Richard G. Benyon

Impacts of tree plantations on groundwater in south-eastern Australia

In some regions dependent on groundwater, such as the lower south-east of South Australia in the Green Triangle, deep-rooted, woody vegetation might have undesirable hydrological impacts by competing for finite, good-quality groundwater resources. In other regions, such as the Riverina in south-central New South Wales, where rising watertables and associated salinisation is threatening the viability of agriculture, woody vegetation might have beneficial hydrological impacts. In response to a growing need to better understand the impacts of tree plantations on groundwater, annual evapotranspiration and transpiration were measured at 21 plantation sites in the Green Triangle and the Riverina. Sources of tree water uptake from rainfall and groundwater were determined by measurements of evapotranspiration and soil water over periods of 2-5 years. In the Green Triangle, under a combination of permeable soil over groundwater of low salinity (<2000 mg L −1 ) at 6-m depth or less, in a highly transmissive aquifer, annual evapotranspiration at eight research sites in Pinus radiata D.Don and Eucalyptus globulus Labill. plantations averaged 1090 mm year −1 (range 847-1343 mm year −1 ), compared with mean annual precipitation of 630 mm year −1 . These plantation sites used groundwater at a mean annual rate of 435 mm year −1 (range 108-670 mm year −1 ). At eight other plantation sites that had greater depth to the watertable or a root- impeding layer, annual evapotranspiration was equal to, or slightly less than, annual rainfall (mean 623 mm year −1 , range 540-795 mm year −1 ). In the Riverina, where groundwater was always present within 3 m of the surface, Eucalyptus grandis Hill ex Maiden trees at three sites with medium or heavy clay, alkaline, sodic, saline subsoils used little or no groundwater, whereas E. grandis and Corymbia maculata (Hook.) K.D.Hill and L.A.S.Johnson trees at a site with a neutral sandy soil and groundwater of low salinity used 380 and 730 mm year −1 of groundwater (respectively 41 and 53% of total annual evapotranspiration). We conclude that commonly grown Eucalyptus species and P. radiata are able to use groundwater under a combination of light- or medium-textured soil and shallow depth to a low-salinity watertable.

Variation in sapwood area and throughfall with forest age in mountain ash (Eucalyptus regnans F. Muell.)

Abstract Previous studies have shown that forest age bears a strong relationship with regional mean annual streamflow from catchments forested with mountain ash in the central highlands of Victoria, Australia. Mean annual streamflow of 585 mm from catchments forested with 30-year-old mountain ash compares with mean annual streamflow of 1195 mm from the same catchments that 27 years previously were forested with old growth mountain ash. About three-quarters of the difference can be explained by a difference in transpiration, and the remainder by a difference in interception loss. Based on earlier observations that differences in transpiration between mountain ash overstoreys aged 50, 90, 150 and 230 years can largely be explained by differences in stand sapwood cross-sectional areas, variation in transpiration with age was accounted for by fitting a log-normal relationship to overstorey sapwood areas measured in six mountain ash stands aged from 7 to 235 years. The sapwood area of the eucalypt overstorey reached a peak of 10.5 m2 ha−1 at about age 15 and then declined gradually to 2.5 m2 ha−1 at age 200. A log-normal relationship fitted to a time series of throughfall measurements from mountain ash forests ranging in age from 1 to 244 years indicates the peak interception loss of 25% to occur at age 30. Interception loss gradually declines to 17% at age 200 years.

