Vegetation,land use and management of the inner-dune zone in the North-Holland Dune Reserve,The Netherlands

The inner-dune zone between the villages of Bakkum and Egmond in the North-Holland Dune Reserve is one of the few remaining examples of the semi-natural boundary between the dunes and the polders. For several centuries this area has been used for various purposes, in particular for grazing. Large-scale excavating, especially after about 1900, has encroached upon the dunes. Vegetation and land-use history were studied and a provisional scheme on the vegetation development is presented. Management policies discussed are non-intervention, mowing in order to start natural processes such as rabbit grazing, and the continuation of traditional livestock grazing. Proposals for external management policies were made in order to maintain or enhance the original transition zone, based on comprehensive physical-geographical studies of the whole inner-dune zone.     

The integration of crop rotation and tillage practices in the assessment of ecosystem services provision at the regional scale

The provision of ecosystem services at the landscape level can be significantly influenced by land management practices. Within an agriculturally dominated case study area in Saxony, Germany, a more detailed land use classification, which includes differentiated information on agricultural management practices, was utilized within the raster-based planning support tool GISCAME. “Management” refers to typical, regional crop rotations and soil tillage practices.The focus of this research was based on an indicator-based approach to assess ecosystem services and the development of land use change (LUC) and land management change (LMC) scenarios. The EuroMaps Land Cover data set was specifically developed for the case study and included remote sensing information for the general land use classification and terrestrial mapping information. Furthermore, statistical data on detailed regional agricultural land management were included. The raster-based planning support tool GISCAME was then used to simulate scenarios and visualize results. The LUC and LMC scenarios showed that the more detailed land use classification provided better output for the prioritization of planning alternatives. Further it enabled a refined assessment of the provisioning services (i) food and fodder provision, (ii) biomass provision, the regulation services, (iii) soil erosion protection, (iv) drought risk regulation, (v) flood regulation, (vi) returns from land-based production (i.e. the market value of biomass provision), and (vii) ecological integrity. The results of this study support the view that the application of improved management measures, such as conservation tillage, can significantly enhance the provision of ecosystem services (e.g. soil erosion protection and drought risk regulation) at the landscape level. The study also indicates that a combination of strategic LUC, such as afforestation and LMC, might be an effective way to enhance regulating services with acceptable trade-offs regarding provisioning services. Our approach presents a refined foundation for ecosystem services assessment, which is designed to better support regional planning and the provision of information to non-experts in the participatory processes. For transfer into other regions, standardized land use and land management classification will have to be defined.     

不同经营管理条件下土壤有机碳及其组分研究进展

土壤有机碳能够反映土壤质量和土壤健康,是当前国内外学者开展研究工作的重点之一.土壤有机碳及其组分的研究已取得了一些成果.本文概述了土壤有机碳分组方面的研究结果,主要讨论了在不同利用类型和不同经营方式条件下,土壤有机碳及其不同组分分解、蓄积、含量、存量、分配比例等的动态变化,并指出了当前土壤有机碳及组分研究面临的一些问题和今后的研究方向.     

The importance of traditional orchards for breeding birds: The preliminary study on Central European example

Orchards are parts of agriculture and apart from their economic role they may preserve biodiversity in highly transformed farmlands. An increasing intensity of orchards management is known to be threat for some species, especially pollinators and birds. On the other hand, the biodiversity of abandoned orchards was hardly investigated. Here, I present a survey of orchards in Poland to estimate how bird's diversities differ in response to the intensity of orchards management. In 2014, 66 orchards of three types – abandoned, extensively and intensively managed – were investigated. Bird species' richness and abundance were found to be highest in abandoned orchards but overall bird diversity and species composition in abandoned did not differ from these found in extensively managed orchards. In abandoned and extensively managed orchards, hollow-dwellers and insectivores (with some rare old-forest associated species) dominated, whereas in intensively managed orchards the most diverse were ground-dwellers. Among the several selected environmental features, the highest impact on bird diversity was related to the tree diversity, abundance of older trees, presence of multilayer understory and heterogeneous surrounding. The preliminary study point that traditional orchards could play a important role for wood-dwelling species in agriculture and because of that the removal or replacement of all traditional orchards by intensively managed orchards should be avoided and needs of orchard protection should be implemented into Agri-Environmental Schemes/High Nature Value farming systems and possibly also into habitat directive of EU. Unfortunately, the number of abandoned and extensively managed orchards is declining from agricultural landscapes, and traditional orchards are replaced by conventional fruit plantations.     

