Growth and mortality in high and low light: trends among 15 shade-tolerant tropical rain forest tree species

1 Past work on tropical rain forest tree seedlings has been dominated by contrasts between strongly light-demanding and strongly shade-tolerant species. We examined patterns of growth and mortality among shade-tolerant tree seedlings in response to light, and investigated the morphological and physiological correlates of high seedling growth and survival rates across species. 2 Seedlings of 15 tree species from Australian tropical lowland forest were grown for up to 1 year in neutral-density shadehouses at three light levels (10%, 0.8% and 0.2% full daylight). All species showed negligible mortality in the 10% and 0.8% shadehouses, but survival was significantly reduced in 0.2% daylight. 3 Seedling survival rate in 0.2% daylight showed no significant relationship with either the dry mass of seed reserves (embryo plus endosperm), or relative growth rates in dry mass (RGR(M) ) in 0.8% and 10% light. 4 The RGR(M) values in 0.8% and 10% daylight were strongly positively correlated, and showed a strong negative correlation with the dry mass of seed reserves. Interspecific variation in low-light RGR(M) was driven by unit leaf rate (rate of accumulation of dry mass per unit area of leaf), whereas interspecific variation in high-light RGR(M) was most closely correlated with leaf area ratio (leaf area per total plant dry mass). 5 Variation in seedling characters in response to light may have important implications for the coexistence of shade-tolerant tropical tree species.