Annual and seasonal variation of NDVI explained by current and previous precipitation across Northern Patagonia

Temporal variation of aboveground net primary production (ANPP) of arid ecosystems has been associated with precipitation regimes with different results. The objective of this paper was to characterize the relationship between interannual variation of annual and seasonal Normalized Difference Vegetation Index (NDVI), as a surrogate for ANPP, and precipitation in the steppes of Northern Patagonia. In 11 sites encompassing a wide range of conditions and vegetation physiognomies, we studied a 20-year monthly data set of NDVI and precipitation. We took into account the precipitation of current, as well as previous periods of variable length. Interannual variation of annual NDVI was little correlated with annual precipitation, either current or previous. In contrast, it was highly and widely correlated with precipitation accumulated during a few months of the previous growing season. Interannual variation of seasonal NDVI was little correlated with current seasonal precipitation. In contrast, it was significantly correlated with precipitation accumulated during previous periods of variable length according to the season and site under consideration. NDVI was more tightly coupled with precipitation in drier ecosystems. Lags of response between NDVI and precipitation provide an opportunity for forecasting ANPP and suggest even longer lags between climatic variation and herbivore performance.     

Soil as a capacitor: Considering soil water content improves temporal models of productivity

Above-ground net primary production (ANPP) in arid and semiarid ecosystems is mainly explained by precipitation (ppt). However, when this relationship is evaluated taking into consideration data from different years in the same site (i.e. temporal models of productivity) the relation is weak, and sometimes it does not exist. In spite of this, the inclusion of previous year's ppt and/or ANPP frequently improves temporal models. In this study we analyze if considering NDVI and mean annual soil water content or transpiration (instead of different combinations of current-year ppt and previous year's ppt or ANPP) improves temporal models of productivity in the Southern Monte (Argentina). Current-year ppt only explained 39.7% of variation in ANPP, while mean soil water content explained 85.3%. The remaining models, which include current-year ppt together with previous-year's ppt or previous-year's ANPP, improve the first model; but the explanatory power of the model based only on mean soil water content is never reached. Our results also show that water losses exceed annual ppt in dry years, whereas the opposite occurs during years with above-average ppt. This carryover effect of soil water indicates that soil acts as a capacitor, accumulating water during wettest years and releasing it during following years.     

Implications of local-scale productivity on compensatory growth in a semi-arid shrubland

Grazing is a dominant determinant of aboveground net primary production (ANPP) and composition of plant communities. However, ANPP can be maintained following grazing due to compensatory growth, the level of which might depend on additional environmental factors, such as precipitation and edaphic conditions. Several studies have shown that along large-scale regional gradients, community-level compensatory ability is positively correlated with ANPP and soil resource availability. However, community-level responses to grazing are also expected to be affected by local-scale heterogeneity in ANPP, particularly under low primary productivity typical to arid environments. Here, we studied the effect of local-scale variations in ANPP on the compensatory growth of an annual community in a semi-arid region. For two consecutive years, ANPP was evaluated following shoot damage in sites with different primary productivity. The results demonstrated that annual ANPP varied significantly among sites and among plots within sites; however, compensatory ability was negatively correlated with annual ANPP, with overcompensation in the least productive patches and under-compensation in the most productive patches. This pattern contradicts the positive correlation between ANPP and compensatory ability commonly found along large-scale productivity ecoclines, suggesting that the effects of ANPP on compensatory ability might be scale-dependent.     

