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     

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.     

Resource partitioning between shrubs and grasses in the Patagonian steppe

Summary Experiments were conducted in the Patagonian steppe in southern South America to test the following hypotheses: (a) grasses take up most of the water from the upper layers of the soil and utilize frequent and short-duration pulses of water availability; (b) shrubs, on the contrary, take up most of the water from the lower layers of the soil and utilize infrequent and long-duration pulses of water availability. Grasses and shrubs were removed selectively and the performance of plants and the availability of soil resources were monitored. Results supported the overall hypothesis that grasses and shrubs in the Patagonian steppe use mainly different resources. Removal of shrubs did not alter grass production but removal of grasses resulted in a small increase in shrub production which was mediated by an increase in deep soil water and in shrub leaf water potential. The efficiency of utilization of resources freed by grass removal was approximately 25%. Shrubs used water exclusively from lower soil layers. Grasses took up most of the water from upper layers but they were also capable of absorbing water from deep layers. This pattern of water partitioning along with the lack of response in leaf nitrogen to the removal treatments suggested that shrubs may be at a disadvantage to grasses with respect to nutrient capture and led to questions about the role of nutrient recirculation, leaching, and nitrogen fixation in the steppe.     

Plant functional types and ecological strategies in Patagonian forbs

Abstract. We identified four major functional types of forbs in the Patagonian steppe, taking into account phenological and morphological traits: (1) shallow-rooted mesophytic species (annuals), (2) shallow-rooted non-mesophytic species, (3) deep-rooted evergreen species, and (4) deep-rooted deciduous species. The major attributes differentiating these groups were the date at which seasonal growth ended, rooting depth, sprouting depth, distance between shoots of the same plant, and degree of ‘mesophytism’. We used Cluster and Principal Components Analyses to identify the groups, and the attributes determining them. Late-growth-cycle types had deep roots and/or high ‘xerophytism’. Late-cycle-xerophytic types had a great sprouting depth, and late-cycle-deep-rooted types had a great distance between shoots of the same plant. On the basis of current knowledge of the structure and functioning of the Patagonian steppe, we suggested three explanations to account for these correlations. 1. Late-cycle forbs survive summer water deficit if they have xerophytic characteristics that reduce transpiration water losses, and/or they have deep roots that increase water uptake. 2. Sprouting depth results from the shift of active buds to dormant buds at the end of the cycle. Summer forbs have a great sprouting depth because only buds which are located deep in the soil survive hot and dry summers. 3. Distant shoots of summer forbs allow them simultaneously to use the high protection against desiccating winds provided by shrubs, and the ample water availability of bare soil patches. All the functional types of forbs depend on winter water recharge to begin their cycles, but each one completes its cycle by using a different portion of the water resources available in spring and summer.     

Differential use of large summer rainfall events by shrubs and grasses: a manipulative experiment in the Patagonian steppe

In the Patagonian steppe, years with above-average precipitation (wet years) are characterized by the occurrence of large rainfall events. The objective of this paper was to analyze the ability of shrubs and grasses to use these large events. Shrubs absorb water from the lower layers, grasses from the upper layers, intercepting water that would otherwise reach the layers exploited by shrubs. We hypothesized that both life-forms could use the large rainfalls and that the response of shrubs could be more affected by the presence of grasses than vice versa. We performed a field experiment using a factorial combination of water addition and life-form removal, and repeated it during the warm season of three successive years. The response variables were leaf growth, and soil and plant water potential. Grasses always responded to experimental large rainfall events, and their response was greater in dry than in wet years. Shrubs only used large rainfalls in the driest year, when the soil water potential in the deep layers was low. The presence or absence of one life-form did not modify the response of the other. The magnitude of the increase in soil water potential was much higher in dry than in humid years, suggesting an explanation for the differences among years in the magnitude of the response of shrubs and grasses. We propose that the generally reported poor response of deep-rooted shrubs to summer rainfalls could be because (1) the water is insufficient to reach deep soil layers, (2) the plants are in a dormant phenological status, and/or (3) deep soil layers have a high water potential. The two last situations may result in high deep-drainage losses, one of the most likely explanations for the elsewhere-reported low response of aboveground net primary production to precipitation during wet years. Received: 23 January 1997 / Accepted: 19 November 1997     

