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

Abstract

Green alder (Alnus alnobetula (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 ρ) 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 (p < 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 c. 2 weeks of RG onset and prior to recorded temperature maxima. This study revealed that, despite daily RG of the clonal shrub Alnus alnobetula 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.

Keywords: alpine treeline; bud burst; climate‐growth relationship; dendrometer; green alder; radial increment.

FIGURE 1

Spatial distribution of the study sites on Mt. Patscherkofel (FL = site close to the forestline, TR‐N = north‐facing treeline site, TR‐S = south‐east‐facing treeline site). Source: TirisMaps Land Tirol (https://maps.tirol.gv.at/synserver?user=guest&project=tmap_master; accessed on 20 August 2025).

FIGURE 2

(a–i) Climate variables, soil water content and soil temperature recorded during the period 2022–2024 at study sites within the treeline ecotone. Mean daily soil water content and daily precipitation sum (a–c). Mean daily air temperature (solid lines) and soil temperature (dotted lines; d–f). Mean daily vapor pressure deficit (VPD; dotted lines) and relative air humidity (RH; solid lines; g–i). Study sites are denoted by black, red and blue lines for the forestline, and the south‐east and north‐facing treeline site, respectively.

FIGURE 3

(a–f) Time series of stem radial variations (daily means±standard deviations) modeled by applying the Gompertz function (a–c). Extracted and modeled daily radial increment are depicted in (d–f). Arrows indicate time of bud burst at the study sites, which are denoted by black, red and blue lines/arrows for the forestline, and the south‐ and north‐facing treeline site, respectively.

FIGURE 4

(a, b) Mean monthly radial stem growth within the treeline ecotone during all three study years (a), and mean radial stem growth during 2022–2024 at all study sites (b). Bars indicate standard deviations.