Monitoring changes of forest canopy density in a temperature forest using high-resolution aerial digital photography

Abstract

A multiannual series of high-resolution small-format aerial digital photography was employed to assess changes in the forest canopy density in a temperate forest. A combination of conventional and adapted techniques of photogrammetry and photo interpretation was used, establishing a specific method. This method has been proved in a twelve-year period (1999-2011) in the core zone of the Monarch Butterfly Biosphere Reserve, in the states of Mexico and Michoacán, in Mexico, employing orthorectified mosaics as base maps to evaluate biennial changes. Photographs were assessed through traditional photointerpretation marking changes on acetates, located over the paper prints, creating new polygons. These were transferred to the orthomosaic directly through the computer screen using the acetates and at least three control points, complying with the principle of radial triangulation. Forest was separated into the following canopy density classes: closed, semi-closed, semi-open, open, and deforested. Accuracy assessment in forest canopy density classification was estimated by field sampling, and employing error matrices being 95%. Since 2003, this method has been used to determine appropriate payment for environmental services. This payment together with an intense interaction with the communities has led to a reduction in forest degradation and deforestation in the core zone.