Assessing Typhoon-Induced Canopy Damage Using Vegetation Indices in the Fushan Experimental Forest, Taiwan

Abstract

Cyclonic windstorms profoundly affect forest structure and function throughout the tropics and subtropics. Remote sensing techniques and vegetation indices (VIs) have improved our ability to characterize cyclone impacts over broad spatial scales. Although VIs are useful for understanding changes in forest cover, their consistency on detecting changes in vegetation cover is not well understood. A better understanding of the similarities and differences in commonly used VIs across disturbance events and forest types is needed to reconcile the results from different studies. Using Landsat imagery, we analyzed the change between pre- and post-typhoon VI values (ΔVIs) of four VIs for five typhoons (local name of cyclones in the North Pacific) that affected the Fushan Experimental Forest of Taiwan. We found that typhoons varied in their effect on forest canopy cover even when they had comparable trajectories, wind speeds, and rainfall. Most VIs measured a decrease in forest cover following typhoons, ranging from −1.18% to −19.87%; however, the direction of ΔVI–topography relationships varied among events. All typhoons significantly increased vegetation heterogeneity, and ΔVI was negatively related to pre-typhoon VI across all typhoons. Four of the five typhoons showed that more frequently affected sites had greater VI decreases. VIs ranged in their sensitivity to detect typhoon-induced changes in canopy coverage, and no single VI was most sensitive across all typhoons. Therefore, we recommend using VIs in combination—for example Normalized Difference Infrared Index (NDII) and Enhanced Vegetation Index (EVI), when comparing cyclone-disturbance-induced changes in vegetation cover among disturbances and across forests.

Disturbance frequency, intensity and forest structure modulate cyclone‐induced changes in mangrove forest canopy cover

“…A site-based study from the Fushan Experimental Forest of northern Taiwan, which experiences roughly one cyclone of at least category 3 every 2 years, also found that cyclone proximity is the chief factor determining remotely sensed cyclone-induced canopy loss of tropical broadleaf forest (Peereman et al, 2020). Given that the effects of cyclones are likely to be consistent across much of the zone of cyclone influence (Ibanez et al, 2018), distance is likely to be the principal factor in modulating cyclone effects on forests across multiple scales, from the local to global scale.…”

“…The NDII is based on the NIR and short-wave infrared (SWIR) bands and is highly sensitive to leaf chlorophyll and water content (Cheng et al, 2006). Both indices have been shown to be good remotely sensed metrics of changes in forest canopy cover and canopy leaf physiology associated with cyclones, including mangroves, although they are limited by their sensitivity to light canopy damage (Peereman et al, 2020;Zhang et al, 2016). Moreover, the SWIR band and the NDII have been used successfully to follow mangrove recovery (Gang et al, 2020;Peneva-Reed et al, 2020).…”

Tropical Cyclone Impact and Forest Resilience in the Southwestern Pacific

“…Immediate impacts of TCs on forests can be particularly devastating with damages ranging from defoliation to extensive trunk snapping or tree uprooting. Most studies focused on the local-or landscape-scale damages caused by a single TC (e.g., [1][2][3][4][5][6][7][8][9]). Analyses of the impacts of multiple TCs on larger regional or global scales are needed to better understand how forests respond to these large-scale disturbances in the context of changing TCs intensity [10].…”

An evaluation of vegetation loss due to the super typhoon Sarika in Hainan Island of China

“…The effect of tropical cyclones on individual trees ranges from defoliation and branch and canopy damage to bole snapping and uprooting. The level of damage depends on the intensity of the cyclone and the resistance properties of trees (Niklas 2000;Butler et al 2012;Uriarte et al 2019;Peereman et al 2020). It can cause severe damage to trees having a large diameter, resulting in bark tearing, drying, branch breaking, falling of several leaves and uprooting (Yin et al 2013;Liu et al 2012;Dunham et al 2000;Gardiner and Quine 2000;Xu et al 2008), and early seed falling.…”