Site assessment for farm forestry in Australia and its relationship to scale, productivity and sustainability

Forest Ecology and Management 171 (2002) 133±152

Site assessment for farm forestry in Australia and its relationship to scale, productivity and sustainability Philip J. Ryana,*, Richard J. Harperb, Michael Laffanc, Trevor H. Bootha, Neil J. McKenzied a CSIRO Forestry and Forest Products, P.O. Box E4008, Kingston, ACT 2604, Australia Department of Conservation, Locked Bag 104, Bentley Delivery Centre, Bentley, WA 6983, Australia c Forestry Tasmania, Perth, Tasmania 7300, Australia d CSIRO Land and Water, G.P.O. Box 1666, Canberra, ACT 2601, Australia

b

Abstract Trees are being established on substantial areas of Australian farmland, often extending into districts where forestry has not been previously practiced. Irrespective of purposeÐwood production, increased water use, erosion control, habitat restoration, carbon sequestration or bio-energy productionÐthe bene®ts of farm forestry depend on tree survival and adequate growth. Site assessment has been mainly used for the prediction of tree performance but it can be extended to form the basis for site-speci®c management. Site assessment needs to provide information on species and management inputs suitable to a site's inherent environmental attributes so that the aim of maximum pro®tability can be achieved while maintaining a sustainable use of resources. A new site concept for farm forestry is presented that re¯ects these factors. This new site concept is de®ned as the integrated effect of environmental factors that in¯uence the potential growth of a particular tree genotype and affect the soil management of that stand of trees to produce a particular product or output. This concept of site is environment-based, independent of the presence of trees, and includes an explicit acknowledgement of a hierarchy of scale (both spatial and temporal) and how it affects what environmental factors need to be assessed. The goal for site assessment for farm forestry is to develop pragmatic, robust and quantitative interpretations from the measured data to aid multiple aspects of plantation management. A pragmatic approach to ®eld survey is required, with data gathering (whether morphological, analytical or electronic) being based on demonstrated bene®t rather than routine prescription, guess-work or tradition. Site assessment needs to be linked to well-designed species trials and permanent growth plots so continual evolution and improvement of edaphic and silvicultural relationships can be incorporated into future modi®cations. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Land evaluation; Plantations; Soil; Climate; Sustainable productivity

1. Introduction Afforestation on cleared agricultural land is emerging as a major new land use in Australia (Wood et al., *

Corresponding author. Tel.: ‡612-6281-8331; fax: ‡612-6281-8239. E-mail address: [email protected] (P.J. Ryan).

2001). This has been promoted as providing a range of economic, environmental and social bene®ts, such as income diversi®cation and increased pro®tability, carbon sequestration, control of land degradation (e.g. salinity and erosion), production of bio-energy, and increased rural employment. A major feature of these new plantations is that they are likely to be integrated with farming activities, unlike much of the current

0378-1127/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 1 1 2 7 ( 0 2 ) 0 0 4 6 8 - 1

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high rainfall area planting in Australia. Signi®cant expansion of farm forestry is expected in dry areas, with trees being planted for their environmental bene®ts, such as reducing groundwater recharge, treating salinised areas or enhancing biodiversity. Salinity is of major concern in these lower rainfall areas with estimates that 4.6 Mha of land is currently salinised, with this increasing to 13.6 Mha by 2050 (National Land and Water Resources Audit, 2001). Farm forestry expansion is occurring in districts where plantation forestry has not been traditionally practised and knowledge on tree performance is lacking. With the large-scale investment in afforestation, it is timely to examine whether currently applied approaches of land evaluation are appropriate and where improvements can be made. This paper will review the status of site assessment for farm forestry in Australia. It will assess (1) recent developments in farm forestry in Australia and impact on site assessment, (2) traditional and new approaches to site assessment, (3) the implicit and explicit roles of scale in site assessment and (4) the capability of land evaluation to facilitate the monitoring of critical soil indicators of forest plantation sustainability. 2. Recent developments in farm forestry in Australia 2.1. New expansion and ownership Farm forestry and private tree plantations are the most rapidly expanding sectors of the forestry industry, and probably also the agricultural sector, in Australia. As at September 2000, the total forest plantation area was 1.5 Mha (502,620 ha of hardwood) with the planting rate accelerating from 30,000 ha/year in 1995 to 125,000 ha/year (516,270 ha total) in 2000 and changing in composition from 63 to 89% hardwood (Wood et al., 2001). The largest plantation expansion since 1995 has been in Western Australia (WA, 59%), followed by the ``Green Triangle'' region of Victoria and South Australia (44%) and Tasmania (40%) (Wood et al., 2001). Continued expansion in the plantation estate is expected with the ``Plantations 2020 Vision'' targeting the establishment of up to 3 Mha of trees on farmland for timber production (Plantation 2020 Vision Implementation Committee, 1997).

This expansion has resulted in a large area of agricultural land being assessed for plantation potential in a short period of time. Unlike other developed countries, Australia does not have a National Soil Survey with mapping at a scale suitable for making management decisions about farm forestry; those maps that do exist over broad areas are often at small scales (McKenzie, 1991). Thus land evaluation for farm forestry has to be undertaken as part of the plantation program. The traditional dominance of state forest agencies and several major private forest companies in owning hardwood and softwood plantations (trees and land) is changing dramatically. Whereas most of the plantation estate existing in 1995 was owned by public agencies, of the new 1999 plantations, 83% is with private tree owners, 10% with joint owners, and only 7% with public tree owners (NFI, 2000). If this trend continues there will be a major shift from public to private tree ownership in all states except NSW. Farm forestry is based around three major groups, with different levels of activity, different information requirements, and likely approaches to site selection. First is the industrial/investment driven forestry group, which includes government business enterprises, who may obtain information from their own research and development or commissioned consultancies. Second are the non-industrial private forestry owners who regard trees as another crop. These individuals are often early adopters of new technology and actively seek out information. Third are the ``Landcare/Bushcare'' government-funded programs, which encourage participants to plant trees on farms primarily for land and nature conservation with commercial bene®ts seen as secondary. The increasing preference for ``Joint Venture'' plantation development (partnerships between a private or public plantation company and private land-owner) means that the trees can be owned by a different organisation than that which owns the land. This is in contrast to most agricultural land use with the tree owner, and not the land owner, responsible for farm forestry land evaluation. This diverse range of owners and participants in farm forestry makes it dif®cult to devise a uniform approach to site assessment but it also offers an opportunity to introduce new approaches to new clients who are willing to accept innovative ideas.

