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Few empirical data exist to examine the influence of regional scale environmental gradients on productivity patterns of plant species. In this paper we analyzed the productivity of several dominant grass species along two climatic gradients, mean annual precipitation (MAP) and mean annual temperature (MAT), in the Great Plains of the United States. We used climatic data from 296 weather stations, species production data from Natural Resource Conservation Service rangeland surveys and a geographic information system to spatially integrate the data. Both MAP and MAT were significantly related to annual above-ground net primary production (ANPP). MAP explained 54 % to 89 % of the variation in ANPP of two C4 short-grasses, Bouteloua gracilis and Buchloë dactyloides, and two C4 tall-grasses, Andropogon gerardii and Schizachyrium scoparium (= Andropogon scoparius). MAT explained 19 % to 41 % of the variation in ANPP of two C4 grasses, B. gracilis and B. dactyloides, and 41 % to 66 % of the variation in ANPP of two C3 grasses, Agropyron smithii and Stipa comata. ANPP patterns for species along both gradients were described by either linear, negative exponential, logistic, normal or skewed curves. Patterns of absolute ANPP (g/m2) for species differed from those of relative ANPP (%) along the MAP gradient. Responses were similar for species with common functional characteristics (e.g. short-grasses, tall-grasses, C3, C4). Our empirical results support asymmetric responses of species to environmental gradients. Results demonstrate the importance of species attributes, type of environmental gradient and measure of species importance (relative or absolute productivity) in evaluating ecological response patterns.

This book is based on a workshop held in Zimbabwe, May 1999, organized by the Department of Research and Specialist Services (Zimbabwe) and the International Board for Soil Research and Management (IBSRAM). Reviewing the current state of knowledge on and the practical aspects of the management of Vertisols in Africa, this book also includes comparative chapters covering other parts of the world, such as India, Australia and Texas (USA).

The concept of magnitude and frequency in geomorphology maintains that the amount of work done on a landscape (the amount of sediment moved) by geomorphic events is determined by the product of the magnitude of the event and the frequency with which that sized event occurs. For many geomorphic systems, it has been found that events of moderate size produce the most sediment transport. This seems not to be the case for aeolian processes on the Southern High Plains. Using meteorological records on visibility reductions caused by blowing dust-a record of the relative amount of dust transported by the wind-at Lubbock, Texas on the Southern High Plains for the period 1947-1989, we find that small magnitude and high frequency events move the most material; that rare, large magnitude events account for considerable transport as well; and, contrary to findings for many other geomorphic systems, moderate events move relatively little sediment. Magnitude and frequency analysis conventionally assumes an identifiable relationship between force applied and the amount of sediment moved. On the Southern High Plains, the soil's resistance to erosion and the vegetation's modification of wind energy are both highly variable in time and space making it unlikely that the sediment transported by two winds of the same speed will be identical. In cases such as these where the resistance to erosion and the effectiveness of the force vary in space and time, magnitude and frequency relations cannot be easily predicted.

Pest risk maps are an important source of decision support when devising strategies to minimize introductions of invasive organisms and mitigate their impacts. When possible management responses to an invader include costly or socially sensitive activities, decision makers tend to follow a more certain (i.e. risk-averse) course of action. We present a new mapping technique that assesses pest invasion risk from the perspective of a risk-averse decision maker. We demonstrate the approach by evaluating the likelihood that an invasive forest pest will be transported to one of the continental US states or Canadian provinces in infested firewood that may be carried by visitors to US federal campgrounds. We test the impact of the risk aversion assumption using distributions of plausible pest arrival scenarios generated with a geographically explicit model developed from data documenting camper travel across the study area. Next, we prioritize regions of high and low pest arrival risk via application of two stochastic ordering techniques that employ, respectively, first- and second-degree stochastic dominance rules, the latter of which incorporates the notion of risk aversion. We then identify regions in the study area where incorporating risk aversion changes a region's pest risk value considerably. While both methods identified similar areas of highest and lowest risk, they differed in how they demarcated moderate-risk areas. Each method provides a tractable way to incorporate decision-making preferences into final risk estimates, and thus helps to better align these estimates with particular decision-making scenarios about an organism of concern. Overall, incorporation of risk aversion helps to refine the set of locations that could be confidently targeted for costly inspections and outreach activities.

