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Climatology - Research - GES

Sustainable futures of Australian temperate forests

An investigation of coupled carbon, water and energy exchanges from hourly to centennial time scales.

Investigators

Project Overview

Australia's forests are a critical natural resource that must be sustainably managed. We will determine the uptake/release of carbon from old growth and regrowth forests and assess the water budgets of the Melbourne water catchment at Wallaby Creek, Victoria, Australia. We aim to understand the current cycles of carbon, water and energy and how these may change over time (hours to centuries). We will integrate our observations with state-of-the-art models to improve our predictions of how forests will respond to change. This will aid our management of forests and forested catchments to ensure sustainable and viable water resources and optimise carbon sequestration. The project is funded from 2004 to 2006.

The full project proposal can be found here (PDF).

The Location and Sites

The site is located at in the Mountain Ash forests (Eucalyptus regnans) of the Wallaby Creek Catchment near Kinglake in Melbourne, Australia.

There are three sites within the Wallaby Creek catchment representing various times since fire.

300 year old 80 year old 20 year old

Mountain Ash Forests (Eucalyptus regnans)

  • Tallest angiosperm, typically 50-90 m at maturity

  • Tallest individual recorded, 114 m height

  • High quality timber species, high productivity

  • > 3 million ha in SE Australia

  • Occurs in Victoria and Tasmania

  • Cool, moist upland sites, 150 to 1100 m altitude, ~1000 mm annual rainfall

Background

Australian native forests are an important component of the Australian landscape, comprising 164 million ha or around 21 per cent of the continent landmass. Temperate open forests cover an area of ~ 5.5 million ha, which is five times greater than the area of plantation forests and therefore represent a potentially important carbon sink [RAC, 1992]. Temperate open forests are also economically important for the forestry industry and are significant in providing areas for recreation and maintaining the health/biodiversity of the crucial in sustaining the amount and quality of drinking water (e.g. Maroondah catchment, Melbourne). For these reasons it important to understand how our forest assets may develop into the future. This important ecosystem is biocomplex and has physical, biological and chemical (biogeochemical) cycles that are coupled across different time scales. Cycles of energy are essential in driving photosynthesis and determining climate and water use. Biogeochemical cycles of carbon, water and nitrogen are important for provision of freshwater, carbon sequestration and forest production. In order to understand how Australian forests will develop in the future we must know how these cycles and the forest as whole will respond to changes in climate, extreme climate events, ecological succession and human disturbance which all occur on different time scales. Unfortunately we have limited understanding of these complex systems at differing spatial and temporal scales [Nikolov & Fox, 1994].

The fundamentals of the carbon cycle within forests are relatively well understood. Previous inventory based estimates have shown the net uptake of carbon of the ecosystem as a whole (Net Ecosystem Exchange or NEE - the net carbon gained by the ecosystem through photosynthetic production minus respiration from plants and soil) decreases with stand age and in old growth forests, carbon cycling is often assumed to be in equilibrium [Carey et al., 2001; Hollinger et al., 1994]. However, young trees are believed to be a net carbon sink because they rapidly sequester carbon as they grow [Kaiser, 2000; RAC, 1992]. Inverse modelling of carbon fluxes shows that Northern Hemisphere old growth forests are a stronger sink of CO2 than calculated from previous inventory studies [Martin et al., 2001] (In Australia we currently do not have a network that allows us to resolve these fluxes using this method). The reasons for high carbon uptake by old growth forests is uncertain, but it has been suggested that high rates of leaf and root turnover contribute to permanent soil carbon pools [Dixon et al., 1994; Schulze et al., 2000]. Therefore old growth forests, in addition to the important role in biodiversity conservation, provide a large carbon store and may act as a carbon sink, keeping carbon dioxide out of the atmosphere [Carey et al., 2001]. Australia's current carbon inventory shows that growth, harvesting and regrowth in managed native forests and plantations has been a net carbon sink for greenhouse gases of 75.8 Mt in 1999 [AGO, 2001]. The role of native open forests in the carbon inventory is uncertain, although they have the potential to contribute a significant carbon sink given their large areal coverage. To reduce this uncertainty, investigations of the carbon cycle in native forests and how it may change with stand age and differing management are required.

