Effects of monsoon precipitation variability on the physiological response of two dominant C4 grasses across a semiarid ecotone

For the southwestern United States, climate models project an increase in extreme precipitation events and prolonged dry periods. While most studies emphasize plant functional type response to precipitation variability, it is also important to understand the physiological characteristics of dominant plant species that define plant community composition and, in part, regulate ecosystem response to climate change. We utilized rainout shelters to alter the magnitude and frequency of rainfall and measured the physiological response of the dominant C₄ grasses, Bouteloua eriopoda and Bouteloua gracilis. We hypothesized that: (1) the more drought-adapted B. eriopoda would exhibit faster recovery and higher rates of leaf-level photosynthesis (A _net) than B. gracilis, (2) A _net would be greater under the higher average soil water content in plots receiving 30-mm rainfall events, (3) co-dominance of B. eriopoda and B. gracilis in the ecotone would lead to intra-specific differences from the performance of each species at the site where it was dominant. Throughout the study, soil moisture explained 40–70 % of the variation in A _net. Consequently, differences in rainfall treatments were not evident from intra-specific physiological function without sufficient divergence in soil moisture. Under low frequency, larger rainfall events B. gracilis exhibited improved water status and longer periods of C gain than B. eriopoda. Results from this study indicate that less frequent and larger rainfall events could provide a competitive advantage to B. gracilis and influence species composition across this arid–semiarid grassland ecotone.     

Semiarid grassland responses to short-term variation in water availability

Standing crop and species composition in semiarid grassland are linked to long-term patterns of water availability, but grasslands are characterized by large single-season variability in rainfall. We tested whether a single season of altered water availability influenced the proportions of grasses and shrubs in a semiarid grassland near the northern edge of the North American Great Plains. We studied stands of the clonal shrub snowberry (Symphoricarpos occidentalis) and adjacent grassland dominated by the native grasses Stipa spartea and Bouteloua gracilis. Rain was excluded and water supplied in amounts corresponding to years of low, medium, and high rainfall, producing a 2 − 4-fold range in monthly precipitation among water supply treatments. There were ten replicate plots of each water treatment in both snowberry stands and grassland. Grass standing crop increased significantly with water availability in grassland but not inside snowberry stands. Total standing crop and shrub stem density increased significantly with water supply, averaged across both communities. In contrast, water had no effect on shrub standing crop or light penetration. In summary, our finding that water has significant effects on a subset of components of grassland vegetation is consistent with long-term, correlational studies, but we also found that a single season of altered water supply had no effect on other important aspects of the ecosystem. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effect of precipitation variability on net primary production and soil respiration in a Chihuahuan Desert grassland

Precipitation regimes are predicted to become more variable with more extreme rainfall events punctuated by longer intervening dry periods. Water‐limited ecosystems are likely to be highly responsive to altered precipitation regimes. The bucket model predicts that increased precipitation variability will reduce soil moisture stress and increase primary productivity and soil respiration in aridland ecosystems. To test this hypothesis, we experimentally altered the size and frequency of precipitation events during the summer monsoon (July through September) in 2007 and 2008 in a northern Chihuahuan Desert grassland in central New Mexico, USA. Treatments included (1) ambient rain, (2) ambient rain plus one 20 mm rain event each month, and (3) ambient rain plus four 5 mm rain events each month. Throughout two monsoon seasons, we measured soil temperature, soil moisture content (θ), soil respiration (R_s), along with leaf‐level photosynthesis (A_net), predawn leaf water potential (Ψ_pd), and seasonal aboveground net primary productivity (ANPP) of the dominant C₄ grass, Bouteloua eriopoda. Treatment plots receiving a single large rainfall event each month maintained significantly higher seasonal soil θ which corresponded with a significant increase in R_s and ANPP of B. eriopoda when compared with plots receiving multiple small events. Because the strength of these patterns differed between years, we propose a modification of the bucket model in which both the mean and variance of soil water change as a consequence of interannual variability from 1 year to the next. Our results demonstrate that aridland ecosystems are highly sensitive to increased precipitation variability, and that more extreme precipitation events will likely have a positive impact on some aridland ecosystem processes important for the carbon cycle.     

