Effects of environmental variables on intra-annual dynamics of radial growth of green alder (Alnus alnobetula) along an elevational transect

Green alder (Alnus alnobetula (Ehrh.) K. Koch) is a tall, multi-stemmed deciduous shrub widespread in the Central European Alps across the treeline ecotone. The focus of this study was (i) to determine key dates of intra-annual radial stem growth (RG), and (ii) to assess environmental factor most closely related to daily RG along an elevational transect. For this purpose, RG was continuously recorded by dendrometers mounted on shoots (n=16–20 dendrometer records per year) at three study plots on Mt. Patscherkofel (1940–2150 m asl) during the growing seasons 2022–2024, and in potted saplings (n=3) at 600 m asl (Botanical garden in Innsbruck, Tyrol, Austria). The Gompertz function was applied to determine phenological dates of intra-annual RG dynamics, i.e., onset, end, duration and time of maximum RG. Daily radial stem increments were extracted from dendrometer traces, and Spearman correlations (ρ) with environmental variables were determined. Results revealed that within the treeline ecotone RG started and ceased around end of June (doy 177±7) and end of August (233±9), respectively. The mean growth duration was found to be 56 ± 9 days. Within the treeline ecotone mild temperatures in May and cold spells during the growing period led to an earlier start and end of RG, respectively. The time of maximum RG was observed in early July (doy 192±8), with about 60 % of the annual increment developing during this month. Although RG in the valley already started in mid-May (doy 134±2) and lasted until mid-October (doy 286±8; duration: 153±7 days), time of maximum RG in 2024 was reached at a similar point in time as within the treeline ecotone (doy 194±13). The environmental factor most closely related to intra-annual RG was soil temperature along the entire elevational transect (ρ=0.371–0.419; P<0.01). Air and cambial temperature were less closely related to RG at all study plots. Precipitation constrained RG at the forest line (ρ=0.549, P<0.001) and at the south-facing treeline site (ρ=0.426, P<0.001). Soil water content and vapour pressure deficit of the air had no significant influence on RG. Results of this study revealed a high degree of plasticity in RG of A. alnobetula, with the RG period spanning two and five months within the treeline ecotone and in the valley, respectively. The greater significance of soil temperature compared to air and cambial temperature for RG is most likely attributable to a non-linear relationship between RG and temperature.

This research was funded by the Austrian Science Fund (FWF), P34706-B.