Defining phenology events with digital repeat photography

Abstract: Phenology is the study of the timing of natural events such as leaf-out, bud burst and senescence relative to climate change. It is important to understand how the climate change will affect the timing of these events as it in turn affects the carbon-, nutrient- and the hydrological cycles. Field observations is the most common way to gather phenology data but the result is dependent on the skill of the observer and is very time consuming. Remote sensing has under the last decades been used more in phenology studies. However the low spatial and temporal resolution from satellite photos makes it hard to study local phenology. Therefore new methods have emerged to complement this. Digital repeat photography is a method which uses regular digital cameras mounted to capture the same area over a longer time period. The captured red, green and blue color values in the photographs can then be used to calculate the appearance of certain phenology events. This study provides a method how to quantitatively define the start and end of the growing season which can be used to indicate climate change. Phenology phases for four different mountain birch stands including canopy and understory in Abisko, Sweden, were studied using data from 2011. The calculated start of the growing season varied with two days between the different stands and follows the field observed phenology. The calculated color values from the photographs had a peak when all leaves were fully developed. The end of the growing season varied by six days between all birch stands. The senescence period was harder to define than the leaf-out event as it includes both the decrease from the colorization of leaves and when leaves fall to the ground.