SEASONAL PHYSIOLOGY OF DOUGLAS-FIR SAPLINGS - RESPONSE TO MICROCLIMATE IN STANDS OF TANOAK OR PACIFIC MADRONE

Plant water potential, leaf conductance, and photosynthesis of sapling s of Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) were monitored monthly in 1986 through 1988. The objective was to describe influence s of microclimate (light, soil water, temperature, and relative humidi ty) associated with young stands of tanoak (Lithocarpus densiflorus [H ook. & Am.] Rehd.) or Pacific madrone (Arbutus menziesii Pursh) on Dou glas-fir physiology. Hardwood shade reduced photosynthetically active radiation to 20 to 38% of full sunlight; as a result, photosynthesis o f Douglas-fir shaded by hardwoods was light-limited throughout the yea r. Soil water potentials (30-cm depth) in summer (July through Septemb er) averaged -0.64 MPa in the presence of hardwoods and -0.23 MPa in t heir absence. Reduced soil water availability in the hardwood stand, c oupled with elevated air temperatures and lower relative humidities, r educed photosynthesis of associated Douglas-fir. Stomatae neared compl ete closure at the following plant water potentials: -2.3 (tanoak), -2 .5 (Douglas-fir), and -3.4 MPa (madrone). Although seasonal rates of p hotosynthesis did not vary greatly among the three species, specific w ater-use patterns were apparent. The abilities of species to minimize water stress were ranked as tanoak > Douglas-fir > madrone.