Differential radial stem growth responses of Alnus alnobetula to short-term cold stress

Green alder (Alnus alnobetula) is a tall, deciduous shrub widespread across the treeline ecotone in the Central European Alps. This study evaluated the impact of growing-season cold stress on radial stem growth (RG) under field conditions and in a controlled environment. RG was recorded by dendrometers on mature shoots (c. 20 yr old; n=18) at the treeline (2140–2150 m asl) on Mt. Patscherkofel during 2023, when several short-term cold spells occurred during the growing season (minimum air temperature (Tair): –1.2°C). In addition, 3–4 yr old saplings (n=5) were exposed to a 2-day cold spell in a climate chamber (minimum Tair: –2.3°C). We hypothesized that a slight frost during the growing season would transiently suppress RG, potentially resulting in a bimodal growth pattern. Contrary to this expectation, two-thirds of mature shoots in the field ceased RG after a mid-season cold spell (day of the year (DOY) 219), and one-third ceased after a late-season cold spell (DOY 241). In contrast, young shoots exposed to an experimental cold spell showed a decline–but no halt–in RG, resulting in significantly lower RG than in controls (P