Measurement and prediction of water consumption by Douglas-fir, Northern California, USA

Douglas-fir (Pseudotsuga menziesii) is an important component of mixed-conifer forests of the Western United States. Changes in forest composition from Douglas-fir encroachment in areas of fire-suppression could have implications for water balance, supply and stream habitats. Increasing episodes of severe drought linked to climate change threaten forest health. Measurements of tree water inform management and biosphere models. Twenty-six trees were instrumented with thermal dissipation sapflow probes. Sites were in Petrolia, California, and Maple Creek, California. Monitoring took place from Summer 2015 through Summer 2016. Daily water use was found to decline steeply over each summer dry period. This may limit the influence of Douglas-fir on soil moisture budgets and streamflow during this time period. Tree size, peak and low flow daily water use averages were statistically related. Little difference was found between drier 2015 and wetter 2016 years. However, larger trees showed greater peak water use in 2015, perhaps from sufficient moisture and greater insolation, and at Petrolia, larger trees showed greater dry period water use in 2016, as greater moisture levels persisted into the summer. Linear mixed effects models of daily integral sap velocity (cm/day) were created with the input variables of solar radiation, vapour pressure deficit (VPD) and soil moisture. The best model using Akaike information criterion scores had fixed effects of solar radiation and VPD. This model was trained on one site and then validated at the second site with goodness of fit tests. The model is provided for estimating Douglas-fir water use.