Oak Mistletoe

(Phoradendron leucarpum)

Peter Kienzler

Introduction

Today most people associate mistletoe with awkward Christmas parties. Kissing under the mistletoe is a tradition which predates the Christmas holiday by thousands of years, and this is just one of a wide variety of ancient traditions involving this family of plants. Myths about these plants appear in Greek, Nordic, and Celtic traditions dating back before written history. When Europeans came to North America they brought their traditions with them but not the European variety mistletoe. Fortunately for European travelers, the American or oak mistletoe (Phoradendron leucarpum) was there to fill that role.

Oak mistletoe does not grow out of the ground. A parasite, an oak mistletoe's roots grow into the bark of trees like oak and maple, often growing high up in the branches. Rather than obtain water and nutrients for itself oak mistletoe steals these vital nutrients from the host tree. The genus name Pharodendron comes from the Greek and means "thief in the tree". Unlike the mistletoe's hosts, which drop their leaves in winter, mistletoe is evergreen. The mistletoe's ability to remain green and healthy even in the harshest winter as all the other plant-life withers around it contributes to the plant's long-held mystique.

Branches of oak mistletoe are woody and grow directly out of the host plant's branches or trunk. The leaves of oak mistletoe have a distinct thick, succulent like, appearance which differentiates it from most host plants. Photo by Peter Kienzler (2020).

Oak mistletoe forming a mass of branches and leaves on a host oak tree (Quercus sp.). Thick masses of mistletoe provide shelter for birds and other wildlife. Photo by Peter Kienzler (2020).

An unidentified ornamental plant on the University of North Carolina Wilmington campus hosting an oak mistletoe. Mistletoe sees higher survivorship in the canopy, but the plant is entirely dependent on birds for seed dispersal. This means that the plant has no control of where it may end up growing. Photo by Peter Kienzler (2020).

Phoradendron leucarpum

formerly P. flavescens and P. Serotinum

Oak Mistletoe, American Mistletoe, Eastern Mistletoe, False Mistletoe, or Hairy Mistletoe

Kingdom: Plantae, all plants

Phylum: Magnoliophyta, the angiosperms or flowering plants

Order: Santalales, hemiparasitic plants including sandalwoods and mistletoes

Family: Viscaceae, true mistletoes

Genus: Phoradendron, the new world mistletoes which parasitize deciduous plants

Species: P. leucarpum, oak mistletoe

Niche and Habitat

In technical terms oak mistletoe is an obligate hemiparasite of deciduous trees and shrubs. Hemiparasitic plants obtain nutrition partially from a host plant and is also capable of obtaining energy by photosynthesis. An obligate hemiparasite can only survive when attached to a particular host. In the case of oak mistletoe, the plant requires a deciduous host. Deciduous plants are a broad group of flowering plants which loose drop their leaves in periods on environmental stress. In the United States deciduous plants tend to loose their leaves during the cold winter months. Oak mistletoe has been documented growing on over 100 species of trees though it often shows preferences for certain species within its range. In North Carolina it is most often found growing on species of maple tree (Acer spp.). In North Carolina Oak mistletoe is most commonly seen the coastal plain and is rarely seen in mountainous areas in the western part of the state. Oak mistletoe grows best when exposed to full sunlight. The plant is not fire tolerant, and is rarely seen in healthy fire regulated habitats. Being poisonous few vertebrates consume the plant's leaves and branches. Several species of caterpillar consume mistletoe. The greater purple hairstreak butterfly caterpillar (Atlides halesus) feeds exclusively on oak mistletoe. White-tailed deer (Odocoileus virginianus) are known to consume oak mistletoe . Mistletoe berries are an valuable winter food source for many birds including cedar waxwings (Bombycilla cedrorum) and eastern bluebirds (Sialia sialis).

Range Data from USDA.gov. Map template generated by amcharts.com.

Evolutionary History

In general parasites have extremely complicated evolutionary histories as they are in constant close competition with the host plant. The seed of the plant is coated in an indigestible glue like substance called viscin, which is incredibly important adaptation. Unlike many other parasitic plants, oak mistletoe does not readily reproduce by fragmentation. It is entirely dependent on seed dispersal to reproduce. If a seed is not deposited on an acceptable host plant it will not survive. If the seed is deposited on an acceptable host plant, often on a high branch exposed to the elements, the seed must remain in place until the roots develop. The special sticky seed assures the plants survival by adhering the seed to the host until roots form.

The haustoria is one of the defining traits of mistletoes. A specialized root, haustoria allow parasitic plants like mistletoe to penetrate the bark of the host. After penetrating the bark the haustoria breaks down the cell-walls in the host's vascular systems. This allows for the free transmission of water and vital nutrients from the host plant to the parasite. The deeper the haustoria grows into the host, the greater the damage inflicted on the host.

Environmental Impacts

As a parasite oak mistletoe can have strong negative impacts on host trees. Infestation by the mistletoe's roots will disrupt both the xylem and the phloem. The xylem being the portion of the plant's vascular system which draws water and nutrients up from the roots. The phloem being the portion of the vascular system which carries sugars created via photosynthesis down from the leaves. Oak mistletoe infestation can result in the loss branches and increases likelihood of root rot. Death of the impacted tree is rare without the presence of additional sources of stress influencing the tree's health. Fortunately, oak mistletoe tends to occur in low densities limiting the impact the plant can have on forests and orchards. Oak mistletoe infesting one tree in a forested area has not been found to increase the likelihood of neighboring trees to become infested. This is due the oak mistletoe's seed dispersal method, which is not particularly efficient compared to other hemiparasitic plants.