Climatic variation and simulated patterns in seedling establishment of two dominant grasses at a semi‐arid‐arid grassland ecotone

Abstract. The objective of this study was to predict the effects of climatic variation at multiple temporal frequencies on seedling establishment by two congeneric C₄ perennial grasses (Bouteloua gracilis and B. eriopoda) at the ecotone between shortgrass steppe grassland and Chihuahuan desert grassland in central New Mexico, USA. The approach was to use a daily time‐step simulation model to determine the occurrence of a recruitment event in each year based upon the amount and timing of soil water required for establishment. Historical weather data were used to predict effects of seasonal and inter‐decadal variation in climate on establishment. A sensitivity analysis was used to predict effects of directional climate change on establishment. Bouteloua gracilis had a broad pattern of simulated establishment from May through September that included periods with high year‐to‐year variation in precipitation. B. eriopoda establishment events occurred primarily in July when precipitation amounts were most reliable. Climatic conditions from 1949 through 1968 were more favorable for B. eriopoda establishment compared to the cooler, wetter conditions from 1969 through 1988 that favored B. gracilis. Establishment of B. eriopoda was lowest in El Niño years whereas B. gracilis establishment was highest in La Niña years. Establishment of B. gracilis was most sensitive to temperature when precipitation was higher than current amounts. The greatest response to temperature by B. eriopoda for all precipitation amounts occurred at cooler temperatures than found currently. These results indicate that climatic variation at multiple frequencies has differential effects on seedling establishment for these two perennial grasses, and may account at least in part for patterns in dominance at this biome transition zone.     

The performance of native and non-native turfgrass monocultures and native turfgrass polycultures: An ecological approach to sustainable lawns

As environmental impacts and life-cycle costs (water use, water, fertilizer, pesticides and mowing) of the constructed landscape come under increasing scrutiny, the development of methods to reduce resource inputs for managed turf are increasingly important. While the use of a number of native turfgrass species as alternatives to non-natives has been previously examined, only a few are presently commercially available, and many species have been perceived as failing to perform to the level required by industry and consumers. Furthermore, the use of polycultures of native turfgrasses has not been extensively investigated, but could offer an alternative to the conventional lawn. We hypothesize that because native short grass species would be expected to be well-adapted to climate and soil conditions in areas with infrequent and erratic rainfall, their performance in terms of drought resistance, resilience to disturbance, and efficient resource use, will exceed that of a non-native species. Moreover, ecological theory predicts that a polyculture of coexisting native species may exhibit additional performance benefits. One native (Bouteloua dactyloides) and one commonly-used, non-native monoculture (Cynodon dactylon) and two native polyculture assemblages (two and seven species including B. dactyloides, Bouteloua gracilis, Bouteloua rigidiseta, Hilaria belangeri, Erioneuron pilosum, Bouteloua hirsuta, Sporobolus vaginiflorus) were compared for leaf density and weed resistance under two irrigation regimens, traffic regimes and mowing intensities. The native turfgrass and turfgrass assemblages generally had 30% higher leaf densities in the early growing season, and up to 50% lower weed density than the non-native monoculture. There was no difference among species tested in response to traffic or irrigation regimens, suggesting that all assemblages could be subjected to both higher traffic frequencies and reduced water inputs. Even though native species can have slower growth rates compared to the non-native species they maintained a higher or equivalent leaf density than the non-native turfgrass. This is a potentially important finding because of the large impact that mowing has on the total carbon footprint of turfgrass, and the corresponding reduction in that carbon footprint that could be realized through a reduced mowing frequency and corresponding reduction in life-cycle cost. This study suggests that regionally adapted native grass species are worth investigating as mono- and polycultures for performance advantages and lower resource inputs, and consequently may be suitable alternatives to conventional non-native turfgrasses in many applications.     