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2.2. Products and markets for farm forestry Farm forestry covers a range of tree growing activities ranging from products with relatively assured markets such as industrial pulp/®bre and lumber to more speculative products such as carbon, bio-energy, essential oils, charcoal and non-traditional purposes such as maintaining biodiversity or aesthetics (Abel et al., 1997). The majority of tree planting has been in single species, industrial plantations in higher rainfall areas of WA, the Green Triangle (SA/VIC) and Tasmania. Products from these plantations are aimed at export pulp and wood ®bre markets with plantation location strongly in¯uenced by distance to export port facilities (Wood et al., 2001). To expand farm forestry beyond this current ``industrial zone'' has required the development of other products and markets (bioenergy, charcoal, essential oils, etc.) that are less reliant on transport distance or have local value. The Kyoto Protocol (Articles 3.3 and 3.4) allows for carbon sequestered in farm forestry, established after 1990, to be considered as emissions credits, which can be traded (Shea, 1998; Schlamadinger and Karjalainen, 2000; Consortium, 2001). Landcare groups, Greening Australia, and Regional Plantation Committees are organisations that have been formed to address land degradation and biodiversity issues. There is current discussion as to developing carbon, salinity and biodiversity markets with tradable credits invested in new plantations but as yet none of these markets have been formally created (Consortium, 2001). 2.3. Previous experience with plantation species may not be relevant for new farm forestry sites Traditional plantation experience in Australia has mainly been with exotic softwoods such as Pinus radiata, Pinus elliottii  Pinus caribaea hybrids and Pinus pinaster planted on large blocks of public land. The native hardwood plantation area in Australia prior to 1995 was limited to 158,000 ha of several fastgrowing species (Eucalyptus globulus, E. grandis, E. nitens and E. regnans) mostly sited on moist, nutrientrich soils for reliable productivity. Large areas of land that are potentially available for farm forestry are not suitable for most of the hardwood and softwood species that have been traditionally grown in planta-

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tions. The major limitations are climatic and in particular annual rainfall, which ranges down to 250± 300 mm/year, and evapotranspiration, that ranges up to 2500 mm/year (Consortium, 2001). Although limited areas of traditional plantation species, such as Tasmanian blue gum and radiata pine, are already being established in these lower rainfall areas, there is the recognition that alternative farm forestry species and planting systems are needed that can be productive on sites with low rainfall (<700 mm), high evaporation, and with infertile and possibly saline soils. Examples of this recognition include the development of the Australian Low Rainfall Tree Improvement Group (ALRTIG) (Bush et al., 2001) and the mallee eucalypt and ``Search'' projects in WA (Bartle, 2001). This trend means that there will be a number of species for which there will be little or no knowledge of speci®c edaphic requirements. This presents a challenge to species-speci®c land evaluation. It is unlikely that farm forestry in the lower rainfall areas will proceed as block-plantations, both to avoid displacement of agriculture but also to best manage the limited water in this environment. In south-western Australia, where cropping is the major economic activity, there are two principal options being assessed (Harper et al., 2001). The ®rst is permanent tree belts or ``alleys'' (Lefroy and Stirzaker, 1999), most likely of short-rotation coppiced species (Bartle, 2001) that can access water from adjacent farmland. The second comprises trees in blocks or strips that are rotated across the landscape (Harper et al., 2000b) to deplete soil water and stop recharge, before the land is returned to agriculture. Land evaluation systems will have to consider these new planting systems and more importantly new issues such as optimising the placement of trees in landscapes to obtain environmental outcomes such as an overall reduction in salinity. 3. Land evaluation for farm forestry Site assessment for farm forestry is one form of land evaluation. General principles for land evaluation are now well established overseas and in Australia (FAO, 1976; McKenzie, 1991; Davidson, 1992) and they apply whether conventional or quantitative digital technologies are used. In broad terms, land evaluation involves a comparison of the requirements of a land

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use with the resources offered by the land (Dent and Young, 1981). The term land suitability is de®ned as the ®tness of a given type of land for a speci®ed land use. It is essential to understand the nature of the proposed land use before undertaking any evaluation. This ensures that the appropriate land attributes are measured and mapped. Land attributes can be considered in two ways:  Land characteristic. An attribute of land that can be measured or estimated (e.g. soil colour, soil texture, slope, etc.).  Land quality. A complex attribute of land that acts in a manner distinct from the actions of other land qualities in its influence on the suitability of land for a specific use. Land qualities take account of interactions between relevant land characteristics. Table 1 lists the key land qualities that in¯uence land suitability for farm forestry. Land suitability can be expressed as the balance of three components that involve these land qualities (1) production or yield, (2) management of inputs and (3) hazard of use (Gibbons, 1976). Various rating or classi®cation systems are possible and most involve keeping two of the above factors constant. For example, land suitability for farm forestry can be classi®ed on the basis of potential productivity (e.g. mean annual increment volume at 10 years) as long as management inputs (fertilisers and

weedicide) and hazards (®re or pests) are assumed to be constant. The evaluation of land prior to the establishment of trees is commonly termed ``site assessment'' and is a well recognised prelude to plantation establishment in both Australia and overseas (Raupach, 1967; Carmean, 1975; Lewis et al., 1976; FAO, 1984; Valentine, 1986; Turvey, 1987; Turner et al., 1990; Turvey et al., 1990). Plantation forestry site assessment has mainly concentrated on the prediction of tree performance, particularly survival and growth (Havel, 1968; Lewis et al., 1976; Turvey et al., 1990; Inions, 1991; McGrath et al., 1991; Edwards and Harper, 1996; Laffan, 1997) for pre-selected species. Site assessment has variously involved combinations of soil, climatic, vegetation and geomorphic (i.e. slope, aspect) information. As described, farm forestry is both moving into regions of Australia where plantation forestry has not been traditionally practised and is considering using tree species that have had little or no commercial development. Thus, empirical site assessment systems already developed such as the technical soil classi®cation for P. radiata (Turvey, 1987; Turner et al., 1990; Turvey et al., 1990) or numerical models for E. globulus (Inions, 1991) will have little direct utility. This transition in farm forestry also limits the application of land suitability methodology (FAO, 1984; Laffan, 1997) because of the lack of knowledge

Table 1 The main land qualities in¯uencing land suitability for farm forestry (adapted from Laffan (1997)) Land quality

Component land characteristics (examples)

Land qualities affecting site productivity Radiation regime Temperature regime Frost hazard Water availability Drainage Nutrient availability Toxicities

Latitude, aspect, annual net radiation (as measure over a landsurface) Elevation, aspect, annual net radiation (as measure over a landsurface) Elevation, landform element, topographic position, landform pattern Soil depth, texture (including coarse fragments), bulk density, macroporosity, aggregate stability Landform element, soil depth, substrate, texture Soil parent material and substrate lithology Sodium, aluminium, boron, iron, etc.

