Leaf litterfall patterns of perennial plant species in the arid Patagonian Monte,Argentina

The objective of this study was to investigate the variation in leaf litterfall patterns of desert plant species in relation to the intra- and interannual variation of precipitation. We collected the leaf litterfall of 12 representative species of the dominant life forms in the arid Patagonian Monte (evergreen shrubs, deciduous shrubs, and perennial grasses) at monthly intervals during three consecutive years. All shrub species showed a marked seasonality in the pattern of leaf litterfall, but the date of the peak of leaf litterfall differed among them. The peak of leaf litterfall in three deciduous and three evergreen shrubs occurred in summer months while in one deciduous shrub and in two other evergreen shrubs the peak of leaf litterfall was in autumn and winter, respectively. In contrast, the leaf litterfall of perennial grasses occurred through the year without a seasonal pattern. In most shrub species, increasing annual precipitation was related to increasing leaf litterfall and the peak of leaf litterfall was positively related to precipitation events occurred some months before, during winter. Moreover, the magnitude of responses in terms of variation in leaf litterfall in relation to interannual variation of precipitation was not the same for all species. Evergreen shrubs showed lower responses than deciduous species. These differences in leaf litterfall patterns were consistent with differences in leaf traits. In conclusion, we found new evidence of species-specific responses of leaf litterfall patterns to precipitation, suggesting that other factors than precipitation may control leaf litterfall in desert plants.     

Bistability and regular spatial patterns in arid ecosystems

A variety of patterns observed in ecosystems can be explained by resource–concentration mechanisms. A resource–concentration mechanism occurs when organisms increase the lateral flow of a resource toward them, leading to a local concentration of this resource and to its depletion from areas farther away. In resource–concentration systems, it has been proposed that certain spatial patterns could indicate proximity to discontinuous transitions where an ecosystem abruptly shifts from one stable state to another. Here, we test this hypothesis using a model of vegetation dynamics in arid ecosystems. In this model, a resource–concentration mechanism drives a positive feedback between vegetation and soil water availability. We derived the conditions leading to bistability and pattern formation. Our analysis revealed that bistability and regular pattern formation are linked in our model. This means that, when regular vegetation patterns occur, they indicate that the system is along a discontinuous transition to desertification. Yet, in real systems, only observing regular vegetation patterns without identifying the pattern-driving mechanism might not be enough to conclude that an ecosystem is along a discontinuous transition because similar patterns can emerge from different ecological mechanisms.     

Relationship between plant nitrogen conservation strategies and the dynamics of soil nitrogen in the arid Patagonian Monte, Argentina

During three consecutive years with contrasting precipitation, we analysed the relationship between strategies of N conservation in the dominant plant functional groups (perennial grasses and evergreen shrubs) of the Patagonian Monte and the main components of N cycling in soil. We hypothesised that the different patterns of N conservation in perennial grasses and evergreen shrubs would have direct consequences for soil-N, inorganic-N release and microbial-N flush in soil. In autumn and late spring of 1999, 2000, and 2001, we assessed N and C concentration in green and senesced leaves, N-resorption efficiency and C/N ratio in senesced leaves of three dominant species of each plant functional group. In the soil associated with species of each plant functional group, we determined N and C concentration, potential-N mineralisation, and the associated microbial-N flush. Slow-growing evergreen shrubs exhibited low N-concentration in green leaves, high N-concentration in senesced leaves and low N-resorption from senescing leaves. In contrast, fast-growing perennial grasses showed high N-concentration in green leaves, low N-concentration in senesced leaves, and high N-resorption from senescing leaves. In evergreen shrubs, the maintenance of long-lasting green leaves with low N-concentration was the most important mechanism of N conservation. In contrast, perennial grasses conserved N through high N-resorption from senescing leaves. Soil-N concentration, potential N-mineralisation, and microbial-N flush in the soil were higher underneath evergreen shrubs than beneath perennial grasses. Observed differences, however, were lower than expected considering the quality of the organic matter supplied by each plant fuctional group to the soil. A possible reason for this relatively weak trend may be the capacity of evergreen shrubs to slow down N cycling through low leaf turnover and the presence of secondary compounds in leaves. Alternatively or simultaneously, the weak relationship between plant and soil N could result from shrubs being able to colonise N-poor soils while grasses may preferably occupy fertile microsites previously influenced by the decomposition pathway of evergreen shrubs. Differences between evergreen shrubs and perennial grasses in the mechanisms of plant N-conservation and in components of N cycling in the underlying soil were consistent over the three years of the study with differing precipitation. Inter-annual differences in N concentration in green leaves and in the microbial-N flush in soil indicate that during the wettest year fast-growing perennial grasses would outcompete slow-growing evergreen shrubs and microorganisms for N uptake.     

