Reaching the Holy Grail During the Tropical Wet Season: Accelerated Daytime Stem Growth and Respiration of Canopy Trees in the Amazon Basin
Kolby Jardine1,2* (email@example.com), Gustavo Spanner2, Edson Augusto2, Aatish Sunder1, Daisy Souza2, Bruno Gimenez2, Robinson Negron-Juarez1, Jeffrey Warren3, Nate McDowell4, Niro Higuchi2, Adriano Lima2, Jeffrey Chambers1,2
1Lawrence Berkeley National Laboratory, Berkeley, CA; 2National Institute for Amazon Research, Manaus, Amazonas, Brazil; 3Oak Ridge National Laboratory, Oak Ridge, TN; 4Pacific Northwest National Laboratory, Richland, WA
Understanding how diurnal respiratory and growth rates of tropical trees respond to temperature in the dry and wet seasons is critical for accurately simulating the future of tropical forest dynamics. While methods to study growth and respiratory processes over diurnal time scales are lacking, this study hypothesized that tree growth rates and their associated respiratory loss rates of carbon dioxide (CO2) are much more sensitive to mild tissue water-stress associated with transpiration than photosynthetic carbon assimilation processes in leaves. The study further hypothesized that stem wood growth and respiration, which accounts for the majority of tree biomass and a large fraction of autotrophic respiration, occurs mainly at night during the dry season and is associated with reduced transpiration rates and refilling of stem water storage capacity. During the day, high transpiration rates and reduced stem water potential result in insufficient daytime turgor pressure required for cell wall expansion and growth. This predicts a negative relationship between stem respiration and temperature. This also predicts that during the wet season, when transpiration is suppressed due to low vapor pressure deficits, increased daytime turgor pressure allows growth and respiration to accelerate as a function of temperature with a positive relationship predicted between stem respiration and temperature. These hypotheses were evaluated by developing a portable system for quantifying real-time stem CO2 efflux rates (ES). Diurnal patterns were summarized in sap flow and ES during the dry and wet seasons in the central Amazon across fast- and slow-growing tree species. While previous work revealed an Amazon “green-up” during the dry season due to leaf flushing, this study suggests that the holy grail for plants is achieved during the wet season, when temperature-stimulated photosynthesis and growth can occur in parallel during the day due to sufficient turgor pressure.