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T.-C. Lin et al. 2010. Immediate effects of typhoon disturbance and artificial thinning on understory light environments
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scattered tree ferns (Alsophila podophylla Hook (Cyatheaceae)) and herbaceous cover. We
measured light availability every five meters along a 300-m transect along the eastern ridge of
experimental watershed #1 established in 1994.
2.2 Methods
Understory light environments were investigated using hemispherical photography
(Rich 1990; Lin and Chiang 2002). Hemispherical photographs were taken at 1.5 m above the
ground, at 5-m intervals in both forests. Photographs were analyzed to estimate global site
factor (GSF), the proportion of solar radiation reaching a given location, relative to a fully
exposed location with no obstructions (Rich 1990) using Hemiview 2.1 (Delta-T 2000).
Two typhoons, Herb in 1996, and Toraji in 2001 were used to evaluate the immediate
effects of typhoon disturbance on understory light environments. According to the Central
Weather Bureau of Taiwan, both typhoon Herb was a strong typhoon (maximum wind velocity
> 51 m.sec-1), and typhoon Toraji was a medium-intensity typhoon (maximum wind velocity
33-51 m.sec-1; Table 1).
3. Result
Light indices significantly increased after both typhoon disturbance and artificial
thinning (Fig. 1, paired t-test all p-values < 0.001). In the unthinned plots, mean post-thinning
GSF was significantly lower than the mean pre-thinning GSF (paired t-test p < 0.01). The effect
of typhoon disturbance was variable, depending on the strength of the typhoon. The mean
percent changes in understory light indices were significantly greater after typhoon Herb (24%)
than after typhoon Toraji, (7.8%) (Fig. 1, Bonferroni multiple comparisons, both p-values <
0.005).
Artificial thinning caused disproportional increases in understory light indices. The 25%
thinning caused an approximately 42% increase 0.25, while the 50% thinning resulted in an
approximately 120% increase reaching 0.36 (Fig. 2). The increase in understory light indices
was greater after the 25% thinning (approximately 42% enhancement) than after the strong
typhoon Herb (approximately 25% enhancement) (one-way ANOVA, p