Refuse dumps from leaf-cutting ant nests reduce the intensity of above-ground competition among neighboring plants in a Patagonian steppe

In arid environments, the high availability of sunlight due to the scarcity of trees suggests that plant competition take place mainly belowground for water and nutrients. However, the occurrence of soil disturbances that increase nutrient availability and thereby promote plant growth may enhance shoot competition between neighboring plants. We conducted a greenhouse experiment to evaluate the influence of the enriched soil patches generated by the leaf-cutting ant, Acromyrmex lobicornis, on the performance of the alien forb Carduus thoermeri (Asteraceae) under different intraspecific competition scenarios. Our results showed that substrate type and competition scenario affected mainly aboveground plant growth. As expected, plants growing without neighbors and in nutrient-rich ant refuse dumps showed more aboveground biomass than plants growing with neighbors and in nutrient-poor steppe soils. However, aboveground competition was more intense in nutrient-poor substrates: plants under shoot and full competition growing in the nutrient-rich ant refuse dumps showed higher biomass than those growing on steppe soils. Belowground biomass was similar among focal plants growing under different substrate type. Our results support the traditional view that increments in resource availability reduce competition intensity. Moreover, the fact that seedlings in this sunny habitat mainly compete aboveground illustrates how limiting factors may be scale-dependent and change in importance as plants grow.     

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.     

Effects of flow regulation, basin characteristics and land-use on macroinvertebrate communities in a large arid Patagonian river

Longitudinal distribution and abundance of macroinvertebrate communities were examined in relation to hydrochemical variables along the Chubut River in the Patagonian Precordillera and Plateau, Argentina. The Chubut River (>1000 km) is the largest river in the area and its basin is subject to multiple uses: agriculture, cattle raising, urbanization and the hydrological regime of the lower section is modified by a reservoir. Quantitative benthic samples were collected at 13 sites in the higher, middle and lower sections of the river basin. Sites were visited four times during 2004 and physicochemical parameters, chlorophyll a and particulate organic matter (POM) were assessed. Ninety-five taxa were collected in the study, with total species richness per site ranging from 5 to 51, and benthos density averaging 299–5024 ind m⁻². Altitude and turbidity were implicated as important factors determining macroinvertebrate assemblages along the river system, and an eutrophication gradient was documented in the regulated/urbanized section of the main river. High turbidity (TSS) and sedimentation limited algal productivity in the middle basin. Below the dam, TSS, total phosphorus (TP) and POM decreased, whereas soluble reactive phosphorus (SRP) and chlorophyll a increased. Macroinvertebrate density increased three fold in this area possibly due to habitat improvement and enhanced trophic resources. Mean species richness did not change below the impoundment; however the community was dominated by gastropods, chironomids and flatworms. The Chubut River is complex and its biotic community reflects the landscape attributes. While benthic composition and density was governed by turbidity and flood disturbance in some river segments, a greater environmental heterogeneity resulted in an unexpected high number of species at the main channel upper basin.     

中国特有濒危植物伯乐树根的生态解剖学研究

伯乐树（Bretschneidera sinensis Hemsl.）是中国特有濒危的第三纪孑遗植物。本研究采用石蜡切片、整体封片、扫描电镜和激光共聚焦扫描等技术研究其根的表面特征和形态结构,从生态解剖学角度揭示其对生境的适应和特殊要求。结果显示伯乐树作为特殊的菌根型木本植物,根尖表面无根毛分化;初生结构包括表皮、皮层和中柱三部分,系原始的发育类型,其皮层明显排列为两轮且存在自然的间隙,皮层内有含黑芥子酶的分泌细胞,中柱内有髓。菌根菌以单菌丝或菌丝网侵入根表皮,并刺激表皮分泌沉积了较厚的无定形物质,入侵后可在皮层间隙内大量分枝,还能进一步形成泡囊结构;菌根菌在寄主细胞内常包围在淀粉粒的周围吸收营养,同时部分菌丝在细胞内被分解形成小泡和碎屑,为寄主细胞提供营养以形成共生关系。根据以上特征,应在就地保护和迁地保育中深入研究适宜伯乐树的土壤条件,以促进菌根的发生和发育。     

