Measuring fractional forest canopy element cover and openness - definitions and methodologies revisited

摘要

Canopy structural parameters are often used to give adequate representation of vegetated ecosystems for various purposes including primary productivity, climate system, water and carbon gas exchanges, and radiation extinction. Canopy structural parameters are usually described using several pseudo-synonymous terms, often measuring different components of vegetation canopies. Standardization in the definitions has fallen short, leading to confusion of terms even in standard text books making the comparison of historic measures futile. Here we clarify concepts that have been used for fractional canopy element cover and openness measures. The fractional canopy element cover and openness concepts considered are canopy closure, canopy cover, canopy openness, crown closure, crown completeness, crown cover, crown porosity, site openness and tilt openness. New methodologies are presented to obtain large scale fractional canopy element cover and openness measures using hemispherical photography. The new methodologies and variations in definitions of fractional canopy element cover and openness concepts are demonstrated using photographic measurements in complex topography. The results indicate that both fractional canopy element cover and openness parameters can be estimated with a few point-based measurements using hemispherical photography. Hemispherical photography is therefore less time, labour and resource intensive, as compared to point based measuring techniques of canopy element cover and openness.\udMost of the commonly and interchangeably used concepts of fractional canopy element cover and openness measures represent physically different structural properties of a vegetated ecosystem.