EPOW - Ecology Picture of the Week

Function and Fate of the Nurse Stump

Western Hemlock (Tsuga heterophylla), Family Pinaceae
Oregon, USA

Explanation:  What is happening to this stump?  In this sequence of photos, a high-cut stump, likely a western redcedar (Thuja plicata), is serving as the platform where a western hemlock has taken root, likely initially blown in as a seed and landing on the top of the stump.  

Over time, the hemlock has used the slowly-rotting stump as a platform for its root system, and perhaps a source of symbiotic fungi to feed its roots.  

Eventually, as in the photo to the right, above, the nurse stump has rotted out entirely, leaving just the stilt roots of the hemlock exposed, supporting the rest of the tree.  

Previously, we encountered forms of stilt-rooted trees that spread their roots to support themselves in uneven or marshy grounds, without use of a supporting platform.  

This is a key ecological function of stumps in some forests, that provide for purchase by regenerating trees, but only some trees, such as the hemlock here, do this.  

The relationship between hemlock and redcedar can be perhaps described as commensalism, as the hemlock benefits but the redcedar receives neither benefit nor harm because it is dead.  Or, perhaps the hemlock can also be described as a vascular plant saprophyte, as it lives on decaying or dead organic material, the redcedar stump, particularly if it uses the stump to derive nutrients; the rotting stump likely absorbs water and rain, so it might also provide water as well for the hemlock.  

And of course, structures other than stumps, such as termite mounds, logs, boulders, and other substrates, also can serve as nurse sites for many plants and trees.  It's a very interesting convergent function found in many parts of the world!  
  

Here, two mature hemlocks have used the stump of an old-growth tree
to establish themselves ... and additional seedling hemlocks have
also become established in the decaying wood of this nurse stump, as well.

  
  
Locations:  All photos in this episode were taken in Oregon, USA.  In the main sequence of photos at the top, the left photo was taken in Tryon Creek State Park in Portland; the middle photo from the Oregon Coast near Waldport; and the right photo from Eagle Fern in Clackamas County.  The above photo is also from Waldport area.