Arid Systems Research Articles

Different microhabitats affect soil macroinvertebrate assemblages in a Mediterranean arid ecosystem

Spatio-temporal variability is a key factor in conservation, management and restoration of ecosystems. Spatial heterogeneity is caused in many cases by organisms that are able to modify their environments. This is especially relevant in arid systems, where organisms such as shrubs and ants create patches of high nutrient availability (fertile islands) surrounded by a low-nutrient matrix. Although variations in structure and physiology among shrubs provoke differences in their effects as fertile islands, whether different microhabitats vary in their influence on animal communities is poorly known. The principal aim of our study is to analyse the effects of different shrub species and Messor harvester ant-nest mounds on the structure of soil macroarthropod assemblages in a strongly seasonal desert location of SE Spain. Shrub microhabitats and ant-nest mounds maintained higher species density, abundance and biomass of soil macroinvertebrates than the surrounding soil matrix. The different microhabitats differed in taxonomic and trophic composition, abundance, and biomass of soil macroinvertebrates, at both litter and belowground levels. Also, variations of invertebrate abundance among microhabitats changed throughout sampling periods. Thus the spatio-temporal heterogeneity of the study site affected the distribution and dynamics of the macroinvertebrate community. The above results indicate that the spatio-temporal mosaic created by microhabitat and seasonal variations on macroinvertebrate assemblages is a relevant issue to be considered in conservation, sustainable management and environmental restoration in heterogeneous arid systems to preserve their biodiversity and ecosystem functioning.

Determinants of parasitic plant distribution: the role of host qualityThis article is one of a collection of papers based on a presentation from the Stem and Shoot Fungal Pathogens and Parasitic Plants: the Values of Biological Diversity session of the XXII International Union of Forestry Research Organization World Congress meeting held in Brisbane, Queensland, Australia, in 2005.

Parasitic plants are less affected by resource constraints than other plants and most exhibit broad host tolerances, occupy large distributional ranges, and produce high numbers of propagules. Yet, parasitic plants are characteristically rare in undisturbed habitats, and patterns of distribution within host populations are often highly nonuniform. Previous work on root and shoot parasites has identified strict germination requirements for many species but, while explaining host ranges and site–microsite preferences for particular species, this cannot account for the highly clumped spatial structure of many parasitic plant populations. Other research has examined the role of seed vectors, but in most systems studied, dispersers are not limiting and their dietary breadth, substrate use, habitat preferences, and distributional ranges exceed the extent of the parasitic plant. Here, I propose the “host-quality hypothesis,” suggesting that variation in the quality of potential hosts can account for nonrandom occurrence patterns of parasitic plants in many systems. “Quality” can relate to access to water, nutrients, or other resources that are generally limiting to hosts, whereby parasites are more likely to establish and survive on hosts with greater access (i.e., higher quality from the parasite’s perspective). Rather than supplanting germination requirements operating at the individual host plant scale or disperser behaviour operating at the landscape scale, this resource-based hypothesis applies at stand and population scales, explaining why some individuals within a stand or population are infected, while other apparently similar hosts are not susceptible. This hypothesis is explored using case studies on root and shoot hemiparasites, and is consistent with a diverse array of findings from a range of temperate and arid systems.

Patterns of accessing variable resources across time and space: Desert plants, animals and people

Key resources in arid and semi-arid systems are highly variable in time and space. Biophysical variability is mainly driven directly or indirectly by rainfall. Arid-dwelling people are also exposed to variability – indirectly from their biophysical environments, and directly as a result of sparse, mobile populations and variable, distant signals from policy and markets. The tradeoffs between the benefits and costs of increasing the scale of resource use are comparatively more critical in shaping system dynamics in arid systems than mesic environments. Understanding the strategies employed to scale resource use is vital for arid-system management. We contribute to this discussion using a conceptual model, based on modern portfolio theory, to explore how variability is managed by diversifying access to resources across space and time. The model is developed using ecological ideas and our Australian case knowledge, but demonstrates general learning on how optimal investment in strategies changes with resource distribution, and with the physical characteristics of the entity (e.g. plant, animal) and resource (e.g. water, nutrients) in question. Using pastoral enterprise, arid-system administration and policy cases, we argue that people and organisations need variability-coping strategies that are comparable with biophysical responses, and that strategy behaviour needs greater acknowledgement in managing arid systems.

