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Quantifying carbon and biomass is relevant information needed in the fight against global warming. Since in Mediterranean ecosystems the agroforestry surface is very large, estimating carbon stocks and their distribution in the different compartments (above ground biomass, root, litter and soil) of these ecosystems is very important. In this work, fixed carbon was quantified in the two most abundant systems of thermophilic shrub of southwestern Iberian Peninsula: Rockroseland ( Cistus ladanifer L.) and Broomland ( Retama sphaerocarpa L.). Biomass was estimated through regression functions from morphology parameters. The results showed that the distribution of carbon among the compartments depends on the species. It was estimated that 34.7 Mg ha −1 of carbon retained in the C. ladanifer system, distributed among the different reservoirs. The shrub system of R. sphaerocarpa stores 24.3 Mg ha −1 of carbon. The carbon stored in biomass was differently also distributed among its components in each species. In C. ladanifer , carbon in above ground biomass is more than 85 % of the total biomass, and 15 % corresponds to carbon in root. However, in R. sphaerocarpa carbon stored in roots goes up to 48 %. These values show that it is important to quantify the carbon stored in all the components of the ecosystem (including the root), and show how important it is to maintain shrubs as reservoirs of carbon in Mediterranean agroforestry.

Understorey vegetation in patches of Retama sphaerocarpa shrubs in semi-arid environments is dependent on the overstorey shrub life history. Community structure changes with shrub age as a result of physical amelioration of environmental conditions by the canopy and organic matter accumulation in the soil. We investigated the effect of the canopy on understorey species diversity in the field and its relationships with the soil seed bank under 50 shrubs from 5 to 25+ years old, and compared species composition in the field in a wet and a dry year. Species composition of the soil seed bank under R. sphaerocarpa shrubs did not differ significantly with shrub age, but seed density increased as the shrubs aged. In the field, community composition changed with shrub age, increasing species richness in a process that depended on the amount of spring rainfall. Our results suggest that the soil seed bank is rather uniform and that the shrub canopy strongly selects which species appear in the understorey. There were seeds of many species present under both young and old shrubs but which only established under old shrubs. This showed dispersal was not limiting species abundance and suggested that the canopy was an important sorting factor for species present in the understorey. Less frequent species contributed the most to patch diversity, and rainfall effectively controlled species emergence. Understorey community composition depended on multiple interspecific interactions, such as facilitation by the shrub and competition from neighbours, as well as on dispersal processes. Facilitation in this environment is a key feature in the structuring of plant communities and in governing ecosystem functioning.

Plants in arid environments cope with stress from excessive irradiance by physiological photoprotection of the photosynthetic apparatus and by structurally reducing the leaf area exposed to the sun (structural photoprotection). We assessed the ecological relevance of structural photoprotection in two plants of contrasting architecture co-occurring in a semi-arid environment, using the three-dimensional canopy model YPLANT. We compared the role of crown geometry in avoiding excessive radiation, analysed the costs of structural photoprotection in terms of reduction of potential carbon gain, and compared these costs with those due to seasonal constraints of photosynthesis and tissue ageing. The results of the model simulations indicated that canopy architecture ofStipa tenacissima(a tussock grass) andRetama sphaerocarpa(a leafless leguminous shrub) minimized the risk of overheating and photo-oxidative destruction of the photosynthetic apparatus with steeply oriented foliage and moderate self-shading. But this structural photoprotection imposed an increased cost in terms of potential carbon gain. Diurnal and seasonal patterns of light interception by the crown of these plants translated into a simulated potential carbon gain only half that of an equivalent, horizontal photosynthetic surface. This reduction in potential carbon gain, due to irradiance avoidance, was similar to that imposed by water shortage.S. tenacissima,which ceases photosynthetic activity during periods of drought, exhibited more structural avoidance of irradiance thanR. sphaerocarpa,which remains active throughout the year. This illustrates the influence of the capacity of plants to utilize light for carbon fixation on the trade-offs between irradiance capture and avoidance. Structural avoidance of excessive radiation efficiently prevents the risk of damage by intense irradiance, has no special maintenance costs, and is biomechanically cheaper than enhanced light harvesting by a horizontal canopy, which points to structural photoprotection as a very effective strategy to cope with high irradiance stress in poor and adverse habitats.

Direct and indirect interactions among plants contribute to shape community composition through above‐ and belowground processes. However, we have not disentangled yet the direct and indirect soil and canopy effects of dominants on understorey species. We addressed this issue in a semi‐arid system from southeast Spain dominated by the legume shrub Retama sphaerocarpa. During a year with an exceptionally dry spring, we removed the shrub canopy to quantify aboveground effects and compared removed‐canopy plots to open plots between shrubs to quantify soil effects, both with and without watering. We added a grass removal treatment in order to separate direct from indirect shrub effects and quantified biomass, abundance, richness and composition of the forb functional group. With watering, changes in forb biomass were primarily driven by indirect shrub effects, with contrasting negative soil and positive aboveground indirect effects; changes in forb abundance and composition were more influenced by direct shrub soil effects with contrasting species composition between open and Retama patches. As community composition was different between open and Retama patches the indirect effects of Retama on forb species did not concern forbs from the open community but forbs from Retama patches. Indirect effects are, thus, important at the functional group level rather than at the species level. Without watering, there were no significant interactions. Changes in species richness between treatments were weak and seldom significant. We conclude that shrub effects on understorey forbs are primarily due to their influence on soil properties, directly affecting forb species composition but indirectly affecting the biomass of the forbs of the Retama patches, and only with sufficient water.

