Productivity and morphological traits of fine roots in forest ecosystems along an elevation gradient of Yakushima Island

Abstract

ABSTRACTWe measured the productivity and morphological traits of fine roots in seven forests between 170 and 1550 m along an elevation gradient on Yakushima Island, Japan. Yakushima Island is continuously covered by forests ranging from the lowland evergreen broad-leaved forests below 1000 m to the upland evergreen coniferous forests above. Mean annual temperature decreases linearly and soil nutrient availabilities (both N and P) also decrease with increasing elevation. We employed an in-growth core method to determine fine-root productivity. Mean fine-root biomass ranged from 0.123 to 0.297 kg m−2 in the lowland broad-leaved forests and from 0.092 to 0.097 kg m−2 in the upland coniferous forests. Mean fine-root productivity ranged from 0.063 to 0.173 kg m−2 yr−1 in the lowland broad-leaved forests and from 0.033 to 0.044 kg m−2 yr−1 in the upland coniferous forests. Both mean fine-root biomass and mean fine-root productivity were significantly greater in middle-elevation broad-leaved forests than in the lower broad-leaved or the higher coniferous forests. Fine-root turnover rate tended to be lower and mean specific root length (SRL) tended to be higher in the coniferous forests. In the upland coniferous forests, trees may adapt to nutrient limitation and/or cooler climate by increasing the life span and SRL of fine roots instead of increasing carbon allocation to fine roots, thereby decreasing fine-root productivity. Fine-root dynamics along the elevational gradient on Yakushima reflected the species shift from broad-leaved to coniferous trees rather than the monotonous reduction of temperature and nutrient availability.