Understorey light environments in a north-east Australian rain forest before and after a tropical cyclone

Abstract

ABSTRACTThis paper describes estimates of canopy openness and associated light availability in the understorey of a north-east Australian rain forest before and immediately after a tropical cyclone. On the basis of 20 hemispherical (fisheye) canopy photographs it was shown that direct, diffuse and total site factors increased significantly as a result of the slight-to-moderate canopy disturbance caused by the cyclone. In the understorey, median total site factors ranged from 2.5–3.4% before the cyclone and from 6.0–8.6% after the cyclone, representing a 2- to 3-fold increase in potential light availability. Following the cyclone, mean relative gap frequencies increased substantially at all altitudes but particularly at canopy positions more than 70° above the horizon. Cyclone-induced canopy disturbance not only reduced the complexity of the understorey light regime but may have also increased the seasonal variability of light within the understorey of the forest during the interval of canopy recovery. The implications of these results for the ecophysiology of understorey tree seedlings and saplings at several temporal scales are discussed.