Patch dynamics of soil biotic feedbacks in the Sonoran Desert

Soil fertility in arid ecosystems is spatially and temporally heterogeneous, due primarily to biotic feedbacks from perennial plants. While the “island of fertility” effect is well-established, little is known about the influence of plant functional characteristics on soil nutrients, or changes in nutrient levels following mortality or microsite succession. To assess these patterns, we studied a functionally diverse Sonoran Desert ecosystem in which all microsite state transitions were determined. The woody species (Ambrosia deltoidea, Larrea tridentata, Parkinsonia microphylla) significantly enhanced soil organic matter, nitrogen and phosphorous levels (referred to generally as fertility) to varying degrees. Effect sizes differed significantly among species but were surprisingly similar. However, following mortality, fertility remained constant beneath Ambrosia and increased significantly beneath decaying Parkinsonia, contrary to expected declines. These results suggest that nutrients are recycled within a microsite, resulting in similar pool sizes beneath live plants across species, regardless of morphological or phonological differences. Microsites occupied by cacti exhibited no differences from bare soil, indicating that cacti may erase islands of fertility. However, nitrate and phosphate levels spiked following mortality of the large cactus, Carnegiea gigantea. In general, fertility effects were correlated with morphological characteristics that should be applicable across most arid systems.

Badger (Taxidea taxus) disturbances increase soil heterogeneity in a degraded shrub-steppe ecosystem

In the western United States, overgrazing, weed invasion and wildfire have resulted in the conversion of shrub-steppe to annual grasslands, with substantial effects on ecosystem function. In these landscapes, badgers disturb large areas of soil while foraging for fossorial animals. Mounds created by badgers contained the lowest concentrations of total carbon, nitrogen and sulphur, mineral nitrogen and mineralizable nitrogen, inter-mound soils had the highest concentrations, and excavation pits had intermediate levels. Soil C:N ratio and pH were greater, and electrical conductivity and soluble Ca2+, Mg2+ and K+ were lower on mound soils compared with either pit or inter-mound soils. Larger pits generally trapped more litter, and increased litter mass equated with greater concentrations of active carbon, but only at the burned sites. Older mounds supported more vascular plants and cryptogamic crusts. Our results demonstrate reduced levels of nutrients and a higher C:N ratio on the mounds compared with either the pits or inter-mounds. Alteration to the homogeneous post-fire landscape by badgers contributes to patchiness in soils and vegetation, which is critical to the functioning of arid systems. Given their effect on soil C:N ratios, mounds may be important sites for recovery of indigenous shrub-steppe plant species.

Social networks in arid Australia: a review of concepts and evidence

Arid systems are markedly different from non-arid systems. This distinctiveness extends to arid-social networks, by which we mean social networks which are influenced by the suite of factors driving arid and semi-arid regions. Neither the process of how aridity interacts with social structure, nor what happens as a result of this interaction, is adequately understood. This paper postulates three relative characteristics which make arid-social networks distinct: that they are tightly bound, are hierarchical in structure and, hence, prone to power abuses, and contain a relatively higher proportion of weak links, making them reactive to crisis. These ideas were modified from workshop discussions during 2006. Although they are neither tested nor presented as strong beliefs, they are based on the anecdotal observations of arid-system scientists with many years of experience. This paper does not test the ideas, but rather examines them in the context of five arid-social network case studies with the aim of hypotheses building. Our cases are networks related to pastoralism, Aboriginal outstations, the ‘Far West Coast Aboriginal Enterprise Network’ and natural resources in both the Lake-Eyre basin and the Murray–Darling catchment. Our cases highlight that (1) social networks do not have clear boundaries, and that how participants perceive their network boundaries may differ from what network data imply, (2) although network structures are important determinants of system behaviour, the role of participants as individuals is still pivotal, (3) and while in certain arid cases weak links are engaged in crisis, the exact structure of all weak links in terms of how they place participants in relation to other communities is what matters.

