An understanding of environmental control on transpiration and its representation in Canadian Land

surface Scheme for two boreal tree species

Shalini Oogathoo

Directeur: Daniel Houle

Co-Directeur: Dan Kneeshaw

26 Avril 2019

1

• Transpiration• important process

• Feeds large amount of water to the atmosphere

2

• Transpiration measured by sapflow techniques and eddy covariance.

Introduction

3

• Adequate representation of transpiration in land surface scheme:

during droughts

• An understanding of the functioning of transpiration (i.e. sapflow) in the natural system:

prior to the growing season during growing season

Introduction

Adds to the understanding of the model pros and cons

4

Temps

ma

i-0

9

juil.

-09

se

pt.-0

9

no

v.-

09

jan

v.-

10

ma

rs-1

0

ma

i-1

0

juil.

-10

se

pt.-1

0

no

v.-

10

jan

v.-

11

ma

rs-1

1

ma

i-1

1

juil.

-11

se

pt.-1

1

no

v.-

11

jan

v.-

12

ma

rs-1

2

ma

i-1

2

juil.

-12

Ra

yon

de

l'a

rbre

(m

m)

0

2

4

6

8

Source: L. Duchesne, 2012

Shrinkage Rehydration

AnnualPlant growthGrowth

• Classification of tree diameter into:

1. winter shrinkage, 2. spring rehydration

(i.e. prior to growing season) and

3. Summer growth.(Tardif et al., 2001)

Three phases:

Introduction

Hypotheses

5

Part I – Hypotheses & Objectives

• VPD and Rad are main drivers of sapflow during the growing season

• Soil water controls sapflow during severe drought events

• Test if VPD and Rad are the main drivers of sapflow

Objectives

• Test the importance of soil water on sapflow

*VPD: vapour pressure deficit; **Rad: solar radiation

• Model may not simulate transpiration (rehydration) prior to the growing season.

• Evaluate the performance of CLASS* for the simulation of transpiration

6

Part II – Hypotheses & Objectives

During growing season

During a severe drought event

• Model may underestimate transpiration during drought period.

• Evaluate the performance of CLASS* for the simulation of soil temperature & soil water

Prior to growing season

*Canadian Land Surface Scheme

Hypotheses Objectives

• Study area: two sites (Laflamme and Tirasse) located in Quebec.

7

Part I – Methodology

Laflamme: Balsam fir (3 trees / 2 probes per tree)Tirasse: Black spruce (3 trees / 2 probes per tree

8

Sapflow

Climate

Soil θ & T

Dendro

Sonde de

températures

Sonde

d’humidité

du sol

(TDR)

Part I – Methodology

Modelling transpiration

9

Part II – Methodology

• Use of Canadian Land Surface Scheme (CLASS)

• Land Surface Scheme for the Canadian Regional Climate Model

• CLASS• Land surface component

• exchange of heat, water and momentum fluxes between land surface and atmosphere

• CLASS inputs:• Climate; Soil; Vegetation

Environmental control during the growing season

10

Part I – Results

VPD and Rad had highest R2

VPD (kPa)

0,0 0,2 0,4 0,6 0,8 1,0

Daily s

ap

flo

w (

cm

)

0

10

20

30

40

50

60

Probe 6

R2: 0.83

VPD (kPa)

0,0 0,5 1,0 1,5 2,0 2,5

Daily s

ap

flo

w (

cm

)

0

10

20

30

40

50

60

70

Probe 3

R2 : 0.81

Relationship of sap flow to vapour pressure deficit (VPD)

11

Balsam fir (2005) Black spruce (2007)

Best-fitted with Gompertz (3 parameters) equation

Part I – Results

Rad (W m-2)

0 200 400 600 800

Daily s

ap

flo

w (

cm

)

0

20

40

60

80

Probe 2

R2 : 0.7

Rad (W m-2)

0 200 400 600 800

Daily s

ap

flo

w (

cm

)

0

20

40

60

80

100

Probe 2

R2: 0.82

Relationship of sap flow to solar radiation (Rad)

12

Balsam fir (2005) Black spruce (2007)

Best-fitted with sigmoidal (3 parameters) equation

Part I – Results

Hourly data: diel hysteresis of sap flow vs VPD and Rad

13

Part I – Results

• Stronger hysteresis with VPD than Rad.

• Hysteresis caused by• Lag between VPD and Rad• Plant and soil water status

Stomatal sensitivity to VPD (a trait of iso/anisohydricity)

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Part I – Results

• Greater slope = higher stomatal sensitivity to VPD (more isohydry).

• Black spruce is more anisohydry than balsam fir.

• Degree of anisohydricityincrease in 2006 due to extreme heat.

Sap flow during drought

15

Part I – Results

• Rad had highest R2 with sapflowduring the drought period, followed by VPD and precipitation.

• Soil water had a low insignificant R2

for both periods.

Simulated & observed soil temperature

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• Underestimates in winter slightly for both.• Overestimates in summer for balsam fir

Part II – Results

Simulated and observed soil water

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• Simulation follows the pattern of observations for both horizons for black spruce, except during winter for horB.

• CLASS converts all liquid water into frozen water

Part II – Results

• For balsam fir: simulation is not good, especially for horizon B

Simulated & observed transpiration prior to and during growing season

18

Balsam fir Black sprucePrior to growing season

CLASS simulate well during growing season, but not prior to growing season

Part II – Results

19CLASS does not simulate well from April to May

Part II – Results

R2 : 0.82 R2 : 0.91

Simulated & observed transpiration prior to and during growing season

Simulated transpiration during drought

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Balsam fir - 2012

Shaded area – drought period

CLASS overestimates transpiration in 2nd

half of drought period

Part II – Results

Conclusion

Hypothesis II was rejected: soil water did not take over control of sapflow

during drought as expected

(suggesting an anisohydric behaviour of balsam fir)

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Our hypothesis I was confirmed: VPD and Rad were found to be the main drivers

of sapflow during the growing season

Conclusion

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• Adequate performance of CLASS for soil temperature

But, important consideration:

soil water varies greatly spatially

• CLASS needs to be improved for the simulation of soil water

• CLASS simulates well transpiration

• CLASS poorly simulates transpiration

During growing season

Prior to growing season &

During drought

Conclusion

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While CLASS overestimates transpiration during drought, other land surface models underestimated transpiration during drought (Ukkola et al., 2016), In spite of the misrepresentation

of anisohydric species in CLASS

Thus, CLASS coupled with Canadian Regional Climate Model will underestimate drought events in future.

Land Surface Scheme being the main driver for climate model, a misrepresentation of transpiration will lead to inaccurate projection of climate model

Merci

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