Some factors affecting water yield from mountain ash (Eucalyptus regnans) dominated forests in south-east Australia

Abstract An experiment in mountain ash forests in Melbournes water supply catchments in south-east Australia investigated the impact on long-term water yield of reducing forest density. Fifty-four per cent of basal area was removed from a 17 ha catchment (Black Spur 1) by patch cutting, and the patches were regenerated with mountain ash. A 50% reduction was implemented in Black Spur 3, an 8 ha catchment, through uniform thinning. Uniform thinning was shown to be more effective in enhancing streamflow than patch cutting. A streamflow increase of 25–30% (130–150 mm year −1 ) was observed after treatment in both catchments. Eleven years later, a treatment effect of 15% was still evident in the selectively thinned catchment (Black Spur 3), but the effect had completely decayed in the patch cut catchment (Black Spur 1). Research by Melbourne Water has established that streamflow is significantly influenced by forest age. It is hypothesised that this relationship, and the observed decay after patch cutting in Black Spur 1, is largely the result of variation in transpiration. To study the relationship between forest age and transpiration in detail, sap velocity was measured over two summers in four mountain ash plots using the heat-pulse method. The trees on these plots were 50, 90, 150 and 230 years old. On days when measurements were made, the mean sap velocity was not significantly different in the 50-, 90- 230-year-old stands, but was significantly smaller by 14% in the 150-year-old stand. Overstorey sapwood area gradually decreased with plot age, and was 57% lower in the 230-year-old plot than in the 50-year-old plot. When combined with the sap velocity measurements, these data indicated that over the six warmest months of the year, transpiration in the 50-year-old plot was 190 mm more than in the 230-year-old plot. These results support a hypothesis that differences in streamflow between 50-year-old and 230-year-old mountain ash forest can largely be accounted for by differences in transpiration. Further heat-pulse studies in young regrowth and in thinned and unthinned 1939 regrowth will be needed if the streamflow changes in Black Spur 1 and Black Spur 3 are to be fully explained.

Quantifying water savings from willow removal in Australian streams

Willows (Salix Spp.), while not endemic to Australia, form dense stands in many stream locations. Australia has been experiencing a long-term drought and potential water extraction by willows is considered a significant problem, although little global scientific evidence exists to support such concerns. The extent of willow occupation in Australian streams has been deemed large enough to warrant investigation of their evapotranspiration rates and quantification of potential water savings from willow removal. Willows situated in-stream (permanent water) and on stream banks (semi-permanent water) were monitored over three summers from August 2005 to May 2008 employing heat pulse velocity sap flux sensors and field measurement of water balance components. A comparative study of native riparian River Red Gum trees was also undertaken. Differences in transpiration flux rates between willows with permanent and semi-permanent access to water were substantial, with peak transpiration of 15.2 mm day(-1) and 2.3 mm day(-1) respectively. Water balance calculations over the three year period indicate that an average potential net water saving of 5.5 ML year(-1)ha(-1) of crown projected area is achievable by removal of in-stream willows with permanent access to water. On stream banks, replacement of willows with native riparian vegetation will have no net impact on site water balances. Results also indicate that under the influence of natural environmental events such as drought, heat stress and willow sawfly infestation, evapotranspiration rates from in-stream willows remain greater than that from open water. These results will have important implications in environmental management of willows and in future water resource allocation and planning in Australia.

Modelling the water balance of effluent-irrigated trees

Irrigation of effluent is an increasingly popular treatment option due to concern about nutrient additions to rivers and coastal waters. Since some studies have shown that irrigation with waste water can lead to contamination of groundwater resources, there is need for a model to predict the fate of irrigated water, salt, and nitrogen that can be applied to a variety of different soils, climates, and crops. We present the development of the water balance part of such a model, APSIM for Effluent, and carry out a comparison against data obtained from an effluent-irrigated plantation of Eucalyptus grandis. Over 10 months, modelled tree water use was within 1.5% of that obtained by sap-flux measurements. When compared over 5 years of the experiment, modelled drainage lay above that estimated by a water balance technique, which was known a priori to underestimate drainage, and was close to that estimated by the chloride mass balance technique. Simulated chloride accumulated in the soil was within the scatter of the observations, although it was consistently at the lower end of the range of the data. There was good agreement between the model predictions and measured chloride concentration distribution with depth in the soil. A considerable amount of water was lost as deep drainage, even for the treatment that aimed to add only enough effluent to replace that lost by evaporation. During 5 years, of the 3370 mm rainfall and 4480 mm effluent received by that treatment, 6710 mm was lost by the various evaporative routes, and 1080 mm was lost by deep drainage.

