COOPER, CLIFTON E.1*, SEAN C. THOMAS2, and WILLIAM E. WINNER1. 1Department of Botany and Plant Pathology, Oregon State University, Corvallis, OR 97331; 2Faculty of Forestry, University of Toronto, Ontario, Canada M5S 3B3. - Gas exchange behavior of old-growth conifer foliage in the Pacific Northwest.
The Wind River Canopy Crane Research Facility near Carson, Washington
provides access to canopy foliage of a 450-year old forest. We use the
crane facility to measure gas exchange parameters of foliage of the
dominant tree species, which include Pseudotsuga menziesii,
Tsuga heterophylla and Thuja plicata. We have done so
since 1996, quarterly, on multiple trees of each species, at upper and
lower canopy positions. We use an LI-6400 Portable Photosynthesis
System to maintain controlled environments around small, intact
branchlets of foliage, and to measure foliar rates of exchange of
CO2 and H2O gases with the atmosphere. We expose
foliage samples to saturating, ambient and zero light levels, and thus
determine rates of maximum and in situ photosynthesis, and dark
respiration. We also record stomatal conductances and transpiration
rates. We then harvest all samples, and by two techniques measure
areas of the foliage exposed to controlled environments: projected
silhouette areas of the intact branchlets, and aggregate one-sided
areas of the disarticulated leaves. We then dry and weigh the samples.
We may thus express gas exchange rates on the basis of branchlet
silhouette area, total leaf area and leaf mass. Results to date show
that Pseudotsuga menziesii maintains the highest photosynthetic
capacity year-round, followed by Tsuga heterophylla and
Thuja plicata. All respiration rates are depressed in winter,
as, to a lesser extent, are photosynthetic capacities. However,
climate is mild enough for the three dominant species to remain
photosynthetically active year-round. Year-to-year variation is
substantial, and may be influenced by climate. We have separately
measured total stand leaf area, and partitioned it according to
species and canopy height. We plan to combine these measurements with
our branchlet-level gas exchange values to produce stand-level
estimates of foliar carbon exchange.
Key words: foliage, photosynthesis, Pseudotsuga menziesii, respiration, Thuja plicata, Tsuga heterophylla