Digital photography for tracking the phenology of an evergreen conifer stand

- A regular digital camera was used as an imaging radiometer.
- The LinearRatio is a reliable method for tracking the phenology of canopy gap fraction and effective plant area index.
- The measured gap fraction phenology was well explained by a theoretical model.

Gap fraction measurements with digital a hemispherical camera were carried out in 2007-2016 in a Scots pine (Pinus sylvestris L.) stand growing on transitional Sphagnum bog in Järvselja, Estonia. The measurements were done at the time when (1) the pines had maximum foliage, or (2) after autumn needle fall or before bud burst. Data for gap fraction calculation were extracted from camera raw files. For data recording, the sensor output signal maximum was kept according to the image brightness histogram within the interquartile range of the sensor dynamic range. A linear conversion method (LinearRatio) was used for image processing. The relative needle loss in autumn (20-30%) increased gap fraction by 0.02-0.05 at view zenith angles from 10° to 60° and decreased the mean plant area index from 1.86 to 1.68 after autumn litter fall. The measurement results were in good agreement with a theoretical gap fraction model simulation and with gap fraction estimates made from cover photos. The measurement and data processing protocol is appropriate for long-term monitoring in permanent sample plots.