NTU Newsletter April 2014  
Spring El Niño Droughts Delay and Impede Vegetation Growth in Taiwan

Researchers at the Department of Geography recently published a study in the international journal Environmental Research Letters. In the article, which was featured in the ERL's special issue "Focus on Extreme Events and the Carbon Cycle" in December, the team details its evaluation of the temporal dynamics of vegetation onset associated with the El Niño/La Niña-Southern Oscillation (ENSO) and the influence of onset time on vegetation productivity in subtropical Taiwan.

Vegetation phenology, which is the investigation of plant lifecycles and their relationships with seasonal climate variations, reflects the impacts of climate change on a terrestrial ecosystem. By analyzing vegetation phenological metrics, including the onset and offset times of summer growing seasons and the length of summer growing seasons, the team found that spring drought caused by ENSO significantly influences vegetation productivity in Taiwan.

The team obtained a time series of photosynthetically active vegetation cover data spanning the decade of 2001-2010 from Moderate Resolution Imaging Spectroradiometer (MODIS) reflectance data and analyzed the data using the phenological analysis toolbox TIMESAT in order to determine the spatial distribution of vegetation phenology in Taiwan.
The analysis showed that a dry spring (February and March) with less than 40 mm of rainfall directly caused the delay of the start of the subsequent growing season and that this seasonal drought impeded vegetation growth in the subsequent season as well. The team attributed this to the delayed impacts of moisture stress related to the preceding ENSO events.

The team discovered strong positive correlations between annual vegetation productivity and both vegetation onset times and lengths of the growing seasons, which it said might indicate that accumulated rainfall in the spring governs annual vegetation productivity.
The team noted that climate forecasts point to coming increases in the frequency and intensity of ENSO-related spring droughts, which could mean dire consequences for ecosystem metabolism.