Soluble phenolics extracted from Larrea divaricata leaves modulate soil microbial activity and perennial grass establishment in arid ecosystems of the Patagonian Monte,Argentina

Plant Ecology - Sheep grazing induces the reduction of perennial grass cover and the increase of shrub cover with high concentration of chemical defences. We analysed the effects of secondary...     

Direct and indirect effects of grazing constrain shrub encroachment in semi‐arid Patagonian steppes

Question: What are the long‐term effects of grazing exclusion on the population structure and dynamics of, and interactions among, three dominant shrub species? Location: Grass‐shrub Patagonian steppe, Chubut, Argentina. Methods: Permanent plots were established in grazed paddocks and paddocks excluded from grazing in representative Patagonian rangelands. Shrub abundance, population size‐structure, short‐term (two 3‐yr periods) and long‐term (matrix models) population dynamics, and neighborhood interactions of three native and codominant shrub species (Mulinum spinosum, Senecio filaginoides and Adesmia volckmanni) were measured and analysed using different statistical approaches. Results: The total density of shrubs was 74% higher in paddocks excluded from grazing, owing mainly to increases in Mulinum (80%) and Senecio (68%) species. However, differences in size structure between ungrazed and grazed paddocks were only detected in Mulinum. Demographic rates differed between shrub species, time‐periods and grazing conditions. In particular, recruitment in the short term (especially in wet years) and population growth rate in the long term (λ) were higher in paddocks excluded from grazing only in Mulinum populations. Senecio populations showed a marginal increase in recruitment and mortality independent of the grazing condition in the wet and dry period. Grazing exclusion modified the balance of neighborhood interactions among the three shrub species. In grazing‐exclusion paddocks, there was a balance between positive and negative interspecific interactions, while in grazed paddocks there were more negative intraspecific and interspecific interactions, resulting in a net negative balance of neighborhood interactions. Conclusions: Our understanding of woody encroachment in arid rangelands can be informed through evaluation of direct and indirect effects of grazing exclusion on the abundance and demography of dominant woody species. In Patagonian arid steppes, the occurrence of woody encroachment in rangelands excluded from grazing can be explained by altered responses in plant‐animal and plant‐plant interactions among shrub species.     

Meaningful traits for grouping plant species across arid ecosystems

Grouping species may provide some degree of simplification to understand the ecological function of plants on key ecosystem processes. We asked whether groups of plant species based on morpho-chemical traits associated with plant persistence and stress/disturbance resistance reflect dominant plant growth forms in arid ecosystems. We selected twelve sites across an aridity gradient in northern Patagonia. At each site, we identified modal size plants of each dominant species and assessed specific leaf area (SLA), plant height, seed mass, N and soluble phenol concentration in green and senesced leaves at each plant. Plant species were grouped according with plant growth forms (perennial grasses, evergreen shrubs and deciduous shrubs) and plant morphological and/or chemical traits using cluster analysis. We calculated mean values of each plant trait for each species group and plant growth form. Plant growth forms significantly differed among them in most of the morpho-chemical traits. Evergreen shrubs were tall plants with the highest seed mass and soluble phenols in leaves, deciduous shrubs were also tall plants with high SLA and the highest N in leaves, and perennial grasses were short plants with high SLA and low concentration of N and soluble phenols in leaves. Grouping species by the combination of morpho-chemical traits yielded 4 groups in which species from one growth form prevailed. These species groups differed in soluble phenol concentration in senesced leaves and plant height. These traits were highly correlated. We concluded that (1) plant height is a relevant synthetic variable, (2) growth forms adequately summarize ecological strategies of species in arid ecosystems, and (3) the inclusion of plant morphological and chemical traits related to defenses against environmental stresses and herbivory enhanced the potential of species grouping, particularly within shrubby growth forms.     