DIETS OF ADULTS AND LAMBS OF DESERT BIGHORN SHEEP DURING YEARS OF VARYING RAINFALL IN CENTRAL ARIZONA

Abstract Effects of unpredictable, variable rainfall in arid systems on diets of adults and lambs of desert bighorn sheep (Ovis canadensis) are poorly understood. We determined associations based on analysis of feces between amounts of rainfall and composition and similarity (overlap and diversity) of diets of adults and lambs, and relationships between diets of adults and lambs, in the Sonoran Desert of central Arizona during 1999–2003. Drought occurred in 2 of 3 years during sampling of feces of adults, and in 1 of 3 years during sampling of feces of lambs. Composition and diversity of diets of adults were comparable during years of drought and normal rainfall, except that diets were not correlated between years when severe drought (31% of long-term average rainfall) followed normal rainfall. Composition and diversity of diets of lambs also were independent of amounts of rainfall, but dietary overlap between and within age groups was greater during years of increasing rainfall compared to previous years...

Insect community response to plant diversity and productivity in a sagebrush–steppe ecosystem

In natural systems, insect diversity and abundance are often positively correlated with plant diversity; however, these relationships may be obscured or complicated by other characteristics of plant communities that influence insect distributions. For example, diversity and abundance of insects may also be related to structural complexity, nutrition, or productivity of plants. In arid systems, primary production is generally enhanced by supplemental moisture, which may provide food resources of greater abundance and quality for herbivores, resulting in greater insect diversity and abundance. Although the association of insect distributions with sites of either high plant diversity or productivity has been studied extensively, the relative importance of these two factors in influencing insect diversity and abundance remains unclear. We examined how insect communities in sagebrush steppe responded to two plant community treatments (a crested wheatgrass monoculture and a diverse community of native shrubs, grasses, and forbs) and three irrigation regimes (summer irrigation, fall/spring irrigation, and ambient precipitation only) on an experimental site designed to test protective cap designs for isolating buried hazardous waste. In both 2000 and 2001, insect diversity and abundance were positively correlated with both plant diversity and irrigation early in the summer; however, by the end of the summer, insect distributions were more strongly influenced by irrigation treatment. Our results suggest that during the dry season in this semi-arid shrub–steppe system, plant diversity may be secondary to moisture effects in influencing insect distributions.

Influence of temporal heterogeneity in nitrogen supply on competitive interactions in a desert shrub community

Soil nutrients in arid systems are supplied to plants in brief pulses following precipitation inputs. While these resource dynamics have been well documented, little is known about how this temporal heterogeneity influences competitive interactions. We examined the impacts of the temporal pattern of N supply on competitive intensity and ability in an N-limited desert shrub community. At our field site, the three codominant shrubs, Atriplex confertifolia, A. parryi, and Sarcobatus vermiculatus, differ in seasonal growth patterns, with A. confertifolia and S. vermiculatus achieving higher growth rates earlier in the growing season than A. parryi. We predicted that these timing differences in maximum growth rate may interact with temporal variation in N supply to alter competitive abilities over time. Seedlings of the two Atriplex species were planted either individually in field plots or as target plants surrounded by neighbor seedlings. After one year of establishment, the same amount of (15)N was applied to plots either as early spring pulses, mid spring pulses or continuously through the second growing season. Competitive effects were observed under continuous and pulsed N supply. Averaged across all target-neighbor treatments, competitive intensity was approximately 1.8-fold greater when N was pulsed compared to when N was supplied continuously, but overall, the outcome of competitive interactions was not influenced by N pulse timing. While the timing of resource supply did not differentially influence the competitive abilities of coexisting species in this system, the temporal pattern of resource supply did alter the intensity of competitive interactions among species. While additional studies in other systems are needed to evaluate the generality of these results, this study suggests that competitive intensity may not necessarily be a direct function of productivity or resource availability as traditionally assumed. Instead, the intensity of competitive interactions in resource-poor systems may depend upon the temporal pattern of resource supply.