Seedling recruitment in a semi‐arid Patagonian steppe: Facilitative effects of refuse dumps of leaf‐cutting ants

Question: What is the influence of refuse dumps of leaf‐cutting ants on seedling recruitment under contrasting moisture conditions in a semi‐arid steppe? Location: Northwestern Patagonia, Argentina. Methods: In a greenhouse experiment, we monitored seedling recruitment in soil samples from refuse dumps of nests of the leaf‐cutting ant Acromyrmex lobicornis and non‐nest sites, under contrasting moisture conditions simulating wet and dry growing seasons. Results: The mean number of seedling species and individuals were higher in wet than in dry plots, and higher in refuse dump plots than in non‐nest soil plots. The positive effect of refuse dumps on seedling recruitment was greater under low moisture conditions. Both the accumulation of discarded seeds by leaf‐cutting ants and the passive trapping of blowing‐seeds seems not explain the increased number of seeds in refuse dumps. Conversely, refuse dumps have higher water retention capacity and nutrient content than adjacent non‐nest soils, allowing the recruitment of a greater number of species and individual seedlings. Conclusions: Nests of A. lobicornis may play an important role in plant recruitment in the study area, allowing a greater number of seedlings and species to be present, hence resulting in a more diverse community. Moreover, leaf‐cutting ant nests may function as nurse elements, generating safe sites that enhance the performance of neighbouring seedlings mainly during the driest, stressful periods.     

Community-wide germination strategies in an alpine meadow on the eastern Qinghai-Tibet plateau: phylogenetic and life-history correlates

In this study, we built up a database of 633 species (48 families, 205 genera) from an alpine meadow on the eastern Qinghai-Tibet plateau. Our objective was to assess the effects of phylogenetic and life-history (life form, perenniality, seed size, dispersal strategy and period) background on the community-wide germination strategies. We found that the seeds of shrubs, perennials, and well-dispersed plants, and the smaller seeds germinated more and comparatively earlier. In one-way ANOVAs, phylogenetic groups explained 12% of the variance in GT (mean germination time for all seeds germinated of each species); life-history attributes, such as seed size, dispersal strategy, perenniality and life form explained 10%, 7%, 5%, and 1% respectively, and dispersal period had no significant effect on GT. Multifactorial ANOVAs revealed that the three major factors contributing to differences in GT were phylogenetic relatedness, seed size and dispersal strategy (explained 4%, 5% and 4% of the interspecific variation independently, respectively). Thus, seeds germination strategies were significantly correlated with phylogenetic and life-history relatedness. In addition, phylogenetic relatedness had close associations and interactions with seed size and dispersal strategy. Then, we think phylogeny and life-history attributes could not be considered mutual exclusively. Seed germination, like any other trait, is shaped by the natural history of the species and by the evolutionary history of the lineage. And a large percentage of the variance remained unexplained by our model, which suggested important selective factors or parameters may have been left out of this analysis. Electronic Supplementary Material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Ontogenetic shifts in interactions of two dominant shrub species in a semi‐arid coastal sand dune system

Question: How does the interaction between two dominant shrub species in a coastal sand dune community change during their life history? Does this interaction influence their population dynamics? Location: A semiarid coastal sand dune system in southeast Spain. Methods: For 3 years we monitored physiological status, growth and reproductive effort of Juniperus phoenicea and Pistacia lentiscus, the dominant shrub species, growing either alone or in close spatial association. We also recorded adult mortality patterns and characterized seedling survival, soil properties and microclimate conditions beneath canopies and in bare ground. Results and conclusions: There was a strong bi‐directional interaction between the two studied species, with a net balance that changed in sign with increasing plant development. While mature individuals facilitated the establishment of seedlings of both species, adult mortality patterns suggested asymmetric competition at later life stages. The interaction with Pistacia negatively affected growth of juniper and contributed to its high mortality rates, while juniper had almost no effect on mature Pistacia individuals. Physiological data suggested that Pistacia had a competitive advantage over juniper, most likely because of differences in rooting patterns and tolerance to salinity, which may determine the source of water available for each species. Community dynamics are governed by facilitation at the seedling stage and shaped by differences in physiological traits in adult plants. Plant‐plant interactions, which are strongly affected by environmental gradients, are important drivers of community dynamics in this system.     