Land qualities affecting plantation management and land degradation Trafficability Slope, surface rock cover, stream density Erosion hazard Slope, stream power index, soil erodibility, dispersion, aggregate stability, organic matter content, texture, porosity, drainage, soil strength Landslide hazard Slope, substrate lithology Flood hazard Elevation, landform element Salinity/salinisation hazard Landform element, landform pattern, substrate lithology Fire hazard Rain seasonality and variability, landform element

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required to determine what land characteristics are critical to speci®c tree species. 4. Current site assessment systems in use in Australia 4.1. South-western Australia The south-west region of WA has a Mediterranean climate with a dominant winter rainfall pattern and summer drought. In areas with >600 mm annual rainfall there has been a rapid expansion of E. globulus plantations (162,691 ha from 1995 to 2000, Wood et al., 2001). Soil and climatic requirements for E. globulus have been reviewed by Inions (1992) and Harper et al. (1998). Recent growth plot studies have related growth and survival to a range of soil and climatic attributes (Harper et al., 1999). Results indicate that climate (rainfall, evaporation), soil volume (estimated by soil depth and occurrence of ferricrete gravel), soil fertility (total N content), and stocking interact to affect the productivity of these plantations. These factors all point to water supply as being the most critical limitation in plantation performance in this region, with increased growth as rainfall increases and evaporation decreases. Many of these soil attributes that are important for E. globulus productivity (soil depth, water holding capacity, fertility and salinity) are not normally measured in standard soil surveys in WA, while those soil attributes that are described (soil pro®le form, texture,

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colour) are at most only poorly related to E. globulus growth. Table 2 presents the farm-scale site assessment procedure used by the WA Department of Conservation and Land Management (CALM) for E. globulus. A similar hierarchical decision making approach, with a series of exclusion rules, has also been developed for the expansion of Pinus pinaster on to farmland (Harper, 1995). 4.2. Tasmania This island state has a cool temperate climate. Over 30,000 ha of three main plantation species (E. globulus, E. nitens and P. radiata) were established from 1995 to 2000 (Wood et al., 2001). One of the major plantation developers, Forestry Tasmania, has adapted the land suitability classi®cation advocated by FAO (FAO, 1976, 1984), which involves a comparison of the requirements of a land use with the resources offered by the land. The de®nition of land suitability (see above) ensures that the appropriate land attributes are measured and mapped. A site suitability classi®cation for plantations in Tasmania (Laffan, 1997) considered land qualities that affect: (1) site productivity, (2) plantation management and (3) land degradation. Four classes of site productivity are de®ned in terms of plantation maximum mean annual volume increment (MAI as m3/ha/year) (Table 3). Productivity class is assessed from the various soil and climatic attributes affecting tree growth; altitude (used as a surrogate for temperature regime), mean annual rainfall (adjusted for water holding capacity of the soil),

Table 2 Farm-scale site assessment for E. globulus plantation development in WA (adapted from Harper et al. (1998, 2000a)) 1 2 3 4 5 6 7 8

Estimate potential productivity (m3 ha 1) from annual rainfall and evaporation Identify farmed areas with slopes <15%, unlikely to be inundated with water. Survey the remainder of the property Survey site at scale of 1:10,000 using air-photo interpretation and ground survey. Undertake ground survey using drill-rigs/back-hoes and EM 38 salinity meters at observation density of 1 observation per hectare Identify sites with shallow (<2 m) soils overlying bedrock, saline soils (EC > 50 m S m 1) and with sand horizons >2 m deep. Do not plant these Identify sites with hardpans within 1 m of the soil surface that can be rectified by ripping. Hardpan includes ferricrete and iron-organic pans. If hard material is basement rock ripping will not increase soil depthÐdo not plant these sites For sites, which have no overt limitations, take 0±10 cm soil samples, using a standard soil sampler. Take 15±20 sub-samples in a Z-shaped pattern. Send samples to laboratory, with location details. These samples will be used to predict fertiliser requirement In sites with <800 mm rainfall integrate trees into landscape as strips, rather than using block plantations. Avoid hill crests and areas that are likely to become saline Undertake economic analysis using land value, likely productivity and risk

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Table 3 Site suitability classi®cation based on site potential productivity classes measured as peak mean annual volume increment (MAI, in m3/ha/ year) modi®ed by the presence or absence of management limitations or degradation hazards for plantations in Tasmania (Laffan, 1997) Suitability class Suitable

Productivity class 1 1a 1b 2

Management limitations or degradation hazards High (MAI > 20) Very high (MAI > 30) High (MAI 20±30) Medium (MAI 15±20)

Negligible or slight to moderate

Marginally suitable

3 >3

Low (MAI 10±15)

Negligible or slight to moderate Severe

Unsuitable

4 >4

Very low (MAI < 10)

Negligible or slight to moderate Severe to very severe

soil drainage characteristics, tree-rooting conditions (effective root depth and ease of root penetration) and nutrient availability (total N and P). There are three classes of site suitability: suitable, marginally suitable and unsuitable (Table 3). They are assessed by taking into account management constraints (fertiliser requirement, traf®cability, soil workability) and land degradation hazards (¯ood risk, erosion risk, landslide risk) in addition to site productivity. It is important to understand that while all the critical land qualities relevant to farm forestry can be described and a number of land characteristics can be measured for each land quality, the actual rankings or rating of these land qualities/characteristics can be speci®c to a region and/or tree species. This is evident in Laffan (1997) where only three species (E. globulus, E. nitens and P. radiata) were considered for plantations across Tasmania.

Application of the Laffan (1997) methodology to provide rapid assessment of plantation potential on exnative forest and ex-agricultural land by Forestry Tasmania has necessitated modi®cations to increase the ef®ciency of ®eld assessment with non-professional teams (staff with only minor soil science training or soil survey experience). The modi®ed ®eld procedure (Laffan, 2000) is currently in operation by Forestry Tasmania (Table 4). 5. Implicit and explicit role of scale in site assessment 5.1. Grain and extent The scale of a biological or physical phenomenon can be de®ned as a combination of two attributes: its

Table 4 Forestry Tasmania's ®eld procedures for assessing plantation potential (Laffan, 2000) Assessing site productivity class and limiting factors 1 Record mean annual rainfall (MAR) and elevation range 2 Record native vegetation type and species, and rock type (geology) 3 Record soil profile features to 0.8 m or impeding layer if shallower (colour, texture, layer depth and designation) 4 Determine soil properties associated with site productivity factors (effective root depth, ease of root penetration, drainage class and nutrient availability) 5 Determine and record site productivity class and limiting factors Assessing site suitability class and limiting factors 6 Determine fertiliser requirement from soil properties 7 Record site features associated with trafficability/soil workability (slope angle, surface rock) 8 Record site features associated with flood and landslide risk 9 Determine soil erodibility class and erosion risk from soil and site features 10 Determine and record site suitability and limiting factor(s)

