Water and nutrient relations of mistletoes at the drought limit of their hosting evergreen oaks in the semiarid upper Yangtze region, SW China

摘要

Does mistletoe infestation influence the position of the dry range limit of oaks? We explored this question and our results contrast most earlier findings on host-mistletoe interaction. The study was located in the SE Himalayas in an environment with 200-360 mm annual precipitation falling mainly during the short monsoon season (hardly any rain during 9-10 month). The hemi-parasite species (HPs)Loranthus delavayiandTaxillus thibetensisare growing onQuercus pannosas.l.at its drought-driven range limit. HPs mostly exhibited lower shoot water potentials than their hosts during both, the dry and the wet season. During the dry season, leaf conductance (g(l)) of the HPs was very low (100-200 mmol m(-2) s(-1)) and mostly lower than in their host. During the wet season, both HPs and oaks reached highg(l)(up to c. 500 mmol m(-2) s(-1)) with higher in HP or no clear differentiation. Leaf delta C-13 in HPs was lower than in the host, suggesting a small limitation of CO(2)uptake by g(l). Both HPs had higher foliage potassium (K) and phosphorus (P) and similar nitrogen (N) content than the host. Thus, counter to abundant literature, HPs consumed extra water (and dissolved nutrients) mainly during the wet season, not adding extra stress to their host trees under extreme drought. Both host trees and mistletoes track the monsoon regime, with peak activity confined to the brief moist period. Under these semi-arid conditions, mistletoes thus, do not seem to affect the drought-driven range limit of these oaks.