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

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.