Estimating the Reduction in Photosynthesis from Sapflow Data in a Throughfall Exclusion Experiment. Rosie Fisher 1, Mathew Williams 1, Patrick Meir 1, Yadvinder Malhi 1 Antonio Lola da Costa 2, Samuel Almeida 3 (1) University of Edinburgh, Edinburgh, Scotland, UK (2) Universidade Federal do Para, Belem, PA, Brazil. (3) Museu Paraense Emilio Goeldi, Belem, PA, Brazil.

Aim –To understand effect of drought on carbon uptake. Complications –Eddy Covariance: impossible at this scale. –Drought physiology =  Stomatal Conductance (g s ) But g s is extremely heterogeneous Solution -INFER  g s from changes in Sapflow (tree water use) -Use g s to predict  photosynthesis rates. How does drought affect carbon exchange?

Estimating g s from Sapflow Data Tree water use depends on g s and Atmospheric Demand. Atmospheric demand depends on: –Temperature –Vapour Pressure Deficit (VPD) –Windspeed (boundary layer conductance) –Leaf Area –Irradiance These environmental variables are vertically heterogeneous Need a model to understand how they are integrated…

Soil Plant Atmosphere Model (SPA) Williams et al Process based simulation model of forest gas exchange Driven by hourly climate data Multi layered canopy meteorology and flux simulations.

(Manaus-1996) SPA Predictions are applicable across many ecosystems

Soil Plant Atmosphere Model (SPA) Williams et al Only these parameters altered between sites: –Leaf Area Index (LAI): LAI 2000 –Leaf Photosynthetic Capacity: Licor 6400 –Hydraulic Properties…..

SPA Water Transport Model  Leaf >  min Soil Roots  soil Stem Leaf 1. Capacitance 2. Soil-Leaf Resistance 3. Soil Water Potential gsgs C uptake

Which hydraulic parameter is the main cause of drought stress? Observed range of swp

Which hydraulic parameter is the main cause of drought stress?

 Hydraulic Resistance may be the cause of drought stress

SPA Water Transport Model  Leaf >  min Soil Roots  soil Stem Leaf 1. Capacitance 2. Soil-Leaf Resistance 3. Soil Water Potential gsgs C uptake Very high 5000 g MPa -1 m -2 Very Low 0.5 Mpa s m 2 mmol -1 Very Wet MPa A. Low hydraulic stress mode

SPA Water Transport Model  Leaf >  min Soil Roots  soil Stem Leaf 1. Capacitance 2. Soil-Leaf Resistance 3. Soil Water Potential gsgs C uptake Standard 2000 g Mpa -1 m -2 Optimised to produce best fit Measured B. Best fitting resistance mode

Optimising hydraulic resistance explains daily sapflow res=9.1res=11.4res=18.5res=16.6 Mpa s m 2 mmol -1 R 2 =0.93

Validation of carbon model

Daily GPP model validation

 Resistance causes p.m. stomatal closure

Carbon cost of drought

Conclusions 1.The control plot shows little evidence of flux limitation below energetic limits. 2.Altering soil to leaf hydraulic resistance successfully simulates sapflow response to drought. 3. The droughted plot shows a probable reduction in carbon uptake of up to 40%

Further Investigation –Develop soil model + predict hydraulic stress from meteorology data. –Test model against measured intermediate variables LWP Stomatal Conductance Do we get the right result for the wrong reason?

Acknowledgements –Raquel Lobo de Vale 1 Leaf Photosynthetic capacity data –Rafael Ferreira da Costa 2 and Alan Braga 3 Leaf Water Potential measurements and data collection –Joao Athaydes 3 Data assimilation and processing.. 1. Instituto Superior de Agronomia, Lisboa, Portugal 2. Museu Paraense Emilio Goeldi, Belem, PA, Brazil 3. Universidade Federal do Para, Belem, PA, Brazil

Control sapflow is close to energetic limit Droughted sapflow is limited by treatment/drought Dry Season

Amazon-Climate Feedback?  CO 2 Uptake by Forests  Temperature  Rainfall  Soil Moisture  Photosynthesis  Climate Change Fossil Fuel Burning Global Climate Model (Hadley Centre, UK)  VPD  Stomatal conductance  Leaf Area

Best Fitting Hydraulic Resistance Model - Can we explain the observed dynamics with this model? Capacitance/ Storage  leaf Soil Roots Soil-Leaf Resistance  soil Stem Leaf Stomata (Gs) Soil Water Potential Fitted using LWP data Optimised to give lowest model-data error From predawn LWP data

The Soil Plant Atmosphere Model (SPA) Williams et al Capacitance/ Storage  leaf Soil Roots 2. Hydraulic Resistance 3.  soil Loss to atmosphere driven by Penman- Monteith Equation Stem Leaf Stomata (Gs) Water moves down a potential gradient through series of resistors >  min Gs optimised to achieve maximum photosynthesis without drying leaves below minimum safe  leaf Gs used to predict Ci + photosynthesis

. Throughfall Exclusion Experiment Caxiuana, Para, NE Brazil.

Field Measurements- do I need this if p says it? Meteorology SapflowLeaf Water Potential

Sapflow Results

Leaf Water Potential at 30m

Sapflow scaling

LWP

Fitting Hydraulic Parameters Low Hydraulic Limitation Mode Fitted Hydraulic Resistance Mode Soil-Leaf Hydraulic Resistance 0.5 (v.low) Found best fitting value for each day. Hydraulic Capacitance5000 (v.high) Fitted from seasonal LWP data Soil Water Potential0.001 (v.high) Estimated from predawn LWP

‘Maximum possible sapflow’, fits Control plot sapflow data, but over-predicts Drought plot sapflow.