The cycle of water within a forest is important and is determined by tree water use, evaporation and runoff. Understanding the ecohydrology of catchments such as the Maroondah catchment is critical given their role in the provision of potable water to large urban populations. Basic hydrological processes for these forests are well understood from comprehensive observations and modelling work (e.g. [Vertessy et al., 1995] which has included research on the effect of stand age on water yields [Cornish and Vertessy, 2001]). However, we do not currently have a good understanding of how the water cycle is coupled with cycles of carbon and energy and how these cycles interact over annual to centennial time scales. Such knowledge is important for the future management of these forests and catchments given anticipated environmental change.

The energy cycle within the forest is critical in driving photosynthesis, evaporation, transpiration, heating of the atmosphere and soil. There is strong coupling between the energy, carbon and water cycles. The amount of energy that is used in evapotranspiration, heating and canopy energy storage, as well as the way in which this energy is partitioned between these fluxes is influenced by the biological (stand age and species composition) and physical characteristics (height, canopy structure) of the forest. For extensive forest ecosystems, this energy flow in turn feeds back to influence climate.

Although several of the individual processes within carbon, water and energy cycles (photosynthesis, respiration, leaf energy balance and turbulent exchange within plant canopies) are well characterised at the leaf level, the complex nature of the coupling between processes has not been examined extensively [Kull, 2002]. A greater understanding of microenvironmental forest processes are needed to be able to scale up from the leaf level to the plant canopy. This is vital if we are to be able to predict the response of whole forests to environmental change. It may be best achieved using integrated observations and ecosystem modelling of the carbon, water and energy balance of forests at a canopy scale. There have been few measurements of the carbon, water and energy processes in Australian forests, making it difficult to both quantify and predict changes with time. Some inventory-style carbon balance estimates have been made in temperate eucalypt forests [Keith et al., 1997], but these do not capture the important ecosystem dynamics and variability.

Recent advances in micrometeorological techniques (namely Eddy Covariance - EC) now allow hourly measurements of carbon, water and energy fluxes from ecosystems. This technique is considered the most robust and accurate for measuring fluxes compared to inventory and inverse modelling approaches [Moncrieff et al., 1997]. To date, there have been few EC studies of native vegetation. Hutley et al. [2000] used EC to quantify water and energy balances of tropical savanna vegetation and Eamus et al. [2001] estimated the carbon sink strength of tropical savanna derived from short periods of measurement. Only one study of southern temperate open eucalypt forests has been conducted, in West Australian Jarrah [Silberstein et al., 2001], however, carbon exchanges were not measured during their study, and measurements were short-term only. As a result of the lack of long-term integrative studies, there is a large uncertainty regarding the carbon, water and energy cycles of temperate (or any) Australian forests, particularly in relation to how they may change with time. Clearly, more rigorous long-term estimates of carbon, water and energy cycles in temperate open forests across various time scales are needed. Such studies are of considerable value in helping to determine the response of forests to environmental change and quantifying Australia's carbon and water balances.

Many of the processes driving water and carbon fluxes at ecosystem level are strongly dependent on seasonal changes and extremes in climate [Grelle et al., 1999]. Seasonal changes of phenology and biomass production significantly affect rates of water and carbon exchanges. Furthermore, extreme events (extreme temperatures, high wind velocity, drought conditions) are not often captured during short-term field measurements yet these non-average conditions have a strong impact on the hydrological and carbon cycles of terrestrial ecosystems [Baldocchi et al., 1997]. It is therefore critical to examine the response of Australian forests over time scales long enough to be relevant to climatic processes issues (seasonal to interannual to decadal). An improved knowledge of the complex land-atmosphere exchanges of carbon, water and energy is vital at time scales encompassing days, seasons, years, and even decades, as well as over spatial scales from a few kilometres to landscapes and will be a major outcome of this proposal. Such knowledge will provide improved parameterisations for predictive ecosystem models and ultimately aid the sustainable management of carbon and water resources within Australian forests.

