Shrub Interspaces Research Articles

Facilitation by Haloxylon persicum Shrubs Enhances Density and Richness of Soil Seed Bank of Annual Plants in a Hyper-Arid Ecosystem.

The soil seed bank is an essential functional component of plant communities. In arid ecosystems, the island-like distribution of shrubs influences the spatial distribution of the soil seed bank. Very little is known about seed banks in deserts of the Middle East. The present study aimed to evaluate the facilitative effects of Haloxylon persicum shrubs on the soil seed bank of annual plants in a sandy desert region in northwestern Saudi Arabia during two consecutive growing seasons (2017-2018 and 2018-2019) with contrasting rainfall. A total of 480 soil samples at 12 stands were collected from two microhabitats, under shrubs and in open areas, soon after the two growing seasons. The germinable seed bank of annual plants was estimated by controlled seedling emergence method. Shrubs significantly facilitated the accumulation of seed bank beneath their canopies after the two growing seasons. In both microhabitats, the size and species richness of soil seed bank were significantly greater after the wet growing season (2018-2019) than following the dry season (2017-2018). The facilitative effects of shrubs were greater following the moister growing season than after the dry season. The effect of shrubs on seed bank-annual vegetation similarity varied between growing seasons, being greater in shrub interspaces than beneath shrub canopies for the dry growing season, while during the wet season, the similarity of the seed bank with standing annual vegetation was greater in sub-canopy microhabitat than in bare soil.

Effects of Shrub Encroachment in the Inner Mongolia Ecotones between Forest and Grassland on the Species Diversity and Interspecific Associations

Shrub encroachment, which is the increase in shrubs or woody plants in grassland, is one of the important ecological problems facing grassland worldwide. The study was conducted in the ecotones between the forest and grassland of Inner Mongolia. Using a simple random sampling method, 40 shrubland sample plots, evenly distributed in the typical forest-grass transition area, were selected for community investigation. According to the steppe shrub encroachment index, the grassland was divided into different levels. The species diversity and interspecific association of different levels of sample plots were statistically analyzed and compared. It was found that the Shannon-Wiener index (H) and Simpson index (P) were negatively associated with the shrub encroachment index (p < 0.01) and gradually decreased with increasing levels of shrub encroachment. When the grassland transitioned to severe shrub formation, the species diversity of the community significantly decreased, and H and P were significantly lower than those of the mild and moderate shrub-steppe, and the lowest values were 1.37 and 0.48, respectively. With the increase in the shrub encroachment index, biomass showed a trend of first increasing and then decreasing. The aboveground biomass of shrub interspace and total aboveground biomass in the severe shrub encroachment steppe were significantly lower than those in the mildly and moderately shrub-steppe, with the lowest values of 101.86 g/m2 and 189.69 g/m2, respectively. Shrub encroachment led to a change in the overall association of shrub communities in the ecotones between the forest and grassland of Inner Mongolia from positive to negative. The vast majority of species pairs in the shrub community showed no significant association, and the interspecies association was relatively weak. The results showed that shrub encroachment would affect community species diversity; mild and moderate shrub encroachment had no significant impact on community species diversity, but severe shrub encroachment significantly reduced the community species diversity. There was no significant effect of shrub encroachment on aboveground biomass, which increased first and then decreased, and the herbaceous plant population played a leading role in grassland productivity. The interspecific association of grassland is loose and weak. The community was in the early stage of unstable succession, and it was possible to continue shrub encroachment or reverse succession into a typical grassland in response to the interference of human or environmental factors.

Soil nutrients mediate the indirect effects of shrub canopy removal: How distance from shrubs affects the herbs and grasses community in a shrub‐encroached grassland

AbstractUnderstanding community processes is essential to predict community dynamics and succession processes. To explore how shrub canopy removal frequency (hereafter ‘removal frequency’) and distance from shrubs affect herbs and grasses community assembly in a shrub‐encroached grassland, we carried out a 4‐year shrub canopy removal experiment with three removal frequencies (no removal, removal once, and removal twice per year) in a Caragana microphylla shrub‐encroached grassland, and assessed community assembly mechanisms directly beneath shrubs (0 m from shrubs, i.e., in the centre of the shrub) and in shrub interspaces (at least 2 m from shrubs) under each removal frequency treatment using both functional trait‐based and phylogenetic‐based approaches. Removal frequency positively affected standard effect sizes (SESs) of the mean pairwise distance (MPD) for multi‐trait, specific leaf area (SLA) and leaf dry matter content (LDMC), denoted by SES.MPDMulti‐trait, SES.MPDSLA, and SES.MPDLDMC, indirectly via regulating soil nitrogen (N) and soil carbon (C) content, while it negatively affected SES.MPDHeight indirectly via regulating soil available phosphorus (P) content. Distance from shrubs negatively affected SES.MPDHeight indirectly via regulating soil available P content, and negatively affected SES.MPDLDMC indirectly via regulating soil C content. The trait‐based approach was more powerful than the phylogenetic‐based one in explaining the responses of community assembly processes to environmental changes. These findings highlight the importance of shrub canopy removal and distance from shrubs in affecting soil nutrient status and consequently community assembly processes, which provides a new insight into how canopy removal or fertile island effects affect community assembly in the shrub‐encroached grassland regions.

