Sunday, March 15, 2009

This old tree.

As one who in recent years has read many, many children’s books, I am aware of a literary convention. Everyone from Pooh and Piglet to Nicholas the Bunny lives in a hollow tree. While it is true that many animals find shelter in trees, rarely do the fictional homes as illustrated in such books resemble a real home for wildlife, known as a wildlife tree.

A real wildlife tree is far from a stately Keebleresque oak, with its dense crown of leafy branches. Rather, it is weathered, dead or dying thing — in short, a wreck. However, these old veterans are a fundamental component of a healthy forest. In southwestern British Columbia, there are at least 56 animal species known to rely on wildlife trees for homes, roosts, or foraging sites, including 42 species of bird, ten species of bat, and other mammals ranging in size from mouse to black bear.

And this points to an important feature — size. A good wildlife tree is large. It should have lofty perches for raptors, and below, crevices and hollows for cavity-nesting species. Its bark should be thick, preferably with loose sheets beneath which bats can roost, butterflies can hibernate, and insect-eating species can hunt. The more holes and hollows the better. Internal rot is desirable, because it makes excavation of nests easier. An indication of rot can be conks or bracket fungi on the exterior, so the more fungally-adorned and lumpy, the more likely a tree will be useful to wildlife.

Excavation, the actual hollowing out, is done by important species known as primary cavity excavators. Woodpeckers are prime examples, but smaller birds such as chickadees and nuthatches also create holes. Many other species, known as secondary cavity users, adopt pre-made holes. These include squirrels, small owls, swallows and several species of duck. Other species are called open nesters. These build nests on branches, or even on the ragged top of a trunk that has been broken by wind.

Even left alone to happily rot, wildlife trees have finite lifespans, and will eventually disintegrate to the point where relatively few animals can use them. In a natural forest, other old growth trees living among them will take their places. It is now recognized that forest management practices should include the identification and preservation of wildlife trees, plus their potential replacements. Additional surrounding trees should also be retained, for an unsheltered tree is more likely to be taken down in the next big wind.

Non-forest wildlife trees, as might been found in a city park or along a country roadside, are often felled either because they are perceived as hazardous, or for firewood. The loss of each is significant, because it may mean the removal of the last homes and feeding sites for many species. Enlightened urban planning now includes the preservation of such trees when possible.

A rotting old tree, with its naked branches, sloughing bark, and fungal infestations is something to be valued, not taken for granted. When Owl’s house blew down, and due to a misunderstanding he was awarded Piglet’s home, Christopher Robin said, “if your own house is blown down, you must go somewhere else, mustn’t you, Piglet? What would you do, if your house was blown down?”

Fortunately for Piglet, Pooh took him in.


Wanderin' Weeta said...

"...with loose sheets beneath which bats can roost..."

I didn't know that. I always think, "caves" or "church steeples". I guess I got my info from kids' books, too.

Good post!

Janet said...

We have a lovely dead tree across the creek from our house. So far it has stood through Hurricane Isabel and multiple nor'easters. It is great to get the binoculars and see who is using some of the hollowed openings.

Kim and Victoria said...

Great post.
I notice the hummingbirds who visit our garden prefer dead or empty branches to rest upon.

Barefootheart said...

And boys. Boys live in hollow trees. See My Side of the Mountain by Jean George.
Nice post.

KaHolly said...

Excellent post today. I wish I had your gift of words. My students and I were discussing this very topic just the other day. Well, I was, they sort of listened. I can add a few more facts to my "dialog"!! Thanks for the info.

Hugh said...

Weeta, Note also there are differences btw roosting sites and hibernating sites and nursery sites. Any given bat species can have a number of different requirements related to these behaviours.

Janet, sounds nice. It's nice to have a biological focal point to watch the year roll by.

K & V, Hummers like to sit in the open to keep an eye on things--especially on each other.

Barefootheart, I had forgotten about that book. I vaguely remember the movie made from it.

KaHolly, Thanks. I barely scratched the surface. A good book is Wildlife Trees in British Columbia by Fenger et al. (five authors). It discusses the trees and fauna of this region in particular, but the ecological principles are broadly relevant.