Conclusion
Our results demonstrate that pine mistletoes are fully carbon-autonomous: they do not provide carbon to the host and are also not supplied with carbon compounds by the host. We also observed that mistletoes are constrained in their photosynthesis by the host when soil water availability is low, most likely due to competition for xylem water – when the competition is released by removing the host needles, the 13C assimilation of the mistletoe increases. This result provides physiological evidences that mistletoes do increase the drought stress of their hosts, resulting in an increased mortality risk during severely dry periods previously proposed. We, therefore, conclude that the hemiparasites live on their own in terms of carbon gain which, however, depends on the water provided by the host tree.