Mistletoe-ing around the tree reduces growth in forests

It might seem romantic, but the last thing a tree wants is an embrace under mistletoe.
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Mistletoe may be rooted in festive legends of fertility and life-giving powers, but it is a more sinister plant than we’d expect from the legends: mistletoe is a parasite. More specifically, it is a hemiparasite that can grow and photosynthesize on its own, or parasitize another plant. As a parasite, mistletoe grows on trees, tapping into the vascular system of the host tree to steal water and carbohydrates. In fact, because mistletoe is directly tapped into the hydraulic system of its host, it rarely regulates its own water loss! Mistletoe is also spreading rapidly throughout Germany, and could have negative implications for forestry.

Viscum album subsp abietis
Viscum album subsp abietis.

So what effect does mistletoe have on host trees, and how does it affect forest stands? In a recent article in Tree Physiology, Chris Kollas and colleagues modelled and measured the effects of mistletoe on tree- and forest stand-level growth in Scots pine (Pinus sylvestris). They used a forest growth model called FORESEE (FORESt Ecosystems in a Changing Environment), which simulates the carbon and water cycles in forests. They found that simulated mistletoe infections reduced forest growth by almost 30%, matching field measurements of the impact of mistletoe on forest growth. They attributed the reduction in growth of infected trees to both the water and carbon being lost to the mistletoe.

What are the implications of these results? Mistletoe can have a large impact on forestry in Europe, and the mistletoe component of the model developed by the authors can be used to predict future impacts of mistletoe on timber volume. This will allow foresters and other ecosystem managers to prepare and plan for the continued spread of mistletoe. Furthermore, since mistletoes increase water stress on trees, and drought events will become more frequent with climate change, the modeling developed by Kollas and colleagues could be used to predict the consequences of the spread of mistletoe on drought stress in trees.

Reference

Kollas, C., Gutsch, M., Hommel, R., Lasch-Born, P., & Suckow, F. (2017). Mistletoe-induced growth reductions at the forest stand scale. Tree Physiology, 1–10. https://doi.org/10.1093/treephys/tpx150


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