Figure 1: A preliminary meta-analysis of the response of stomatal conductance to elevated [CO2] comparing responses among functional groups. A response of 1 indicates no change. Stomata of eucalypts appear to be more sensitive to high [CO2] than conifers or broadleaf deciduous species, indicating potentially larger water savings at high [CO2].
The whole tree chambers serve two purposes. They allow us to grow relatively large trees in a controlled environment where we can regulate the temperature, humidly, water availability and CO2 concentration. They allow us to monitor whole tree CO2 and water fluxes as the trees grow. In this respect they are unique and are providing valuable information at a spatial scale that would otherwise be unobtainable.
Not widely used in forestry due to its susceptibility to pests. However its wide natural distribution and rapid growth make it suitable as a research tool.
The upper part of the chamber is sealed from the soil and roots so we can measure canopy and root/soil fluxes independently. Air is circulated through a large heat exchanger which serves two purposes it cools the air but also condenses out excess moisture resulting from tree transpiration. A small amount of fresh air is constantly added to the chamber into which pure CO2 is metered. The system acts as a null balance and co2 is added to compensate for photosynthesis by the tree. A root barrier to depth of 1m prevents lateral movement of shallow roots ensuring that soil properties are influenced by the target tree and not neighbours. Soil moisture is monitored with various instruments to a depth of 4 m.
Similar sized trees at time of measurement (about 15 m2 leaf area).
Daytime CO2 flux (PAR>100umol m-2 s-1) averaged over two week bins then averaged by treatment (n=6 then 3) to give the SEM (error bars)
Due to the way that humidity was regulated by using a cold trap held at just below target chamber dew point to both remove excess heat and excess water from the air the actual equilibrium humidity in a chamber is dependant to some extent on the way the tree/chamber partitions incoming radiation load between latent and sensible heat. Large trees with high transpiration rates tend to self cool to some extent and so there is less demand for active cooling and the absolute humidity in the chamber is slightly higher. Elevated chambers had a VPD about 15% lower than ambient chambers the difference developed as the trees became large.
Leaf level measurements made periodically throughout the experiment showed a ratio of ITE that matched expectations from the Ball–Berry model.
PAR >300 umol m-2 s-1 with data in 2hr bins from sunrise. Each point is the mean of 3 chambers during that 2hr bin. Data cover the period
Effects of rising CO2 concentration on water use efficiency of Eucalyptus saligna - Craig Barton
Effects of rising CO 2 concentration on water use efficiency of Eucalyptus saligna Craig Barton M. Adams , J. Conroy, R. Duursma, D. Eamus, D. Ellsworth, S. Linder, B. Medlyn, D. Tissue, R. McMurtrie CCRSPI Conference 2011, Melbourne
Hawkesbury Forest Experiment <ul><li>Established to investigate the impacts of climate change on Australian trees . </li></ul><ul><li>Integrated program of experimental research and modelling. </li></ul><ul><li>Little known about the response of Eucalypts to elevated CO 2 . </li></ul><ul><li>Many Australian forest and woodland systems characterised by poor soils and frequent droughts. </li></ul>
Stomatal conductance response to elevated [CO 2 ] Medlyn et al unpublished
Whole Tree Chambers <ul><li>Designed and first used in Northern Sweden. </li></ul><ul><li>They isolate trees in a controlled environment for CO 2 exposure and whole-tree measurements. </li></ul><ul><li>Chambers track outdoor conditions. </li></ul><ul><li>Allows us look at interactive effects of elevated CO 2 and drought. </li></ul><ul><li>Growing under local climate. </li></ul>
Eucalyptus saligna <ul><li>Sydney Blue Gum </li></ul><ul><li>Fast growing mainly coastal tree that has commercial plantation use. </li></ul><ul><li>Prefers warm humid climatic conditions </li></ul><ul><li>Mean precip 900-1800mm p.a. </li></ul><ul><li>Planted April 2007. </li></ul>
Experimental design <ul><li>12 whole tree chambers </li></ul><ul><li>6 run at ambient [CO 2 ] A (390 ppm) </li></ul><ul><li>6 run at elevated [CO 2 ] E (630 ppm) </li></ul><ul><li>Half of each set subjected to periodic drought. </li></ul>
Whole tree chambers Described in Medhurst et al 2006 PC&E and Barton et al 2010 Ag.For. Met Fresh air inlet 1 air change per hour Root barrier Heat exchanger floor 6 m condensate CO 2 addition
Whole-tree fluxes The system can resolve responses to short term fluctuations in light. Afternoon depression of carbon uptake present. CO 2 fluxes are very similar Water loss is much lower in the elevated CO 2 tree. Barton et al 2010 Agricultural and Forest Meteorology 150 :941-951
Effect of CO 2 on Instantaneous Transpiration Efficiency <ul><li>ITE = instantaneous transpiration efficiency </li></ul><ul><li>= A / E (mmol CO 2 mol -1 H 2 O) </li></ul><ul><li>Carbon Assimilation / Water used </li></ul><ul><li>According to Ball-Berry model, A and E are related: </li></ul>
Implications <ul><li>If we assume that stomatal conductance does not acclimate to [CO 2 ] then the ratio of ITEelev / ITEamb will equal Ca-elev / Ca-amb </li></ul><ul><li>In our case 630/390 = 1.6 </li></ul><ul><li>ITE should increase by 60% </li></ul>
Conclusions <ul><li>Strong down-regulation of photosynthesis resulting in little if any “ fertilisation effect” of elevated CO 2 </li></ul><ul><li>Data supports Ball-Berry model at leaf and canopy scale. </li></ul><ul><li>No acclimation of stomatal response to CO 2 </li></ul><ul><li>Implications for models used to predict forest growth (3PG, GDAY, Cabala) </li></ul><ul><li>VPD is important to results. </li></ul>
Investigating the Impacts of Climate Change on Australia’s Forests Craig Barton The Hawkesbury Forest Experiment M.Adams, B. Amiji, J. Conroy, R. Duursma, D.Eamus, D. Ellsworth, S. Linder, M. Löw, B. Medlyn, J. Parsby, D. Tissue, R. McMurtrie, et al Thank You