What is the Role of Mistletoe in Our Environment?

Phoradendron californicum
Desert mistletoe in an ironwood tree

Kendall Kroesen, Habitats Program Manager

Desert mistletoe (Phoradendron californicum) is a native, parasitic plant that grows on several species of Sonoran Desert trees and shrubs. Tucson Audubon recommends leaving it in trees because it is such a useful resource for birds and other wildlife, providing food, shelter and sometimes nest-building locations. In particular, it is a key resource for phainopeplas.

Scientists we consult generally support this view, saying the mistletoe does not do significant harm to its host trees. However, more generally in the Tucson region there is a strong bias against it. People believe that as a parasite, it must be killing trees.

While we have not seen definitive studies on our species of mistletoe, we believe that this native species has co-evolved with the native trees that host it. There is no scarcity of these trees so it is clear they have not been eliminated by parasitic plants. We also notice many trees that seem to tolerate infestation well, and other trees that have died with no mistletoe infestations.

Now a new study out of Australia suggests the important role mistletoe can play in its ecosystem. Researchers wondered, as some have suggested, if the concept of "keystone species," can be applied to mistletoe. Keystone species are ones that play a key ecological role in their environment, disproportionate to their abundance.

Desert mistletoe berries are eaten by several birds
To study this, they studied forest areas where they removed all the mistletoe, and compared them to control areas where mistletoe remained and others where there was naturally no mistletoe. They studied all the areas before and after removal of mistletoe from the experimental plots.

Briefly here are their results. "Three years after mistletoe removal, treatment woodlands lost, on average, 20.9 per cent of their  total species richness, 26.5 per cent of woodland-dependent bird species and 34.8 per cent of  their woodland- dependent residents, compared with moderate increases in control sites and no  significant changes in mistletoe-free sites."

They suggest that, in this case, the mistletoe is providing nutrient enrichment of the forest floor via litter-fall "promoting species richness, driving small-scale heterogeneity in productivity and food availability for woodland animals." These results seem to support the designation of mistletoe as a keystone species.

Tucson Audubon hopes to generate a civil conversation about mistletoe in our region, what effect it may be having on our landscape trees and what effect removing it may be having on our environment, particularly on birds.

The article is published in the Proceedings of the Royal Society, Biological Sciences, September 2012, volume 279, number 1743. To see the journal article in full, see http://rspb.royalsocietypublishing.org/content/279/1743/3853.full.pdf+html.