Journal of Tropical Forest Science 26(4): 532–542 (2014) Dong TL et al. 532 © Forest Research Institute Malaysia SITE CONDITIONS FOR REGENERATION OF HOPEA ODORATA IN NATURAL EVERGREEN DIPTEROCARP FOREST IN SOUTHERN VIETNAM TL Dong 1, 2 , CL Beadle 1, 3 , R Doyle 1 & D Worledge 3 1 Tasmanian Institute of Agriculture and School of Land and Food, The University of Tasmania, Private Bag 98, Hobart, Tasmania 7001, Australia 2 Silviculture Research Institute, Vietnamese Academy of Forest Science, Duc Thang, Bac Tu Liem, Hanoi, Vietnam; dong. tran@vafs.gov.vn 3 CSIRO Ecosystem Sciences, Private Bag 12, Hobart, Tasmania 7001, Australia Received July 2013 DONG TL, BEADLE CL, DOYLE R & WORLEDGE D. Site conditions for regeneration of Hopea odorata in natural evergreen dipterocarp forest in southern Vietnam. Matching species to suitable sites is important in reforestation. This study investigated the site conditions that support regeneration of Hopea odorata, a valuable timber species, in a secondary evergreen natural forest. Stand structure, light intensity at seedling level and soil condition were examined in three representative 50 × 50 m plots. The upper canopy was dominated by four dipterocarps: H. odorata, Shorea roxburghii, Anisoptera costata and Dipterocarpus alatus. The prevailing stand structure supported vigorous germination but not development of seedlings of all four species. Low light levels near the forest floor were the major constraint on seedling development of H. odorata. There was no regeneration when the mean per cent transmitted incident daily photosynthetic active radiation (PAR) was 2.2%; seedling germination but not development was supported when PAR was 6.6%; regeneration and development occurred when PAR was 11.4%. The soils were slightly acidic with low clay and high sand contents and low nutrient concentration, but this was apparently not a constraint on growth given adequate light conditions. The results suggest that the re-establishment of H. odorata on degraded sites using nurse crops should be possible provided that high levels of shading are avoided. Keywords: Gap regeneration, photosynthetically active radiation, site requirement INTRODUCTION Valuable tropical timber species have a history of overharvesting (Lamb 2011). One of these species is Hopea odorata , a late successional tropical dipterocarp which is naturally distributed in many South-East and South Asian countries (Prosea Foundation 1993). Its timber is valued because of its durability and resistance to insects, and use for weight-bearing construction. Hopea odorata is currently assessed as vulnerable in the IUCN Red List of threatened species (IUCN 2012). In Vietnam, it is listed as a priority species in need of immediate conservation (Hong 2012). Under natural conditions, usually in riparian and moist forest, H. odorata grows with other dipterocarps which eventually occupy the upper storey and become dominant. Natural regeneration occurs where there is shade, suggesting that seedlings are shade tolerant (Sakai et al. 2009). Young trees become more light demanding (Thao 1995). While shade is considered important for germination and initial establishment within the understorey (Kettle 2009), it is associated with low rates of growth, rates increasing when light intensities are increased (Appanah 1998). Continuous shade can compromise survival (Kamaluddin & Grace 1993). There have been many attempts using enrichment planting or planting with nurse crops to re-establish H. odorata for either timber or conservation (Weinland 1998). In Vietnam, it has been a major species in reforestation programmes, but many of these have failed because of the use of inappropriate planting sites and silvicultural treatments (Tam 2007). Nutrient deficiency and soil compaction led to poor seedling establishment on degraded rainforest soils in Sabah (Nussbaum et al. 1995). In lowland