ENSO effects on the transpiration of eastern Amazon trees

Abstract

Tree transpiration is important in the recycling of precipitation in the Amazon and might be negatively affected by El Niño–Southern Oscillation (ENSO)–induced droughts. To investigate the relative importance of soil moisture deficits versus increasing atmospheric demand (VPD) and determine if these drivers exert different controls over tree transpiration during the wet season versus the dry season (DS), we conducted sap flow measurements in a primary lowland tropical forest in eastern Amazon during the most extreme ENSO-induced drought (2015/2016) recorded in the Amazon. We also assessed whether trees occupying different canopy strata contribute equally to the overall stand transpiration ( T stand ). Canopy trees were the primary source of T stand . However, subcanopy trees are still important as they transpired an amount similar to other biomes around the globe. Tree water use was higher during the DS, indicating that during extreme drought trees did not reduce transpiration in response to low soil moisture. Photosynthetically active radiation and VPD exerted an overriding effect on water use patterns relative to soil moisture during extreme drought, indicating that light and atmospheric constraints play a critical role in controlling ecosystem fluxes of water. Our study highlights the importance of canopy and subcanopy trees to the regional water balance and highlights the resilience to droughts that these trees show during an extreme ENSO event. This article is part of a discussion meeting issue ‘The impact of the 2015/2016 El Niño on the terrestrial tropical carbon cycle: patterns, mechanisms and implications’.

Foliar water uptake in Amazonian trees: Evidence and consequences

“…Here, we measured upper-canopy trees as these account for a very high proportion of the total forest biomass and transpiration (Brum et al, 2018). Here, we measured upper-canopy trees as these account for a very high proportion of the total forest biomass and transpiration (Brum et al, 2018).…”

New insights into the variability of the tropical land carbon cycle from the El Niño of 2015/2016

“…This suggests that trees may be prioritizing carbon production at the expense of risking great hydraulic damage. The largest trees were found to be responsible for the greatest proportion of transpiration flux [25] and were also shown to have the greatest change in mortality rate as a consequence of drought [27]. Shenkin et al [27] found a relationship between drought-induced mortality rates and tree heights in Amazonian trees, but a weaker relationship with crown exposure, suggesting a limited role for atmospheric demand and a strong impact of water supply on the risk of death.…”

“…What is critical to interpreting changes in the signals from any of these variables over time is a clear understanding of which El Niñ o-mediated changes in climate are responsible for the observed variation in tree growth and carbon cycling. Brum et al [25] find that atmospheric water vapour deficit (VPD), rather than changes in soil moisture and precipitation, is the key driver of increasing dry season transpiration during the 2015/16 El Niñ o in a site in eastern Amazonia. Fontes et al [28] find sharp drops in water flux through trees, associated with drops in both leaf and xylem safety margins.…”

Drought Effects on Tectona grandis Water Regulation Are Mediated by Thinning, but the Effects of Thinning Are Temporary

“…Plantation trees experience similar meteorological effects as upper canopy trees experience in secondary forests. Changes in vapor pressure deficit (VPD), temperature, precipitation, and radiation have all been shown to affect plant water use at hourly, daily, and monthly timescales (Bretfeld et al, 2018;Brum et al, 2018). There are often species-specific thresholds where plant water use begins to decline in response to high VPD, radiation, or temperature (Sinacore et al, in review), which often leads to reduced tree water use and stand transpiration during droughts (Brum et al, 2018;Zhang et al, 2018).…”

“…Changes in vapor pressure deficit (VPD), temperature, precipitation, and radiation have all been shown to affect plant water use at hourly, daily, and monthly timescales (Bretfeld et al, 2018;Brum et al, 2018). There are often species-specific thresholds where plant water use begins to decline in response to high VPD, radiation, or temperature (Sinacore et al, in review), which often leads to reduced tree water use and stand transpiration during droughts (Brum et al, 2018;Zhang et al, 2018). With predicted increases in temperature, in particular, understanding at what threshold tree water use begins to decline at high temperatures and VPDs is crucial for modeling future stand and landscape level responses to a changing climate.…”