Mistletoes are the largest group of aerial parasitic plants, with over 1600 species worldwide. They survive by feeding off the sap of their host tree, and are adept at mimicking the appearance of that tree. Because they are long-lived – they can survive over 30 years – it can take decades to notice the damage they are inflicting on their host.

Currently the scientific community is divided over whether mistletoe is a menace or a valuable part of the ecosystem. Anne Griebel from Western Sydney University in Australia and her colleagues have analysed some of the positive and negative effects of mistletoe infection, and how climate change might alter the current balance.

When mistletoe establishes itself on a branch, it immediately sets to work stealing carbon, nutrients and water from its host. As long as there is plenty of water, both tree and mistletoe can survive for quite some time. The extra water drawn up by the mistletoe even increases evaporative cooling. Meanwhile, birds and insects benefit from the berries as a food source and the dense mistletoe brooms as an ideal habitat.

"From a biodiversity perspective mistletoe is debated as a keystone species, and the upsides clearly outweigh the loss of carbon storage," said Griebel.

But if water becomes scarce, particularly if there is a prolonged drought, the picture isn't so rosy. The host tree closes its stomata to prevent further water loss, which may eventually starve the tree to death or cause irreparable fractures across the tree's water transport network, which can also kill the tree. For a sickly tree like this, mistletoe decreases evaporative cooling, and with an overall high infection rate within a forest the ecosystem eventually transitions into a positive feedback loop that enhances warming.

Already the northern range of mistletoe distribution is increasing as climate warms. Recent studies have shown a rise in mistletoe infection in Australia, the US and in many parts of Europe, including the Swiss Alps, which are seeing increased mistletoe-related mortality.

"Since both mistletoe presence, and longer and more intense droughts are increasing in the near future, we expect a further increase in mortality rates of mistletoe-infected stands," said Griebel, whose work is published in Environmental Research Letters (ERL) .

Clearly we need to keep an eye on mistletoe as our climate changes, and potentially intervene to avoid large-scale forest die-back in some locations. Scientists are developing remote sensing methods to monitor mistletoe infections, but it is early days and for the time being fieldwork is the only truly reliable way.

As to whether you should seek out some mistletoe for your Christmas festivities, it all depends on whether mistletoe is thriving where you live, and whether your sympathies lie with the tree, or the birds and insects who make mistletoe their home.

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