Approach

The overall objectives of this study are to understand the complex coupling of carbon, water and energy cycles within Australia's temperate forests over various temporal scales in order to assess the impact of future environmental change. We will measure hourly fluxes of carbon, water and energy above the forest using the EC technique on a tall tower over a period of more than 3 years. Concurrent measurements of meteorological variables and component processes will be made. Our approach is to combine continuous flux measurements of these cycles on a multi-year time basis with ecological process interpretation and modelling. This will allow us to understand the complexity of these systems and incorporate this into our models to improve future simulations.
The site will be located within the Maroondah Water catchment that supplies Melbourne's drinking water. The catchment is an excellent example of temperate eucalypt forest and is unique because it has intact old growth stands with individual trees as old as 300 years. The forest is primarily Mountain Ash (Eucalyptus regnans) and has been the site of intensive hydrological research. Our proposed research will provide ongoing information on canopy scale water budgets that will be used by the CRC for Catchment Hydrology. In addition, areas of the surrounding catchment are host to commercial clear felling practices. Operations over the past century have subsequently allowed forests to regrow and there are many areas of homogeneous forest of varying age. We will investigate these sites to examine decadal to centennial changes to carbon, water and energy cycles. There are two major project objectives:

Objective 1: To quantify the carbon, water and energy exchanges in a temperate forest and the factors regulating them over hourly to inter-annual time scales.

Objective 2: Establish the carbon, water and energy cycles of different aged forest stands and investigate how they change over successional time scales (decadal to centennial).

Measurements

  • A 110m tall tower in the old growth Mountain Ash forest has been established using the Eddy Covariance technique to measure the carbon, water and energy fluxes. Hourly measurements of fluxes, meteorological variables and component processes will be used to examine canopy scale processes and mechanisms controlling fluxes. The site will become a long-term research site.

  • We will measure the input fluxes of net radiation (Rn, Wm-2) and output fluxes of water vapour (LE, Wm -2), atmospheric heating (H, Wm-2) and heating of the soil (G, Wm-2) every 30 minutes. These components define the energy balance (Rn=H+LE+G). The net radiation is comprised of incoming and outgoing shortwave and longwave radiation fluxes and these will be measured independently in order to quantify changes in radiative exchanges and the efficiency of radiation trapping of the forest (i.e. albedo). To measure the soil heat flux (G) we will use the combination method and utilise a spatial array of soil heat flux plates and soil temperature sensors. Net Ecosystem Exchange (NEE, g C m-2 s-1) of carbon will be measured.

  • EC instrumentation will include a 3D-sonic anemometer and a fast response CO2/H2O open path infrared gas analyser (IRGA). We will use profile based CO2 storage measurements, below canopy EC measurements and chamber-based techniques to assess the "true" nocturnal flux.

  • A full suite of environmental and climate measurements will be made concurrently with the flux measurements and will include photosynthetically active radiation, air temperature, vapour pressure, atmospheric pressure, soil moisture content, wind speed (at various height to define roughness lengths), wind direction (to determine fetch) and phenology. We will also measure energy stored within the biomass using thermocouples to measure leaf, stem and trunk temperatures. We will also measure CO2 and water vapour storage within the canopy air volume (an important term during stable conditions) by establishing a closed path CO2/H2O analyser that will sequentially sample a vertical profile through the canopy.

  • We will measure components of the carbon balance (soil, stem, leaf) and its change over time, along with additional key model variables for integrative measurement/modelling studies.

  • Additional component processes that define the water balance will be made. We will partition the water balance of the forest, which is of fundamental importance for the hydrology of the catchment and is of primary interest to catchment hydrology research. We will measure water inputs using raingauges. Soil moisture changes will be followed during the season using TDR type probes.

Data

Real time data can be found here. For longer time series and quality controlled data please contact the authors.

