A modified Jarvis-Stewart model for predicting stand-scale transpiration of an Australian native forest

Rates of water uptake by individual trees in a native Australian forest were measured on the Liverpool Plains, New South Wales, Australia, using sapflow sensors. These rates were up-scaled to stand transpiration rate (expressed per unit ground area) using sapwood area as the scalar, and these estimates were compared with modelled stand transpiration. A modified Jarvis-Stewart modelling approach (Jarvis 1976), previously used to calculate canopy conductance, was used to calculate stand transpiration rate. Three environmental variables, namely solar radiation, vapour pressure deficit and soil moisture content, plus leaf area index, were used to calculate stand transpiration, using measured rates of tree water use to parameterise the model. Functional forms for the model were derived by use of a weighted non-linear least squares fitting procedure. The model was able to give comparable estimates of stand transpiration to those derived from a second set of sapflow measurements. It is suggested that short-term, intensive field campaigns where sapflow, weather and soil water content variables are measured could be used to estimate annual patterns of stand transpiration using daily variation in these three environmental variables. Such a methodology will find application in the forestry, mining and water resource management industries where long-term intensive data sets are frequently unavailable

Seasonal variations in tree water use and physiology correlate with soil salinity and soil water content in remnant woodlands on saline soils

Ecophysiological studies of remnant woodlands in saline environments are scarce. We investigated seasonal fluctuations in soil water and salinity together with leaf and branch traits (area-based maximum assimilation (Amax), foliar nitrogen, specific leaf area (SLA) and Huber value (Hv)) and sap velocities of Eucalyptus macrorhyncha at four semi-arid sites in south-eastern Australia. Summer and winter soil salinities (10 cm depth) were 15-35 dS m⁻¹ and 8-10 dS m⁻¹ respectively. Gravimetric soil water content in the upper 20 cm was 2-5% in summer and 7-23% in winter, resulting in a significant inverse correlation between soil water and soil salinity. We found significant correlations between soil conditions and plant traits and function across seasons. Soil water content was significantly correlated with foliar N, SLA, Hv and maximum sap velocity while soil salinity was significantly correlated with Amax, Hv and maximum sap velocity. Correlations indicate co-variation of soil conditions and plant physiology in response to environmental conditions such as solar radiation and vapour pressure deficit (D). E. macrorhyncha tolerates the dual stresses of high salinity and low soil water during summer. While the plants appeared unhealthy, our data show that remnant vegetation can remain functional even in close proximity to saline scalds.9 page(s

Leaf age-related and diurnal variation in gas exchange of kauri (Agathis australis)

New Zealand kauri (Agathis australis) (D.Don) Lindl. is a large and long-lived tree species endemic to the species-rich forests of the north of the North Island. Agathis australis are culturally and ecologically significant, but little is known about their ecophysiology. In particular, environmental drivers of fluxes of carbon and water for A. australis trees have not been quantified. We measured leaf gas exchange to explore the effect of leaf age, tree size, foliar nitrogen concentration, photosynthetically active radiation (PAR) and vapour pressure deficit (D) on assimilation rates (A) and stomatal conductance (gs). We also measured carbon isotope discrimination of leaves and applied an optimal stomatal behaviour model. Both gs and A were highest for year one leaves (130 mmol m−2 s−1 and 5 μmol m−2 s−1, respectively) then declined with leaf age to < 80 mmol m−2 s−1 and < 3 μmol m−2 s−1, respectively, in 4–5-year-old leaves. Instantaneous water use efficiency (A/gs) was highly variable, but there was no leaf age-related pattern. Our diurnal results indicate that A. australis gs peaks early in the day (before 0900 h at 250 mmol m−2 s−1) and A is comparatively low, remaining below 9 μmol m−2 s−1 throughout the day. Overall, water use efficiency is low based on intrinsic water use efficiency and the stomatal model. Isotopic analysis indicated moderate water use efficiency over the life of leaves compared to other temperate conifers. This information is valuable for modelling carbon and water fluxes of A. australis and for improving our understanding of the threat of summer droughts to these forest giants

Latent heat fluxes during two contrasting years from a juvenile plantation established over a waste disposal landscape

Isa A.M. Yunusa, Sigfredo Fuentes, Anthony R. Palmer, Catriona M.O. Macinnis-Ng, Melanie J.B. Zeppel, Derek Eamu

Woody thickening: a consequence of changes in fluxes of carbon and water on a warming globe

Understanding patterns, rates and controls of water and CO2 exchange between land surfaces and the atmosphere is central to the sciences of meteorology, ecology, hydrology, ecophysiology, forestry and related endeavours. Measurements involving sapflow sensors, eddy covariance and remote sensing have contributed substantially to our understanding of these issues. In this talk, we apply a combination of methods in order to apply a soil-plant-atmosphere model to the question: what is causing the globally observed phenomenon of woody thickening? The density of woody plants in arid and semi-arid regions is increasing regionally and globally (Fensham et al., 2005, Hoffman et al., 1999, Bowman et al. 2001, Burrows et al. 2002). This can be deduced from analyses of tree-ring widths, forest inventory data, aerial photo-interpretation and from long-term monitoring sites (Spiecker et al., 2003). Potential causes of woody thickening have been extensively discussed in the past. Mechanisms that have been proposed include the (a) Walther model, which invokes competition for water and nutrients among the deeper roots of woody plants versus the shallower roots of shrubs and grasses; (b) a role for changes in the timing, intensity and frequency of fire; and (c) changes in herbivory by large herbivores. Such thickening may have a large impact on regional CO2 budgets, atmospheric CO2 concentration and ecosystem function and regional water budgets. We propose an alternative mechanism to explain woody thickening based upon changes in water and carbon fluxes within the soil-plant-atmosphere continuum resulting from a change in global atmospheric conditions. Such a mechanism is global in reach, appears consistent with a number of phenomena and has several testable predictions, which we briefly discuss.Derek Eamus, S Fuentes C Macinnis-Ng, A Palmer, D Taylor, R Whitley, I Yunusa, M Zeppe