Tenacissima Tussocks Research Articles

Interacting water, nutrients, and shrub age control steppe grass‐on‐shrub competition: Implications for restoration

AbstractUnderstanding the factors that control grass–shrub interactions and coexistence is critical to the design of dryland management and restoration strategies. Using Stipa tenacissima tussocks as adult neighbor and Olea europaea var. sylvestris as target woody species, we performed a 5‐year‐long, large‐scale manipulative experiment to investigate the independent and combined effects of water and nutrient availability, and the modulating effects of O. europaea ontogeny on the net outcome of grass–shrub interaction. At two contrasting dryland sites, we conducted experimental plantings of Olea seedlings on two microsites: contiguous to a Stipa tussock (Stipa microsite) and on the intertussock bare‐soil areas (Open microsite), and manipulated resource availability by combining nutrient and water addition. The experiment followed a full factorial design, with all treatment combinations applied at each site. We monitored survival, height, and stem basal diameter of Olea individuals over a study period of 5 years. All across the wide range of conditions considered, negative effects of Stipa on Olea largely prevailed during the first 2 years after the plantation. We found that competition was stronger in the drier steppe, where extra inputs of water favored neutral interactions. Conversely, nutrient addition increased competition strength, particularly when combined with water inputs, pointing to the contrasting role of different resources and the importance of interactions between them in the control of plant–plant interaction outcome. The competition effects of Stipa faded with time and gradually shifted to neutral interaction as Olea individuals aged. Our results are particularly relevant to guide restoration efforts in formerly wooded xeric grasslands and rangelands.

Predicted effects of climate change on a Mediterranean keystone plant species

Keystone plant species are known to provide various essential ecosystem services. The Mediterranean keystone species Stipa tenacissima improves soil functioning and facilitates the establishment of different plant species, so it is used in restoration programmes. In this paper, we aimed to predict the impacts of climate (aridity) and soil texture (sand content) on the establishment and development of S. tenacissima (number, cover and width of tussocks) and on soil properties underneath S. tenacissima tussocks. This research demonstrated a significant positive effect of S. tenacissima on LFA (landscape function analysis) indices, revealing the importance of this key species in the soil stabilization, water infiltration and nutrient cycling. Similarly, soil nutrient contents (organic carbon and total nitrogen) and enzyme activities (s-glucosidase and phosphatase) were highly improved underneath S. tenacissima compared to bare ground areas. However, increases of aridity and sand content showed strong negative effects on soil organic carbon, total nitrogen and activities of the hydrolytic enzymes both on bare ground areas and underneath S. tenacissima tussocks. Furthermore, we found that the climate is the main controller of the number of S. tenacissima tussocks, cover and width. Soil sand content influences the colonization of S. tenacissima, as it affects soil water potentials. In general, this study demonstrates the non-sustainability of S. tenacissima under a warmer and drier climate, which will negatively affect its ability to create islands of fertility in South Mediterranean grasslands.

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

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. 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. 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. 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.

Effects of biological soil crusts on a Mediterranean perennial grass (Stipa tenacissima L.)

In the last decades, there has been a growing interest on evaluating the effects of biological soil crusts (hereafter, biocrusts) on vascular plants. The purpose of this paper was to investigate the influence of biocrusts on Stipa tenacissima, key species in arid and semi-arid environments of the Mediterranean Basin. In fact, we explored the impact of biocrusts removal underneath S. tenacissima on soil chemical properties and leaf mineral uptake. Indeed, we studied the effect of mixed biocrusts on the seed germination of S. tenacissima under laboratory conditions. The research showed a greater availability of soil resources correlated with enhanced leaf nutrient levels of bioessential elements in the presence of intact biocrusts compared to fragmented biocrusts underneath S. tenacissima tussocks. We suggest that this is related to the loss of soil moisture as a result of biocrusts removal. In addition, we found that mixed biocrusts favored seed germination and viability of S. tenacissima. By improving soil chemical properties and leaf nutrient uptake, biocrusts can provide enhanced growth conditions for Stipa tenacissima. These findings with the positive effect on seed germination suggest that biocrusts can be used in the restoration of S. tenacissima.

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.

Shrub encroachment does not reduce the activity of some soil enzymes in Mediterranean semiarid grasslands

Shrub encroachment is a worldwide phenomenon with implications for desertification and global change. We evaluated its effects on the activities of urease, phosphatase and β-glucosidase in Mediterranean semiarid grasslands dominated by Stipa tenacissima by sampling 12 sites with and without resprouting shrubs along a climatic gradient. The presence of shrubs affected the evaluated enzymes at different spatial scales. Soils under S. tenacissima tussocks and in bare ground areas devoid of vascular plants had higher values of phosphatase and urease when the shrubs were present. For the β-glucosidase, this effect was site-specific. At the scale of whole plots (30 m × 30 m), shrubs increased soil enzyme activities between 2% (β-glucosidase) and 22% (urease), albeit these differences were significant only in the later case. Our results indicate that shrub encroachment does not reduce the activity of extracellular soil enzymes in S. tenacissima grasslands.

