Abstract
Green alder (Alnus alnobetula) is currently the most expanding shrub species in the Alps. Because dense thickets impair tree establishment, understanding how climate affects shrub growth is essential for predictions of treeline dynamics. We evaluated ring width data from >50 A. alnobetula stems sampled at treeline on Mt. Patscherkofel (Central European Alps, Austria) to identify main climatic drivers and influence of climate warming on radial stem growth (RG). We also compared RG of A. alnobetula with RG of the co-occurring treeline conifer Swiss stone pine (Pinus cembra). We addressed our questions through calculation of response functions and evaluation of climate in years showing exceptional growth deviations. Response function analyses and evaluation of growth trends during 1991–2020 revealed that RG of A. alnobetula is significantly and directly related to summer temperatures. Precipitation in January also showed a direct relationship to RG, indicating effects of frost drought on RG. Surprisingly, nitrogen fixing A. alnobetula showed strikingly lower RG compared to P. cembra, and the latter also responded more strongly to the increase in summer temperature in the course of climate warming. We explain these findings by different carbon allocation strategies, i.e., preference of “vertical” stem growth in late successional P. cembra vs. favoring “horizontal” spread in the pioneer shrub A. alnobetula.
Highlights
Published: 11 March 2022In the European Alps global change, especially land abandonment and the decrease in grazing pressure in recent decades, initiated expansion of green alder (Alnus alnobetula (Ehrh.) K
This study revealed predominant influence of air temperature during summer on radial stem growth (RG) of A. alnobetula
If other abiotic and biotic factors are disregarded, ongoing climate warming will promote further spread of A. alnobetula across the alpine treeline ecotone as long as (i) adequate winter precipitation minimizes the effects of frost drought and (ii) the occurrence of shoot damage due to late frost events in a warming induced lengthened growing season (e.g., [79])
Summary
In the European Alps global change, especially land abandonment and the decrease in grazing pressure in recent decades, initiated expansion of green alder (Alnus alnobetula (Ehrh.) K. A. alnobetula is currently the most expanding shrub species in the Austrian, French, Italian and Swiss. In the Alps A. alnobetula is naturally restricted to avalanche slide path, screes and steep, north-facing slopes exhibiting high water availability [9–11], expansion into subalpine grasslands and well-drained areas [12] is enhanced by its clonal growth, high seed production and the symbiosis with N2 -fixing actinobacteria [13,14] and ectomycorrhizal fungi [15]. Expansion of A. alnobetula, which forms 2–4 m tall canopies, causes considerable environmental changes that have mostly a negative effect on the conservation of vascular plant diversity [4,5,16] and drive grasslands and meadows into N-saturated, species-poor shrubland [17]. The life span of A. alnobetula individuals is approximately 60 years (maximum age 110 years [18]), it may persist for centuries due to its ability of clonal growth by layering [19]
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