Understanding how UV radiation interacts with prevailing climatic conditions and litter quality to determine leaf litter decomposition is fundamental for understanding soil carbon cycling pathways and ecosystem functioning in drylands. We carried out a field manipulative experiment to investigate how litter quality (labile and nitrogen-rich Retama sphaerocarpa vs. recalcitrant and nitrogen-poor Stipa tenacissima), position (on the ground vs. standing) and different UV radiation levels (UV pass vs. UV block) affect litter decomposition rates at two semiarid Mediterranean steppes with contrasting climates (continental vs. maritime) in a fully factorial experimental design. As expected, Retama litter decomposed faster than that of Stipa, and litter placed on the ground decayed faster than standing litter. However, and surprisingly, contrasting effects of UV radiation on litter decomposition were observed between the two sites. At the continental site, UV radiation increased litter decay constants by 21 % on average, although the contribution of photodegradation was larger when litter was placed on the ground rather than in standing litter. At the maritime site, decay constants were 15 % larger in the absence of UV radiation regardless of litter position. Significant litter type × UV exposure radiation and litter type × position interactions indicate that photodegradation contributes more to litter decomposition under less favorable moisture and substrate availability conditions for microbial decomposers. Our results emphasize the need to consider interactions between moisture availability, litter quality and UV radiation in litter decomposition models to fully understand litter decomposition impacts on soil carbon cycling and storage in drylands under climate change.

Facilitation by nurse plants plays an important role in determining community composition in severe environments. Although the unidirectional effect of nurses on beneficiary species has received considerable research interest, nurse-mediated interactions among beneficiary species (so-called indirect interactions) are less known. Consequently, community composition in nurse plant systems is generally considered as a simple consequence of the facilitative effect of the nurse even though beneficiary species may significantly contribute to community assembly and modulate the direct nurse effects on the community. In an observational study we assessed nurse effects and nurse-mediated beneficiary interactions in two contrasting nurse plant systems in dry environments using a newly developed framework. We quantified plant–plant interaction intensity using the relative interaction index (RII) at the community and species level for three Retama sphaerocarpa shrub size-classes in a semiarid shrubland and four Arenaria tetraquetra cushion plant communities differing in aspect and elevation in dry alpine gravel habitats. The observed RII was split into nurse and beneficiary effects, and related to individual mass, species frequency and abundance using generalized linear mixed models. Results showed predominantly positive nurse effects and negative beneficiary interactions. The effect size of nurse plants, however, was significantly higher than the effect size of beneficiary species in both systems. Individual plant mass and abundance of species was dependent on the combined effects of nurse and beneficiary species whereas species occurrence was related to nurse effects only. Despite evident differences, the semiarid and alpine nurse plant systems showed strong funcional parallelisms. We found interdependence between the effects of nurse and beneficiary species on beneficiary plant assemblages emphasizing their combined role on community assembly in both systems. Our results highlight the need to consider indirect interactions to understand fully plant community dynamics.

Aims Nurse plants facilitate the establishment of other species under their canopies through improvement of microclimate and soil conditions beneath their canopies. We aimed to test whether the effect of nurse soil on understorey communities was independent of the canopy effect, and to what extent soil properties (i.e., nutrients, pH, moisture, temperature, aggregates) can predict understorey biomass and diversity. Methods In a field experiment, we extracted three soil blocks (20 × 20 × 15 cm) under the canopy of eighteen Retama sphaerocarpa shrubs --grouped into small, medium, and large canopy sizes ( n  = 6). Soil blocks were distributed in a stratified-random design, so that each shrub received back three soil blocks (one from under each Retama canopy size). At the end of the growing season plant biomass, abundance, diversity and soil properties were recorded. Results Canopy and soil independently affected understorey community structure. Biomass and diversity increased under large Retama canopies irrespective of soil origin, whereas biomass was higher -and plant diversity lower- in soils from large Retama shrubs irrespective of canopy size. Biomass and species abundance were influenced more by soil than by canopy. There was, however, complementarity between these factors, as biomass increased in soils with high moisture and low temperature, while diversity was highly influenced by canopy size. Conclusions Soil, rather than microclimate amelioration by the nurse, was the main driver of understorey community structure, soil moisture playing a pivotal role. This is highly relevant as drought is expected to increase in drylands worldwide.

