Phenology
Phenology is the study of the timing of vegetation life-cycle events, such as budburst, flowering, leaf colouring, leaf fall, ect.
Change in plant phenology impacts the whole ecosystem functioning and productivity. For example, change in flowering impacts the timing and the intensity of the pollen season.
Plant phenology can be studied from ground observation (see here) but, as well, estimated through satellite/airborne sensors.
Unit: day of the year (DOY)