. Where primary forest fragments are >8.5 ha, secondary forest area declines less rapidly with increasing distance from primary forest up to 1.5 km. Marginality inferred by soil potential and slope had no bearing on regeneration, except at the coarsest of spatial scales where regeneration is a proxy for primary forest cover. Conclusion Findings underline the need to conserve even modest rainforest patches as propagule reservoirs enabling regeneration. Marginality per se may have a limited role in regeneration. As most secondary forest was an extension of primary forest, its unique conservation value relative to that of primary forest may likewise merit reconsideration.