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Fig. 1. Scale extent and grain for some common topographic maps, DEMs for Australia, Africa and the world, plus two global vegetation data sets (LAC and GAC). The extent of topographic maps can be increased by physically appending them together (indicated by the arrows) while their grains are based on minimum depicted area (Reid, 1988). The 1:2 M soils map refers to the atlas of Australian soils (Northcote et al., 1960±1968). The Australian 2.5 min, 8 min DEMs, the African 6 min DEM and the world 0.58 DEM come from Booth (1996b, 1998).

grain and its extent (Wiens, 1989) in both spatial and temporal dimensions. Grain is de®ned as the size of the ®nest resolved spatial or temporal unit for which data are collected or presented. Extent is de®ned as the largest spatial or temporal unit over which data at grain resolution can accumulate. For example, a digital elevation model (DEM) can have an extent of 50,000 ha and a grain of 650 m2. A traditional soil survey map may have the same extent (50,000 ha) but its grain is implicit in the physical dimension of the map and printing resolution of lines. Fig. 1 displays the scales associated with standard published land classi®cation maps in Australia. A single paper soillandscape map published in NSW would have an extent of 247,500 ha and a grain of 20±27 ha based on the smallest polygon that can convey useful information (Reid, 1988). The spatial extent can be expanded by physically combining multiple maps (indicated in Fig. 1 by the arrows), but this becomes cumbersome and requires large available spaces.

GIS has overcome some of the scale limitations of traditional maps, especially the extent. Grid or raster GIS coverages also have an obvious grain size. There is a DEM covering Australia at a grain of 9 arc-second  9 arc-second (7 ha). The satellite path®nder vegetation dataset summarizes normalised difference vegetation index (NDVI) values at a global extent (GAC) every month at a grain of 247,500 ha (Fig. 1). There are still, however, practical limitations to extant-grain combinations. These latter remotesensed images highlight the combination of both spatial and temporal dimension of scale to site assessment although the temporal aspect (and its interaction with spatial processes) has traditionally been poorly understood. Fig. 2 highlights some of the biological and physical phenomena that can affect an individual plantation age class in Australia using a temporal±spatial graph (adapted from a similar ®gure in Schneider (1998)). The grain and extent of a phenomenon such as an El

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Fig. 2. A temporal±spatial scale diagram containing several environmental factors affecting a typical hardwood (HW) plantation age class establishment in Australia, adapted from a similar ®gure in Schneider (1998). Site evaluation processes of (1) site/soil description and sampling and (2) extension of this plot data to the whole age class area are superimposed.

NinÄo event can be conveyed in both a spatial and temporal context in Fig. 2a. A minimum time interval required to identify an El NinÄo event would be 2 years (grain) while such an event would continue for no

more than 8 years (extent). These are represented as the lower and upper limits of the vertical ellipse axis in Fig. 2a. The spatial extent of an El NinÄo event can be up to and beyond the Paci®c rim while the grain has to

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at least cover the area from Australia to South America (horizontal ellipse axis). Typical areas planted to hardwoods across Australia (HW age class) in recent years would range from a minimum area of 20 ha to a full extent of 100,000 ha. Such an age class would a minimum life of 7 years and maximal life of 30 years (Fig. 2a). Wild®re is a common feature of the Australian environment and can be represented in a similar manner in Fig. 2a. An interpretation of this temporal±spatial graph would be that any current or future hardwood plantation age class in Australia will experience at least one El NinÄo event over all of its area because its temporal grain and extent is greater than of a typical El NinÄo event but its spatial grain and extent is less. Whereas wild®re incidence is far greater (temporal grain and extent being much smaller than that for the HW age class) and its spatial extent can exceed that of the HW age class. Neither of these environmental perturbations (drought or wild®re) is accounted for in current site assessments for farm forestry in Australia. The process of site assessment can be represented in this temporal±spatial framework to illuminate some important scale issues. Fig. 2b represents site assessment as a two-stage process; the ®rst is the actual site/ soil description and sampling, while the second is the extrapolation of these various short-term small area measurements to larger spatial extents, and less commonly, temporal extents equivalent to the HW age class. The inherent scale of these two aspects of site assessment is quite distinct. Soil sampling occurs usually at a minimum grain size of a soil core (e.g. 50 mm diameter and 0.1 m long or 2  10 4 m3) increasing to a soil pit (0.75 m3), while a single ``site'' description using the Australian standard methodology (McDonald et al., 1990) has a grain size of 314±1256 m2. Such a site/soil description and sampling may take anything from 15 min to 2 h to complete (Fig. 2b). In contrast, the ®nal site assessment product would cover at least 50 ha and up to a majority of the planned HW age class, and take from several weeks to nearly a year to complete (Fig. 2b). The process of ``scaling-up'' the ®ne-grain samples (1) to the full extent of the HW age class (2) (as indicated by the diagonal arrow in Fig. 2b) can be done spatially using the traditional qualitative processes of soil mapping using air-photo interpretation or the more quantitative processes of spatial modelling

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(Gessler et al., 1995; McKenzie and Ryan, 1999; Ryan et al., 2000). However, what is often ignored in the site assessment process is the temporal scaling-up of site/ soil attributes to time intervals relevant to a HW age class. For example, extending the soil depth, texture and other site attributes to estimate the water supply capacity of the site and/or potential impact on groundwater of the growing plantation, or the availability of key nutrients to the stand over time. This temporal scaling-up can be achieved either by modelling, such as with the forest physiological models 3-PG (Landsberg and Waring, 1997; Landsberg and Coops, 1999) or ProMod (Battaglia and Sands, 1997), or by monitoring of representative sites for key dynamic soil attributes over the life of the plantation. McKenzie (1991) and McKenzie et al. (1995) discussed the development of ``soil reference sites'' which can be applied to this situation. Long-term monitoring unfortunately seems to be an anathema to commercial forestry enterprises or research funding agencies so temporal scaling-up will depend increasingly on modelling. 5.2. Implicit scale in environment factors used for site assessment Investment in farm forestry requires some form of prediction of the climate that is likely to occur over the putative rotation, and in particular the incidence of extreme events such as water de®cits (droughts), strong winds or frosts. There are implicit scale characteristics inherent with many of the environment factors used for site assessment for farm forestry. Climate varies over large extents and so requires coarse-grain assessment to provide adequate spatial models, although ®ne-grain variation does occur in topographically diverse regions. Climate is temporally dynamic so these spatial models need to be updated regularly. Such is the design of the current weather satellites, which provide global coverage at a grain of 1 km2 twice a day. To overcome this high temporal variation, climate data are usually summarised to long-term annual averages at the expense of information implicit in the variance such as the El NinÄo/La NinÄa cycles, however, it is feasible to develop longterm water balances or estimate return periods of drought events of a particular severity. In contrast to climatic data, topographic data has negligible temporal variation and the development of

