Tropical Tree Water Availability and Sourcing

Authors

Jeffrey Warren¹* (warrenjm@ornl.gov), Cynthia Wright^1,2, Rutuja Chitra-Tarak³, Bruno Gimenez⁴, Robinson Negrón-Juárez⁵, Adam Collins³, Kurt Solander³, Gustavo Spanner⁴, Regison Oliveira⁴, Valdiek da Silva Menezes⁴, Alfonso Zambrano², Jeffrey Chambers⁵, Niro Higuchi⁴, Stuart Davies², Nate McDowell⁶, Alex Neild¹, Sarah Ottinger¹

Institutions

¹Oak Ridge National Laboratory, Oak Ridge, TN; ²Smithsonian Tropical Research Institute, Washington DC; ³Los Alamos National Laboratory, Los Alamos, NM; ⁴National Institute of Amazonian Research, Manaus, Brazil; ⁵Lawrence Berkeley National Laboratory, Berkeley, CA; ⁶Pacific Northwest National Laboratory, Richland, WA.

URLs

• https://ngee-tropics.lbl.gov/

Abstract

As the global climate warms, tropical forests increasingly experience extreme heat and recurrent drought conditions, which can lead to declining growth rates and mortality under extreme conditions. Spatial patterns of soil water availability and plant water sourcing depth, which depend on rainfall, soil texture, and topography, are important for maintaining tree function during drought. Trees employ hydraulic strategies to combat drought, including stomatal regulation, stem water storage, drought-deciduousness, niche positions on the landscape, and deep rooting. Strong evidence suggests that hydraulically vulnerable, yet deeply rooted species are less sensitive to drought, which is reflected in potentially high growth and low mortality following drought events. Rooting depth can be inferred through analysis of stable water isotopes in precipitation, ground water, soils, and tree xylem. To understand the depth of tree water sourcing across the hyperdiverse tropics, researchers assessed stable isotopes during seasonal drought in dozens of species across wood-density and life-strategy spectra of old-growth forests in Panama and Brazil. To understand water availability, researchers deployed sensors for soil water content and soil water potential across the topographic gradient of each field site. Root distribution and structural and hydraulic traits were also assessed to investigate tree level drought strategies. Data will be used to model water availability and extraction patterns and to link plant water sourcing depth to other functional plant traits to enable scaling. Results from the upland K34 tower in the central Amazon Forest indicate that a vast number of fine roots are in the upper soil, with 45% in the upper 5 cm. However, observations from the K34 soil pit revealed roots growing at depths up to 10 m, indicating some species are accessing these deep soils. During a month-long drought, upper soils dried toward the wilting point and water extraction shifted to deeper layers. An analysis indicated that the upper 2 to 3 m could sustain evapotranspiration demand. New soil water measurements across the K34 topography indicate differential rates of soil drying and water availability, and thus differential tree hydraulic stress during drought.

Water stable isotope samples are currently under analysis to assess species-trait assemblages capable of accessing deeper water. This ongoing work is poised to leverage data collection during the next El Niño event, forecasted for 2023 to 2024. Future measurements of soil water availability and tree water sourcing are planned pantropically, including in Malaysia.