Forest Canopies
  

The Pacific Northwest of the United States has natural stands of old-growth forest many over 500 years old.  The Pseudotsuga-Tsuga forest type is generally thought to regenerate after fire when Douglas fir (Pseudotsuga menziesii) invades and provides the forest cover.   Douglas fir is long-lived and becomes dominant in the upper canopy of the forest.  Douglas fir is generally unable to set seed in the highly organic ground floor that develops as the forest grows but other species, particularly western hemlock (Tsuga heterophylla), western red cedar (Thuja plicata) and true firs such as grand fir (Abies grandis) colonize beneath the Douglas fir canopy.  Generally theses species, with the exception of western red cedar, are short lived compared with Douglas-fir but they are able to regenerate continuously where Douglas-fir does not.  We are interested in the processes that maintain old-growth forest structure and also in how important aspects of forest structure that enhance biodiversity can be re-created in younger forests that have been commercially managed.

We study two questions about old-growth forest.  (1) How can Douglas-fir survive for much longer than many other conifer species?  Our research has focussed on the way that foliated shoots reiterate on the branches and even from the trunk of Douglas-fir.  Other long-lived species such as Sequoia sempervirens and Sequoiadendron giganteum also have extensive foliage reiteration from branches and trunks and our research is focussing on the physiological processes affected by this reiteration.  (2) What affects seedling growth of the trees that grow beneath the Douglas-fir canopy?  We are particularly interested in the balance required for shady conditions that may allow seedlings to develop without being out competed by ground vegetation, and patches of sunlight that may enable seedlings to grow into large trees.

We are working with colleagues Charlie Halpern, Doug Sprugel and Bob Edmonds in designing an experiment to test possible silvicultural options for application in the City of Seattle Cedar River Watershed, the major supplier of water to the city.  The watershed is forested and most of the forest was clear cut between 50 and 30 years ago and either naturally seeded or was planted.  The forest that has grown is now quite uniform and the managers of the watershed wish to increase biodivesity.  The question is how to treat the present forest - whether to thin, and if so how, in clumps or to thin from below in a uniform way.