Semi‐arid grazing systems and climate change: a survey of present modelling potential and future needs

Summary Sustainable land use under climate change requires detailed knowledge of the system dynamics. This applies particularly for the management of domestic livestock in semi‐arid and arid grazing systems, where the risk of degradation is high and likely climate change may have a strong impact. A suitable way to assess potential future trends of these complex systems is through the application of simulation models. We reviewed 41 models published between 1995 and 2005 simulating semi‐arid and arid livestock grazing systems. The models were categorized according to the model aim and type, their temporal and spatial scale, and several indicators of model complexity. Additionally, we developed a list of model requirements for adequately simulating the effects of climate change. Based on these requirements, we evaluated the potential of current models to simulate impacts of climate change and determine important shortcomings. Three general model types could be distinguished, namely state and transition, difference and differential equation, and rule‐based models. Over time, we found that the number of models aiming to improve management strategies increased, while there were fewer models that aimed to understand system dynamics. This was accompanied by a trend to simplify model descriptions of hydrological relationships. Important shortcomings of current models included the impact of increased CO2 levels on plant productivity and the ability to resolve changes in intra‐annual precipitation patterns. The consideration of both external drivers is crucial under climate change, hence sustainable long‐term decision making is currently lacking important information. Synthesis and applications. Sustainable livestock management in semi‐arid and arid systems requires knowledge about effects of future climate change to adjust livestock density adequately. Producers’ experiences from current weather conditions are not necessarily transferable to future conditions, thus models could help to support management. However, an analysis of current models has shown that few existing models are able to assess the impacts of the predicted climate change. Therefore, we call for the development of new dynamic grazing models that provide land managers with the necessary tools to face the threat of future climate changes.

The human role in changing river channels

Direct consequences of the human role, where human activity affects river channels through engineering works including channelization, dam construction, diversion and culverting, have been long recognised [Marsh, G.P., 1864. Man and Nature or Physical Geography as Modified by Human Action. Charles Scribner, New York; Thomas Jr., W.L., (ed.) 1956. Man's Role in Changing the Face of the Earth. Chicago, University of Chicago Press, Chicago.]. The less obvious indirect effects of point and reach changes occurring downstream and throughout the basin, however, are much more recently appreciated, dating from key contributions by Strahler [Strahler, A.N., 1956. The nature of induced erosion and aggradation. In W. L. Thomas (Ed.), Man's Role in Changing the Face of the Earth. University of Chicago Press, Chicago, 621–638.], Wolman [Wolman, M.G., 1967. A cycle of sedimentation and erosion in urban river channels. Geografiska Annaler 49A, 385–95.], Schumm [Schumm, S.A., 1969. River metamorphosis. Proceedings American Society of Civil Engineers, Journal Hydraulics Division 95, 255–73.], and Graf [Graf, W.L., 1977. The rate law in fluvial geomorphology. American Journal of Science, 277, 178–191.]. These are complemented by effects of alterations of land use, such as deforestation, intensive agriculture and incidence of fire, with the most extreme effects produced by building activity and urbanisation. Changing river channels are most evident in the channel cross-section where changes of size, shape and composition are now well-established, with up to tenfold increases or decreases illustrated by results from more than 200 world studies. In addition the overall channel planform, the network and the ecology have changed. Specific terms have become associated with changing river channels including enlargement, shrinkage and metamorphosis. Although the scope of adjustment has been established, it has not always been possible to predict what will happen in a particular location, because of complex response and contingency. The ways in which changes in cross-section relate to reach and network changes are less clear, despite investigations showing the distribution of changes along segmented channels. When considering the human role in relation to changing river channels, at least five challenges persist. First, because prediction of the nature and amount of likely change at a particular location is not certain, and because the contrasting responses of humid and arid systems needs to be considered, modelling is required to reduce uncertainty, as was first emphasised by Burkham [Burkham, D.E., 1981. Uncertainties resulting from changes in river form. American Society Civil Engineers Proceedings, Journal Hydraulics Division 107, 593–610.]. Second, feedback effects incorporated within the relationship between changes at channel, reach and network scales can have considerable implications, especially because changes now evident may have occurred, or have been initiated, under different environmental conditions. Third, consideration of global climate change is imperative when considering channel sensitivity and responses to threshold conditions. Fourth, channel design involving geomorphology should now be an integral part of restoration procedures. This requires, fifthly, greater awareness of different cultures as a basis for understanding constraints imposed by legislative frameworks. Better understanding of the ways in which the perception of the human role in changing river channels varies with culture as well as varying over time should enhance application of design for river channel landscapes.

