remote sensing Tropical Cyclone Impact and Forest Resilience in the Southwestern Paciﬁc

: Tropical cyclones (TCs) can have profound effects on the dynamics of forest vegetation that need to be better understood. Here, we analysed changes in forest vegetation induced by TCs using the normalized difference vegetation index (NDVI). We used an accurate historical database of TC tracks and intensities, together with the Willoughby cyclone model to reconstruct the 2D surface wind speed structure of TCs and analyse how TCs affect forest vegetation. We used segmented linear models to identify signiﬁcant breakpoints in the relationship between the reconstructed maximum sustained wind speed (Wmax) and the observed changes in NDVI. We tested the hypothesis that the rate of change in damage caused by TCs to forest and recovery time would increase according to Wmax thresholds as deﬁned in the widely used Safﬁr–Simpson hurricane wind scale (SSHWS). We showed that the most signiﬁcant breakpoint was located at 50 m/s. This breakpoint corresponds to the transition between categories 2 and 3 TCs in the SSHWS. Below this breakpoint, damages caused to forest vegetation and the time needed to recover from these damages were negligable. We found a second breakpoint, with a sharp increase in damages for winds >75 m/s. This suggested that extremely intense tropical cyclones, which might be more frequent in the future, can cause extreme damages to forest vegetation. Nevertheless, we found high variation in the observed damages and time needed to recover for a given Wmax. Further studies are needed to integrate other factors that might affect the exposure and resistance to TCs as well as forests’ capacity to recover from these disturbances. Conceptualization, B.D., C.M., M.D. and M.M.; methodology, B.D., M.M., C.M., T.I. and M.D.; software, B.D.; validation, T.I. and C.M.; formal analysis, C.M., T.I. and M.M.; resources, T.I.; data curation, M.D. All authors have read and agreed the of the manuscript.


Introduction
Tropical cyclones (TCs), also referred to as hurricanes in the North Atlantic and Northeast Pacific and typhoons in the Northwest Pacific, generate high speed winds and heavy rainfalls over large areas. 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].
Increasingly available and accurate remote-sensed vegetation index time series and TC trajectories and characteristics databases now allow to analyse the impacts of TCs on forests and their recovery at large spatio-temporal scales (e.g., [11][12][13]). Maximum sustained wind speed (Wmax) appears to be a key parameter to predict changes in vegetation indices. Yet, the direct relationship between local Wmax and the observed vegetation indices dynamics has, to our knowledge, not been explored. We suggest that this could be tackled using simple cyclone models that allow reconstructing the 2D wind speed structure (e.g., [14]) from TC characteristics available in TC databases.
The Saffir-Simpson Hurricane Wind Scale (SSHWS, [15]) is one of the most commonly used TC classifications. It uses Wmax to anticipate the potential damages caused by TCs. SSHWS ranges from category 1 TCs (Wmax between 33 and 42 m/s) which are predicted to mostly damage foliage, to category 5 TCs (Wmax ≥ 70 m/s) which are predicted to cause extensive snapping or uprooting of shrubs and trees (Table 1). On top of that classification, category 3 or higher TCs (Wmax ≥ 50 m/s) are considered as major TCs that can cause devastating to catastrophic damages (https://www.nhc.noaa. gov/aboutsshws.php, accessed on 22 February 2022). Yet, to our knowledge, the potential damages anticipated by the SSHWS have not been confronted concerning actual damages on a regional scale and on multiple storms over a long time period. Here, we used high-resolution historical database of TC tracks and intensities (IB-TrACS, [16]), together with the Willoughby cyclone model [17] to reconstruct the 2D surface wind speed structure of TCs to analyse how TCs affect forest vegetation. Our analyses cover six archipelagos of the south-west Pacific region over the 2000-2020 period. Our understanding of the impacts of TCs on forests in the islands of this region is relatively poor as most studies in this region have been conduced in Australia and on a global scale most studies have focused on the most intense cyclones in the North Atlantic Basin and to a lesser extent in the North West Pacific Basin [18]. We expect that damage caused by TCs to forests and the time needed to recover from these damages would increase with increasing Wmax. We test the hypothesis that the rate of change in damage caused by TCs to forests and the recovery time would increase according to Wmax thresholds as defined in the SSHWS. More particularly, we expect that winds generated by major, category 3 or higher, TCs would result in particularly intense damages.

