Understory Light Availability and Spatial Variation

Abstract

Photosynthetically active radiation is a critical resource for understory plants and its availability and heterogeneity plays a major role in seedling regeneration and survival. This thesis examines various methods to quantify understory light availability, temporal dynamics and spatial variation in a shaded environment. Comparing indirect estimates to direct measurements by BF3 sensors, we found that hemispherical image analysis (HIA) overestimated the proportion of total and diffuse light transmitted to the understory. Instantaneous BF3 readings were correlated to daily integrative BF3 estimates, but were subject to outliers depending on sky condition. The median of several ten-minute means performed better than other indirect estimates. We used the BF3 sensor to develop a new definition of sunflecks and to quantify the distribution of sunfleck duration and peak intensity in our study plot. In addition, we characterized spatial variation in understory light using both HIA and a mobile BF3 sensor system. Our results highlight temporal variation in spatial patterns, even for diffuse transmittance over a short period of time.