Drought-induced delays in stem hydraulic development shape gas exchange and growth recovery in Douglas fir

The limiting factors of tree recovery from drought, particularly the coordination between carbon sources and sinks, remain poorly understood. In this study, juvenile Douglas fir (Pseudotsuga menziesii) were exposed to 28 d of mild or severe drought, followed by 35 d of recovery. We continuously monitored CO₂ and H₂O fluxes in shoots and roots to derive gas exchange and carbon accumulation, while measuring basal area to estimate stem growth and sapwood development. To identify underlying mechanisms of drought recovery, we periodically measured nonstructural carbohydrates (NSC), midday water potential (Ψmd), and foliar abscisic acid (ABA). We found no evidence that ABA or Ψmd limited gas exchange recovery, with stomatal conductance recovery instead related to drought-induced reductions in sapwood development. While carbon accumulation ultimately recovered to control levels following mild stress, severe stress caused persistent impairments, ultimately reducing carbon accumulation by 51%, with stem growth similarly affected. We found no evidence of growth being limited by NSC, which remained abundant. However, we suggest that drought-induced limitations to stem development govern this pattern. This became clear when considering the diurnal growth cycle, where daytime growth was largely absent in trees after exposure to severe drought despite accounting for up to 30% of total growth in control trees. Daytime growth appeared to depend on sufficient sapwood area, which likely buffered xylem tension to support growth conditions. Our findings suggest drought-induced reductions of stem hydraulic development constrain the recovery of gas exchange and growth. Further, altered diurnal growth patterns may explain prolonged productivity declines in forests following drought.