Fine-root mass, growth and nitrogen content for six tropical tree species

Although fine roots might account for 50% of the annual net primary productivity in moist tropical forests, there are relatively few studies of fine-root dynamics in this biome. We examined fine-root distributions, mass, growth and tissue N and C concentrations for six tree species established in 16-year-old plantations in the Caribbean lowlands of Costa Rica in a randomized-block design (n = 4). The study included five native species (Hyeronima alchorneoides, Pentaclethra macroloba, Virola koschnyi, Vochysia ferruginea and Vochysia guatemalensis) and one exotic (Pinus patula). Under all species >60% of the total fine-root mass to one meter deep was located in the uppermost 15 cm of the soil. Fine-root live biomass and necromass (i.e., the mass of dead fine roots) varied significantly among species but only within the uppermost 15 cm, with biomass values ranging from 182 gm−2 in Pinus to 433 gm−2 in Hyeronima plots, and necromass ranging from 48 gm−2 in Pinus to 183 gm−2 in Virola plots. Root growth, measured using ingrowth cores, differed significantly among species, ranging from 261 gm−2 yr −1 in Pinus to 891 gm−2 yr −1 in Hyeronima. Turnover rates of fine root biomass ranged from 1.09 to 2.03 yr −1 in Virola and Hyeronima plots respectively. Fine-root biomass was significantly and positively correlated with fine-root growth (r = 0.79, P < 0.0001), but did not correlate with fine-root turnover (r = 0.10; P = 0.20), suggesting that fine-root accumulation is a function of growth rate rather than mortality. Fine-root longevity was not correlated (r = 0.20, P = 0.34) and growth was negatively correlated with root N concentration across species (r = -0.78, P < 0.0001), contrary to reported trends for leaves, perhaps because N was relatively abundant at this site.