Patterns of nitrogen conservation in shrubs and grasses in the Patagonian Monte,Argentina

We analysed the main plant strategies to conserve nitrogen in the Patagonian Monte. We hypothesized that the two main plant functional groups (xerophytic evergreen shrubs and mesophytic perennial grasses) display different mechanisms of nitrogen conservation related to their structural and functional characteristics. Evergreen shrubs are deep-rooted species, which develop vegetative and reproductive growth from spring to late summer coupled with high temperatures, independently from water inputs. In contrast, perennial grasses are shallow-rooted species with high leaf turnover, which display vegetative growth from autumn to spring and reproductive activity from mid-spring to early-summer, coupled with precipitation inputs. We selected three evergreen shrubs (Larrea divaricata Cav., Atriplex lampa Gill. ex Moq. and Junellia seriphioides (Gilles and Hook.) Moldenke) and three perennial grasses (Stipa tenuis Phil., S. speciosa Trin. and Rupr. and Poa ligularis Nees ex Steud.), characteristic of undisturbed and disturbed areas of the Patagonian Monte. N concentration in expanded green and senesced leaves was estimated in December 1997 (late spring) and June 1998 (late autumn). Deep-rooted evergreen shrubs displayed small differences in N concentration between green and senesced leaves (low N-resorption efficiency), having high N concentration in senesced leaves (low N-resorption proficiency). Shallow-rooted perennial grasses, conversely, showed high N-resorption efficiency and high N-resorption proficiency (large differences in N concentration between green and senesced leaves and very low N concentration in senesced leaves, respectively). The lack of a strong mechanism of N resorption in evergreen shrubs apparently does not agree with their ability to colonize N-poor soils. These results, however, may be explained by lower N requirements in evergreen shrubs resulting from lower growth rates, lower N concentrations in green leaves, and lower leaf turnover as compared with perennial grasses. Long-lasting N-poor green tissues may, therefore, be considered an efficient mechanism to conserve N in evergreen shrubs in contrast with the mechanism of strong N resorption from transient N-rich tissues displayed by perennial grasses. Evergreen shrubs with low N-resorption efficiency provide a more N-rich substrate, with probably higher capability of N mineralization than that of perennial grasses, which may eventually enhance N fertility and N availability in N-poor soils.     

Utilization of food resources by small and medium‐sized mammals in the Monte Desert biome,Argentina

Theoretical models of species coexistence between desert mammals have generally been based on a combination of food and microhabitat selection by granivorous rodents. Although these models are applicable in various deserts of the world, they cannot explain resource use by mammals in Neotropical deserts. The present study examines diet composition in a mammal assemblage in the Monte desert, Argentina. The results show that two main strategies are used by these mammals: medium‐sized species (hystricognath rodents: Dolichotis patagonum, Lagostomus maximus, Microcavia australis and Galea musteloides; and an exotic lagomorph: Lepus europaeus) are herbivores, whereas small‐sized species (a marsupial: Thylamys pusillus; and sigmodontine rodents: Graomys griseoflavus, Akodon molinae, Calomys musculinus, Eligmodontia typus) are omnivorous. Small mammals also show a tendency towards granivory (C. musculinus), insectivory (A. molinae and T. pusillus) and folivory (G. griseoflavus).     

Mycorrhizal associations in the Patagonian steppe,Argentina

The mycorrhizal status of plant species in north-west Patagonia was examined. Communities representative of Patagonian steppe and marshes were compared with respect to the mycorrhizal status of their species.Most of both native and exotic plant species sampled were arbuscular mycorrhizal (AM). The percentage of species with mycorrhizal association was higher for perennial herbs and shrubs than for annual herbs. The higher ratio of mycorrhizal/nonmycorrhizal (NM) species found for dicotyledons than for monocotyledons, could reflect the presence of a considerable number of NM monocotyledons in the marsh. The mycorrhizal status of plants differed slightly between the steppe and the marsh. In the steppe, native AM species were more frequent than in the marsh. In contrast, in the marsh, the NM species were proportionally more represented than in the steppe. The Juncaceae and Cyperaceae, which include hydrohytic NM plants, accounted for many of these differences. Moreover, the dominant species in the marshes, Juncus arcticus, is a NM species.In the present study, most of species belonging to the same taxonomic family tended to have the same mycorrhizal associations, in agreement with studies on plants from other regions. Exceptions to this general behaviour were observed in the families Cyperaceae, Scrophulariaceae, Berberidaceae and Amaryllidaceae. The most represented families in which mycorrhizal behaviour differed between species of the same family were Asteraceae, Fabaceae and Poaceae. Senecio neaei (Asteraceae) and Boopis australis (Calyceraceae) showed facultative mycorrhizal behaviour.     

