Quaking aspen
Quaking aspen. Photo: William W. Dunmire

Trees like alders, poplars, birches and willows are dubbed “pioneer species” in forestry. They earn that nickname for often being the first trees to colonize sites disturbed or damaged by landslides, fires, floods or clear cuts.

Pioneer species grow rapidly and establish new canopies faster than competing vegetation. They can mature at a young age and produce an abundance of seeds to help the species claim even more territory. In the case of aspens and other poplars, cut one down and you may soon find yourself in a thick forest of suckers springing up from roots of the cut tree.

These life strategies gives pioneer species a competitive edge in the fight for sunlight, water, soil nutrition and space to grow. Rapid growth, however, comes with a price.

In general, pioneer species tend to be weaker wooded, less decay resistant and shorter lived than other types of trees. As pioneer species age, they become increasing susceptible to branch breakage and stem decay.

You wouldn’t think this would be in the long term interest of the trees, but it’s all relative. A 60-year-old aspen is very much at the end of its lifespan. And those large cottonwoods topping out at 100+ feet tall? They’re a lot younger than they look.

A pioneer tree in decline is a sad sight to behold, but all that limb breakage and leaf loss serves a purpose.

The best and most nutritious soils on a disturbed site may have been scraped off, covered over, burned up or washed away. As pioneer species fall apart, they return organic material to the soil, create wildlife habitat, absorb and retain water, and prime the site for colonization by a greater diversity of other, more long-lived trees.

So what does this all mean for the urban forest?

For starters, pioneer species have their rightful place in the urban landscape. Many are found growing naturally in riparian areas or forest edges where they provide vital environmental benefits..

Secondly, it’s important to consider these species’ natural tendencies before planting them. Let’s face it, trees prone to storm damage, pests and diseases, and aggressive reproductive strategies aren’t going to be good choices for a downtown.

So here’s the deal: pioneer species are best reserved for natural areas or restoration sites. The more attractive ones, such as white birches, weeping willows and quaking (also known as ‘trembling’) aspens, should be planted sparingly as specimen trees in parks or people’s yards.

Understanding pioneer species is another fine example of why we should all strive to plant the right tree in the right place!