Mistletoe Mystery

Hemlock Dwarf Mistletoe (Arceuthobium tsugense)
Hemlock Dwarf Mistletoe (Arceuthobium tsugense) growing on Western Hemlock (Tsuga heterophylla).

I’ve been looking for Dwarf Mistletoe (Arceuthobium sp.) at Miracle Beach Provincial Park for years now – I know that it is there, growing on the Western Hemlock (Tsuga heterophylla). My wife Jocie has found it several times and has brought it back to the house for me to photograph but for one reason or another I’ve never managed to get around to taking a photograph of it.

This Monday I finally found some! We’ve had a couple of days of very strong winds and that always brings down trees and branches in the park. While out for a walk I spotted a large branch of Western Hemlock lying on the road with some strange growths on the wood. You may see large clumps of “witches broom” that are a sign that the mistletoe is growing on the tree but Mistletoe is generally so high up that you rarely get to see it up close.

Hemlock Dwarf Mistletoe (Arceuthobium tsugense)
Hemlock Dwarf Mistletoe (Arceuthobium tsugense) is parasitic on Western Hemlock.

So the next challenge is identifying it to species, notoriously difficult with mistletoes. Pojar and MacKinnon lists Western Dwarf Mistletoe (Arceuthobium campylopodum)as parasitic on a “wide range of conifers… [but] most commonly on western hemlock” so there is a good fit there. eFlora also lists A. campylopodum as a good candidate and the range map shows records of this species on Vancouver Island. The other two species listed in eFlora (A. douglasii and A. americanum) prefer different host trees (Dougla-fir and Pinus sp. respectively) and show no records on Vancouver Island. The simple key in Illustrated Flora of BC also tends to favour A. campylopodum in that the stems are over 2 mm thick and over 3 cm long.

So – Western Dwarf Mistletoe it is, right? Well, a little more research and a suggestion from Jamie Fenneman leads to the likelihood that this mistletoe is actually Hemlock Dwarf Mistletoe (Arceuthobium tsugense) which makes sense since it is parasitic on Western Hemlock – Jamie has identified his sample picture for A. campylopodum as A. tsugense in eFlora. Washington State University has a good overview on several mistletoes that is useful for identification. Again, the size and colour of A. tsugense, as well as its host tree, distinguish it from other mistletoes in British Columbia. However, Calflora states that A. tsugense is undergoing name changes and is now called A. campylopodum (!) so we’re back where we started!

Hemlock Dwarf Mistletoe (Arceuthobium tsugense)
Is it or isn't it? Hemlock Dwarf Mistletoe (Arceuthobium tsugense) or Western Dwarf Mistletoe (Arceuthobium campylopodium)?

This has turned out to be more complicated than I thought! I think that part of the problem is that these mistletoes are going through name changes and new species are being classified due to genetic research. At this point, I’m happy to call this mistletoe¬† Hemlock Dwarf Mistletoe (Arceuthobium tsugense) due to its association with Western Hemlock.

I love walking after wind storms, you never know what you’ll find and what you’ll learn. There’s lot going on up in the forest canopy!