Vegetation changes and human occupation in the Patagonian steppe, Argentina, during the late Holocene

Vegetation changes during the late Holocene are interpreted from four fossil pollen sequences from two caves at the Los Toldos archaeological locality, Santa Cruz province, Argentina. Taphonomic processes are particularly taken into account in order to analyze the effects on the fossil pollen records of biotic factors such as human occupation and animals, and abiotic ones such as volcanic ash fall. Fossil pollen assemblages are interpreted using local modern pollen data. The main vegetation change occurred at ca. 3750 uncal b.p., when a shrub steppe of Asteraceae subf. Asteroideae with Schinus, Ephedra frustillata and a high proportion of grasses was replaced by a shrub steppe of Colliguaja integerrima and Asteraceae subf. Asteroideae. This change is synchronous with an archaeological record change and could be related either to moderate climatic variations or the effects of ash fall on the environment. Plant communities similar to the present-day ones were established in the Los Toldos area from ca. 3750 uncal b.p.     

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.     

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.     

Seasonal changes in the thermal tolerances of the toad Rhinella arenarum (Bufonidae) in the Monte Desert of Argentina

We studied the thermal tolerances of Rhinella arenarum during the dry and wet seasons of the Monte Desert in San Juan Province, Argentina. This toad had differences in CT_max between dry and wet seasons, and the CT_max values were higher in the wet season (Austral summer). Operative temperature, body temperature, environmental maximal temperature, and relative humidity were related to CT_max, suggesting seasonal acclimatization of R. arenarum. Additionally, the CT_max recorded for R. arenarum was 36.2 °C, and the maximum ambient temperature recorded during the toads' activity time was 37 °C. Also, the CT_min recorded for R. arenarum was 5.3 °C and the minimum environmental temperature recorded was 7.2 °C. The wide thermal tolerance range recorded and the relationship between tolerance limits and the environmental extremes indicate that seasonal acclimatization is an effective mechanism by which toads can raise their thermal tolerance, allowing them to survive in the challenging conditions of the Monte Desert. Additional studies are needed to understand the relationship between the thermal tolerance of this desert amphibian and the environmental parameters that influence its thermal physiology.     

Seed supply as a limiting factor for granivorous bird assemblages in the Monte Desert,Argentina

Seed availability may limit the size of granivorous bird populations, particularly in desert environments. Seasonal and annual fluctuations in the abundance of granivorous birds were studied at three sites in the Northern Monte Desert, Argentina. The way in which these changes were related to seed supply was also studied. Granivore abundance and biomass, as well as seed abundance, were greater in winter. Changes in total granivore abundance were mostly because of variations in numbers of the most mobile bird species, all of which were found to have similar patterns of variation in their abundance. Evidence suggests that during the non‐breeding season, the most mobile species are able to track patches of high seed availability over long distances, whereas the abundance of less vagile species largely depends on the local ecological conditions. Seed supply is a proximate factor that limits the abundance of granivorous birds in the Monte Desert during the winter, but does not limit species richness. Granivore abundance was not correlated with seed supply in spring.     

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.     

