The effect of wind and precipitation on vegetation and biogenic crust covers in the Sde-Hallamish sand dunes

Vegetation and biogenic crust covers play an important role in sand dune stabilization, yet there is a lack of high temporal and spatial resolution data on sand dune cover. A field experiment, aimed at measuring the dynamics of biogenic crust and vegetation in sand dunes, was conducted at the Sde-Hallamish sand dunes in the northwestern Negev Desert, Israel, from July 2008 to August 2010. The climate of the Sde-Hallamish sand dunes is arid (the mean annual precipitation over the past 13 years is 61 mm), and the dunes are linear and partially stable, mainly due to the presence of biogenic crust and partially due to the presence of vegetation. In July 2008, 10×10 m plots on the four dune habitats (crest, interdune, north slope, and south slope) were treated as follows: (i) removal of vegetation and biogenic crust, (ii) removal of biogenic crust only, (iii) removal of vegetation only, (iv) partial removal of biogenic crust and vegetation, and (v) control plot. The surface coverage of sand, biogenic crust, and vegetation was monitored on a monthly basis, using a remote-sensing technique especially developed for the Sde-Hallamish sand dunes. It was found that strong wind events, with durations of several days, accounted for the coverage changes in biogenic crust and vegetation. The response to precipitation was much slower. In addition, no rehabilitation of biogenic crust and vegetation was observed within the experiment time period. The changes in biogenic crust cover were not necessarily related to changes in dune dynamics, since often an increase in biogenic crust cover is a result of wind erosion that exposes old crust that was buried under the sand; wind hardly erodes biogenic crust at all due to its high durability to wind action. The Sde-Hallamish dunes seem to have become more active as a result of a prolonged drought during the past several years. The field experiment reported here indicates that biogenic crust cover exhibits large seasonal variations that are not necessarily related to the growth of new crust but rather to the exposure of old buried crust.