Climatology - Research - GES

• 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

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.

• 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.

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