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