Spring phenology of Swiss grasslands under climate change variability and trends since 19

Phenological observations provides important information for the agricultural management of grasslands, especially in spring, when a clear trade-off between increasing biomass and decreasing forage quality emerges as plant development progresses. For this reason, and in view of the impacts of climate change on agroecosystems and the needs for adapting farming activities to altered climatic conditions, there is much interest for understanding the long-term trends of key phenological stages. In this contribution we first provide an overview of 30 years of phenological observations carried out since 1995 at more than 50 sites located in Western Switzerland and covering an altitudinal belt ranging between 400 and 1400 m above sea level. Using the so-called Dactylis glomerata (cocksfoot) equivalent stage to assess the average plant development in these grasslands, we show that full heading has been advancing by about 2 days per decade over the 30 years of observations. Secondly, we demonstrate that observed dates of full heading and associated trends are very well matched by estimates obtained with a simple phenological model. We use this model, along with models for determining the end of the snow season and the beginning of the vegetation period, to extend the assessment of climate change impacts on grassland spring phenology in Western Switzerland back to 1961. We report on shifts and associated trends calculated along the altitudinal transect covered by our network. Finally, we compare the development of grassland phenology with the one of spring phases of other plant species and discuss our results in the context of the overall temperature evolution in Switzerland. We also emphasize a possible association of the variability in the spring phenology of grasslands with the long-term evolution of large-scale atmospheric circulation modes, and the implications of projected future changes in climatic conditions for the agricultural utilization of permanent grasslands.