Signs of frost drought in stem diameter variations

Frost drought refers to the chronic or acute desiccation of trees exposed to high evaporative pressures while being rooted in cold or frozen soils. This phenomenon has been known for more than a century but is still poorly characterized. Summer desiccation manifests itself as long-term stem contractions. Similar contractions have been reported in winter. In this study, we investigated the causes of total winter stem contraction (WSC) using 14 years of dendrometer data from evergreen ( P.abies ) and deciduous ( L.decidua ) mature trees growing along an elevational transect (from 800 to 2200 m asl) in the Swiss Alps. Results indicated that WSC varied between 30 μ m and 1478 μ m and were strongly dependent on species, elevation, and tree height. Moreover, the magnitude of contractions was strongly associated with stem contractions subsequent to freeze–thaw events ( Δ F). We suggest that both Δ F and WSC are the consequences of water losses due to ice blockage associated frost drought, occurring when the distal parts of the tree are thawed and transpiring, while the larger basal parts remain frozen, thus inhibiting water uptake and creating a hydraulic imbalance. • Winter stem contraction (WSC) is associated with freeze–thaw cycles. • Freeze–thaw cycles contractions ( Δ F) are linked with evaporative conditions. • Ice blockage frost drought is likely the main driver of Δ F and therefore WSC.