Interactions between plant nutrients,water and carbon dioxide as factors limiting crop yields

Biomass production of annual crops is often directly proportional to the amounts of radiation intercepted, water transpired and nutrients taken up. In many places the amount of rainfall during the period of rapid crop growth is less than the potential rate of evaporation, so that depletion of stored soil water is commonplace. The rate of mineralization of nitrogen (N) from organic matter and the processes of nutrient loss are closely related to the availability of soil water. Results from Kenya indicate the rapid changes in nitrate availability following rain.<br>Nutrient supply has a large effect on the quantity of radiation intercepted and hence, biomass production. There is considerable scope for encouraging canopy expansion to conserve water by reducing evaporation from the soil surface in environments where it is frequently rewetted, and where the unsaturated hydraulic conductivity of the soil is sufficient to supply water at the energy limited rate (e.g. northern Syria). In regions with high evaporative demand and coarse-textured soils (e.g. Niger), transpiration may be increased by management techniques that reduce drainage.<br>Increases in atmospheric [CO₂] are likely to have only a small impact on crop yields when allowance is made for the interacting effects of temperature, and water and nutrient supply. <br>     

Urban expansion and its consumption of high-quality farmland in Beijing,China

China faces the challenge of using limited farmland to feed more than 1.3 billion people. Accelerated urbanization has exacerbated this challenge by consuming a large quantity of high-quality farmland (HQF). It is therefore essential to assess the degree to which urban expansion has preferentially consumed HQF, and discern the mechanism behind this. We found urban areas in Beijing to expand at speeds of 48.97 km²/year, 21.89 km²/year, 62.30 km²/year and 20.32 km²/year during the periods 1986–1995, 1995–2000, 2000–2005 and 2005–2020, respectively. We developed an indicator of HQF consumption due to urban expansion, representing the ratio of HQF consumed to its proportion of overall farmland, and found its values were 2.21, 1.57, 1.99 and 1.10 for 1986–1995, 1995–2000, 2000–2005 and 2005–2020, respectively. Thus, although HQF has been overrepresented in the farmland consumed by Beijing's urbanization, this phenomenon has decreased over time. Centralized expansion has contributed greatly to consumption of HQF. Topography and distances to urban and water bodies determine the relative consumption of HQF in urbanization.     

Impact of land use intensity on sandy desertification: An evidence from Horqin Sandy Land,China

It has been widely accepted that high land use intensity contributes to sandy desertification in Northern China. However, it is surprising that it is even not easy to find an explicit definition and proper indicator for the term “land use intensity” in the sandy desertification related literatures. To better understand the association between sandy desertification and its driving factors, especially in the context of the synergistic roles of both climatic and anthropogenic variables, an indicator for land use intensity was developed in this paper for quantifying the relative role of land use output on different kinds of vegetation cover. The results indicated land use intensity increased rapidly from 1980 to 1995, obviously decreased from 1995 to 2000, and increased slowly after 2000 as a result of the continued increase of socioeconomic pressures and the fluctuating changes in vegetative cover.An ARDL model was established and applied to analyze the correlation between the area of bare sand land and land use intensity with the influence of annual precipitation in temporal dimension. The area of bare sand land was significantly affected by land use intensity in the long run. However, in the short run, the area of bare sand land was mainly impacted by the auto-correlation effect of itself in temporal dimension and by the annual precipitation. The improvement of the sand dunes obviously reduced the risk of sandy desertification under the condition of specific land use intensity.Before analyzing the correlation between land use intensity and sandy desertification, specific skills were applied to improve the model, including data interpolation and regression verification, since there would be regression biases resulting from the acquisition and processing of time-series data of different sources. Results showed that the area of bare sand land (S_t)and the land use intensity (LI_t) were time-independent. Lag terms of these two variables were added to improve original model.Changes in socioeconomic pressures and ecological restoration projects had obvious influences on land use intensity. Along with the steady increase of socioecological pressures on land and the changes in vegetative cover resulting from various ecological protection strategies and ecological restoration projects, shifts in land use intensity followed a different trend with the population and the social-economic demands. Nonetheless, the continuous increase of socioeconomic demands will increase land use intensity in the long run, so that the risk of sandy desertification remains high.     

