EPOW - Ecology Picture of the Week

Holiday Snow With a Lesson

Forest Clearcuts
Cascade Mountains, Washington, USA

Explanation:  Happy holidays, everyone!  For those of us that live in "snow country," enjoy some sledding, skiing, snowball-throwing, and some beautiful snowy scenery where you are.

But ... take a look at this holiday's photos.  Why is the snow so blotchy?  

We are flying over the western slope of the Cascade Mountains in Washington state, USA.  Below us is a mix of private and public lands ... with number clearcuts and logging roads carved out of the thick forest cover.  

Clearcuts and related forest openings change the local climate.  Without a dense overstory cover of tree canopies, snow adheres on these open patches when the rest of the landscape is snow-free.  Snow, however, can disappear faster from clearcuts than from under the forest canopy.  During summer, it is typically hotter in clearcuts than under the forest canopy.  Overall, soil and air temperature is far more variable in clearcuts than under the forest canopy.  

Large commercial forestry clearcuts, with associated
logging roads, invite snow to linger and tend to
change the local climate pattern.

This has implications for many ecological processes.

In one study, Holmes and Zak (1999) found that clearcutting hardwood forests led to a significant loss of nitrogen due to leaching.  But the form and fate of nitrogen from clearcutting seems to vary by type of forest ecosystem (Grenon et al. 2004).

Snow has a high albedo, or reflectance.  Snow on a clearcut reflects much of the winter sun's energy, whereas the darker forest canopy absorbs much of the heat. This likely leads to a "positive feedback effect" in clearcuts, resulting in cooler temperatures and lingering snow.  In turn, this results in an increase in water yield from clearcuts, but this can vary by topography, elevation, and forest type, too.

Not a timber clearcut per se, this straight line of snow across a landscape in British Columbia, Canada, is a powerline corridor, kept tree-free.   As evident in this photo, powerline corridors and openings also show similar temperature patterns as do clearcuts.

Greater snow depth in clearcuts might impede movement of wildlife and can hide critical forage on the forest floor.  But during summer, a flush of grass, forbs, and shrubs are well known to attract deer and other ungulates.  

Other wildlife, such as marten and spotted owls tend to avoid clearcuts or use them in far less proportion than their availability.  

And the visual aspects of clearcuts is yet another topic.   

So as we change the structure and climate of patches of forest, the implications are mixed.  Whether they are friend ... or foe ... depends on what is desired -- timber, wildlife habitat, snowmelt runoff, or a snowy scene.
 

Forest clearcut in the Klamath Mountains of
northwestern California.  Makes for cold field work!

Information:
   Grenon, F., R.L. Bradley, and B.D. Titus.  Temperature sensitivity of mineral N transformation rates, and heterotrophic nitrification: possible factors controlling the post-disturbance mineral N flush in forest floors.  Soil Biology and Biochemistry 36(9):1465-1474.
   Holmes, W.E., and D.R. Zak.  1999.  Soil microbial control of nitrogen loss following clear-cut harvest in northern hardwood ecosystems.  Ecological Applications 9(1):202-215.
   Steventon, J.D., and J.T. Major.  1982.  Marten use of habitat in a commercially clear-cut forest.  Journal of Wildlife Management 46(1):175-182.