The significance of episodic rains for reproductive phenology and productivity of trees in semiarid regions of northwestern Venezuela

The semiarid regions of northwestern Venezuela have extremely low and highly unpredictable precipitation, yet these conditions support species with contrasting phenology and leaf longevity. Episodic rains significantly increased leaf water potential (from –5 to –2.5 MPa) in several species and, in some cases, triggered flowering, leading us to hypothesize that the coexistence of species with contrasting phenology is due to differences in their ability to utilize small rainfall events. Irrigation treatments were used to simulate brief rainfall events, and the response of three species (Erythrina velutina [deciduous], Croton heliaster [semideciduous], and Capparis odoratissima [evergreen]) was monitored over a period of 14 months. To partition the effects of water reaching the canopy versus the soil, irrigation was supplied either in the form of mist to the canopy or by minisprinklers near the base of the trees. Nonirrigated trees were used as controls. Productivity (estimated as aboveground litter production) and water potential were enhanced by soil irrigation in two species. However, in the evergreen species canopy irrigation had a greater effect on water relations and productivity than soil irrigation, as indicated by higher predawn water potential, higher total annual flower (40 g m^–2 year^–1) and fruit (5 g m^–2 year^–1) production, and longer leaf longevity (410 days in control trees versus 520 days in canopy-irrigated trees). Canopy irrigation augmented flower and fruit production in all three species. Our findings suggest that reproductive phenology in these species is driven by episodic rains and that evergreen species may sustain productivity by their ability to make use of water deposited on leaf surfaces.     

Species trait syndrome drives the leaves’ functional variations of dominant grasses to modifications in summer water supply

Climate change models predict a strong reduction of average precipitation, especially of the summer rainfall, and an increase in intensity and frequency of drought events in the Mediterranean region. The research aim was to understand how four dominant grass species (Arrhenatherum elatius, Cynosurus cristatus, Elymus repens, and Lolium perenne) in sub-Mediterranean meadows (central Apennines, Italy) modulate their resource acquisition and conservation strategies to short-term variation of the pattern of summer water supply. During summer 2016, using a randomized block design, we tested the effect of three patterns of summer water supply, differing in water amount and watering frequency, on leaf area, leaf dry mass, specific leaf area (SLA), leaf senescence, and plant height. Our results showed that dominant grass species can modulate their strategies to variation of the pattern of summer water supply, but the response of leaf traits and plant height is mediated by the set of functional characteristics of the species. E. repens and A. elatius, with summer green leaves, lower SLA, later flowering period, and deeper roots, were less influenced by changes in water amount. C. cristatus and L. perenne, which display acquisitive strategies (persistent leaves, higher SLA values), earlier flowering, and shallower roots were more influenced by changes in the pattern of summer water supply. Our results suggest that a short-term decrease in water availability might affect primarily species with trait syndromes less adapted to face summer drought.

     

Effect of long-term nitrogen fertilization on mycorrhizal fungi associated with a dominant grass in a semiarid grassland

We studied the diversity of arbuscular mycorrhizal fungi (AMF) in semiarid grassland and the effect of long-term nitrogen (N) fertilization on this fungal community. Root samples of Bouteloua gracilis were collected at the Sevilleta National Wildlife Refuge (New Mexico, USA) from control and N-amended plots that have been fertilized since 1995. Small subunit rDNA was amplified using AMF specific primers NS31 and AM1. The diversity of AMF was low in comparison with other ecosystems, only seven operational taxonomic units (OTU) were found in B. gracilis and all belong to the genus Glomus. The dominant OTU was closely related to the ubiquitous G. intraradices/G. fasciculatum group. N-amended plots showed a reduction in the abundance of the dominant OTU and an increase in AMF diversity. The greater AMF diversity in roots from N-amended plots may have been the result of displacement of the dominant OTU, which facilitated detection of uncommon AMF. The long-term implications of AMF responses to N enrichment for plant carbon allocation and plant community structure remain unclear.     

