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IT HAS has a rather wicked reputation. As well as encouraging illicit kissing
at Christmas, mistletoe is seen as a devastating tree parasite that often kills
its host. But at least one kind of mistletoe does its host some good.

“It’s been assumed that all mistletoes are bad and should be eradicated,”
says Ron van Ommeren from Northern Arizona University in Phoenix. The weedy
shrub’s roots burrow into the xylem of other plants to steal their water and
nutrients. Dwarf mistletoe in particular is renowned for devastating pine
trees.

But Ommeren and his team have found that the mistletoe Phoradendron
juniperinum, which preys on juniper, helps attract birds that spread the
tree’s seeds. Junipers only produce large numbers of berries once every few
years, when they attract huge flocks of birds. In the intervening years they
have a pitiful berry crop incapable of attracting birds at all.

By studying about 20 sites of juniper stands on the Colorado Plateau, Ommeren
found that mistletoe fruit helped attract seed-spreading birds in the low-berry
years. Over three years, twice as many juniper seedlings grew in areas affected
by mistletoe, he will report in an upcoming issue of the journal
Oecologia. And this mistletoe didn’t kill its host—it only stunted
some of the branches.