Light gradient partitioning by tropical tree seedlings in the absence of canopy gaps

To explore the importance of light availability for seedling growth in low light environments, we examined light-dependent growth, biomass allocation and mortality of tree seedlings growing in sites with 0.2-6.5% full sun, the range of light commonly encountered in the understory of closed canopy, lowland tropical forests. We transplanted seedlings of the canopy tree species, Dipteryx panamensis, Virola koschnyii, and Brosimum alicastrum into second-growth forest and native tree plantations at La Selva Biological Station, Costa Rica. We assessed seedling survival, growth, and seedling light environments bimonthly for 14 months. Plants were harvested at the end of the study to assess leaf area, total biomass, biomass partitioning and root growth. Survivorship of all species exceeded 60% across all microsites, although both D. panamensis and B. alicastrum had lower probabilities of survival in the darkest microsites. All species showed a strong positive relationship between light availability and growth, increasing in total biomass as light increased. However, the strength of the growth response differed among species causing a change in the rank order of species growth rates as light availability increased. Although D. panamensis showed the lowest growth rates in the darkest microsites, a strong response to increasing light led to a cross-over in performance, such that D. panamensis had the highest growth rate at the highest light levels studied. These data suggest that resource gradient partitioning could occur even in low light environments (0.2-6.5%). Given the limited range of light regimes sampled (i.e., non-gap microsites), our data demonstrate that growth of tropical tree seedlings beneath closed canopies is highly sensitive to light availability and that shade-tolerant species vary in these responses. Our results show that understory light heterogeneity, in the absence of canopy gaps, can significantly affect recruitment processes for shade-tolerant tree species.