The genetic diversity of bacterial populations’ nodulating Retama sphaerocarpa grown in the soils of Maamora cork forest (Morocco) was examined. ERIC-PCR fingerprinting of 30 strains distributed them in 2 groups, of which a representative strain from each group was studied by multilocus sequence analysis of the 16S rRNA, atpD , and gyrB genes . The two representative strains RSM25 and RSM32, grouped with “ Microvirga tunisiensis” . This is the first description of Retama nodule bacteria as members of the genus Microvirga . A nodC -based phylogeny confirmed that the two representative strains RSM25 and RSM32 are affiliated with symbiovar mediterranense. The 2 strains were capable of nodulating not only R. sphaerocarpa but also R. monosperma, R. dasycarpa and L. luteus , and unable to nodulate Phaseolus vulgaris, Vachellia gummifera, Cicer arietinum, Vigna unguiculata and Glycine max. The inoculation of R. sphaerocarpa with RSM25 or RSM32 produced a 1.22-, and 1.36-fold increase in the dry weight of the plants compared to those grown in the presence of 0.05% KNO 3 . The 2 strains used monosaccharides and disaccharides as the sole C source, but fructose, glucose, galactose, arabinose and starch were not used. They were unable to grow on glycine as a N source. Phosphate solubilization and siderophore production was not detected, but IAA or IAA-related compounds were produced. The alkaline pH of the sampling site soil in the Maamora forest where Retama grows could explain why Microvirga was the dominant species in the root nodules of the plants.

In Mediterranean environments, gully erosion is responsible for large soil losses. It has since long been recognized that slopes under vegetation are much more resistant to soil erosion processes compared to bare soils and improve slope stability. Planting or preserving vegetation in areas vulnerable to erosion is therefore considered to be a very effective soil erosion control measure. Re-vegetation strategies for erosion control rely in most cases on the effects of the above-ground biomass in reducing water erosion rates, whereas the role of the below-ground biomass is often neglected or underestimated. While the above-ground biomass can temporally disappear in semi-arid environments, roots may still be present underground and play an important role in protecting the topsoil from being eroded. In order to evaluate the potential of plant species growing in Mediterranean environments to prevent shallow mass movements on gully or terrace walls, the root reinforcement effect of 25 typical Mediterranean matorral species (i.e. shrubs, grasses herbs, small trees) was assessed, using the simple perpendicular model of Wu et al. (Can Geotech J 16:19-33, 1979). As little information is available on Mediterranean plant root characteristics, root distribution data were collected in SE-Spain and root tensile strength tests were conducted in the laboratory. The power root tensile strength-root diameter relationships depend on plant species. The results show that the shrubs Salsola genistoides Juss. Ex Poir. and Atriplex halimus L. have the strongest roots, followed by the grass Brachypodium retusum (Pers.) Beauv. The shrubs Nerium oleander L. and the grass Avenula bromoides (Gouan) H. Scholz have the weakest roots in tension. Root area ratio for the 0-0.1 m topsoil ranges from 0.08% for the grass Piptatherum miliaceum (L.) Coss to 0.8% for the tree Tamarix canariensis Willd. The rush Juncus acutus L. provides the maximum soil reinforcement to the topsoil by its roots (i.e. 304 kPa). Grasses also increase soil shear strength significantly (up to 244 kPa in the 0-0.1 m topsoil for Brachypodium retusum (Pers.) Beauv.). The shrubs Retama sphaerocarpa (L.) Boiss. and Anthyllis cytisoides L. are increasing soil shear strength to a large extent as well (up to 134 and 160 kPa respectively in the 0-0.10 m topsoil). Whereas grasses and the rush Juncus acutus L. increase soil shear strength in the topsoil (0-0.10 m) to a large extent, the shrubs Anthyllis cytisoides (L.), Retama sphaerocarpa (L.) Boiss., Salsola genistoides Juss. Ex Poir. and Atriplex halimus L. strongly reinforce the soil to a greater depth (0-0.5 m). As other studies reported that Wu's model overestimates root cohesion values, reported root cohesion values in this study are maximum values. Nevertheless, the calculated cohesion values are used to rank species according to their potential to reinforce the soil.

Biological soil crusts (BSCs) are a key biotic component of dryland ecosystems worldwide. However, most studies carried out to date on carbon (C) fluxes in these ecosystems, such as soil respiration, have neglected them. We conducted a 3.5-year field experiment to evaluate the spatio-temporal heterogeneity of soil respiration in a semiarid Stipatenacissima steppe and to assess the contribution of BSC-dominated areas to the annual soil respiration of the whole ecosystem. We selected the six most frequent microsites in the study area: Stipa tussocks (ST), Retama sphaerocarpa shrubs (RS), and open areas with very low (< 5% BSC cover, BS), low, medium and high cover of well-developed BSCs. Soil respiration rates did not differ among BSCdominated microsites but were significantly higher and lower than those found in BS and ST microsites, respectively. A model using soil temperature and soil moisture accounted for over 85% of the temporal variation in soil respiration throughout the studied period. Using this model, we estimated a range of 240.4-322.6 g C m⁻² y⁻¹ released by soil respiration at our study area. Vegetated (ST and RS) and BSC-dominated microsites accounted for 37 and 42% of this amount, respectively. Our results indicate that accounting for the spatial heterogeneity in soil respiration induced by BSCs is crucial to provide accurate estimations of this flux at the ecosystem level. They also highlight that BSC-dominated areas are the main contributor to the total C released by soil respiration and, therefore, must be considered when estimating C budgets in drylands.