Semi-arid Mediterranean Ecosystems Research Articles

Biological soil crusts modulate nitrogen availability in semi-arid ecosystems: insights from a Mediterranean grassland

Biological soil crusts (BSCs) greatly influence the N cycle of semi-arid ecosystems, as some organisms forming them are able to fix atmospheric N. However, BSCs are not always taken into account when studying biotic controls on N cycling and transformations. Our main objective was to understand how BSCs modulate the availability of N in a semi-arid Mediterranean ecosystem dominated by the tussock grass Stipa tenacissima. We selected the six most frequent soil cover types in the study area: S. tenacissima tussocks (ST), Retama sphaerocarpa shrubs (RS), and open areas with very low (BS), low (LC) medium (MC) and high (HC) cover of well developed and lichen-dominated BSCs. The temporal dynamics of available N dynamics followed changes in soil moisture. Available NH 4 + -N did not differ between microsites, while available NO 3 - -N was substantially higher in the RS than in any other microsite. No significant differences in the amount of available NO 3 - -N were found between ST and BS microsites, but these microsites had more NO 3 - -N than those dominated by BSCs (LC, MC and HC). Our results suggest that BSCs may be inhibiting nitrification, and highlight the importance of this biotic community as a modulator of the availability of N in semi-arid ecosystems.

Survival costs of reproduction in a short‐lived perennial plant: Live hard, die young

According to life-history theory, reproductive investments involve costs in terms of growth, future fecundity, and/or survival. However, studies to date have often failed to detect costs of reproduction, with survival costs among the less documented. We investigated the cost of reproduction in Helianthemum squamatum (Cistaceae), a short-lived perennial of semiarid Mediterranean environments. After experimental flower removal, we evaluated next season's growth, reproduction, and survival of the plants. We also monitored an indicator of plant physiological status (F(v)/F(m)) and leaf nutrient concentration at key phenological stages during reproduction. Survival rate in deblossomed plants was significantly higher than in control plants. As far as we know, this is the first experimental evidence of a survival cost of reproduction in a perennial plant. In contrast, no cost to growth or reproduction was found during the next season, and no significant differences in F(v)/F(m) or leaf nutrients were found between control and deblossomed plants. Helianthemum squamatum's success in semiarid Mediterranean ecosystems seems to rely on a persistent seed bank, combined with a sustained high reproductive output at the expense of survival. We conclude that this strategy might be more common than previously thought among short-lived shrubby plants growing in stressful Mediterranean areas.

Indication of antagonistic interaction between climate change and erosion on plant species richness and soil properties in semiarid Mediterranean ecosystems

AbstractWe analyzed the consequences of climate change and the increase in soil erosion, as well as their interaction on plant and soil properties in semiarid Mediterranean shrublands in Eastern Spain. Current models on drivers of biodiversity change predict an additive or synergistic interaction between drivers that will increase the negative effects of each one. We used a climatic gradient that reproduces the predicted climate changes in temperature and precipitation for the next 40 years of the wettest and coldest end of the gradient; we also compared flat areas with 20° steep hillslopes. We found that plant species richness and plant cover are negatively affected by climate change and soil erosion, which in turn negatively affects soil resistance to erosion, nutrient content and water holding capacity. We also found that plant species diversity correlates weakly with plant cover but strongly with soil properties related to fertility, water holding capacity and resistance to erosion. Conversely, these soil properties correlate weaker with plant species cover. The joint effect of climate change and soil erosion on plant species richness and soil characteristics is antagonistic. That is, the absolute magnitude of change is smaller than the sum of both effects. However, there is no interaction between climate change and soil erosion on plant cover and their effects fit the additive model. The differences in the interaction model between plant cover and species richness supports the view that several soil properties are more linked to the effect that particular plant species have on soil processes than to the quantity and quality of the plant cover and biomass they support. Our findings suggest that plant species richness is a better indicator than plant cover of ecosystems services related with soil development and protection to erosion in semiarid Mediterranean climates.

