Phenology and stem growth dynamics of Betula pendula Roth. in the Spanish Pyrenees

Key message We established silver birch phenology in the Pyrenees. We highlight actual evapotranspiration as the main driver and the altitudinal effect within a north–south phenological pattern. Abstract Research into the phenology of trees is one of the most important issues in making reliable predictions of possible range shifts due to current climate change. Under the current climate change, warming and water stress can threaten tree performance by altering phenological events, specially at the edge of the southern especies distribution. We aimed at establishing bud burst, leaf and stem growth phenology of the Betula pendula in the Spanish Pyrenees, to determine the environmental cues triggering phenological events and to assess the effect of climate and soil water balance on annual growth cycle. The most important cues for bud burst are heat accumulation and long days that reduced the thermal time to bud burst. Leaf unfolding, the period of stem cambial activity, and the variation of the stem radial growth rates throughout the year are mainly regulated by the actual evapotranspiration. Leaf yellowing and abscission are advanced when hydric stress is pronounced shortening leaf longevity. The increase in spring temperatures brings forward the date of bud burst and the start of stem growth. However, the increase in the duration of hotter droughts causes an advance in the date of cessation of birch growth. From the southern Pyrenees to the boreal birch populations, there is a marked latitudinal gradient of phenological events, which are not linear except for the maximum stem growth rate. In the Pyrenees, the growing season is longer than in the boreal forests, but ring thickness is similar.