Enhanced understanding of water cycling processes of dwarf shrubs using high-precision lysimeters and climate manipulations

Throughout European mountains, changes in livestock production systems since the 1950s have resulted in the gradual segregation between more accessible, flatter, and productive grasslands with intensified fodder production, and more remote, steeper, and less productive meadows used for extensive grazing, and some abandoned. After cessation of grazing in subalpine grasslands, secondary succession promotes the gradual colonization of species and functionally diverse herbaceous communities by shrubs. Although shrub encroachment is considered a ‘Plant Functional Type transformation’, our knowledge about the impact of climate change on shrub encroached ecosystems is still limited. Mechanistic analyses of alpine grassland responses to drought have focused on carbon fluxes, and a few studies have targeted components of the ecosystem water budget or nutrient cycling. However, these studies are focused on herbaceous functional groups, and shrubs are usually neglected. Moreover, despite the prevalence of this original climate change driver in mountains, snow manipulations are still rare.

To improve understanding of nitrogen and water cycling processes of shrubs with expected increased drought and advanced snowmelt, small high-precision lysimeters (SFL®, Meter Group AG, Munich, Germany) were used to analyze the effects and mechanisms of climate change on shrub species. In a garden experiment in the LTSER-site Stubai Valley (970 m a.s.l.), Tyrol Austria, two congeneric shrubs contrasting a deciduous (Vaccinium myrtillus) and evergreen (Vaccinium vitis-idaea) were planted into 16 lysimeters. In a split-plot design of 3.5m x 3.5m each, two plots were subject to either (1) control, (2) earlier snowmelt, or (3) summer drought treatments.

The manipulative experiments indicate that a shortening of the period with snow cover at the end of winter affects soil freezing and hence, soil nitrogen (N) and carbon (C) availability. Results further highlight the interacting effects of climate manipulations on key plant traits, and their consequences for N- and water availability. Furthermore, summer drought seems to additionally affect biogeochemical cycling and evapotranspiration for both investigated shrub types. This study's results reveal the importance of addressing the impact of shrub encroachment not only from a land management perspective but also to increasingly raise awareness about climate change effects on shrubs. Moreover, it provides valuable insights into challenges and chances of growing shrubs in lysimeters, being a promising approach for future climate impact studies. The study was conducted as part of the LUCSES project, ANR-FWF (ANR-20-CE91-0009 and FWF-I 4969-B).