Glomalin-related soil protein in a Mediterranean ecosystem affected by a copper smelter and its contribution to Cu and Zn sequestration

The amount of glomalin-related soil protein (GRSP), a glycoprotein produced by arbuscular mycorrhizal fungi (AMF), its contribution to the sequestering of Cu and Zn in the soil, and the microsite variation of other soil traits (pH, water-stable aggregates—[WSA], soil organic carbon—[SOC]) was studied in a semi-arid Mediterranean ecosystem near a copper smelter and affected by deposit of metal-rich particles since 1964. Rhizospheric (R) and non-rhizospheric (NR) soil of four representative plants ( Argemone subfusiformis, Baccharis linearis, Oenothera affinis and Polypogon viridis) was analyzed. The results showed a strong variability in GRSP (6.6–36.8 mg g − 1 ), Cu content (62–831 mg kg − 1 for the total Cu and 5.8–326 mg kg − 1 for the available Cu) and pH (4.2–5.5) in the different plant and rhizospheric zones analyzed. A strong relationship between the GRSP with the soil Cu and Zn contents was found ( r = 0.89 and 0.76 for Cu and Zn respectively, p < 0.001). The GRSP-bound Cu ranged from 3.76 to 89.0 mg g − 1 soil and represents 1.44–27.5% of the total Cu content in soil. Moreover, the WSA reached 89% in P. viridis R. For this plant, the C contained in GRSP represented up to 89% of SOC, and this coincided with the most extreme conditions of soil degradation within the ecosystem (the highest content of heavy metals and low pH values). This study provides evidence on the role of the GRSP in Cu and Zn sequestration and suggests a highly efficient mechanism of AMF to mitigate stress leading to stabilization of soils highly polluted by mining activities.

Environmental gradients and community attributes underlying biodiversity patterns of semi-arid Mediterranean grasslands

Protection and/or establishment of forest plantation have been used as a management strategy to conserve and stop the deterioration of semi-arid Mediterranean grasslands ecosystems, producing a mosaic of vegetation types. This study was intended to investigate the changes in grassland community in response to protection and forest tree plantation practice as well as to explore the underlying environmental gradients responsible for the observed differences or similarities among these vegetation types. Two multivariate analysis methods including discriminate analysis and non-metric multi-dimensional scaling were used to quantify changes in community composition and attributes following different management practices (free grazing, protection with open grassland, sparse and dense forest tree plantations). This was investigated using species frequency, species abundance, or habitat characteristics. The study results showed that habitat types differed significantly between each other and were significantly separated using multivariate approaches. Discrimination based on habitat characteristics and species composition indicated that protection (or grazing) and light (or shade) explained more than 90% of the observed variability in community changes in response to the protection and forest tree plantation. Also, results indicated that shade effect can be attributed to tree canopy cover and/or litter accumulation on the ground. It could be hypothesized that protection from grazing and afforestation resulted in complex environmental gradients of which shade, litter accumulation as well as protection from grazing disturbance are major constituents. A careful manipulation of protection and afforestation can be used to create a spatially different environmental gradients leading to greater habitat diversity as well as a greater species diversity, and better conservation means of grassland in semi-arid areas.

Do plant clumps constitute microbial hotspots in semiarid Mediterranean patchy landscapes?

Three semiarid Mediterranean patchy landscapes were investigated to test the existence of a microsite effect (i.e. plant canopy vs. inter-canopy) on soil microbial communities. Surface soil samples were independently taken from both microsites under naturally changing conditions of humidity and temperature through the year. In gypsiferous soils covered with a shrub steppe, improved physical and chemical soil properties were registered underneath the plant canopy, where the densest and most active microbial communities were also detected (e.g. microbial biomass C averaged 531 and 202 mg kg −1 in canopy and inter-canopy areas, respectively). In calcareous perennial tussock grasslands, either growing on soils over limestones or alluvial deposits, the microsite effect was not so marked. Soil humidity, temperature and total organic C were homogeneously distributed over the landscape conditioning their uniform microbial activity under field moisture conditions (ATP content averaged 853 and 885 nmol kg −1 in canopy and intercanopy areas, respectively). However, readily mineralizable C and microbial biomass C were preferentially accumulated in soils underneath the tussocks determining their larger potential microbial activity (e.g. C hydrolysis capacity under optimal conditions). In conclusion, plant clumps either functioned as microbial hotspots where enhanced microbially driven ecosystem processes took place or as microbial banks capable of undergoing a burst of activity under favourable climatic conditions. Our results provide experimental evidence of a non-patchy distribution of certain soil microbial properties in semi-arid Mediterranean patchy ecosystems.

