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Leaf Conductance during the Final Season of a Senescing Aspen Branch

United States Department of Agriculture Forest Service, Rocky Mountain Forest and Range Experiment Station, Fort Collins, Colorado 80526

Leaf conductance, transpiration, and environmental conditions were measured on two aspen (Populus tremuloides Michx.) branches in a natural stand, using an automatic cuvette system. Fortuitously, leaves on one branch senesced about 10 days early, allowing comparison between a senescing branch and a normal branch. Terminal bud development was retarded on the senescent branch, and a portion of the branch eventually abscised about 20 centimeters from the end. Roughly 1% to 2% of the other branches on the study tree and adjacent trees of that clone also senesced and were dead the following spring.

Although no visual symptoms of senescence were observed until September, stomatal behavior was atypical shortly after leaves were fully expanded. During July and August, leaf conductances under full sunlight were higher on the branch which senesced than on the branch which was normal, reaching values greater than 1.0 centimeters per second, and conductance was highly variable.