Monday, April 06, 2009

Nature Quiz of the Week 26

Setting: Last October, just east of Snoqualmie Pass.

1. What species of tree is this?

This is a red alder (Alnus rubra). Note the bright color of the inner bark that gives it its scientific name.

2. What happened to it?

The damage to the bark is typical of an ungulate rub. Because of the height on the tree (which is a little difficult to ascertain in this picture) I believe it was from an Elk (Cervus elaphus).

3. Approximately how old is the damage?

The damage was fairly recent. As the wound on the tree ages the color will dull significantly, also note the fresh bark that has fallen on top of the vegetation on the ground. There are several leaves on top of the fallen bark which suggests it probably hadn't occurred within the past few hours but it had definitely occurred that fall. I have not studied the relevant clues enough (e.g. how quickly the color dulls in the bark) to feel comfortable being too much more specific than that. But take a look at Dancingfrog's interesting analysis in the comments.

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Dancingfrog said...

This looks like bear rub on red alder less than 24 hours before the photo.

Jonathan said...

Hi Dancingfrog,

Thanks for posting your answers. Would you care to share why you came to those conclusions?

Dancingfrog said...

Naw, this isn’t bear. This is antler rub.

I interpreted the upper marks on the trunk as claw marks because at least some have thin pointed ends. The upper marks though appear to be antler stabs and slashes. I particularly note the curved nature of the upper marks and how they could reveal the radius of head movement of an animal indicating a long-neck. Looking closer I see that the tree is on a slope and the animal approached it from the lower side.

The trunks are stump sprouts in a logged site. I’m estimating that the tree is about 7 inches diameter-at-breast-height. A sprout having an already intact root system can grow pretty fast and therefore the site was probably originally disturbed maybe ~10 years before. The trunk is olive with horizontal lenticels and red inner bark characteristic of Alnus rubra, here the fast growing tree is without its customary white lichens of older red alders. As the tree attempts to heal over the injury, at the healing edge of the cambium a ridge will form. This ridge provides a concentrated rubbing point for an animal. It is also where the tree receives the most pressure when rubbing and it most susceptible for loosing bark and wood. So the chips on the ground are a combination of the slough bark/wood of the healing ridge and perhaps some minor antler, although most of the antler sites visible in the photo look older. The rub site has been used by the animal multiple times, but for less than a year, because the diameter of the intact wood of the bole is almost equal to the diameter of the outside (not many attempts by the tree to re-heal). There is small lateral rub spots on the side of the trunk suggesting processes or tine of the antler rubbing. And maybe an remote stab on the next bole to the right visible near the edge of the picture.

I notice in the background left that thinner saplings have additional damage. Like I’ve seen from moose slashing antlers back and forth. It’s more likely moose rub.

Two pieces of evidence suggested to me that the activity was recent. The inner bark of red alder oxidizes to the darker red color after exposure to the atmosphere. You can see that the boundary of cambium is still light indicating that there has not been enough time passed for the freshly exposed inner bark to oxidize. The second piece of evidence is on the ground. Bent and broken herbaceous plants have not withered or deteriorated at injury sites. Wood chips are atop herbaceous plants that had been growing all year and there is no modified, redirected growth toward the light by the affected plants.

If this were bear there would be much more trampling at the base of the tree. A larger ungulate would stand a greater distance from the tree while interacting with it, and therefore have less impact near its base.

Jonathan said...

Wow. I'm impressed with the detail of your observation.

When you say that the trunks are stump sprouts, do you mean they are shoots from a cut stump? How can you tell? I'm also curious how you can tell that it is a logged site (it is, though I suppose most places around here are...)

dancingfrog said...

Too me it looks like a logged site for these reasons:
1. The forest floor has an intact humus layer (not bare soil), with herbaceous plants characteristic of forest understories, yet…
2. All the trees in the photo are relative young. (Tree diameter does not necessarily correspond to age, but depends on species shade tolerance and origin of the growth (seed or vegetative growth from a stump sprout, for instance); and
3. There is open canopy in the background.

This combination of features could happen from a blowdown in a forest, but

4. There is no large diameter woody material on the ground.
5. The Alder in question has at least two shoots from appears to be a cut stump.
6. The stump appears to cut because the top has some profiles with straight edges. A live Alder would not break off at the base along straight lines.
7. The Alder stump was live at the time of the breakage because at least two very vigorous shoots grew after the breakage.
8. The shoots are vigorous because they are relatively large, yet the stump is hardly decayed.