An increasing trend in the ratio of transpiration to total terrestrial evapotranspiration in China from 1982 to 2015 caused by greening and warming

摘要

The ratio of transpiration to total terrestrial evapotranspiration (T/ET) plays an important role in the hydrological cycle and in the energy budgets between the land and the atmosphere. Although China has experienced substantial climate warming and vegetation restoration (i.e., greening) over the past decades, the response of T/ET to the changing climate and environmental factors is poorly understood. Here, we apply a model-data fusion method that integrates the Priestly-Taylor Jet Propulsion Laboratory (PT-JPL) model with multivariate observational datasets (transpiration and evapotranspiration) to quantify the relative contributions of multiple factors to the T/ET trend for the terrestrial ecosystem of China from 1982 to 2015. Validation against the observational data indicates that the PT-JPL model performed well. The multi-year average T/ET was estimated to be 0.56 +/- 0.05 in China. The T/ET of the forest ecosystems (0.65-0.72) was generally higher than that of the non-forest ecosystems (0.41-0.60). T/ET increased remarkably at a rate of 0.0019 yr(-1) (P 0.01) during the study period. Leaf area index increased significantly over the period, by 0.0031 m(2)m(-2) yr(-1). It appears that greening and climate change were the most likely causes of the increasing T/ET in China, directly explaining 57.89% and 36.84% of the T/ET trend, respectively. Particularly, in the subtropical-tropical monsoonal region, greening directly contributed 24.43% to the T/ET trend whereas climate change contributed 60.95%. The influences of greening and climate change on T/ET trends are mutually reinforcing. Additionally, partial correlation analyses between the climate-driven T/ET and the climate variables indicate that warming (0.04 degrees C yr(-1), P 0.01) was the major driving force of the climate-induced interannual variability of T/ET across the whole study area (R = 0.84), especially in the subtropical-tropical monsoonal region (R = 0.89). Our results may help elucidate the interactions between terrestrial ecosystems and the atmosphere within the context of long-term global climate changes.