Dipterocarp seedling growth in rain forest canopy gaps during six and a half years

The growth of seedlings of 11 species of Dipterocarpaceae in artificial canopy gaps of different sizes in a lowland evergreen dipterocarp rain forest in Sabah has been followed for 77 months. Three species were abundant and studied in most detail. The main objective was to analyse the foresters' observation, on which silviculture is based: as gap size increases, species that are more light-demanding win the race to fill the gap. Hopea nervosa seedlings had higher survival in closed forest than those of the other two well represented species, Parashorea malaanonan and Shorea joherensis. From seedling demography these two species groups can be called shade-tolerants and light-demanders respectively. At 40 months the seedlings tallest at gap creation had increased their height advantage in all gap sizes. These were mainly H. nervosa. By 53 months, seedlings of Shorea johorensis had grown ahead of H. nervosa in all but closed forest and tiny gaps of 6% and 8% canopy openness (1 and 4 mol m(-2) day(-1) photosynthetically active radiation (p.a.r.) respectively). By 77 months they were even further ahead. Thus S. johorensis has a more flexible response. It is better able to use the extra p.a.r. of larger gaps (ca. 10%, openness or more; 10 mol m(-2) day(-1) p.a.r. or more), and is a light-demander in a second sense, whereas H. nervosa is light-indifferent in this sense. Seedlings of P. malaanonan unexpectedly failed to show rapid height growth in the larger gaps at 53 and 77 months. This species alone suffered very serious apical damage by herbivory. The 11 species under study occurred in various mixtures. To analyse success in gap-filling in the forest they were grouped by timber density into two classes. With increasing gap size, one group, six light hardwood species, known to be light-demanders, grow progressively ahead of the other group, four medium hardwood species, known to be shade-tolerant. P. malannonan, also a light hardwood, grows only slowly because of herbivory. We find no evidence for fundamental niche differentiation. All species showed increasing rates of growth with increasing gap size. Species that responded more slowly might succeed in situations where they alone occur.