The Burning of Biocrusts Facilitates the Emergence of a Bare Soil Community of Poorly-Connected Chemoheterotrophic Bacteria With Depressed Ecosystem Services

Wildfires destabilize biocrust, requiring decades for most biological constituents to regenerate, but bacteria may recover quickly and mitigate the detrimental consequences of burnt soils. To evaluate the short-term recovery of biocrust bacteria, we tracked shifts in bacterial community form and function in Cyanobacteria/lichen-dominated (shrub interspaces) and Cyanobacteria/moss-dominated (beneath Artemisia tridentata) biocrusts one week, two months, and one year following a large-scale burn manipulations in a cold desert (Utah, USA). We found no evidence of the burned bacterial community recovering to a burgeoning biocrust. The foundational biocrust phyla, Cyanobacteria, dominated by Microcoleus viginatus (Microcoleaceae), disappeared from burned soils creating communities void of photosynthetic taxa. One year after the fire, the burned biocrust constituents had eroded away and the bare soils supported the formation of a convergent community of chemoheterotrophic copiotrophs regardless of location. The emergent community was dominated by a previouly rare Planococcus species (family Planococcaceae, Firmicutes) and taxa in the Cellulomonadaceae (Actinobacteria), and Oxalobacteraceae (Betaproteobacteria). Previously burnt soils maintained similar levels of bacterial biomass, alpha diversity, and richness as unburned biocrusts, but supported diffuse, poorly-interconnected communities with 75% fewer species interactions. Nitrogen fixation declined at least 3.5-fold in the burnt soils but ammonium concentrations continued to rise through the year, suggesting that the exhaustion of organic C released from the fire, and not N, may diminish the longevity of the emergent community. Our results demonstrate that biocrust bacteria may recover rapidly after burning, albeit along a different community trajectory, as rare bacteria become dominant, species interconnectedness diminishes, and ecosystem services fail to rebound.

Sagebrush Wildfire Effects on Surface Soil Nutrient Availability: A Temporal and Spatial Study

Core Ideas In sagebrush ecosystems, wildfire effects on surface soil nutrient availability varied spatially. Wildfire increased surface soil nutrient availability long‐term, mostly in shrub canopies. Soil nutrients of greatest increase with long‐lasting elevated levels included mineral N and Mn. Wildfires occurring in Artemisia (sagebrush) ecosystems can temporarily increase soil nutrient availability in surface soil. Less is known, however, on how soil nutrient availability changes over time and microsite post‐wildfire. In October 2013 a wildfire approximately 30 km north of Reno, Nevada, occurred in an Artemisia tridentata ssp. vaseyana (mountain big sagebrush) community. We quantified soil nutrient availability of surface soil (0–10 cm) by microsite (shrub interspace and shrub canopy), treatment (burned and unburned), and temporally (monthly from November 2013 through May 2015). Changes in nutrient availability, due to wildfire, largely occurred in shrub canopy microsites. For most months and relative to unburned shrub canopies, burned shrub canopies had elevated availability of mineral N (dominantly NH4+‐N), bicarbonate‐extractable P, DTPA‐extractable Mn, and solution‐phase Ca+2, Mg+2, SO4–2, K+, and ortho‐P. Burned shrub canopies also had, for most months, far less N mineralization potentials than unburned shrub canopies. The large increase in Mn availability in burned shrub canopies significantly correlates with mineral N and solution‐phase Ca+2 and SO4–2, suggesting release via pyrolysis of soil organic matter. Except for a slight increase in solution‐phase Mg+2, SO4–2, and ortho‐P for particular months, there is little evidence for nutrient translocation from burned canopies to adjacent interspaces during the study. Density of the exotic annual grass Bromus tectorum increased from 5 to 10% cover pre‐wildfire to about 60% in the post‐wildfire growing season attesting to the importance of elevated soil nutrient availability to its growth.

