Tenacissima Steppes Research Articles

Marked Range Regression and Possible Alteration of Distribution of the Dupont’s Lark Chersophilus duponti in Tunisia: Conservation Consequences of Vanishing Alfa Grass Stipa tenacissima Steppes in North Africa

The effect of global warming and desertification on bird populations of semi-arid North African ecosystems has been little studied, although ecosystems in those areas are suffering dramatic changes. Dupont’s lark is one of the most endangered passerines in Europe, but it is also considered scarce in North Africa, where its range and numbers are only well known for Morocco. We analyzed the current distribution and population size of Dupont’s lark in Tunisia and compared the current figures with older data. To assess the presence of the species in the patches of adequate habitat that we found, we elicited territorial calls by broadcasting the males’ territorial songs during early breeding season (N = 123, ≈40 h). Fieldwork (45 persons/day) and analysis of satellite images were combined to determine the current minimal extent of occurrence and area of occupancy, following IUCN definitions. In the only habitat where we found the species (well-preserved pure or mixed alfa patches in the Feriana-Kasserine region), the extent of occurrence in and effective area of occupancy were small (56.3 and 33.2 km2, respectively), particularly when compared with previous estimates (presence of the species in adequate habitat over ca. 89,000 km2). The species has not been detected at all in a large area in the southern part of its potential range, where additional surveys are urgently needed to locate possible remnant small and isolated populations that could persist, as suggested by two recent records of the species there. The breeding population of Dupont’s lark in Tunisia is estimated to be fewer than 600 song birds (335–577). The drastic reduction of range and numbers of this species must have been caused by the disappearance or degradation of alfa grasslands due to agricultural development, overgrazing, and increased aridity.

Soil fertility indicators and soil stoichiometry in semi-arid steppe rangelands

Soil fertility depends on vegetation cover, climatic conditions and soil-specific edaphic factors that regulate transformation processes of plant residues and organic matter. Soil physicochemical characteristics in drylands negatively affect evolutionary process of soil materials resulting in fertility loss. This study investigated the variability of soil physicochemical parameters and fertility estimates in three types of semi-arid steppe rangelands of North Africa, viz. Stipa tenacissima, Artemisia herba-alba and Atriplex halimus. The effect of soil parameters on the evolution of soil fertility was appraised using soil organic carbon (SOC), available phosphorus (AP) and C:P ratio as fertility indicators. In two semi-arid regions with haplic calcisols, soil was sampled in six replicates at each steppe rangeland and a control (bare soil). Using standard protocols, each sample was analyzed to determine pH, electrical conductivity (EC), SOC, AP, C:P ratio, total and active CaCO3. All the soil physicochemical parameters tested, except total CaCO3, showed positive increases in A. halimus and S. tenacissima steppe rangelands. The variation of pH and EC values among rangelands was significant, with A. halimus rangelands had significantly the highest scores and A. herba-alba rangelands the lowest scores. The redundancy analysis showed that the edaphic factors triggering significant increases in scores of soil fertility indicators, when compared to the control, were active CaCO3, EC and pH. These physicochemical parameters positively determined the accumulation of AP and SOC, especially in A. halimus rangelands. The high values of stochiometric C:P ratio were associated to soil characteristics of S. tenacissima and A. herba-alba rangelands. Our findings suggest that soil physicochemical parameters of semi-arid steppe rangelands - compared to bare soil - influenced the evolution of soil fertility and stoichiometric C:P ratio. The type of steppe vegetation differently affects the physicochemistry and stoichiometry of the soil.

Litter as a filter for germination in semi-arid Stipa tenacissima steppes

Woody species play a key role in Stipa tenacissima steppes, they affect ecosystem functioning, facilitate establishment of other plants and increase plant richness. Among the mechanisms involved in facilitation, effect of litter on seed germination and seedling establishment is largely underexplored. We studied the properties of the litter layer under five woody species common in Stipa tenacissima steppes, and assessed its effect on the germination of seeds of two key species with contrasting ecological strategy and seed morphology (the perennial grass Brachypodium retusum and the resprouting shrub Pistacia lentiscus). Litter accumulation was highest under Quercus coccifera and Pinus halepensis, and lowest under Rhamnus lycioides. Pistacia lentiscus and Rosmarinus officinalis showed intermediate levels. Litter accumulation hampered germination, particularly when seeds were sown over the litter. Litter of all species but Rhamnus lycioides reduced germination when seeds were under the litter. Litter extracts had no effect on germination. The effect of litter was largely independent of seed type. Our results show that litter has an overall negative effect acting as a physical barrier for. Litter may influence the balance between facilitation and interference. Its relation with species traits and its impact on habitat heterogeneity and community assemblage deserves further attention.

