High drought-tolerance of an Eastern Amazon forest: results from a large scale rainfall exclusion Gina Cardinot 1,2, Daniel Nepstad 2,3, Missy Holbrook 4 and David Ray 2,3  The Woods Hole Research Center Funded by: 1 Federal University of Rio de Janeiro 2 IPAM-Amazon Institute of Environmental Research 3 Woods Hole 3Woods Hole ResearchCenter 3 Woods Hole 3Woods Hole Research Center 4 Harvard University

Many Amazon forests are resistant to drought

to identify thresholds beyond which key aspects of forest structure and function would be altered in important ways how is drought severity related to flammability? how is tree mortality influenced by prolonged drought? Goals: What will happen to the Amazon rainforest as rainfall declines?

A decrease in rates of stem diameter growth. Stem increment is one of the most sensitive components of NPPA decrease in rates of stem diameter growth. Stem increment is one of the most sensitive components of NPP Increase tree mortalityIncrease tree mortality A decrease in leaf water potential and increase of xylem damage by cavitationA decrease in leaf water potential and increase of xylem damage by cavitation Drought-induced reduction in leaf area may increase susceptibility to fire in large forest areasDrought-induced reduction in leaf area may increase susceptibility to fire in large forest areas

leaf-level physiological measurements, tree-level water balance determinations (e.g. sapflow) xylem proprieties

Floresta Nacional do Tapajós map Seca Floresta Project (rainfall exclusion experiment)

Study Site Layout Dry Plot Wet Plot

 Paired 1 ha plots  The panels cover ~75% of the ground area and drain into elevated gutters. 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

Results

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

Deep soil water

Nov/ 2000 Oct/ 2001 Dendrômetros.  1.7 Mg/ha/yr less carbon accumulation  25% of the Amazon, this would mean 170 million tons of C remaining in the atmosphere each year

Nov/ 2000 Tree mortality Beginning in May 2002, the middle of the third exclusion period, mortality rates for trees 10-30cm Dbh became elevated in the treatment plot relative to the control. In November 2002 nearing the end of the dry season, trees with >30cm Dbh began to show a similar response.

Tree mortality after 3 exclusion years

Leaf water potential - Predawn

Sap flux density

Figura 12. Instrumento para as medidas de LAI.

Xylem – specific conductivity

Vulnerability curves Percent Lost of Condutivity (PLC) vs Leaf water potential

 The forest shows a complex responses to drought  These data also help us to refine our forest models