Release-induced changes in needle morphology and physiology, and crown-level photosynthesis of Norway spruce in a drained peatland forest

The capacity of suppressed trees to respond to increased resource availability following selection harvest remains debated. Increased light can promote sun-type needle development and, together with greater water and nutrient availability, may enhance photosynthetic capacity. However, photosynthetic rate may not increase as much due to increased vapor pressure deficit. We investigated how selection harvesting affects needle morphology and crown photosynthesis of previously suppressed Picea abies trees in a drained peatland. We assessed whether released trees develop sun-acclimated foliage and how physiological acclimation influences carbon assimilation. Leaf gas exchange measurements from upper and lower crown positions were used to parameterize the pyAPES model, which was applied to estimate and integrate crown level carbon assimilation. Under the experimental light conditions (230 µmol m⁻² s⁻¹), Vcmax and Jmax were lower in released trees than in control trees. However, released trees exhibited higher total crown assimilation than control trees. Total assimilation was approximately 4.0-fold higher in the upper crown and 6-fold higher in the lower crown of released trees, resulting in a 4.6-fold increase at the whole-crown level six years after harvesting. Overall, the findings demonstrate that leaf-level physiological responses alone may not capture stand-level carbon dynamics following forest management interventions.