The growth and yield of trees, as of other plants, in arid areas depend on several limiting factors including low and irregular rainfall with frequent drought periods, high temperature and intense radiation, and poor soil conditions, including low moisture-holding capacity, high infiltration rate, and low content of organic carbon. The bioclimatic aridity of a site depends on the relative amounts of water gained from rainfall and lost by evaporation and transpiration. Various ratios of these climatic measurements have been used, but they will not be reviewed here. Temperature and its annual fluctuations were recognized in subdivisions, and rainfall regimes were represented by colours. Extensive animal breeding occurs, and rainfed agriculture is increasing, although irrigated agriculture may be considered more logical.
Vegetation is taller and denser with more trees than in the arid zone.
For the purpose of this report an exact definition is unnecessary because tree-planting techniques and constraints do not change abruptly. However, most of the examples are taken from Gujarat, where tree planting is actively pursued. Although there are considerable variations in climate, soil, topography, and demography within these areas, many of the obstacles to tree planting are common within a country. All components include a strong human dimension, and it is important to emphasize the potential contribution that local people can make. Development projects are intended to be for people, but they should also be designed to operate with people.