Self-Pruning Trees

June 18, 2006

By Clare Nunamaker

One characteristic of many young plantation forests, including those here in Scotland, is the overabundance of dead, low-hanging tree branches. But as the trees age in such stands, many of their dead limbs will, depending on environmental factors and the tree species, be self-pruned.

Self-pruning of trees varies both with the species and environment factors. Certain species, such as Ponderosa pine and Douglas-fir, are adept at self-pruning. Take a look at the trees in a mature or old-growth grove dominated by these species, and you'll see many trees with no limbs for quite a long way up the tree trunk.

Self-pruning is also a response to the environment. In open areas, for example, you'll notice that conifers often have live branches all the way down to the ground, or nearly so. When the tree is getting adequate sunlight, water, and nutrients, there's really no reason for it to self-prune.

But if the same tree were growing in even a moderately dense stand where it would compete with its neighbors for limited resources, you'd see self-pruning occur. Take a look and you'll find that the pruning-adept Douglas-fir tends to shed more branches in a dense forest but hangs onto more branches if open grown.

Researchers propose that self-pruning is an adaptation to fire. If a mature tree has shed its lower limbs, it is much more difficult for a low-intensity fire to climb into the canopy and damage the tree's foliage.

Knowing which trees tend to self-prune in shady conditions can be useful when making forest management decisions regarding fire risk. Pruning by hand takes a lot of time and effort, and favoring trees that can do some of that work for you can make a lot of sense.

Self-pruning trees are certainly not a panacea, however. It can take them many years in shaded conditions to begin to shed their lower limbs. And in high-risk areas, such as around buildings and alongside roads, people create openings that let in sunlight, making trees less likely to self-prune.

Still, as part of an overall strategy of reducing fire risk on yourproperty, it's worth remembering that not all trees are created equal.

You can now view Tree Tips on the web. Go to www.nwwg.org, click on Resources and click on Tree Tips.

You can contact Clare Nunamaker at forestry@nunamaker.com. Clare is a California Registered Professional Forester currently living near Edinburgh, Scotland.