Tracking the long-term structure changes of a mature deciduous broadleaf forest stand using digital hemispherical photography

Hemispherical photography provides permanent records of forest canopy structure. We analysed digital hemispherical images taken during the period of 2007–2018 in a mature silver birch stand located in Järvselja, Estonia. The stand was thinned in 2004. Understory trees were removed in the spring of 2018. Images were processed using the LinearRatio_SC method. Effective plant area index L^eff during the leafless phenophase increased as a result of tree growth from 0.92 to 1.24 and understory cutting was not detectable. During the full foliage condition L^eff increased from 3.6 in 2008 to 5.8 in 2017. After removal of understory trees from the stand L^eff decreased, and repeated measurements in the summer of 2018 estimated the plant area index range 4.5 < L^eff < 4.8. The results are in agreement with the expected changes following forest growth and demonstrate that LinearRatio_SC is a suitable method for the estimation and long-term monitoring of forest canopy properties from digital hemispherical images.