Shrubs as a source of spatial heterogeneity—the case of Retama sphaerocarpa in Mediterranean pastures of central Spain

Microbial interactions play an important role in regulating the effects of plant species on soil bacterial diversity

Plant species and microbial interactions have significant impacts on the diversity of bacterial communities. However, few studies have explored interactions among these factors, such the role of microbial interactions in regulating the effects of plant species on soil bacterial diversity. We assumed that plant species not only affect bacterial community diversity directly, but also influence bacterial community diversity indirectly through changing microbial interactions. Specifically, we collected soil samples associated with three different plant species, one evergreen shrub (Rhododendron simsii) and the other two deciduous shrubs (Dasiphora fruticosa and Salix oritrepha). Soil bacterial community composition and diversity were examined by high-throughput sequencing. Moreover, soil bacterial antagonistic interactions and soil edaphic characteristics were evaluated. We used structural equation modeling (SEM) to disentangle and compare the direct effect of different plant species on soil bacterial community diversity, and their indirect effects through influence on soil edaphic characteristics and microbial antagonistic interactions. The results showed that (1) Plant species effects on soil bacterial diversity were significant; (2) Plant species effects on soil microbial antagonistic interactions were significant; and (3) there was not only a significant direct plant species effect on bacterial diversity, but also a significant indirect effect on bacterial diversity through influence on microbial antagonistic interactions. Our study reveals the difference among plant species in their effects on soil microbial antagonistic interactions and highlights the vital role of microbial interactions on shaping soil microbial community diversity.

Effect of Leaf Litter from Cistus ladanifer L. on the Germination and Growth of Accompanying Shrubland Species

Most communities with the presence of Cistus ladanifer are characterised by the low richness of accompanying species, with C. ladanifer, in most cases, exceeding 70% of the coverage of woody species. This fact could be due to the allelopathic activity attributed to compounds present in the leaves of C. ladanifer, which may have a negative effect on the germination and growth of woody species that share its habitat. One of the possible ways of incorporating allelopathic compounds to the soil is the degradation of leaf litter. Therefore, the aim of this study was to determine how the presence of leaf litter from C. ladanifer affects accompanying species. Under controlled conditions, we analysed the effect of C. ladanifer leaf litter on the germination and growth of seedlings of five species that share their habitat with C. ladanifer (Retama sphaerocarpa, Cytisus multiflorus, Lavandula stoechas, Cistus salviifolius, and Cistus crispus). Additionally, the effect of leaf litter on the species itself, C. ladanifer, has been studied. The experiments were designed with different concentrations of leaf litter (UL) and leaf litter from which the compounds with allelopathic activity were extracted (WL). The results show that such effect greatly depends on the analysed species, with L. stoechas being the most negatively affected species. On the other hand, C. multiflorus and C. salviifolius were only negatively affected at the stage of seedling growth. The results reveal the involvement of leaf litter in the allelopathic activity attributed to C. ladanifer and that its presence has a negative influence on the germination and growth of accompanying woody species. This shows the need to delve into the potential relevance of allelopathy as an interaction that determines the composition, structure and dynamics of a community.

Phosphorus and Nitrogen Modulate Plant Performance in Shrubby Legumes from the Iberian Peninsula

We investigated the impact of phosphorus nutrition on plant growth and biological nitrogen fixation in four leguminous plants in the Tribe Genistea. The main objective of the study was to analyze Phosphorus and Nitrogen use efficiency under drought. We also tested for the effects of rhizobial inoculation on plant performance. Plants inoculated with Rhizobium strains isolated from plants of the four species growing in the wild were cropped under controlled conditions in soils with either low P (5 µM) or high P (500 µM). The experiment was replicated in the presence and absence of plant irrigation to test for the effects of drought stress of inoculated and non-inoculated plants under the two P levels of fertilization. Low-P treatments increased nodule production while plant biomass and shoot and root P and N contents where maximum at high P. Low P (5 µM) in the growing media, resulted in greater N accumulated in plants, coupled with greater phosphorus and nitrogen uptake efficiencies. Drought reduced the relative growth rate over two orders of magnitude or more, depending on the combination of plant species and treatment. Genista cinerea had the lowest tolerance to water scarcity, whereas Genista florida and Retama sphaerocarpa were the most resistant species to drought. Drought resistance was enhanced in the inoculated plants. In the four species, and particularly in Echinospartum barnadesii, the inoculation treatment clearly triggered N use efficiency, whereas P use efficiency was greater in the non-inoculated irrigated plants. Nodulation significantly increased in plants in the low P treatments, where plants showed a greater demand for N. The physiological basis for the four species being able to maintain their growth at low P levels and to respond to the greater P supply, is through balanced acquisition of P and N to meet the plants' nutritional needs.

