Understory light availability in a forest ecosystem is the function of forest structure, density, species composition, age, management practices as well as location (latitude).  It is of particular concern in the maintenance of biodiversity, and sustainability of ecosystems with light demanding species such as slash and longleaf pine and the associated understory. We studied canopy light transmittance and understory light availability in slash pine and longleaf pine ecosystems over a range of density, basal area, and proportional species composition in north-west and central Florida sites using Digital Hemispherical Photographs (DHPs). The DHPs were analysed using CAN_EYE V 5.0 (INRA, Avignon, France). 

Results/Conclusions

Slash pine and longleaf pine were observed to differ in canopy light transmittance characteristics in terms of gap fraction (fraction of absorbed Photosynthetically Active Radiation i.e. fAPAR) and plant area index (PAI). Slash pine in general had smaller gap fraction (measured daily integrated black sky fAPAR = 0.77 to 0.36) and greater effective PAI 57(0.45 to 5.4)  leading to lower canopy light transmittance as compared to longleaf pine (measured daily integrated black sky fAPAR: 0.81 to 0.31, Effective PAI 57: 0.56 to 1.8). The observations ranged over basal area values of 20 to 160 and 20 to 120 ft²/acre for slash pine and longleaf pine, respectively. The gap fraction was correlated to basal area in both species. The values varied depending on the proportion of the two species in mixed slash-longleaf pine ecosystems.  The study shows that for a given basal area, understory light availability differs with species and their relative proportion in mixed stands.  The observations have important implications for management of these ecosystems which are being managed using basal area regulation approach. Thus basal area-based management approaches need to account for light transmittance behavior of the constituent species so that optimum understory light regime could be created.