Belowground zone of influence in a tussock grass species

Belowground plant competition is strong and mediated by resource depletion as well as by high variety of exudates. Both factors contribute to active root segregation, affecting neighbours’ growth. In field and greenhouse experiments we investigated the belowground zone of influence of Stipa tenacissima, a tussock-forming grass which is the dominant species in many semiarid communities of the western Mediterranean Basin. Sometimes tussocks show a fringe nearly devoid of annual plants, most likely caused by competition or interference. Fringe size was a function of water availability, so that the more water available the smaller the fringe and vice-versa. Aboveground mass of annual plants was higher in gaps than in the fringe, but root mass was higher in the fringe. More species emerged in soils from gaps than in soils from the fringe, and productivity was ten times higher in soils from gaps than in soils from the fringe. Growth of barley plants was inhibited in the vicinity of S. tenacissima tussocks. S. tenacissima produced a belowground zone of influence around the tussock through resource depletion, particularly water, but also likely through root exudates. Both mechanisms might inhibit the establishment of other annual and grass species within the fringe. Fringe area has important effects for plant establishment, influencing population and community dynamics in these semiarid environments.

Ecohydrological adaptation of soils following land abandonment in a semi‐arid environment

AbstractRepresentative land use types were selected in southeast Spain to investigate the redistribution of soil water in relation to vegetation development and land abandonment.Simulated rainfall experiments were performed during 2 days on each of the four selected surfaces: ploughed, a 3‐year fallow field, a 12‐year abandoned field with shrubs and a semi‐natural field with Stipa tenacissima tussocks. Each experiment consisted of five runs of artificial rain. Soil moisture dynamics were measured by time domain reflectometry (TDR), as well as runoff and ponding on the plot surface. Also the movement of the wetting front was measured after each run.On the ploughed field, ponding and local runoff were less important with shallow and homogeneous wetting fronts. The other land uses showed increased heterogeneity in wetting front development, with soil water pockets under the plants that moved down at greater velocities than under the bare inter‐spaces. It is hypothesized that S. tenacissima and possibly Artemisia herba‐alba adapt their environment by capturing water through stem–root flow systems to divert water to deeper soil layers to reduce evaporation. Furthermore it is suggested that during vegetation development soil heterogeneity increases, as expressed by the water infiltration patterns observed, and that the bare inter‐patches are important in the infiltration process. Copyright © 2010 John Wiley & Sons, Ltd.

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.

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.

Bulk leaf δ18O and δ13C reflect the intensity of intraspecific competition for water in a semi‐arid tussock grassland

We investigated the extent to which plant water and nutrient status are affected by intraspecific competition intensity and microsite quality in a monodominant tussock grassland. Leaf gas exchange and stable isotope measurements were used to assess the water relations of Stipa tenacissima tussocks growing along a gradient of plant cover and soil depth in a semi-arid catchment of Southeast Spain. Stomatal conductance and photosynthetic rate decreased with increasing intensity of competition during the wet growing season, leading to foliar delta(18)O and delta(13)C enrichment. A high potential for runoff interception by upslope neighbours exerted strong detrimental effects on the water and phosphorus status of downslope S. tenacissima tussocks. Foliar delta(15)N values became more enriched with increasing soil depth. Multiple stepwise regression showed that competition potential and/or rhizosphere soil depth accounted for large proportions of variance in foliar delta(13)C, delta(18)O and delta(15)N among target tussocks (57, 37 and 64%, respectively). The results presented here highlight the key role that spatial redistribution of resources (water and nutrients) by runoff plays in semi-arid ecosystems. It is concluded that combined measurement of delta(13)C, delta(18)O and nutrient concentrations in bulk leaf tissue can provide insight into the intensity of competitive interactions occurring in natural plant communities.

Abiotic conditions, neighbour interactions, and the distribution of Stipa tenacissima in a semiarid mountain range

Abiotic conditions and interactions among plants are important factors determining plant geographical distributions. With the aim of analyzing the distribution of a perennial grass species in a semiarid mountain range, we experimentally tested above- and belowground effects of shrubs on the grass along a gradient of aridity. We measured growth, physiology and reproductive output of Stipa tenacissima tussocks surrounded by intact or manipulated neighbour shrubs. Performance of Stipa improved under mesic conditions, but the net effect of shrubs on Stipa did not change along the gradient. Belowground, we did not find significant effects, suggesting belowground resource partitioning between tussocks and shrubs. Aboveground net effects of shrubs on Stipa were generally positive and independent from abiotic conditions. The improved performance of Stipa at the mesic end of gradient suggests that its absolute physiological optimum lies out of its realized range. Overall, the arid end of the geographical distribution of Stipa in this mountain range may be determined by environmental severity. The coincidence of the upper limit of Stipa with the lower tree limit suggests that the more mesic end of Stipa distribution may depend on other factors; most likely competition with trees, which, unlike shrubs, could be excluding Stipa tenacissima from forests.

