Summary 1.A dense understorey of annual and perennial herbs grow under the canopy of Retama sphaerocarpa shrubs in semiarid environments of south-east Spain, influencing plant productivity and diversity at a regional scale. We investigated the facilitation by the shrub on its understorey in field and laboratory experiments with Barley designed to explore the mechanisms of interaction between both vegetation layers and their spatial variation. 2.There was a gradient of spatial heterogeneity in soil chemical fertility under the shrub canopy, with organic matter and soil nitrogen contents higher at the centre than at the edge of the canopy. Dry mass production of Barley was also higher in soils from intermediate positions, and lower in soils from both the centre and edge of the canopy. 3. In the field, pots sown with Barley placed near the centre, at an intermediate position and at the edge of the canopy of Retama shrubs showed significant differences in productivity, suggesting a mulching effect of the canopy that also affects seedling establishment. 4. Micro-climatic measurements showed significant differences in total radiation reaching the soil, mean air and soil temperatures and maximum temperature among different positions in the understorey, increasing radially from the centre to the edge of the canopy. 5. These results and field observations suggest that the optimal association of climatic factors under the canopy would combine with a high soil fertility mediated by litter decomposition to increase biomass production mid-way between the centre and the edge of the canopy. Overstorey and understorey thus interact to increase nutrient retention locally, which benefits both the shrub and the herb layer.

Retama sphaerocarpa shrubs in semi‐and environments often have a dense understorey of annual and perennial herbs forming so‐called “islands of fertility” The effect of the canopy on soil fertility and microclimate and the combined effect of canopy and litter on species diversity and productivity were assessed under Retama spliaerocarpa shrubs in a semi‐and environment in southeast Spain Soil chemical properties differed significantly among three positions under the canopy, particularly between inner and outer positions The potential mineralization rate of organic matter was significantly higher in soils from an intermediate position under the canopy than in soils from either the centre and the edge Soil chemical fertility and estimated soil seed bank were highest also in soil at an intermediate position and lowest in soil from the edge The understorey flora was favoured by the lower temperature and irradiation and increased soil fertility under the shrubs canopies Species emerging from the soil seed bank separated clearly into two groups which occupied inner and outer positions under the canopy Different levels of addition of Retama litter significantly decreased species richness and the number of emerged seedlings In the field, pots placed near the centre, at an intermediate position and at the edge of the canopy of Retama shrubs differed significantly in species richness and biomass production Overall, micro‐climatic conditions combined with increased chemical fertility of the soil and inhibitory effects of litter to produce a large biomass of herbs at intermediate positions between the centre and the edge of the canopy. The high proportion of litter from annual species in that position increases the mineralization rate and hence nutrient dynamics in a process which also could benefit the shrub

The main aims of this study were, ®rstly, to adapt the rainfall interception model of Rutter et al. (Agric. Meterology, 1971, 9, 367±384) to individual plants of two semiarid shrubs (Anthyllis cytisoides L. and Retama sphaerocarpa (L.) Boiss.) and a tussock grass (Stipa tenacissima L.) and secondly, to understand how the different canopy structures in¯uence rainfall partitioning by individual plants. The selected species represent contrasting canopy types typical of vegetation of semiarid areas. Free throughfall coef®cients were estimated from ®eld measurements of low volume rainfall events and vertical photographs taken beneath the plant canopy. Canopy drainage curves were measured by continuous weighing of wetted plants. Canopy boundary layer conductances were calculated by measuring the evaporation of water from wet canopies. Field measurements of gross rainfall, throughfall and stem¯ow were taken for each rainfall event for A. cytisoides and R. sphaerocarpa. The Rutter type model of rainfall interception was adapted for individual shrubs and tested with measured rainfall events showing a good agreement between observed and predicted values for R. sphaerocarpa and for A. cytisoides.The interception model was then run to simulate interception loss during actual rainfall events, using atmospheric conditions measured every 5 s. The results from this simulation showed signi®cant differences in interception loss between species, which can be explained by differences in canopy drainage and boundary layer conductance, and are caused primarily by the structural differences in their canopies. R. sphaerocarpa gave lower interception than the other two species, S. tenacissima gave higher interception, while A. cytisoides had an intermediate value. The low interception loss by R. sphaerocarpa can be explained by its low total area index, thus, high free throughfall and high canopy drainage rate per unit projected canopy area. On the other hand, S. tenacissima and A. cytisoides, show a low free throughfall and drainage rate per unit projected canopy area because of their higher aerial biomass density. The ecological implications of these adaptations are discussed. #

Dewfall is widely recognized as an important source of water for many ecosystems, especially in arid and semiarid areas, contributing to improve daily and annual water balances and leading to increased interest in its study in recent years. In this study, occurrence, frequency and amount of dewfall were measured from January 2007 to December 2010 (4 years study) to find out its contribution to the local water balance in a Mediterranean semiarid steppe ecosystem dominated by scattered tussocks of Stipa tenacissima (Balsa Blanca, Almería, SE Spain). For this purpose, we developed a dewfall measurement method, ‘The Combined Dewfall Estimation Method’ (CDEM). This method consists of a combination of the potential dewfall model, i.e. the single‐source Penman–Monteith evaporation model simplified for water condensation, with information from leaf wetness sensors, rain gauge data, soil surface temperature and dew point temperature. To assess the reliability of the CDEM, dewfall was measured in situ using weighing microlysimeters during a period of 3 months. Daily micrometeorological variables involved in a dewfall event were analysed in order to assess the significance of dewfall at this site. Dewfall condensation was recorded on 78% of the nights during the study period. Average monthly dewfall duration was 9.6 ± 3.2 h per night. Average dewfall was 0.17 ± 0.10 mm per night and was mostly dependent on dewfall duration. Dewfall episodes were longer in late autumn and winter and shorter during spring. Annual dewfall represented the 16%, 23%, 15% and 9% of rainfall on 2007, 2008, 2009 and 2010, respectively. Furthermore, when a wet period was compared to a dry one, the dewfall contribution to the water balance at the site was found to be 8% and 94%, respectively. Our results highlight the relevance of dewfall as a constant source of water in arid ecosystems, as well as its significant contribution to the local water balance, mainly during dry periods where it may represent the only source of water at the site. Copyright © 2013 John Wiley & Sons, Ltd.

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