Conservation

Oak mistletoe is a least concern species with a large native range and a high level of adaptability. It is considered a pest by orchardists and can create problems for people when a tree in the front yard becomes infested. Intolerant of fire, high rates of infestation in natural settings can be managed with therapeutic controlled burns. Some experts believe that high rates of mistletoe infestation in a forest may be a useful indicator of poor fire management. While oak mistletoe needs to be controlled in some settings, it also provides valuable ecological services. Mistletoe offers critical micro-habitat for a variety of mammals, birds, and insects, some of them threatened or endangered. In areas where these species are of concern, a healthy presence of mistletoe should be maintained.

Friends or Foes?

Oak Mistletoes are a literal drain on the maples, oaks, locusts, and other trees that host them. The presence of oak mistletoe does benefit under-story plants. Unlike the trees they infect, mistletoe does not reabsorb nutrients into the body of the plant before dropping its old leaves and extra branches. This advances the nutrient cycle by returning nutrients horded in the trees to the forest floor, where the nutrients can be consumed by under-story plants.

In late fall oak mistletoe produces pearly white berries which remain on the plant through the winter. This food source is highly valued by overwintering birds. Each berry contains the seed coated in a sticky indigestible chemical. In exchange for a winter snack the bird spreads the mistletoe seed with its leavings. Since the bird drops the seed randomly, many seeds never find an adequate host plant and do not grow. The bird can also carry the seed over very large distances. This allows mistletoe to disperse seeds without causing a destructive infestation in a single woodland. When a seed is deposited on a proper host the sticky chemical holds the seed to the tree until the roots can form.

Mature oak mistletoe form dense chaotic growths, providing nesting space for a variety of arboreal or tree living animals. These dense masses branches offer animals effective shelter from the elements and cover from predators. Southern flying squirrels (Glaucomys volans), owls, and a variety of song birds have been seen using mistletoe as nesting sights. Tree bats (Lasiurus spp.) are also known to roost inside witch brooms.

Does video of the organism exist?

Web Resources for Audience to Explore

12 Things to Know about Mistletoe- A series of fun facts from the National Wildlife Federation about mistletoes in America.
A Natural History of Mistletoe- A Scientific American article discussing the complicated interaction these plants have within there ecosystems.
Pucker Up! Why Do People Kiss Under the Mistletoe?- An article from Live Science about cultural and historical importance of mistletoes.
Mistletoe: A Merry Parasite- An article from the Alliance for the Chesapeake Bay. The article provides a solid breakdown of the unique adaptions seen in oak and dwarf mistletoe.
MISTLETOE - A PARASITE WITH PANACHE - Courtesy of Arborist Now, this web-page provides a thoughtful look at a plant many people find frustrating.

Resources

Calscape. (n.d.). Retrieved from California Native Plant Society: https://calscape.org/Phoradendron-leucarpum-(American-Mistletoe)?srchcr=sc5a51809028d37

Citizen's United to Protect the Maurice River. (2010). Plant profile: Phoradendrum leucarpum. Retrieved from Plants of Southern New Jersey: https://www.cumauriceriver.org/botany/Phle.html

Davis, R. (2020). Mistletoe: a merry parasite. Retrieved from Alliance for the Chesapeake Bay: https://www.allianceforthebay.org/2017/12/mistletoe-a-merry-parasite/

Dwarf Mistletoe. (2010). Retrieved from Colorado State Forest Service: https://csfs.colostate.edu/forest-management/common-forest-insects-diseases/dwarf-mistletoe/

Goughtery, A. V. (2013). Spatial distribution of eastern mistletoe phoradendron leucarpum, viscaceae in an urban environment. Journal of the Alabama Academy of Science, 84(3). Retrieved from https://go-gale-com.liblink.uncw.edu/ps/i.do?p=ITOF&u=wilm99594&id=GALE|A372028367&v=2.1&it=r&sid=summon

Great purple hairstreak. (2015, July 11). Retrieved from Carolina Nature: http://www.carolinanature.com/butterflies/gphairstreak.html

Johnston, M. C. (2015, July 11). Oak Mistletoe (pharodendron leucarpum. Retrieved from Carolina nature: https://www.carolinanature.com/trees/phle.html

Johnston, M. C. (2018, November 26). Lady Bird Johnson Wildflower Center. Retrieved from University of Texas Austin: https://www.wildflower.org/plants/result.php?id_plant=phle14

Johnston, M. C. (n.d.). Plants database: Phoradendron leucarpum. Retrieved from USDA Natural Resource Conservation Service: https://plants.usda.gov/core/profile?symbol=PHLE14

Norton, L. (2010, December 21). Pucker up! Why do people kiss under mistletoe. Retrieved from Live Science: https://www.livescience.com/32901-why-we-kiss-under-mistletoe.html

Phoradendron leucarpum. (n.d.). Retrieved from North Carolina State University Extension Gardener Plant Toolbox: https://plants.ces.ncsu.edu/plants/phoradendron-leucarpum/common-name/mistletoe-american/

Quinn-Davidson, L. (2018, February 22). Science Thursday: mysterious mistletoe. Retrieved from Fire Adapted Communities Learning Network: https://fireadaptednetwork.org/science-thursday-mysterious-mistletoe/

Sytsma, K. J. (n.d.). Santalales: plant order. Retrieved from Encyclopedia Brittannica: https://www.britannica.com/plant/dwarf-mistletoe

Waters, H. (2012, December 21). The Natural History of Mistletoe. Retrieved from Scientifc American: https://blogs.scientificamerican.com/culturing-science/mistletoe-natural-history/

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