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.

Linking population density and habitat structure to ecophysiological responses in semiarid Spanish steppes

We have studied the underlying factors responsible for the heterogeneous ecophysiological status of a semiarid Stipa tenacissima L. steppe in a subcatchment of SE Spain by assessing population composition and habitat structure of S. tenacissima stands. To do this, we measured and estimated 18 variables (11 biotic and seven abiotic) in 15 plots randomly distributed in the subcatchment, and then zoned this area by plot affinity using PCA. This analysis produced three sectors determined mainly by S. tenacissima cover and soil depth variables. The linear relationship fitted between S. tenacissima tussock biomass and tussock density in monospecific stands (both logarithmic) indicated a curve close to −1, suggesting that the system is close to the maximum constant yield state. Ecophysiological measurements (gas exchange, fluorescence and individual leaf area index) were taken in two periods with different water availability in a representative plot in each sector. The intraspecific competition (inferred from the density dependence of green biomass) and rock outcrops were the main factors influencing the ecophysiological status in the study area. While, in the wet season, intraspecific competition regulated water consumption in zones where S. tenacissima tussocks (monospecific stands) are dominant, during the dry season, stands in zones with extensive rock outcrops and stone cover (tussocks in “soil pockets”) had no access to non-rainfall water gains because of the adjacent bare soil, and so in these stands, gas exchange was lower and photoinhibition higher. This article stresses the importance of considering the connection between tussocks and bare-ground interspace in the functional and structural analysis of semiarid steppes.

Stipa tenacissima as a nurse plant of the endemic species Haplophyllum bastetanum near Granada, SE Spain

Question: What is the role of Stipa tenacissima as a nurse plant of the rare and threatened endemic species Haplophyllum bastetanum?Location: Semi‐arid steppe of the Guadix‐Baza basin (SE Spain).Methods: In the two H. bastetanum populations known, we (1) estimated the number of individuals and (2) analysed the influence of the factors ‘site’ (S1 = Hernán Valle, S2 = Alicún), ‘microhabitat’ (inside and below the canopy of the S. tenacissima tussocks vs. open areas), ‘life stage’ (vegetative, reproductive), ‘aspect’ (N, S, E, W, flat) and ‘damage’ (yes or no), in relation to the distribution of individuals and to the height, number of flowers, number of capsules, and edaphic variables.Results: The total number of individuals was estimated at 3135. S. tenacissima was found to have a positive and significant effect on the distribution of H. bastetanum, more pronounced in the sample taken at S2 than S1, where 83.8% of the plants were growing in association with S. tenacissima. Similarly, there were more H. bastetanum individuals that became reproductive inside S. tenacissima tussocks than in the open areas, and H. bastetanum individuals had a greater production of flowers and fruits, especially in the sample taken at S1. Organic carbon, nitrogen, and carbon:nitrogen ratio were related to the height and number of flowers and capsules of the plants.Conclusion: Although new patches were established, with a larger number of individuals, H. bastetanum continues to be Critically Endangered (CR). The results support the hypothesis of the facilitation effect of S. tenacissima on this rare, endemic species. Facilitation of species of interest in conservation is an aspect that has not previously been clearly reported. We propound this important process as a valuable tool for rehabilitation of populations of threatened plant species in conservation biology projects.

Small-scale spatial variation in soil CO 2 efflux in a Mediterranean semiarid steppe

Soil respiration is one of the main processes of loss of organic C, but few studies so far have evaluated losses from arid and semiarid soils as compared with other ecosystems. Spatial variation in CO 2 efflux is especially important in semiarid areas, where the distribution of ecological factors and organisms is markedly patchy. We have evaluated the small-scale spatial variation of soil respiration in a semiarid Stipa tenacissima steppe from late winter to early summer. We selected the five most frequent soil cover types in the study site: upslope and downslope of S. tenacissima tussocks (SU and SD surface units, respectively), patches of the perennial grass Brachypodium retusum (BR surface unit), biological crusts (BC surface unit), surface earthworm casts (EC surface unit) and bare ground (BG surface unit). The dynamics of soil CO 2 efflux clearly reflected the changes in soil moisture. We observed significant differences in CO 2 efflux between surface units, with BR and BG showing maximum and minimum CO 2 efflux rates, respectively. Soil temperature increased during the course of the study period, showing significant differences between the most shaded (SU and SD) and the other surface units (BC, BG, BR and EC), with higher temperatures in the latter group. Soil CO 2 efflux was poorly correlated with soil temperature, but significant non-linear relationships were found for SU, BR and BC surface units. Rainfall simulation significantly enhanced CO 2 efflux in SU, BR and EC surface units. Our results highlight the importance of the spatial variation of both vegetation and surface soil features for the estimation of CO 2 efflux in semiarid ecosystems. They also suggest that estimations based only on vegetated and bare-ground areas can be rather conservative if the sampling effort is not high enough to capture the spatial variability in CO 2 efflux in the latter areas.