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DEMs has enabled detailed quantitative terrain analysis (Moore et al., 1991; Wilson and Gallant, 2000) over large extents. The more pertinent problem for topographic data is determining the appropriate grainsize. Fig. 1 displays the grain of the current Australian DEM as a 9 arc-second  9 arc-second (7 ha) area. While this may be adequate for a large percentage of Australia, which is relatively ¯at, it becomes inadequate in the more dissected hills and mountains where a grain of 25±2500 m2 is necessary to capture the elevation variation within a hillslope (Hutchinson and Gallant, 2000). Again the quality of the information required will depend on its use; if the site assessment and management system requires information on likely water erosion hazard or localised movement of water to site trees in more favourable hydrological conditions more detailed topographic information will be required. Variation in soil attributes that affect land qualities such soil water availability or nutrient supply can occur at ®ne-grain scale within hillslopes. Thus, a map such as the atlas of Australian soils (Northcote et al., 1960±1968) with a grain of 8000 ha (Fig. 1) cannot convey any quantitative information on speci®c soil attributes or land qualities because the grain size incorporates far too much variation, and the mapping units contain an array of soils. Site assessment for farm forestry needs to have an explicit recognition of scale factors affecting the environmental variables or land qualities that are to be related to plantation productivity and management. To achieve this requires a reconsideration of the ``site concept'' used as the basis for site assessment. 6. A new site concept for farm forestry 6.1. Site as an integrated effect of environmental factors The site concept in forestry has been used to integrate environmental factors in¯uencing tree performanceÐterms such as ``site quality'' and ``site index'' have various de®nitions but usually rely on tree measurements (Carmean, 1975; Grey, 1980). This traditional site concept is inadequate when trees are absent at the time of planting, as is the case for most farm forestry sites. When plantations were established on forested land, site evaluation systems took into

account the abundance and vigour of the natural vegetation (e.g. Havel, 1968). Although many practitioners still attempt to classify land in terms of its former natural vegetation cover, this approach is haphazard at best when the land has been almost totally cleared for agriculture. A more practical de®nition of ``site'' would be the integrated effect of environmental factors that in¯uence the potential growth of a particular tree genotype and affect the soil management of that stand of trees to produce a particular product or output. This concept of site is environment-based, independent of the presence of trees, and hierarchical in scale (Fig. 3). Scale is critical to this new site concept and its application. At continental or global scales the site concept can be reduced to climatic factors. Such a site assessment for E. globulus at Australian and global extents has been done by Booth (1996a,b). The ranges of seven important climatic factors de®ne descriptions of this type. Descriptions for several 100 species are included in the forestry compendium global module (CAB International, 2000). However, if the site assessment is required to produce information for site and soil management then the scale has to be ®ne-grained enough to adequately quantify the soil nutrient and water regimes and to assess degradation hazard (Fig. 3) (these three land qualities are discussed further below). It is this complex interaction of environmental variables working at different scales affecting plantation productivity and sustainable management that has made forest site such a dif®cult entity to quantify. New digital technologies have facilitated the implementation of this site concept. GIS enables the integration of environmental coverages such as (1) spatial interpolation of long-term climate data from meteorological records, (2) geological and remotely sensed geophysical data, (3) digital elevation models (DEMs) and quantitative terrain attributes and (4) spatial models of speci®c soil attributes or land qualities produced from soil pro®le data with known geo-position. The integration of such diverse datasets for native forest management has been discussed by Ryan et al. (2000), and it will be possible to adapt such an approach to farm forestry site selection. Each general environmental attribute presented in Fig. 3 can be further re®ned to produce quanti®able GIS coverages. For instance, some of the important climatic factors for plantation forestry include radiation, rainfall

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Fig. 3. A new site concept for farm forestry land evaluation. This site concept can be applied to the multiple goals of farm forestry; maximum wood productivity, optimum soil management, and minimum soil degradation.

(long-term mean, seasonalityÐsummer versus winter dominance or uniform/bimodal and variability), temperature (mean monthly maximum and minimum temperature, long-term absolute minimum), duration and timing of the frost period, evaporation, and vapour pressure de®cit. Most of these climatic attributes can be estimated spatially using computer programs that provide derived surfaces of average values (e.g. ANUCLIM, McMahon et al., 1996) or estimates of daily values of periods up to several decades (e.g. Data Drill,

Jeffrey et al., 2001) for discrete locations. Similarly, geology, geomorphology, and terrain can be further characterised to produce a set of critical environmental coverages. An example of the partial implementation of the new site concept (Fig. 3) would be a GIS with digital coverages given in Table 5 where the four main environmental attributes (climate, geology, geomorphology and terrain) have been characterised by individual ``primary'' coverages that are readily available

Table 5 Translation of environmental attributes presented in the new site concept (Fig. 3) into primary and secondary GIS coverages Environmental attribute