Precipitation timing and soil heterogeneity regulate the growth and seed production of the invasive grass red brome

Invasions of exotic grasses are increasing the spatial extent and temporal frequency of wildfire. Mitigation efforts to prevent wildfire depend on understanding the biophysical agents that drive spatial and temporal distributions of exotic grasses. We examined the growth and reproduction of the exotic grass red brome (Bromus rubens L.) in response to precipitation timing in the fall period, fertile-island topography, and fire history in the Mojave Desert. Tiller density, shoot height, shoot biomass, and seed production were measured. Early precipitation timing (mid-October versus late November) triggered early germination and led to greater red brome density (1.5-fold), shoot height (1.9-fold), shoot biomass (6.8-fold) and seed production (3.9-fold). Soil cores collected from beneath shrubs (fertile-islands) versus shrub inter-spaces also produced red brome with enhanced shoot height (1.5-fold), and seed production (1.9-fold). The legacy effect of wildfires on soils (10 years after the fires) had no significant effect on the growth or reproduction of red brome. Soils beneath shrubs had significantly less gravel and higher mineral nitrogen (N) concentrations than soils from shrub inter-spaces. Soil gravel % was negatively correlated with red brome density, height, biomass and seed production while mineral N tended to have weak positively correlations with red brome density and seed production. These results suggest that the growth and reproduction of red brome is influenced by fall precipitation timing and soil characteristics. Management planning for fire intervention should consider fall precipitation patterns, soil characteristics and shrub topography as important determinants of red brome growth and fire potential.

Direct and indirect effects of native range expansion on soil microbial community structure and function

Summary Analogous to the spread of non‐native species, shifts in native species’ ranges resulting from climate and land use change are also creating new combinations of species in many ecosystems. These native range shifts may be facilitated by similar mechanisms that provide advantages for non‐native species and may also have comparable impacts on the ecosystems they invade. Soil biota, in particular bacteria and fungi, are important regulators of plant community composition and below‐ground ecosystem function. Compared to non‐native plant invasions, there have been relatively few studies examining how soil biota influence – or are influenced by – native species range shifts. Here, we examined how a native range‐expanding sagebrush species (Artemisia rothrockii) affects below‐ground abiotic conditions and microbial community structure and function using next‐generation sequencing coupled with other biotic and abiotic soil analyses. We utilized a range‐expansion gradient, together with a shrub removal experiment and structural equation models, to determine the direct and indirect drivers of these interconnected processes. Sagebrush colonization increased bacterial and archaeal richness and diversity and altered community composition across the expansion gradient. Soil organic C and N and soil moisture increased with sagebrush presence; however, results varied across the expansion gradient. We found no relationship between sagebrush and soil pH; however, pH strongly influenced microbial richness and diversity. Microbial (substrate‐induced) respiration was influenced by soil organic N, as well as microbial diversity and functional group relative abundances, highlighting direct and indirect effects of sagebrush on microbial community structure and function. Microbial community composition of soils after 4 years of sagebrush removal was more similar to communities in shrub interspaces than underneath shrubs, suggesting microbial community resilience. Synthesis. Our results suggest that native range expansions can have important impacts on soil biological communities, soil chemistry and hydrology which can further impact below‐ground ecosystem processes such as nutrient cycling and litter decomposition. The combination of high‐throughput sequencing and structural equation modelling used here offers an exciting yet underutilized approach to understanding how both native and non‐native species’ range expansions may affect the structure and function of soil ecosystems.

Biological Soil Crust Response to Late Season Prescribed Fire in a Great Basin Juniper Woodland

Expansion of juniper on U.S. rangelands is a significant environmental concern. Prescribed fire is often recommended to control juniper. To that end, a prescribed burn was conducted in a Great Basin juniper woodland. Conditions were suboptimal; fire did not encroach into mid- or late-seral stages and was patchy in the early-seral stage. This study evaluated the effects of the burn on biological soil crusts of early-seral juniper. Fire reduced moss cover under sagebrush and in shrub interspaces. Mosses were rare under juniper; their cover was unaffected there. Lichens were uncommon under juniper and sagebrush and therefore not significantly impacted there. Their cover was greater in shrub interspaces, but because the fire was spotty and of low intensity, the effects of burning were minimal. Compared with unburned plots, the biomass of cyanobacteria was diminished under juniper and sagebrush; it was reduced in the interspaces in both burned and unburned plots, presumably in response to generally harsher conditions in the postburn environment. Nitrogen fixation rates declined over time in juniper plots and interspaces but not in sagebrush plots. Although fire negatively affected some biological soil crust organisms in some parts of the early-seral juniper woodland, the overall impact on the crusts was minimal. If the intent of burning is to reduce juniper, burning of early-seral juniper woodland is appropriate, as most affected trees were killed. Control of sagebrush can likewise be accomplished by low-intensity, cool season fires without eliminating the crust component. Intense fire should be avoided due to the potential for greater encroachment into the shrub interspaces, which contain the majority of biological soil crust organisms. Burning early-seral juniper may be preferred for controlling juniper encroachment on rangeland.

