Size-related shifts in carbon gain and growth responses to light differ among rainforest evergreens of contrasting shade tolerance

Recent work suggests that plant size affects light requirements and carbon balance of juvenile trees, and such shifts may be greater in light-demanding species than in their more shade-tolerant associates. To explore the physiological basis of such shifts, we measured juvenile light interception, carbon gain and growth of four subtropical Australian rainforest trees differing in shade tolerance, comparing individuals ranging from 13 to 238 cm in height, across a wide range of understory environments. We hypothesized that even in a standardized light environment, increasing sapling size would lead to declines in net daily carbon gain of foliage and relative growth rates (RGR) of all species, with declines more pronounced in light-demanding species. Crown architecture of individuals was recorded using a 3-dimensional digitizer, and the YPLANT program was used to estimate the self-shaded fraction of each crown and model net carbon gain. Increased sapling size caused a significant increase in self-shading, and significant declines in net daily carbon gain and RGR of light-demanding species, while such ontogenetic variations were minimal or absent in shade-tolerant species. Additionally, differences in the slope of the relationship between light and RGR led to crossovers in RGR among shade-tolerant and light-demanding species at low light. Our results show that the magnitude of ontogenetic variation in net daily carbon gain and RGR can be substantial and may depend on successional status, making it unsafe to assume that young seedling performance can be used to predict or model responses of larger juvenile trees.