Comparing common methods for assessing understory light availability in shaded-perennial agroforestry systems

Regulating the shade provided by overstory trees is important in the management of shaded-perennial agroforestry systems. In order to compare the merits of commonly used light-assessment techniques that could potentially be useful to farmers and extensionists and to quantify the extent of shading in multistrata agroforestry systems, understory photosynthetically active radiation (PAR) was measured beneath 28 single-species and four mixed-species stands of trees in Costa Rica. Stand age varied from 1.5 to 20 years, with uniform age trees within stands. Canopy gap fraction was estimated by three methods in each stand: densiometer, visual index, and hemispherical photography. The gap fractions so derived were compared as estimators of mid-day PAR transmission. Canopy architectural parameters such as crown height, crown diameter, canopy coverage, and stand density were measured and compared in regression models for their ability to predict PAR transmission. As expected, stand characteristics alone did not accurately predict PAR transmission. Of the techniques, gap fractions using densiometers were the most predictive, except under conditions of low stand density (<500 trees per hectare) and open canopies, where estimates from hemispherical photograph were better. Shade assessment using densiometers may provide an adequate, dependable guide for overstory management in multistrata systems and could be useful to farmers willing to adjust their management techniques.