Diurnal patterns of leaf transpiration/stomatal conductance, leaf water potential, and sap flow in the Central Amazonian: Are patterns driven by an imbalance between leaf transpiration and sap flow?
Monday, 6 June 2016
Clarissa Fontes1, Kolby Jardine2, Todd E Dawson3, Bruno Gimenez4, Niro Higuchi4, Jennifer A. Holm2, Jeffrey Q Chambers1 and NGEE Tropics Manaus Pilot Study Team, (1)University of California Berkeley, Berkeley, CA, United States, (2)Lawrence Berkeley National Laboratory, Berkeley, CA, United States, (3)University of California Berkeley, Center for Stable Isotope Biogeochemistry, Berkeley, CA, United States, (4)INPA National Institute of Amazonian Research, Manaus, Brazil
Abstract:
Tropical forests cycle more carbon and water than any other terrestrial biome, but are highly sensitive to warming climates. Improvement of terrestrial carbon and water cycling predictions requires a better understanding of the physiological mechanisms underlying forest response to warming. Here, we present in situ observations from the Central Amazon that demonstrates leaf temperature to strongly control both water and carbon plant physiological processes. In situ measurements of net photosynthesis and stomatal conductance peaked in the morning and showed a clear mid-day/post-midday depression associated with temperatures passing the optimal value for net photosynthesis (~32 °C). Despite assumptions of a significant time delay, tight temporal correlations were found between sap velocity (1m), leaf transpiration (25 m), and leaf temperature (25 m). A useful metric for evaluating plant-water status is through leaf water potential (ΨL), which reflects the balance between sap flow and transpiration, and in the tropics is assumed to reach a minimum during mid-day. Here we show that despite high leaf temperatures throughout mid-day to early afternoon, dramatic recoveries (20-70%) in ΨL occur around mid-day and post-mid-day in Central Amazonian trees. This is potentially due to an increase in the sap flow/transpiration ratio caused by a decrease of stomatal conductance after 10:00 am. This study helps to elucidate the processes behind diurnal patterns of plant-water relations in the tropics. It can serve as a benchmark for dynamic vegetation models and calls into question the general assumption that ΨL reaches a minimum during mid-day and is tightly coupled with leaf temperature.