Atmospheric nitrogen deposition in the northern Chihuahuan desert: Temporal trends and potential consequences

Rates and impacts of atmospheric nitrogen (N) deposition are poorly understood in arid land ecosystems where soils are typically low in plant available N. To address this issue, we quantified long-term trends in N deposition and estimated its impact on plant community structure in the northern Chihuahuan desert of Central New Mexico, USA. Annual and seasonal rates of N deposition were strongly positively correlated with precipitation. When precipitation effects were removed statistically, N deposition increased at an annual rate of 0.049 kg ha⁻¹ yr⁻¹ between 1989 and 2004. Based on two independent fertilization studies at our desert grassland field site, continued atmospheric inputs are likely to increase grass cover, decrease legume abundance, and may favor blue grama (Bouteloua gracilis) at the expense of the current dominant species, black grama (Bouteloua eriopoda). We conclude that, although arid lands have low rates of N deposition and are primarily water limited, observed trends in N deposition rates may lead to significant changes in plant community structure.     

Biomass production, evapotranspiration and water use efficiency of arid rangelands in the Northern Cape, South Africa

Annual above-ground net primary production (ANPP), evapotranspiration (ET) and water use efficiency (WUE) of rangeland have the potential to provide an objective basis for establishing pricing for ecosystem services. To provide estimates of ANPP, we surveyed the biomass, estimated ET and prepared a water use efficiency for dwarf shrublands and arid savanna in the Riemvasmaak Rural Area, Northern Cape, South Africa. The annual production fraction was surveyed in 33 MODIS 1 km² pixels and the results regressed against the MODIS f_PAR product. This regression model was used to predict the standing green biomass (kg DM ha⁻¹) for 2009 (dry year). Using an approach which combines potential evapotranspiration (ET₀) and the MODIS f_PAR product, we estimated actual evapotranspiration (ET_a). These two models (greening standing biomass and ET_a) were used to calculate the annual WUE for 2009. WUE was 1.6 kg DM mm⁻¹ ha⁻¹ yr⁻¹. This value may be used to provide an estimate of ANPP in the absence of direct measurements of biomass and to provide a comparison of the water use efficiency of this rangeland with other rangeland types.     

内蒙古温带草原典型草地生态系统生产力对水分在不同时间尺度上的响应

水分是干旱、半干旱区草地生态系统生产力的主要限制因素,但水分如何影响生产力的季节内变异,以往研究相对不足。本研究以内蒙古温带典型草原为研究对象,基于多通道自动原位监测箱系统获得的生态系统日尺度总初级生产力(GPP)及遥感植被归一化指数估算年尺度的地上净初级生产力数据,在不同时间尺度上研究了水分对生产力的影响。结果表明:年降水量对该典型草原草地生态系统地上净初级生产力年际变异无显著影响,但土壤水分显著影响GPP的季节变异,土壤水分解释了GPP季节变异的22%;其他环境因子对生产力的季节变异也有一定影响;温度是生态系统处于干旱时GPP的主要限制因素,GPP随温度的升高而降低;而辐射是生态系统处于湿润时GPP的主要限制因素,GPP随辐射的升高而升高。本研究结果将有利于提升未来降水格局改变对草地生态系统生产力影响的认识。     

The significance of atmospheric nutrient inputs and canopy interception of precipitation during ecosystem development in piñon–juniper woodlands of the southwestern USA

In arid ecosystems, widely spaced vegetation and prolonged dry periods may enhance canopy capture of nutrients from dry deposition. Additionally, differences in precipitation type, plant canopy architecture, and soil nutrient limitation could affect canopy exchange of atmospherically derived nutrients. We collected bulk precipitation and throughfall underneath piñon pine (Pinus edulis) and one-seed juniper (Juniperus monosperma) along a substrate age gradient to determine if canopy interception or throughfall chemistry differed among tree species, season, or substrate age. The Substrate Age Gradient of Arizona consists of four sites with substrate ages ranging from 1 ky to 3000 ky-old, which exhibit classic variations in soil nitrogen (N) and phosphorus (P) availability with substrate age. Greater nutrient inputs below canopies than in intercanopy areas suggest throughfall contributes to the “islands of fertility” effect. Canopy interception of precipitation did not differ between tree species, but was greater in the summer/fall than winter/spring. We found that net canopy retention of atmospherically derived N was generally greater when N availability in the soil was low, but retention also occurred when N availability was relatively high. Taken together, our results were inconclusive in determining whether the degree of soil nutrient limitation alters canopy exchange of plant growth-limiting nutrients.     

