Abstract
The vegetative season length plays an important role in the annual carbon production of plants. It is determined by both the bud burst date and the leaf senescence date. Whereas the budburst is an event easily observable (a binary observation indicating if the bud is burst or not), the leaf decolouration is more subjective due to the progressive and long aspect of the process. The observation of the beginning of colour changing can be more or less late (from light green to yellow) according to the observers that could lead to a false shift of observations among the observers (Liu et al, 2021). Our objective is to develop tools to homogenise these observations.In the working group of the TEMPO network (https://tempo.pheno.fr/soere-tempo_eng/) on the leaf senescence, during two vegetative seasons (from the end of leaf expansion to the end of leaf fall), leaves were collected monthly in summer and weekly or biweekly during autumn in shaded and sunned canopy positions, on different tree species including forest (Fagus sylvatica, Quercus robur, Quercus petraea, Carpinus betulus and poplar hybrid (Populus deltoides × Populus nigra)) and fruit (Malus domestica and Prunus avium) species and in different locations in France for some species. The colour of theses leaves was immediately measured thanks to a PANTONE CAPSURE™ including the complete Munsell® colour chart. The leaf chlorophyll content was also estimated with a chlorophyll meter on the same leaves. From the combination of the colour measurements in the different locations or years, a reference colours chart has been identified for each species. This chart combines i) the main summer reference colours which could be different among trees within a same species according to the growing conditions of trees; ii) the colour when leaf senescence starts that we have defined as the colour from which leaves have lost 50% of their chlorophyll content and iii) the main colours during the leaf senescence.The perspective of the project is to extend this approach for a large number of species to build a standard tool to measure the leaf senescence in trees and even beyond in all perennial plants in various ecological context. For example, the development of these reference colour charts could help to homogenise the leaf senescence observations in professional networks (ex: RENECOFOR for forest species in France) or in citizen sciences projects (ex: observatoire des saisons).