Connecting gas exchange, carbohydrate storage, and growth dynamics during variable drought stress and recovery in Douglas fir

Tree mortality from increasing drought events have been reported worldwide, with the potential to threaten both forest health and services such as timber production. Tree species resilience to drought events is dependent on a range of inter-connected mechanisms, such as gas exchange and hydraulic regulation, which further affect processes such as non-structural carbohydrate (NSC) storage, growth, and stress signaling. The effect of drought on these numerous plant processes have been well studied, however knowledge gaps remain regarding how these processes respond in concert to different drought intensities, as well as the recovery ability following stress release. In a greenhouse setting, we investigated how variable drought intensities (control, mild, severe) affected photosynthetic assimilation (Anet), NSC storage, and growth at multiple timepoints throughout a five-week drought and five-week recovery period in two-year-old Douglas fir (Pseudotsuga menziesii). Here, we show how system-wide physiological stress tracks drought intensity, with severe drought leading to an earlier cessation of Anet, greater utilization of starch to increase soluble sugars, and reduced growth as compared to the mild and control treatments. This study offers a holistic view of plant-physiological responses to drought stress intensity, and underscores how physiological damage sustained during severe drought events can have long-lasting impacts on forest health.