Elevational response in leaf and xylem phenology reveals different prolongation of growing period of common beech and Norway spruce under warming conditions in the Bavarian Alps

Abstract

Alpine ecosystems face multiple environmental impacts caused by extraordinary rapid warming. One of the impacts caused by rising temperature is the prolongation of the growing season. To determine temperature-driven variation in leaf and xylem phenology, we studied Norway spruce and European beech along two elevational transects in the Northern Limestone Alps, southern Germany. Phenology was observed, and microcore samples were taken regularly from May to October 2011. Microcore thin sections were analyzed for expanding, wall thickening and mature cells in the forming tree ring. Leaf phenology and xylem phenology were compared in respect of onset dates, elevational responses and aspects. Both tree species showed similar responses to elevation in early spring leaf and xylem phenological phases. Delayed start dates and advanced end dates led to shortened growing seasons with elevation for both leaf and xylem development. Elevational responses of phenological phases were similar to each other throughout the growing season in spruce, but only at the beginning of the season for beech. Xylem growth periods from cell expansion until the end of lignification were strongly reduced with elevation for beech, especially the period in which new cells were formed. This was mainly due to an earlier end of xylem cell formation in higher elevations. For spruce, the length of the xylem cell growth period decreased less with elevation. Colder conditions in high elevations seemed to lead to a longer period of cell maturation in spruce. We conclude that with warmer temperature conditions, the tree-ring growth period could be lengthened more in beech compared to spruce, as long as it is not limited by drought stress or unfulfilled chilling requirements.