Yield-SAFE: A parameter-sparse, process-based dynamic model for predicting resource capture, growth, and production in agroforestry systems

1. Silvoarable agroforestry (SAF) is the cultivation of trees and arable crops on the same parcel of land. SAF may contribute to modern diversified land use objectives in Europe, such as enhanced biodiversity and productivity, reduced leaching of nitrogen, protection against flooding and erosion, and attractiveness of the landscape. Long-term yield predictions are needed to assess long-term economic profitability of SAF.

2. A model for growth, resource sharing and productivity in agroforestry systems was developed to act as a tool in forecasts of yield, economic optimization of farming enterprises and exploration of policy options for land use in Europe. The model is called Yield-SAFE; from “YIeld Estimator for Long term Design of Silvoarable AgroForestry in Europe”. The model was developed with as few equations and parameters as possible to allow model parameterization under constrained availability of data from long-term experiments.

3. The model consists of seven state equations expressing the temporal dynamics of: (1) tree biomass; (2) tree leaf area; (3) number of shoots per tree; (4) crop biomass; (5) crop leaf area index; (6) heat sum; and (7) soil water content. The main outputs of the model are the growth dynamics and final yields of trees and crops. Daily inputs are temperature, radiation and precipitation. Planting densities, initial biomasses of tree and crop species, and soil parameters must be specified.

4. A parameterization of Yield-SAFE is generated, using published yield tables for tree growth and output from the comprehensive crop simulation model STICS. Analysis of tree and crop growth data from two poplar agroforestry stands in the United Kingdom demonstrates the validity of the modelling concept and calibration philosophy of Yield-SAFE. A sensitivity analysis is presented to elucidate which biological parameters most influence short and long-term productivity and land equivalent ratio.

5. The conceptual model, elaborated in Yield-SAFE, in combination with the outlined procedure for model calibration, offers a valid tool for exploratory land use studies.

How disturbances and management practices affect bird communities in a Carpathian river ecosystem?

We studied how interactions between disturbances, succession, human alterations and other habitat and landscape attributes affect bird community patterns in a lower reach of a large West Carpathian river Váh with complex disturbance and alteration histories. Breeding-bird communities, their habitats (54 variables) and surrounding landscapes (11 metrics) were sampled using standardized point counts with limited distances at 40 riparian sites divided among two transects along a 12.9 km river stretch. The most frequent and abundant birds were generalists typically associated with forest edge habitats, such as Parus major, Sylvia atricapilla, Fringilla coelebs, Oriolus oriolus, Phylloscopus collybita, Sturnus vulgaris, Turdus merula and Luscinia megarhynchos. Abundances show significant increase at the lower transect responding apparently to greater size and heterogeneity of riparian habitats and more abundant food supply linked to more diverse and intense human influences in a suburban zone. Both indirect (NMDS) and direct ordination (CCA) revealed remarkably large number of evenly important factors underlying riparian bird-habitat interactions. It suggests considerable environmental heterogeneity and complexity of these interactions as a likely outcome of long and complex disturbance and alteration histories of the area. Yet structure and relative importance of first two gradients (longitudinal and lateral linkages) remains simple and stable, complying well with predictions of river continuum concept and stream ecosystem theory. Of the nine statistically significant variables most strongly correlated with first two CCA axes, percentages of Helianthus tuberosus, footpaths, fields, Calystegia sepium and steep banks uphold our hypotheses predicting significant effects of invasive species, visitor disturbances, agricultural land use and unaltered river banks/bed on bird community composition and structure. A small but significant contribution of patch size standard deviation within a 150 m radius to the CCA model provides the sole evidence for a hypothesized effect of adjacent terrestrial landscapes on riparian bird communities over different scales of patch size variability. Percentages of fields, invasive species and vertical banks along with average tree diameter were also the most productive predictors across our generalized linear models of riparian bird species diversity, evenness and abundance. Adopting an integrated river basin management, maintaining or restoring both longitudinal and lateral connectivity, taking advantage of natural processes and traditional management practices to maintain or restore riverine biodiversity can be recommended by our data in support of more systematic and evidence-based decision-making.     