内蒙古半干旱草原土壤水分对降水格局变化的响应

在全球气候变化背景下, 未来我国北方半干旱地区的降水格局将呈现出季节与年际间降水波动增强和极端降水事件增加的趋势。水分是半干旱草原的主要限制因子, 降水格局变化导致的土壤水分状况的改变必然对生态系统的结构和功能产生显著的影响。该研究选取内蒙古多伦和锡林浩特两个典型半干旱草原群落, 通过分析2006-2013年的降水和多层次土壤(0-10 cm, 10 cm, 20 cm, 30 cm和50 cm)含水量连续观测数据, 研究降水格局变化对土壤水分状况及其垂直分布的影响, 特别是土壤水分对降水事件的脉冲响应过程。结果表明: 两个站点的土壤含水量均呈现显著的季节及年际间波动, 其中土壤表层 0-10 cm水分波动更剧烈。锡林浩特50 cm处土壤含水量波动较大, 主要由于春季融雪的影响。年际间多伦和锡林浩特生长季土壤表层0-10 cm土壤含水量与降水量存在显著的正相关关系, 下层(10-50 cm)土壤含水量与降水量相关性不显著。研究发现小至2 mm的降水事件就能够引起两个站点表层0-10 cm土壤含水量的升高, 即该地区有效降水为日降水量> 2 mm。表层0-10 cm土壤含水量对独立降水事件的脉冲响应可通过指数方程很好地拟合。降水事件的大小决定了降水后表层0-10 cm土壤含水量的最大增量和持续时间, 同时这个脉冲响应过程还受到降水前土壤含水量的影响, 但该过程中并未发现植被因子(叶面积指数)的显著影响。降水后水分下渗深度及该深度的土壤含水量增量主要由降水事件的大小主导, 同时受到降水前土壤含水量的影响。在多伦和锡林浩特, 平均每增加1 mm降水, 下渗深度分别增加1.06和0.79 cm。由此作者认为, 在内蒙古半干旱草原, 降水事件大小和降水前土壤干湿状况是影响土壤水分对降水响应的主要因素, 而植被因子的影响较小。     

Responses of soil moisture to precipitation pattern change in semiarid grasslands in Nei Mongol,China

Aims Under global climate change, precipitation patterns were predicted to change with larger seasonal and annual variations and more extreme events in the semiarid regions of northern China. Water availability is one of the key limited factors in semiarid grasslands. Changes in precipitation patterns will inevitably affect ecosystem structure and function through soil water condition. Our objective was to investigate the response of soil water content to changes of precipitation pattern, especially its pulse response to precipitation events.
Methods Two semiarid steppe sites (Duolun and Xilinhot) in Nei Mongol were chosen and meteorological stations were installed to monitor precipitation and soil volumetric water content (VWC) at five soil depths (0-10 cm, 10 cm, 20 cm, 30 cm, 50 cm) from 2006 to 2013. The pulse response of VWC at 0-10 cm to an individual precipitation event was simulated by an exponential equation.
Important findings Significant seasonal and inter-annual variations of VWC were observed at the Duolun and Xilinhot sites. VWC at 50 cm soil layer in Xilinhot showed an obvious increase during the early spring due to the influences of snow melting. Mean surface (0-10 cm soil layer) VWC was significantly correlated with annual precipitation across eight years, but VWC in the deeper soil layers (10-50 cm) were not impacted by precipitation. We also found that the precipitation event larger than 2 mm could induce a significant increase in surface (0-10 cm soil layer) VWC, and could be regarded as an effective precipitation in this region. The maximum increment of surface VWC after the events and lasting time (T_lasting) were determined by the event size, while showed negatively linear correlations with the initial soil water content before the events. Vegetation development (leaf area index) did not show significant impacts on the responses of surface soil moisture to precipitation pulses. The infiltration depth of rain water was also determined by rain size and pre-event soil moisture. In average, soil water can infiltrate 1.06 cm and 0.79 cm deeper in Duolun and Xilinhot with 1 mm more precipitation, respectively. Therefore, our results suggest that the event size and pre-event soil moisture were the most important factors affecting response patterns of soil moisture to rain events in semiarid ecosystems.     