城市污水土地处理系统中优先有机污染物的生态行为及其调控对策

城市污水生态工程土地处理系统是一个多功能、多目标的净化污水,保护水体实现废水资源化的适用技术系列。如果条件合适,设计合理,科学运行和管理得当,它能对发展经济和保护环境起到积极的作用。应当指出,优先有机污染物及其生态环境问题可能成为推广应用生态工程土地处理系统的一个限制因素。本文从理论与实践的结合上论述净化功能、生态效应和生态风险评价等一系列复杂的基本性问题,主要目的是为制定生态工程土地处理系统优先有机污染物的调控对策提供科学依据。     

Soil as a mediator in plant‐plant interactions in a semi‐arid community

Abstract. Competition and facilitation may occur simultaneously in plant communities, and the prevalence of either process depends on abiotic conditions. Here we attempt a community‐wide approach in the analysis of plant interactions, exploring whether in a semi‐arid environment positive or negative interactions predominate and whether there are differences among co‐occurring shrub species. Most shrubs in our plot exerted significant effects on their understorey communities, ranging from negative to positive. We found a clear case of interference and another case where the effect was neutral, but facilitation predominated and the biomass of annuals under most shrubs in our community was larger than in gaps. Effects on soil water and fertility were revealed as the primary source of facilitation; the build‐up of soil organic matter changed soil physical properties and improved soil water relations. Facilitation by shrubs involved decoupling of soil temperature and moisture. Sheltering from direct radiation had an effect on productivity, but significant differences in understorey biomass did not parallel understorey light environment. A positive balance of the interaction among plants, essentially mediated by changes in soil properties, is the predominant outcome of plant interactions in this semi‐arid community.     

Seasonal nitrogen uptake strategies in a temperate desert ecosystem depends on N form and plant species

• Symbiotic plants might be able to regulate a limited nitrogen (N) pool, thus avoiding and reducing competition for resources, through the uptake of different chemical N forms. Our aim was to see whether coexisting herbs showed preference for different forms of N in a temperate desert.
• We conducted a situ experiment using the ¹⁵N labeling method in the Gurbantunggut Desert of Northwestern China dominated by Erodium oxyrrhynchum, Hyalea pulchella, Nonea caspica and Lactuca undulata during their growing period (April and May).
• Four desert herb species preferentially relied on ¹⁵N‐NO₃ for their N nutrition. Multi‐factor analysis of variance (ANOVA) analysis results showed that species, N forms, months, and soil depths strongly affected N uptake rate. The uptake rate by herbs was higher in May than in April, and higher at 0–5 cm than at 5–15 cm soil layers. Erodium oxyrrhynchum, N. caspica and L. undulata showed different preference on N form over months. Erodium oxyrrhynchum and L. undulata changed their uptake preference from more ¹⁵N‐Glycine in April to more ¹⁵N‐NH4 in May.
• Although the N uptake rate of four desert herbs varied across different soil depths and months, all species absorbed more inorganic N compared with organic N. The higher preference for ¹⁵N‐NO₃ and ¹⁵N‐NH4 over ¹⁵N‐Gly possibly reflects adaptation to different N forms in temperate desert.

     

Relative performance of clumped vs. experimentally isolated plants in a South African winter-rainfall desert community

We conducted a factorial removal experiment in the arid strandveld onthe west coast of South Africa to test the hypothesis that perennial specieswith a preference for occurring in multi-species clumps shouldderive benefits from their association into clumps. Contrary to our hypothesis,we obtained evidence of competition for water in the clumped non-succulentspecies studied in the form of depressed water potentials. We were not able todetect any effect on leaf water contents associated with isolation, suggestingthat clumped plants are able to compensate physiologically in response tocompetitive stress. We also found that isolating individuals had no effect(positive or negative) on branch growth. Finally, we showed that isolatingindividuals exposed them to a far greater risk of damage by wind or animals. Inlight of these results we conclude that the spatial arrangement of plants inthis community does suggest a situation where the benefits associated withoccurring in clumps exceed any competitive costs.     

