Abstract
El Niño events generate periods of relatively low precipitation, low cloud cover and high temperature over the rainforests of Southeast Asia, but their impact on tree physiology remains poorly understood. Here we use remote sensing and functional trait approaches—commonly used to understand plant acclimation to environmental fluctuations—to evaluate rainforest responses to an El Niño event at a site in northern Borneo. Spaceborne measurements (i.e. normalised difference vegetation index calculated from Moderate Resolution Imaging Spectroradiometer data) show the rainforest canopy greened throughout 2015, coinciding with a strengthening of the El Niño event in Sabah, Malaysia, then lost greenness in early 2016, when the El Niño was at its peak. Leaf chemical and structural traits measured for mature leaves of 65 species (104 branches from 99 tree canopies), during and after this El Niño event revealed that chlorophyll and carotenoid concentrations were 35% higher in mid 2015 than in mid 2016. Foliar concentrations of the nutrients N, P, K and Mg did not vary, suggesting the mineralisation and transportation processes were unaffected by the El Niño event. Leaves contained more phenolics, tannins and cellulose but less Ca and lignin during the El Niño event, with concentration shifts varying strongly among species. These changes in functional traits were also apparent in hyperspectral reflectance data collected using a field spectrometer, particularly in the shortwave infrared region. Leaf-level acclimation and leaf turnover could have driven the trait changes observed. We argue that trees were not water limited in the initial phase of the El Niño event, and responded by flushing new leaves, seen in the canopy greening trend and higher pigment concentrations (associated with young leaves); we argue that high evaporative demand and depleted soil water eventually caused leaves to drop in 2016. However, further studies are needed to confirm these ideas. Time-series of vegetation dynamics obtained from space can only be understood if changes in functional traits, as well as the quantity of leaves in canopies, are monitored on the ground.
Highlights
El Niño events bring periods of low rainfall and reduced cloud cover to tropical rainforests that are typically supplied with plentiful water (Lopes et al 2016), but the responses of tropical rain forests to these events remain poorly understood
Spaceborne measurements show the rainforest canopy greened throughout 2015, coinciding with a strengthening of the El Niño event in Sabah, Malaysia, lost greenness in early 2016, when the El Niño was at its peak
We argue that trees were not water limited in the initial phase of the El Niño event, and responded by flushing new leaves, seen in the canopy greening trend and higher pigment concentrations; we argue that high evaporative demand and depleted soil water eventually caused leaves to drop in 2016
Summary
El Niño events bring periods of low rainfall and reduced cloud cover to tropical rainforests that are typically supplied with plentiful water (Lopes et al 2016), but the responses of tropical rain forests to these events remain poorly understood. There is evidence of greening of tropical tree canopies during dry periods in the Amazon based on optical remote sensing (Saleska et al 2007). One explanation for this greening is that trees continue to access water through deep root systems (Huete et al 2006) and acclimatise to greater direct solar irradiance (i.e. reduced cloud cover) by up-regulating photosynthetic and protective pigments (Saleska et al 2007, Saleska et al 2016). Interpretation of remotely sensed data is challenging and there is continued debate over tropical forest responses to variability in temperature, irradiance and precipitation (Samanta et al 2010, Morton et al 2014, Bi et al 2015)
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