Consequences of a severe drought on spatial patterns of woody plants in a two-phase mosaic steppe of Stipa tenacissima L.

The establishment and development of the plants of S. tenacissima L. organize a source–sink pattern of water and sediments from bare soil areas to tussocks on mountain slopes forming small terraces or terracettes upslope of tussocks. We hypothesized that terracettes enhance woody plant survival compared with woody plants on bare soil. We examined this hypothesis at three locations in south-eastern Spain after a severe drought from 1993 to 1995. The spatial association of individual plants and the pattern of plant mortality support the hypothesis that the terracettes formed by S. tenacissima tussocks facilitate woody plants. The prediction that facilitation dominates interference during stress periods such as drought was supported by mortality patterns. Drought effects alone do not explain spatial patterns of woody plants in the two-phase mosaic steppe of Stipa tenacissima.

Microsite and mycorrhizal inoculum effects on the establishment of Quercus coccifera in a semi-arid degraded steppe

Alpha grass ( Stipa tenacissima) steppes are widely distributed within the semi-arid areas of the Mediterranean Basin, and represent a degraded stage of vegetation. We evaluated the effect of S. tenacissima tussocks and nursery inoculation with mycorrhizae on the survival of the native shrub Quercus coccifera. Experiments were carried out in three steppes located in semi-arid southeastern Spain. Survival during the first months after plantation was significantly higher in the surroundings of S. tenacissima tussocks than in open areas between the tussocks. We did not find any effect of nursery inoculation on seedling survival. This effect may be related to the fact that the used fungal strain did not survive the drought summer conditions in the study area. Our results support the idea of a facilitative effect of S. tenacissima on introduced shrubs, which may be related to improved environmental conditions in tussock microsites. However, this effect may not be enough for the establishment of seedlings in years with below-average rainfall and a strong summer drought.

Infiltration, penetration resistance and microphytic crust composition in contrasted microsites within a Mediterranean semi-arid steppe

In semi-arid areas with sparse vegetation cover, runoff generated in the open areas is crucial for the maintenance of vegetated patches. Microphytic crusts play a major role in this redistribution of water, thus influencing ecosystem functioning and dynamics. We investigated the effects of alpha grass (Stipa tenacissima L.) on the composition of the microphytic crusts, surface soil compaction, and infiltration in a semi-arid steppe of SE Spain. The microphytic crust composition differed between the upslope of S. tenacissima tussocks (tussock microsites) and the inter-tussock areas with sparse vascular plant cover (open microsites), with more moss cover in the tussock microsite, and more cyanobacteria and lichens in the open microsite. The surface soil compaction was higher in the open microsite. Variables related with infiltration showed a clear microsite effect, with higher infiltration rate and less time required by first drop to percolate in the tussock microsite. Partial correlation analysis showed a significant negative relationship between the cyanobacteria cover and the infiltration rate, and both the cyanobacteria cover and the percentage of bare soil showed a significant positive relationship with the time required for first drop to percolate. Our results reinforce the idea that open microsites act as sources of water for S. tenacissima tussocks. This study helps to understand the interactions between microphytic crusts and vascular plants in semi-arid environments.

POTENTIAL FOR USING FACILITATION BY GRASSES TO ESTABLISH SHRUBS ON A SEMIARID DEGRADED STEPPE

In arid and semiarid environments, isolated vegetative patches establish islands of fertility in which facilitation is a dominant interaction between plant species. These patches may provide favorable microsites for revegetation with desirable species in areas where traditional revegetation procedures fail. Alpha grass (Stipa tenacissima) steppes are widely distributed within the semiarid areas of southern Europe and northern Africa and represent a degraded stage of climax vegetation. In this study, we analyzed the effects of S. tenacissima tussocks on the survival, growth, and ecophysiological features of experimentally planted seedlings of Medicago arborea, Quercus coccifera, and Pistacia lentiscus in three sites in a semiarid region in southeastern Spain. Our main objective was to test whether S. tenacissima was able to facilitate shrub establishment in semiarid degraded steppes. Soils under S. tenacissima tussocks had higher organic matter content and water availability than those from open areas. Sti...