草原化荒漠植被草本层片植物对人工施加氮素的响应

水分是干旱区生态过程的第一非生物限制因子,土壤氮由于直接影响着生物生产力和土壤生态过程而被认为是仅次于水分的限制因子。利用人工施加氮肥的控制试验研究了荒漠植被草本层片植物在多度、物种丰富度、高生长以及地上部分生物量等群落学特征对不同施肥处理的响应。结果表明,在施肥量分别为12.5、25、50 g·m-2时,样方植物多度和物种丰富度在施肥当年和第二年较对照均有不同程度的降低,且施肥量越高,降低越明显,这一结果支持来自典型草地和森林植被生物多样性对氮素增加或氮降沉增加响应的结论,即氮素的增加会使生物多样性减少。地上部分生物量对施氮肥的响应在施肥当年(年降水量为多年平均值的1.4倍)和第二年(年降水量为多年平均值的0.7倍)表现出相反的规律,即施肥当年随着施肥量的增加生物量显著增加,而在第二年则显著减小,说明水肥耦合同样有利于荒漠生态系统生物生产力的提高。氮素对植物高生长的影响不明显,各施肥梯度与对照相比均无显著性差异(P>0.05),这可能主要取决于荒漠草本植物本身固有的生物学特性。     

Contrasting nutrient-capture strategies in shrubs and grasses of a Patagonian arid ecosystem

Shallow-rooted grasses and deep-rooted shrubs dominate arid ecosystems where nitrogen is concentrated in the upper layers of the soil and water is distributed throughout. Analysis of mineral nitrogen and absorption patterns using a tracer indicated that shrubs in Patagonia absorbed nutrients from the lower, relatively nutrient-poor layers of the soil. Are they, consequently, at a competitive disadvantage with grasses that have the opposite pattern? Studies of nitrogen economy indicated that shrub and grass species have similar N-use efficiency but that they achieve it through opposite mechanisms. Shrubs have a conservative N economy absorbing annually only small fraction of their N content, whereas grasses have a more open N economy. This study about N-capture strategies in conjunction with previous studies about water-use by shrubs and grasses in the Patagonian Steppe suggest a coupling of N and water-capture strategies. Our findings have implications for the response of arid and semiarid ecosystems to global warming, nitrogen deposition, and biodiversity change. For example, climate change scenarios predict, for most arid regions, decreases in moisture availability that will result in a reduction in deep water, which in turn will reduce shrub density and result in a less conservative nitrogen economy.     

Post-fire primary production and plant community dynamics in chaparral stands exposed to varying levels of nitrogen deposition

High levels of atmospheric nitrogen (N) deposition to southern California chaparral shrublands may interact with fire to affect biomass production and plant species composition during secondary succession. To determine the potential interactions between post fire recovery and N deposition we compared rates of aboveground net primary production (ANPP), shrub growth, and the relative abundance of Adenostoma fasciculatum, other sub-dominant shrubs, and herbaceous species of three chaparral stands exposed to different levels of atmospheric N deposition over the first 3 years of post-fire succession. Our data suggest that rates of ANPP (gdw m(-2) month(-1)) and aboveground N storage (gN m(-2) month(-1)) for these chaparral stands were not related to N deposition even though sites exposed to high levels of N deposition had significantly higher rates of shrub growth (gdw plant(-1) month(-1)) and N uptake (gN plant(-1) month(-1)). However, high-N stands were composed of larger shrubs with a lower density, and this trade-off between shrub size and density may explain the low correlation between N deposition and post-fire ANPP. Differences in relative plant species abundance between sites were significantly correlated with N deposition exposure, where stands exposed to high N deposition had a lower relative abundance of A. fasciculatum and a higher relative abundance of other shrub and herbaceous species. While many factors can affect rates and patterns of post-fire recovery, these results suggest that chronic exposure to N deposition may significantly alter plant growth and species composition in successional chaparral stands.     

