Peter Isaac

Special- savanna patterns of energy and carbon integrated across the landscape

Savannas are highly significant global ecosystems that consist of a mix of trees and grasses and that are highly spatially varied in their physical structure, species composition, and physiological function (i.e., leaf area and function, stem density, albedo, and roughness). Variability in ecosystem characteristics alters biophysical and biogeochemical processes that can affect regional to global circulation patterns, which are not well characterized by land surface models. We initiated a multidisciplinary field campaign called Savanna Patterns of Energy and Carbon Integrated across the Landscape (SPECIAL) during the dry season in Australian savannas to understand the spatial patterns and processes of land surface–atmosphere exchanges (radiation, heat, moisture, CO2, and other trace gasses). We utilized a combination of multiscale measurements including fixed flux towers, aircraft-based flux transects, aircraft boundary layer budgets, and satellite remote sensing to quantify the spatial variability across a continental-scale rainfall gradient (transect). We found that the structure of vegetation changed along the transect in response to declining average rainfall. Tree basal area decreased from 9.6 m2 ha−1 in the coastal woodland savanna (annual rainfall 1,714 mm yr−1) to 0 m2 ha−1 at the grassland site (annual rainfall 535 mm yr−1), with dry-season green leaf area index (LAI) ranging from 1.04 to 0, respectively. Leaf-level measurements showed that photosynthetic properties were similar along the transect. Flux tower measurements showed that latent heat fluxes (LEs) decreased from north to south with resultant changes in the Bowen ratios (H/LE) from a minimum of 1.7 to a maximum of 15.8, respectively. Gross primary productivity, net carbon dioxide exchange, and LE showed similar declines along the transect and were well correlated with canopy LAI, and fluxes were more closely coupled to structure than floristic change.

Spatial and Temporal Variations in Fluxes of Energy, Water Vapour and Carbon Dioxide During OASIS 1994 and 1995

This paper introduces the micrometeorological field campaigns known asOASIS (Observations At Several Interacting Scales) and then summarizesseveral companion studies that have used the OASIS dataset. Instrumentedtowers, aircraft and atmospheric sondes were used for measurements overthree paired sites (crops and pastures), approximately equi-spaced along an88-km transect in south-eastern New South Wales, Australia, during the australsprings of 1994 and 1995. Measurements included standard meteorologicaldata and the fluxes of solar and net radiation, sensible heat, water vapour andthe greenhouse gases CO2, N2O, CH4. Descriptions of the site, andthe spatial and temporal variations of climate fields and fluxes, are presented.There were strong contrasts in fluxes and surface conductances, evaporationratios and water use efficiencies between the 1994 drought year and the normalrainfall year of 1995. Despite greater incoming solar radiation in 1994 associatedwith less cloud cover, net radiation was lower than in 1995 because of greateroutgoing thermal radiation caused by higher surface temperatures. In 1994 dailysensible heat fluxes were about 50% higher and evaporation rates about half thosefor 1995. Rainfall in the three-month growing season prior to the field campaignswas the key determinant of leaf area index, surface conductances and the fluxes ofsensible and latent heat and CO2. Antecedent rainfall distribution also controlled variation in fluxes and surface properties along the transect within each year. There was a net loss of CO2 to the atmosphere at the drier central sites in 1994, and a net uptake at the wetter north-eastern sites. Both sites recorded uptake of CO2 in 1995, but values were lower at the central site than at the north–east site due to the strong rainfall gradient along the transect in the three months prior to each fieldcampaign. Differences in fluxes between crops and pastures at each site were smallerthan between sites.

Estimation of Regional Evapotranspiration by Combining Aircraft and Ground-Based Measurements

Two methods are examined for combining measurements from instrumented aircraftand towers to estimate regional scale evapotranspiration. Aircraft data provided spatially averaged values of properties of the surface, the evaporative fraction and maximum stomatal conductance. These quantities are less sensitive to meteorological conditions than the turbulent fluxes of heat and water vapour themselves. The methods allowed aircraft data collected over several days to be averaged and thus to reduce the random error associated with the temporal under-sampling inherent in aircraft measurements. Evaporative fraction is estimated directly from the aircraft data, while maximum stomatal conductance is estimated by coupling the Penman–Monteith equation to a simple model relating surface conductance to the incoming shortwave radiation and specific humidity saturation deficit. The spatial averages of evaporative fraction and maximum stomatal conductance can then be used with routine tower data to estimate the regional scale evapotranspiration. Data from aircraft flights and six ground based sites during the OASIS field campaign in south–east New South Wales in 1995 have been used to check the methods. Both the evaporative fraction and the maximum stomatal conductance derived from the aircraft data give information on the spatial variability of the surface energy budget at scales from 10 to 100 km. Daily averaged latent heat fluxes estimated using these methods for the OASIS study region agree with the available observations in quasi-stationary conditions or in weakly non-stationary conditions when the data from several aircraft flights are averaged to reduce the impact of short term imbalances in the surface energy budget.

How do leaf and ecosystem measures of water-use efficiency compare?

The terrestrial carbon and water cycles are intimately linked: the carbon cycle is driven by photosynthesis, while the water balance is dominated by transpiration, and both fluxes are controlled by plant stomatal conductance. The ratio between these fluxes, the plant water-use efficiency (WUE), is a useful indicator of vegetation function. WUE can be estimated using several techniques, including leaf gas exchange, stable isotope discrimination, and eddy covariance. Here we compare global compilations of data for each of these three techniques. We show that patterns of variation in WUE across plant functional types (PFTs) are not consistent among the three datasets. Key discrepancies include the following: leaf-scale data indicate differences between needleleaf and broadleaf forests, but ecosystem-scale data do not; leaf-scale data indicate differences between C3 and C4 species, whereas at ecosystem scale there is a difference between C3 and C4 crops but not grasslands; and isotope-based estimates of WUE are higher than estimates based on gas exchange for most PFTs. Our study quantifies the uncertainty associated with different methods of measuring WUE, indicates potential for bias when using WUE measures to parameterize or validate models, and indicates key research directions needed to reconcile alternative measures of WUE.

Narrowband spectral indices for the estimation of chlorophyl along a precipitation gradient

Narrowband spectral indices have largely been used for the estimation of biochemical attributes of vegetation. However, most of the proposed indices were developed using a limited number of species and locations. In this study, a strong precipitation gradient in northern savannas of Australia was used to develop narrowband spectral indices for the estimation of leaf chlorophyll content and verify the spatial generality of the indices when applied to different species from different environmental conditions. Simple ratio indices (SRI) and normalized difference indices (NDI) were developed for samples taken from seven study sites along the transect. The results showed a limited transferability of the indices along the transect. Therefore, there is a need for more site-specific indices for large scale estimation of chlorophyll in the study area.