Study Area
The study area encompasses 76 islands in six archipelagos (New Caledonia, Vanuatu, Fiji, Samoa, Tonga, Wallis and Futuna) located in the South West Pacific basin in longitude 152.3°E-162.5°W and latitude 10.0°S-30.0°S. Small islands, i.e., those less than 5 km long and/or wide were omitted. In this region, TCs mostly occur between November and May during austral summer [19]. On average, 6-8 tropical storms occur in the South West Pacific Basin between November and May with a peak of activity in February-March [20]. There are 2 to 5 tropical storms per decade on land for the considered islands with maximum cyclone densities west of Vanuatu and north of New Caledonia extending into the Coral Sea, between 10°S and 25°S fading off toward Samoa [21].

Forest Vegetation
Changes in forest vegetation induced by TCs were analysed using the Normalized Difference Vegetation Index (NDVI), one of the most widely used satellite-based vegetation indices [22]. Chlorophyll pigments contained in leaves are responsible for photosynthesis and absorb red light (RED), whereas healthy leaf tissues reflect strongly in the near infrared region (NIR) of the light spectrum. The NDVI is computed as the normalized ratio between RED and NIR reflected by the vegetation and captured by the sensor of the satellite (Equation (1)). The NDVI ranges from −1 to 1 and is positively correlated with the density and health of the vegetation (i.e., the higher the NDVI, the denser or healthier the vegetation) with negative NDVI values corresponding to water areas.
We used the NDVI from the MOD13Q1 products supplied by the MODIS (Moderate Resolution Imaging Spectroradiometer) sensor that was launched by NASA onboard the Terra satellite in 1999. The MOD13Q1 products have provided NDVI values every 16 days since February 2000 at a 250 m pixel spatial resolution. We extracted NDVI of forested areas using the "tree cover, broadleaved, evergreen, closed to open (>15%)" vegetation class from the global land cover map produced by the European Spatial Agency (https://www.esa.int/Applications/Observing_the_Earth/Space_for_our_ climate/ESA\_global_land_cover_map_available_online, accessed on 22 February 2022) and resampled at 250 m to match the NDVI resolution. Prior to the analyses, all pixels covered by clouds or water were removed using the pixel reliability files provided by MOD13Q1.

Tropical Cyclones
We used the Saffir-Simpson hurricane wind scale (SSHWS, Table 1, [15]) to classify TC intensities. TC trajectories and characteristics were extracted from the IBTrACS database ( [16], https://www.ncdc.noaa.gov/ibtracs/, accessed on 22 February 2022). This database provides the positions of each TC center and their maximum 10 min-sustained wind speed (Wmax) every 6 h. We only considered cyclones reaching category 1 or higher (Wmax ≥ 33 m/s). A total of 74 TCs with Wmax ≥ 33 m/s crossed the studied area between 2000 and 2020 ( Figure 1). Following Vincent et al. [23], we then used the Willoughby cyclone model [17] to reconstruct the 2D idealized surface wind speed structure generated by each TC over the 20 year period studied (2000-2020). The Willoughby model (Equation (2)) allows to reconstruct at a given time (t) the 2D surface wind speed structure of a TC based on both the latitude of its center (Φ) and the maximum sustained wind speed (Wmax) as provided by the IBTrACS database.
In Equation (2) RMW is the radius of maximum wind (i.e., the distance r from the center where wind speed reaches Wmax), Wr is the radial wind speed generated at distance r from the center of the TC.
The Willoughby model generates an exponential increase in wind speed from the center of the TC (the "eye") to the RMW where winds reach their maximum speed (Wmax, Figure 2). Beyond RMW, wind speed decreases exponentially as a function of Wmax and Φ. According to this model, the most powerful TC over the studied area and study period (i.e., the category 5 TC Winston in 2016) generated winds ≥ 17 m/s up to 250 km off the TC center. Because TCs can move fast, we performed a linear interpolation of the original six-hour Φ and Wmax provided by the IBTrACS database to reconstruct the 2D surface wind speed structure of each TC every 15 min at a 250 m spatial resolution. Finally, we computed for each TC the maximum speed of winds that affected each pixel.