内蒙古典型草原地上生物量的空间格局及其气候成因分析

内蒙古典型草原区的格点地上生物量与格点的纬度和经度均存在显著正相关关系,呈现北高南低、东高西低的空间分布格局;各气象站点所在地的草地地上生物量与多年平均年降水量之间存在显著正相关关系,与多年平均年均温之间存在显著负相关关系,而与多年平均年干燥度之间亦存在显著负相关关系,且在干燥度介于1~1.5的地区,地上生物量对干燥度变化的响应非常敏感;草地地上生物量的空间分布格局主要是在水热条件共同作用下形成的,年降水量的近东西向分布决定地上生物量分布的近东西向分异特点,而气温和降水的共同作用决定地上生物量分布的近南北向分异特点。     

Spatial carbon and nitrogen distribution and organic matter characteristics of biological soil crusts in the Negev desert (Israel) along a rainfall gradient

In arid and semi-arid areas biological soil crusts are main contributors to C and N-cycles and the origin of organic matter. Nevertheless systematic studies on the spatial distribution of total organic carbon (TOC) and nitrogen (N) and a characterization of crust organic matter composition are missing. To describe the spatial distribution of TOC and N we examined three soil depths and three relief positions along a steep rainfall gradient. In addition the molecular composition of organic matter was characterized by Pyrolysis-field ionization mass spectrometry.TOC and N concentrations decreased with increasing depth, the effects of the relief followed no clear trend. Surprisingly the amount of TOC and N decreased with increasing rainfall. Stable organic matter compounds were reduced with increasing rainfall. Topcrusts (0–2 mm) showed a relative enrichment in bacteria, as indicated by proportionally larger contents in N-acetylmuramic acid (m/z 167 + 276) than the subcrusts (2–40 mm). These were enrichment in cyanobacteria, as indicated by proportionally larger contents of hexadecadienoic acid (m/z 252).We conclude that the spatial distribution of TOC and N is related to sampling depth and annual precipitation. Organic matter composition and the main biomass contributors in crusts are successfully identified by pyrolysis-field ionization mass spectrometry.     

A non-destructive method to estimate biomass in arid environments: A comment on

Numerous ecological studies in grasslands and shrublands have used plant cover as a surrogate of aboveground net primary production (ANPP), biomass or biovolume. Regression equations between biomass and plant cover involve taking into account a wide range of vegetation cover. This comment assesses the consequences for biomass estimations of the method presented by Flombaum and Sala [Flombaum, P., Sala, O.E., 2007. A non-destructive and rapid method to estimate biomass and aboveground net primary production in arid environments. Journal of Arid Environments 69, 352–358.], which consists in removing portions of individuals to achieve the desired plant cover estimation. We found that alteration of architecture of individuals could lead to an overestimation of biomass by a factor ranging from 1.05 to 3.16, depending on the extent of the reduction of the plant cover.     

Ecological effects of experimental drought and prescribed fire in a southern California coastal grassland

How drought and fire disturbance influence different levels of biological organization is poorly understood but essential for robust predictions of the effects of environmental change. During a year of severe drought, we conducted a prescribed fire in a Mediterranean-type coastal grassland near Irvine, California. In the weeks following the fire we experimentally manipulated rainfall in burned and unburned portions of the grassland to determine how fire and drought interact to influence leaf physiological performance, community composition, aboveground net primary productivity (ANPP) and component fluxes of ecosystem CO₂ exchange and evapotranspiration (ET). Fire increased leaf photosynthesis (A_net) and transpiration (T) of the native perennial bunchgrass, Nassella pulchra and the non-native annual grass, Bromus diandrus but did not influence ANPP or net ecosystem CO₂ exchange (NEE). Surprisingly, drought only weakly influenced A_net and T of both species but strongly influenced ANPP and NEE. We conclude that despite increasing experimental drought severity, prescribed fire influenced leaf CO₂ and H₂O exchange but had little effect on the component fluxes of ecosystem CO₂ exchange. The differential effects of prescribed fire on leaf and ecosystem processes with increasingly severe drought highlight the challenge of predicting the responses of biological systems to disturbance and resource limitation.     

