Abstract

Shrublands in semi-arid regions are heterogeneous landscapes consisting of infertile bare areas separated by nutrient rich vegetated areas known as resource islands. Spatial patterns in these landscapes are structured by feedbacks driven by the transport of water and nutrient resources from the intershrub space to areas below shrubs, and the retention of these resources to locally drive productivity and tight biogeochemical cycles. Most understanding of plant-soil feedbacks is based predominantly on studies of low topographic gradient landscapes, and it is unclear whether the patterns of association between soils and vegetation, and the autogenic processes that create them, also occur on more steeply sloping terrain. Here we analyze the spatial patterns of soils, vegetation and micro-topography on hillslopes of contrasting lithology (one granite at 16^∘, one schist at 27^∘) in the Sonoran desert foothills of the Catalina Mountains. We also describe a method of extracting vegetation density from terrestrial laser scanning point-cloud data at 5 cm × 5 cm scales, and find that it correlates well with soil organic carbon measurements. Vegetation was associated with microtopographic mounds (relative to the mean slope) extending 0.3 m downslope and 1.8 m (schist) and 0.9 m (granite) upslope on the study hillslopes. Soils below the shrub canopies exhibited 2-3 times more soil organic matter and 2-4 times higher hydraulic conductivity than the interspaces. Soils enriched with organic matter were found to extend at least two canopy radii downslope of woody shrubs, but not upslope. These plumes were clearest in the lower-gradient granite site where vegetation mounds created distinct patterns of microtopographic convergence and divergence. At the steeper schist site microtopography appeared to have a weaker control on topographic flow accumulation. Collectively our findings suggest that the spatial structure of association between soils and microtopography and vegetation on these slopes exhibit many of the features observed in lower-gradient areas. However microtopography and soils are more asymmetric along the downslope axis of the hillslopes than lower-gradient areas, and vary with lithology. Alluvial and colluvial processes are likely more important in shaping vegetation and soil dynamics on hillslopes, and these factors need further consideration in scaling results to the landscape level.

Citation

Harman C.J., Lohse K.A., Troch P.A., Sivapalan M. (2014): Spatial patterns of vegetation, soils and microtopography from terrestrial laser scanning on two semi-arid hillslopes of contrasting lithology. Journal of Geophysical Research: Biogeosciences 119(2): 163-180. DOI: 10.1002/2013JG002507

This Paper/Book acknowledges NSF CZO grant support.