Large conifer trees in the Pacific Northwest, USA (PNW) use stored water to extend photosynthesis, both diurnally and seasonally. This is particularly important during the summer drought, which is characteristic of the region. In the PNW, climate change is predicted to result in hotter, drier summers and warmer, wetter winters with decreased snowpack by mid-century. Understanding seasonal bole water dynamics in relation to climate factors will enhance our ability to determine the vulnerability of forests to climate change. Seasonal patterns of bole water content in old-growth Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) trees were studied in the Cascade Mountains of western Oregon, USA. Relative water content (RWC) was monitored hourly in three 400+ and three ~150 years-old trees using permanently mounted dielectric devices for 10 years. RWC increased during the late spring and early summer to maximum levels in August then decreased into fall and remained low over winter. The difference between minimum RWC in the winter and maximum in mid-summer averaged 4.5 and 2.3% for the older and younger trees, respectively, across all years. RWC closely followed growth and was positively correlated with air and soil temperature, vapor pressure deficit and photosynthetically active radiation, but lagged plant available soil water. The progressive decrease in RWC seen each year from mid-summer through fall was attributed to net daily loss of water during the summer drought. The marked increase in RWC observed from spring to mid-summer each year was hypothesized to be the period of embolism repair and water recharge in elastic tissues. We conclude that bole water content is an integral part of tree water dynamics enabling trees to extend carbon assimilation into drought periods and during periods when cold soil inhibits water uptake by roots, an adaptation that could benefit the survival of large PNW trees under climate change.
Keywords: Pseudotsuga menziesii; basal area increment; bole water content; dielectric measurement; relative water content; soil moisture; summer drought.