Early development of three planted indigenous tree species and natural understorey vegetation in artificial gaps in an Acacia mangium stand on an Imperata cylindrica grassland site in South Kalimantan, Indonesia

Early performance of two dipterocarp species Anisoptera marginata and Shorea parvifolia, and a long-living pioneer species Peronema canescens (Verbenaceae) planted in artificial gaps (size 260 m2) and surrounding untreated stands was studied in a fast-growing plantation of Acacia mangium on an Imperata cylindrica grassland site in South Kalimantan, Indonesia. Forty seedlings of each species were planted at one-meter intervals in lines across each of the five gaps, starting and ending under closed stand. Survival, height and diameter (d0.05) increments were measured, and the effect of gap opening on the composition and abundance of understorey vegetation (grass, shrub and native tree seedlings and saplings) was studied. 19 months after planting, average survival rates were 97% for A. marginata, 94% for P canescens and 71% for S. parvifolia, with no statistical differences between gap and closed stand. Substantial mortality and damage of dipterocarps were caused by wild boars; minor damage by dieback (for S. parvifolia) and insect pests (for A. marginata). Early growth was clearly influenced by distance from gap centre and light conditions; the growth of seedlings was greater the nearer the seedlings were situated to centre and the higher the level of daily photosynthetic photon flux density (PPFD) was. Gap opening increased the growth of shrub species Chromolaena odorata, but not that of Imperata grass. It also increased the density and height growth of saplings of native pioneer and secondary tree species. Seedling density increased both in closed stand and actual gaps, but was higher inside gaps. Results indicate good prospects for diversifying the composition of fast-growing forest plantations on severely degraded former forest lands and integrating slow-growing valuable species in plantation programs. Both in-depth ecophysiological studies on species-specific growth requirements, and practical oriented research on silvicultural options and economics need further studies.