Leaf growth dynamics in four plant species of the Patagonian Monte, Argentina

Studying plant responses to environmental variables is an elemental key to understand the functioning of arid ecosystems. We selected four dominant species of the two main life forms. The species selected were two evergreen shrubs: Larrea divaricata and Chuquiraga avellanedae and two perennial grasses: Nassella tenuis and Pappostipa speciosa. We registered leaf/shoot growth, leaf production and environmental variables (precipitation, air temperature, and volumetric soil water content at two depths) during summer-autumn and winter-spring periods. Multiple regressions were used to test the predictive power of the environmental variables. During the summer-autumn period, the strongest predictors of leaf/shoot growth and leaf production were the soil water content of the upper layer and air temperature while during the winter-spring period, the strongest predictor was air temperature. In conclusion, we found that the leaf/shoot growth and leaf production were associated with current environmental conditions, specially to soil water content and air temperature.     

Ecological strategies in a Patagonian arid steppe

The vegetation in the Coironal arid steppe consists of grasses and shrubs. The objective of this paper was to test Walter's hypothesis that woody vegetation and grasses compete for water in the upper layers of the soil, but woody vegetation has exclusive access to a source of water at deeper levels.Analysis of root profiles and patterns of leaf and soil water potential led us to accept the hypothesis for this arid steppe. Additional information on phenology and on the ability of the major grass species to respond to watering permitted to identify two ecological strategies corresponding to grasses and shrubs. Grasses behave as opportunists having always leaves ready to grow as soon as water becomes available. They have a shallow root system and are able to respond very rapidly to increases in soil water availability. In contrast, woody species have a clear-cut periodic pattern of growth and dormancy. They possess thick horizontal roots running below 35–40 cm and utilized water stored in lower layers of the soil.A diagrammatic model summarizes the role of periodic and opportunistic species upon water circulation in the ecosystem. The effect of changes in the proportion of the two groups upon water dynamics is also discussed.Nomenclature follows Nicora (1978) and Cabrera (1971). Acknowledgements. We would like to thank Instituto Nacional de Tecnología Agropecuaria for its valuable support. This work was also supported by Subsecretaría de Ciencia y Técnica and Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET). E. H. Satorre, A. Nuñez and M. Aguiar helped in data collection in the field and in laboratory sample processing.     

Leafing patterns and leaf traits of four evergreen shrubs in the Patagonian Monte, Argentina

We assessed leafing patterns (rate, timing, and duration of leafing) and leaf traits (leaf longevity, leaf mass per area and leaf-chemistry) in four co-occurring evergreen shrubs of the genus Larrea and Chuquiraga (each having two species) in the arid Patagonian Monte of Argentina. We asked whether species with leaves well-defended against water shortage (high LMA, leaf longevity, and lignin concentration, and low N concentration) have lower leaf production, duration of the leafing period, and inter-annual variation of leafing than species with the opposite traits. We observed two distinctive leafing patterns each related to one genus. Chuquiraga species produced new leaves concentrated in a massive short leafing event (5–48 days) while new leaves of Larrea species emerged gradually (128–258 days). Observed leafing patterns were consistent with simultaneous and successive leafing types previously described for woody plants. The peak of leaf production occurred earlier in Chuquiraga species (mid September) than in Larrea species (mid October–late November). Moreover, Chuquiraga species displayed leaves with the longest leaf lifespan, while leaves of Larrea species had the lowest LMA and the highest N and soluble phenolics concentrations. We also observed that only the leaf production of Larrea species increased in humid years. We concluded that co-occurring evergreen species in the Patagonian Monte displayed different leafing patterns, which were associated with some relevant leaf traits acting as plant defenses against water stress and herbivores. Differences in leafing patterns could provide evidence of ecological differentiation among coexisting species of the same life form.     

Limpet grazing on a physically stressful Patagonian rocky shore

Many theories of consumer control of communities have come from studies conducted in relatively benign, temperate zone rocky intertidal systems. Here, we examine gastropod grazing and the maintenance of bare space on a dry, wind-swept rocky shore of Patagonia, Argentina. Two limpet species are the primary intertidal grazers. Siphonaria lessoni dominates mid and high intertidal zones, while Nacella magellanica dominates the lower zone. In all zones, limpet densities are positively correlated with bare space and the occurrence of cracks. Tethering experiments revealed that: (1) physical stress sets the upper distribution limit of both limpets, (2) predators, such as oyster catchers, regulate Nacella populations and may restrict them to cracks and vertical surfaces, and (3) desiccation stress appears to drive similar crack distribution patterns of Siphonaria in the upper intertidal. Experimental removal of limpets in each intertidal zone indicated that limpets have: (1) no detectable effect in the high intertidal where physical forces dominate community organization, (2) weak impacts at mid-elevations as grazing only limited the abundance of fleshy algae with physical forces again dominating community structure, and (3) relatively stronger, but still weak impacts in the low zone. These results suggest that grazing impacts on Argentine rocky shores are weak in comparison to the physical stresses (e.g. high winds, low humidity) that largely determine structure in this system. The dominance of physical forcing in this system occurs despite having similar grazer densities to other temperate, but comparatively wet, rocky shorelines (e.g. British Isles) where top-down control is strong.     

Plant phenology,leaf traits and leaf litterfall of contrasting life forms in the arid Patagonian Monte,Argentina

Question: Do coexisting plant life forms differ in overall phenology, leaf traits and patterns of leaf litterfall? Location: Patagonian Monte, Chubut Province, Argentina. Methods: We assessed phenology, traits of green and senesced leaves and the pattern of leaf litterfall in 12 species of coexisting life forms (perennial grasses, deciduous shrubs, evergreen shrubs). Results: We did not identify differences in phenology, leaf traits and patterns of leaf litterfall among life forms but these attributes contrasted among species. Independent of the life form, the maintenance of green leaves or vegetative growth during the dry season was mostly associated with leaves with high leaf mass per area (LMA) and high concentration of secondary compounds. Low LMA species produced low litterfall mass with low concentration of secondary compounds, and high N concentration. High LMA species produced the largest mass of leaf litterfall. Accordingly, species were distributed along two main dimensions of ecological variation, the dimension secondary compounds in leaves ‐ length and timing of the vegetative growth period (SC ‐ VGP) and the dimension leaf mass per area ‐ leaf litterfall mass (LMA ‐ LLM). Conclusions: Phenology, leaf traits and leaf litterfall varied among species and overlapped among life forms. The two dimensions of ecological variation among species (SC ‐ VGP, LMA ‐ LLM) represent distinct combinations of plant traits or strategies related to resource acquisition and drought tolerance which are reflected in the patterns of leaf litterfall.     

Acceptability of plant species along grazing gradients around watering points in Tunisian arid zone

Water points provide excellent sites for studying overgrazing effects on plant communities in dry areas. Distance from water can be considered like a surrogate of grazing pressure being high near the water and low away from it. The main aim of this study is to investigate overgrazing effects on acceptability of fodder plants along a grazing gradient around three natural watering points. To achieve this goal, we classified spontaneous plants according to their acceptability degree and we followed their cover, richness and density as well as the grazing value along a grazing gradient around these wells, using phyto-ecological studies during the spring 2004 and 2006. Main results show that very palatable plants (mainly constituted by annuals) are more dominant in both the closed and the more disturbed transect areas around wells. The unpalatable plants dominate sites with moderate disturbance around wells. Ligneous palatable species obviously have a lower degree of disturbance. During the studied seasons the grazing gradient around wells 1 and 2, the oldest ones, seemed to exert a feedback upon the grazing intensity.     

