Seeded pioneer die-offs reduce the functional trait space of new-growth coastal dune forests

Abstract

The planting or seeding of pioneer species to promote restoration apparently contributes little to the establishment of late-successional species, despite the common assumption that they facilitate forest regeneration. We evaluate the consequences of planting pioneers for coastal dune restoration by measuring plant traits (specific leaf area, wood density, δ13C and maximum plant height), species composition and canopy openness in three rehabilitating forests of different ages, where a pioneer species (Acacia karroo) was seeded, and one undisturbed old-growth forest. We surveyed woody species composition in 2010 and in 2015 to assess how changes in A. karroo influences community structure of rehabilitating forest. Our results showed that the number of adult individuals of A. karroo decreased progressively with forest age, indicating that the demise of individuals of A. karroo opens canopy gaps in rehabilitating sites. This accorded with a significantly higher variation of canopy openness levels in the oldest rehabilitating site. Rehabilitating sites tended to progress towards a reduced trait space as they aged, contrasting with the old-growth forest that showed an expanded trait space. Communities located at high levels of canopy openness were dominated by species with lower values of specific leaf area and wood density, indicative of acclimation to high light conditions and fast-growing strategies. Our results suggest that changes in light availability due to canopy gap formation, can act as an environmental filter which may deflect forest regeneration trajectories. We show that coupling a trait-based approach with environmental measurements can give insight in regeneration trajectories of rehabilitating sites and, therefore, better inform restoration practices. Preventing the formation of large canopy gaps in restoration programs using pioneers as a regeneration pathway may facilitate the natural recovery of degraded forest.