Impact of the 2015 El Niño event on Borneo: Detection of drought damage using solar-induced chlorophyll fluorescence

　Remotely sensed solar-induced chlorophyll fluorescence (SIF) is applicable as an indicator of changing photosynthetic activity in terrestrial ecosystems. The vegetation of Borneo has previously been affected by drought and fire during El Niño events. Changes in satellite-based SIF data during an El Niño event in Borneo in 2015 were examined using three satellites-GOSAT, GOME-2, and OCO-2-covering the whole island and its southern and northern areas, respectively. Relationships between environmental factors and vegetation damage, precipitation, fire incidence, vegetation indices, and gross primary production (GPP), which were determined using machine-learning methods, were also examined for the period 2007–2018. SIF tended to be low in dry seasons, even in normal years, possibly because of increased drought stress and/or a higher incidence of fires with less precipitation. During the dry season of 2015, there were significant reductions in SIF in southern Borneo where fires were frequent. Other vegetation indices and GPP were also lower. Serious drought conditions with frequent fires during the El Niño event might have caused ecological degradation throughout Borneo, with a significant decrease in SIF.