Spatial patterning of resource accumulation in a 22 year-old water harvesting project in the Chihuahuan Desert

In 1975, catchment dikes were constructed on an area of degraded rangeland within the Jornada Experimental Range, southern New Mexico. The project was abandoned in 1979 after several intensive amendments including application of biosolids and seeding treatments failed to sustain vegetation. In 1997, this project was revisited when an aerial photograph revealed an established vegetation community. The objective of this study was quantify detailed spatial patterns of resources within this 22-year old rehabilitation experiment in order to gain insight into the past and present biotic and abiotic mechanisms at work in the autogenic rehabilitation of this site. The main hypothesis was that the pattern of resource accumulation was a direct result of distance from the catchment dike structures. A conservatively estimated 32% of biosolid aggregates >2mm in diameter remained in the soil 18 years after initial application. The absence of any clear association between patterns of established vegetation and amounts of recovered biosolid suggests this amendment had little effect. Significant differences in basal vegetation cover by meter distance from dikes were observed; although the pattern was different than hypothesized. Increases in basal vegetation cover were not incremental with increasing meter distances from dikes. Based on vegetation measurements, current plant species composition is the result of natural successional processes. Current vegetation patterns appear to result from the same processes that govern the assembly of naturally occurring, banded vegetation patterns observed in many arid and semi-arid environments. Spatial patterns of soil properties, as well spatial distribution of the soil seed bank, mirror vegetation patterns. Self-generating processes were triggered by the construction of the catchment dikes along topographic contours perpendicular to sheet-flow of run-off. This changed the dynamics of the site from to one that continually dispersed diffuse resources, to one that facilitated resource accumulation. Once a critical amount of soil nutrients and seeds accumulated from obstruction to wind and water movement over the soil surface, accumulated seeds could germinate and establish. As more plants survived the harsh site conditions; positive feedbacks between vegetation and soil resources resulted in the successful rehabilitation of this seriously degraded area.