Jordi Cortina

POSITIVE, NEGATIVE, AND NET EFFECTS IN GRASS–SHRUB INTERACTIONS IN MEDITERRANEAN SEMIARID GRASSLANDS

Current theoretical models and field evidence suggest that facilitation and interference act simultaneously in the field, but there is little information on their joint dynamics under varying abiotic conditions. We evaluated spatial and temporal variations in the positive, negative, and net effects of the tussock grass Stipa tenacissima on the shrub Pistacia lentiscus in Mediterranean semiarid grasslands. We performed a field experiment in which positive (microclimatic amelioration and water inputs from runoff) and negative (belowground competition) effects were experimentally manipulated under contrasting levels of stress. The environment provided by S. tenacissima (tussock treatment) improved the survival and physiological status of planted P. lentiscus seedlings. The elimination of shade and competition from S. tenacissima significantly decreased and increased, respectively, seedling performance as compared to the tussock treatment. We found evidence that microclimatic amelioration is the main positive...

Do positive interactions increase with abiotic stress? A test from a semi-arid steppe

Theoretical models predict that the relative importance of facilitation and competition may vary inversely across gradients of abiotic stress. However, these predictions have not been thoroughly tested in the field, especially in semi–arid environments. In this study, we evaluated how the net effect of the tussock grass Stipa tenacissima on the shrub Pistacia lentiscus varied across a gradient of abiotic stress in semi–arid Mediterranean steppes. We fitted the relationship between accumulated rainfall and the relative neighbour index (our measures of abiotic stress and of the net effect of S. tenacissima on P. lentiscus, respectively), which varied across this gradient, to a quadratic model. Competitive interactions dominated at both extremes of the gradient. Our results do not support established theory. Instead, they suggest that a shift from facilitation to competition under high abiotic stress conditions is likely to occur when the levels of the most limiting resource are so low that the benefits provided by the facilitator cannot overcome its own resource uptake.

Pines and oaks in the restoration of Mediterranean landscapes of Spain: New perspectives for an old practice — a review

Pines have been extensively used for land restoration in the Mediterranean basin and in other parts of the world, since the late 19th century. The theoretical basis supporting pine utilisation was its stress-tolerant and pioneer features, and their attributed role of facilitating the development of late-successional hardwoods in the long-term. In the present work, the use of pines and hardwoods in forest restoration is discussed in the frame of the current disturbance regime and social demands for Mediterranean forests. Large pine plantations have recently disappeared because of their sensitivity to fire (e.g., Pinus nigra) or because of the short fire-intervals (e.g., Pinus halepensis). Combined pine and oak plantations are proposed for degraded land restoration on the basis of the complementary features of both groups of species. Seeding and containerised seedling plantation, soil amendments and plantation techniques to reduce transplant shock are evaluated for reforestation under water-stressing conditions, on the basis of several experiments performed in eastern Spain. Both P. halepensis and Quercus ilex are tested.

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...

Small-scale Environmental Heterogeneity and Spatiotemporal Dynamics of Seedling Establishment in a Semiarid Degraded Ecosystem

In semiarid environments, surface soil properties play a major role in ecosystem dynamics, through their influence on processes such as runoff, infiltration, seed germination, and seedling establishment. Surface soil properties usually show a high degree of spatial heterogeneity in semiarid areas, but direct tests to evaluate the consequences of this heterogeneity on seedling establishment are limited. Using a combination of spatial analysis by distance indices (SADIE) and principal components analysis (PCA) we quantified the spatiotemporal patterns of seedling survival of a Mediterranean native shrub (Pistacia lentiscus) during the first 3 years after planting on a semiarid degraded site in southeastern Spain. We used a variation partitioning method to identify environmental variables associated with seedling survival patterns. Three years after planting, only 36% of the seedlings survived. During the first summer, one-third of the seedlings died, with secondary major mortality in the 3rd summer after planting. The spatial pattern of survival became strongly clumped by the end of the first summer, with clearly defined patches (areas of high survival) and gaps (areas of low survival). The intensity of this pattern increased after subsequent high-mortality periods. Of the 14 variables, the ones most strongly coupled to seedling survival were bare soil cover, sand content, and soil compaction. These findings contribute to our understanding of the linkages between the spatial heterogeneity of abiotic factors and the response of plant populations in semiarid degraded ecosystems and can be used to optimize restoration practices in these areas.