Primary coverage

Climate

Mean annual rainfall Net radiation Mean annual potential evaporation

Geology

Lithology Gamma radiometric K Electro-conductivity

Terrain

Slope Contributing area

Processed or secondary coverage

Coverage type

Prescott index

Grid Grid Grid

Parent rock class Regolith stability

Compound topographic index, stream power index

Polygon Polygon Grid Grid Grid Grid

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or cheap to produce, and mostly numerical. These primary coverages can be further processed to ``secondary'' coverages that can be more readily interpretable. For example, the ratio of mean monthly rainfall and mean monthly evaporation can be used as a simple soil water balance (Prescott index) (Prescott, 1948; McKenzie and Ryan, 1999), lithology classes from a geology map can be reinterpreted as parent rock codes (Turvey, 1987; Turner et al., 1990) or regolith stability classes (Murphy et al., 1998), both of which have indirect relationships to plantation productivity and management. Any of the individual spatial coverages for climate, geology and terrain, or combinations of the above, could be used to stratify the potential plantation area as long as there is suf®cient variation in that environment attribute across the region of interest and the individual data sets are reasonably accurate. Scale issues are inherent in Table 5 in that one can progress down the list and at each primary or secondary coverage determine whether this attribute has suf®cient spatial variation to warrant its inclusion and quanti®cation for the region of interest. Small regions may have insuf®cient variation in rainfall across their extent but have enough relief to produce variation in evaporation. Similarly, smaller areas may have uniform climate, geology and terrain and vary only in soil properties. In this manner the general site concept in Fig. 3 can be re®ned to speci®c applications. Existing site classi®cation systems, such as the technical soils classi®cation (Turvey, 1987; Turner et al., 1990), Inions (1991), and Laffan (1997), can be compared using Fig. 3 as a basic framework. The more dif®cult task is determining what land qualities and component land characteristics should be quanti®ed and how spatial coverages of these attributes can be produced. 6.2. Land qualities The three key land qualities included in Fig. 3 (soil water, soil nutrient and soil degradation) are purposely simpli®ed to re¯ect the three components of land suitability (Gibbons, 1976) (production or yield, management of inputs and hazard of use) and the general grouping of land qualities used by Laffan (1997) and listed in Table 1. These general land qualities can be further re®ned where and when there is knowledge of speci®c edaphic adaptations of the plantation species

of interest. Tables 1, 2 and 4 give examples of these component land characteristics for known plantation species. There is a lack of knowledge of the speci®c edaphic adaptations of each of the new species being considered for farm forestry in low rainfall regions of Australia. This means that site assessment has to target the most critical land qualities in a generic manner as possible. It is here that the existing forest physiological models are helpful because they have had to develop through a similar process. The simplest of these models (3-PG and ProMod) have reduced their land quality inputs to two; (1) plant available soil water holding capacity and (2) a soil fertility index (Battaglia and Sands, 1997; Landsberg and Waring, 1997). These will be discussed further below, but ®rst it is useful to consider the scale of our sampling unit (``1'' in Fig. 2b) because this will affect what attributes to measure. 6.3. Soil individual The value of a set of measurements depends on effective sampling. Every sample should represent a de®ned body or class of soil. The soil unit or individual to be characterised requires speci®cation beyond that provided by standard site and pro®le descriptions, especially where it needs to be related to tree stand growth. The dimensions of the soil individual of interest should be clearly de®ned, and for statistical purposes should be kept constant in any quantitative investigation (McKenzie et al., 2000b). The soil individual has two functional roles in plantation forestry; it is the medium within which the trees obtain nutrients and water, and the surface over which water, machines and humans must pass. The ®rst is a volumetric attribute, while the second is a sur®cial attribute; that is, one that has a geometric surface and an atmosphere±soil interface. The volume of soil accessible to tree roots has a dominant control on site suitability because it determines the total water store, and to a lesser extent nutrient store available to trees. 6.3.1. Effective regolith volume Boardman (1988) suggested that soil volume was probably the most critical soil attribute that should be assessed in site surveys. The simplest surrogate for soil volume is soil depth to a root impeding layer (Turner et al., 1990), however, it is important to realise

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that the roots of many eucalypts (and other tree species) can grow below the solum into weathering C or D horizons, saprolite or unconsolidated sediments. Hence the term ``regolith'' is more appropriate rather than soil in describing this volume. In WA, E. globulus stands roots can grow down to 8 m and more through a lateritised granitic saprolite within 4±6 years of planting to use available water (S. Crombie and R. Harper, unpublished data). Regolith volume is not only affected by horizontal and vertical dimensions of the soil individual, but also by the inherent matrix components. The most obvious aspect of this matrix is particle size or texture. It is important to characterise the volume of coarse fragments and/or presence of root impediments such as hardpans, massive or coarse structure, and macroporosity. It is the matrix components that modify a total regolith volume to an effective volume. The determining factor here is the ability of roots to exploit a volume of regolith so in many cases there are species-speci®c adaptations which determine the importance of impediments root growth and function (e.g. sodicity, pH, bulk density or redox potential). The ``effective regolith volume'' can therefore be de®ned as the below-ground volume that individual trees, within a soil individual, can exploit over time to obtain water and nutrients. This three-dimensional measure provides the basis of scaling-up individual soil sample analyses from cores and pro®les to a spatial scale more appropriate to tree stands and is therefore important for both estimating the soil water and fertility regimes. 6.4. Soil water regime When the soil water regime is characterised, the main attributes to quantify are the capacity of the soil to receive, store and drain water, the presence and depth to a water table and its salinity. 6.4.1. Soil water-holding capacity Drought deaths are periodically reported in areas with shallow soil, and where root exploration of subsoils is limited. Examples of where tree growth has been affected on sites with limited regolith volume include deaths of Pinus radiata on shallow soils in dissected river valleys in south-western WA (McGrath et al., 1991), and more recently in Eucalyptus globulus

145

plantations in WA (Harper et al., 1999) and Victoria. Effective regolith volume is a critical attribute to quantify the soil water store. Once this volumetric boundary is de®ned it can be further re®ned to estimate plant available water-holding capacity (PAWC). This can be measured directly by relating soil volumetric moisture content to matric potential (pF curves) or estimated indirectly via pedo-transfer functions (PTFs) where simple soil attributes such as texture, bulk density and structure are used in empirical models to estimate PAWC (Williams et al., 1992). PAWC is a static variable and in some cases it may be important to assess the dynamics of soil water movement. For this it is necessary to determine saturated conductivity (Ks). As with PAWC, Ks can be measured directly or estimated using PTFs (McKenzie and Jacquier, 1997; O'Connell and Ryan, 2002). In wetter environments, the ability of the soil to drain water becomes more critical than overall storage capacity. 6.4.2. Groundwater In low rainfall environments the ability to store and deliver water to tree roots is critical. Groundwater with a salinity of <10 dS m 1 can be utilised by tree roots within the capillary fringe (Stirzaker et al., 2002). Non-saline groundwater can allow trees to grow in low rainfall environments (George, 1991), however, the hydrology of the catchment will determine whether this groundwater will be consumed, diminished or unaffected by tree water usage over time (Stirzaker et al., 2002). Certain tree species can utilise water with salinity >10 dS m 1, but the important criteria is how plant water use changes as salinity increases and how long these water use rates can be maintained (Marcar et al., 1995). With the use of revegetation to reduce the onset of salinity a new set of issues for site evaluation have emerged so as to optimise the positioning of trees in landscapes and to determine the proportion of catchments that need to be replanted with perennials to control recharge (Stirzaker et al., 2002). Similarly, there has been extensive debate on the relative bene®ts of recharge compared with discharge plantings. Morris and Thompson (1983) demonstrated in Victoria that trees planted in recharge areas were more effective in controlling salinity, and the limited long-term impact of discharge plantings was demonstrated in multi-species eucalypt plantations on salt seeps after