Long‐term trends in restoration and associated land treatments in the southwestern United States

Restoration treatments, such as revegetation with seeding or invasive species removal, have been applied on U.S. public lands for decades. Temporal trends in these management actions have not been extensively summarized previously, particularly in the southwestern United States where invasive plant species, drought, and fire have altered dryland ecosystems. We assessed long‐term (1940–2010) trends in restoration using approximately 4,000 vegetation treatments conducted on Bureau of Land Management lands across the southwestern United States. We found that since 1940, the proportions of seeding and vegetation/soil manipulation (e.g. vegetation removal or plowing) treatments have declined, while the proportions of prescribed burn and invasive species treatments have increased. Treatments in pinyon‐juniper and big sagebrush communities declined in comparison to treatments in desert scrub, creosote bush, and riparian woodland communities. Restoration‐focused treatment objectives increased relative to resource extraction objectives. Species richness and proportion of native species used in seeding treatments also increased. Inflation‐adjusted costs per area rose 750% for vegetation/soil manipulation, 600% for seeding, and 400% for prescribed burn treatments in the decades from 1981 to 2010. Seeding treatments were implemented in warmer and drier years when compared to the climate conditions of the entire study period and warmer and wetter years relative to several years before and after the treatment. These results suggest that treatments over a 70‐year period on public lands in the southwestern United States are shifting toward restoration practices that are increasingly large, expensive, and related to fire and invasive species control.     

The unique and endangered Campo Rupestre vegetation and protected areas in the Iron Quadrangle,Minas Gerais,Brazil

The Iron Quadrangle (IQ) in Brazil is one of the world's largest reserves of iron ore, and besides its economic importance, the region also plays a key role in nature conservation with the endemic Campo Rupestre vegetation. Despite a large number of protected areas (PAs) offering refuge for the remaining Campo Rupestre vegetation in the IQ, studies on the current situation of these natural remnants, and the real functionality of these PAs are scarce. This study aimed to map the remaining natural phytophysiognomies of the IQ, evaluating the proportion of Campo Rupestre on ferruginous and non-ferruginous substrates in PAs. We applied the object-oriented classification using Sentinel 2A and 2B satellite images, vegetation indexes, iron oxide index and terrain attributes, with field-collected validation points. We identified thirteen types of land use and cover, of which five represented natural phytophysiognomies. The most common phytophysiognomies in the IQ are Seasonal Forests and Cerrado (all types included), accounting for more than 60% of the total area. Campo Rupestre on ferruginous and non-ferruginous substrates accounts for 5.4% of the total mapped area only. On the other hand, about 42.5% of the Campo Rupestre (ferruginous or not) are protected and correspond to 17.7% of the PAs. We show that an appropriate mapping of the phytophysiognomies in the IQ region is key for improving landscape ecology knowledge and future conservation planning, especially of the endangered Campo Rupestre vegetation.     

Hoofbeats From the Currituck Outer Banks: A Study of the Corolla Wild Horse Fund Adoption Program

Research on the adoption and relinquishment of horses, both domestic and wild, remains limited. As a result, little is known about adopters, their adopted horses, and their adoption experience. This study surveyed and interviewed 17 adopters of Colonial Spanish mustangs through the Corolla Wild Horse Fund (CWHF). Together, they adopted 22 horses of varying ages and genders from 2002 to 2012. The participants, who had a range of previous experience with and knowledge of horses, were generally very satisfied with their horses and their adoption experience. Being able to adopt a gentled/trained mustang and receiving support from the CWHF during the adoption process played key roles in adoption success. Additionally, participants’ strong desire to preserve a perceived endangered species or national treasure appeared to be a major reason for adopting a Colonial Spanish mustang and served as motivation for making the adoption successful. The results of the study provide insights into ways to improve the number and success of adoptions through other equine programs, especially the Bureau of Land Management's wild horse and burro program.     

Fine sediment trapping in two mangrove-fringed estuaries exposed to contrasting land-use intensity,Palau, Micronesia

A comparative study was undertaken of the fate of fine sediment in the Ngerikiil and Ngerdorch mangrove-fringed estuaries in Babeldaob Island, Palau, Micronesia, in 2002. The mangroves comprised 3.8% of each catchment area, and in both systems, they trapped about 30% of the riverine sediment. Mangroves are important buffers protecting fringing coral reefs from excessive sedimentation. The sediment yield was significantly higher in the Ngerikiil River catchment (150 tons km^–2 yr^–1) that has been extensively cleared and farmed, than in Ngerdorch River catchment (1.9 tons km^–2 yr^–1) that was still relatively pristine during the study period.This revised version wa published online in March 2005 with corrections to the issue cover date.     