岷江上游干旱河谷灌丛群落土壤生态酶化学计量特征

PDF HTML阅读 XML下载 导出引用 引用提醒 岷江上游干旱河谷优势灌丛群落土壤生态酶化学计量特征 DOI: 10.5846/stxb201401270202 作者: 作者单位: 中国科学院生态环境研究中心,中国科学院生态环境研究中心城市与区域国家重点实验室,中国科学院生态环境研究中心城市与区域国家重点实验室 作者简介: 通讯作者: 中图分类号: 基金项目: 国家自然科学基金(31170581,41101270) Patterns of ecoenzymatic stoichiometry in the dominant shrubs in the semi-arid Upper Minjiang River Valley Author: Affiliation: Research Center for Eco-environmental Science, Chinese Academy of Sciences,Research Center for Eco-environmental Science, Chinese Academy of Sciences, Fund Project: 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:与碳(C)、氮(N)和磷(P)分解矿化相关的土壤生态酶的相对活性可以揭示微生物生长和代谢过程的能量限制。尽管干旱生态系统愈来愈受到生态学家的关注,但在土壤水分有效性限制和斑块状分布的植被背景下,探讨土壤酶活性化学计量特征,及其与土壤物理、化学因子和土壤微生物群落结构的关系的相关研究仍十分缺乏。通过调查岷江干旱河谷不同水分梯度4个研究样地(两河口、飞虹、撮箕和牟托)灌丛及其丛间空地的土壤物理化学性质、微生物群落组成和土壤生态酶活性,探讨不同水分梯度样地间及灌丛和空地间土壤酶活性计量的分布特征。结果显示,岷江干旱河谷土壤C : N : P酶活性化学计量表现出了与全球尺度上酶活性化学计量的相对特异性(C : N : P = 3 : 2 : 2 vs. 1 : 1 : 1)。整体上,岷江干旱河谷区域土壤微生物生长代谢存在一定程度的N限制。虽然酶活性化学计量特征在所有空地和灌丛土壤之间总体未表现出显著差异,但对各研究地的进一步分析表明,两个相对干旱的两河口和飞虹样地表现出更为明显的N限制,而两个相对湿润的撮箕和牟托样地则存在一定的P限制。这种样地间的差异性主要受土壤水分有效性的驱动。逐步回归分析表明,土壤含水量是不同样地间土壤C : N酶活性比(ln(BG) : ln(NAG+LAP))和N : P酶活性比(ln(NAG+LAP) : ln(AP))的决定性影响因子。而土壤含水量与C : N酶活性比具有显著正相关关系,与N : P酶活性比表现为显著负相关关系。此外,土壤真菌、菌根真菌和真菌 : 细菌比对土壤酶活性化学计量的影响,说明了真菌和菌根真菌类群对干旱河谷养分循环的重要作用。 Abstract:The activities of soil ecoenzymes in relation to the mineralization of soil organic carbon, nitrogen, and phosphorus can reveal stoichiometric and energy limitations on the growth and metabolism of soil microbes. Despite the fact that arid and semi-arid ecosystems have received considerable attention in recent years, studies on the patterns of soil ecoenzymatic stoichiometry and the relationships among soil physiochemical and microbial community factors are still rare. In this study, we investigated the variations of soil physiochemical properties, microbial community composition, and EEAs by comparing soils in dominant shrubs and in the shrub-interspaces at four sites (Lianghekou, Feihong, Cuoji, and Moutuo). These study sites were expected to differ with respect to the level of water stress along the semi-arid valley in the upper Minjiang River, Sichuan province. The patterns of ecoenzymatic stoichiometry of C : N : P were different in the semi-arid Minjiang River valley as compared to those in the global pattern (C : N : P=3 : 2 : 2 vs. 1 : 1 : 1). Overall, there may be some degree of nitrogen limitation of microbial metabolism in this semi-arid region, which was supported by the relative activities of β-glucosidase compared to β-N-acetylglucosaminidase + leucine aminopeptidase and β-N-acetylglucosaminidase + leucine aminopeptidase to phosphatase. The different patterns of enzymatic stoichiometry between the soils in shrubs and in the shrub interspaces were not obvious. However, the limitation of C, N, and P resources in each individual site was somewhat site specific and may be due to the soil water availability. The two relatively drier sites (Lianghekou and Feihong) were more constrained by nitrogen availability, while the two relatively wetter sites (Cuoji and Moutuo) were more constrained by phosphorus availability. The variations of soil enzymatic stoichiometric patterns were affected by nutrient availabilities and soil water content as shown by the Pearson product-moment correlation analysis. The results of stepwise regression further revealed that the variations of enzyme ratios, such as C : N and N : P, were driven primarily by soil water content. Soil water content was positively correlated with the C : N enzyme ratio and negatively correlated with the N : P enzyme ratio. We also observed that arbuscular mycorrhizal fungi (AMF) and the fungi-to-bacteria ratio positively correlated with the C : N enzyme ratio, but negatively with the N : P enzyme ratio, which may imply that the enrichment of fungi and AMF communities will benefit C and P cycling by influencing enzyme activities. Our results suggest that any predicted decrease in soil water with climate change will probably further constrain the availability of N and P in this N-limited ecosystem in the semi-arid region. Moreover, this study also highlights the important role of fungi and AMF communities in meditating C and P cycling in this semi-arid region. 参考文献 相似文献 引证文献