Colonization processes in semi-arid Mediterranean old-fields

Semi-arid Mediterranean ecosystems undergo a slow process of secondary succession after disturbance which is dependent on rainfall. We tested the hypothesis that colonization of abandoned lands in semi-arid SE Spain was slowed down by seed dispersal and species-specific responses to climatic variability. In field and glasshouse experiments we analysed germination of the two main colonizer species, their appearance in the field as seedlings, their relationships with climate and the resulting population structure. Seed germination of the two dominant legume species, Anthyllis cytisoides and Retama sphaerocarpa, was enhanced by scarification, but triggered in the field by temperature and rainfall, respectively. There was no evidence of allelopathic effects affecting germination in the field. Population structure reflected high inter-annual variability in shrub establishment, which was related to rainfall in spite of self-thinning and other processes that modeled current shrub numbers. Colonization of abandoned lands in semi-arid, Mediterranean environments in SE Spain seems to be primarily limited by seed dispersal, but also affected by climatic variability, which produced temporally separated recruitment events.

The Rhizobia Nodulating Shrubs for Revegetation of Arid Lands: Isolation of Native Strains and Specificity of the Plant – Rhizobia Interaction by Cross Inoculation Tests

Medicago strasseri, M. citrina, Colutea arborescens, and Dorycnium pentaphyllum are legume shrubs potentially useful for the revegetation of semiarid Mediterranean ecosystems and for animal grazing. As nodulation with specific rhizobia and/or mycorrhizal fungi, may become essential for the settlement and growth of legumes, the objective was to ascertain the specificity/promiscuity of the plant–rhizobia interaction, by detecting the occurrence of infective rhizobia in the soils to be revegetated and by cross inoculation tests. Native rhizobia for the target species, were isolated from nondisturbed native areas. The four species were inoculated with four semiarid soils located in central Spain, that we intend to revegetate. Specific rhizobia for M. strasseri and M. citrina were detected in three of the four soils. The soil that did not show infective rhizobia had received sewage-sludge from a waste-water treatment plant, and levels of heavy metals were high. None of the 4 soils had specific rhizobia for C. arborescens and D. pentaphyllum. The cross inoculation tests revealed that M. strasseri and M. citrina belonged to the same symbiotic grouping as alfalfa. On the other hand C. arborescens and D. pentaphyllum only nodulated with their own specific native rhizobia, and the rhizobia from C. arborescens and D. pentaphyllum were not infective with any of the tested species. For M. strasseri the effectiveness of the native rhizobia was significantly higher than the effectiveness of the infective rhizobia from the soils to be revegetated. Consequently for M. strasseri, C. arborescens and D. pentaphyllum, inoculation with native strains (I.MsS1, I.CA and I.DP respectively) would be advisable.

Short- and intermediate-term effects of site and plant preparation techniques on reforestation of a Mediterranean semiarid ecosystem with Pinus halepensis Mill

A 5-year study of the reforestation of a semiarid Mediterranean ecosystem with Pinus halepensis is presented. Reforestation of Mediterranean semiarid ecosystems is challenging as a result of harsh environmental conditions and historical human exploitation leading to degradation. New techniques have to be developed in order to increase survivorship and growth that integrates nursery and field treatments. A total of 60 treatments resulting from 12 site × 5 plant preparation techniques were tested. Plant preparation consists of mycorrhizing in nursery, extra fertilizing in nursery, planting in containers in the nursery, planting in the field with a polypropylene tube, and control plants (bred in plastic bags). Soil preparation consists of hole, subsoiling without amendment, subsoiling with composted organic amendment added in the surface or in the furrow of plantation and subsoiling with fresh organic amendment added in the surface or in the furrow of plantation. Each one of these treatments was tested with a hydrophilic acrylic copolymer and without it. Survivorship was very high (0.91) given the prevailing environmental conditions. Subsoiling promoted higher survival than holes, and plants bred in containers had lower levels of survivorship than control ones. Protecting seedlings with polypropylene tubes had no effect on survivorship. The most effective treatment for promoting growth was organic amendment, the effect of which increased until the fourth year. Protection with tubes was very effective in increasing height, although only in the short-term. Subsoiling also promoted faster growth than holes, although the effect was less noticeable after 3 years and was, in any case, less effective than organic amendment. Other treatments promoting faster growth were the use of fresh organic waste as opposed to composted waste and its incorporation in the plantation furrow rather than as a layer on the surface. The addition of hydrophilic acrylic copolymer reduced growth during the first months of the reforestation. The fertilization of plants in the nursery and inoculation with spores of mycorhyzal fungi did not improve seedling growth. Plants bred in containers performed worse than control plants bred in individual plastic bags. The findings suggests the importance of nutrient supply in semiarid environments, an aspect that has been previously neglected in favor of water supply. Organic amendments reinforce both and can be recommended as long as adequate environmental controls are in place.

