Using dendrochronology to trace the impact of the hemiparasite Tristerix chodatianus on Andean Polylepis trees

The high Andean forests of the genus
Polylepis (Rosaceae) are threatened by extinction due
to anthropogenic effects such as timber extraction,
burning, and overgrazing. Some species are also
affected by Tristerix chodatianus (Loranthaceae), a
hemiparasitic plant which induces progressive host
damage. To understand this hemiparasitic process, we
evaluated the impact of T. chodatianus on growth and
the wood anatomy of Polylepis flavipila using growth
rings and digital image processing. We found that P.
flavipila has a xeromorphic wood anatomy and that the
ecological indices of mesomorphism and vulnerability
decrease at higher elevations to avoid embolisms and
vessel cavitation. Tristerix chodatianus causes
anatomical changes in the host wood, mainly in the
last 13 years measured. Alterations include a reduction
in the vessel density, and an increase in the vessel
diameter and Vulnerability Index, all of which are
more evident in downstream sections of parasitized
branches. These changes in the xylem tissues increase
the vulnerability of the parasitized branches to
embolisms and cavitation, thus leading to progressive
death of the tree crown. Our study confirms that the
analysis of the wood anatomy of parasitized branches
can be used to determine the years of colonization of a
tree branch by a hemiparasite and that it can be a useful
tool to monitor the phytosanitary state of parasitized
Polylepis trees.