PHOTOSYNTHESIS OF NINE PIONEER<i>MACARANGA</i>SPECIES FROM BORNEO IN RELATION TO LIFE HISTORY

Abstract

Early successional (pioneer) tropical tree species are characterized by a suite of ecophysiological and life history traits; however, little is known of the relationships between these groups of traits, and their consequences for species' distribution patterns in diverse tropical forests. This study investigated leaf-level ecophysiological variation in seedlings of nine sympatric, pioneer tree species of Macaranga (Euphorbiaceae) from Borneo, grown at three light levels (high: ∼19 mol/d; medium: ∼7 mol/d; low: ∼3.6 mol/d). A multivariate analysis of traits associated with species' successional status was used to rank species according to life history variation, and then to investigate patterns of covariation in seedling ecophysiological and life history traits. Ecophysiological traits varied significantly among the nine species. On a leaf area basis, dark respiration (Rd-area) in high-light seedlings ranged from 0.51 to 0.90 μmol CO2·m−2·s−1, and light saturated net photosynthesis (Amax-area) ranged from 7 to 13 μmol CO2·m−2·s−1. Amax-mass and Rd-mass were strongly negatively correlated with leaf mass per unit area (LMA). Among species, Amax-mass and Rd-mass were strongly positively correlated for high-light grown seedlings, reflecting a trade-off between high assimilation rates and respiratory costs. Within species, Amax-area, Rd-area, gs, LMA, and photosynthetic light compensation point were significantly greater in high-light grown plants for all species. Due to the high plasticity of LMA, Amax-mass and Rd-mass were only weakly influenced by light growth conditions, suggesting that resource allocation patterns that maximize photosynthetic ability are critical to survival and growth in low light for these species. Principal components analysis (PCA) of ecophysiological traits for the nine species revealed a continuum of variation from species with relatively low Amax, low gs, and high LMA to species with the opposite traits. The primary axis of the PCA of life history traits was strongly related to variation in shade tolerance and seed mass. The second life history axis distinguished among the more shade-intolerant species. The PCAs of ecophysiological and life history traits were not completely concordant due to variation in life history traits among high Amax species. Amax-mass and LMA, were correlated with a successional ranking of the species. The study shows how a suite of inter-related ecophysiological and life history traits can result in a diversity of pioneer tree ecologies.