Mistletoe: The vampire of the forest

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When it's not the holiday posterplant, mistletoe spends its days sucking the life out of trees worldwide.

Mistletoe is basically a vampire - but one of those an anti-hero type vampires. Yes, I was surprised to learn that the same forest shrub that we love to smooch under every December is a parasite that spends its days sucking the "lifeforce" from trees round the globe. Out of roughly 1,400 species of mistletoe, most are hemiparasites , meaning they depend on host trees for minerals and water but still harvest energy from the sun in their leaves. Many view the plants as a pest, but that's starting to change.

"Even though they can be really hard on a tree, they can also be really important to wildlife," says David Shaw, a forest health specialist at Oregon State University. While stealing hard-earned resources from trees, the bushy brooms that mistletoe creates provide food and shelter to birds, bugs, and a few mammals. Recent research suggests this mix of thieving and generosity - they could be essential to the health and prosperity of an ecosystem.

Mistletoe's parasitism starts with poop and exploding berries. Mistletoe bushes clump on branches like something out of a Dr. Seuss book. Their parasitism is airborne. Birds eat their berries, which are coated in gluey material called viscin. The birds poop all over the forest, and thanks to the viscin, the mistletoe seeds in said poop stick to branches. Once firmly attached to the branch, mistletoe sprouts and drills down into the branch until it reaches the tree's veins. It sticks a haustorium (basically a straw) in and sips the tree's mineral and water cocktail.

Another group of mistletoes, dwarf mistletoes, does things a bit differently. In a dramatic twist on mistletoe reproduction, their seeds explode, literally. The blast zone can reach up to 15 feet. Seeds stick to saplings and wedge themselves into the tree's innards, infecting the entire tree, and sprouting sometimes years later. These guys are full parasites, taking sugar, water, and minerals from the tree. "Dwarf mistletoe is freaky, freaky, freaky stuff," says David Watson, an ecologist at Charles Sturt University in Australia. "Its [shoots] look like miniature asparagus."

Eventually, the mistletoe bush grows, blooms, and forms berries, and the cycle begins anew. Mistletoes infections can kill individual trees and stands of trees, and most mistletoe species attack specific tree species. Death by mistletoe is slow, though, and takes years, even decades. "Its' very slow, but its methodical," says Shaw. Against fire or an axe, mistletoe is defenseless, though. Unfortunately, that means suppressing fire or cutting down all the pretty uninfected trees can cause mistletoe outbreaks. "Mistletoe infections can be a symptom of larger problems," notes Shaw.

Generally, mistletoes won't overrun a forest, and taking mistletoe out of a forest could be a big mistake. In a recent study, Watson's lab removed mistletoe from swaths of eucalyptus forest in southern Australia, and three years later, those areas lost 26.5 percent of woodland bird species and 34.8 percent of their woodland residents overall.

Weirdly, this didn't seem to affect the birds feeding on mistletoe. That doesn't make any sense. Instead, most of the suffering species ate insects on the forest floor. As a parasite, mistletoe could care less about recycling resources, so its leaves are chock full of nutrients when they hit the ground - a buffet for bugs. "It's a bit like a dripping tap. It's constantly raining down high volumes of high-quality, nutrient-rich mulch," explains Watson. From the looks of it, mistletoe is a keystone species and plays a crucial role in that forest ecosystem.

While the intricacies of that role might not be exactly the same in every forest, it's starting to look like mistletoe is kind of a big deal in a lot of places. On Zimbabwe's savannas, mistletoe increases the number and diversity of leaf-eating bugs. Mistletoes on mesquite trees in central Mexico have been linked to a greater abundance of tropical bird species. Shaw's lab is now crowdsourcing information about which birds use mistletoe in western forests and how they use it. Preliminary data suggests that the plant might be pretty important for birds that eat berries in the winter like iconic Western bluebirds (though it's a bit soon to say for sure).

In Oregon, though dwarf mistletoe can do serious damage, it can also create habitats for endangered northern spotted owls by making trees gnarly and contorted, creating nooks and crannies for nests. "It fundamentally changes the architecture of forest canopies," says Watson.

While not every one is so optimistic about mistletoe in forests, it's clear that it and other parasites might be bigger players in ecosystems that we suspect. "They're these sneaky little buggers," says Watson. "The more we look at them the more we realize they really are calling the shots in many food webs."

Perhaps it's a parasite's world, and we just live in it.