Growth and carbon partitioning of tropical tree seedlings in contrasting light environments

Irradiance at the ground level in tropical forests varies between very low in the understorey to high in forest gaps. Irradiance influences growth directly through differences in carbon gain, and indirectly through differences in carbon partitioning. In this paper we present an analysis of the morphological and physiological factors influencing the growth rates of tropical tree seedlings with different ecological strategies. A compilation was made of published data on relative growth rates and biomass allocation of species in contrasting light environments. The compilation encompasses 43 studies and 194 species. All morphological characteristics show ontogenetic trends. Stem mass ratio (SMR) increases with plant mass, mainly at the expense of the leaf mass ratio (LMR). A concomitant decrease in specific leaf area (SLA) leads to reduced leaf area per unit plant mass (LAR), and hence, a lower relative growth rate (RGR) when plants increase in size. Pioneers grow faster than shade-tolerant species, not only in high light but also in low-light conditions. In low light LAR explains interspecific differences in RGR, whereas in high light differences in net assimilation rate (NAR) are more important. We then use these data and additional information on physiological processes to model carbon balances for the average pioneer and shade-tolerant species in both a low- and a high-light environment. Simulated photosynthetic carbon gain is higher in pioneers than in shade-tolerant species, in both light environments. This is due to a larger leaf area in low light, acid the combination of a larger leaf area and higher photosynthetic rates in high light. Patterns in biomass fractions and carbon partitioning are strikingly different. More than half of the assimilates are partitioned to the leaves (for respiration, the replacement of shed leaves and additional growth), a third is partitioned to the roots and only 15% to the stem. Stems are not a great burden on the carbon balance, due to their low respiration rates and long lifespan. At the whole plant level, respiration consumes the largest share of assimilates, followed by net growth. About 15% of the assimilates are invested in the replacement of shed plant parts. The ability of shade-tolerant species to tolerate shade, apparently reduces the maximal possible growth rate in high light. In contrast, specialization for growth in a high-light environment does not preclude reasonable growth in low light, but probably at the cost of an increased risk of mortality.