Analysis
We analysed the damages caused by TCs to forest vegetation and the recovery time from these damages. For each forest pixel exposed to wind generated by a category 1 or greater TC, we computed the damage caused by the cyclone passages as the relative changes in NDVI: where NDV I t0 is the NDVI value measured just before the passage of the TC at t 0 , NDV I ti is the NDVI value measured at t i just after the passage of the TC with i starting at the first NDVI acquisition date after the passage of the TC and then every 16 days over the following year. Recovery times from these damages were estimated as TR, the time t i needed to recover at least 95% of the pre-cyclone NDVI value (NDV I t0 ). Many ∆NDV I ti values were missing because of the lack of acquisition or because of the presence of clouds. We filled in missing values using the linear interpolation function na.approx from the zoo R package.
We then used two types of model to predict damages (∆NDV I t1 ) and recovery time (TR) as a function of the maximum sustained wind speed (W max ): (i) Segmented linear models with fixed W max breakpoints corresponding to the boundaries of the categories of the Saffir-Simpson hurricane wind scale (hereafter, SSHWS model), and (ii) segmented linear models with free number and location of W max breakpoints (hereafter, optimized model). For the optimized models, we tested different numbers of breakpoints ranging from 1 to 5 breakpoints and then kept the models that minimized the Bayesian information criterion (BIC), i.e., the models that exhibited the best compromise between the number of parameters and likelihood. Segmented linear models were adjusted using the segmented function of the segmented R package [24] and BIC values were computed using the BIC function available in the R base package. We tested the significance of the presence of breakpoints (i.e., the presence of a non-constant regression parameter in the linear predictor) using the davies.test function [24].
Reconstructed W max and observed ∆NDV I t1 are available online (http://preditropic. ird.nc, accessed on 22 February 2022). Moreover, it let the user forecast the ∆NDV I t1 following a simulated TC based on the optimized model.

Results
According to the 2D reconstruction of the wind speed structure of TCs, almost all forest pixels (95.6%) located in the studied area were exposed at least once to maximum sustained wind speed ≥ 33 m/s over the last 20 years (Table 2). A third of these forest pixels (33.1%) were exposed at least three times to such wind speeds ( Table 2). Most intense TCs are relatively infrequent. Only 29 TCs reached categories 4 or 5 and less than a quarter (21.3%) of the forest pixels were exposed at least once to maximum sustained wind speed ≥ 58 m/s (category 4). This proportion fell down to 5% when we only considered category 5 cyclone winds (i.e., ≥70 m/s, Table 2).
For both damages and recovery time, optimized segmented linear models (for which the number and position of breakpoints were optimized) performed much better than SSHWS segmented linear models (for which the number and position of breakpoints were fixed following the SSHWS), with ∆BIC = −28.35 and −44.33, respectively (Tables 3 and 4). In both cases the optimized models identified two significant breakpoints. The first breakpoints, located at 50.00 m/s for damages as swhown in Figure 3a and 50.48 m/s for recovery time as shown in Figure 3c, corresponded to the transition between category 2 and 3 TCs. Below 50.00 m/s observed damages were negligible ( 1% decrease in NDVI on average) and the corresponding recovery time was less than two weeks. Table 2. Coverage of forest pixels exposed to different intensity and frequency of TCs between 2000 and 2020. For instance, in the first row we can read that most forest pixels (95.6%) were exposed to at least one category 1 or higher TC and in the third row that a third of forest pixels (33.1%) were exposed to at least three category 1 or higher TCs. For Wmax greater than 50 m/s, observed damages to forest vegetation started to increase with an average at ∼1% of NDVI loss for each 1 m/s increase in Wmax (Figure 3a). This increase in damages resulted in a sharper increase in recovery time with an average rate of almost four days per 1 m/s increase in Wmax (Figure 3c). Another breakpoint was identified within the category 5 TC category at 75 m/s (75.70 m/s for damages and 76.64 m/s for recovery time). Above this breakpoint, damages and recovery times sharply increased. On average >40% decrease in NDVI and >150 days of recovery time were observed for the strongest winds (78 m/s).

