Are invasive mistletoes killing Ziziphus spina-christi?

Abstract

The once widespread and abundant Middle Eastern tree, Ziziphus spina-christi (Rhamnaceae—common name = Christ's thorn), has a new threat to its survival, the invasion and heavy parasitism (ca. 80% of trees) by the hemiparasitic mistletoe Plicosepalus acaciae. This mistletoe, which largely parasitizes Acacia tree species, has spread rapidly from its original range in the southern desert region of Israel into the rest of the country since the 1960s. The reason for the rapid invasion of this mistletoe (and increase in abundance in its native range) is purported to be the rapid increases in the populations of the main disperser of the mistletoe's fruit, the yellow-vented bulbul Pycnonotus xanthopygos. This bird breeds in human settlements, which have become substantially larger and more widespread in recent years. Because mistletoes are known to be profligate users of water, we hypothesized that the reason for the widespread senescence and mortality of Ziziphus spina-christi was the higher water stress placed on parasitized trees. We found no significant effect of mistletoe infestation on Z. spina-christi current water status. However, we found that the trees with higher levels of infestation have more dry branches, indicating that the water status of the Z. spina-christi in the longer term has been compromised by the mistletoe infestation. Moreover, infested trees had significantly lower fruit production than uninfested trees. In contrast, mistletoes produced fruit more successfully on trees where infestation was heavier. We conclude that mistletoe has a significant negative impact on Z. spina-christi and that control of its disperser, the yellow-vented bulbul, is the most effective means of limiting this problem.

Keywords:

• parasitism
• mistletoes
• water stress
• plant invasion