Disproportionate effects of non‐colonial small herbivores on structure and diversity of grassland dominated by large herbivores

The response of semiarid grasslands to small, non‐colonial herbivores has received little attention, focusing primarily on the effects of granivore assemblages on annual plant communities. We studied the long‐term effects of both small and large herbivores on vegetation structure and species diversity of shortgrass steppe, a perennial semiarid grassland considered marginal habitat for small mammalian herbivores. We hypothesized that 1) large generalist herbivores would affect more abundant species and proportions of litter‐bare ground‐vegetation cover through non‐selective herbivory, 2) small herbivores would affect less common species through selective but limited consumption, and 3) herbivore effects on plant richness would increase with increasing aboveground net primary production (ANPP). Plant community composition was assessed over a 14‐year period in pastures grazed at moderate intensities by cattle and in exclosures for large (cattle) and large‐plus‐small herbivores (additional exclusion of rabbits and rodents). Exclusion of large herbivores affected litter and bare ground and basal cover of abundant, common and uncommon species. Additional exclusion of small herbivores did not affect uncommon components of the plant community, but had indirect effects on abundant species, decreased the cover of the dominant grass Bouteloua gracilis and total vegetation, and increased litter and species diversity. There was no relationship between ANPP and the intensity of effects of either herbivore body size on richness. Exclusion of herbivores of both body sizes had complementary and additive effects which promoted changes in vegetation composition and physiognomy that were linked to increased abundance of tall and decreased abundance of short species. Our findings show that small mammalian herbivores had disproportionately large effects on plant communities relative to their small consumption of biomass. Even in small‐seeded perennial grasslands with a long history of intensive grazing by large herbivores, non‐colonial small mammalian herbivores should be recognized as an important driver of grassland structure and diversity.     

Geographic patterns of simulated establishment of two Bouteloua species: implications for distributions of dominants and ecotones

Abstract. Our overall objective was to use a soil water model to predict spatial patterns in germination and establishment of two important perennial C₄-bunchgrasses across the North American shortgrass steppe and desert grassland regions. We also predicted changes in establishment patterns under climate change scenarios. Bouteloua gracilis dominates the shortgrass steppe from northeastern Colorado to southeastern New Mexico. Bouteloua eriopoda dominates desert grasslands in central and southern New Mexico. Germination and establishment for each species were predicted at 16 sites along the gradient using a daily time step, multi-layer soil water model (SOILWAT) to determine the percentage of years that temperature and soil water criteria for germination and establishment were met. Percentage of years with predicted establishment decreased from north to south for B. gracilis, but increased from north to south for B. eriopoda, comparable to observed dominance patterns. The 95 % confidence interval around the point at which simulated establishment were equal for the two species was near the location of the shortgrass steppe-desert grassland ecotone where both species are abundant. The intersection in percentage of years with establishment for the two species was predicted to move further north when climate was scaled using three Global Circulation Models (GCMs), indicating a possible northward expansion of B. eriopoda. Our results suggest that recruitment by seed may be an important process in determining, at least in part, the geographic distribution of these two species. Changes in climate that affect establishment constraints could result in shifts of species dominance that may or may not be accompanied by changes in species composition.     

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.     

Size of Precipitation Pulses Controls Nitrogen Transformation and Losses in an Arid Patagonian Ecosystem

Arid ecosystems receive precipitation pulses of different sizes that may differentially affect nitrogen (N) losses and N turnover during the growing season. We designed a rainfall manipulation experiment in the Patagonian steppe, southern Argentina, where we simulated different precipitation patterns by adding the same amount of water in evenly spaced three-small rainfall events or in one-single large rainfall event, three times during a growing season. We measured the effect of the size of rainfall pulses on N mineralization and N losses by denitrification, ammonia volatilization, and nitrate and ammonia leaching. Irrigation pulses stimulated N mineralization (P < 0.05), with small and frequent pulses showing higher responses than large pulses (P < 0.10). Irrigation effects were transient and did not result in changes in seasonal net N mineralization suggesting a long-term substrate limitation. Water pulses stimulated gaseous N losses by denitrification, with large pulses showing higher responses than small pulses (P < 0.05), but did not stimulate ammonia volatilization. Nitrate leaching also was higher after large than after small precipitation events (P < 0.05). Small events produced higher N transformations and lower N losses by denitrification and nitrate leaching than large events, which would produce higher N availability for plant growth. Climate change is expected to increase the frequency of extreme precipitation events and the proportion of large to small rainfall events. Our results suggest that these changes would result in reduced N availability and a competitive advantage for deep-rooted species that prefer nitrate over ammonia. Similarly, the ammonium:nitrate ratio might decrease because large events foster nitrate losses but not ammonium losses.     

