Drought Induced Fine Root Growth and Canopy Green-up of Tropical Dry Zone Vegetations in Sri Lanka

Abstract

Fine roots in forest soils have important implications for global carbon (C) balance, but processes underlying this C sink are not well understood. This study evaluates year round dynamics of fine roots in a tropical dry mixed evergreen forest and an arboretum in the dry zone of Sri Lanka. Monthly soil core samples (up to 25 cm depth) were collected randomly to cover a whole annual cycle of the two sites. The soils were air dried, sieved (< 2 mm), and fine roots (≤ 2 mm) were separated by handpicking coupled with a water floating technique. Then, fine root biomass and C density were calculated using oven dry weight. Annual mean fine root biomass of the dry zone forest and the arboretum were found to be 5.72 ± 0.57 t/ha and 7.88 ± 0.81 t/ha, respectively, with C densities of 2.69 ± 0.27 t/ha and 3.7 ± 0.38 t/ha, respectively. Thus, dry zone arboretum showed a higher growth and biomass, and hence a C pool of fine roots, than the dry zone forest, possibly due to a younger forest stand with fast fine root turnover rate. In both sites during the dry spell, there was an increased production of fine roots and a simultaneous leaf flush on the canopy with a green-up. The increased fine root growth during the dry season generally allows the trees to absorb more water under water-stressed situations. These events may be due to an undisclosed survival mechanism of such ecosystems under drought, which needs further studies.