News and Media

Video of news report from Channel Ten Melbourne (Monday 12th April 2005)

Photos

Jason climbing 110m tower on first ascent Jason (left) and Lindsay (right) on tower

Kenichi (left) and Ian McHugh (right) sampling root biomass Lindsay (left) and Danni Martin (right) taking soil CO2 flux measurements

Jason climbing 110m tower on first ascent  

Outputs

Wood, Stephen (2005) Change in Leaf Area Index (LAI) and Tree Characteristics of Different Age Mountain Ash Stands to Determine Effects on Water Yield . Third year project. PDF 478K.

Useful Links

References

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Amthor, J.S., J.M. Chen, J.S. Clein, S.E., et al., Boreal forest CO2 exchange and evapotranspiration predicted by nine ecosystem process models: Intermodel comparisons and relationships to field measurements, Journal Geophysical Research, 106, 33623-22648, 2001.

Baldocchi, D., E. Falge, L.H. Gu, et al., FLUXNET: A new tool to study the temporal and spatial variability of ecosystem-scale carbon dioxide, water vapor, and energy flux densities, Bul.Am. Met. Soc., 82 (11), 2415-2434, 2001.

Baldocchi,D.D., C.A. Vogel, and B. Hall, Seasonal variation of carbon dioxide exchange rates above and below a boreal jack pine forest, Agricultural and Forest Meteorology, 83, 147-170, 1997.

Carey,E., A. Sala, R. Keane, et al., Are old forests underestimated as global carbon sinks?,Glb. Chg. Biol.,7(4),339-344, 2001

Cornish, P.M., and R.A. Vertessy, Forest age-induced changes in evapotranspiration and water yield in a eucalypt forest, Journal of Hydrology, 242 (1-2), 43-63, 2001.

Dixon, R., S. Brown, R. Houghton, et al., Carbon Pools and Flux of Global Forest Ecosystems, Science, 263, 185-190, 1994.

Eamus, D., L.B. Hutley, and A.P. O'Grady, Daily and seasonal patterns of carbon and water fluxes above a north Australian savanna, Tree Physiology, 21 (12-13), 977-988, 2001.

Grelle, A., A. Lindroth, and M. Molder, Seasonal variation of boreal forest surface conductance and evaporation, Agricultural and Forest Meteorology,98-99 , 563-578, 1999.

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Hutley, L. B., A. P. O'Grady, and D. Eamus. Monsoonal influences on evapotranspiration of savanna vegetation of northern Australia. Oecologia 126: 434-443, 2001.

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Keith H., R.J. Raison, K.L. Jacobsen. Allocation of carbon in a mature eucalypt forest and some effects of soil phosphorus availability. Plant Soil 196: 81-99, 1997.

Kull, O. Acclimation of photosynthesis in canopies: models and limitations. Oecol 133: 267-279, 2002.

Martin, P., G.J. Nabuurs, M. Aubinet, et al., Carbon sinks in temperate forests, Ann. Rev. Energy Env., 26, 435-465, 2001.

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Peel, M.C., Vertessy, R.A., & Watson, F.G.R. (2002) Generating water yield curves for forest stands in the Thomson catchment for inclusion in the Integrated Forest Planning System. Final report to Natural Resources & Environment, Victoria, 33 pp.

Peel, M.C., Watson, F.G.R., Vertessy, R.A., Lau, J.A., Watson, I.S., Sutton, M.W., & Rhodes, B.G. (2000) Predicting the water yield impacts of forest disturbance in the Maroondah and Thomson catchments using the Macaque model, Cooperative research Centre for Catchment Hydrology, Rep. No. 2000-00/14. Melbourne. 71 pp.

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Watson, F.G.R. & Vertessy, R.A. (1996) Estimating leaf area index from stem diameter measurements in Mountain Ash forest. Cooperative Research Centre for Catchment Hydrology. Melbourne. Report 96/7, November 1996, 102 pp. (PDF of version with old front cover) (Errata) .

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Acknowledgements

This project was funded from 2004 to 2006 from the Australian Research Council under project number DP0451247.


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