Seed longevity and fire: post‐germination responses of Rumex acetosella L. in northwest Patagonian grasslands (Argentina)

The cosmopolitan herb Rumex acetosella forms persistent soil seed banks and increases in cover after fire. We investigated how the interaction between seed age and fire affects seedling growth by exposing different‐aged seeds to heat, smoke, charcoal, and ash treatments. We measured growth of germinated seedlings that were transplanted and allowed to grow for 65 days in a greenhouse. Seedlings from seeds >8 years old did not reach an appropriate radicle length for transplantation. Seedling growth decreased with increasing temperature of the heat treatment. As seed age increased, growth decreased with smoke and charcoal, and increased with ash treatment. Height was negatively correlated with seed age. Our results suggest that fire and seed age could affect demographic responses of R. acetosella seedling populations. Post‐fire recruitment could be partially favored by the positive effect of nutrient input from ash on seedling growth. High fire intensities, however, would be detrimental to seedling vigor.     

腾格里沙漠人工植被区固沙灌木影响深层土壤水分的动态模拟研究

灌木是我国沙漠地区主要的优势植物类型,固沙灌木的存在有益于沙丘的固定进而有利于退化沙漠生态系统的生态恢复。基于腾格里沙漠沙坡头地区50多年的人工植被区长期观测研究,建立生态-水文模型模拟了该地区固沙灌木盖度和深层土壤水分的动态变化过程,结果表明,固沙灌木的建立改变了沙区原有的生态-水文过程,在固沙灌木建立40多年后,固沙灌木的盖度和深层土壤水分达到了新的平衡状态。灌木盖度逐渐稳定在(10±0.9)%左右,而深层土壤水分稳定在(2.58±0.2)%左右。因此,在年均降雨量为186mm的腾格里沙漠沙坡头地区,土壤水分的最大植被承载力为:灌木和生物土壤结皮的盖度分别维持在10%和60%,深层土壤水分维持在3%左右。     

Regional productivity mediates the effects of grazing disturbance on plant cover and patch‐size distribution in arid and semi‐arid communities

Patch‐size distribution and plant cover are strongly associated to arid ecosystem functioning and may be a warning signal for the onset of desertification under changes in disturbance regimes. However, the interaction between regional productivity level and human‐induced disturbance regime as drivers for vegetation structure and dynamics remain poorly studied. We studied grazing disturbance effects on plant cover and patchiness in three plant communities located along a regional productivity gradient in Patagonia (Argentina): a semi‐desert (low‐productivity community), a shrub‐grass steppe (intermediate‐productivity community) and a grass steppe (high‐productivity community). We sampled paddocks with different sheep grazing pressure (continuous disturbance gradients) in all three communities. In each paddock, the presence or absence of perennial vegetation was recorded every 10 cm along a 50 m transect. Grazing effects on vegetation structure depended on the community and its association to the regional productivity. Grazing decreased total plant cover while increasing both the frequency of small patches and the inter‐patch distance in all communities. However, the size of these effects was the greatest in the high‐productivity community. Dominant species responses to grazing explained vegetation patch‐ and inter‐patch‐size distribution patterns. As productivity decreases, dominant species showed a higher degree of grazing resistance, probably because traits of species adapted to high aridity allow them to resist herbivore disturbance. In conclusion, our findings suggest that regional productivity mediates grazing disturbance impacts on vegetation mosaic. The changes within the same range of grazing pressure have higher effects on communities found in environments with higher productivity, markedly promoting their desertification. Understanding the complex interactions between environmental aridity and human‐induced disturbances is a key aspect for maintaining patchiness structure and dynamics, which has important implications for drylands management.     

Leaf litterfall, fine-root production, and decomposition in shrublands with different canopy structure induced by grazing in the Patagonian Monte, Argentina

