In the Amazon basin, an estimated 25–50% of precipitation is recycled back to the atmosphere through forest transpiration, with important implications for the interactions between the biosphere and atmosphere. In this study, we present in situ field observations of environmental (direct solar radiation, air temperature (Tair) and vapor pressure deficit (VPD)) and physiological (sap velocity (Vs), stomatal conductance (gs), and leaf water potential (ΨL)) variables and their correlations with leaf temperature (Tleaf) during the 2015–2016 El Niño-Southern Oscillation (ENSO). In order to observe the interactions between physiological variables and fast changing environmental conditions, we collected high temporal frequency data (15–60 min) in two primary rainforest sites located in the Eastern (Santarém) and in the Central (Manaus) Amazon. Since the 2015–2016 ENSO event was the warmest period in the Amazon forest over the past 13 years, we expected peak Tleaf to increase and subsequently hysteretic behavior of water use vs. Tleaf to become more pronounced. In this study, we explored the mechanisms that regulate tree transpiration and the diurnal hysteresis patterns between physiological and environmental variables to contrast different tree species responses to the extreme 2015 dry season (ENSO) and a normal 2017 dry season (“control scenario”).
Despite expectations of a significant delay due to the large vertical distance between the observations of sap velocity (Vs) and leaf temperature (Tleaf), the two variables tightly track each other, during the day and night. On some days during the 2015 dry season, the differences between Tleaf and Tair were close to 8°C for some species (average difference between Tleaf and Tair for all species: 1.65 ± 1.07°C). For the first time in the Amazon forest, the quantitative differences and the hysteresis pattern between Tleaf and Tair were demonstrated and compared during the 2015 (ENSO) and 2017 (“control scenario”) dry seasons. The relationship between Tleaf and Tair was significantly different between these two periods and, in general, Tleaf was higher than Tair during the middle morning to early afternoon. The use of the variable Tleaf together with Tair is extremely important to ecophysiological observations due to the differences in terms of magnitude and temporal patterns. Also, Tleaf is an important variable to estimate the true water vapor pressure gradient between the substomatal cavity and the boundary layer of the air near the leaf surface (ΔVPD).
Sap velocity displayed species-specific diurnal hysteresis patterns that were strongly linked to gs and ΔVPD and reflected by changes in Tleaf. In the morning, gs was linearly related to Tleaf and sap velocity displayed a sigmoidal relationship with Tleaf. In the afternoon, stomatal conductance declined as Tleaf approached a daily peak, allowing ΨL to begin recovery, while sap velocity declined with an exponential relationship with Tleaf. Hysteresis indices (Tleaf : Tair and Tleaf : ΨL) were much more pronounced during the ENSO event than during a typical dry season and varied between species, which reflects species-specific capacitance and tree hydraulic traits. The clockwise hysteresis in Vs-Tleaf and Vs-ΔVPD was evident with morning periods showing higher temperature sensitivities than afternoon and night periods and in this study is referred to as the “gs effect”. In Manaus, the scatter plot of Vs-direct solar radiation revealed a counterclockwise hysteresis pattern, on the same day as the Vs-ΔVPD clockwise hysteresis. For the same direct solar radiation values, higher Vs values in the afternoon were observed relative to the morning period, and in this study this pattern is referred to as the “VPD effect.”
Current climate change scenarios indicate warmer temperatures and the potential for more extreme droughts in the tropics, such that a mechanistic understanding of the water cycle from individual trees to landscapes is needed to adequately predict future changes in forest structure and function. In this study, we contrasted physiological responses of tropical trees during a normal dry season with the extreme dry season due to the 2015–2016 El Niño-Southern Oscillation (ENSO) event. We quantified high resolution temporal dynamics of sap velocity (Vs), stomatal conductance (gs) and leaf water potential (ΨL) of multiple canopy trees, and their correlations with leaf temperature (Tleaf) and environmental conditions [direct solar radiation, air temperature (Tair) and vapor pressure deficit (VPD)]. The experiment leveraged canopy access towers to measure adjacent trees at the ZF2 and Tapajós tropical forest research (near the cities of Manaus and Santarém). The temporal difference between the peak of gs (late morning) and the peak of VPD (early afternoon) is one of the major regulators of sap velocity hysteresis patterns. Sap velocity displayed species-specific diurnal hysteresis patterns reflected by changes in Tleaf. In the morning, Tleaf and sap velocity displayed a sigmoidal relationship. In the afternoon, stomatal conductance declined as Tleaf approached a daily peak, allowing ΨL to begin recovery, while sap velocity declined with an exponential relationship with Tleaf. In Manaus, hysteresis indices of the variables Tleaf-Tair and ΨL-Tleaf were calculated for different species and a significant difference (p < 0.01, α = 0.05) was observed when the 2015 dry season (ENSO period) was compared with the 2017 dry season (“control scenario”). In some days during the 2015 ENSO event, Tleaf approached 40°C for all studied species and the differences between Tleaf and Tair reached as high at 8°C (average difference: 1.65 ± 1.07°C). Generally, Tleaf was higher than Tair during the middle morning to early afternoon, and lower than Tair during the early morning, late afternoon and night. Our results support the hypothesis that partial stomatal closure allows for a recovery in ΨL during the afternoon period giving an observed counterclockwise hysteresis pattern between ΨL and Tleaf.
Contacts (BER PM): Daniel Stover SC-23.1, Daniel.Stover@science.doe.gov (301-903-0289)
PI Contact: Bruno O. Gimenez, Smithsonian Tropical Research Institute (STRI), firstname.lastname@example.org
Kolby J. Jardine, Lawrence Berkeley National Laboratory (LBNL), Climate and Ecosystem Sciences Division, email@example.com
This material is based upon work supported as part of the Next Generation Ecosystem Experiments-Tropics (NGEE-Tropics) funded by the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research through contract No. DE-AC02-05CH11231 to LBNL, as part of DOE’s Terrestrial Ecosystem Science Program. Additional funding for this research was provided by Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES).
Gimenez B., Jardine K., Higuchi N., Negrón-Juárez R., Sampaio-Filho I., Cobello L., Fontes C., Dawson T., Varadharajan C., Christianson D., Spanner G., Araújo A., Warren J., Newman B., Holm J., Koven C., McDowell N., Chambers J., (2019) Species-Specific Shifts in Diurnal Sap Velocity Dynamics and Hysteretic Behavior of Ecophysiological Variables During the 2015-2016 El Niño Event in the Amazon Forest Front. Plant Sci. 10:830. https://doi.org/10.3389/fpls.2019.00830