Effects of woody vegetation patches on species composition in Stipa tenacissima steppes

Stipa tenacissima steppes cover 70,000 km2 in the Mediterranean basin. In these habitats, resprouting shrubs form woody patches that were removed in the past to promote fiber and forage production. Patches are recovering after decades of abandonment. Patches enhance plant richness and affect ecosystem functioning. Yet, they have been commonly considered as a single entity, paying scarce attention to patch heterogeneity, patch dynamics and their impact on community composition. We describe the physical and biotic structure of 450 patches in 15 catchments along a climate gradient in southeastern Spain, and analyze the drivers of overstory and understory composition and recruitment of patch-forming species. Patches were formed by one to six species (Quercus coccifera, Juniperus oxycedrus, Rhamnus lycioides, Ephedra fragilis, Pistacia lentiscus and Osyris lanceolata). Climate determined the species composition in each patch, their physical characteristics and the recruitment of patch-forming species. Species cover and richness depended on the dominant species. Our results suggest that patches dominated by Quercus coccifera and Juniperus oxycedrus may decline and patches dominated by Rhamnus lycioides and Ephedra fragilis may expand as climate warms. They also provide new insights on the interactions in patch communities very relevant for the conservation and management of S. tenacissima steppes.

Effects of deferring grazing on the floristic diversity of Stipa tenacissima Loefl. ex L. rangelands in central Algerian steppe

AbstractThe present work aims to study the effect of deferring grazing on the floristic diversity of Stipa tenacissima Loefl. ex L. rangelands in the central Algerian steppe in two different bioclimatic stages, namely semi‐arid and arid. This work is part of a perspective to know the floristic characteristics of an area excluded from grazing and an unrestricted rangeland, in two different environments in order to highlight the factors that are conceivably responsible for the diversification of flora in the steppe ecosystems. In the present study, a number of 96 floristic surveys were performed during the spring season of the years 2014, 2015 and 2016. The methodology adopted is a subjective sampling. The minimum area retained is 100 m2 defined by the method of Braun Blanquet and de Bolos (1957, Annales Estacion Experimental de Aula Dei, 5, 266). This method has been used to study the floristic diversity represented by the floristic richness, Shannon index, the equitability, the biological and the phytochoric type. The climatic correction according to the altitudinal gradient was applied to determine the bioclimatic stage. The results from the surveys indicate that the rangelands at the studied sites contain 109 species, 27 families and 63 genera out of which 79 species and 22 families are in the semi‐arid bioclimatic stage. A two‐way analysis of variance was employed to examine the effect of the bioclimatic stage and the pasture land put at rest. Variance analysis of the diversity indices showed that the bioclimatic stage has a significant impact on the floristic richness and flora diversity of the S. tenacissima steppe (p ≤ 0.01) with a 5% threshold; on the other hand, the protection of these ecosystems does not have a remarkable influence on the floristic diversity (p > 0.05). In recent years, climatic disturbances have resulted in changes in vegetation, even in rested areas located between the study localities.

Landscape Change in the Steppe of Algeria South-West Using Remote Sensing

Abstract Landscape dynamics is the result of interactions between social systems and the environment, these systems evolving significantly over time. climatic conditions and biophysical phenomena are the main factors of landscape dynamics. Also, currently man is responsible for most changes affecting natural ecosystems. The objective of this work is to study the dynamics of a typical landscape of western Algeria in time and space, and to map the distribution of vegetation groups constitute the vegetation cover of this ecosystem. as well as using a method of monitoring the state of a fragile ecosystem by remote sensing to understand the processes of changes in this area. The steppe constitutes a large arid area, with little relief, covered with low and sparse vegetation. it lies between the annual isohyets of 100 to 400 mm, subjected to a very old human exploitation with an activity of extensive breeding of sheep, goats, and camels. Landsat satellite data were used to mapping vegetation groups in the Mecheria Steppe at a scale of 1: 300,000. Then, a comparison was made between the two maps obtained by a classification of Landsat-8 sensor Operational Land Imager (OLI) acquired on March 18, 2014, and Landsat-5 sensor Thematic Mapper (TM) acquired on April 25, 1987. The results obtained show the main changes affecting the natural distribution of steppe species, a strong change in land occupied by the Stipa tenacissima steppe with 65% of change, this steppe is replaced by Thymelaea microphylla, Salsola vermiculata, lygeum spartum and Peganum harmala steppe. an absence from the steppe Artemisia herba-alba that has also been replaced by the same previous steppes species. The groups with Quercus ilex and Juniperus phoenicea are characterized by a strong regression that was lost 60% of its global surface and transformed by steppe to stipa tenacissima and bare soil.

