Abstract
Dwarf shrubs are a key functional group of the high-elevation vegetation belt. Despite their ecological relevance and high sensitivity to environmental changes, the hydraulic adaptations and species-specific variations in drought tolerance within this growth form are mostly unknown. Here, we assess the xylem vulnerability to cavitation of important character species of the Alpine dwarf shrub heaths in Central Europe. Due to the high percentage of nonfunctional xylem areas in these species, vulnerability curves were gained by an innovative staining approach with safranin, validated by hydraulic and xylem anatomical measurements. The loss of 50% conducting xylem area appeared in the range of -1.78 to -2.91 MPa. Midday plant water potential during an intense summer drought remained above these critical thresholds but was correlated with cavitation resistance. No trade-off between hydraulic safety and efficiency was detected across analyzed species. We conclude that the characteristic clustered occurrence of species in the heterogeneous mountain terrain (mainly interpreted as a consequence of varying snow cover dynamics) is also reflected in species-specific adjustments in xylem hydraulics. The interspecific variety in vulnerability thresholds and plant water potentials during summer drought indicates different hydraulic strategies and adjustments in water relations between these co-occurring shrubs.
Highlights
Dwarf shrubs are an important functional group of high mountain vegetation, and several essential character species of subalpine and alpine plant communities belong to this specific growth form (Körner, 2003)
Note: Parameters Ψ12, Ψ50 and Ψ88 correspond to the water potential at 12%, 50%, and 88% loss of conductivity/loss of conductive area, respectively, and CI 2.5% and CI 97.5% indicate the confidence interval for each parameter; n is the number of pooled samples per vulnerability curve
A direct comparison of the vulnerability curve based on the proportion of stained xylem with the curve gained by classical hydraulic measurements proved that the staining method revealed reliable results
Summary
Dwarf shrubs are an important functional group of high mountain vegetation, and several essential character species of subalpine and alpine plant communities belong to this specific growth form (Körner, 2003). These water shortages are especially prevalent in skeletal soils with thin humus layers and can amplify the effects of the overall extreme physical environment on mountain plants (Körner, 2003) These harsh environmental conditions include low temperature, persisting snow cover, intensive radiation and high wind load, and apply to the characteristic dwarf shrub heaths in the Central Alps (Cernusca, 1976). Dominant species of these plant communities mainly belong to the family Ericaceae, which show clonal reproduction and form extensive networks of subterranean rhizomes. Six dwarf shrub species characteristic of the subalpine zone of the Central European Alps (Arctostaphylos uva-ursi (L.) Spreng., K. procumbens (L.) Gift & Kron, Calluna vulgaris (L.) Hull, Erica carnea L. [all evergreen], and V. myrtillus L., and Vaccinium gaultherioides Bigelow [deciduous]) were selected and analyzed by hydraulic measurements, staining experiments, and Ψ measurements during a persistent summer drought
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