S.J. Sochacki

Bio-mitigation of carbon following afforestation of abandoned salinized farmland

As the global demand for food continues to increase, the displacement of food production by using agricultural land for carbon mitigation, via either carbon sequestration, bioenergy or biofuel is a concern. An alternative approach is to target abandoned salinized farmland for mitigation purposes. Australia, for example, has 17 million ha of farmland that is already or could become saline. At a representative, salinized, low rainfall (350 mm yr−1) site at Wickepin, Western Australia, we demonstrate that afforestation can mitigate carbon emissions through either providing a feedstock for bioenergy or second generation biofuel production and produce salt‐tolerant fodder for livestock. A range of factors markedly affect this mitigation. These include hydrological conditions such as salinity, site factors such as slope position and soil properties and a range of silvicultural factors such as species, planting density and age of the planting. High density (2000 stems ha−1) plantings of Eucalyptus occidentalis Endl. produced a mean total biomass of 4.6 t ha−1 yr−1 (8.5 t CO2‐e ha−1 yr−1) averaged over 8 years. Atriplex nummularia Lindl. produced a mean total biomass of 3.8 t ha−1 yr−1 (6.9 t CO2‐e ha−1 yr−1) averaged over 4 years and approximately 1.9 t ha−1 yr−1 of edible dry matter annually to 8 years of age. With differences in salt tolerance between E. occidentalis and A. nummularia, we propose an integrated approach to treating salinized sites that takes salinity gradients into account, replicates natural wetland ecosystems and produces both fodder and biomass. Continued mitigation is expected as the stands mature, assuming that growth is not affected by the accumulation of salt in the soil profile. Such carbon mitigation could potentially be applied to salinized farmland globally, and this could thus represent a major contribution to global carbon mitigation without competing with food production.

Evaluating a sustainability index for nutrients in a short rotation energy cropping system

In dryland environments 3–5 year rotations of tree crops and agriculture represent a major potential bioenergy feedstock and a means to restore landscape hydrologic balances and phytoremediate sites, while maintaining food production. In soils with low natural fertility, the long‐term viability of these systems will be critically affected by site nutrient status and subsequent cycling of nutrients. A nutrient assimilation index (NAI) was developed to allow comparison of species and tree component nutrient assimilation and to optimize nutrient management, by quantifying different strategies to manage site nutrients. Biomass, nutrient export and nutrient use efficiency were assessed for three short rotation tree crop species. Nutrient exports following harvest at 3 years of high density (4000 trees ha−1) were consistently higher in Pinus radiata, with values of 85 kg ha−1 of N, 11kg ha−1 of P, and 62 kg ha−1 of K, than Eucalyptus globulus and Eucalyptus occidentalis. Component NAI was generally in the order of leaf

Managing water in agricultural landscapes with short-rotation biomass plantations

Bioenergy production using woody biomass is a major climate change mitigation strategy but is often considered in terms of competitive effects on water. This paper describes the use of a short‐rotation biomass system (Phase Farming with Trees PFT or ‘Kamikaze Forestry’) to manage water in dryland farming systems where this has accumulated below the root zone and has on and off‐site environmental impacts. This excess water can be utilized for growth by deep‐rooted, high‐density biomass plantations inserted as short rotations into agricultural land. The objective is to promote rapid growth and mining of deep stored water through strategies such as high planting densities, the use of fast‐growing species or fertilization each of which increases leaf area. Once the water is used, the trees are harvested and excess water is allowed to build up again in the subsequent cropping phase. Biomass production and water depletion were measured in a five‐year rotation of trees inserted into a dryland (367 mm yr−1 mean annual rainfall) cereal farming system in south‐western Australia. Both were markedly affected by tree age, planting density, and landscape position on a very minor slope. The greatest biomass production was achieved with high‐density (4000 stems ha−1) plantings of Eucalyptus occidentalis and Eucalyptus globulus in lower landscape positions. High‐density plots of these species in mid and upper landscape positions succumbed to drought after 3–4 years, but depleted available soil water to depths of >8 m, equivalent to 771 mm of stored available water. These results suggest that biomass yield can be readily manipulated through planting density and site selection. Moreover, biomass production can produce positive water management co‐benefits.

The development of reforestation options for dryland farmland in south-western Australia: a review

Current forest industries in south-western Australia are based on regrowth natural eucalypt forests and Pinus and Eucalyptus spp. plantations, and restricted to areas with >600 mm y−1 annual rainfall. Dryland farming systems have been developed across 20 million ha in a zone with 300–600 mm y−1 annual rainfall and a Mediterranean climate. This zone is beset with land degradation problems, such as salinity and wind erosion, and there has been considerable effort in the last three decades to develop reforestation options to stabilise the landscapes. Traditional forestry approaches using pulp wood or sawlog production in this zone have been limited by unfavourable economics driven by modest tree growth rates, large transport costs to processing and export facilities, and high labour costs. Given that salinity results from a disruption of the landscape water balance, reforestation has represented a major component in attempts to tackle the problem. Issues with reforestation include (1) obtaining sufficient scale of activity to impact watershed water balances, (2) obtaining a hydrological response without displacing farm production and rural communities and (3) gaining payment for non-forest benefits. This paper reviews the approaches that have been used to integrate trees into the dryland farming (300–600 mm y−1 annual rainfall) systems of south-western Australia, and have resulted in at least 113 286 ha of reforestation. These included both traditional (pine and eucalypt sawlogs) and new (sandalwood, biodiversity restoration and carbon mitigation through bioenergy and carbon sequestration) projects. Ongoing investment has centred on carbon sequestration as this represents one of the few profitable options for the management of dryland salinity in this region. Approaches developed in this region to encourage farmland reforestation will be applicable in other dryland regions, particularly with the interest in using the land-sector to meet climate mitigation targets.

Seasonal Timing for Estimating Carbon Mitigation in Revegetation of Abandoned Agricultural Land with High Spatial Resolution Remote Sensing

Dryland salinity is a major land management issue globally, and results in the abandonment of farmland. Revegetation with halophytic shrub species such as Atriplex nummularia for carbon mitigation may be a viable option but to generate carbon credits ongoing monitoring and verification is required. This study investigated the utility of high-resolution airborne images (Digital Multi Spectral Imagery (DMSI)) obtained in two seasons to estimate carbon stocks at the plant- and stand-scale. Pixel-scale vegetation indices, sub-pixel fractional green vegetation cover for individual plants, and estimates of the fractional coverage of the grazing plants within entire plots, were extracted from the high-resolution images. Carbon stocks were correlated with both canopy coverage (R2: 0.76–0.89) and spectral-based vegetation indices (R2: 0.77–0.89) with or without the use of the near-infrared spectral band. Indices derived from the dry season image showed a stronger correlation with field measurements of carbon than those derived from the green season image. These results show that in semi-arid environments it is better to estimate saltbush biomass with remote sensing data in the dry season to exclude the effect of pasture, even without the refinement provided by a vegetation classification. The approach of using canopy cover to refine estimates of carbon yield has broader application in shrublands and woodlands.