Mistletoes are semi-parasitic plants attached to shoots or stem of host poplar trees. They absorb water and nutrients from the vascular system of their hosts and have their own photosynthesis system. The water use of mistletoes can be influenced by the water status of their hosts besides abiotic environmental conditions. However, there is a lack of studies on the dynamics of mistletoe water utilization in relation to both biotic and abiotic factors.  

The water utilization of host trees and mistletoe varies with the microenvironment conditions of the canopy. Experiment is needed to understand the relations. 

The research group of Plant Physiological Ecology from Institute of Applied Ecology (IAE) of the Chinese Academy of Sciences took Changbai Mountain Forest Ecosystem Research Station as a platform.

By building a canopy platform at 20 m above the ground, they monitored the dynamic changes of sap flow of Viscumcoloratum (Kom.) Nakai (Loranthaceae) bysap flow system in combination with continuous measurements of microclimatic variables and volumetric water content (VWC) of its host tree branch xylem in the summer 2015.  

They found that the host tree VWC exhibited substantial fluctuations during sunny days but lower VWC of the host did not negatively affect the sap flow of V. coloratum.  

Hourly and daily mean transpiration rates (E_sap) of V. coloratum calculated from sap flow measurements showed strong positive correlations with photosynthetic photon flux density (PPFD) and vapor pressure deficit (VPD) measured in close vicinity to the point of mistletoe attachment. 

The mean E_sap of V. coloratum was substantially higher than that of their host during clear days (4.55 ± 0.54 vs 2.01 ± 0.15 kg m⁻² day⁻¹). Moreover, the mistletoe-to-host transpiration ratio was not constant but became increasingly larger with the increase of PPFD or VPD on both hourly and daily bases, suggesting a weaker control of water loss in the mistletoe in comparison to its host species.  

The strong dependence of mistletoe E_sap on micrometeorological variables and its decoupling from the host tree xylem water status suggests that the development of dense tree canopy functions as a potential mechanism for the host trees in reducing the competitive water use of mistletoes.  

These findings have important implications for the interactions between mistletoe species and their host trees in temperate forests. 

The results has been accepted by the journal Tree Physiology entitled "Microenvironment in the Canopy Rivals the Host Tree Water Status in Controlling Sap Flow of a Mistletoe Species in Tree Physiology". 

The work was supported by NNSFC, one hundred person project of CAS, (A) and Major projects of Ministry of science and technology.