Sunday, February 18, 2007

Under the Mistletoe

Along with the Christmas holly, laurel, rosemary, yews, boxwood bushes and, of course, the Christmas tree, mistletoe is an evergreen displayed during the Christmas season and symbolic of the eventual rebirth of vegetation that will occur in spring. But perhaps more than any other of the Christmas evergreens, it is a plant of which we are conscious only during the holidays. One day we're kissing under the mistletoe, and next day we've forgotten all about it

When the Christmas decorations come down, mistletoe fades from our minds for another year, receding into the mists of mythology, rituals and enigma. Particularly in regions where the plant is not native (or is rare), most people do not even realize that mistletoe does not grow on the ground, but rather on trees as a parasitic shrub.
So writes David Beaulieu over at About Landscaping.

Of course, in the chaparral and other oak-growing regions of the Arizona hills, most of the mistletoe (and, believe me, there is a lot) isn't really obvious because both it and the oaks are evergreens. In our special spring live oak-leaf-shedding season, only the mistletoe keeps its leaves. Then you get a good picture of just how much of a tree is oak and how much is guest (above and below.)

Mistletoe is a semi-parasite; those evergreen leaves are busy converting atmospheric CO2 into cellular hydrocarbons to make more leaves, flowers and berries. However, the plant depends upon its host for both water and the soil minerals dissolved in the sap.

Christmas aside, in the popular imagination, mistletoe is not just a pest, but a tree killer to be destroyed. My ecologist Sson wonders about how many years mistletoe really cuts off an infested tree's life. As he points out in ecology lingo, "it isn't a good survivial strategy to kill the host." The top pictures are of the parasites in emory oaks with trunks in the 9" to 1' diameter range, which represents many years' growth. The scrub oak in my yard was fully grown 20 years ago when we moved here; I don't see any signs that the plants or their parasites are failing. On the other hand, major drought just might have a big impact, with the two plants fighting for what little water is available.

Note the contrast in color and shape of the oak leaves and the mistletoe. Also very visible in the picture below -- the special knot formed by roots of the guest plant. (It was always my speculation that the reason the Celtic Druids were so hung up on mistletoe was that the oak knot made a fine cudgel. Wrong, of course. The experts cite the rareity of oak mistletoe in Europe, as well as the magic of its evergreen leaves on apple and other barren deciduous trees in mid-winter.) Just FYI, the fuzzy brown ball on the oak branch below is another parasite -- this one, an insect larva. Oaks host many interesting flora and fauna!

The other mistletoe I've seen in the Prescott area is a leafless plant that infests ponderosa pines; a host pine resembles trees covered with mosses in wetter climes. A different variety grows on mesquites, ironwoods and other acacias down at lower desert elevations. One of my favorite birds, the phainopepla, follows the mistletoe, from desert to uplands in season, to enjoy the little white berries as they ripen.

Of course, when the oak (or a branch) dies, the mistletoe is doomed, as the bones above attest.


Desert Cat said...

The trees in the lower desert that are capable of shedding branches in response to drought (mesquites and palo verdes) sometimes appear to shed branches specifically to rid themselves of heavy mistletoe infestations.

Perhaps it is the same mechanism. Perhaps the response is "hey I'm a little overextended here--let's drop this demanding branch", and the mistletoe is doomed.

Prescottstyle said...

Hello Granny J. What I would like to know is how mistletoe starts? Does it come in by air, Is it passed from tree to tree by birds? It's everywhere, so it must have a common thread.
I love reading Walking Prescott, because I do just that, walk Prescott, alot! I see a lot of your subjects in my daily jaunts downtown to my office.

I'll be posting some of my new stuff soon. I've been shooting the local scenery lately with my view camera, once I get the digital scans back from the lab in Tempe I can post some. Tally ho!

Granny J said...

Clever trees, those acacias, Mr. Cat (any relation to my comic favorite, Bucky Katt?)

Basically, Mr. Style, the answer is "bird poop". BTW, you put me to shame with talk of a view camera. Here I am thinking of a fancier point-and-shoot with a decent zoom lens so that I can catch a raven or two with enough pixels to really see. But I am looking forward to your new pictures.

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