Spatial patterns of soil moisture as affected by shrubs, in different climatic conditions.

Abstract

Abstract. At three study sites, representing Mediterranean, semi-arid and mildly-arid climatic conditions, the effect of shrubs on the spatial patterns of soil moisture was studied. At each site soil moisture was measured, on hillslopes, at the vicinity of 8 shrubs. For each shrub the measurements have been taken at 3 microenvironments, i.e. under the shrub (US), at the margins of shrub (MS) and between shrubs (BS). At the microenvironments US and MS the measurements were taken at 3 directions: upslope, downslope and sideslope of the shrubs. At all sampling points soil samples were taken from 3 depths: 0-2, 2-5 and 5-10 cm. In addition, rock fragments cover percentage near the shrubs was determined. A soil moisture pattern was found, around each shrub, which is composed of a radial gradient and a downslope gradient. The radial gradient is expressed by soil moisture decreasing from the US microenvironment, in all directions, through the MS towards the BS microenvironment. The US microenvironment has a 'spatial advantage' of higher soil moisture content due to (1) relatively higher infiltration rate, (2) capture overland flow from the BS area upslope that shrub and (3) low evaporation rate because of the shading effect. The downslope gradient is expressed by decreasing soil moisture from the upslope direction of each shrub (MS and US microenvironments) towards the downslope direction of that shrub (MS and US microenvironments, respectively). This gradient is controlled by the relatively high content of rock fragments near the shrubs at their upslope direction. Such rock fragments spatial distribution is attributed to (1) the detachment and transport of rock fragments by sheep and goats trampling and (2) the effect of shrub on the continuity of overland flow and sediment transport. The effect of rock fragments is similar to that of shrubs regarding increasing infiltration and decreasing evaporation rate. The relatively high soil moisture at the upslope direction of each shrub enhances annuals growth producing a positive feedback loop: soil moisture--annuals growth--trampling. This sequence maintains the typical rock fragments spatial organization and contributes to the sustainability of the grazing system. At all the study sites at the US microenvironment there is a trend of decreasing soil moisture with increasing soil depth. At microenvironments MS and BS soil moisture increases with soil depth. The results are of great relevance for rehabilitation strategies as they suggest that in order to combat desertification in degraded semi-arid and mildly-arid areas, where the main land use is grazing, both shrubs and rock fragment should be kept at their present spatial distribution.