Mistletoe is a unique plant that grows high in tree branches due to a specialized parasitic relationship. It is classified as a hemi-parasite, meaning it produces its own food through photosynthesis but must steal water and mineral nutrients from a host tree to survive. This life cycle ensures that mistletoe is always found attached to the bark of another woody plant, never rooted in the ground.
How Mistletoe Seeds Travel
The process of tree infestation begins with the mistletoe’s fruit and the involvement of local bird populations. Mistletoe produces small, often brightly colored berries that are attractive to fruit-eating birds, such as the Phainopepla in North America or the Mistletoebird in Australia. These birds consume the berries, which contain a single seed surrounded by a sticky, gelatinous layer called viscin.
Viscin is a specialized, glue-like substance rich in glucose that surrounds the seed. Because the birds’ digestive tracts are relatively simple, the seeds pass through quickly and remain intact. When the bird defecates or wipes its beak, the sticky seed is deposited directly onto a tree branch, where the viscin quickly dries and firmly cements the seed to the host’s bark.
The Parasitic Connection to the Host Tree
Once securely adhered to the branch, the seed begins to germinate. The emerging structure, known as the hypocotyl, grows across the bark until it finds a suitable point of entry. This initial growth develops into a specialized, root-like structure called the haustorium, which is the physical and physiological bridge between the two plants.
The haustorium works by physically penetrating the host tree’s bark and then growing inward towards the vascular tissue. This penetration is a combination of mechanical force and enzymatic action that helps break down the tree’s outer layers. The mistletoe’s haustorium ultimately connects directly to the tree’s xylem, which is the host’s water-conducting tissue.
The parasite actively siphons water and dissolved mineral nutrients, such as nitrogen and potassium, directly from the host tree’s xylem stream. This connection allows the mistletoe to bypass the need for a soil-based root system. It uses these stolen resources to support its growth while still generating necessary sugars through its own green leaves.
Physiological Effects on Infected Trees
The constant siphoning of water and minerals by the mistletoe creates significant physiological stress on the host tree. Mistletoe is not part of the tree’s internal feedback loop, meaning it continues to demand resources even when the host is under drought conditions and attempting to conserve water. This unchecked water loss can leave the tree more vulnerable to drought and other environmental stressors.
Infected trees often exhibit reduced growth in height and diameter as the host diverts resources to the parasite. A common visible result of infestation is the formation of dense, unnatural growths of branches known as “witches’ brooms.” These brooms are a reaction by the tree to the parasitic infection, leading to a localized proliferation of shoots that further diverts the tree’s energy.
Heavy or long-term infestations cause branches to become swollen, weakened, and more susceptible to breaking, especially during high winds. The continuous deprivation of nutrients and water can lead to dieback in the crown of the tree, starting from the top and moving downward. This cumulative stress and structural damage can severely compromise the overall health of the tree and potentially lead to its death.