Determining the phenological stages of Nestos-Delta (Alnus glutinosa) riparian forest (Natura 2000 site) by using radiation transmittance data

Trees’ phenology is a key factor for most forests’ physiological processes. The precise definition of the start-end dates and the duration of each phenological stage is critical. In the present study, radiation data were used from pyranometers installed at the Alnus glutinosa riparian forest of Nestos-Delta (Natura 2000 site), within the implementation of LIFE-PRIMED project. The pyranometers were placed inside the stand (below the trees’ canopy) and also in a forest opening. The radiation transmittance coefficient τ (unitless) was estimated on a daily basis for the period October 2019 to December 2020. τ can be used for the forest’s phenological stages determination. Four stages were identified. The first concerns the development of leaves and starts at mid April until mid June, as τ decreases with a rate of about 0.0071 d-1. At the second stage (mid June to mid September) the canopy is fully developed and effective and τ presents almost stable low values (0.13). During the third stage (mid September to late December) senescence and gradual rejection of leaves occurs as τ increases with a rate of 0.0062 d-1. During the last stage of the leafless canopy (late December to mid April), τ becomes maximum (0.6) and stable.