Hail event rather than cold stress, has profound effects on the carbon-water coupling process of Hevea brasiliensis in Xishuangbanna

Abstract

The annual pattern of the carbon-water coupling of rubber trees (Hevea brasiliensis) has never been explored on the plant and ecosystem scales, and the responses of the carbon-water coupling to cold stress and hail events remain unknown. We continuously measured the sap flow density (Js), diameter at breast height (DBH), and the carbon and water flux of rubber plantations in Southwest China from 2012 to 2016, to compare the water-use efficiency (WUE) of rubber plantations by considering different scales (plant and ecosystem) and methods (eddy covariance and isotope techniques) and to reveal the adaptation strategy of rubber trees to cold stress and hail events from the perspective of WUE. We found the followings: (1) high correlation in the annual WUE pattern were noted on the plant and ecosystem scales (r=0.74, P<0.05), with higher WUE values obtained during the rainy seasons than during the dry seasons; (2) The rubber trees responded to cold stress with the reduction of the strength of the carbon-water coupling, which was characterized by a lower ratio of growth rate of DBH (Rdbh) to Js during the cold stress period (slope=0.007, P>0.05) compared with after the cold stress (slope=0.03, P<0.05); and (3) The hail event severely disrupted the WUE pattern of rubber trees, which was characterized by a weak relationship between Rdbh and Js after experiencing the hail event (slope=0.01, r=0.35, P>0.05) than without experiencing the hail event (slope=0.07, r=0.63, P<0.05). We concluded that the response of rubber trees to the frequent cold stress might involve the long-term adaptation by short-term decoupling of WUE; however, the effects of the occasional hail event on the growth of rubber trees were more profound.