The Effect of the 2015/16 El Niño on Hydraulic Characteristics of Central Amazonian Trees

Extreme drought events in the Amazon forest have been more frequent in recent decades and are commonly associated with the El Niño phenomenon. Such an extreme drought occurred in 2015/16 and offered a unique opportunity to enhance our understanding of how Amazon trees respond to severe water deficit. How the plants respond physiologically to such extreme short changes between water supply and demand, caused by climate anomalies, is still unknown. Here, we present in situ observation of pressure-volume and vulnerability curves, monthly leaf water potential, leaf temperature and sap velocity from four Amazonian tree species during the 2015/16 El Niño. We show that three out off four species in Central Amazon crossed their hydraulic threshold (e.g. reached their 50% loss of hydraulic conductivity - P50, their leaf turgor loss point or both) during the El Niño. This is potentially due to reduced amounts of soil water available for plants, increased leaf temperatures and decreased air relative humidity. Precipitation during August- October/2015 was below 100mm and 35% lower than in 2010, the strongest drought ever recorded. In addition, leaf temperatures became higher reaching 47oC. This combination of conditions resulted in the inability of these trees to cool their leaves via transpiration, leading to turgor loss (wilting point) that resulted in combined heat and water stress damage to many leaves. In addition, sap velocity in all of the trees studied, decreased considerably during the driest months and leaf water potentials became more negative, even during the predawn, leading species to lose 50% or more of their hydraulic conductivity. Our results corroborate the findings that frequent droughts will lead to a combination of physiological stresses that potentially increase the risk of tree mortality linked to hydraulic failure. Furthermore, our results demonstrate how vulnerable Amazonian tree species are to extreme drought events and improve our understanding of how changes in climate could affect species distribution, tree mortality and abundance in the tropics.