Effect of Changing Light Availability on Nutrient Foraging in Tropical Deciduous Tree-Seedlings

Abstract

We evaluated the relationship between the capacity to respond to changes in light and the ability to search and exploit nutrient-rich soil patches (foraging). Seedlings of four tropical tree species (Caesalpinia platyloba, Celaenodendron mexicanum, Cordia alliodora and Heliocarpus pallidus) were exposed to a factorial combination of high (H) and low (L) photon flux density with fertilized (+ Fp) and unfertilized nutrient patches (- Fp) for 42 days. After this initial growth period, half of the plants from H were transferred to L (HL treatment), and half of the plants from L were transferred to H (LH) for another 42 days. When plants were transferred from high light to low light (HL), their productivity and RGR (relative growth rate) were lower than plants under high light, but when plants were transferred from low light to high light (LH), they reached similar or higher RGR (and sometimes biomass) compared to plants under continuously high light. The study species differed in their ability to explore and to benefit from the fertilized patch, depending on characteristics such as root length and root density in the fertilized patch and the potential relative growth rate of the species. The slow-growing species C. mexicanum showed no response to nutrient patches because it was unable to locate and produce roots in the patch. This species responded only to light, with slightly higher plant dry biomass and growth rate under HH and HL than under LH and LL, with thinner leaves (higher specific leaf area) under LL and LH. Fast-growing species had a higher ability than slow-growing species to forage for patchily distributed soil nutrients. This ability was maintained when plants experienced light increases (LH plants) but not when plants were growing under shaded conditions. In addition, the lack of the fertilized patch caused a strong reduction in RGR in the most productive light conditions (HH), so that HH plants had as low an RGR as LL and HL plants when grown without a fertilized patch. This response indicates a strong interaction between light capture and soil foraging behaviour.