PARENT SESSION
Poster Session #39: Light Relations.
Wednesday, August 7. Presentation from 5:00 PM to 6:30 PM. Exhibit Hall B & C, TCC

Measuring sapling-available photosynthetically active radiation (PAR): a comparison of methods.

Hogarth, Leah^*,1, Kobe, Richard¹, ¹ Michigan State University, East Lansing

ABSTRACT- Efficient measurement of plant-available light is critical in ecological studies. Although there are several methods for estimating PAR, comprehensive comparisons are lacking. We evaluated six of these methods over understory (< 1% canopy openness) to large gap conditions (approx. 20%) in a northern hardwood forest in Northwestern lower Michigan, during June to September 2000. Methods included direct long-term measurements with gallium-arsenide photodiodes calibrated against quantum sensors, hemispherical canopy photos taken with conventional and digital cameras, and percent openness (DIFN) measured with Licor LAI 2000 as a single point above the sapling crown or the average of three horizontal (across crown) or three vertical (along leader) readings. We measured PAR with these methods for 26 white ash (Fraxinus americana) and 23 sugar maple (Acer saccharum) saplings and also evaluated the methods with respect to predicting growth. Overall, photodiode measurements best predicted growth. Under low light, LAI 2000 DIFN was best correlated with photodiode measurements. However, DIFN overestimated light availability in southern gap positions, which were better characterized by calculated total transmittance (TT) from canopy photos. Digital canopy photo TT was better correlated with photodiode measurements than conventional film TT; however digital photos overestimated light availability, especially at low light. Thus, if using indirect methods, low light is best measured with Licor's LAI 2000, but PAR in gaps is better approximated by calculated TT from digital canopy photos.

KEY WORDS: light, growth, forest, PAR