1. Drought resistance of Amazon trees reflects a low vulnerability to cavitation: results from a large scale rainfall exclusion experiment. Gina Cardinot 1,2,, Missy Holbrook 3 & Daniel Nepstad 2,4  The Woods Hole Research Center Funded by: 1 Federal University of Rio de Janeiro 2 IPAM-Amazon Institute of Environmental Research 3 Harvard University 4 Woods Hole Woods Hole ResearchCenter 4 Woods Hole Woods Hole Research Center

2.  In terrestrial ecosystems, plants form the principal pathway for water movement from soil to atmosphere, and their ability to supply leaves with water is a prerequisite for CO2 uptake.  However, the capacity of the xylem to transport water from the soil to the leaves is itself at risk during periods of drought due to the increased potential for cavitation.  Studies of xylem hydraulic properties in the field are important tools for understanding how plants respond to ecosystem changes.

3.  Our research focuses on the effects of drought on the water transport capacity of Amazonian rainforest trees.  Our goal is to understand the extent to which cavitation results in a loss of hydraulic conductivity, potentially leading to stomatal closure, leaf shedding, and tree death.

4. Floresta Nacional do Tapajós-Santarém/PA Projeto Seca Floresta (throughfall exclusion experiment) Our research was carried out within the context of the world’s largest rainfall exclusion experiment. Eastern Amazônia; Pará State/Brazil)

5. Study Site Layout Dry Plot Wet Plot two 1-ha plots, of which one is a control and one in which rainfall reaching the forest floor has been reduced by 50% since 2000.

8.  The oxisol soil is highly weathered and clayey in texture. The water table is more than 100 m deep.  The panels cover ~75% of the ground area.  They are left in place during the ~6 month wet season between January and July.  The treatment prevents ~50% of incoming precipitation from entering the soil.  Baseline measurements were made for 1 yr prior to throughfall exclusion, beginning in Jan 1999  Rainfall exclusion since 2000

9. Coussarea racemosa - Rubiaceae Miconia engensis - MelastomataceaePoecilanthe effusa - Fabaceae Escheweilera pedicellata - Lecythidaceae. We chose four species which are common in this area

10. Field pressure-drop flow meter: Xylem specific conductivity Ks=F/L(∆P*Asw) Leaf specific condutictivity Kl=F/L(∆P*LA)

11. Results

12. Soil Water (TDR): Surface depletion in Yr 1; depletion at depth Yr 2. TDR probes under construction. Daily rain

13. Figura 12. Instrumento para as medidas de LAI.

14. Sap flux density

15. Vulnerability curves Percent Lost of Conductivity (PLC) vs Leaf water potential PLC 50% = -3.9 MPa PLC 50% = -3.6 MPa PLC 50% = -4.9 MPa

16. Xylem – specific conductivity

17. Leaf hydraulic conductivity a. b. c. a. b. * * * *

18. PLC--Native embolism

19. Leaf water potential - Predawn

21.  Drought resistance exhibited by Amazon tropical rainforest trees allows them to maintain physiological function and xylem continuity despite the marked seasonality in rainfall in the eastern Amazon.  The forest shows a complex responses to drought  Pleistocene climatic change - Plant respond as specie  These data also help us to refine our forest models

22.  THANKS! David Ray, Holbrook Lab-Peter Melcher, William Riker, Paul Lefebvre, Paulo Brando, Wanderley Rocha, Williams Castro and field technicians (Levi Seixas, Osvaldo Portela e Roberto Baena). CAPES fellowship to Cardinot