Selective sheep grazing in the Patagonian Monte induces the reduction of total and perennial grass cover, species replacement within life forms, and the increase in dominance of long-lived evergreen woody plants with slow growth rates and high concentration of secondary compounds in leaves. We hypothesized that these changes in the canopy structure induced by sheep grazing will affect the mass, chemistry and decomposability of leaf litter and fine roots. We selected two sites in the Patagonian Monte, representative of ungrazed and grazed vegetation states. At each site, we assessed canopy structure (total cover and absolute and relative grass and shrub cover), monthly leaf litterfall, and fine-root biomass and production in the upper soil (15 cm). We also estimated the rates of mass, C, soluble phenolics, lignin and N decay in litterbags containing both leaf litter and fine roots of each site under field conditions during two consecutive years. The ungrazed site exhibited higher total plant cover, absolute and relative grass- and shrub-cover than the grazed one. Leaf litterfall was lower at the grazed site than at the ungrazed site. Fine-root production did not vary between sites. Leaf litter and fine root tissues had higher concentration of secondary compounds at the grazed than at the ungrazed site. However, fine roots showed lower mass and C decay than leaf litter, attributable to the predominant secondary compound (lignin and soluble phenolics, respectively). Leaf litter decomposed slower but released more N during decay at the ungrazed than at the grazed site, probably due to its low concentration of secondary compounds. We concluded that changes in canopy structure induced by grazing disturbance such as those explored in our study could reduce leaf litterfall mass and increase the concentration of secondary compounds of both leaf litter and fine roots leading to slow N release to soil during decay.     

Land degradation affects shrub growth responses to precipitation in a semiarid rangeland of north‐eastern Patagonia (Argentina)

Arid land degradation diminishes the proportion of precipitation conducted to infiltration and increases the proportion lost to run‐off and evaporation. Consequently, we expect that the effects of annual precipitation on shrub growth vary with land degradation as a result of changes in soil available water. Chuquiraga avellanedae is the dominant shrub and the main indicator of land degradation in semiarid rangelands of north‐eastern Patagonia. We chose two communities with a different degree of land degradation: an herbaceous steppe with shrubs (HSS) and a degraded shrub steppe (SS). Vegetative growth of C. avellanedae was determined nondestructively using a double‐sampling approach. Soil water content was estimated for the two communities using a soil water balance model. Linear regressions were used to evaluate the relationships between shrub growth and (i) annual precipitation and (ii) mean available water during the period of high vegetative growth in the soil layer that each plant community concentrates their roots. In SS, with elevated clay content, there were more roots of C. avellanedae in the upper layers of soil while in HSS, with coarse‐textured soil, C. avellanedae had more roots in deeper layers. Vegetative growth of C. avellanedae, both in HSS and SS communities, was positively related to annual precipitation but, for a given precipitation, C. avellanedae presented higher vegetative growth in HSS than in SS. We also found a positive relationship between vegetative growth and soil available water, and this relationship did not differ between communities. SS presented lower water availability because of lower infiltration rates. Our results showed that, irrespective of the degree of land degradation, plants respond directly to water content of the soil layers where most roots are present at a specific window of time.     

Production and turnover rates of shallow fine roots in rangelands of the Patagonian Monte, Argentina

Selective sheep grazing in arid rangelands induces a decrease in total cover and grass cover and an increase in the dominance of shrubs. Both life forms differ in aboveground and belowground traits. We hypothesized that grazing disturbance leads to the replacement of grass by shrub fine roots in the upper soil, and this is reflected in changes in the seasonal dynamics of shallow fine roots at the community level. In two sites representative of non-grazed and grazed vegetation states in the Patagonian Monte, we assessed the canopy structure, and the fine root biomass, N concentration, production, and turnover during two consecutive years. The non-grazed site exhibited higher total, grass, and shrub cover than the grazed site. The grazed site had larger or equal fine root biomass than the non-grazed site except for late spring of the second year. This could be associated with the ability of shrubs to develop dimorphic-root systems occupying the soil freed by grasses at the grazed site, and with the larger contribution of grass than shrub fine roots in relation to an extraordinary precipitation event at the non-grazed site. This was consistent with the N concentration in fine roots. Fine root production was positively correlated to temperature at the grazed site and with precipitation at the non-grazed site. Fine root turnover did not differ between sites. Our results indicate that grazing leads to a shifting in the seasonality and main climatic controls of fine root production, while fine root turnover is mostly affected by changes in soil water conditions.     