Reproductive biology in a tropical shrubland of Venezuelan Guayana

Plant sexual systems were evaluated in a tropical shrubland community of Venezuelan Guayana. Of 89 plant species, 73.0% were hermaphrodite, 12.4% monoecious, 10.1 % submonoecious, and 4.5 % dioecious. Temporal variation of sexual expression showed that hermaphrodite species were mostly adichogamous and monoecious, submonoecious species dichogamous (protandrous andprotogynous). Herkogamy was an important mechanism promoting cross-pollination. Of 38 hermaphrodite species, 39.5 % were herkogamous. Plant sexual systems were not associated with life form. Most species were pollinated mostly by large and medium-size bees. Pollinator specificity and pollen load transportation, were higher for monoecious species than for hermaphrodite, submonoecious and dioecious ones. Both pollinator specificity and pollen transportation were not significantly different among representatives of the latter three sexual systems. Plant sexual systems were significantly associated with dispersal syndromes; dioecious species had bird-dispersal syndromes, the other sexual systems mostly anemochory, epizoochory and endozoochory; but many hermaphrodites were anemochorous.     

Pattern and process in Norwegian upland grasslands: a functional analysis

Abstract. Four classes of functional and morphological plant traits – established strategies (the CSR scheme sensu Grime 1979), life‐forms (sensu Raunkiaer 1934), morphology, and regenerative strategies – are used as tools for explaining vegetation gradients at summer farms in the mountains of western Norway. These farms are assembly points for free‐ranging domestic grazers, and differ floristically and ecologically from the surrounding heath or woodland vegetation. DCA and TWINSPAN are used to relate major gradients in a floristic data set from 12 summer farms to two sets of explanatory variables: (1) environmental variables representing physical factors, plot position, soils, and land use, and (2) the 4 classification schemes. The main floristic gradient parallels a spatial gradient from the centres of the farms to the surrounding vegetation. A functional interpretation based on the concurrent use of the 2 sets of explanatory variables suggests that the gradient is one of decreasing disturbance and increasing environmental stress caused by decreasing grazing and manure effects away from farms. Partial CCA is used to investigate the relationships between the 4 functional/morphological plant trait classes. The 4 classification schemes are partially redundant, and do not represent different trends of specialization within the landscape. There is no strong evidence for decoupling of the traits of the vegetative and regenerative phases within the data. The combination of general process‐based theories and specific plant attribute responses enhances the generality and interpretability of the study.     

Distribution of plant species at a biome transition zone in New Mexico

Question: Is there a difference in plant species and life form composition between two major patch types at a biome transition zone? Are subordinate species associated with different patch types at the shortgrass steppe — Chihuahuan desert grassland transition zone? Is this association related to differences in soil texture between patch types and the geographic range of associated species? Location: central New Mexico, USA. Methods: Patches dominated by either Bouteloua gracilis, the dominant species in the shortgrass steppe, or Bouteloua eriopoda, dominant species in the Chihuahuan desert grasslands, were sampled for the occurrence of subordinate species and soil texture within a 1500‐ha transitional mosaic of patches. Results: Of the 52 subordinate species analysed, 16 species were associated with B. gracilis‐dominated patches and 12 species with B. eriopoda‐dominated patches. Patches dominated by B. gracilis were richer in annual grasses and forbs, whereas patches dominated by B. eriopoda contained more perennials forbs and shrubs. Soils of B. gracilis‐dominated patches had higher clay and lower rock contents compared with soils of B. eriopoda‐dominated patches. Differences in species characteristics of the dominant species as well as differences in soil texture between patch types contribute to patch‐scale variation in composition. The association of species to patch types was not related to their geographic range and occurrence in the adjacent biomes. Conclusions: Patch types at this biome transition zone have characteristic life‐form and species composition, but species are associated to patch types due to local constraints, independently from their affinity to the adjacent biomes.