Drivers and dynamics of foliar senescence in temperate deciduous forest trees at their southern limit of distribution in Europe

Research on autumn phenology is very important for understanding and simulating the future growth of temperate deciduous forests. This is especially needed at the southern edge of the temperate zone, where climate change impacts are particularly intense. We studied foliar senescence timing for mature stands of Fagus sylvatica L., Populus tremula L., Betula pendula Roth, Quercus petraea (Matt.) Liebl. and Quercus robur L. at the southern edge of European temperate forests. First, we analysed long-term series (1997-2019) of senescence timing in southern France. Then, we compared a more detailed four-year dataset (2017-2020) of senescence dynamics and its correlations to meteorological conditions for stands in northern Spain and Belgium, with the latter area representing the core distribution of the species. In contrast to other temperate regions, no significant delaying trends in the timing of autumn phenology were detected in the long-term in southern temperate forests, which were characterized by high inter-site variability. In the short-term (4 years), species-specific correlations with meteorological conditions were found, with, for example, the senescence of F. sylvatica being affected by temperature while the senescence of Q. petraea was affected by both temperature and precipitation. Autumn dynamics differed between the core and southern areas of distribution of the species in the study period. In particular, while late season conditions affected senescence timing in the southern populations, the senescence dynamics of the core populations was mostly influenced by the legacy of spring - early summer conditions and, overall, more affected by precipitation than southern populations. Our data fill important knowledge gaps on the functioning of temperate deciduous forests at the southern limit of distribution in Europe.