Determinants of whole-plant light requirements in Bornean rain forest tree saplings

1. Shade tolerance, defined as the ability to survive and grow under low light, varies markedly among tree species. However, the role of low-light growth responses in determining shade tolerance is unclear, as are the effects of non-light resources such as soil nutrients. 2. A conceptually simple field measure of shade tolerance is the whole-plant light compensation point (WPLCP), evaluated as the x-intercept of the relationship between growth and incident light integrated over a long time interval. Here we compare WPLCP for growth and survivorship of saplings of Bornean tree species differing in shade tolerance, and evaluate the importance of various physiological and morphological traits in predicting WPLCP. We also examine both phenotypic and evolved differences in WPLCP between tree saplings growing on two distinct soil types at Sepilok Forest Reserve, Sabah, Malaysia. 3. Growth-based estimates of WPLCP showed essentially a 1 : 1 correspondence to threshold light levels for survivorship. At higher light, more light-demanding species showed higher growth, resulting in a steeper slope of the relationship between relative growth rate (RGR) and light availability than in more shade-tolerant species. This resulted in significant crossovers in the RGR-light relationship among species. 4. Dark respiration (R-d) was the single best predictor of WPLCP; other leaf traits such as leaf nitrogen and photosynthetic capacity were correlated with, but excluded as predictors of, WPLCP in multiple regression analyses. 5. Although soil type had no consistent phenotypic effect on WPLCP, evolved responses among species were pronounced: species associated with the nutrient-poor, drought-prone, sandstone-derived soils had higher WPLCP values than alluvial soil specialists in phylogenetically controlled comparisons. 6. Our results indicate that minimum light levels for growth do not diverge from those for survivorship, and do not support the view that low-light survivorship solely determines shade tolerance. Our analyses also suggest that R-d is the strongest determinant of whole-plant light requirements in tropical tree saplings, and thus may be an easily measured surrogate of WPLCP and shade tolerance. 7. Prediction of tree species resource requirements is crucial for understanding forest dynamics and promoting ecology-based forest management and restoration, particularly in diverse tropical forests where data on the resource requirements of most species are not available. Easily measured surrogates of resource requirements (e.g. R-d as a predictor of shade tolerance) will contribute to this goal, as will an improved understanding of the interactive effects of multiple resources on tree performance.