Potential impacts of aridity on structural and functional status of a southern Mediterranean Stipa tenacissima steppe

Over the last decades, there has been a growing interest in evaluating the predicted potential impacts of future climatic conditions on ecosystem functioning. Ecological indicators developed to assess ecosystem functioning strongly rely on soil and vegetation variables.In the current investigation, we evaluated the structural and functional status of a southern Mediterranean Stipa tenacissima steppe along an increasing aridity gradient. We estimated also the predicted effects of aridity on their functioning.The wide variation of LFA indices, vegetation structural attributes and soil properties among the different study sites revealed structural and functional heterogeneity in S. tenacissima steppes with the increasing aridity. Correlations obtained between LFA indices and most patch attributes showed the importance of vegetation structural attributes on the functioning of S. tenacissima steppes. On the other hand, correlations found between LFA indices and most measured soil variables verified the significance of LFA indices in monitoring soil functioning in southern Mediterranean S. tenacissima steppes.The strong negative relationships observed between aridity and stability, nutrient cycling indices demonstrated that increasing aridity can reinforce erosive processes and deteriorate nutrient cycling (i.e. aridity decreased soil carbon, nitrogen and enzymatic activities).Similarly, aridity was negatively related to vegetation structural attributes including patch cover and landscape organization index. This finding suggests that any increase in aridity can promote the loss of perennial vegetation cover.The current investigation highlighted that predicted increasing aridity will dramatically affect the functioning of southern Mediterranean S. tenacissima steppes, especially in the more arid areas.

Assessment of species diversity and state of Stipa tenacissima steppes

North African steppes are subjected to extreme degradation resulting in the reduction of their surface, genetic erosion of resources, and decrease in biodiversity. Stipa tenacissima steppes, which constitute one of the most representative vegetation types in the driest areas of the Mediterranean basin, are continuously degrading. With the aim of contributing to a better knowledge of the floristic composition and diagnosing the state of degradation of these steppes, we conducted a phytoecological analysis of 10 S. tenacissima sites in Tunisia. Floristic inventory compiled a systematic list of 46 vascular plant species belonging to 43 genera and 26 families. Species richness ranged from 4 to 18 species per 900 m2. Total vegetation cover was moderate and fluctuated between 22.8% and 49.9%. Our results revealed also a decreasing trend in species richness with increasing elevation (\rho = -0.585). Indeed, species richness was negatively correlated with slope (\rho = -0.19) and positively correlated with sand content (\rho = 0.262). Biological types were dominated by chamaephytes; this chamaephytization is due to the phenomenon of aridization and overgrazing. Moreover, the low species cover and the appearance of nonpalatable species highlighted the vulnerability of these steppes to degradation.

Multi-criteria participative evaluation of Pinus halepensis plantations in a semiarid area of southeast Spain

Semiarid areas host several hotspots of biodiversity, and provide vital ecosystem services. Afforestation has been extensively used to restore these areas. However, studies on the impact of afforestation on the provision of ecosystem goods and services are scarce. We have evaluated changes in the provision of ecosystem services after afforestation in a semiarid area in southeastern Spanish. Through a multi-criteria analysis, Pinus halepensis plantations were compared to four unrestored landscape units (grasslands, Stipa tenacissima steppes, shrublands and abandoned agricultural fields), using six ecological and socioeconomic criteria and fourteen indicators. The weights of criteria and indicators were obtained from the opinion of 38 stakeholders. The values of each indicator per landscape unit were obtained from empirical data and bibliographic sources. From the stakeholders’ perspective, regulating and supporting services and biodiversity were more important than provisioning, cultural and economical services. Pine plantations enhanced ecosystem services as hydrological and climatic regulation, and aesthetic value, but decreased the provision of other services as forage productivity, water availability, and small and big game habitat. Pine plantations provided the highest levels of ecosystem services. The multi-criteria and participatory assessment allowed managers to check the overall impact of their decisions, and it represents an efficient tool for reciprocal learning and negotiation.