Invasive species and climate change: <Emphasis Type="Italic">Conyza canadensis</Emphasis> (L.) Cronquist as a tool for assessing the invasibility of natural plant communities along an aridity gradient

The predicted reduction in precipitation in the eastern Mediterranean due to climate change may expose the natural plant communities to invasive species. We assessed whether natural plant communities along an aridity gradient in Israel were resistant to invasion by considering differences in abiotic conditions and community characteristics in these regions. We considered Conyza canadensis as a model plant as it is a common invader in the region. We examined the mechanisms and functional traits of both the plant communities and C. canadensis that promote or discourage invasion. Study sites represented a rainfall gradient with four ecosystem types: mesic Mediterranean, Mediterranean, semiarid and arid. Our results showed that the mechanisms of community invasion resistance varied along the aridity gradient. At the arid and semiarid sites, water deficiency impaired the establishment of C. canadensis. At the mesic Mediterranean site, plant competition had a negative effect on C. canadensis performance, thus greatly reducing the likelihood of its establishment. We conclude that a decrease in regional precipitation due to climate change may not affect intrinsic resistance characteristics of natural plant communities to invasion in the area.     

Potential of Schinopsis lorentzii for dendrochronological studies in subtropical dry Chaco forests of South America

The lack of knowledge about species with well-delimited annual rings has hampered the development of dendrochronological records in the subtropical Chaco region of South America. In this contribution, we present the first tree-ring chronology of Schinopsis lorentzii (Anacardiaceae), a dominant species in the semi-arid Chaco. Cross sections were collected near Las Lajitas, Salta, Argentina, and processed following the methods commonly used in dendrochronology. Annual growth variations between radii from a single individual and between radii from different trees were highly correlated. To determine the climatic parameters that control radial growth, we compared annual tree-ring variations against regional temperature and precipitation records. Correlation functions indicate that tree growth is highly influenced by spring–summer rainfall variations, which represent more than 80% of the total annual precipitation. The chronology, which covers the interval from 1829 to 2004, provides a context for the unprecedented increase in precipitation since the mid-1970s in the region. The climatic-sensitivity of S. lorentzii provides a unique opportunity to reconstruct precipitation variations during past centuries in the extensive semiarid regions of subtropical South America.     

内蒙古西鄂尔多斯荒漠区降水入渗氘同位素特征

探讨我国干旱半干旱地区大气降水在土壤剖面中的时空分布特征将为西鄂尔多斯荒漠退化生态系统恢复和维持提供科学依据.本研究利用氘同位素技术研究了内蒙古西鄂尔多斯荒漠的大气降水、土壤水、地下水中的氘同位素值(δD),运用二元线性混合模型计算降水对各层土壤水的贡献率,并结合土壤含水量分析了不同降水条件下土壤剖面各层土壤水δD的时空分布特征.结果表明: 雨后9 d内,小雨(0～10 mm)影响0～10 cm土壤含水量和土壤水δD值,对表层土壤(0～10 cm)的贡献率在30.3%～87.9%;中雨(10～20 mm)影响0～40 cm土壤含水量和土壤水δD值,对0～40 cm土壤水的贡献率为28.2%～80.8%;大雨(20～30 mm)和特大暴雨(＞30 mm)影响0～100 cm土壤含水量和土壤水δD值.降水对100～150 cm深层土壤水δD值影响不显著.西鄂尔多斯荒漠土壤水δD介于大气降水δD与地下水δD之间,表明西鄂尔多斯荒漠土壤水主要来源于大气降水与地下水.在同一降水强度下,表层土壤水(0～10 cm)受降水的直接影响显著,随着土壤深度的增加,土壤水δD变化幅度降低,100～150 cm深层土壤水δD基本趋于稳定.降水强度越大,对土壤水δD影响的时间越长,影响的土壤深度也越深.     

