Understory light at the fushan experimental forest in Northeastern Taiwan: Watershed and landscape perspectives

Understory light regimes have a major impact on seedling growth and patterns of forest regeneration. Many researchers have reported understory light regimes for relatively small areas, and then extrapolated their results to the landscape level with little knowledge of landscape-level heterogeneity. We used hemispherical photography to characterize understory light regimes at both the watershed and landscape scales within the Fushan Experimental Forest in northeastern Taiwan. We established 20 transects, 7 of which were located in Experimental Watershed #1 and the other 13 were scattered across an area of 150 ha at the Fushan Experimental Forest. Understory light was not normally distributed across the Fushan Experimental Forest, with some microsites having very high light levels due to typhoon-induced canopy damage. Median understory light levels exceeded 15% of the incident sunlight at both the watershed and landscape scales. These light levels are much higher than those reported for many mature tropical and temperate forests in other parts of the world, where understory light levels of < 5% of incident sunlight are common. Aspect, rather than spatial scale, had the largest impact on undercanopy light. In forests with rough topography, utilizing transects that run from the ridge to the valley is more likely to adequately characterize spatial heterogeneity than plots or a few longer transects.