Variations in landscape patterns and vegetation cover between 1957 and 1994 in a semiarid Mediterranean ecosystem

The aim of this study was to analyze the main processes that determine changes in landscape patterns and vegetation cover from 1957-1994 to develop a model for land cover dynamics. Land cover and landscape patterns were assessed and compared using aerial photographs taken in 1957, 1985, and 1994. Over this period, tall grass steppe and arid garrigues increased by 6% and 4%, respectively, while crop fields decreased by 15% and tall arid brush remained the same. Over the same period, tall grass steppes and arid garrigues became less fragmented. Changes in land use were triggered by socioeconomic forces, which were constrained by the underlying structure of the physical landscape. The best preserved vegetation (tall arid brushes) was concentrated at higher elevations, with a pronounced slope, not oriented towards the sea, and in volcanic substrate. Communities tended to be better preserved further away from towns and at lower house densities. Tall grass steppe was present on more gradual sea-oriented slope and in calcareous substrate, and increased at higher elevations, although not far from the town but away from high anthropogenic influence. Previous studies have revealed that traditional land uses of this landscape, particularly grazing, favoured the transition from tall arid brush to tall grass steppe. In this study, we analyzed to what extent the underlying structure of the physical landscape imposes limitations to the vulnerability to human activity of the main vegetation types. According to the data on the probability of vegetation transition over the 37-year period, the shift from tall arid brush to tall grass steppe appeared to be favoured by gradual slopes. Tall arid brush recovered from either arid garrigues or tall grass steppes at steeper slopes. Thus, steep terrain had a favourable effect on the formation of brushwood and more gradual terrain favoured tall grass steppe. The prevalent trends were confirmed by a projection of a transition matrix over 100 years.

Analysing arbuscular mycorrhizal fungal diversity in shrub-associated resource islands from a desertification-threatened semiarid Mediterranean ecosystem

The diversity of arbuscular mycorrhizal (AM) fungal species was analysed in vegetation patches dominated by Pistacea lentiscus, one of the most representative shrub species from Mediterranean regions and a target plant currently used in revegetation programmes. The number of AM fungal spores in soil samples taken directly from the target ecosystem was relatively low, and the only AM fungal species recognised in the native soil was Paraglomus occultum. To bait the whole population of native AM fungi, trap cultures were established using different sources of AM fungal inoculum. External AM-mycelium appears to be the main source of AM inoculum in this ecosystem. After 3 years of trap culturing, only five distinguishable AM fungal species, Glomus mosseae, Glomus claroideum, Glomus viscosum, Glomus constrictum and Paraglomus occultum, were morphologically identified, indicating a relatively low AM fungal species richness. These isolates were also genetically characterised by sequence analysis of a portion of their SSU rRNA and their phylogenetic relationships were established . A functional diversity bioassay revealed the effectiveness of G. constrictum and G. mosseae as inoculants for the target shrub species.

Mycorrhizal dependency of a representative plant species in mediterranean shrublands ( Lavandula spica L.) as a key factor to its use for revegetation strategies in desertification-threatened areas

Lavender plants ( Lavandula spp.) are small woody shrubs which belong to the natural succession in certain plant communities of semiarid mediterranean ecosystems in the southeast of Spain and they thrive in areas which are threatened by desertification. Lavender plants form arbuscular mycorrhizas and have been described as mycorrhizal-dependent species, but they are also known to display a stimulated root phosphatase activity in conditions of low-nutrient supply. Consequently, a series of microcosm experiments were carried out in order to identify the relative importance of the mycotrophic habit in comparison with phosphatase production as mechanisms which account for plant establishment and nutrient uptake, using four soils typical of the degraded area of study. It was found that lavender plants were obligatorily mycorrhizal species under the prevailing conditions of their natural habitat. The specific activity of root-associated acid phosphatase in lavender was enhanced in P-deficient, non-mycorrhizal, plants, but apparently this stimulated phosphatase activity did not result in an increased ability of the plant to take up P. Low organic phosphate content in the test soils may account for the absence of plant response to acid phosphatase activity. However, it is much more likely that this ineffectiveness is due to the fact that P-deficiency is concomitant, in the degraded test habitats, with deficiencies in other nutrients. Thus plant growth is being limited by an overall lack of nutrient supply. Mycorrhizal inoculation largely improved P, N and K uptake thereby restoring the biochemical cycling of plant nutrients. In conclusion, lavender plants must be mycorrhizal in order to thrive in the degraded soils from desertification threatened areas in typical mediaterranean ecosystems.

