Question: What environmental factors and physiological traits determine the juvenile survival rates of 11 dipterocarp species (Dipterocarpaceae) co-occurring in a Bornean tropical rain forest?
Location: Tropical northwestern Borneo.
Methods: In 248 quadrats, distributed over 12 ha, the juvenile (height< 2.0 m) survival of 11 shade-tolerant dipterocarp species was monitored for one year. The probability of survival for each species was expressed as a logistic function of plant height and three environmental factors: canopy openness, soil water potential and conspecific individual density. The correlation between survival under shaded conditions and several physiological traits of juveniles was tested by normal Pearson correlation and a phylogeny based manner using independent contrasts.
Results: Seven species had increased survival under more open canopies, one species had increased and two species had decreased survival at drier sites. Six species had higher survival as the density of conspecifics increased, whereas two species had lower survival. The magnitudes of the effects for the three environmental factors were similar. However, the interspecific difference in survival was largely determined by a dependence of small seedlings on the light environment. Survival under low light was correlated positively with root dry mass and negatively with total leaf area of juveniles.
Conclusions: Responses of seedlings to the fine-scale gradient of light availability were considerably different among the 11 species. Species that suffered increased mortality under low light had a set of structural traits that would potentially allow rapid growth under bright conditions. Differential performance along the light gradient may contribute to the stable coexistence of these species.