The effect of shrub canopy upon surface temperatures and evaporation in the Negev Desert

Abstract

Although known as 'islands of fertility' or 'resource islands', information regarding the effect of shrubs upon microclimate in deserts is scarce. Here we report on measurements of evaporation and temperatures that were carried out in and around a pair of shrubs at the Nizzana research site in the western Negev Desert during 1993-94 and during the growing season (NovemberMarch) of 1994-95 and 1996 -1997. Whereas evaporation was measured monthly using mini-atmometers (10 cm diameter and 10 cm tall) at an exposed site and under and around the shrub (at the eastern, southern, western and northern aspects), temperature was measured under a shrub canopy, at its northern aspect, and at an exposed habitat. Evaporation was aspect dependent with increasing rates in the following order: exposed > south-facing > west-facing ≈ east-facing > north-facing > under canopy. Except from the northern aspect, the under-canopy habitat showed substantially lower rates of evaporation in comparison with all other habitats. The differences between the under-canopy and the exposed habitat were larger during wintertime (with the under-canopy habitat having 0·53 times the evaporation rate than that of the exposed habitat) although higher differences in temperatures characterized both habitats in summertime (up to 14·4°C in summer as compared with 6·9°C only in winter). The results were explained by extended surface wetness that characterized the under-canopy habitat following rainstorms. While already being dried out at the exposed habitat, surface wetness at the under canopy habitat persisted for several days afterwards, resulting, following one rainstorm, in vapour pressure of 2·15-2·39 kPa in comparison with only 0·82-0·83 kPa of the exposed habitat. The substantially lower evaporation rates that characterize the under-canopy habitat may thus play a pivotal role in providing preferential conditions for lush under-canopy annual growth.

The use of microbiotic crusts as biomarkers for ponding, subsurface flow and soil moisture content and duration

“…Indeed, whereas the northern aspect of the shrub and especially the under-canopy habitat provided efficient shading, no shading was provided by the southern aspect while only partial shading, during low-evaporation hours, was provided by the western (during the morning hours) and eastern (during the afternoon hours) aspects. As expected, the lower evaporation rates monitored at the under-canopy habitat (being 0.53 that of a sun-exposed habitat; see Kidron, 2009) resulted also in higher and longer moisture content at the upper 0-40 cm soil layer of SH-UC (Kidron, 2010).…”

“…These findings were in agreement with evaporation data. When the evaporation at the different habitats around the shrub was monitored, substantially lower evaporation rates were recorded at the under-canopy and the northern aspect, while the remaining aspects either were not (at the southern aspect) or were only slightly (at the western and eastern aspects) affected (Kidron, 2009). Indeed, whereas the northern aspect of the shrub and especially the under-canopy habitat provided efficient shading, no shading was provided by the southern aspect while only partial shading, during low-evaporation hours, was provided by the western (during the morning hours) and eastern (during the afternoon hours) aspects.…”

Linking surface and subsurface properties of biocrusted and non-biocrusted habitats of fine-grained fluvial sediments (playas) from the Negev Desert

“…Although time-consuming, direct measurements by weight provide the most exact values of the amount of water within the soil sample, serving therefore as the standard calibration method for all other methods, such as moisture sensors connected to data loggers (Yoder et al, 1998) or neutron gauges (Leib et al, 2003), which are used in turn for the calibration of moisture obtained using remote sensing methods (Famiglietti et al, 1999). It is worth mentioning, that water loss following soil disturbance and the manual sampling is negligible due to a b the fact that sampling is carried out within minutes after digging and due to low evaporation rates during the rainy season (of <2 mm d -1 ; see Kidron, 2009). In order to account for the difference in texture, the volumetric water content was then calculated based on the average bulk density as determined by pressure plates (Kidron et al, 2002).…”

Quercus ilex L. carbon sequestration capability related to shrub size

“…In arid and semiarid areas of the Mediterranean region where water is the main limiting factor, any amelioration of the environmental physical conditions may have an important impact upon the habitat water regime (Cernusca and Seeber 1981;Hennessy et al 1985;Breshears et al 1998;Kieft et al 1998;Snyder et al 2006;Kidron 2009). Shrub structure defines patterns of light-capturing areas, and the air temperature buffering effect of the canopy (Hennessy et al 1985;Scoles and Archer 1997;Snyder et al 2006).…”