Growth rate overrides the benefit of extended growing season from boreal to semiarid conifers

: Climate warming lengthens the productive season in extratropical forests. Yet there is inconclusive evidence on whether this in turn elicits enhanced tree growth and forest carbon sequestration. In this study, we used cellular observations of conifer wood formation spanning boreal to semiarid forests to quantify the relative importance of cambial growth rate versus season length in driving annual cell production, the key process underlying woody growth. We found that rate is the primary driver of annual cell production. Furthermore, whereas the growing season lengthens with annual temperature, growth rate exhibited an optimum at ~6°C, beyond which increasingly dry conditions compromise wood production gains. The negative impact of warming on intra-annual growth dynamics can thus largely offset the benefit of longer growing season on future forest carbon storage.