Seed Banks in Arid Wetlands with Contrasting Flooding, Salinity and Turbidity Regimes

Aquatic plant communities in arid zone wetlands underpin diverse fauna populations and ecosystem functions yet are relatively poorly known. Erratic flooding, drying, salinity and turbidity regimes contribute to habitat complexity, creating high spatial and temporal variability that supports high biodiversity. We compared seed bank density, species richness and community composition of aquatic plants (submergent, floating-leaved and emergent) among nine Australian arid zone wetlands. Germinable seed banks from wetlands within the Paroo and Bulloo River catchments were examined at nested scales (site, wetland, wetland type) using natural flooding and salinity regimes as factors with nondormant seed density and species richness as response variables. Salinity explained most of the variance in seed density (95%) and species richness (68%), with flooding accounting for 5% of variance in seed density and 32% in species richness. Salinity-flooding interactions were significant but explained only a trivial portion of the variance (<1%). Mean seed densities in wetlands ranged from 40 to 18,760 m−2 and were highest in wetlands with intermediate levels of salinity and flooding. Variability of densities was high (CVs 0.61–2.66), particularly in saline temporary and fresh permanent wetlands. Below salinities of c. 30 g l−1 TDS, seed density was negatively correlated to turbidity and connectivity. Total species richness of wetlands (6–27) was negatively correlated to salinity, pH and riverine connectivity. A total of 40 species germinated, comprising submergent (15 species), floating-leaved or amphibious (17 species), emergent (6 species) and terrestrial (6 species) groups. Charophytes were particularly important with 10 species (five Chara spp., four Nitella spp. and Lamprothamnium macropogon), accounting for 68% of total abundance. Saline temporary wetlands were dominated by Ruppia tuberosa, Lamprothamnium macropogon and Lepilaena preissii. Variable flooding and drying regimes profoundly altered water quality including salinity and turbidity, producing distinctive aquatic plant communities as reflected by their seed banks. This reinforces the importance of hydrology in shaping aquatic biological communities in arid systems.

Small-scale habitat use and assemblage structure of ground-dwelling beetles in a Patagonian shrub steppe

