Steve Burgess Seminar Presentation 22nd September

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  • 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)