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15 years (Greenwood et al., 1994). Responses in many cases may be catchment speci®c. Waterlogging can occur in soils where the rate of in®ltration is less than that received from rainfall and various up-slope contributions. The pattern of waterlogging is often related to landscape position, with waterlogged sites most likely in lower slope positions and at the breaks of slope. Waterlogging can lead to anaerobic conditions, mortality of tree roots, and paradoxically drought deaths. Trees in such sites can also suffer from wind-throw. There are three issues related to waterlogging that should be considered in site survey. First is the estimation of the degree of waterlogging from observations of soils and landscapes, particularly as in Australia many soil features are relict from previous climates and cannot necessarily be unequivocally interpreted in terms of soil hydrologic regime. Second, waterlogging may recur at infrequent intervals as a result of climatic conditions and here the return frequency of critical events may be uncertain. Finally, there are differences between species in terms of their response to waterlogging, and these are often not clear so any interpretation of site attributes should be circumspect. 6.5. Soil fertility regime In many ways the soil fertility regime is the hardest of the land qualities to quantify because it has many potential nutritional dimensions that are linked with biological processes. It is therefore useful to distinguish the two aspects of soil fertility; nutrient supply (quantity), and nutrient intensity (McKenzie, 1991), particularly in relation to what can be measured during a site assessment survey. Nutrient supply is best characterised by determining: (1) Total soil nutrient stocks for phosphorus (P), nitrogen (N) and carbon (C) within for the effective regolith volume. An example would be total soil phosphorus density in kg/m2 to the rootimpeding layer. (2) Exchangeable cations (Ca, Mg, K, Na and Al) to estimate soil base saturation, exchangeable acidity, effective cation exchange capacity and sodium exchange percentage for the effective regolith volume.

These soil attributes are similar to PAWC in that they require knowledge of the effective regolith volume and are relatively stable. They can be used to stratify the plantation on the basis of potential fertility and are also relatively straightforward to measure as part of a site assessment survey as long as laboratory facilities are available and affordable. Two simple indices of nutrient intensity useful in site assessment are pH and electrical conductivity in 1:5 soil±water suspensions. Nutrient intensity also includes measurements of dynamic soil fertility properties that are in¯uenced in part by the nutrient supply but also by other environmental factors such as temperature, soil moisture and biological activity. These attributes are affected by processes in surface soil horizons and therefore less a function of the effective regolith volume. Intensity factors, such as mineralisation rates and availability indices, are strongly timedependent and often vegetation-dependent, and thus of little practical use at the site assessment phase. As with soil water, which is also highly temporally variable, knowledge of nutrient availability or mineralisation rates may be best estimated via simulation modelling. For example, Polglase et al. (in press) have produced the model SNAP to estimate soil N-mineralisation using more readily available data. Another approach to handle dynamic fertility attributes is to determine high risk (or high potential return) areas and initiate a periodic soil monitoring but this is only practical after an area been accepted for plantation development. The vertical arrow in Fig. 2b represents both of these temporal scaling-up approaches. 6.6. Soil management and conservation practice for sustainable farm forestry Whereas in the past, site assessment has mainly been used for the prediction of tree performance, it can be extended to form the basis for site-speci®c management (Table 1) and quantifying the risks associated with these activities (degradation hazard). Land evaluation for soil management and conservation practice is more advanced than that for quantifying soil water or nutrient regimes, although knowledge of the latter two qualities will impact on decisions for fertilisation, traf®cability and erosion control. Many of the techniques used for horticultural and other intensive agriculture can be readily used (Dent and Young, 1981;

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Davidson, 1992; McKenzie, 1991). The interpretation of land characteristics for plantation operational prescriptions does require some knowledge of the main plantation operational practices such as site cultivation; ripping, mounding and fertiliser requirements of individual tree species (Valentine, 1986). Many of the land characteristics that affect site traf®cability or degradation hazard (water and wind erodibility, soil strength, compaction hazard) are sur®cial attributes in contrast to the volumetric attributes important for soil water and fertility. These sur®cial attributes are strongly scale-dependent and require characterisation both within the soil individual and in relation to surrounding environment. This latter contextual analysis can be facilitated by quantitative terrain analysis because it can quantify the surface geometry (e.g. slope, curvature and contributing area), which can then be used to estimate how water would move over such surfaces (compound topographic index or soil wetness index and stream power index) (Moore et al., 1991; McKenzie et al., 2000a). Measurements within the soil individual scale vary from rock outcrop and micro-topography (geometry) to surface soil properties that characterise resistance to deformation (aggregate stability, shear strength, penetration resistance, bulk density and texture). These different attributes can be combined within a GIS environment to give various interpreted themes (Ryan et al., 2000). Increased forest productivity within the constraints of effective soil management and minimal degradation forms a basis for sustainable plantation management. Data collected at the site evaluation stage should, therefore, provide information for an array of other decisions, ranging from site-species matching, establishment practice, to fertilisation requirements through the ensuing rotation. Pro®tability will be increased by applying inputs where they are needed, rather than on a uniform, or prescription basis. Sites with unmanageable constraints (e.g. with shallow or saline soils) can be avoided. Sites with manageable constraints (e.g. poor fertility, poor drainage, root constraints from shallow hardpans) can be treated as required. Similarly, erosion risk can be predicted, and appropriate strategies recommended. The ef®cient selection of soil indicators for sustainable plantation management requires a foundation of spatial site assessment to determine what land quali-

147

ties or soil properties are critical to forest productivity and management of that site. Site assessment also facilitates the selection of where, and at what scale, these indicators (or suitable surrogates) can be ef®ciently measured and monitored across the landscape. 7. Use of species trials, permanent growth plots, and simulation models The expansion of farm forestry in low rainfall zones of Australia has required assessment of alternative plantation species for which there is little previous experience in plantations or knowledge of their respective edaphic requirements other than the sites where they occur naturally. The Australian Low Rainfall Tree Improvement Group (ALRTIG) (Bush et al., 2001) and the ``Search'' projects in WA (Bartle, 2001) are examples of how this situation is being approach in a systematic manner. ALRTIG has identi®ed several species that are particularly suited to 400±600 mm rainfall environments. Australian ALRTIG species include Eucalyptus cladocalyx, E. occidentalis, E. sideroxylon/tricarpa, E. camaldulensis, and Corymbia maculata, as well as several mallee eucalypts (E. angustissima, E. horistes, E. kochii ssp. kochii, E. kochii ssp. plenissima, E. loxophleba ssp. lissophloia and E. polybractea). P. pinaster and P. brutia are also been included as the only softwoods (Bush et al., 2001). Initial site evaluation at the continental scale has been done using individual species climatic modelling (Booth, 1996a,b). However, the only way to build up detailed knowledge on the speci®c edaphic requirements of these species has been to design a standard species trial and replicate this trial across the targeted rainfall zone in southern Australia. Each trial site will have detailed site characterisation with a minimum site-soil dataset based on the concepts outlined above. Such ``soil reference sites'' associated with well-design standard trials and other permanent growth plots are considered as a critical component to improve land evaluation in Australia (McKenzie, 1991; McKenzie et al., 1995). Information obtained from these trials and permanent growth plots will be crucial for future farm forestry site assessment. The data can also be use to calibrate forest physiological models that can then simulate potential productivity of extended areas.

