New Zealand mangroves as a model system for studying tree

carbon and water relations Jarrod Cusens and Sebastian Leuzinger

H2O CO2

Carbon and water are tightly coupled

Transpiration contributes ca. 80-90% of terrestrial evapotranspiration

Forests and carbon

• Forests/trees dominate global terrestrial carbon cycle

• Almost all carbon that enters terrestrial systems passes through trees

• About 70-90% aboveground C is in forests

Transpiration clouds over the

Amazon

Transpiration clouds over the

Amazon

Studying natural systems

Observational

• Complex

• Unpredictable

• Uncontrollable

Greenhouse experiments

• Not always representative

• E.g. seedlings and saplings

Costly

Swiss Canopy Crane Project

Mangrove survival

• The intertidal zone is harsh and a stressful place for plants

• The two major stressors are:

1. Salt stress

2. Water logging anoxic soils

Mangrove survival strategies

• Salt exclusion at the roots via ultrafiltration

• Salt excretion at the leaves

• Pneumatophores (aerial roots)

• Vivipary

• Successive cambia

http//www.nzpcn.org.nz Photo: John Sawyer

Mangrove survival

• Salt exclusion at the roots (about 90% of the salt)

• Salt excretion at the leaves (40% of remaining salt)

• Pneumatophores (aerial roots)

• Vivipary

• Successive cam

Mangrove survival

• Salt exclusion at the roots (about 90% of the salt)

• Salt excretion at the leaves (40% of remaining salt)

• Pneumatophores (aerial roots)

• Vivipary

• Successive cam

http://en.wikipedia.org/wiki/Mangrove

Mangrove survival

• Salt exclusion at the roots (about 90% of the salt)

• Salt excretion at the leaves (40% of remaining salt)

• Pneumatophores (aerial roots)

• Vivipary

• Successive cam

Mangrove survival

• Salt exclusion at the roots (about 90% of the salt)

• Salt excretion at the leaves (40% of remaining salt)

• Pneumatophores (aerial roots)

• Vivipary

• Successive cam

http//www.nzpcn.org.nz Photo: John Barkla

http//www.nzpcn.org.nz Photo: John Barkla

Mangrove survival

• Salt exclusion at the roots (about 90% of the salt)

• Salt excretion at the leaves (40% of remaining salt)

• Pneumatophores (aerial roots)

• Vivipary

• Successive cambia with secondary growth (84.9% of trees and shrubs that exhibit this are water or salt stressed)

Robert et al. 2011 PLOSOne 6:1 1-10

Why mangroves?

1. Mangroves are not classically water stressed because they grow in the tidal zone

2. Tidal inundation with salt water induces periodic and predictable stress conditions

3. No extremes in temperature (i.e. no freezing)

4. Little or no nutrient limitation

5. Forests are monospecific so there is no interspecific competition

6. Their canopies are easily accessible even when trees are mature

Monospecific

Tide line

Low canopy at maturity

New Zealand mangroves

• Avicennia marina subsp. australasica

• Most wide-spread species globally

• In contrast to global trends NZ mangroves are expanding/spreading

Two areas of interest 1. Water

– Water relations of trees in relation to diurnal, tidal and seasonal rhythms and, environmental conditions?

– Do mangroves use alternative water uptake mechanisms?

2. Carbon

– What factors limit growth in mangroves on various temporal scales and environmental conditions?

– How much carbon do they store as they grow?

Study Design • Three different sensors for water use

• Stem growth + NSC

• Climatic sensors

• Soil sensors

Site selection

• We had several criteria

• The middle of the whole system Eddy-flux

• Uniform tree size

• Minimal edge effects

Environmental variables

• Sunlight

• Temperature

• Humidity

• Soil moisture

• Rainfall

• Salinity

• Tide height and timing

Water relations sensors A. Sap-flow

− Sap-flow sensors

B. Leaf water-potential − ZIM-probes

C. Stem-diameter fluctuation − Dendrometers

http://www.zim-plant-technology.com

ZIM-probes for leaf water-potential

ZIM-probes

3 x Sap-flow

3 x Dendrometer

Base station

Three trees with three of each sensor

Aluminium scaffolding system

Powered with a wind generator

Central logger and battery pack

Alternative water uptake

salt water ca. -2.5 MPa

dry air ca. -90 Mpa

xylem water potential ca. -3 Mpa ?

leaf water potential ca. -3.5 Mpa

root water potential ca. -2.7 Mpa ?

according to cohesion theory:

Alternative water uptake

salt water ca. -2.5 MPa

dry air ca. -90 Mpa

xylem water potential ca. -3 Mpa ?

leaf water potential ca. -3.5 Mpa

root water potential ca. -2.7 Mpa ?

according to cohesion theory:

alternative theory: water uptake through hygrophillic mucilage plugs to avoid such low xylem water potentials? Active water transport through xylem mucilage linings?

Alternative water uptake

salt water ca. -2.5 MPa

dry air ca. -90 Mpa

xylem water potential ca. -3 Mpa ?

leaf water potential ca. -3.5 Mpa

root water potential ca. -2.7 Mpa ?

according to cohesion theory:

alternative theory: water uptake through hygrophillic mucilage plugs to avoid such low xylem water potentials? Active water transport through xylem mucilage linings?

but: water in meta-stable status when under -3 Mpa !

Freshwater is abundant in the atmosphere at night

Freshwater is abundant in the atmosphere at night

So why not make use of it?

• Epistomal mucilage plugs?

• Xylem mucilage linings?

Zimmermann et al (2007) Protoplasma 232: 11–34

Carbon and growth • When do mangroves grow? • Two main measurements

– Stem growth – Non-structural carbohydrates

NSCs CO2

NSCs

NSCs

Stem

dia

met

er

Time

Stem growth + NSC? St

em d

iam

eter

Time

Stem ‘stasis’ + NSC St

em d

iam

eter

Time

Stem

dia

met

er

Time

What’s happening here?

And what’s happening here? St

em d

iam

eter

Time

Some early data

Inverse of turgor

Looking ahead

• Litterfall already started

• Fertilisation planned in the next year

• Eddy-flux installation this year

• Multifactor: CO2 enrichment/FACE x Warming x Fertilisation – To date only short CO2 experiments have been done

in mangroves

• Mangrove ‘LTER’? – characterisation of the whole system/multidisciplinary

AND

More mud!

Thanks

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Track our data in real time:

http://haze.decentlab.com/aut-deployment-001/

Blog:

http://thehipecologist.wordpress.com/

@jcusens