Growing-season cold spells and photoperiod are linked to radial growth cessation in the alpine treeline shrub Alnus alnobetula

Green alder (Alnus alnobetula) is a tall shrub that dominates the alpine treeline ecotone in the Central European Alps, yet its radial growth (RG) response to growing-season cold spells is unclear. We used dendrometers to quantify effects of cold spells on RG of mature shoots at the treeline (2,150 m a.s.l.) in a year with several such events, and conducted complementary climate-chamber experiments on juvenile shoots. We hypothesized that abrupt temperature drops to near 0°C or slight freezing would prematurely halt RG (H1), and that, due to photoperiodic constraints, late-season cold spells would be more effective in terminating RG (H2). At the treeline, an early-season event (DOY 207) caused only a temporary halt in RG, contradicting H1. A mid-season event (DOY 219) halted RG in two thirds of mature shoots, and the remainder ceased after a late-season event (DOY 241), supporting H2. These divergent RG responses among individuals likely reflect differences in physiological status, such as carbohydrate reserves. In climate-chambers, an early-season experimental frost (−2.2°C) reduced but did not halt RG in juvenile shoots, whereas a later-season cold spell halted RG. Thus, in A. alnobetula, RG resilience to cold spells is linked to photoperiod and possibly developmental stage.