氮素和水分添加对贝加尔针茅草原土壤氨氧化微生物群落结构的影响（英文）

应用PCR-DGGE技术研究了连续2年氮素、水分添加处理对贝加尔针茅草原土壤氨氧化细菌(AOB)16S r RNA基因和古菌(AOA)amo A基因遗传多样性的影响。结果表明,土壤硝化势在水分添加低氮(N30)和非水分添加高氮(N300)处理下较对照显著升高(P0.05),在水分添加高氮(N200、N300)处理下较对照急剧下降(P0.05)。不同处理下氨氧化细菌的群落结构变化较氨氧化古菌敏感,氮素添加显著改变了氨氧化微生物的群落结构,在高氮素处理下,氨氧化细菌的多样性指数有下降的趋势,而古菌的多样性则有升高的趋势,表明氨氧化古菌在高氮素下生长比较活跃。发育树分析的结果表明,非水分添加处理下,氨氧化细菌的群落由亚硝化螺菌属的Cluster 1、Cluster 3和Cluster 4组成,水分添加处理下由亚硝化螺菌属Cluster 3、Cluster 4和亚硝化单胞菌属的Cluster 6组成,而氨氧化古菌群落在水分添加处理下主要由泉古菌门的Cluster 1、Cluster 2和water lineage组成,在非水分添加处理下主要由泉古菌门的Cluster 1、Cluster 2和Cluster 5组成,两种水分输入机制下,氨氧化微生物群落的不同组成说明,水分在调节氨氧化微生物群落结构方面有重要作用。此外,氨氧化细菌群落多样性与全氮,NH4+和NO3-的含量呈显著负相关,而古菌群落与其呈显著正相关,说明氨氧化细菌与古菌对氮素和水分添加有不同的且互补的反应机制。     

Effects of fertilization on population density and productivity of herbaceous plants in desert steppe

In order to investigate the impacts of fertilization on population density and productivity on herbaceous plants in desert steppe, nitrogen （N）, phosphorus （P）, and N-P addition experiments were performed. Each fertilizer treatment included four addition levels, i.e., 0, 5, 10, and 20 g/m2. The results indicated that population density decreased as fertilization levels increased regardless of the sort of fertilizer. More specifically, total density as well as density ofArtemisia capillaris, Allium polyrhizum, and Enneapogon brachystachyus decreased significantly in 20 g/m2 treated plots, as compared with the control plots. Fertilization effects on aboveground and root biomasses were extremely similar to that found in population density; that is, both total aboveground biomass and aboveground biomasses for A. capillaris, A. polyrhizum, and E. brachystachyus were negatively correlated with increasing fertilization levels, with all determination coefficients （R2） greater than 0.80. Therefore, in the case of desert regions （annual precipitation 〈180 mm）, fertilization would inhibit population density and productivity of herbaceous plants.     

干旱半干旱地区草地碳循环关键过程对降雨变化的响应

未来全球气候变化背景下,全球或局部地区的降雨量及降雨时间分布将发生一定的变化.草地生态系统在全球碳收支中作用显著,对气候变化的反馈起着重要的作用.但由于草地多处于干旱半干旱地区,受到水分条件的限制,对降雨变化响应敏感,其碳源汇功能表现出很大的不确定性.为了更好地预测未来全球气候变化背景下草地的碳源汇功能及其对气候变化的进一步反馈,有必要深入研究决定草地碳源汇功能的两大碳循环关键过程--净初级生产力和土壤呼吸对降雨变化的响应特征及机制.本文对国内外有关草地生产力和土壤呼吸如何对降雨量、降雨强度、降雨频率和间隔时间进行响应的相关研究成果进行了综述,在此基础上指出了目前研究存在的不足,并对未来相关的重点研究方向进行了探讨和展望.     