Biological soil crust influence on germination and rooting of two key species in a Stipa tenacissima steppe

Background and aims In Mediterranean steppes, Stipa tenacissima tussocks facilitate the establishment of vascular plants. We hypothesized that this effect may partially reflect the indirect interaction between Stipa tenacissima, biological soil crusts (BSC), and seeds. Methods We explored the relationship between BSC composition and soil surface conditions (surface roughness and hydrophobicity by using the water drop penetration time test), and seed germination and seedling rooting in aS. tenacissima steppe in southeastern Spain. We explored the causal factors of seed germination at two spatial scales and used SADIE index to represents the soil surface heterogeneity. Results Microsites strongly differed in BSC compositionand soilsurface conditions. Germination of two key species, Pistacia lentiscus and Brachypodium retusum, was not affected by BSC type. In contrast, rooting was lower on soil from open areas covered by BSC than on soil from open areas dominated by bare soil and soil collected under the tussocks. The effect was similar in both species. Lichens were probably responsible for the decrease in rooting. Conclusions Our results suggest that lichen cover and the cover of bare soil and mosses may hamper and facilitate rooting, respectively. By affecting seedling rooting, BSC may contribute to the facilitative effect of Stipa tenacissima.

Hydrobal: An eco-hydrological modelling approach for assessing water balances in different vegetation types in semi-arid areas

In semiarid areas, water is a limited resource and its management is a challenge. Water-balance models can improve the management of water resources by determining the effect of vegetation type on the soil–water balance and aquifer recharge. Here, we present HYDROBAL, an eco-hydrological modelling approach for assessing the water balance with a daily resolution. HYDROBAL is suitable for investigating the temporal variability in soil–water content determined by vegetation water uptake as a function of climatic conditions. The processes, mechanisms, and water flows involved in soil moisture changes are modelled based on daily rainfall and micrometeorological variables and used to predict changes in daily soil–water content. The model outputs include actual evapotranspiration, runoff, and aquifer recharge (deep percolation). The model was applied in a semi-arid area of south-eastern Spain, with six vegetation cover types: bare soil (B), open Stipa tenacissima steppe (St), thorn shrubland (S), dry grassland (G), and Aleppo pine (Pinus halepensis) afforestation of S and G (AS and AG, respectively). A dynamic evaporative coefficient (k) was calibrated for each vegetation type to estimate the soil–water consumption. The model was verified in base on its ability to predict the daily measured soil moisture content in plots with different vegetation types. Comparison between the estimated and measured soil moisture contents (θmodel vs. θTDR) indicated good model performance for all vegetation cover types in both wet and dry periods. High value of the coefficient of determination in the linear regressions for θmodel=ƒ (θTDR) demonstrate the accuracy of the hydrological model. All correlations between measured and predicted soil–water content were strong and significant (R2>0.69, p<0.001)

Évaluation de l’effet du nettoyage des touffes sur la régéneration de la steppe à Alfa (Stipa tenacissima) mise en défens dans l’ouest de l’Algérie

Assessment of the effect of the cleaning of the tufts on the regeneration of the Alfa (Stipa tenacissima L.) steppe put into defens in western Algeria. Since several decades the Algerian steppe has been facing an acute problem of degradation induced by the combined effects of anthropic and natural factors. The main factors at the origin of this degradation are climate change, grazing and clearing. Plant associations with Stipa tenacissima show an alarming regression ; their range has been reduced by more than 45 % during the last 50 years. The average density of Stipa tenacissima is presently only 4350 plants per hectare against around 14 000 in 1975. This is especially due to the effect of sheep grazing which limits the regeneration of Stipa tenacissima. At such a rate the Stipa tenacissima steppe will disappear within a few decades, and species without pastoral interest will settle and the area become quite sensitive to both wind and water erosion. Despite the mobilized resources, results obtained in the struggle against this phenomenon remain so far much mitigated and reveal the inefficiency of the adopted approaches and methods. To save what remains of this ecosystem, an experiment based upon establishing exclosures and cleaning tufts of their fatras gave appreciable results. These practices allowed an increase in phytomass of 869 kg of dry matter per hectare and per year and an average annual increase in height of 16 cm.

Biological Soil Crust Microsites Are the Main Contributor to Soil Respiration in a Semiarid Ecosystem

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 Stipa tenacissima 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.

