The effects of climate change on the flowering phenology of alder trees in Southwestern Europe

Global warming impacts plant phenology and the effect of climate change will be more intensely experienced at the edges of a plant’s distribution. This work focuses on the climatic range occupied by Iberian alder (Alnus lusitanica Vít, Douda & Mandák). The Iberian Peninsula constitutes the Southwestern edge of the global chorological distribution of European black alder (Alnus glutinosa (L.) Gaertn. s. l.), and some of the warmest and driest conditions for alder population are located in the centre of Spain. The critical temperature-relevant periods that regulate the reproductive phenology of alder were analysed using a statistical-based method for modelling chilling and forcing accumulation periods in temperate trees. Our results reveal that autumn chilling was the most important thermal accumulation period for alder in a Mediterranean climate, while forcing requirements are satisfied in a short period of time. Autumn temperatures were significantly correlated with the timing of flowering, and chill units during this season directly influence start-dates of alder flowering. A positive trend was observed in the timing of pollen seasons, meaning a slight delay of alder flowering in central Spain. This coincided with autumn warming during the period 2004-2018. If this warming trend continues, our results predict a delay in the start-date of flowering by around 3-days for every degree increase in maximum autumn temperatures according to the most optimistic emission scenarios.