Northern Chihuahuan Desert Research Articles

The population dynamics of an endemic collectible cactus

Astrophytum is one of most collected genera in the cactus family. Around the world several species are maintained in collections and yearly, several plants are taken from their natural habitats. Populations of Astorphytum capricorne are found in the northern Chihuahuan desert, Mexico, and as many endemic cactus species, it has a highly restricted habitat. We conducted a demographic study from 2008 to 2010 of the northern populations found at Cuatro Ciénegas, Mexico. We applied matrix population models, included simulations, life table response experiments and descriptions of the population dynamics to evaluate the current status of the species, and detect key life table stages and demographic processes. Population growth rate decreased in both years and only 4% individual mortality can be attributed to looting, and a massive effort is needed to increase seedling recruitment and reduce adult mortality. The fate of individuals differed between years even having the same annual rainfall mainly in accentuated stasis, retrogression and high mortality in all size classes, which coupled with low seed production, no recruitment and collection of plants are the causes contributing to population decline, and hence, increase the risk in which A. capricorne populations are found. Reintroduction of seedlings and lowering adult mortality are urgently needed to revert the alarming demographic condition of A. capricorne populations.

Soil property variation within an aridisol in Big Bend National Park, Texas, USA

The ecological and edaphic characteristics that dictate the existence and extent of typical vegetation cover types on Tornillo loam (Ustifluventic Haplocambid), a soil common to the northern Chihuahuan Desert region within the boundaries of Big Bend National Park (BBNP) and prone to erosion, are not well-known. The objective of this study was to evaluate near-surface soil physical and chemical properties as potential explanatory characteristics for different patchy vegetation types [i.e., bare soil, creosote (Larrea tridentate) flat, and grass] on Tornillo loam in the northern Chihuahuan Desert region of BBNP in southwest Texas. Results showed that soil bulk density in the top 5cm and penetration resistance in the top 20cm were similar (P>0.05) among all three vegetation cover types, but soil particle-size distributions in the top 20cm differed significantly (P<0.05). Extractable soil Na and soil pH in the top 6cm were both greater (P<0.05) in the bare-soil areas (482mgkg−1 and 8.9, respectively) than in the grass patches or creosote flats, which did not differ (115mgkg−1 and 8.5, respectively). Future research should further investigate interactions between near-surface soil chemical characteristics, particularly soil Na concentration and pH, and past and future restoration activities to promote revegetation and control erosion.

Grassland to shrubland state transitions enhance carbon sequestration in the northern Chihuahuan Desert.

The replacement of native C4 -dominated grassland by C3 -dominated shrubland is considered an ecological state transition where different ecological communities can exist under similar environmental conditions. These state transitions are occurring globally, and may be exacerbated by climate change. One consequence of the global increase in woody vegetation may be enhanced ecosystem carbon sequestration, although the responses of arid and semiarid ecosystems may be highly variable. During a drier than average period from 2007 to 2011 in the northern Chihuahuan Desert, we found established shrubland to sequester 49 g C m(-2) yr(-1) on average, while nearby native C4 grassland was a net source of 31 g C m(-2) yr(-1) over this same period. Differences in C exchange between these ecosystems were pronounced--grassland had similar productivity compared to shrubland but experienced higher C efflux via ecosystem respiration, while shrubland was a consistent C sink because of a longer growing season and lower ecosystem respiration. At daily timescales, rates of carbon exchange were more sensitive to soil moisture variation in grassland than shrubland, such that grassland had a net uptake of C when wet but lost C when dry. Thus, even under unfavorable, drier than average climate conditions, the state transition from grassland to shrubland resulted in a substantial increase in terrestrial C sequestration. These results illustrate the inherent tradeoffs in quantifying ecosystem services that result from ecological state transitions, such as shrub encroachment. In this case, the deleterious changes to ecosystem services often linked to grassland to shrubland state transitions may at least be partially offset by increased ecosystem carbon sequestration.

Modelling the shrub encroachment in a grassland with a Cellular Automata Model

Abstract. Arid and semi-arid grasslands of southwestern North America have changed dramatically over the last 150 years as a result of shrub encroachment, i.e. the increase in density, cover and biomass of indigenous shrubby plants in grasslands. Numerous studies have documented the expansion of shrublands in the southwestern American grasslands; in particular shrub encroachment has occurred strongly in part of the northern Chihuahuan desert since 1860. This encroachment has been simulated using an ecohydrological Cellular Automata model, CATGraSS. It is a spatially distributed model driven by spatially explicit irradiance and runs on a fine-resolution gridded domain. Plant competition is modelled by keeping track of mortality and establishment of plants; both are calculated probabilistically based on soil moisture stress. For this study CATGraSS has been improved with a stochastic fire module and a grazing function. The model has been implemented in a small area in Sevilleta National Wildlife Refuge (SNWR), characterized by two vegetation types (grass savanna and creosote bush shrub), considering as encroachment causes the fire return period increase, the grazing increase, the seed dispersal caused by animals, the role of wind direction and plant type competition. The model is able to reproduce the encroachment that has occurred in SNWR, simulating an increase of the shrub from 2% in 1860 to the current shrub percentage, 42%, and highlighting among the most influential factors the reduced fire frequency and the increased grazing intensity.

Long‐term mark‐and‐recapture study of a freshwater mussel reveals patterns of habitat use and an association between survival and river discharge

Summary Climate change and human population growth threaten the supply of fresh water for human use and freshwater biodiversity. Long‐term studies are necessary to identify the effects of such temporal trends on biological and ecological phenomena; however, the collection of long‐term data can be costly and time‐consuming. We investigated the effect of hydrological variation over time on population dynamics in a perennial river of the northern Chihuahuan Desert, using an imperilled freshwater mussel (Popenaias popeii) as a model. We conducted a 15‐year mark‐and‐recapture study, and distance sampling, to estimate demographic parameters while accounting for habitat heterogeneity and changes in river discharge. Recapture probability varied between microhabitats, and survival was positively correlated with river discharge. Survival and the finite rate of population growth were relatively stable over time. