Multiple-year assessment of phenological plasticity within a beech (Fagus sylvatica L.) stand in southern Germany

•Significant differences in spring and autumn phenology were observed between years.•LAI seasonal pattern correlated with leaf unfolding, leaf coloring and leaf fall.•Beech trees did not show a fixed phenotypic temporal pattern during the study.

Phenological traits such as unfolding of leaves in spring and onset of leaf coloring in autumn are powerful data that can be used to assess the local adaptation of plants to regional climate conditions and to track climate change in nature. Phenotypic variability in beech has been widely studied, but most of the investigations emphasize the variation between populations. The aim of this study was to explore phenotypic responses at the individual level within a beech population, comparing in a stand the annual phenological patterns as well as leaf area index (LAI) courses for three consecutive years (2010–2012). Thirty adult beech trees were randomly selected in a forest in southern Germany for which the seasonal pattern in LAI and different stages of leaf unfolding, leaf coloring and leaf fall were recorded biweekly/three times a week by hemispherical photographs and visual observations, respectively. Based on their phenological and LAI patterns we were able to analyze in detail the phenological variation of individual trees between seasons over a three year study. Our main results confirm that LAI based on hemispherical photographs was correlated with the ground phenological observations. Surprisingly, they suggest that the phenological patterns of individual beech trees compared to their stand neighbors were not necessarily fixed; instead they showed annually very plastic phenotypic responses. Not having a fixed phenological pattern may be a strategy to cope with a wide range of environmental conditions in an ever-changing climate.

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