Mistletoe in Oregon: Forest Robber or Robin Hood?

Mistletoes, or “tree thieves” as they are known to some, are a diverse group of parasitic flowering plants that range across much of the globe, including Corvallis—just look up as you drive to OSU from downtown, and you’ll see the telltale round, fluffy plants in the canopies of our local trees. Often regarded as a pest by foresters—or a useful ploy to hopeful lovers—these plants use host trees as a surrogate root system, hijacking precious water and nutrients. This can cause a reduction in growth and in seed production, and, in cases of severe infection, premature mortality for hosts.

Less well known, however, is that mistletoes are widely considered by biologists to be a keystone species whose presence often affords surprising benefits to a wide range of forest inhabitants.

In a recently published study, David Watson, an ecologist at Charles Sturt University in Albury, New South Wales, and his colleagues removed more than 40 tons of mistletoe from Australian woodlands to monitor the effects on local wildlife. What they found after three years was that more than one-third of all woodland-dependent species in the study sites disappeared along with the mistletoe. The effect was particularly strong for birds, with the number of species present reduced by more than one-quarter.

Scientists have known for years that mistletoe provides important habitat for many species of birds and even some mammals. A 2006 analysis found that of the 330 species of tree nesting birds in Australia, 245 utilize mistletoe for nesting sites—that’s 74 percent! One of the more surprising findings of Dr. Watson’s study, however, was the indirect impact of mistletoe removal on ground-dwelling species.

Although untested in this study, Dr. Watson’s explanation for this effect is related to soil enrichment caused by mistletoe. A typical plant reabsorbs the nutrients bound within its leaves before casting them to the forest floor in order to conserve resources. In the case of mistletoe, however, which derives nutrients from host trees, resources are less limited and its shed leaves tend to have a higher nutritional content. This influx of nutrients nourishes soil organisms at the base of the food web and gradually spreads throughout the forest community.

Here in the Pacific Northwest a native species called dwarf mistletoe plays a complex role in the health of coniferous forests. Dwarf mistletoes alter the structure of forest canopies, may reduce the abundance of beneficial fungi associated with infected trees, and can increase the risk of high severity wildfires. On the other hand, a study conducted in southwestern Oregon found that 90 percent of the federally endangered northern spotted owls’ nesting sites occurred within mistletoe infested limbs known as “witches brooms.”

And so the question remains: Is mistletoe the robber or the Robin Hood of the forest?

“It’s both.” says Dave Shaw, OSU’s resident forest health expert. “The robber takes from those that depend on individual tree growth. Mistletoes become Robin Hood once you connect them to the forest ecosystem in general.”

Historically, science has focused on a very simplified view of nature which requires that the various components be separated from their context and scrutinized in a laboratory. In isolation the effects of any relationship can seem skewed. In contrast, ecological studies that try to capture the complexity of interactions on a larger scale—say, the entire forest community—can yield new and unexpected results.

In this case, as Dr. Shaw explains, “Mistletoes are an example of where you can have too much of a good thing. Where possible, it is nice to consider the benefits of mistletoe, but when necessary, management can be good.”

As we find time and again within the natural world, the key to successful management lies in a balanced approach.

by Mike Vernon