Multiscale patterns of arbuscular mycorrhizal fungal abundance and diversity in semiarid shrublands

Abstract The distribution of arbuscular mycorrhizal (AM) fungal abundance and diversity across multiple scales, and the factors that drive spatial patterns, remains largely unknown in arid ecosystems. We examined multiple measures of AM fungal abundance, as well as spore diversity and community composition, at microsite (1 m2), local (1 ha), and regional (5000 ha) scales in semiarid shrublands. At the microsite scale, hyphae, spores, and glomalin-related soil protein were more abundant underneath shrub canopies, but unvegetated shrub interspaces had similar amounts of viable propagules, spore diversity, and spore community composition compared to canopies. Significant local and regional scale variation in abundance, diversity, and community composition were correlated with variation in soil organic matter, climate, and soil phosphorus concentration. We observed high alpha, beta, and gamma spore diversity and significant spatial autocorrelation of communities. This study demonstrates how multiple indicators of Glomeromycotan abundance and diversity vary differentially in natural systems and how soil and climate factors are important drivers of spatial patterns.

Long‐term redevelopment of resource islands in shrublands of the Great Basin, USA

Soil resource availability in semi‐arid and arid shrubland ecosystems is highly heterogeneous and includes patterns of accumulation primarily beneath shrubs as opposed to shrub interspaces. These resource islands contribute to ecosystem resilience after natural disturbances such as fire, yet very little is known regarding their redevelopment following soil disturbance and shrub re‐colonization. Cultivation involves the removal of native vegetation and mixing of soils both vertically and horizontally. The old fields in this study offered a unique look at the long‐term redevelopment of resource islands under shrubs where cultivation was abandoned nearly a century ago. Using adjacent pairs of previously cultivated and native shrubland from three soil series, we sampled surface soils (0–5 cm) in microsites under shrubs and in the interspaces between them to examine if the soil fertility in old fields (C, N, P, Ca, Mg, K) had regained similar microsite patchiness to the native shrubland, if the values of each soil fertility measure in old fields were different from native shrubland, and if the overall microsite fertility under shrubs in old fields was different from non‐disturbed microsites. We found that while most of the resource island patterning had redeveloped, the content of each fertility measure had not recovered to pre‐disturbance levels. Further, the recovery was different between the soil series and between sites with different dominant shrub species. There were also differences between sites within the same soil series, suggesting that historical cultivation practices may influence resource island recovery in multiple ways. Overall, under‐shrub microsite fertility in previously cultivated areas was distinct from comparable under‐shrub microsites in native areas in two out of the three soil series. These findings suggest that while patterning may redevelop within 90 years, it may take over a century for resource island fertility to fully re‐establish in some formerly cultivated soils.

Influence of aboveground vegetation on seed bank composition and distribution in a Great Basin Desert sagebrush community

The quantity, composition, and spatial dispersion of seed banks can greatly affect community dynamics. While seed banks of hot deserts have been studied extensively, little is known about seed banks in cold deserts, in particular the relationship between the seed bank and the aboveground vegetation. We investigated the relationship between the seed bank and aboveground vegetation and the effect of microhabitat (shrub interspace or beneath shrub) and aboveground community phase (high or low perennial bunchgrass cover) on the seed bank of a Great Basin Desert sagebrush community. The seed bank and aboveground vegetation differed in their most dominant species, resulting in moderately dissimilar species compositions as determined by Sørensen's similarity index and Bray–Curtis distance. In contrast, comparing the seed bank species composition to the aboveground vegetation structure (functional groups) using non-metric multidimensional scaling (NMDS) revealed a correspondence between the two communities. Shrub seed densities were higher beneath shrubs. Neither microhabitat nor community phase explained variation in total seed density or species richness. Therefore, our measures of the aboveground vegetation did not influence seed density across functional groups or species richness, and the similarity between the seed bank and aboveground vegetation varied depending on the aboveground organizational level used in comparisons.

