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 (T_air): –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 T_air: –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<0.05). The results of this study revealed that (i) mature shoots of Alnus alnobetula can exhibit divergent RG responses to growing season cold spells, ranging from short-term suppression to complete cessation, and (ii) age-specific responses exist, as RG in young shoots did not cease after an experimental frost. The findings suggest that individual- and age-specific sensitivities of RG to growing season cold stress ensure persistence and facilitate establishment of Alnus alnobetula in the harsh and highly variable alpine treeline environment.

This research was funded in whole by the Austrian Science Fund (FWF; grant-DOI: 10.55776/P34706).