Contrasting elevational patterns of forest structure across the Taiwan Strait in monsoon Asia: the potential role of typhoon disturbance

Background/Question/Methods

Understanding factors controlling ecosystem structure and function is key to predicting the effects of climate change on vegetation distribution and carbon sequestration. Species diversity, biomass, and tree height of forest ecosystems are frequently found to decrease with increasing elevations except where drought stress is more severe at low than high elevations. Typhoon disturbance is similar to drought stress in that it could reverse the common elevational trend in species diversity, biomass, and tree height. Here we report elevational trends (800-3000 m) in species richness and tree height/biomass of humind forests in Taiwan, an island experiencing an average of 0.9 typhoons annually. We ask if the elevational trend is reversed in Taiwan in contrast to the common elevational trend observed in Wuyishan (500-2100 m), a site at comparable latitudes but approximately 300 km inland across the Taiwan Strait and therefore well beyond the reach of typhoons.

Results/Conclusions

Tree species richness follows the common elevational trend; it decreases linearly with increasing elevations in both Taiwan and Wuyishan. However, there are increasing trends for biomass (12 to 4 Mg/ha, r = 0.57, p = 0.0013) and mean tree height (11.5 to 5.2 m, r = 0.83, p < 0.001) from low to high elevations in Taiwan but a decreasing trend in Wuyishan (12 to 2 m, r = -0.68, p < 0.001). We hypothesize that typhoon disturbance plays a key role driving the revered elevational trend in Taiwan based on two lines of evidences: 1) the variation in tree height at low elevations is smaller than that at high elevations in Taiwan, suggesting typhoons weaken as they move upwards, and 2) elevational patterns of water availability are similar between Taiwan and Wuyishan, suggesting drought stress is not likely a factor. This study is likely the first to report an increase in tree height/biomass with elevations in subtropical humid forests, and highlights the potential role of tropical cyclone in driving elevational patterns of forest structure. Climate change induced alterations in tropical cyclone regime may affect elevational patterns of forest structure and carbon sequestration in regions experiencing frequent cyclone disturbance.