Desert pavements as indicators of soil erosion on aridic soils in north-east Patagonia (Argentina)

Desert pavements are prominent features of many geomorphic surfaces in arid and semiarid lands. In the semiarid soils of north-eastern Patagonia, gravel cover in the shrub interspace areas of shrub-dominated communities is generally high, and contrast with that of grass-dominated patches where gravel cover is either absent or negligible. In the present study we analyze the relationship between soil erosion and desert pavement formation, in three sites, the upper, middle and lower slope positions of a flank pediment where well-conserved soils served as reference areas. We used the gravel cover and mass, as well as the thickness of the remnant A horizon, as determined by the depth of the Bt horizon of a Xeric Calciargid, as measures of soil erosion. Surface gravel at four cardinal points in respect to mounds associated with shrub-clumps was collected and the depth to the Bt horizon was determined. The mean thickness of the A horizon in the well-conserved soils were 11.3, 10.0 and 13.5 cm for the upper, middle and lower slope positions, respectively. For the same positions, the mean coarse fragment contents (> 2.0 mm) in the 0–10 cm depth of the A horizon in the well-conserved soils were 144, 92 and 119 g kg −1, and the mean surface gravel mass in the eroded patches were 5.3, 3.1 and 4.7 kg m −2. Surface gravel mass and depth of the remnant A horizon gave different estimates of the magnitude of soil erosion in the flank pediment. Thus, the mean/maximum soil loss, as determined by the mean gravel mass on the soil surface for the upper, middle and lower slope positions were, 28.3/68.2, 27.0/63.8 and 31.5/56.4 mm, respectively. These figures increased to a mean of 50.0, 52.5 and 82.0 mm for the same positions when soil loss was determined as the difference between the thickness of the A horizon of the well-conserved soil and that of the remaining A horizon in eroded patches. The loss of the A horizon by wind and water erosion seems to initiate the change from grass steppe to a stable shrub steppe characterized, in the shrub interspaces, by well-developed desert pavements. The strong correlation between surface gravel mass and the thickness of the remaining A horizon indicates that accelerated soil erosion has played an important role in the formation of desert pavements, and that desert pavements are good indicators of the extent and intensity of the erosion process in the Punta Ninfas area.

Spatial and temporal patterns of water availability in a grass–shrub ecotone and implications for grassland recovery in arid environments

AbstractEncroachment of woody shrubs into historic desert grasslands is a major problem throughout the world. Conversion of grasslands to shrub‐dominated systems may result in significant alteration of biogeochemical processes and reduced resource availability in shrub interspaces, making grassland recovery difficult. Soil petrocalcic horizons, which are common in arid and semi‐arid regions globally, have been shown to dramatically alter plant water availability. To assess how soil water dynamics are affected by woody encroachment in petrocalcic soils under contrasting precipitation patterns, we conducted a 3‐year replicated study in a mixed shrub–grass system measuring soil water in unvegetated interspaces and under mesquite canopies. Plots were instrumented with time‐domain reflectometry (TDR) moisture probes, both above and within the soil petrocalcic horizon. Soils under both cover types maintained large increases in available water content for several months during a wetter than normal winter and summer (increases of 0·08–0·16 m3 m−3). Interspace soils absorbed significantly greater quantities of water during the winter and retained more water into the spring than soils under shrubs. In contrast, soils under shrubs initially absorbed greater volumes of water during and following summer rains. Differing seasonal dynamics were attributed to interactions between the unique properties of soils with petrocalcic horizons and canopy‐induced variability in evapotranspiration. Observed patterns of plant‐available water do not support the hypothesis of greater resource availability under shrubs. Similar or greater water availability in shrub interspaces indicates that concentration of soil water under shrubs may not be a process limiting grass recovery on these soils. Published in 2010 by John Wiley & Sons, Ltd.

Arbuscular mycorrhizal fungal abundance in the Mojave Desert: Seasonal dynamics and impacts of elevated CO 2

Arbuscular mycorrhizal fungi (AMF) play important roles in ecosystem processes. However, little is known about AMF abundance in arid, nutrient-poor environments like the Mojave Desert of North America. We conducted two AMF studies: one examined AMF responses to elevated atmospheric CO 2 at the Nevada Desert FACE (Free-Air-Carbon dioxide-Enrichment) Facility (NDFF), and the second examined seasonal dynamics at nearby sites. In both studies, AMF measurements (root colonization, extra-radical hyphal [ERH] length, and two measures of glomalin-related soil protein [GRSP]) and environmental factors (root length, soil water content, and precipitation) were measured across microsites (beneath shrubs, shrub interspaces) for two species ( Larrea tridentata, Ambrosia dumosa). Elevated CO 2 did not significantly affect AMF measurements. Other NDFF studies show no change in fine root production under elevated CO 2 but show increased available nitrogen. We infer that additional fixed carbon under elevated CO 2 is not allocated to soil resource foraging. However, AMF varied seasonally. ERH seasonally declined across species and microsites, but GRSP declined only beneath L. tridentata. Our results combined with previous results indicate that drought negatively affects AMF root colonization. Robust inter-relationship among AMF measurements only occurred between the two measures of GRSP, indicating that resources are independently allocated to different AMF structures.