Using Tree Detection Algorithms to Predict Stand Sapwood Area, Basal Area and Stocking Density in Eucalyptus regnans Forest

Managers of forested water supply catchments require efficient and accurate methods to quantify changes in forest water use due to changes in forest structure and density after disturbance. Using Light Detection and Ranging (LiDAR) data with as few as 0.9 pulses m−2, we applied a local maximum filtering (LMF) method and normalised cut (NCut) algorithm to predict stocking density (SDen) of a 69-year-old Eucalyptus regnans forest comprising 251 plots with resolution of the order of 0.04 ha. Using the NCut method we predicted basal area (BAHa) per hectare and sapwood area (SAHa) per hectare, a well-established proxy for transpiration. Sapwood area was also indirectly estimated with allometric relationships dependent on LiDAR derived SDen and BAHa using a computationally efficient procedure. The individual tree detection (ITD) rates for the LMF and NCut methods respectively had 72% and 68% of stems correctly identified, 25% and 20% of stems missed, and 2% and 12% of stems over-segmented. The significantly higher computational requirement of the NCut algorithm makes the LMF method more suitable for predicting SDen across large forested areas. Using NCut derived ITD segments, observed versus predicted stand BAHa had R2 ranging from 0.70 to 0.98 across six catchments, whereas a generalised parsimonious model applied to all sites used the portion of hits greater than 37 m in height (PH37) to explain 68% of BAHa. For extrapolating one ha resolution SAHa estimates across large forested catchments, we found that directly relating SAHa to NCut derived LiDAR indices (R2 = 0.56) was slightly more accurate but computationally more demanding than indirect estimates of SAHa using allometric relationships consisting of BAHa (R2 = 0.50) or a sapwood perimeter index, defined as (BAHaSDen)½ (R2 = 0.48).

Use of a forest sapwood area index to explain long‐term variability in mean annual evapotranspiration and streamflow in moist eucalypt forests

Mean sapwood thickness, measured in fifteen 73 year old Eucalyptus regnans and E. delegatensis stands, correlated strongly with forest overstorey stocking density (R2 0.72). This curvilinear relationship was used with routine forest stocking density and basal area measurements to estimate sapwood area of the forest overstorey at various times in 15 research catchments in undisturbed and disturbed forests located in the Great Dividing Range, Victoria, Australia. Up to 45 years of annual precipitation and streamflow data available from the 15 catchments were used to examine relationships between mean annual loss (evapotranspiration estimated as mean annual precipitation minus mean annual streamflow), and sapwood area. Catchment mean sapwood area correlated strongly (R2 0.88) with catchment mean annual loss. Variation in sapwood area accounted for 68% more variation in mean annual streamflow than precipitation alone (R2 0.90 compared with R2 0.22). Changes in sapwood area accounted for 96% of the changes in mean annual loss observed after forest thinning or clear-cutting and regeneration. We conclude that forest inventory data can be used reliably to predict spatial and temporal variation in catchment annual losses and streamflow in response to natural and imposed disturbances in even-aged forests. Consequently, recent advances in mapping of sapwood area using airborne light detection and ranging will enable high resolution spatial and temporal mapping of mean annual loss and mean annual streamflow over large areas of forested catchment. This will be particularly beneficial in management of water resources from forested catchments subject to disturbance but lacking reliable long-term (years to decades) streamflow records.

