You’ve seen Witch’s Broom in your walks, but most people have no idea what it is, or why it is there, and they often just think it a bird or squirrel nest – and it could be. But where does it come from?
A mass of many causes
A Witch’s Broom is a growth of the tree itself. However, the branches and needles are dwarfed and distorted into a distinct, often dense, mass that is very different from normal growth. Brooms can have a variety of causes including infection by fungi or viruses or by other pathogens (some of which are spread by insects). It can also be induced by an injury, by mistletoe colonization, or may also be an apparently spontaneous genetic mutation occurring in the absence of any of the already-mentioned potential causes.
Common to all of these causes is that they introduce additional cytokinin, a plant hormone, to the affected part of the tree. Cytokinin and auxin, another important plant hormone, control plant growth and cell division. As the balance of cytokinin to auxin is disrupted, it causes almost uncontrollable shoot growth which results in a mass of sticks, needles and twigs all coming from a single point on the tree. This creates what looks like a nest, or the end of an old stick broom, hence the name.
Different tree, different broom
If you’re in K-Country in an area where you’re noticing a lot of brooms in spruce trees, you may be seeing the effects of Spruce Witch’s Broom Rust. This is a rust fungus with the scientific name of Chrysomyxa arctostaphyli, and as the common name suggests, it affects spruce trees (Picea). This particular type of Witch’s Broom is distinctive in that the needles of the broom are a yellow or orangey colour. Oddly enough, they are shed in fall to regrow in May or June. The small one to the right (with a glove for scale) is just starting to colour up. Later on, spore-producing pustules will form on the broom needles and release yellow or orange spores, along with a distinct sweet, earthy odor. These spores blow around in the wind to potentially infect other trees – and probably make people sneeze, too.
Brooms in pines
If you’re in a part of K-Country where you’re seeing a lot of brooms in lodgepole pines (Pinus contorta), you may be seeing the effects of colonization by Lodgepole Pine Dwarf-Mistletoe, Arceuthobium americanum. The plant itself (not the broom it induces) is pictured at right. Mistletoe is a flowering, parasitic plant. This is the only mistletoe in Alberta. It only grows on lodgepole pine and jack pine. It can take several years from the time a mistletoe seed lands on and sticks to a pine branch to when it actually forms a visible plant and then longer yet to when it develops enough to bloom – at least 7 years, according to a local study. When the plant itself does become noticeable, it consists of a cluster of short, yellowish or orangey, leafless stems emerging from the pine branch. These are tipped with clusters of tiny flowers without petals. Mistletoe is spread by the sticky seeds being forcibly ejected from the seed capsules and landing on surrounding pine branches. Much more noticeable than the plant itself are the brooms that it can also induce on affected lodgepole pines. Again, these brooms are distorted growths of the lodgepole pine (distinct from the mistletoe plant itself) but they are induced by the effects of mistletoe colonization.
A part of nature
Brooms often don’t appear to have severely adverse effects on the tree, and trees can have more than one broom. The tree to the right has 11 brooms in it, all highlighted in red in the photo. Brooms range in size from 1 foot across to much, much bigger. Some brooms can be upward of 2 meters in size. Brooms that have fungal or viral causes or are related to mistletoe colonization will often occur in clusters, with up to 50 broom-bearing trees in a small area. By contrast, brooms that result from random genetic mutations, unassociated with the various types of causes that have been discussed, are typically isolated finds in an otherwise normal population of that species.
Those relatively rare instances of genetic mutation unrelated to any disease or infestation are of interest to certain horticulturalists. In areas where it’s allowed, entire brooms or cuttings from them are sometimes collected, grafted to root stocks and cultivated. This tests whether genetically stable and desirable dwarf forms can be propagated from them.
But ecologically impactful
Brooms themselves are actually ecologically important here. Some insects such as moths or spiders use them as nests, and some animals nest in them as well. Northern flying squirrels often hollow out ones high up in trees to use as nests. Magpies have been know to do this as well.
Read about some of the other fascinating plants of Kananaskis Country here!