Partial recovery of large seed arrival following ecological restoration in fragmented tropical rainforests

Forest restoration success depends crucially on the reinitiation of
ecological processes such as seed arrival that drive natural regeneration.
We know little about whether, by increasing and diversifying local seed
sources to alleviate seed limitation, and attracting animal frugivores to
alleviate dispersal limitation, restoration could shift seed arrival rates
and species composition in the direction of mature rainforests,
particularly for large-seeded species in degraded fragments. We tested
this by examining seed arrival in 189 1 m2 seed fall traps within 63 seed
trapping stations across sites ecologically restored (through maximum
diversity mixed-native species planting 13-21 years ago) and those left
unrestored within degraded rainforest fragments, and in mature “benchmark”
rainforests, in India’s Western Ghats. We found that overall, large-seeded
species arrival rates in restored sites was comparable with the benchmark,
and was higher than unrestored sites. Seeds arriving through dispersal
(from fruiting trees absent in the overstory) also showed a recovery
towards benchmark levels, but were dominated by alien species abundant in
the matrix habitat. Restoration also had no effect on species richness and
composition of all arriving seeds, with restored sites closely resembling
unrestored sites and remaining distinct from benchmark sites. Further,
restoration did not alleviate the effect of fragment isolation on large
seed arrival rates, which declined with distance from benchmark forests
similarly in restored and unrestored sites. Our results suggest that
restoration can increase local availability and dispersal of large seeds,
and highlights the value of diverse native species restoration plantings
within degraded forests and promoting their use instead of alien tree
species in the surrounding matrix. However, our findings also show that
recovery of natural regeneration can remain slow or uncertain in restored
forest fragments, and underscore the need for sustained monitoring and
evaluation of restoration over multiple decades.