Water stress and recovery dynamics of physiological function and growth in juvenile Pinus radiata.

Abstract

Forests worldwide face increasingly frequent, prolonged and severe droughts driven by climate change, causing widespread tree dieback and productivity losses. Yet predicting how trees recover their water-carbon balance and growth after non-lethal drought remains challenging. To address this, we subjected juvenile Pinus radiata D.Don-a drought-tolerant, strongly isohydric conifer-to moderate (14 weeks) and extended (20 weeks) dry-down periods. We continuously monitored stem radial growth and water reserves for over 6 months with high-resolution dendrometers, alongside weekly measurements of leaf gas exchange, capturing fine-scale dynamics of the water-carbon balance and growth during stress and recovery. We recorded a complete recovery of leaf function and stem growth in all plants after drought release, but the recovery rate depended on drought duration: plants under prolonged stress recovered more slowly. Stomatal conductance returned to control levels after $\sim$1.5 and 2.5 weeks following moderate and extended drought, respectively, whereas net $\mathrm{CO_{2}}$ assimilation recovered within $\sim$1.5 weeks regardless of drought duration. In contrast, cambial activity resumed rapidly, within a few days up to a week, as soon as stem water reserves were refilled. Growth recovery was rapid even in plants that experienced very low water potentials and nearly 2 months in a neutral or negative carbon balance state. Wood formation resumed ahead of photosynthesis recovery, reflecting a decoupling between carbon source and sink processes. Although drought reduced total radial growth, aboveground biomass gain in stressed plants remained comparable to that of well-watered controls, even for those with a growing period halved because of drought. This high degree of growth resilience arose through compensatory growth, with post-stress growth rates 1.4-2.4 times higher than pre-stress rates. These findings provide new insights into drought responses and recovery in a conifer with conservative water regulation and will improve model predictions of juvenile-tree resilience under future climates.