Interactions between plant-growth-promoting rhizobacteria (PGPR), arbuscular mycorrhizal fungi and Rhizobium spp. in the rhizosphere of Anthyllis cytisoides, a model legume for revegetation in mediterranean semi-arid ecosystems.

Arbuscular mycorrhizal (AM) fungi, Rhizobium bacteria and plant-growth-promoting rhizobacteria (PGPR) were isolated from a representative area of a desertified semi-arid ecosystem in the south-east of Spain. Microbial isolates were characterized and screened for effectiveness by a single-inoculation trial in soil microcosms. Anthyllis cytisoides L., a mycotrophic pioneer legume, dominant in the target mediterranean ecosystem, was the test plant. Several microbial cultures from existing collections were also included in the screening process. Two AM fungi (Glomus coronatum, native, and Glomus intraradices. exotic), two Rhizobium bacteria (NR4 and NR9, both native) and two PGPR (A2, native, and E, exotic) were selected. A further screening for the appropriate double and triple combinations of microbial inoculants was then performed. The parameters evaluated were biomass accumulation and allocation, N and P uptake, N2 -fixation (15 N) and specific root length. Overall, G. coronatum, native in the field site was more effective than the exotic G. intraradices in co-inoculation treatments. In general, our results support the importance of physiological and genetic adaptation of microbes to the whole environment, thus local isolates must be involved. Many microbial combinations were effective in improving either plant development, nutrient uptake, N2 -fixation or root system quality. Selective and specific functional compatibility relationships in plant response between the microbial inoculants, were observed. Despite the difficulty of selecting a multifunctional microbial inoculum, appropriate microbial combinations can be recommended for a given biotechnological input related to improvement of plant performance. This could be exploited in nursery production of target plant species endowed with optimized rhizosphere/mycorrhizosphere systems that can be tailored to help plants to establish and survive in nutrient-deficient, degraded habitats. The relevance of this microbial-based approach in the context of a reclamation strategy addressed to environmental sustainability purposes is discussed.

Recycling of olive oil by-products: Possibilities of utilization in animal nutrition

Olive tree culture is especially important in the Mediterranean area, Spain being first as regards total culture surface (2 121 181 ha) and number of productive trees (189 000). Both olive tree culture and olive oil industry produce large amounts of by-products. It has been estimated that pruning produces 25kg of by-products (twigs and leaves) per tree per year. Leaves represent 5% of the weight of olives in oil extraction. On the other hand, the olive oil industry produces 35kg of solid waste (crude olive cake) and 1001 of liquid waste (oil mill wastewaters) per 100kg of treated olives. Such substantial amounts of by-products may have harmful effects on the environment. Consequently, many alternative utilizations of by-products have been considered. One important alternative from the quantitative point of view is their utilization as a source of nutrients for animals. Information concerning chemical and nutritional characteristics of the various types of olive tree culture by-products and, particularly, of those by-products resulting from the new industrial procedures applied to olive oil extraction must be generated in order to achieve efficient uses of such by-products in animal feeding. The possibilities of different chemical and biological methods for the nutritive evaluation of olive tree culture and olive oil industry by-products have also to be investigated. Olive tree culture and olive oil industry by-products could play a crucial role as sources of local feeds for small ruminants. They may also contribute to the development of efficient and environmentally conservative extensive animal production systems within Mediterranean semi-arid ecosystems.

The dependence of the productivity of a semi-arid mediterranean hill pasture ecosystem on climatic fluctuations

The results of a 6-year grazing trial in open Tabor oak woodland, dominated by annual plants, were used for the determination of the dependence of productivity on climatic fluctuations. Stepwise multiple regressions between October–November, December–January and February–March rainfall and minimum December–January temperatures resulted in high correlation coefficients, explaining 76% of the variance in the unfertilized, and even 99% of the variance in the more intensively improved and fertilized, pastures. Legumes were more dependent on total rainfall but also highly responsive to N or P dressings, whereas grasses were more responsive to early rains, especially after N dressings. This is the first study in any mediterranean annual grassland where such close correlations between climate and productivity could be determined. In the drier years this productivity is typical of semi-desertic biomes but in more favorable rainfall years it is well within the range of the subhumid woodland-grassland biome. This great responsiveness to climatic fluctuations was apparently of large adaptive importance in the evolution of these grasslands and has contributed much to their striking resilience and persistence as dissipative structures, “creating order through fluctuation”. It was concluded that this great climatic dependence, as a major bottleneck for rational livestock production, can be overcome only by improving the efficiency of moisture utilisation by introducing deeper-rooted, drought resistant and summer-green folder shrubs and trees.