Temperate and tropical rangelands are the most extensive land type on earth. These lands are biologically diverse and support the livelihoods of millions of households. In most rangelands, there is not sufficient precipitation for agriculture, and so families use livestock to essentially turn sunlight into food. Historically, the inhabitants of these areas had to contend with droughts, fires, livestock raids, and other stressors and shocks. Animals evolved physical and behavioral adaptations such as high heat tolerance and migration, and people adopted behaviors such as transhumant movement and developed cultural norms that minimized exposure to stresses. Flexibility came to be a defining feature of pastoral communities, and their adaptive capacity was great.

During the 20th century, adaptive capacity in semi-arid and arid systems began to erode. Changes in land use and fragmentation limited movements by wild and domestic ungulates and reduced the forage they could acquire, human-wildlife conflicts increased, and wildlife consumption expanded. Land use intensified through expanded agriculture, increased livestock stocking, and changes in water supplies. Land tenure changes, such as colonial domination, subdivision, the creation of parks, and privatization reduced transhumance. Government policies forced changes that induced sedentarization, which reduced options available to pastoralists to adapt to environmental change. Human population growth severely limited the capacity of pastoralists to adapt, and some pastoralists themselves reduced their adaptive capacity, for example, by settling permanently to be near schools and services.

The 21st century will bring unprecedented climate change to semi-arid and arid systems (at least within human history). Our use of fossil fuels is releasing the carbon long-stored in oil and gas into the atmosphere. Higher carbon concentration (and concentrations of other gases) is increasing the earth's greenhouse effect. In general, scientists have projected that in the 21st century temperatures will increase, precipitation will increase in some places and decrease in others, the variation in precipitation will increase between years, and the frequency of climatic shocks such as drought will increase.

These stressors will reduce further the adaptive capacity of coupled semi-arid and arid systems. One means of increasing the likelihood that households, enterprises, and land managers will be able to adapt to future changes is if future conditions can be anticipated. A quantitative method of anticipating future conditions is through using simulations, where inputs into simulations are modified as predicted in change scenarios. We sought a means of simulating future changes in rangelands across the globe. The simulation needed to be responsive to changes in precipitation and temperature through time. We developed a process-based simulation model that is spatially explicit and of moderate complexity. The model allows global analyses of native rangelands and the changes they may undergo. The tool we developed is a global rangeland model called G-Range.

G-Range Design Goals and Concepts
Several goals and constraints guided the design of G-Range. These include that we sought:

  • A simulation tool for global rangelands that captures main primary production, and its dynamics;
  • A tool of moderate complexity, one that could be useful to a new user in a week or less;
  • A structure that includes simulating changes in all rangelands across the globe within a single executed process;
  • A monthly time-step in simulations; 3
  • Representation of global vegetation at least at the scale of herbaceous, shrubs, and trees;
  • The ability for the proportions of those different kinds of plant types to change over time;
  • Simulations that may span from about 5 to 100 or more years;
  • The ability to include natural or management modifications to rangelands, such as through fire or fertilization;
  • Programming structures that will allow the software to be parallelized for use on multiprocessor clusters or networks, although making the software run parallel was not part of this effort;
  • Greater concern for clarity in logic and ease of use than in conserving hard drive space or algorithmic elegance;
  • Portability in the G-Range code, allowing simulations to be done on a variety of platforms (e.g., Windows, Linux cluster);
  • A graphical user interface (GUI) that is weakly linked to G-Range. G-Range should be able to be run in batch model, without input from a user, and related to that;
  • Parameter files used, rather than inputs from a GUI, allowing simulations to be made without using the interface;
  • Output should be straightforward spatial surfaces, without complex summary analyses. Those post-simulation analyses may more readily be done in other packages based on output from G-Range.

A Strong Foundation
G-Range was not intended to be programmed 'from scratch.' Numerous grassland and rangeland simulation models have been developed. We explored a variety of models, to different degrees. Some models are quite complex, making global application unwieldy. Other models are too simple, not allowing for scenario analyses of the types we intend. Some simulation models are pointbased, inappropriate for a spatially explicit global simulation model. Some models use simple rules to infer changes, which can be unrealistic. More practically, some were judged out-of-date based on their Web sites. Others appeared 'closed source' rather than 'open source' packages, such that the suitability of the software for our use would be difficult to judge without requests to the authors. Some were commercial products and were excluded from consideration.

We selected Century to serve as the foundation for the soil modeling and physiological aspects of the G-Range model. There were both scientific and practical reasons for this. Some of the main uses of G-Range are related to climate change, and the potential for rangelands to sequester carbon. Century is the model most commonly used to explore questions about soil organic carbon in agroecosystems around the world, and is used very often in rangelands as well. Century has been in development for more than 20 years, and has been assessed by comparing its results to observed data many timesMore practically, Century was begun and continues to be developed at our home laboratory, the Natural Resource Ecology Laboratory at Colorado State University. Lastly, the authors of Century were supporting of our efforts to use it as a foundation for G-Range. Aspects of G-Range were influenced by SAVANNA as well, which is authored by Dr. Michael Coughenour, also of our laboratory. Lastly, plant population modeling and some other aspects of G-Range are new contributions.