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

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.

Role of DIN and DON in nitrogen cycle in a humid subtropical forest catchment in northeast Taiwan

<p>Although global models of nitrogen (N) cycling typically focus on nitrate of ecosystem N saturation, dissolved organic nitrogen (DON) is the dominant form of nitrogen export from many watersheds. In previous hypotheses, DON dynamics in the watersheds was treated as being functionally equivalent to inorganic N forms. However, unlike inorganic N, the dynamics of N contained within organic molecules is controlled not only by direct biological demand for N, but also by heterotrophic demand for the reduced C, to which N is attached. During 2016-2018, we evaluated the DON release hypothesis and the passive carbon vehicle hypothesis by comparing streamwater DON, DOC, and DIN concentrations across Fushan experimental forested watershed in the northeast Taiwan. We found that (1) the export of the Fushan Experimental Forest (FEF) is N saturated and (2) the altering nature of the DON release hypothesis and passive carbon vehicle hypothesis between non-event days and typhoon events. Results show that DON concentrations change systematically with increasing nitrate concentrations in all surveys. Among which, DON concentration correlates negatively with nitrate concentration in non-event days but positively during typhoon events. Our results support the coupling between DIN, DON, and DOC concentrations in forested watersheds that are subject to high rates of anthropogenic N loading. In non-event days, the N-containing dissolved organic matter may be in a labile form of carbon. Thus, alleviating heterotrophic N limitation may result in a decrease in DON export (passive carbon vehicle hypothesis), while during typhoon events, DON losses increase as demand for labile N forms attenuates (DON release hypothesis). These hypotheses are not mutually exclusive but represent the potentially contrasting roles of DON within C and N cycles. Our study suggests that bioavailability assays and addition experiments will present variations in the direct biological demand for N and heterotrophic demand for the reduced C, which is informative and necessary for characterizing the processes controlling DON export.<br><br></p><p><strong>Keywords:</strong> DON, DIN, N saturation, DON release hypothesis, passive carbon vehicle hypothesis</p>

Typhoon effects on litterfall in a subtropical forest

The litterfall in a subtropical broadleaf forest within the Fushan Experimental Forest in northeastern Taiwan was monitored for 9 years. Mean annual litterfall was very sensitive to typhoon frequency and intensity, ranging from 3 to 11 Mg·ha–1·year–1. Litterfall was significantly higher in years with strong typhoons than in years without typhoons, and the number of strong typhoons explained 82% of interannual variation in litterfall. Nutrient-use efficiency (dry mass/nutrients in litterfall) was high for N, but low for P compared with other tropical forests. This result supports the idea that the study forest is P limited but not N limited. Nutrient loss via litterfall represents a large percentage of aboveground biomass, especially during years with strong typhoons (e.g., 19%–41%, 15%–40%, 5%–12%, for N, P, and K, respectively). Forests that experience infrequent wind disturbance (e.g., temperate or boreal forests) can gradually regain any lost nutrients prior to the next disturbance; this is different from the situation observed in the Fushan Experimental Forest. At Fu-shan the pattern of not responding to typhoons with a flush of new growth appears to be an adaptation to the frequency with which there are multiple typhoons affecting the forest in a single year. Nutrient loss in litterfall caused by frequent typhoon disturbances appears to limit tree growth and contributes to the very low canopy height of the Fushan Experimental Forest.