Lignin and enhanced litter turnover in tree plantations of lowland Costa Rica

We quantified the effect of overstory species composition on forest floor dynamics in lowland Costa Rica. Aboveground litter production and forest floor mass were measured over 1 year in 16-year-old single-species plantations established in a randomized complete block design with four blocks. The tree species investigated (=experimental treatments) were Hyeronima alchorneoides, Pentaclethra macroloba, Pinus patula subsp. tecunumanii, Virola koschnyi, Vochysia ferruginea, and Vochysia guatemalensis. Organic matter fluxes in litterfall, which averaged 10.1 Mg ha−1 year−1, were similar to those reported from comparable Neotropical plantations, whereas nitrogen fluxes in litterfall (up to 210 kgN ha−1 year−1) were high. Litter production was significantly greater beneath Hyeronima and V. ferruginea than beneath Virola, primarily due to high rates of non-woody litter production by Hyeronima, large amounts of twig litter production by V. ferruginea, and low production of both fractions by Virola. V. ferruginea had significantly more branch litter on the ground than did any other species, whereas Hyeronima had the largest accumulations of non-woody (mostly leaf) litter. Accumulations of woody litter correlated closely with rates of branchfall: branches ≤1 cm in diameter persisted on the forest floor for 0.9 year on average, independent of species. Rates of decay of non-woody litter, in contrast, varied significantly among species, with fastest rates (2.8 year−1) observed beneath Pentaclethra and the slowest decay rates (1.5 year−1) in plots of V. guatemalensis. Contrary to expectations, litter decay rates increased with increasing lignin contents, paralleling results of a concomitant study that found a significant negative correlation between litter lignin and surface-soil organic matter content. Our results demonstrate that different overstory-tree species generate significantly different forest floors, in terms of mass, composition and dynamics. These differences affect soil restoration, tree regeneration and other ecological processes important to selecting species for tropical plantation establishment and reforestation efforts.