Hierarchy of responses to resource pulses in arid and semi-arid ecosystems

In arid/semi-arid ecosystems, biological resources, such as water, soil nutrients, and plant biomass, typically go through periods of high and low abundance. Short periods of high resource abundance are usually triggered by rainfall events, which, despite of the overall scarcity of rain, can saturate the resource demand of some biological processes for a time. This review develops the idea that there exists a hierarchy of soil moisture pulse events with a corresponding hierarchy of ecological responses, such that small pulses only trigger a small number of relatively minor ecological events, and larger pulses trigger a more inclusive set and some larger ecological events. This framework hinges on the observation that many biological state changes, where organisms transition from a state of lower to higher physiological activity, require a minimal triggering event size. Response thresholds are often determined by the ability of organisms to utilize soil moisture pulses of different infiltration depth or duration. For example, brief, shallow pulses can only affect surface dwelling organisms with fast response times and high tolerance for low resource levels, such as some species of the soil micro-fauna and -flora, while it takes more water and deeper infiltration to affect the physiology, growth or reproduction of higher plants. This review first discusses how precipitation, climate and site factors translate into soil moisture pulses of varying magnitude and duration. Next, the idea of the response hierarchy for ecosystem processes is developed, followed by an exploration of the possible evolutionary background for the existence of response thresholds to resource pulses. The review concludes with an outlook on global change: does the hierarchical view of precipitation effects in ecosystems provide new perspectives on the future of arid/semiarid lands?     

Do changes in rainfall patterns affect semiarid annual plant communities?

Question: Climate change models forecast a reduction in annual precipitation and more extreme events (less rainy days and longer drought periods between rainfall events), which may have profound effects on terrestrial ecosystems. Plant growth, population and community dynamics in dry environments are likely to be affected by these changes since productivity is already limited by water availability. We tested the effects of reduced precipitation and fewer rain events on three semiarid plant communities dominated by annual species. Location: Three semiarid plant communities from Almería province (SE Spain). Methods: Rain‐out shelters were set up in each community and watering quantity and frequency were manipulated from autumn to early summer. Plant productivity, cover and diversity were measured at the end of the experimental period. Results: We found that a 50% reduction in watering reduced productivity, plant cover and diversity in all three communities. However, neither the 25% reduction in watering nor changes in the frequency of watering events affected these parameters. Conclusions: The lack of response to small reductions in water could be due to the identity and resistance of the plant communities involved, which are adapted to rainfall variability characteristic of arid environments. Therefore, a rainfall reduction of 25% or less may not affect these plant communities in the short term, although higher reductions or long‐term changes in water availability would probably reduce productivity and diversity in these communities.     

Soil carbon flux following pulse precipitation events in the shortgrass steppe

Pulses of water availability characterize semiarid and arid ecosystems. Most precipitation events in these ecosystems are small (≤10 mm), but can stimulate carbon flux. The large proportion of carbon stored belowground and small carbon inputs create the potential for these small precipitation events to have large effects on carbon cycling. Land-use change can modify these effects through alteration of the biota and soil resources. The goal of our research was to determine how small precipitation events (2, 5, and 10 mm) affected the dynamics of soil carbon flux and water loss in previously cultivated Conservation Reserve Program (CRP) fields and undisturbed shortgrass steppe. Total carbon loss and duration of elevated carbon flux increased as event size increased in all field types. Time since cultivation increased in importance for carbon flux as event size increased. A comparison of water loss rates to carbon flux suggests that water is limiting to carbon flux for the smallest events, but is less limiting for events above 5 mm. We also describe how water availability interacts with temperature in controlling carbon flux rate. We conclude that small precipitation events have the potential for large short-term losses of carbon in the shortgrass steppe.