Figure 1. Percent total soil nitrogen concentration beneath an average-size mesquite (Prosopis juliflora) shrub. Santa Rita Experimental Range, Arizona, as a function of canopy distance and depth (Klemmedson and Barth, 1975).

Figure 2. Some known or suspected interactions among common components, processes, or phenomena characteristic of mesquite grasslands of the Tamaulipan biotic province. Lines with arrowheads indicate a positive influence or increase stimulated by a two-way interaction in the direction of the arrow. Lines with balls indicate a decrease, inhibition, or negative influence in the direction of the ball. The diagram is by no means complete or accurate. It is intended only to convey the potential complexity and our lack of understanding of the essential interactions. The key question is how to quantify the relationship between arrow "A", mesquite's direct impact on the area of open grassland available and the degree to which mesquite contributes (indirectly) to arrow "B", the input of mineralized nitrogen to open grassland. This relationship determines and defines the role of mesquite in a particular system during a particular time period.