Journal of Tropical Ecology (2005) 21:317–328. Copyright © 2005 Cambridge University Press doi:10.1017/S0266467405002269 Printed in the United Kingdom Pair-wise competition-trials amongst seedlings of ten dipterocarp species; the role of initial height, growth rate and leaf attributes E. V. J. Tanner ∗1 , V. K. Teo ∗ , D. A. Coomes ∗ and J. J. Midgley† ∗ Department of Plant Sciences, University of Cambridge, Cambridge CB2 3EA, UK † Botany Department, University of Cape Town, P. Bag Rondebosch, 7701 South Africa (Accepted 7 October 2004) Abstract: To investigate whether seedlings of ten dipterocarp species differed significantly in terms of growth and mortality or whether species were not significantly different and could be considered ecologically similar, seedlings were grown, two per pot, in two experiments: (1) where the two seedlings were of equal height (30 cm); and (2) where one seedling was 10 cm shorter than the other. Seedlings were grown in a shade house with 15% above- canopy light in a 50:50 forest soil–sand mixture and were watered frequently; pots were placed so that seedling density was 130 seedlings m −2 of ground. In the first experiment there were 45 pairwise combinations of species when seedlings were 30 cm tall (AB, AC, AD, ... . BC, BD ... IJ; where A, B, C ... J signify different species); each combination was replicated 10 times so there were 450 pots with 900 seedlings. In the second experiment there were 100 pairwise combinations of species and size e.g. Aa (30 cm A with 20 cm a), Ab (30 cm A with 20 cm b), each combination was replicated 10 times hence there were 1000 pots with 2000 seedlings. After 22 mo 79% of the initial 2900 seedlings survived; on average they had grown 42 cm (i.e. to 72 cm tall from their initial 30 cm). The most frequent outcome of competition-trials between different sized individuals (784 of 1000 trials) was that the initially taller seedling of each pair ‘won’ (it was the taller or surviving seedling). When 900 of these trials (setting aside, Aa, Bb, Cc etc.) were analysed as 45 comparisons between species with different sized individuals (Ab and aB are one interspecific comparison for these purposes), initial height determined the outcome in 23 cases (even in some competitions between light hardwood species and heavy hardwood species); in 6 cases a species (mostly light hardwoods) behaved as a ‘dominant’ – they usually won even if they were smaller initially. We found few significant differences between species in: initial seedling heights; leaf nitrogen concentrations; and specific leaf areas when they were grown in similar conditions, and these attributes were not correlated with growth rates. The similarity of seedlings of different species meant that often a height difference of just 10 cm was enough to determine the outcome of a pairwise competition-trial in high seedling densities and light equivalent to that in forest gaps. Key Words: competition, Dipterocarpaceae, Dryobalanops, Hopea, Parashorea, seedlings, Shorea, size, tropical trees INTRODUCTION The understanding of determinants of forest dynamics and species co-existence can be polarised into the following two extremes – chance versus niche (Brokaw & Busing 2000). Chance is the most important determinant of success in the neutral model; interspecific differences (such as for minimum or maximum light requirements) are considered to be small and thus most species fall within a few broad guilds. These differences are considered to be relatively unimportant in explaining dynamics and 1 Corresponding author. Email: EVT1@cam.ac.uk co-existence (Hubbell 2001); instead, chance events, such as those related to the vagaries of phenology, predation, or dispersal and how these may limit seedling recruitment in gaps are thought to be more important (Hubbell et al. 1999). In contrast, in the more traditional model, species- specific differences are considered significant and are important in explaining dynamics and co-existence. In this model, a diversity of gap sizes and frequencies is needed to facilitate co-existence (see review Sheil & Burslem 2003). To test these ideas forest ecologists are asking questions such as (1) are there niche differences amongst forest trees? and (2) what are the main factors, both biotic and abiotic, which determine ecological