The shift from plant-plant facilitation to competition under severe water deficit is spatially explicit

The stress-gradient hypothesis predicts a higher frequency of facilitative interactions as resource limitation increases. Under severe resource limitation, it has been suggested that facilitation may revert to competition, and identifying the presence as well as determining the magnitude of this shift is important for predicting the effect of climate change on biodiversity and plant community dynamics. In this study, we perform a meta-analysis to compare temporal differences of species diversity and productivity under a nurse plant (Retama sphaerocarpa) with varying annual rainfall quantity to test the effect of water limitation on facilitation. Furthermore, we assess spatial differences in the herbaceous community under nurse plants in situ during a year with below-average rainfall. We found evidence that severe rainfall deficit reduced species diversity and plant productivity under nurse plants relative to open areas. Our results indicate that the switch from facilitation to competition in response to rainfall quantity is nonlinear. The magnitude of this switch depended on the aspect around the nurse plant. Hotter south aspects under nurse plants resulted in negative effects on beneficiary species, while the north aspect still showed facilitation. Combined, these results emphasize the importance of spatial heterogeneity under nurse plants for mediating species loss under reduced precipitation, as predicted by future climate change scenarios. However, the decreased water availability expected under climate change will likely reduce overall facilitation and limit the role of nurse plants as refugia, amplifying biodiversity loss.

Factors driving species assemblage in Mediterranean soil seed banks: from the large to the fine scale

BACKGROUND AND AIMS

Many studies have analysed the mechanisms that determine plant coexistence in standing vegetation, but the determinants of soil seed bank species assemblies have rarely been studied. In gypsum soil communities, aerial vegetation and seed banks are tightly connected in space and time, but the mechanisms involved in their organization may differ. The aim of this study is to understand the relative importance of biotic and abiotic factors controlling soil seed bank composition and structure.

METHODS

Persistent and complete (i.e. persistent plus transient) soil seed banks were investigated at two spatial scales in a very species-rich semi-arid community dominated by annuals. A water addition treatment equivalent to 50 % annual increase in average precipitation (abiotic factor) was applied for two consecutive years, and the relationships of the soil seed bank to the biological soil crust (BSC), above-ground vegetation and the presence of Stipa tenacissima tussocks (biotic factors) were simultaneously evaluated.

KEY RESULTS

As expected, the standing vegetation was tightly related to seed abundance, species richness and composition in both seed banks. Remarkably, BSC cover was linked to a decrease in seed abundance and species richness in the persistent seed bank, and it even determined complete seed bank composition at the fine spatial scale. However, this effect disappeared at coarser scales, probably because of the high spatial heterogeneity induced by BSCs. In contrast to findings on standing vegetation, Stipa and the irrigation treatment for two consecutive years had no effect on soil seed banks.

CONCLUSIONS

Soil seed bank assemblies in our semi-arid plant community were the result of above-ground vegetation dynamics and of the direct filtering processes on seed fate operated by the spatially heterogeneous BSCs. Cover of BSCs was negatively correlated with seed abundance and species richness, and affected seed species composition in the soil. Changes in species composition and enrichment when the BSC cover is low suggest that BSCs promote a fine scale niche differentiation in the soil seed bank and thereby potentially enhance species coexistence and high species diversity in these communities.

Competitive effect of a native-invasive species on a threatened shrub in a Mediterranean dune system

The canopy shade of the Retama species has been widely reported to ameliorate the environmental conditions in the understory, thus facilitating other species' establishment. The shading effect of the native-invasive leguminous shrub Retama monosperma (L.) Boiss on the endangered Thymus carnosus Boiss was analysed to determine a positive or negative net effect. Data was taken in all four seasons, representing contrasting light and water availability in a Mediterranean coastal dune ecosystem (SW Spain). The morphological and physiological status of sun-exposed T. carnosus plants growing in open areas versus shaded plants growing under R. monosperma were measured seasonally. Leaf mass area, leaf area index and pigment content showed typical sun-shade responses. In contrast, sun-exposed T. carnosus displayed higher stem water potential, transpiration rate and water use efficiency, both intrinsic and integrated, denoting low tolerance to the presence of R. monosperma. Five years after the measurements, canopy cover had decreased and mortality was higher in shaded plants, thus confirming the competitive effect of R. monosperma on T. carnosus. R. monosperma arises as a competitor for endangered T. carnosus communities, consequently reinforcing its invasive behaviour. This species-specific shrub study demonstrates that eventual beneficial effects of Retama canopy may be overridden by competition in the understory, particularly in the case of species well-adapted to high light and low water levels.

Effects of 'target' plant species body size on neighbourhood species richness and composition in old-field vegetation

Competition is generally regarded as an important force in organizing the structure of vegetation, and evidence from several experimental studies of species mixtures suggests that larger mature plant size elicits a competitive advantage. However, these findings are at odds with the fact that large and small plant species generally coexist, and relatively smaller species are more common in virtually all plant communities. Here, we use replicates of ten relatively large old-field plant species to explore the competitive impact of target individual size on their surrounding neighbourhoods compared to nearby neighbourhoods of the same size that are not centred by a large target individual. While target individuals of the largest of our test species, Centaurea jacea L., had a strong impact on neighbouring species, in general, target species size was a weak predictor of the number of other resident species growing within its immediate neighbourhood, as well as the number of resident species that were reproductive. Thus, the presence of a large competitor did not restrict the ability of neighbouring species to reproduce. Lastly, target species size did not have any impact on the species size structure of neighbouring species; i.e. they did not restrict smaller, supposedly poorer competitors, from growing and reproducing close by. Taken together, these results provide no support for a size-advantage in competition restricting local species richness or the ability of small species to coexist and successfully reproduce in the immediate neighbourhood of a large species.