Maximum Sustained Wind Speed
We observed high deviations around these trends. Standard deviation for the observed damages was about 20% and tended to slightly decrease with increasing wind speed (Figure 3b). Standard deviation in recovery time was about 30-35 days below 50.00 m/s, but then sharply increased reaching on average ∼90 days for category 5 TCs (Figure 3d). Furthermore, over 20% of the studied pixels did not recover after one year. One important outcome of our modelling is the production of spatial maps of damage and recovery time. An example is illustrated in Figure 4 which shows the observed and best-modelled (optimized model) damages and recovery caused by the category 5 TC Winston (2016) on Viti Levu (Fiji).

Discussion
Our results partly support the use of the Saffir-Simpson hurricane wind scale (SSHWS) to anticipate the damages caused by TCs to forest vegetation. Simpson [15] suggested that winds < 50 m/s mostly resulted in minor defoliation and we indeed found that such winds resulted in negligible damages to forest vegetation ( 1% decrease in NDVI). Our modelling showed that the most significant breakpoint was located at 50 m/s. This breakpoint corresponded to the transition between category 2 and 3 TCs identified by Simpson [15] above which trees start to experience major damages including extensive defoliation, branch breaking, tree uprooting, and bole snapping. This is in agreement with field observation in the south-west Pacific. For instance, forest plots located on Tutuila (American Samoa) and exposed to winds of 12-14 m/s generated by TC Heta (2003-2004) exhibited < 5% uprooting or bole snapping for trees larger than 10 cm [25]. Forest plots located on a neighboring island (Ta'u) exposed to reconstructed wind of 60-62 m/s from the TC Olaf (2005) exhibited from 20% to almost 100% uprooting or bole snapping for trees larger than 10 cm depending on the species [5]. Intermediate damages were observed on the island of Vava'u (Tonga) with about 20% uprooting or bole snapping for trees larger than 10 cm in forest plots affected by reconstructed wind of 51 m/s from TC Waka (2001) [26].
Model optimization highlighted another significant breakpoint located around 75 m.s, i.e., within the highest category of the SSHWS (that begins at 70 m/s for category 5). When this breakpoint was reached, sharp increases in both the damages caused by TCs and the recovery time were observed. These high intensity winds (>75 m/s) resulted on average in a reduction in NDVI greater than 40%. While this breakpoint relies only on a few points, it feeds the debate about updating TC intensity scales to better fit increasing TC intensity (e.g., [27,28]). Indeed, such extremely intense TCs occurred during the last decade and may reflect the possible intensification of these disturbances under human-induced global warming (e.g., [10,29]). For instance, after typhoon Haiyan (2013), one of the most intense TCs ever recorded worldwide, severely impacted the Philippines, Lin et al. [30] suggested creating a new category 6 for TCs generating winds ≥ 80 m/s. However, based on the most recent analyses [10], it is not clear that increased TC intensities have indeed occurred during the past 40 years (1979-2017) in the Western Pacific. On the same line, while Winston (2016) generated sustained wind speed > 80 m/s in the South Pacific, i.e, one of the strongest winds on record in the South Pacific, it remains to be explored whether such intensity records may increase in the future under global warming.
Reconstructed maximum wind speed was a good predictor of observed mean damages caused by TCs and mean recovery time. However, we observed large variations around mean trends. These large variations may be partly explained by the topography of the studied mountainous islands. Indeed, wind exposure varies greatly with topographical position, which result in very patchy damages in topographically complex areas [3,8,31]. The TC regime could also play a role in the effects of these disturbances on forests. Forests that have been historically exposed to frequent intense TCs are likely to be acclimated or adapted to these disturbances and thus could be more resistant to TCs than forests that have been historically rarely exposed to these disturbances [13]. Different levels of resistance of different forest types also probably explain the large variation in observed damages for a given maximum sustained wind speed. For instance, for a given wind speed, damages are expected to be lower in dry forests than in wet forests because dry forests are shorter [32] and composed of species that have on average harder wood than in wet forests [33]. Indeed, shorter tree height and higher wood density can provide higher mechanical resistance to wind and are often associated with less important cyclone-induced damages [5,18,34]. Finally, large variation in the observed damages for a given maximum sustained wind speed could be explained by different TC translation speeds with slower TCs (longer residence time) which should be more destructive than faster ones [35].
Variation in recovery time substantially increased with maximum sustained damage, especially after category 3-5 TCs. Below category 3 cyclones, damages mostly consisted in defoliation and recovery in production of new leaves. However, category 3 and more cyclones caused greater damages such as branch breaking, uprooting, and bole snapping. Several processes allowed recovery from these damages, including growth of surviving trees, growth of new trees from seed or sapling banks, and growth of new trees from new colonization events. Resprouting of surviving trees, i.e., the production of new vegetative growth from above or below ground dormant buds, is often observed after disturbances by TCs [36][37][38]. Resprouting capacity probably varies depending on habitat type and species composition. For instance, dry forest species might be better resprouters than wet forest species [39]. The rapidity of the establishment of new trees should be affected by the amount of forest and its spatial configuration. For the same level of damages, we expect that recovery time would be shorter in more forested and less fragmented landscapes compared to less forested and more fragmented ones. However, we cannot know whether the recovery in NDVI was due to the growth of trees or other growth forms. Likewise, we cannot know whether the recovery in NDVI was due to the growth of native species or to the establishment of non-native, potentially invasive, species. Invasion by non-native plant species following disturbance by TCs [40] which can slow down or block forest recovery is also more likely in more fragmented areas [41].

