EPOW - Ecology Picture of the Week

Forest Edge

Douglas-fir (Pseudotsuga menziesii) forest & clearcut
Cascade Mountains, Washington State, USA

Explanation:  Last week we explored how sea level rise can cause the demise of a coastal forest.  This week we are well inland ... in the Cascade Mountains of southern Washington state, USA ... and exploring a different kind of disturbance:  conditions on the edge of a forest stand.

The above photos -- both stitched panoramas -- are two views of this forest edge.  What created this edge is obvious; the private land to the right was very recently clear-cut logged.  

What results from this kind of large patch cutting? 

Look at the top photo, above.  It is a striking illustration of how such an abrupt edge can alter microclimate, with deep shade and protection in the forest interior to the far left and full sunlight and exposure to the far right.  I recorded a temperature difference of some 8 degrees between the forest interior and the open clear-cut.  In ecology, this is called a "high-contrast edge."

This has great implications for the kinds of plants that can grow (or regrow), and the insects and vertebrate animals that can occupy each of these kinds of habitats.  Many wildlife species closely associated with forest cover, including Northern Spotted Owls, generally will not occur in the clear-cut space for most of their life function such as breeding and nesting.  

Now look at the bottom photo.  It illustrates that, along this kind of high-contrast edge, you typically find a great deal of woody debris on the forest floor ... and it piles up because of forestry practices that remove the merchantable timber but that leave the "slash" behind ... and also because of "blow-down" that occurs from wind, ice, and storms that hammer the edge of the forest.

In fact, the trees left along the edge were ones that grew up in the protection of their neighbors in the dense forest ... but once exposed, they are now vulnerable to wind breakage because the trees are not "wind-firm."  Watch the following brief time-lapse movie I made of how these trees flex and bend even in low wind ... (in stronger winds, especially under a snow or ice load, these trees may break or blow down, causing the edge to "creep" further into the forest):