Vegetation Patterns, Changes, and Interactions with Wind Erosion in an Arid Grassland Ecosystem

Arid grassland degradation involves shrub expansion and declines in grasses, and negatively affects these ecosystems; however, the conversion process is not well understood. This research conducted in an arid grassland included four objectives: 1) to examine spatial distributions of grasses and shrubs; 2) to examine relationships between vegetation cover and wind dynamics; 3) to examine effects of wind erosion on grass and shrub photosynthesis; and 4) to examine effects of wind erosion on vegetation changes. All grass and shrub locations and dimensions were recorded on eighteen 10x20m vegetation distribution plots, and patterns were examined using spatial statistics. Three treatment blocks were established, each with four 25x50m grass-cover reduction plots (100%, 75%, 50%, and 25%) and a control plot. Meteorological towers were used to record wind speeds and directions on plots, and wind profile data were used to calculate shear stress ratios (SSRs), indicating momentum absorbed by vegetation. Vegetation measures of lateral cover and size of gaps between plants were recorded on each plot. Shrub photosynthesis rates were monitored on 100 0rass-reduction areas (T-100%), and grass and shrub photosynthesis rates were monitored immediately downwind of T-100% and controls. Photosynthesis rates for grasses buried to depths of 5 or 10cm were monitored. Grass and shrub fractional cover was also recorded downwind of T-100% and controls over time. Spatial patterns analyses showed clumped grass distributions, random shrub distributions, and grasses distributed evenly with respect to shrubs (indicating competition). Gap size was a slightly better predictor of SSRs than lateral cover, and a significant, positive logarithmic relationship between gap size and SSRs was observed. II Removal of grasses led to greater shrub photosynthesis rates on and downwind of T- 100% areas, while grass photosynthesis rates were not affected by increased wind erosion or burial. Grass cover decreased and shrub cover increased on plots downwind of T- 100%; while grass cover increased and shrub cover decreased on controls. Significant logarithmic relationships between vegetation changes and dust flux from upwind areas were observed. These results indicate increased wind erosion following disturbances provides shrubs with a competitive advantage over grasses and is a mechanism for shrub encroachment in arid grasslands.

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