Interactions Between Enrichment Planted Seedlings and Naturally Occurring Trees in Selectively Logged Lowland Dipterocarp Forest

Old-growth forests in Southeast Asia are dominated by trees of the Dipterocarpaceae family which are targeted by selective logging. Their traits (supra-annual mast fruiting, limited dispersal, and recalcitrant seeds that form no seed bank) mean they can have poor natural regeneration rates in some selectively logged forests. Enrichment planting is used to attempt to overcome this recruitment limitation and increase restoration success. However, it is still unclear what factors influence the performance of planted seedlings. Here, we analyse the growth and survival between 2012 and 2015 of 721 enrichment line-planted seedlings from 16 species of dipterocarps within the selectively logged forest of the Sabah Biodiversity Experiment, alongside the location, size and identity of nearly 5000 naturally occurring trees within 10 m of focal planted seedlings. We analysed the survival and growth of enrichment planted dipterocarp seedlings in relation to three properties of the surrounding naturally occurring vegetation: (1) canopy openness; (2) the abundance of naturally occurring dipterocarps (proportion of total basal area); (3) the presence of nearby (< 10 m) large trees (basal area of the largest tree as a proportion of the total). Survival and growth rates of enrichment planted seedlings were positively associated with canopy openness and total basal area of surrounding trees. These results were consistent between the two planting cohorts. Increased survival and growth of enrichment planted seedlings in areas of Sabah Biodiversity Experiment with higher canopy openness (up to around 40%) is consistent with understory light as a limiting resource. The unexpected higher survival and growth of enrichment planted seedlings in forest areas with higher basal area of unlogged trees may be explained by the creation of patches that are better or worse for tree growth, with areas less heavily impacted by logging containing both more naturally occurring trees and providing better conditions for the survival and growth of planted seedlings.