Disturbance of the biological soil crusts and performance of Stipa tenacissima in a semi-arid Mediterranean steppe

Biological soil crusts (BSC) play a major role in water and nutrient fluxes in semi-arid and arid areas, affecting the establishment of vascular plants and contributing to the spatial arrangement of vegetated and open areas. However, little is known regarding their effects on the performance of extant vegetation. By using experimental manipulations (surface soil cutting and herbiciding), we evaluated the effect of the physical structure and the biotic component of smooth biological soil crusts on soil moisture dynamics, and on the nutrient and water status, growth rate, and reproductive effort of Stipa tenacissima tussocks in a semi-arid steppe. Soil moisture content was weakly reduced after cutting the soil surface and was not affected by herbicide application. Cutting and herbiciding the biological soil crust had no effect on most morpho-functional and reproductive traits measured in S. tenacissima tussocks. The integrated water use efficiency of this species, as measured by 13C natural enrichment, decreased when the biotic community of biological soil crusts was killed with herbicide. In the S. tenacissima steppe studied, killing the BSC and breaking the continuity of the structural crust had a weak effect on the short-term performance of this species, but our results suggest that BSC exert a control on slope hydrology beyond that provided by physical soil crusts.

Changes in soil properties and vegetation following livestock grazing exclusion in degraded arid environments of South Tunisia

In this study, characteristics of vegetation and soil properties under continued grazing and exclusion of livestock for 6 and 12 years were examined in a degraded Stipa tenacissima steppe in South Tunisia. Exclosures enhance the total plant cover, the dry matter yield, the number of species per unit area and the Shannon–Wiener diversity. Some palatable species were frequently found inside the protected site. In the continually grazed site, these species are being replaced by less desirable species. Contents of soil organic matter, total nitrogen, Ca 2+ and K + in soils, as well as, water infiltration rate and basal soil respiration showed an increasing trend as time of grazing exclusion increased, from minimum values in the continually grazed area (Gr) to the maximum levels in 12 years protected area (12ex), while there was an opposite trend for Na + concentration, EC, pH and soil hydrophobicity values. The results suggested that excluding grazing livestock on the arid degraded steppes has a great potential to restore vegetation and soil. Therefore, it must be encouraged as an alternative to stop further degradation and to combat desertification in arid and semi arid ecosystems.

Acacia salicina, Pinus halepensis and Eucalyptus occidentalis improve soil surface conditions in arid southern Tunisia

Despite low growth rates, plants in arid areas have a strong ability to modify soil surface properties affecting ecosystem processes and community dynamics. But our knowledge on species effects on soil properties in these areas comes largely from observational studies, increasing the risk of confounding factors and precluding estimations of rates of change. We evaluated changes in soil surface properties underneath Acacia salicina, Pinus halepensis and Eucalyptus occidentalis in a 10-year-old common garden experiment established on a degraded Stipa tenacissima steppe in southern Tunisia. The three species tested improved soil properties compared to those of open areas. Acacia salicina ranked first as soil modifier as the soil underneath this species showed higher total organic carbon, total nitrogen, available phosphorus, soil CO 2 efflux and infiltration rate, and lower soil hydrophobicity than soil in open areas. The richness of vascular plants was higher under A. salicina than under the other types of cover. This species showed higher ability to improve microsite conditions and foster succession. Short rotations of A. salicina could thus be employed for the restoration of degraded S. tenacissima steppes provided that other aspects of its ecology are controlled. Pinus halepensis represents a good alternative when native species are a priority, albeit facilitative ability is lower.

Indices based on surface indicators predict soil functioning in Mediterranean semi-arid steppes

Methodologies based on indicators occupy a prevalent place when assessing ecosystem functioning and monitoring desertification processes because they are affordable yet do not compromise accuracy. The landscape function analysis (LFA), developed in Australia by David Tongway (CSIRO), uses soil surface indicators to assess the condition of a given ecosystem by producing three numerical indices (stability, infiltration and nutrient cycling) reflecting the status of basic soil functions. None of the previous studies aiming to validate the LFA indices have explored how they relate to surrogates of soil functioning using a large number of test sites capturing different climatic and soil conditions. We aimed to do so using data gathered in 29 Stipa tenacissima steppes in Spain. The nutrient cycling index was strongly correlated with soil variables related to microbial activity and nutrient cycling, such as soil pH, total soil N and P, soil respiration and phosphatase and β-glucosidase activities. Strong correlations between the infiltration index and both soil compaction and the water holding capacity of soils were found. The stability index was also significantly correlated with most of the soil variables evaluated. These relationships were evident in both gypsum and calcareous soils. Our results indicate that the LFA indices may be employed as surrogates of soil variables related to nutrient cycling and water infiltration in semi-arid S. tenacissima steppes. The LFA methodology has an enormous potential to assist land managers and policy makers in the establishment of cost-effective desertification monitoring and restoration programs in semi-arid environments.