Conclusions
We found that damages caused to forest vegetation by TCs based on NDVI changes were negligible for winds <50 m/s, i.e., for category 1-2 TCs (SSHWS). This is consistent with recent conclusions reached by Peereman et al. [13] on global mangroves. Above this breakpoint, reduction in NDVI increased linearly with increasing wind speed which likely resulted from increasing major damages to vegetation, including branch breaking, tree uprooting, and bole snapping. A second breakpoint, with a sharp increase in damages for winds > 75 m/s suggested that extremely intense TCs can cause extreme damages to forest vegetation. While reconstructed maximum sustained wind speed was a good predictor of damages and recovery time, we observed large variations in the resistance and recovery of vegetation for a given wind speed. We hypothesize that these variations can be further explored by integrating other spatial factors such as topographical exposure, forest types, TC regime, and landscape structure as well as more detailed surface wind structures that are not represented in analytical models. Such enhanced wind structures may come from the recent advances on very intense wind retrievals from satellite imagery [42]. Whether the frequency of intense TCs will increase compared to weaker cyclones under climate change needs further investigations, especially in the South Pacific [10] and will require further advances in climate cyclone modelling in the future scenario [43]. Finally, we found that >20% of the studied forest pixels did not fully recover their pre-cyclone NDVI values after one year. Further studies are needed to assess whether or not these areas can recover to a forest state in a longer time period or whether they may shift toward non-forest states.

Conflicts of Interest:
The authors declare no conflict of interest.

Abbreviations
The following abbreviations are used in this manuscript: