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In arid regions throughout the world, shallow phreatic aquifers feed natural oases of much higher productivity than would be expected solely from local rainfall. In South America, the presence of well-developed Prosopis flexuosa woodlands in the Monte Desert region east of the Andes has puzzled scientists for decades. Today these woodlands provide crucial subsistence to local populations, including descendants of the indigenous Huarpes. We explore the vulnerability and importance of phreatic groundwater for the productivity of the region, comparing the contributions of local rainfall to that of remote mountain recharge that is increasingly being diverted for irrigated agriculture before it reaches the desert. We combined deep soil coring, plant measurements, direct water-table observations, and stable-isotopic analyses ( 2 H and 18 O) of meteoric, surface, and ground waters at three study sites across the region, comparing woodland stands, bare dunes, and surrounding shrublands. The isotopic composition of phreatic groundwaters (δδ 2 H: −−137‰‰ ±± 5‰‰) closely matched the signature of water brought to the region by the Mendoza River (−−137‰‰ ±± 6‰‰), suggesting that mountain-river infiltration rather than in situ rainfall deep drainage (−−39‰‰ ±± 19‰‰) was the dominant mechanism of recharge. Similarly, chloride mass balances determined from deep soil profiles (>6 m) suggested very low recharge rates. Vegetation in woodland ecosystems, where significant groundwater discharge losses, likely >100 mm/yr occurred, relied on regionally derived groundwater located from 6.5 to 9.5 m underground. At these locations, daily water-table fluctuations of ∼∼10 mm, and stable-isotopic measurements of plant water, indicated groundwater uptake rates of 200-–300 mm/yr. Regional scaling suggests that groundwater evapotranspiration reaches 18-–42 mm/yr across the landscape, accounting for 7-–17%% of the Mendoza River flow regionally. Our study highlights the reliance of ecosystem productivity in natural oases on Andean snowmelt, which is increasingly being diverted to one of the largest irrigated regions of the continent. Understanding the ecohydrological coupling of mountain and desert ecosystems here and elsewhere should help managers balance production agriculture and conservation of unique woodland ecosystems and the rural communities that rely on them.

1. The study of plant-pollinator interactions in a network context is receiving increasing attention. This approach has helped to identify several emerging network patterns such as nestedness and modularity. However, most studies are based only on qualitative information, and some ecosystems, such as deserts and tropical forests, are underrepresented in these data sets. 2. We present an exhaustive analysis of the structure of a 4-year plant-pollinator network from the Monte desert in Argentina using qualitative and quantitative tools. We describe the structure of this network and evaluate sampling completeness using asymptotic species richness estimators. Our goal is to assess the extent to which the realized sampling effort allows for an accurate description of species interactions and to estimate the minimum number of additional censuses required to detect 90% of the interactions. We evaluated completeness of detection of the community-wide pollinator fauna, of the pollinator fauna associated with each plant species and of the plant-pollinator interactions. We also evaluated whether sampling completeness was influenced by plant characteristics, such as flower abundance, flower life span, number of interspecific links (degree) and selectiveness in the identity of their flower visitors, as well as sampling effort. 3. We found that this desert plant-pollinator network has a nested structure and that it exhibits modularity and high network-level generalization. 4. In spite of our high sampling effort, and although we sampled 80% of the pollinator fauna, we recorded only 55% of the interactions. Furthermore, although a 64% increase in sampling effort would suffice to detect 90% of the pollinator species, a fivefold increase in sampling effort would be necessary to detect 90% of the interactions. 5. Detection of interactions was incomplete for most plant species, particularly specialists with a long flowering season and high flower abundance, or generalists with short flowering span and scant flowers. Our results suggest that sampling of a network with the same effort for all plant species is inadequate to sample interactions. 6. Sampling the diversity of interactions is labour intensive, and most plant-pollinator networks published to date are likely to be undersampled. Our analysis allowed estimating the completeness of our sampling, the additional effort needed to detect most interactions and the plant traits that influence the detection of their interactions.

Fil: Vazquez, Diego P.. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza. Instituto Argentino de Investigaciones de las Zonas Áridas. Provincia de Mendoza. Instituto Argentino de Investigaciones de las Zonas Áridas. Universidad Nacional de Cuyo. Instituto Argentino de Investigaciones de las Zonas Áridas; Argentina. Universidad Nacional de Cuyo. Facultad de Ciencias Exactas y Naturales; Argentina. University of Freiburg; Alemania

The structure of mutualistic networks is likely to result from the simultaneous influence of neutrality and the constraints imposed by complementarity in species phenotypes, phenologies, spatial distributions, phylogenetic relationships, and sampling artifacts. We develop a conceptual and methodological framework to evaluate the relative contributions of these potential determinants. Applying this approach to the analysis of a plant-pollinator network, we show that information on relative abundance and phenology suffices to predict several aggregate network properties (connectance, nestedness, interaction evenness, and interaction asymmetry). However, such information falls short of predicting the detailed network structure (the frequency of pairwise interactions), leaving a large amount of variation unexplained. Taken together, our results suggest that both relative species abundance and complementarity in spatiotemporal distribution contribute substantially to generate observed network patters, but that this information is by no means sufficient to predict the occurrence and frequency of pairwise interactions. Future studies could use our methodological framework to evaluate the generality of our findings in a representative sample of study systems with contrasting ecological conditions.

1. Fire represents a frequent disturbance in many ecosystems, which can affect plant-pollinator assemblages and hence the services they provide. Furthermore, fire events could affect the architecture of plant-pollinator interaction networks, modifying the structure and function of communities. 2. Some pollinators, such as wood-nesting bees, may be particularly affected by fire events due to damage to the nesting material and its long regeneration time. However, it remains unclear whether fire influences the structure of bee-plant interactions. 3. Here, we used quantitative plant-wood-nesting bee interaction networks sampled across four different post-fire age categories (from freshly-burnt to unburnt sites) in an arid ecosystem to test whether the abundance of wood-nesting bees, the breadth of resource use and the plant-bee community structure change along a post-fire age gradient. 4. We demonstrate that freshly-burnt sites present higher abundances of generalist than specialist wood-nesting bees and that this translates into lower network modularity than that of sites with greater post-fire ages. Bees do not seem to change their feeding behaviour across the post-fire age gradient despite changes in floral resource availability. 5. Despite the effects of fire on plant-bee interaction network structure, these mutualistic networks seem to be able to recover a few years after the fire event. This result suggests that these interactions might be highly resilient to this type of disturbance.

Fil: Ojeda, Ricardo Alberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza. Instituto Argentino de Investigaciones de las Zonas Áridas. Provincia de Mendoza. Instituto Argentino de Investigaciones de las Zonas Áridas. Universidad Nacional de Cuyo. Instituto Argentino de Investigaciones de las Zonas Áridas; Argentina

Subterranean rodents are considered major soil engineers, as they can locally modify soil properties by their burrowing activities. In this study, the effect of a subterranean rodent of the genus Ctenomys on soil properties and root endophytic fungal propagules in a shrub desert of northwest Argentina was examined. Our main goal was to include among root endophytic fungi not only arbuscular mycorrhiza but also the dark septate endophytes. We compared the abundance of fungal propagules as well as several microbiological and physicochemical parameters between soils from burrows and those from the surrounding landscape. Our results show that food haulage, the deposition of excretions, and soil mixing by rodents’ burrowing promote soil patchiness by (1) the enrichment in both types of root endophytic fungal propagules; (2) the increase in organic matter and nutrients; and (3) changes in soil edaphic properties including moisture, field capacity, and texture. These patches may play a critical role as a source of soil heterogeneity in desert ecosystems, where burrows constructed in interpatches of bare soil can act, once abandoned, as “islands of fertility,” promoting the establishment of plants in an otherwise hostile environment.

Key messageThe yield of epicuticular waxes of Bulnesia retama was increased mainly by water stress, and marginally by UV-B. Natural populations from the most stressful ecoregions showed higher productivity and plasticity.Bulnesia retama is a tree/shrub endemic to the arid and semi-arid zones of South America. This species produces outer stem waxes that can be used in industry, and has been exploited in the past degrading natural populations. Considering the need to diversify marginal dryland economies by making sustainable use of local resources, we aimed to study the productivity of outer stem waxes of natural populations of B. retama from three eco-regions with different environmental stress (rainfall and altitude). We studied the wax productivity of the three populations in response to water stress and UV-B. We conducted two pot experiments with seedlings propagated from seeds of the different environmental backgrounds. We regulated water availability by differential irrigation and manipulated UV-B using selective absorbance filters. We collected stem waxes by the traditional method of brushing dry stems. We found that water stress was the main promoter of stem wax production in this species, while the effect of UV-B was marginal, and was only detected in combination with water stress. Seedlings from the most stressful eco-regions showed the highest stem wax productivity and were the most plastic to variations in environmental conditions. Environmental stress, particularly water stress, was the determining factor in the outer stem wax yield of this species, and in the productive potential of different natural populations, which could be related to ecotypes with different wax yield potential. This knowledge can be used for exploiting this resource sustainably, to select ecotypes for cultivation, and to develop productive varieties through classical breeding.

Abstract The genus Larrea has an amphitropical distribution in North and South American deserts, and its phylogeny remains unresolved. This genus is conspicuous and specious within the Monte Desert, the largest, although understudied, southern South American dryland. Larrea presents an interesting case for phylogenetic studies due to its paternally inherited chloroplasts, its species hybridize in nature, and although nominal species are morphologically distinct, hybrids might be cryptic. We analysed ITS2 nuclear (nDNA) and rbcL chloroplast (cpDNA) sequences of the bifoliolate section, Bifolium, including L. tridentata (Lt) from North America, and its South American congeners: L. cuneifolia (Lc) and L. divaricata (Ld), and sequences of the multifoliolate Larrea section: L. ameghinoi (La), L. nitida (Ln), and a morphological hybrid swarm. We aligned and analysed sequences from 111 individuals collected at 31 populations sampled along the range of each species. The nDNA revealed 56 haplotypes, and median-joining and maximum likelihood reconstructions provided clear separation among species and suggested hybridization between Lc-Ld. The nuclear phylogeny showed that the section Larrea diverged earlier than Bifolium, within which Lc diverged first, meanwhile, consistent with previous studies, Lt forms a monophyletic group sister to Ld. Comparatively, cpDNA was less variable, with only six haplotypes shared between Ln-Lc and Ln-La, and rarely between Ld-Lc. Our results emphasize the significance of separately considering nuclear and plastid evolutionary signals when reconstructing unresolved relationships. While nuclear markers clarified phylogenetic relationships and cryptic hybridization among Larrea species, the chloroplast revealed the retention of widespread ancient polymorphisms, which were conserved in populations of distinct species. Each marker provided insights into particular evolutionary patterns, highlighting that genetic variation may be more influenced by hybridization and mode of chloroplast inheritance than previously recognized.

QUESTION: For a desert where winter is the driest, harshest season we asked: does the effect of dominant shrubs (Bulnesia retama) on annual species depend on (1) the functional traits of the latter, (2) the season of the year, or (3) the activity of livestock? LOCATION: A low‐density goat farm in central‐northern Monte Desert, Argentina. METHODS: We estimated the effect of shrubs using a log response ratio based on annual species population sizes underneath shrub canopies and in open spaces. We collected density data of annual species in 18 visits between Aug 2010 and Apr 2013 in permanent 50‐cm square areas laid out according to a split‐plot design, in which the activity of livestock (fenced and unfenced; plots were 10‐m squares) was the main factor, and microsite type (shrub and open) was the subordinate factor, with 20 replicates for each combination. We also gathered data on eight functional traits (characterizing whole plants, leaves, roots and seeds) from annual plants collected in the study site following standardized protocols. RESULTS: Annual species with acquisitive attributes (high specific leaf area, intermediate‐to‐low leaf dry matter content, large leaves and high specific root volume) were more benefited by shrubs compared to species with the opposite, relatively conservative attributes. Facilitative influences of shrubs were pervasive during winter, while competitive influences increased in frequency during summer, when total plant density was higher. This pattern was not affected by livestock. CONCLUSIONS: The outcome of species interactions depended on the interplay between plant strategies and abiotic stress: facilitated species were mostly acquisitive, and shrub facilitation was more important during the harshest season (winter). Specific root volume, along with widely used functional traits (specific leaf area, leaf dry matter content, leaf size) delineated such strategies. This underlines the importance of considering below‐ground traits when studying plant–plant interactions. Single traits achieved superior explanatory power of shrub effects than composite ones (i.e. principal component axes based on single traits), reinforcing the idea that single functional traits are themselves meaningful indicators of complex physiological trade‐offs that ultimately affect community structure and dynamics.