The effect of Stipa tenacissima tussocks on some soil surface properties under arid bioclimate in the southern Tunisia

In arid and semi—arid areas of the Mediterranean basin, open steppes are dominated by alfa grass (Stipa tenacissima L.) tussocks which play a crucial role in maintaining ecological ecosystem integrity. In this study, we analyze the effects of S. tenacissima tussocks on soil properties in an arid S. tenacissima steppe in Southern Tunisia. Our main objective is to test whether S. tenacissima microsite is able to ameliorate soil surface properties in semiarid degraded steppes. Soils under S. tenacissima tussocks have higher organic matter, nitrogen content, phosphorus and CO2 efflux than those from open areas. S. tenacissima tussocks improve also physical soil features by increasing soil water content and infiltration capacities underneath the canopy. This study reinforces the idea that fertile patches and particularly S. tenacissima tussocks, by their positive effect on the soil, can be of particular importance for successful restoration in degraded arid and semiarid steppes.

Ecosystem structure and soil‐surface conditions drive the variability in the foliar δ13C and δ15N ofStipa tenacissimain semiarid Mediterranean steppes

AbstractWe evaluated the effects of ecosystem composition and structure (species richness and diversity, cover and spatial attributes of vegetation), abiotic factors (climate and topographical features) and the condition of the bare‐ground areas (evaluated using soil‐surface indicators) on the performance ofStipa tenacissima[evaluated using foliar δ13C, δ15N, nitrogen concentration and the carbon‐to‐nitrogen (C:N) ratio] in 15 steppes of SE Spain. Foliar δ13C values ofS. tenacissimashowed a low degree of variation in the studied steppes, with average values ranging from −24.1 to −22.9‰. Higher variation was found in the δ15N values, which ranged from −5.5 to −2.4‰. The nitrogen concentration and the C:N ratio varied between 5.0 and 8.0 mg g−1, and between 55.4 and 85.3, respectively. The δ13C values became less negative with increasing spatial aggregation of perennial vegetation, while the C:N values increased with increasing perennial vegetation cover. The δ15N values became more negative with increasing infiltration in the bare‐ground areas, but the nitrogen concentration was not related to any of the environmental variables measured. Our results suggest that the relative importance of ecosystem structure and soil‐surface conditions in the bare ground areas was higher than that of abiotic factors as determinants of the performance ofS. tenacissima. The results also show that even subtle changes in these ecosystem features may lead to modifications in plant performance in semiaridS. tenacissimasteppes, and thus to modifications in the associated ecosystem functions in the mid‐ to long‐term.

Remnant shrubs in Mediterranean semi-arid steppes: effects of shrub size, abiotic factors and species identity on understorey richness and occurrence

Isolated shrub patches are a key element for community structure and dynamics in semi-arid ecosystems, and may act as “hot spots” of understorey species diversity. Despite its importance, the relationships between shrub patch characteristics and understorey species richness, and the relative importance of these characteristics against other factors driving understorey species richness, are still poorly understood. We studied perennial species richness under the canopy of late-successional shrubs in semi-arid Stipa tenacissima steppes of SE Spain. The main aim of this study was to evaluate the relationship between shrub size and understorey richness, and to test if this relationship is modified by species identity. We also aimed to know the relative importance of shrub patch characteristics and abiotic factors as predictors of the distribution of understorey species under shrub canopies. Altitude and geographical co-ordinates were able to significantly explain the patterns of shrub occurrence and abundance in the study area. Understorey species richness was significantly related to the size of individual shrubs according to a simple power relationship for all evaluated species. Slope values of linear regressions with log-transformed data, ranging between 0.22 and 0.37, did not differ between species. Site characteristics, patch characteristics and patch species identity significantly explained the occurrence of species on patches. However, the relative importance of site characteristics was higher than that of patch characteristics and species identity in explaining these patterns. Our results agree with the general expectations of the theory of island biogeography and complement previous studies that emphasise the importance of late-successional shrubs in semi-arid Stipa steppes. Environmental management activities within these steppes should promote the conservation of remnant shrubs, as well as its introduction with restoration activities.