Studies in arid systems have often focussed on the origin and maintenance of vegetation patchiness, but little is yet known about how microhabitat structure created by woody plants affects soil macroarthropods. We studied patterns of small-scale habitat use by darkling and ground beetles (Coleoptera: Tenebrionidae and Carabidae) in a two-phase shrub steppe mosaic, in NW Patagonia, Argentina. Beetles were sampled using pitfall traps during the early (December–January) and late (February–March) activity season (3675 trap-days). Traps were placed in three microsites: shrub centre, shrub periphery and off-shrub bare-soil areas. We also experimentally tested effects of aerial cover and litter accumulation, two key features of shrub patches, on beetle activity and assemblage structure. We captured 1415 individuals, the majority being omnivorous tenebrionids (67%) and predaceous carabids (28%). All recorded species (five tenebrionids plus two carabids) occurred in all three microsites and in both trapping seasons. Overall beetle activity, biomass and diversity were consistently greater in shrub microsites than in bare-soil areas. Mean diurnal soil temperatures were higher in bare soil (37.6 °C) than beneath shrubs (23.6 °C). Tenebrionid activity decreased markedly in the late season, coinciding with the increased frequency of daily maximum soil temperatures over 30 °C (shrub) and 50 °C (bare soil). Carabid activity did not vary with season, reflecting a shift in species dominance not found among tenebrionids. Addition of artificial shelters to bare-soil patches revealed differing perceptions of microhabitat cover by beetle groups, carabids being less selective than tenebrionids. In contrast, litter manipulation did not significantly alter microhabitat use by beetles. We conclude that shrubs act as ‘keystone structures’ providing sheltered habitat to ground-dwelling macroarthropods. Thus, disturbances and management actions altering shrub cover may have unwanted, community-wide impacts on biodiversity and ecosystem processes.

Plant nitrogen capture in pulse‐driven systems: interactions between root responses and soil processes

Summary Soil resources are heterogeneously distributed in natural systems. In arid systems, for example, soil nitrogen (N) is often supplied in pulses. Mechanisms influencing the ability of a species to exploit N pulses through the season, however, are poorly understood despite the strong potential for temporal variation in N supply to impact growth, survival and competitive interactions in these systems. We examined how plant physiological and soil processes interacted to influence the ability of two dominant perennial Atriplex species to capture N from pulses occurring at different times in the growing season. 15N‐labelled pulses were applied to the two Atriplex species in early and mid spring. Sequential time harvests were used to quantify changes in plant 15N content, root length, root length relative growth rate, root N inflow rates, microbial biomass 15N, soil water content and soil inorganic 15N pools. Path analysis and structural equation modelling were used to quantify the relative importance of different root parameters for plant N capture from pulses and to evaluate the degree to which these root responses interacted with soil processes to influence plant N capture. Plant N capture was greatest when pulses coincided with high root length relative growth rates. Declining availability of total inorganic N through a pulse had a limited effect on N capture. This was partly because soil pools were removed from the soil system four times faster than soil pools, allowing sufficient supply to roots, although total inorganic N pools declined. Microbial immobilization rates did not change significantly through a pulse and did not influence plant N capture during a pulse. Instead, plant N capture during a pulse was limited by total root length and uptake capacity per unit root length, which in turn was affected by plant N demand and soil water content. Understanding interactions between root responses, soil processes and pulse timing provides insight into mechanisms underlying competitive interactions and diversity maintenance in pulse‐driven systems.

Seasonal timing of N pulses influences N capture in a saltbush scrub community

Limiting resources are generally available in brief temporal pulses in arid systems. We compared the abilities of dominant shrubs in a saltbush scrub community to capture N from pulses and evaluated whether N capture and partitioning within this community is influenced by the seasonal timing of pulses. Based on previous research in agronomic systems we predicted that the ability of a species to capture N following a pulse would depend on when the pulse occurred in relation to plant growth rate and N demand. Supporting this hypothesis, Atriplex confertifolia and Sarcobatus vermiculatus, which had greater growth rates early in the growing season compared to Atriplex parryi, captured more N from early spring pulses than A. parryi. Atriplex parryi, which had higher growth rates later in the growing season, captured more N from mid- and late spring pulses than the other species. These temporal differences in N capture among species, however, also depended on the magnitude of the N pulse. These results suggest that temporal variation in N availability may differentially impact competitive abilities of coexisting species and potentially facilitate species coexistence in arid systems.