Patterns of plant species richness in pasture lands of the northeast United States

Pasture lands are an important facet of land use in the northeast United States, yet little is known about their recent diversity. To answer some fundamental questions about the diversity of these pasture lands, we designed a broad survey to document plant species richness using an intensive, multi scale sampling method. We also wanted to learn whether environmental (soils or climate) or land management variables could help explain patterns of species richness. A total of 17 farms, encompassing 37 pastures, were sampled in New York, Pennsylvania, Vermont, Maryland, Massachusetts and Connecticut during July and August 1998. We positively identified a total of 161 different plant species across the study region. Species richness averaged 31.7±1.1 on pastures. Infrequent, transient species that were mostly perennial and annual forbs accounted for 90% of the species richness. Except for a subjective rating of grazing intensity, land management methods were not good predictors of species richness. Over time, it appears that grazing neither reduces nor increases species richness in pastures. Of the environmental variables measured, only soil P explained a significant amount of the variation in species richness. Soil P was inversely related to species richness at the 1m² scale. Percent SOM was positively associated with species richness at this scale, although weakly. At larger spatial scales, we suggest that patterns of species richness are best explained by the species diversity of soil seed banks, or seed rain, and stochastic recruitment of these species into existing vegetation.     

Identification of hotspots of at-risk terrestrial vertebrate species in the south-central Great Plains of North America: A tool to inform and address regional-scale conservation

Biodiversity is being lost at an unprecedented rate, and resources for conservation efforts are limited. This is particularly problematic in the Great Plains of North America, where land-cover conversion for agriculture and energy production has reduced habitat for many species. In the U.S. portion of the Great Plains, a growing human population and a concomitant increasing need for food, fiber, and energy have caused landscape transformations that have resulted in over 700 vertebrate species currently being listed by state and federal conservation agencies as being at-risk in this region. Conservation efforts for such a large number of species will be most efficient when applied to areas with large numbers of these species, but such areas have never before been identified. We overlaid range maps created by the U.S. Geological Survey’s Gap Analysis Program for terrestrial vertebrate species to identify hotspots of high concentrations of U.S. state-defined Species of Greatest Conservation Need (SGCN; species that have identified as being rare or otherwise vulnerable enough to warrant conservation action in a given state) in the short- and mixed-grass prairie ecoregions of the southern and central Great Plains of the United States. We identified hotspots for species currently listed as SGCN as well as those pending designation, and a combined (current and pending) group. We then used data from the U.S. Geological Survey’s Protected Areas Database on land ownership and from the U.S. Department of Agriculture’s National Agricultural Statistics Service on land use/land cover to quantify the types of land ownership and land use/land cover types in hotspots to give land managers necessary information to address conservation of at-risk species in the Great Plains. Sufficient data were present for examination of 289 at-risk terrestrial vertebrate species. Hotspots of these species were located mostly on state- or federally-managed land in eastern New Mexico, Colorado, and west Texas. The current hottest hotspots were associated with areas with more natural/less anthropogenic forms of land use/land cover; areas with the lowest numbers of SGCNs had proportionately more cropland and less grassland than did hotspots. Identifying regional hotspots of at-risk biodiversity, and describing land use/land cover features associated with such areas, offers an opportunity to take a multi-species approach in more precisely establishing areas of conservation concern in the U.S.     

Grazing effects on the species‐area relationship: Variation along a climatic gradient in NE Spain

