This document discusses methods for measuring plant transpiration and its effects on hydrology. It describes how transpiration varies among ecosystems and vegetation types, and how measuring transpiration is key to understanding links between plant functioning and hydrology. Various techniques are presented for measuring transpiration from above, below, and directly from plants using methods like heat pulse probes. Recent work applying these methods to studies of bamboo water use is also summarized.

1. Measuring transpiration to
understand plant function
and its effect on water
resources
Steve Burgess

2. 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

3. Dryland Salinity:
17 million Ha affected by 2050
Source: www.napswq.gov.au
Dryland salinity risk in south-west
Western Australia 2000
Source:NLWRA (2001a)

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. Heat Ratio Method
T1
Because of its
symmetric
Heater configuration,
the HRM can
resolve zero
T2 flows
Zero sap
flow

12. Heat Ratio Method
Flow velocity (V)
is
T1
logarithmically
related to the
ratio of
Heater
temperature
increases up-
and downstream
T2 from a heater
V = thermal diffusivity x Ln T1
probe distance T2

13. Installation

14. Correction for
wounding Marshall’s ‘ideal’ for 5 cm h-1
2.8
2.6
Sensitivity to heat 2.4
Wound = 0.30 cm
V = 45 cm h-1
in the sap stream is 2.2 y = 0.011x + 1.48
r 2 = 0.996
reduced Actual for 5 cm h-
v 1/v 2
2
1.8 1
Wound = 0.17 cm
1.6
V = 5 cm h-1
1.4 y = 0.001x + 1.12
r 2 = 0.993
1.2
1
Conductive 0 10 20 30 40 50 60 70 80 90 100
xylem Time following heat pulse (s)
Heat ratio is attenuated by wounding and
the ratio is no longer constant for a given
velocity – at odds with Marshall’s theory
Vessels blocked by Numerical modelling allows for corrections
drilling/mechanical that account for the thermal behaviour of
damage steel probes and of non-conductive wood

15. Heat ratio method -accuracy
Thermometric sap flow (cm3 h-1) 400
Average ± S.E.
350
1:1 line
300
250
200
150
100
50
0
0 100 200 300 400
Gravimetric sap flow (cm3 h-1)

16. 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

17. 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

18. 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

19. Do fungal
endophytes act as
wicks?
Micrographs by Adeline Fabre
This is actually a Douglas Fir leaf

20. 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

21. 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

22. 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

23. 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

24. Total daily sap flow
3.0
b
2.5 b
d -1)
2.0
-2
E L (kg m
1.5
a a
1.0
0.5
0.0
1-Jul-2004 2-Jul-2004
Non-riparian riparian

25. 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

26. 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

27. 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

28. 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)