The Different Timing of Exposure to Drought Stress Differentially Affects Phenology and Growth in Goat Willow

Periods of drought can substantially alter the phenology of forest tree species, influencing growth dynamics as well as thetiming of budburst and leaf senescence. This study investigated whether the plant response varies depending on whenduring the growing season drought stress occurs.Female individual of goat willow ( Salix caprea L.) was propagated by hardwood cuttings, resulting in 151 ramets assigned toeight experimental groups. Seven groups were subjected to drought treatments at different intervals between early Marchand mid-July 2021, while the eighth served as a well-watered control. The same treatment schedule was repeated in 2022with increased drought intensity. Each plant remained under drought conditions until visible stress symptoms appeared,and the total mass (container included) decreased to about half of its initial value, after which plants were rehydrated andirrigated regularly for the rest of the season. Control plants were watered continuously.During autumn 2021, the onset of leaf senescence in the first treatment group was delayed by four days compared with thecontrol. By the following spring (2022), this group flushed about five days earlier than the control, whereas groups 6 and 7flushed approximately two days later. In autumn 2022, all drought-treated groups showed delayed leaf senescence, mostnotably those exposed later in the season—by up to 30 days on average compared to the control.These results indicate that the timing of drought stress strongly modulates leaf phenology in Salix caprea , exerting bothbeneficial and adverse effects on height growth and flower development.