Differential responses of sap flow rates in Pseudotsuga menziesii, Thuja plicata, and Tsuga heterophylla to canopy position, seasonal variability, and climate

Background/Question/Methods

Water use in mature forests is typically dominated by canopy position, with the largest trees transpiring the majority of water.  Douglas-fir (Pseudotsuga menziesii), western redcedar (Thuja plicata), and western hemlock (Tsuga heterophylla) are dominant tree species on mesic Pacific Northwest sites. Water use of Douglas-fir has been used to describe stand-level water use; however, water use of mixed stand is less understood.  Sap flow measurements indicate photosynthetic activity, and differences in flow can help infer diurnal and seasonal patterns in C fixation. This study aims to measure diurnal and seasonal water use among three conifers at various canopy positions. In July 2010, Douglas-fir (1 large dominant, 1 co-dominant), western hemlock (1 large dominant, 1 intermediate), and western redcedar (1 large dominant, 1 co-dominant, 1 intermediate, 1 suppressed) were instrumented with sap flow modules, dendrometer bands, and soil moisture probes. Measurements were taken at half-hour intervals from July 2010 through February 2012. Local air and soil temperature, solar radiation, relative humidity, and wind speed were measured. We compared continuous measurements of climatological and soil conditions with daily sap flow patterns of the target trees to determine general differences in sap flow rates between species.

Results/Conclusions

During July 2010, western redcedar had greater total daily sap flow than western hemlock or Douglas-fir. Douglas-fir sap flow exceeded that of western hemlock and western redcedar on cool, cloudy days, while redcedar sap flow rates were highest on warm, sunny days. Western hemlock had the lowest daily sap flow of the three species, although it maintained some level of nighttime sap flow, perhaps indicating recovery of bole-stored water or night-time transpiration. Daily patterns also varied, with Douglas-fir and western hemlock flow beginning earlier, dipping midday, presumably due to stomatal closure, and increasing again late afternoon. In contrast, western redcedar began transpiration later but maintained high levels throughout the day. Western redcedar showed a stronger drought response than Douglas-fir, experiencing a gradual decline in daily sap flow during drought periods. During January 2011, western hemlock consistently had the greatest sap flow, indicating it was the most photosynthetically active in the winter. Douglas-fir demonstrated little winter activity, while western redcedar showed increasing levels of sap flow when temperatures were high for multiple days. Seasonal and daily differences in sap flow of the three species describe a more complicated water use pattern than studies focusing only on the response of dominant Douglas-fir.