Influence of resource pulses and perennial neighbors on the establishment of an invasive annual grass in the Mojave Desert

Invasion by exotic annual grasses is one of the most significant threats to arid ecosystems in the western USA. Current theories of invasibility predict plant communities become more susceptible to invasion whenever there is an increase in the amount of unused resources. The objective of this field study was to examine how resource pulses and temporal variation in resource demand by the native shrub vegetation influences establishment of the invasive annual grass Schismus arabicus. Water and nitrogen were applied as pulses in early spring, mid-spring, or continuously throughout the growing season to plots containing either Atriplex confertifolia or Atriplex parryi shrubs. The effect of resource pulses on Schismus density and biomass was highly dependent on the seasonal timing of the resource pulses and the identity of the neighbor shrub. When resource pulses coincided with high rates of resource capture and growth of the native vegetation, density and biomass of Schismus was reduced. Schismus establishment was greater under continuous resource supply compared to pulsed resource supply, likely because more soil resources were available at a shallow depth when resources were supplied at a continuous low rate. These results suggest that the establishment of invasive annual grasses in arid systems can be influenced by the magnitude and spatial distribution of resource pulses in addition to the seasonal timing of resource pulses.

The role of ultraviolet radiation in litter decomposition in arid ecosystems

In arid ecosystems, as much as 75% of solar radiation that penetrates the atmosphere hits the surface of the soil. The combination of high irradiance, high temperature, and low moisture puts constraints on the activity and organization of microbial communities. To separate the direct effects of UV absorbance on litter decomposition from the indirect effects of microbial selection, we placed mixed cohorts of senescent piñon ( Pinus edulis) and juniper ( Juniperus monosperma) litter into triplicate microcosms assigned to four treatments: UV irradiated (0.6 mW/cm 2 UV-A and UV-B for 12 h/day), with and without water additions, and non-irradiated with and without water additions. After 26 weeks, mass loss rates did not differ significantly among treatments with 90% organic matter remaining for piñon litter and 60% for juniper. The distribution and abundance of functional groups as assessed by FTIR spectra did not reveal any differences in relation to treatment, but differences were observed in the quantity and quality of dissolved organic carbon (DOC) extracted from the samples. The amount of DOC extracted increased by 56% for piñon litter and decreased by 32% for juniper in the UV-only treatment compared to initial values. Litter treated with UV and water had the lowest concentration of DOC with a decrease of 69% for juniper litter and 28% for piñon. The largest concentration of reactive phenols was found in the UV-only treatment with a 309% increase for piñon litter and a 10% reduction for juniper when compared to initial values. The treatments receiving water in the presence or absence of UV showed a similar response for both litter types, a reduction of 20–30% in phenolic concentration. Five extracellular enzyme activities, used, as indicators of microbial activity, were higher in the treatments that received water, but activities did not show an interaction with UV irradiation. The results suggest that UV radiation alone, or in combination with microbial activity, was as effective at decomposing litter as microbial activity alone. Thus, solar radiation can be an important contributor to litter degradation in arid systems.

Distribution of Soil Organic and Inorganic Carbon Pools by Biome and Soil Taxa in Arizona

Arid systems represent an important component of the global soil C budget in that they cover 12% of the global land area and contain nearly 20% of global soil C stocks, both organic (SOC) and inorganic (SIC). The objectives of this study were to quantify SOC and SIC stocks in Arizona biomes, using Arizona as a model system for arid lands. Biome distribution was extracted from the Arizona Gap Analysis Project spatial vegetation dataset (GAP), while soil C data were extracted from the Arizona State Soil Geographic Dataset (STATSGO) at a scale of 1:250 000, and the western Yavapai County Soil Survey Geographic Dataset (SSURGO) at a scale of 1:24 000. Soil data were converted from a polygonal vector format to a raster format, and a raster‐based method used to estimate SOC and SIC stocks by biome. Statewide, STATSGO soil C stocks indicate Arizona contains 0.5 and 1.5 Pg of SOC and SIC, respectively, with 27% of the SOC in pinyon‐juniper biomes (PJ), and 34% of SIC in creosotebush‐bursage biomes (CB). A comparison of soil C estimates between datasets indicates significantly greater estimates of biome SOC and SIC using SSURGO data relative to the STATSGO data. SSURGO soil C estimates varied considerably between the raster‐based and soil taxa based method of data aggregation. Soil taxa data exhibited large intra‐unit variation in each biome. In addition, soil C differed substantially between biomes by soil taxa (e.g., Haplargid SOC of 4.0 and 13.5 kg m−2 in the paloverde‐cacti (PC) and montane pine (MP) forest biomes, respectively). Raster based soil C estimations incorporate the spatial distribution and areal land cover of each soil type within a biome, providing a more accurate representation of soil C stocks.

