Measuring transpiration to understand plant function

and its effect on water resources

Steve Burgess

Water use by plants is central to the hydrological cycle

60% of rainfall returns to the atmosphere via evaporation and transpiration

Transpiration varies among ecosystems but can be 30% of rainfall

Vegetation type and functioning affects local and regional hydrology

Dryland Salinity:17 million Ha affected by 2050

Source: www.napswq.gov.au

Source:NLWRA (2001a)

Dryland salinity risk in south-west Western Australia 2000

Mountain ash forests of Melbourne’s catchments

• Mountain ash (Eucalyptus regnans) are the tallest flowering plant

• Mountain ash grow in even aged stands• Younger stands use more water than

older stands due to greater leaf area• Logging or disturbance leads to

regeneration with younger trees, decreasing stream flow

Perth’s drinking water

• 57% of Perth’s water comes from groundwater aquifers

• The largest, Gnangara mound, has 23,000 Ha of Pinus pinaster planted directly above

• People feared that P. pinaster used more water than the native Banksia woodland they replaced, reducing recharge of the aquifer

• Recent transpiration studies suggest this is not true

Kenya’s water resources

• Reafforestation aims to increase forest cover in Kenya from 1.7% to 10% in two decades

• Extra water used by newly planted forests may reduce stream flow, affecting water supplies

• But, reafforestation does not always reduce stream flow- Californian redwoods harvest more fog water than they use

Measuring transpiration• Measuring transpiration is key to linking

vegetation functioning to hydrology• Transpiration can be measured in all the

compartments of the soil-plant-atmosphere continuum.

• From above: atmospheric methods include eddy flux techniques

• From below: soil techniques include neutron and capacitance probes

• Directly: plant-based techniques include using heat as a tracer of xylem sap flow

Using heat to measure xylem sap flow

• Heat can be used as a tracer for water flow• Constant heating methods use a lot of power

and often require heating of tissues to be fairly uniform – not possible for all plant types

• Heat pulse methods use very little power and can make targeted point estimates of sap flow in plants of all kinds of different morphologies

Heat pulse methods

• Earlier methods had poor resolution at low flow rates (e.g. at night)

• Since night-time is 50% of the year, it matters whether flows are non-zero or not and the earlier techniques could not distinguish this

• Night-time transpiration can reach rates 40% of daytime maximum and should not be ignored in water balance equations

Heat ratio method

• In 1996, the University of Western Australia, CSIRO and ICRAF developed a heat pulse method called the heat ratio method (HRM)

• The HRM has excellent resolution for a wide range of flow rates including zero and reverse flows

• We have spent 10 years developing expertise and experience with this method

Because of its symmetric configuration, the HRM can resolve zero flowsT2

T1

Heat Ratio Method

Zero sap flow

Heater

V = thermal diffusivity x Ln T1probe distance T2

T2

T1

Heater

Heat Ratio Method

Flow velocity (V) is logarithmically related to the ratio of temperature increases up-and downstream from a heater

Installation

1

1.2

1.4

1.6

1.8

2

2.2

2.4

2.6

2.8

0 10 20 30 40 50 60 70 80 90 100

Time following heat pulse (s)

v1/v

2

Wound = 0.17 cm V = 5 cm h-1

y = 0.001x + 1.12r 2 = 0.993

Wound = 0.30 cm V = 45 cm h-1

y = 0.011x + 1.48r 2 = 0.996

Correction for wounding

Heat ratio is attenuated by wounding and the ratio is no longer constant for a given velocity – at odds with Marshall’s theory

Numerical modelling allows for corrections that account for the thermal behaviour of steel probes and of non-conductive wood

Sensitivity to heat in the sap stream is reduced

Vessels blocked by drilling/mechanical damage

Conductive xylem

Actual for 5 cm h-1

Marshall’s ‘ideal’ for 5 cm h-1

Heat ratio method -accuracy

0

50

100

150

200

250

300

350

400

0 100 200 300 400

Gravimetric sap flow (cm3 h-1)

Ther

mom

etric

sap

flow

(cm

3 h-1

) Average ± S.E. 1:1 line

Why is hydraulic redistribution important?

• Species with dimorphic root systems (lateral and tap roots) can pump water into dry soil to absorb nutrients in the dry season

• They can also sequester water into deeper layers in the wet season

Foliar absorption of water

• Roots can lose as well as absorb water• Leaves can absorb as well as lose

water• Example from the Californian coastal

redwoods Sequoia sempervirens

How does water get into leaves?

• Recent paper by Schreiber showing that polar pathways in cuticle matrix increase in permeability with increasing humidity

• Temperature can also influence wax structure and cuticular permeability

• What about stomata? Hints from the response of old leaves versus young leaves

Do fungal endophytes act as wicks?Micrographs by Adeline Fabre

This is actually a Douglas Fir leaf

Using sap flow to screen multiple species

• We’ve used sap flow to screen different species’ response to summer rain

• This provides lots of information on rooting depth, summer dormancy, year round water use etc.

• We measured 12 different sclerophyllous tree and shrub species growing at Corrigin, Western Australia

Bamboo

• Nearly 1500 species• Broad range of habitats – tropical,

subtropical, montane• 6.3 million km2 of natural forest

containing bamboos in Asia• 920,000 km2 of bamboo-dominated

rainforest in Amazonia

Bamboo

• Used in semi-exploited natural stands and intensively managed plantations

• Globally significant sink for CO2

• Helps in soil/water conservation• Wide range of products including edible

shoots

Bamboo water use

• Kleinhenz and Midmore (2000) reviewed 200 scientific publications on bamboo agronomy and concluded that there is an absence of information on bamboo water use worldwide.

• At UWA, student Nick Hogarth has done water use studies on Bambusa arnhemica

Total daily sap flow

aa

bb

0.0

0.5

1.0

1.5

2.0

2.5

3.0

1-Jul-2004 2-Jul-2004

EL

(kg

m-2

d-1)

Non-riparian riparian

At the riparian site:

• Sap flow more than double• Photosynthesis more than double• Height of culms was the same• Hydraulic conductivity the same• Leaf area: sapwood area was lower

Leaf area adjustments

• Bambusa arnhemica adjusts stomatal conductance and leaf area according to water availability to maintain homeostasis

• Because of such adjustments, differences in total water use by Bambusa arnhemica at different water sites may depend mostly on seasonal changes to leaf area

• Differences between bamboo and deeper rooted Eucalypts may be most apparent in the dry season

This week’s work

• Formal comparisons between bamboo and Eucalyptus and bamboo have only just begun

• Such information will be essential in predicting what will happen to Kenya’s water resources if there is a shift towards planting bamboo in place of Eucalyptus

Acknowledgements

•Nick Hogarth and Tim Bleby for the bamboo field measurements•Todd Dawson, Chin Ong, Neil Turner, Ahmed Khan, Craig Beverly, Hans Lambers, Alastair Grigg for some of the graphics•CRC for Plant-Based Solutions to Dryland Salinity•The Australian Research Council (ARC)•Australian Centre for International Agricultural Research (ACIAR)