Moisture pulses, trace gas emissions and soil C and N in cheatgrass and native grass-dominated sagebrush-steppe in Wyoming, USA

Effects of large-scale weed invasion on the nature and magnitude of moisture-pulse-driven soil processes in semiarid ecosystems are not clearly understood. The objective of this study was to monitor carbon dioxide (CO2) and nitrous oxide (N2O) emissions and changes in soil carbon (C) and nitrogen (N) following the application of a water pulse in Wyoming big sagebrush (Artemisia tridentata ssp. wyomingensis) communities dominated by the exotic annual grass cheatgrass (Bromus tectorum) and by the native perennial grass western wheatgrass (Pascopyrum smithii). Sampling locations were established in shrub interspaces dominated by B. tectorum and P. smithi and beneath shrub canopies adjacent to interspaces dominated by B. tectorum and P. smithi, where no grass was present. Soils were classified as fine-loamy, mixed, Borollic Haplargids. Soil samples (0–10cm) and air samples were collected at 0, 4, 8, 24, 49, 72, and 216h following additions of 25.4mm of water. Soil samples were analyzed for dissolved organic carbon (DOC), microbial biomass carbon (MBC), extractable ammonia (NH4+), extractable nitrate (NO3−), and dissolved organic nitrogen (DON). Grass species induced differences in soil nitrification, N2O and CO2 emissions, and the quantity and timing of labile C available to microbial populations responding to increased moisture availability. In the first 8-h phase after wetting P. smithii soils had the greatest CO2 emissions compared to other soils but B. tectorum soils had the greatest N2O emissions and the greatest increases in CO2 emissions relative to before wetting. Microbial biomass C in B. tectorum interspace soils increased rapidly but the response was short-lived despite sufficient water availability. After the first 8h of soil response to wetting, the observed MBC declines in B. tectorum interspace coincided with disproportional DOC and DON concentration increases. Similar DOC and DON increases were also observed in B. tectorum soils beneath shrub canopy. In contrast, DOC and DON concentrations in P. smithii soils remained unaffected by soil wetting and small MBC increases observed during the first 8-h phase did not decline as rapidly as in B. tectorum interspace soils. In conclusion, summer drying-wetting cycles that occur frequently in areas invaded by B. tectorum can accelerate rates of nitrification and C mineralization, and contribute significantly to trace gas emissions from sagebrush-steppe grasslands. With frequent summer rainfall events, the negative consequences B. tectorum presence in the ecosystem can be significant.

Nutrient Availability in Rangeland Soils: Influence of Prescribed Burning, Herbaceous Vegetation Removal, Overseeding with Bromus Tectorum, Season, and Elevation

Soil nutrient availability influences plant invasions. Resin capsules were used to examine soil nutrient bioavailability along 2 sagebrush–grassland elevation transects in the east Tintic Range (Utah) and Shoshone Range (Nevada). In the fall of 2001, treatments were applied to 3 replicate plots at each site, which included prescribed burning, herbaceous vegetation removal, and controls. Cheatgrass (Bromus tectorum L.) was overseeded in small subplots within each treatment. Following treatments in each plot, resin capsules were installed at 15-cm depth in a shrub interspace and a B. tectorum–overseeded area. Nutrient availability was integrated during late fall to spring and spring to late fall for 2 years. Herbaceous vegetation removal increased availability of nitrate (Nevada and Utah) and Ca and Mg (Nevada only) but only during the second sampling period (growing season). Availability of K and ortho-P (Nevada and Utah) and nitrate (Nevada only) was greater on prescribed burned plots. For Utah, availability of ortho-P, K, Ca, Mg, and Fe generally increased with increasing elevation. Availability of Ca, Mg, K, and Fe was greatest during late fall to spring integration periods for Nevada. Overseeding with B. tectorum interacted with the burn treatment to influence availability of Ca, Mg, and Fe (Nevada sites only). Patterns of nutrient availability can be explained by a combination of decreased root uptake in relation to mineralization, differences in soil water content with season and elevation, and nutrient release from vegetation and soil as a consequence of prescribed burning. Herbaceous vegetation removal and burning can raise nitrate availability and increase risk of invasion by nitrophilic species such as B. tectorum. Nutrient availability can be out of phase with plant growth; plants capable of taking up nutrients during cold periods may have a competitive advantage. Resin capsules have utility in quantifying the effects of treatments on the availability of many soil nutrients.