On spatial variability of above-ground forest biomass

Abstract The topography-related variation and plot-to-plot variation of above-ground forest biomass in a central Appalachian watershed and in a southeast Australian watershed and its surroundings were compared. The Appalachian site was covered with typical Appalachian hardwoods, ca 85 years old, and the Australian site was covered with an eucalyptus forest ( Eucalyptus regnans ), ca 53 years old, when the study was initiated. Both forests are unmanaged. Biomass data were collected on 112 800 m 2 plots at the Appalachian site and on 30 plots of the same size at the Australian site. The oven-dry above-ground biomass average was 322 t/ha at the Appalachian site and 476 t/ha at the Australian site. At the Appalachian site, the single plot biomass varied from minus 67% to plus 85% of the mean, and at the Australian site the corresponding numbers are minus 50% and plus 61%. At both sites, the coefficient of variation shows a more or less steady value when the number of plots ( n ) exceeds 20. For n = 20, coefficients of variation are 28% and 22% for the Appalachian site and for the Australian site, respectively. The smallest above-ground biomass per unit area was observed on southwest-facing slopes at the Appalachian site and on west-facing slopes at the Australian site. The greatest above-ground biomass per unit area was observed on east-facing slopes at the Appalachian site and on south-facing slopes at the Australian site. In general, the sites facing east and the poles are more productive than those facing west and the equator.

Predicting long-term streamflow variability in moist eucalypt forests using forest growth models and a sapwood area index

A major challenge in surface hydrology involves predicting streamflow in ungauged catchments with heterogeneous vegetation and spatiotemporally varying evapotranspiration (ET) rates. We present a top-down approach for quantifying the influence of broad-scale changes in forest structure on ET and hence streamflow. Across three catchments between 18 and 100 km2 in size and with regenerating Eucalyptus regnans and E. delegatensis forest, we demonstrate how variation in ET can be mapped in space and over time using LiDAR data and commonly available forest inventory data. The model scales plot-level sapwood area (SA) to the catchment-level using basal area (BA) and tree stocking density (N) estimates in forest growth models. The SA estimates over a 69 year regeneration period are used in a relationship between SA and vegetation induced streamflow loss (L) to predict annual streamflow (Q) with annual rainfall (P) estimates. Without calibrating P, BA, N, SA, and L to Q data, we predict annual Q with R2 between 0.68 and 0.75 and Nash Sutcliffe efficiency (NSE) between 0.44 and 0.48. To remove bias, the model was extended to allow for runoff carry-over into the following year as well as minor correction to rainfall bias, which produced R2 values between 0.72 and 0.79, and NSE between 0.70 and 0.79. The model under-predicts streamflow during drought periods as it lacks representation of ecohydrological processes that reduce L with either reduced growth rates or rainfall interception during drought. Refining the relationship between sapwood thickness and forest inventory variables is likely to further improve results. This article is protected by copyright. All rights reserved.

Comment on Wood et al. 2008, 'Impacts of fire on forest age and runoff in mountain ash forests'

Wood et al. (2008; FPB 35) concluded their measurements of evapotranspiration (ET) in Eucalyptus regnans F.Muell. forest at Wallaby Creek, Victoria showed that ET differs only slightly between regrowth and oldgrowth, contrary to the findings of previous research. We assert that the conclusions of Wood et al. are invalid and argue that Wood et al. substantially overestimated annual transpiration and rainfall. Monthly whole-forest ET measured by Wood et al. using eddy covariance in a 296-year-old stand sum to ~700 mm year–1; consistent with rainfall of 721 mm year–1 recorded nearby by the Bureau of Meteorology. However, the Wood et al. conclusions were based on 1077 mm annual transpiration at this site, which appears to be estimated from a few months of heat pulse velocity measurements. Transpiration alone cannot be 54% higher than whole-forest ET because the latter includes transpiration, rainfall interception and evaporation from the forest floor. We believe Wood et al. made errors in scaling heat pulse velocities to whole-stand annual transpiration. Their rainfall of 1175 mm year–1 averages 62% higher than at three Bureau of Meteorology and Melbourne Water sites nearby. The paper also contains inaccuracies in reporting of the literature and numerous other errors.