Spatial patterns of surface soil properties and vegetation in a Mediterranean semi-arid steppe

In arid and semi-arid areas with sparse vegetation cover, the spatial pattern of surface soil properties affects water and nutrient flows, and is a question of considerable interest for understanding degradation processes and establishing adequate management measures. In this study, we investigate the spatial distribution of vegetation and surface soil properties (biological crusts, physical crusts, mosses, rock fragments, earthworm casts, fine root accumulation and below-ground stones) in a semi-arid Stipa tenacissima L. steppe in SE Spain. We applied the combination of spatial analysis by distance indices (SADIE) and geostatistics to assess the spatial pattern of soil properties and vegetation, and correlation analyses to explore how these patterns were related. SADIE analysis detected significant clumped patterns in the spatial distribution of vegetation, mosses, fine root accumulation and below-ground stone content. Contoured SADIE index of clustering maps suggested the presence of patchiness in the distribution of earthworm casts, fine roots, below-ground stone content, mosses and biological crusts. Correlation analyses suggested that spatial pattern of some soil properties such as biological crusts, moss cover, surface rock fragments, physical crusts and fine roots were significantly related with above-ground plant distribution. We discuss the spatial arrangement of surface soil properties and suggest mechanistic explanations for the observed spatial patterns and relationships.

Small-scale spatial variation in soil CO2 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 CO2 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 CO2 efflux clearly reflected the changes in soil moisture. We observed significant differences in CO2 efflux between surface units, with BR and BG showing maximum and minimum CO2 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 CO2 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 CO2 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 CO2 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 CO2 efflux in the latter areas.

Climatic trends, disturbances and short-term vegetation dynamics in a Mediterranean shrubland

Fire and erosion are two major disturbances affecting Mediterranean ecosystems. Both of them are closely related to climate. There is evidence of decreasing precipitation in the Mediterranean, particularly during summer. There are also indications of an increased variability in the rainfall distribution. Climatic changes, though show high heterogeneity at a local scale. Based on these observations, we have evaluated the following hypotheses for the Region of Valencia (East Spain). (1) During the past three decades, climatic conditions have become more favourable for wildfires and high erosivity rainfall events. We have used 30-year climate records from 97 meteorological stations to examine this. Results indicate that in general the hypothesis is true, although trends are spatially dependent. (2) The effect of high intensity rain on burned land may substantially affect short-term ecosystem composition and function, and thus successional trajectories. Based on a plot scale study, we have assessed nutrient and vegetation dynamics after burning a pyrophytic community dominated by gorse (Ulex parviflorus). Erosion following high intensity rainfall affects physicochemical soil properties. As a consequence, plant cover is reduced and specific composition affected, changing the previous relationship between obligate seeder and resprouter species.

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.

Does Pinus halepensis facilitate the establishment of shrubs in Mediterranean semi-arid afforestations?

Aleppo pine (Pinus halepensis) dominates forest ecosystems in dry and semi-arid areas of the Mediterranean Basin. In addition to its natural distribution, this species has been used extensively in afforestation projects during this century. The improvement in soil fertility and microclimate provided by the P. halepensis canopy is known to facilitate the establishment of perennial grasses, but little is known about the interactions of this species with woody species in natural stands and afforestations under semi-arid climate. In this study, we analyze the effects of the P. halepensis canopy on the survival and ecophysiological features of experimentally planted seedlings of several Mediterranean shrub and tree species in two semi-arid afforestations of SE Spain. Soils under P. halepensis (pine microsite) showed small differences in nutrient content relative to those outside the canopy (open microsite). Radiation reaching pine microsites was four times lower than in open microsites. Soil moisture did not show significant differences between both microsites during most of the year. One year after planting, survival of the introduced seedlings was significantly higher in the open microsites than in the pine microsites. Predawn and midday water potentials and chlorophyll fluorescence of seedlings measured before the summer did not show any microsite effect. Our results suggest that the changes in understorey microclimate associated with P. halepensis are not sufficient to facilitate the establishment of shrubs under semi-arid conditions.