A Leguminous Shrub ( Caragana microphylla) in Semiarid Sandy Soils of North China

Caragana microphylla Lam., a pioneer leguminous shrub species for vegetation re-establishment, is widely distributed in the semi-fixed and fixed sandy lands of the Horqin region. Some soil chemical and physical properties were measured under the canopy of C. microphylla and in the adjacent open areas to determine the effects of individual shrubs on soil properties. The influence of isolated C. microphylla on chemical and physical properties of the topsoil was significantly different between plots under the shrub canopy and in the shrub interspaces. Beneath the shrub canopy greater amounts of fine particle fractions, a higher water-holding capacity, and a lower bulk density, as well as higher aboveground and belowground litter biomass were found. Soil organic C and total N concentrations were 23%-31.6% and 14%-27.2% higher under the shrub canopies than in the shrub interspaces, respectively, giving rise to ”islands of fertility”. In a desertified sandy grassland ecosystem, C. microphylla was believed to play a major role in organic C sequestration, N accumulation, and the hydrologic cycle. Additionally, it has been found to be of ecological importance for vegetative restoration and reversal of desertification.

Effects of elevated CO2 on fine root dynamics in a Mojave Desert community: a FACE study

AbstractFine roots (≤1 mm diameter) are critical in plant water and nutrient absorption, and it is important to understand how rising atmospheric CO2 will affect them as part of terrestrial ecosystem responses to global change. This study's objective was to determine the effects of elevated CO2 on production, mortality, and standing crops of fine root length over 2 years in a free‐air CO2 enrichment (FACE) facility in the Mojave Desert of southern Nevada, USA. Three replicate 25 m diameter FACE rings were maintained at ambient (∼370 μmol mol−1) and elevated CO2 (∼550 μmol mol−1) atmospheric concentrations. Twenty‐eight minirhizotron tubes were placed in each ring to sample three microsite locations: evergreen Larrea shrubs, drought‐deciduous Ambrosia shrubs, and along systematic community transects (primarily in shrub interspaces which account for ∼85% of the area). Seasonal dynamics were similar for ambient and elevated CO2: fine root production peaked in April–June, with peak standing crop occurring about 1 month later, and peak mortality occurring during the hot summer months, with higher values for all three measures in a wet year compared with a dry year. Fine root standing crop, production, and mortality were not significantly different between treatments except standing crop along community transects, where fine root length was significantly lower in elevated CO2. Fine root turnover (annual cumulative mortality/mean standing crop) ranged from 2.33 to 3.17 year−1, and was not significantly different among CO2 treatments, except for community transect tubes where it was significantly lower for elevated CO2. There were no differences in fine root responses to CO2 between evergreen (Larrea) and drought‐deciduous (Ambrosia) shrubs. Combined with observations of increased leaf‐level water‐use efficiency and lack of soil moisture differences, these results suggest that under elevated CO2 conditions, reduced root systems (compared with ambient CO2) appear sufficient to provide resources for modest aboveground production increases across the community, but in more fertile shrub microsites, fine root systems of comparable size with those in ambient CO2 were required to support the greater aboveground production increases. For community transects, development of the difference in fine root standing crops occurred primarily through lower stimulation of fine root production in the elevated CO2 treatment during periods of high water availability.

Soil erosion as affected by shrub encroachment in northeastern Patagonia

Soil erosion is the primary cause of irreversible loss of soil productivity on most rangelands. In northeastern Patagonia, the increase in soil erosion has been closely associated with the increase in shrub cover in the grass or shrub-grass steppes. We used rainfall simulation to compare infiltration and sediment production from patches of grass, shrub-grass, and shrub steppes of the Punta Ninfas range site. Bare soil and gravel covers were higher and litter cover was lower in the shrub steppe than in the shrub-grass and the grass steppes. In the shrub inter-spaces of the shrub steppe, bulk density was greater and macroporosity and soil organic matter were lower (P less than or equal to 0.05) than in the mounds beneath shrubs and in the grass and shrub-grass areas. Infiltration rate was 60 to 65% lower in the shrub steppe than in the grass and shrub-grass steppes, respectively. On the contrary, total sediment production and concentration were higher (P less than or equal to 0.05) in the shrub steppe as compared to the grass and the shrub-grass areas. Gravel cover was the variable that best predicted infiltration and sediment production. The organic matter content of the sediment, mostly litter, in the shrub and the shrub-grass steppes were similar and greater (P less than or equal to 0.05) than in the grass steppe. Runoff litter removal may represent one of the processes that drive the transition from shrub-grass to shrub steppes. High rates of sediment removal, mainly litter, from the shrub interspaces of the shrub steppe may limit the natural recovery of the soil physical and hydrological properties. These degraded patches fail to capture incident rainfall and restrict the possibilities for the recovery of perennial grasses favoring the dominance of shrubs.