Long-term flowering intensity of European tree species under the influence of climatic and resource dynamic variables

Mast flowering and seeding is a well-known reproductive strategy of tree species with many ecological consequences regulating synchronous year-to-year flowering intensity variations at the population level. In contrast to flowering timing, the effects of climate change on flowering intensity across space, time, and species are largely unexplored. In this study, a long-term data set on flowering intensities for eight common tree species (Alnus glutinosa, Fagus sylvatica, Larix decidua, Picea abies, Pinus sylvestris, Pseudotsuga menziesii, Quercus petraea, and Quercus robur) in Germany was reassembled to analyse flowering mechanisms and strategies by applying GAMLSS (Generalised Additive Models for Location Scale and Shape) models together with climatic data (temperature, precipitation, and drought) and various time-lagged effects. All species showed increasing flowering intensities in the period 1954–2019. The flowering intensity of Larix and Pinus differed significantly across their respective ecological provenances. Time series revealed higher synchrony among broadleaf than conifer species, although correlation coefficients of both their flowering intensities generally increased over time. GAMLSS modelling mainly explained flowering intensities well, with R² ranging between 0.58 (Pseudotsuga) and 0.25 (Alnus). Flowering intensity of almost all species was significantly influenced by flowering in previous years, indicating autocorrelative influences pointing to resource depletion and accumulation. Growing season temperature was modelled to be the main factor among weather cues, with the general pattern of flower masting being correlated negatively with temperature two years before masting and positively with temperature one year before masting. In addition, the short-term drought estimated by Standardised Precipitation-Evapotranspiration Index (12 months) increased flowering intensity in almost all cases. Therefore, it can be inferred that the heavy flowering of European common tree species has been regulated by sufficient resources and prevailing optimal climatic conditions.