Summer water use by mixed-age and young forest stands, Mattole River, northern California, U.S.A
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Resource managers have noted a decline in summer flow levels in the last decade in the Mattole River watershed, Humboldt County, California. Reduced river flows pose a threat to endangered coho and chinook salmon in the watershed, as stream heating is inversely proportional to discharge. While the cause of the reduced flow is unclear, several factors have been cited: increased groundwater pumping from residential development in the area, regional climate shifts tied to global warming, and the recovery of forest cover after widespread deforestation in the 1950s and 1960s. The goal of this project was to gain insight into the effect of stand age and composition on forest water consumption. Quantitative information on tree and stand level transpiration was collected in order to inform comprehensive hydrologic budgets being developed for the Mattole River watershed under existing conditions and resulting from prospective forest management activities. Granier thermal dissipation probes were inserted into 18 Douglas-fir (Pseudotsuga menziesii) trees in mixed and even-aged stands in order to record water use over the course of the 2008 summer dry season. Trees ranged in size from 10 to 91 cm diameter at breast height (DBH). A tight relationship was found between sapwood area ( cm2) and water use (liters/season, y = 7.68x – 638.6, r2 = .86). Strong positive relationships were also found between DBH (cm) and water use (y = 92.40x – 1068.4, r2 = .90), and for basal area ( cm2) and water use (y = 1.261x + 241.57, r2 = .94). The relationship between basal area and water use was much steeper for the youngest trees (y = 3.42x – 233.15. r2 = .76), indicating a steep increase in water use with increasing tree size at the lower end of the size range. This information was used to model stand level water use with the current composition and under future scenarios using Forest Vegetation Simulator. Results indicate that the water use of Mattole River forests will decline in coming decades as the high numbers of young (< 5 cm DBH) trees decline from canopy closure and stem suppression.