ENHS Nature Trails February 2004 Article 2

President's Column: "What our recent snow storm teaches us about why our dominant forests are coniferous"
by David Wagner

The New Year's day snow storms had a dramatic impact on many parts of our territory. It shut down many roads and kept folks close to home. The most dramatic impact must have been felt by those people who lost electricity after the wet, heavy snow caused branches to fall on power lines. The effects of this were reported by Reida Kimmel in the last issue of Nature Trails. Shut off from power and travel, it caused isolated people to feel their isolation and call up their self reliance. Eventually, the branches brought down by that storm will be gathered up to be stacked close by to build fires to keep warm when the next storm hits. The affliction generates the cure.

In the last issue of Tree Time, the newsletter of the Mount Pisgah Arboretum, the arboretum's caretaker and site manager, Tom LoCascio reported the damage caused by the snowstorm in one of our favorite parks. He observed that the majority of the damage occurred to the deciduous trees, the oaks, maples, and Oregon ash. Another term for these deciduous trees is hardwoods. However, their hard wood seems to make them more vulnerable to damage than the soft wood of the Douglas fir and true fir. Tom's comments made me think again about how our climate has shaped the character of our forests and woodlands. It is a lesson taught to me over many years. Early in my botanical career, I hosted one of my major mentors on a trip into northern Idaho. His home territory was in the forests of eastern U.S., rich in diversity of hardwoods. As we looked over the dense sea of greencaps from a high point, he said to me, "What a paleozoic landscape!" His exclamation was based on the observation that, although in a temperate zone, like his home state of Michigan, what dominated the hillsides in Idaho were cone bearing trees, trees whose close ancestors dominated the earth millions of years before the hardwoods arrived on the scene. Conifers were on the hillsides before dinosaurs rose up out of the swamps. Moving from swamps to arctic ridges in paleozoic times, conifers evolved a structure for later ages. These trees of venerable lineage dominate the forests of Oregon too. Douglas fir and hemlock, western red cedar and silver fir, white pine and spruce: these are the trees of our mountains. Why don't we have our ridges covered with maples and birches, oaks and sweet gums, sycamores and ashes? The snow has part of the answer.

The conifer trees like Douglas fir and spruce have adapted to snow falling on their branches. They have a slender and steep profile, so that when the snow falls on them, it sloughs off as the branches tip gracefully downwards. There is a single, central trunk with a sharp point held stiffly upwards, surrounded by small, radiating branches at the top, and gradually longer, downward pointing branches arranged below. That's the physical structure that accepts snowy seasons. The oaks and ash and maples have a branch structure that angles upwards and outwards. It's great for a big load of broad leaves to catch the sun's rays in spring and summer. But it also catches a big load of snow in the off season. When the snow load comes too fast and heavy, the branches can't handle it and break. We live in an area where the snow falls often enough to suppress dominance of trees with spreading branches. The differential vulnerability of these spreading, hardwood trees was demonstrated in the recent snow fall. Even more vulnerable are broadleaf evergreen trees, like madrone and tanoak. These trees are presently abundant only to the south of us where heavy snowfall in low mountains is much less frequent.

This is the physical part of why conifers like Douglas fir dominate our forests. There's a physiological aspect that is likewise tied to our climate. That part of the story I'll tell next month.

[ Back ]

Page last modified: 21 March 2004
Location: http://biology.uoregon.edu/enhs/archive/feb04/feb042.html
E-mail the WebSpinner: cpapke@gmail.com