Consistency in the Damage Effects Among Typhoons

The consistent decrease in VI values following all five typhoons (with very few exceptions, see Table 3.4) suggest that all the VIs can generally capture typhoon-induced losses in vegetation cover (Ayala-Silva & Twumasi, 2004; Kang et al., 2005; Lee

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et al., 2008; Rossi et al., 2013; Tsai & Yang, 2016; Hu & Smith, 2018). However, we find weak correlations in ΔVIs between different typhoon events (ρ < 0.4, Table E2), indicating that typhoons range in their effects on vegetation cover. Although this is not surprising because typhoons differ in intensity, duration, trajectory and occurrence time relative to plant phenology, the inconsistency among vegetation indices highlights that results derived from one or a few disturbance events are unlikely to represent general trends in disturbance effects. Indeed, our results show that the successive Typhoons Soudelor and Dujuan did not have consistent effects on vegetation although they had comparable paths, wind speeds, and directions (Figure 3.1, Figure 3.2, Table 3.1). Additionally, the very weak negative correlations of the ΔVIs between typhoons Soudelor and Dujuan (Table E2) indicate that the two typhoons had different effects on the vegetation cover, as observed in other sites subject to successive cyclones (Elmqvist et al., 1994; Burslem et al., 2000; Liu et al., 2018).

Powerful cyclones, such as Hurricanes Hugo and Maria in Puerto Rico, Typhoon Herb in Taiwan, and Cyclone Larry in northeastern Australia, attract scientific study of their ecological effects (Brokaw & Grear, 1991; Lodge et al., 1991; Walker, 1991; Lin et al., 2003b; Kang et al., 2005; Grimbacher et al., 2008; Lee et al., 2008; Metcalfe et al., 2008; Turton, 2008; Liu et al., 2018; Uriarte et al., 2019; Hall et al., 2020⁾. This high prevalence of studies has advanced our understanding of cyclone ecology, especially in relation to the most powerful and damaging storms (reviewed by Everham &

Brokaw, 1996; Lugo, 2008; Lin et al., 2020). However, in this study, Typhoon Herb was considered to be the most powerful typhoon in several decades (Lin & Jeng, 2000).

Typhoon Herb had the greatest wind speed among the five typhoons studied, while Nari was most intense in terms of precipitation. Considering these two storms, the resultant changes in vegetation index values (ΔVIs) were only weakly correlated.

Future increases in the frequency of the most intense cyclones are predicted (Knutson et al., 2010; Knutson et al., 2015; Walsh et al., 2016). However, our results suggest that conclusions drawn from studies that document the effects of a single or a few intense cyclones are insufficient for predicting the effects of future cyclones.

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The fact that there were small or no changes in VIs associated with Typhoon Herb is somewhat surprising. In addition to the potential influence of image quality on our analyses, the timing of Typhoon Herb is probably the most important factor. Typhoon Herb occurred in the summer (late July) when the forest was in the middle of the main growing season, and the two images were approximately two months apart. Thus, plant growth could be substantial during the period, potentially confounding the detection of typhoon-caused decreases in vegetation cover by VIs. This contrasts with Typhoon Dujuan, which caused the largest decreases in VIs. Typhoon Dujuan occurred near the end of the main growing season (April to September) so that although the two images were also approximately two months apart, there was little vegetation growth during the period. As a result, typhoon-induced vegetation loss can be better detected with the VIs. Thus, image timing in relation to plant growth phenology should be considered when examining disturbance-induced changes in vegetation cover using VIs.

Not only were the overall effects inconsistent among typhoons, there was large variation in linear regression results, which showed that typhoon damage–topography relationships were inconsistent among typhoons (Table 3.6 and Table F1). Topography was better at explaining disturbance distribution across the landscape for Typhoons Herb and Nari, the typhoons that passed the closest to the FEF, but they were not the storms that caused the greatest degree of vegetation damage. This result suggests that topography–vegetation damage relationships vary with cyclone distance and that topography is a key determinant of vegetation damage only when typhoons are very close to the study site, despite the magnitude of the damage. It also suggests that factors other than cyclone distance determine the severity of typhoon-induced vegetation cover damage.

Nevertheless, there were some consistencies across typhoons. First, the canopy generally recovered quickly as many VIs returned to their pre-disturbance values within a year (Figure 3.6). This result is consistent with the report of the rapid recovery of the FEF observed following Typhoon Bilis using NDVI (Kang et al., 2005). However,

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recovery of a VI does not imply total canopy recovery as LAI and litterfall typically do not recover within a year of damage in the FEF (Lin et al., 2017). Second, the relationship between high pre-disturbance NDVI and strong NDVI loss observed by Kang et al. (2005) was also detected for all five typhoons in this study despite their differences in paths and intensities (Table E1). This pattern may be explained by the higher aerodynamic drag of dense canopies as suggested by Harrington et al. (1997), who reported a similar relationship between pre-disturbance LAI and LAI loss (see also Herbert et al., 1999). Third, cyclones are disturbance agents which induce heterogeneity in forest landscapes (Lugo, 2008; Lin et al., 2011). Secondary tree falls and defoliation may have led to the increased heterogeneity observed following almost all typhoons examined here. Finally, most ΔVIs were positively correlated between typhoons (Table E2), except for Soudelor–Dujuan, suggesting that different typhoons could have similar effects although the strength of the relationships varied greatly among typhoons and VIs.