Decoupling of Radial Growth Phenology From Temperature Constraints in the Clonal Shrub <i>Alnus alnobetula</i> at the Alpine Treeline

Green alder (<i>Alnus alnobetula</i> (Ehrh.) K. Koch) is a tall shrub that is widespread across the treeline ecotone in the Central European Alps. This shrub forms dense monospecific thickets and spreads rapidly through clonal propagation. Understanding its growth dynamics and climatic sensitivity is therefore essential for assessing its potential spread under changing environmental conditions. The study focused on determining the key dates of intra-annual radial stem growth (RG) within the treeline ecotone on Mt. Patscherkofel (1940-2150 m a.s.l., Tyrol, Austria) and the influence of climate variables on daily RG. RG was continuously recorded during 2022-2024 by dendrometers and modeled using the Gompertz function to determine key phenological dates of intra-annual RG, i.e., onset, end, duration, and time of maximum RG. Additionally, daily RG was extracted from dendrometer traces, and correlations (Spearman <i>ρ</i>) with environmental variables were performed. Bud burst was found to occur 28 ± 3 days before the onset of RG, indicating that carbon reserves are initially allocated toward leaf development. RG started and ceased at the end of June (doy [day of the year] 177 ± 7) and the end of August (doy 233 ± 9), respectively. The RG duration amounted to 56 ± 9 days and primarily occurred in July. Significant relationships (<i>p</i> < 0.001) found between temperature variables and daily RG at all study sites underscore the importance of temperature for RG. However, in all study years, the maximum RG (doy 192 ± 7) was already observed within <i>c</i>. 2 weeks of RG onset and prior to recorded temperature maxima. This study revealed that, despite daily RG of the clonal shrub <i>Alnus alnobetula</i> being temperature-dependent, its RG phenology is decoupled from prevailing temperature and differs from that of trees. This is most likely due to its deciduous nature and the preferential carbon allocation for clonal propagation, which promotes horizontal spreading within the treeline ecotone.