This paper focuses on the development of the OKANOLA project to provide research, education and demonstration to stimulate the development of winter rape as a major profitable rotational crop with winter wheat in the Oklahoma and Kansas, USA. Information on the major insect pests of rape and winter wheat and the challenges in their management are also presented.

This chapter presents findings from a study of 31 urban agriculture (UA) sites in Houston (Texas, USA), which examined the site objective, plot access regime, land tenure, cultivation practices, labour, capital, and harvest destination. Results show, among others, that the all-access regime is considerably more prevalent than allotments; churches are desirable locations for UA; and obtaining a consistent workforce is the most commonly cited challenge for UA organizers.

This chapter questions the use of sustainability discourses in urban agriculture projects, with a comparative study of Austin, Texas and La Habana, Cuba. Faced with a food security crisis in the 1990s, Cuba turned to a diversified and mostly organic agricultural system in order to feed its habitants. Austin also has a well-established community garden programme and a lot of positive press for its sustainable agriculture programmes. The case studies may seem different: the Austin case revolves around community resentment at the establishment and practices of urban farms, while the work in Cuba focuses on the inability of government-led sustainable production to meet the needs of many low-income communities. However, some comparable dynamics can be found. In both Austin and La Habana, the ways in which agricultural practices are deemed sustainable in the cultural and political discourses is predicated upon the ways in which food is produced, with less regard to continuing issues of food insecurity that urban agriculture has yet to redress. Still less attention is paid to the cultural politics of food: the ways in which race, class and gender can shape access to food choices, as well as have the capacity to shape political discourse around food. What this study finds in Austin and La Habana are similar sets of agricultural practices, as well as similar discourses towards their value for the community and the nation. The study also finds comparable outcomes in terms of the ways in which food can expose spatial and social divisions.

This chapter focuses on the breeding of low-chill peach cultivars adapted to the subtropics, although they may be grown in temperate environments (in the absence of spring freezes that kill the bloom) or in tropical highlands. Topics covered are: reasons for breeding low-chill cultivars; history of low-chill cultivar development; current low-chill breeding programmes (New South Wales and Queensland, Australia; São Paulo, Brazil; Mexico; South Africa; Taiwan; Thailand; and California, Florida and Texas, USA); common objectives in low-chill peach breeding (low chilling requirement, time of flowering, flower bud density, blind nodes and bud drop, fruit development period, fruit shape, fruit firmness and disease resistance); and management of low-chill breeding populations (hybrid seed production, seedling populations, and testing advanced selections).

This chapter describes a multi-farm regional modelling approach and its characteristics, and discusses why one might want to use such an approach. Regional-level modelling is useful for investigating broader issues about the regional implications for new technology or agricultural policy when there are market and resource interactions between the farms. Aggregation procedures for forming the representative farms are illustrated. Two case studies in the Edward and Ogallala Aquifer regions of Texas are included to illustrate applications of such regional models. In these two areas, water issues and resource substitution possibilities create the need to examine interactions among farms and therefore use a regional model.

This paper describes a range of state-level issues and responses to groundwater law. It begins with a description of groundwater law, both for the USA in general and for the three states (Kansas, Nebraska and Texas) in particular. It then follows with an account of groundwater allocation law problems faced by Kansas due to groundwater mining. The next section addresses groundwater issues and innovations in each state: (1) in Kansas, a water reuse project and an aquifer storage and recovery project; (2) in Nebraska, two types of interstate conflicts, one dealing with antiexportation statutes and the other dealing with allocation of an interstate river and the surface-groundwater interaction with that river; and (3) in Texas, questions about the continued efficacy of its Rule of Capture groundwater doctrine and about the advisability of moving groundwater long distances within the state.