Seasonal Physiology of Douglas-fir Saplings: Response to Microclimate in Stands of Tanoak or Pacific Madrone

Abstract

Abstract Plant water potential, leaf conductance, and photosynthesis of saplings of Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) were monitored monthly in 1986 through 1988. The objective was to describe influences of microclimate (light, soil water, temperature, and relative humidity) associated with young stands of tanoak (Lithocarpus densiflorus [Hook. & Arn.] Rehd.) or Pacific madrone (Arbutus menziesii Pursh) on Douglas-fir physiology. Hardwood shade reduced photosynthetically active radiation to 20 to 38% of full sunlight; as a result, photosynthesis of Douglas-fir shaded by hardwoods was light-limited throughout the year. Soil water potentials (30-cm depth) in summer (July through September) averaged -0.64 MPa in the presence of hardwoods and - 0.23 MPa in their absence. Reduced soil water availability in the hardwood stand, coupled with elevated air temperatures and lower relative humidities, reduced photosynthesis of associated Douglas-fir. Stomatae neared complete closure at the following plant water potentials: -2.3 (tanoak), -2.5 (Douglas-fir), and -3.4 MPa (madrone). Although seasonal rates of photosynthesis did not vary greatly among the three species, specific water-use patterns were apparent. The abilities of species to minimize water stress were ranked as tanoak > Douglas-fir > madrone. For. Sci. 40(1):59-82.