Mistletoe is often recognized by its clustered green growth high in the canopy of trees, especially visible in winter after the host tree has shed its leaves. This persistent growth is physically and biologically connected to the tree. Mistletoe is a flowering plant that relies on other trees for survival. The relationship between mistletoe and its host directly influences the health and survival of the tree it attaches to.
Defining the Relationship: Hemiparasitism
Mistletoe is classified as an obligate hemiparasite, meaning it must live on another plant to survive, but it still produces some of its own food. The “hemi” designation indicates that the plant is photosynthetic, possessing green leaves that capture sunlight to produce sugars. Mistletoe handles its own carbon needs, unlike a full parasite.
Its reliance on the host tree is for water and dissolved mineral nutrients, which it extracts directly from the host’s vascular system. This gives mistletoe a significant advantage, particularly in environments where water is scarce. A mistletoe infestation can be especially stressful for a host tree during periods of drought because the mistletoe maintains a high rate of water use that the host tree cannot match.
The Mechanism of Connection: The Haustorium
The physical connection facilitating resource transfer is a specialized structure called the haustorium. This structure develops from the mistletoe seed after it germinates on the host branch. The haustorium penetrates the outer bark and grows inward, functioning like a modified root system.
The haustorium locates and integrates with the host’s water-transporting tissues, specifically connecting to the xylem. The xylem is the system of tubes responsible for moving water and minerals from the roots up to the canopy. This integration allows the mistletoe to intercept the flow of water and nutrients intended for the host’s own tissues, ensuring a steady supply of resources.
Consequences for the Host Tree
A mistletoe infestation can be detrimental to the host tree, especially when the infestation is heavy or the tree is already stressed. The constant siphoning of water and minerals leads to reduced growth and vigor. This resource depletion is magnified during dry periods because the mistletoe’s high water demand exacerbates water stress on the tree.
The tree often reacts to haustorium penetration with swelling at the point of attachment, which can compromise the branch’s structural integrity. The infection may also trigger the development of dense, abnormal masses of shoots known as “witches’ brooms.” These formations can weigh down limbs, increasing the risk of breakage, and a severe infestation can eventually lead to branch dieback or the death of the entire tree.
Spread and Life Cycle
Mistletoe relies primarily on birds for the dispersal of its seeds to new host trees. The plant produces berries consumed by various bird species. The seeds within these berries are coated in a sticky substance called viscin.
When birds excrete the seeds or wipe their beaks on a branch, the viscin adheres the seed firmly to the host tree’s bark. This sticky coating allows the seed to remain in place until it germinates. Once attached, the seed forms a haustorium to establish its parasitic connection and complete its life cycle.