COS 239-3 - A shift from light-limited to water-limited sap velocity in the Central Amazon during an extreme drought

Thursday, August 18, 2022

10:30 AM – 10:45 AM EDT

Presenting Author(s)

LM

Lin Meng

Lawrence Berkeley National Laboratory, United States

Slides (PDF)

Coauthor(s)

JC

Jeffrey Chambers

Lawrence Berkeley National Laboratory, United States
CK

Charles Koven

Lawrence Berkeley National Laboratory, United States
GP

Gilberto Pastorello

Lawrence Berkeley National Laboratory, United States
BG

Bruno Gimenez

National Institute of Amazonian Research (INPA), Brazil
KJ

Kolby Jardine

Lawrence Berkeley National Laboratory, United States
YT

Yao Tang

Lawrence Berkeley National Laboratory, United States
NM

Nate G. McDowell

Pacific Northwest National Laboratory, United States
RN

Robinson Negron-Juarez

Lawrence Berkeley National Laboratory, United States
ML

Marcos Longo

Lawrence Berkeley National Laboratory, United States
AA

Alessandro Araujo

Embrapa Amazônia Oriental, Tv. Dr. Enéas Piheiro, Brazil
JT

Javier Tomasella

Centro Nacional de Monitoramento e Alertas de Desastres Naturais - CEMADEN, Brazil
CF

Clarissa Fontes

University of California, Berkeley, United States
MM

Mikey Mohan

University of California, Berkeley, United States
NH

Niro Higuchi

National Institute of Amazonian Research (INPA), Brazil

Background/Question/Methods

Transpiration in tropical forests plays a critical role in regulating the global water cycle and climate. Transpiration is often considered to be light- but not water-limited in humid tropical rainforests due to abundant soil water, even during the dry seasons. As the climate becomes warmer and drier, it is more likely that a threshold could be reached after which soil moisture, instead of light availability, becomes a primary regulator of transpiration. The record-breaking 2015-16 El Niño drought provided a unique opportunity to examine whether transpiration is constrained by water under severe lack of rainfall. We measured sap velocity, soil water content, and meteorological variables in an old-growth upland forest in the Central Amazon throughout the 2015-16 drought.

Results/Conclusions

We found a rapid decline in sap velocity and in its temporal variability during the drought compared to the regular dry season. Such changes were accompanied by a marked decline in soil moisture and an increase in temperature and vapor pressure deficit. Sap velocity was largely limited by net radiation during the wet season; however, it shifted to be primarily limited by soil moisture during the drought. The threshold in which sap velocity became dominated by soil moisture was at 0.33 m3/m3 (around -150 kPa in soil matric potential), below which sap velocity dropped steeply. Our study provides evidence for a soil water threshold on transpiration in a moist tropical forest, suggesting a shift from light limitation to water limitation under future climate characterized by increased temperature and an increased frequency, intensity, duration and extent of extreme drought events. The ability of tropical forests in the Amazon to survive in the future largely depends on their acclimation and adaptation to drier conditions.