Plant productivity, predation, and the abundance of black-tailed jackrabbits in the Chihuahuan Desert of Mexico

The abundance of black-tailed jackrabbits (Lepus californicus) can fluctuate dramatically. We used data from the Chihuahuan Desert to test the relative strength of top-down (predation) or bottom-up (food availability) limiting forces. Predictions for the top-down hypothesis were, 1) a positive relationship between coyote (Canis latrans) and jackrabbit abundance (numerical response) and 2) a positive relationship between percent occurrence of jackrabbits in coyote scats and jackrabbit abundance (functional response). Predictions for the bottom-up hypothesis were, 1) plant productivity is directly related to precipitation, 2) jackrabbit abundance is positively related to precipitation and plant productivity, and 3) changes in abundance of jackrabbits over the reproductive season will be directly related to precipitation and plant productivity. We found a limited numerical response but no functional response of coyotes to jackrabbit abundance. Forb productivity was significantly related to annual precipitation levels (r² = 0.69, p = 0.002). Grass productivity was related to annual precipitation (r² = 0.34, p = 0.028). Jackrabbit abundance (r² = 0.38, p = 0.002) and changes in abundance (r² = 0.73, p < 0.001) were significantly related to precipitation and forb and grass productivity. We conclude that precipitation levels and plant productivity affect jackrabbit abundance more than predation levels.     

Vertical distribution of fine roots of Tamarix ramosissima in an arid region of southern Nevada

To increase our understanding of soil water and nitrogen use strategies of invasive Tamarix ramosissima during dry seasons, the vertical distributions of fine roots and their associations with soil properties were examined in the Virgin River floodplain, southern Nevada, United States. We measured morphological traits of fine roots, such as fine-root mass density, fine-root length density, specific root length and specific root area at 10 cm increments to a depth of 2 m. Soil properties were analyzed at 20 cm increments. More than 60% of fine root length and biomass was concentrated at depths between 20 and 60 cm. Soil nitrogen (N) concentration had strong and positive relationships with fine-root mass and length densities, suggesting that the fine-root distribution may be influenced by soil N availability. A weak but positive relationship was observed between soil moisture and fine-root mass density. Soil salinity had no relationship with root morphological traits. These findings suggest that T. ramosissima fine roots may contribute to N uptake from the upper soil layers during dry seasons. This might be an important advantage over native riparian tree species in arid riparian areas of the southwestern United States.     

Effects of simulated storm sizes and nitrogen on three Chihuahuan Desert perennial herbs and a grass

Establishment and growth of three perennial herbs and a small tussock grass were studied in an experiment that provided simulated rainfall of 6 mm week⁻¹ or 25 mm once per month and nitrogen fertilization in combination with the different simulated rainfall regimes. Wild onion, Allium macropetalum, failed to establish in plots receiving 25 mm month⁻¹ simulated rainfall. The perennial composite, Bahia absinthifolia, occurred at higher densities in plots that were not irrigated but there were no differences in biomass in any of the irrigation or fertilization treatments. Desert holly, Perezia nana, failed to establish in nitrogen fertilized plots and developed higher abundance and biomass in plots receiving 25 mm month⁻¹. Nitrogen fertilization had either no effect or an adverse effect on the perennial herbs. The tussock grass, Dasychloa pulchella exhibited highest abundance and biomass with 6 mm week⁻¹ added water plus nitrogen. Since global climate change will affect both rain storm frequency and size and atmospheric nitrogen deposition, the results of this study are applicable to understanding vegetation responses climate change.