Can intercropping with the Chinese medicinal herbs change the water use of the aged rubber trees?

Intercropping the medicine herbs (i.e., Amomum villosum and Clerodendranthus spicatus) with rubber tree in Xishuangbanna prefecture of southwestern China is regarded as a promising solution to reduce the negative hydrological effects which were caused by the monocultural cultivation of rubber tree and improve the sustainability of aged rubber plantation. However, the water-use characteristics of rubber tree in these agroforestry systems (AFSs) were still rarely reported, and our current understanding of the below-ground competition in such tree and herb AFSs is also insufficient. Therefore, we adopted the techniques of stable isotopes to study the water use efficiency (WUE; through leaf δ13C) and the water-absorbing patterns (through δ2H and δ18O) of the aged rubber trees and the intercropped herbs for the investigation of their water competition, and we also measured the contents of their leaf carbon (C), nitrogen (N) and phosphorus (P) to check the competition effects on the nutrient status of rubber tree. As the results of Bayesian mixing model (MixSIAR) suggested, the aged rubber trees in the rubber monoculture still absorbed the surface and shallow soil water (0–15 cm depths) mainly, same as the intercropped herbs in these AFSs, but rubber trees in the AFSs prefer to absorb the middle and deep soil water (below 15 cm depths). Such phenomenon verified that the water-absorbing patterns of these aged rubber trees were still flexible. However, the leaf δ13C of the rubber trees in these AFSs indicated their WUE was not improved through the intercropping, and thus the soil water content decreased in the AFSs. In short, intercropping the shallow roots medicine herbs (i.e., A. villosum and C. spicatus) can help the aged rubber trees use deeper soil water, and thus help the rubber tree and intercrops form complementary water-absorbing patterns in the pronounced dry season, but there is no improvement in their WUE and the soil water conditions. In addition, the decreased leaf N of rubber trees in the AFSs suggests that the N absorption of rubber trees was limited. Therefore, intercropping the medicine herbs with rubber tree still need more consideration on the design of such AFSs, especially in terms of the soil water conservation and management.