Questions: Does grazing have the same effect on plant species richness at different spatial scales? Does the effect of spatial scale vary under different climatic conditions and vegetation types? Does the slope of the species‐area curve change with grazing intensity similarly under different climatic conditions and vegetation types? Location: Pastures along a climatic gradient in northeastern Spain. Methods: In zones under different regimes of sheep grazing (high‐, low‐pressure, abandonment), plant species richness was measured in different plot sizes (from 0.01 to 100 m2) and the slope of the species‐area curves was calculated. The study was replicated in five different locations along a climatic gradient from lowland semi‐arid rangelands to upland moist grasslands. Results: Species richness tended to increase with grazing intensity at all spatial scales in the moist upland locations. On the contrary, in the most arid locations, richness tended to decrease, or remain unchanged, with grazing due to increased bare soil. Grazing differentially affected the slope (z) of the species‐area curve (power function S=c A^z) in different climatic conditions: z tended to increase with grazing in arid areas and decrease in moist‐upland ones. ß‐diversity followed similar pattern as z. Conclusions: Results confirm that the impact of grazing on plant species richness are spatial‐scale dependent. However, the effects on the species‐area relationship vary under different climatic conditions. This offers a novel insight on the patterns behind the different effects of grazing on diversity in moist vs. arid conditions reported in the literature. It is argued that the effect of spatial scale varies because of the different interaction between grazing and the intrinsic spatial structure of the vegetation. Variations in species‐area curves with grazing along moisture gradients suggest also a different balance of spatial components of diversity (i.e. a‐ and ß‐diversity).     

Nutrients,Conductivity and Plankton in a Landscape Approach to a Pampean Saline Lowland River (Salado River,Argentina)

Nutrients, conductivity and other physical and chemical parameters were measured seasonally in the main channel and in tributaries of the Salado River basin during the period March 1997–June 1999. The sampling began in a low water period and later included flood events. High water events were associated with high proportions of allochthonous compounds (polyphenols, suspended solids and organic matter). Nutrients and conductivity were related to hydrological conditions and different land uses in the catchment. A relationship was found between the land-use and nutrient concentrations due to the inflow of diffuse sources from agricultural lands. High nutrient concentrations and conductivity recorded in the headwaters were, respectively, related to the intensive agriculture in this area and the inflow of saline groundwater, drained by canals from endorheic basin to the main channel. Their effects on the middle and lower reaches were related to the discharge and inputs of other sub-catchments. Nutrients, sulphates, alkalinity and conductivity declined downstream towards the river mouth. The consequences for the plankton community of these spatio-temporal changes in the chemical characteristics are discussed. The heterogeneity of the Salado River is related to seasonality, land use and the geomorphological features of the basin.     

江西新建县土地利用时空动态特征分析

在RS与GIS支持下,以江西省新建县为研究区,采用与土地利用动态度相关的一系列时空特征指数,研究了1991、1996和2000年3个时间段上土地利用的时空动态特征.结果表明,单一土地利用动态中建设用地变化动态度最大;1991～2000年间研究区域综合土地利用动态度年均变化速度为0.165%,其中前5年变化速度比后4年快,为后者1.5倍多.1991、1996和2000年土地利用程度综合指数分别为243.68、244.08和244.87,稍高于全国平均水平23192.新建县1996～2000年各种土地利用类型总的空间转移量(转出或转入)比前一时段有所下降,土地利用变化逐渐由双向转移向单向不平衡转移转化.     

Introduction and conclusions for a Discussion entitled Land resources: on the edge of the Malthusian precipice?

Introduction and conclusions for a Discussion hosted by the Ciba Foundation and entitled Land resources: on the edge of the Malthusian precipice? Discussion held on the 4th and 5th December 1996, and organized and edited by D. J. Greenland, P. J. Gregory and P. H. Nye. <br>     

The economic determinants of land degradation in developing countries

The following paper investigates the economic determinants of land degradation in developing countries. The main trends examined are rural households'' decisions to degrade as opposed to conserve land resources, and the expansion of frontier agricultural activity that contributes to forest and marginal land conversion. These two phenomena appear often to be linked. In many developing areas, a poor rural household''s decision whether to undertake long-term investment in improving existing agricultural land must be weighed against the decision to abandon this land and migrate to environmentally fragile areas. Economic factors play a critical role in determining these relationships. Poverty, imperfect capital markets and insecure land tenure may reinforce the tendency towards short-term time horizons in production decisions, and may bias land use decisions against long-term land management strategies. In periods of commodity booms and land speculation, wealthier households generally take advantage of their superior political and market power to ensure initial access to better quality resources, in order to capture a larger share of the resource rents. Poorer households are confined either to marginal environmental areas where resource rents are limited, or only have access to resources once they are degraded and rents dissipated.<br>Overall trends in land degradation and deforestation are examined, followed by an overview of rural households'' resource management decisions with respect to land management, frontier agricultural expansion, and migration from existing agricultural land to frontiers. Finally, the discussion focuses on the scope for policy improvements to reduce economic constraints to effective land management. <br>