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

Abstract

Summary 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. 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. 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 (RGRM) in 0.8% and 10% light. The RGRM 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 RGRM was driven by unit leaf rate (rate of accumulation of dry mass per unit area of leaf), whereas interspecific variation in high‐light RGRM was most closely correlated with leaf area ratio (leaf area per total plant dry mass). Variation in seedling characters in response to light may have important implications for the coexistence of shade‐tolerant tropical tree species.