Global patterns and sources of dissolved organic matter export to the coastal zone: Results from a spatially explicit, global model

Here we describe, test, and apply a system of spatially explicit, global models for predicting river export of three dissolved organic matter (DOM) components: dissolved organic carbon (DOC), dissolved organic nitrogen (DON), and dissolved organic phosphorus (DOP). The DON and DOP models represent the first attempt to model DON and DOP export in a spatially explicit, global manner. DOC, DON, and DOP models explain 88%, 77%, and 91% of the variability in DOC, DON, and DOP yield (kg C, N, or P km−2 yr−1) from validation basins, respectively, and all models are relatively bias free. When applied globally, these models predict that 170 Tg C yr−1, 10 Tg N yr−1, and 0.6 Tg P yr−1 are exported by rivers to the coastal zone as DOC, DON, and DOP, respectively. Because predicted spatial patterns of export for DOC, DON, and DOP are all largely driven by water runoff, geographic distributions of high and low fluxes are fairly consistent across elements, with high fluxes of DOC, DON, and DOP generally predicted for high runoff systems and low fluxes predicted for arid systems. However, there are important regional differences in predicted rates of DOC, DON, and DOP export due to anthropogenic inputs of DON and DOP and wetland influence on DOC.

Re‐analysis of meta‐analysis: support for the stress‐gradient hypothesis

Summary Using meta‐analysis, Maestre et al. (2005, Journal of Ecology, 93, 748–757) recently rejected the predictions of the stress‐gradient hypothesis for arid and semi‐arid environments in entirety. It was concluded that neither positive nor negative effects of neighbours increased with abiotic stress and that different theoretical models are now needed. In light of this sweeping conclusion, we re‐examined the analytical approach and explored the general synthetic power of meta‐analysis. Detailed statistical re‐analyses demonstrated that some of the meta‐analyses of Maestre et al. were robust. However, more rigorous data selection criteria, changing gradient lengths between studies and covariance in response effects did not support their original conclusions. Additionally, application of more rigorous data selection criteria did allow us to detect a significant and consistent positive effect of neighbours, which suggests that facilitation is important at many points along stress gradients. Careful evaluation of the studies used by Maestre et al. also revealed serious limitations. Many studies included in the meta‐analyses were not conducted along stress gradients, did not identify a stress gradient within the study, focused on invasive species or were not peer reviewed. Most importantly, however, gradient lengths were not quantified and appeared to differ dramatically among studies. This crucial source of variation was not accounted for statistically nor in the interpretations. Meta‐analyses are useful tools for synthesis and description but are inherently limited by the appropriateness of the data selected. Unfortunately, in this particular instance, the data available to and selected by Maestre et al. did not adequately test the stress‐gradient hypothesis and cannot thus reject its value for understanding the organization of plant communities in arid systems. The ecological implication of our synthesis is that meta‐analytical summary statistics may not always tell the whole story. Alternative interpretations of differences in effect sizes (or lack thereof) are possible because studies will vary in their ability to test specific predictions of a hypothesis, and furthermore, a certain level of judgement is required to infer the relative importance of certain ideas to synthetic progress within a discipline.