Patch structure and dynamics in a Patagonian arid steppe

Two patch types were recognized in the Occidental District of the Patagonian arid steppe: i) shrubs encircled by a ring of tussock grasses, and ii) tracts of scattered tussocks. Completeness of the ring of grasses around the three dominant shrubs was a function of shrub size. Average completeness was 62, 71 and 83%, respectively for the three dominant shrubs (Senecio filaginoides, Mulinum spinosum and Adesmia campestris). A model for the cyclic dynamics of the two patch types was proposed. It includes a building phase (grass ring construction), a mature phase (maximum ring completeness) and a degenerate phase. In this last phase, triggered by shrub death, completeness of the ring progressively decreases until remnant grasses become undistinguishable from the scattered tussocks patch type. Ring formation occurred independently of shrub species. Grass species were differentially associated to the two patch types and to rings of different shrub species. Cyclical patch dynamics influenced the pattern of resource utilization, since the shrub-ring patch, with a share of only 18% of cover, contributed 44% of the total primary productivity.Abbreviations (ST) Shrub-ring patch - M asymptote, maximum completeness - D parameter related to the initial lag - B rate of increase in ring completeness - z shape parameter - r radius     

Evolution of local facilitation in arid ecosystems

In harsh environments, sessile organisms can make their habitat more hospitable by buffering environmental stress or increasing resource availability. Although the ecological significance of such local facilitation is widely established, the evolutionary aspects have been seldom investigated. Yet addressing the evolutionary aspects of local facilitation is important because theoretical studies show that systems with such positive interactions can exhibit alternative stable states and that such systems may suddenly become extinct when they evolve (evolutionary suicide). Arid ecosystems currently experience strong changes in climate and human pressures, but little is known about the effects of these changes on the selective pressures exerted on the vegetation. Here, we focus on the evolution of local facilitation in arid ecosystems, using a lattice-structured model explicitly considering local interactions among plants. We found that the evolution of local facilitation depends on the seed dispersal strategy. In systems characterized by short-distance seed dispersal, adaptation to a more stressful environment leads to high local facilitation, allowing the population to escape extinction. In contrast, systems characterized by long-distance seed dispersal become extinct under increased stress even when allowed to adapt. In this case, adaptation in response to climate change and human pressures could give the final push to the desertification of arid ecosystems.     

Spatial interactions and resilience in arid ecosystems

We present a mathematical analysis of the consequences of spatial interactions between vegetation patches by means of water flow for the functioning of arid systems. Our model results suggest that spatial exchange of water improved the resilience to disturbances and increased the resistance to human-induced loss of cover. Furthermore, spatial exchange of water allowed vegetation to persist at lower rainfall levels compared to systems that lack spatial interactions. Our analysis stresses the general importance of spatial interactions for the functioning of ecological systems.     

Interactive effects of grazing and shrubs on the annual plant community in semi‐arid Mediterranean shrublands

Question: We studied the interactive effects of grazing and dwarf shrub cover on the structure of a highly diverse annual plant community. Location: Mediterranean, semi‐arid shrubland in the Northern Negev desert, Israel. Methods: Variation in the biomass and plant density of annual species in the shrub and open patches was monitored during four years, inside and outside exclosures protected from sheep grazing, in two contrasting topographic sites: north and south‐facing slopes that differed in their dominant dwarf shrubs species: Sarcopoterium spinosus and Corydothymus capitatus, respectively. Results: Above‐ground biomass, density and richness of annual species were lower under the canopy of both shrub species compared to the adjacent open patches in the absence of grazing. Grazing reduced the biomass of annuals in open patches of both topographic sites, but not in the shrub patches. On the north‐facing slope, grazing also reduced plant density and richness in the open patches, but increased plant density in the shrub patches. At the species level, various response patterns to the combined effects of grazing and patch type were exhibited by different annuals. Protection against the direct impacts of grazing by shrub cover as well as species‐specific interactions between shrubs and annuals were observed. A conceptual mechanistic model explaining these interactions is proposed. Conclusion: In semi‐arid Mediterranean shrublands grazing and dwarf shrub cover interact in shaping the structure of the annual plant community through (1) direct impacts of grazing restricted to the open patches, (2) species‐specific facilitation/ interference occurring in the shrub patches and (3) subsequent further processes occurring among the interconnected shrub and open patches mediated through variation in seed flows between patches.