Multiple periglacial refugia in the Patagonian steppe and post‐glacial colonization of the Andes: the phylogeography of Calceolaria polyrhiza

Aim We perform a phylogeographical study of an endemic Patagonian herbaceous plant to assess whether geographical patterns of genetic variation correspond to in situ Pleistocene survival or to glacial retreat and post‐glacial expansion. We also seek to determine the locations of potential glacial refugia and post‐glacial colonization routes. Location Southern Andes and Patagonian steppe. Methods We used Calceolaria polyrhiza, a widely distributed Patagonian herbaceous plant that occurs mainly in the understorey of Nothofagus rain forests and in the arid Patagonian steppe, as our model system. The chloroplast intergenic spacer trnH–psbA was sequenced for 590 individuals from 68 populations. Sequence data were analysed using phylogenetic (maximum parsimony, maximum likelihood and Bayesian inference) and population genetic (spatial analyses of molecular variance, mismatch distributions and neutrality tests) methods. Nested clade phylogeographic analyses, and divergence time estimates using a calibrated molecular clock, were also conducted. Results A total of 27 haplotypes identified in the present study clustered into four primary genealogical lineages, revealing three significant latitudinal phylogeographical breaks. The two high Andean lineages probably split first, during the late Miocene, and the Patagonian lineage split around 4 Ma, coincident with the establishment of the Patagonian steppe. Within each haplogroup, major diversification occurred in the Pleistocene. The Patagonian groups show a pattern consistent with a rapid post‐glacial expansion and colonization of the Andean flanks, achieved independently by four lineages. The highest haplotype diversity was found along a longitudinal transect that is remarkably congruent with the limit of the ice‐sheet extension during the Greatest Patagonian Glaciation. A north‐east expansion is evident, which is probably associated with the ‘Arid Diagonal’ fluctuations. Main conclusions Glacial climate fluctuations had a substantial impact on the diversification, distribution and demography of the study species. A scenario of multiple periglacial Pleistocene refugia and subsequent multiple recolonization routes, from eastern Patagonia to the Andean flanks, may explain the phylogeographical patterns observed. However, current genetic structure also preserves the imprints of older events that probably occurred in the Miocene and Pliocene, providing evidence that multiple processes, operating at different spatial and temporal scales, have moulded biodiversity in Patagonia.     

Environmental heterogeneity and habitat use of the red viscacha rat in drylands of west-central Argentina

Environmental heterogeneity shapes the traits of natural populations, including space use and burrowing behavior. We studied the red viscacha rat (Tympanoctomys barrerae), a semifossorial rodent with patchy distribution in drylands of west-central Argentina, to provide ecological data on habitat and burrow features of this specialist species. We collected data on 165 burrows and 55 individuals from 2017–2018 and observed correlations between different vegetation types, vegetation diversity, and physical attributes (e.g., soil hardness). A high number of individual red viscacha rats was negatively associated with herbs. The number of burrow entrances was positively associated with harder soils and negatively associated with vegetation cover. Our results suggest that the species' patchy distribution is strongly associated with environmental heterogeneity of these arid environments, and consequently provide an essential step towards conserving their populations across their geographic range.     

Extraordinary abundance of hydrocorals (Cnidaria,Hydrozoa, Stylasteridae) in shallow water of the Patagonian fjord region

During two scuba-diving expeditions in 2005 and 2006, stylasterids were documented and sampled from fjords and channels in the Central Patagonian Zone, Chile. At 15 of a total of 33 sampling sites we found colonies of Errina antarctica. We discuss the observed distribution patterns and the variability of colony shapes. In some regions we found E. antarctica to occur in extraordinary high densities on primary and secondary hard substratum below 10 m. In the archipelago Madre de Dios we discovered large colonies of E. antarctica covering areas of more than 10,000 m² with coverage exceeding 80%. The dense accumulations of E. antarctica have reef-like structure and form complex habitats. At one site we observed extensive mechanical damage to the corals that may result from coral extraction by divers and/or from boat anchors. Slow growth, fragility, scarceness and the lack of knowledge on their ecology make these spectacular and unique biocenoses very susceptible for damages and require protection and further studies.