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Simple forest growth models, such as 3-PG and ProMod (Battaglia and Sands, 1997; Landsberg and Waring, 1997), offer the ability to directly relate site attributes to biomass production. The newer models attempt to encapsulate most of the environmental factors presented in Fig. 3 as inputs affecting forest productivity. Two key soil attributes are required to run both models. The ®rst is plant available soil waterholding capacity (PAWC) and the second a fertility index. McKenzie and Ryan (1999) and Ryan et al. (2000) have shown how such spatial soil models can be developed over forest areas using modern digital technologies, so this presents the opportunity to operate these forest growth models spatially within a GIS environment (Mummery and Battaglia, 2001; Tickle et al., 2001). A modi®ed version of 3-PG (3-PGS) allows potential forest productivity to be checked against actual forest productivity by using satellite remote sensed data (NDVI) to estimate leaf area index (Coops et al., 1998; Coops, 1999). This is another important means to enable this physiological model to be run spatially to monitor and predict actual and future forest growth (Coops, 1999). There are, however, several limitations with both 3PG and ProMod that require further development for their full spatial application to most farm forestry sites. The respective soil fertility indices need better de®nition as to what aspects of nutrient supply they represent. The soil water sub-model needs to allow for lateral redistribution due to landscape geometry and soil properties. Presence of groundwater and its salinity need to be incorporated into the soil water submodel and linked with tree growth. Some of these features are currently under development. However, the power of these physiological forest growth models is their relative simplicity and modest input requirements so further modi®cations need to be parsimonious. Although these models do not explicitly account for various stochastic events such as drought, insect predation or wild®re they do offer the ability to estimate potential productivity with given climate and soil data and thus they are becoming an important tool in site assessment for farm forestry. An example is the incorporation of the ProMod model into the Farm Forestry Toolbox CDROM (Private Forests Tasmania, 2001).

Other simulation models for catchment scale soil water, groundwater and other hydrological interactions with tree stands are being applied to farm forestry assessment (Stirzaker et al., 2002). These simulation models will play an increasingly important role in site assessment for farm forestry because they offer an ef®cient way to assess the impact of planting trees on the hydrological cycle, from the soil individual to the catchment scale, without having to do longterm site monitoring. This is critical in the low rainfall agriculture zone of Australia where all land uses are competing for a limited fresh water resource. 8. Principles for future site assessment for farm forestry in Australia Several principles for site assessment are clear from the rapid expansion of farm forestry in Australia:  A primary premise of all site assessment is that methods should be explicit, quantitative, consistent and repeatable. It is also recognised that logistic constraints determine the time and money that can be spent on site assessment. Competitive private plantation developers will not adopt time-consuming assessment procedures no matter how reliable or accurate they are.  The large variation in climatic and land resource attributes across Australia requires a robust, multiscaled system; from region to field. Site assessment should, in principle, cover this range of scale. This also means that data should be collected at meaningful spatial resolutions that can be readily scaledup to the extent of plantations.  Critical parameters important for plantation site assessment include climate (especially solar radiation, water availability and temperature), soil physical and chemical attributes, topography, biological factors such as pests and diseases risk, and genetic characteristics of particular trees. Determining soil physical factors, particularly availablewater-holding capacity, is necessary for carrying out reliable water balance calculations in low rainfall zones.  Farm forestry development requires the assessment of new species for which there is little prior knowledge. In these cases, the recommended approach to site assessment is to undertake coarse scale climatic

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modelling using available data such as the CABI Forest Compendium Global Module. This should be followed with a well-designed, standard set of species trials which have detailed site characterisation. Within these homoclimes, potential productivity can be estimated using simple forest physiological models (3-PG or ProMod). Site assessment procedures can be further developed to fine scale resolution as results from the species trials became available.  The relationships between land attributes, soil management practice, and tree response are often not quantified. Thus activities such as fertilisation, cultivation or tree thinning are often still applied by prescription, or guesswork, rather than actual requirement.  New digital technologies will facilitate the implementation of quantitative site assessment. GIS enables the integration of environmental coverages such as (1) spatial interpolation of long-term climate data from meteorological records, (2) geological and remotely sensed geophysical data, (3) DEMs and quantitative terrain attributes and (4) spatial models of specific soil attributes or land qualities produced from soil profile data with known geo-position. Simulation models for forest productivity and soil±water±tree interactions at catchment scales are becoming available as GIS applications and are thus an increasingly important component of site assessment.

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ence to the factors affecting the productivity, management and hazard of the tree species of interest. The spatial context of the GIS data and hierarchical nature of the site concept does require special consideration of scaling issues, such as what is the representative elementary volume for measurement and how soil data should be extended across landscapes in a statistically valid manner. Modern spatial technologies can facilitate some of this procedure, but it is important to understand that we face considerable uncertainty in the speci®c adaptations of individual tree species and their productive potential in these agricultural environments. Therefore, there is still need for well-designed species trials across a range of wellcharacterised sites. Results from these trials can be fed back into site assessment procedures and used in simulations models for rapid extension beyond the trial sites. This forms the basis for continual evolution and improvement of site assessment procedures for farm forestry in Australia. Acknowledgements This work has been supported by the RIRDC/ LWRRDC/FWPRDC Joint Venture Agroforestry Program (JVAP) and the Australian Natural Heritage Trust. Thanks to Nico Marcar and Tom Jovanovic for comments on an earlier draft and John McGrath, Bob Gilkes and Keith Smettem for stimulating discussions.

9. Conclusions Farm forestry is expanding rapidly in Australia and there are large amounts of public and private money being invested in this sector. Site assessment has a critical role in ensuring that this industry is viable and sustainable. There are potentially large returns, both economical and environmental, in expanding the area of trees within Australia's major river catchments. The challenge is to adapt our current land evaluation techniques to take account of the differences between tree crops (especially native eucalypts) and more familiar agricultural crops, and to measure the critical land qualities in a quantitative, ef®cient manner. Site assessment can become a process of collation of spatial environmental data and its analysis with refer-

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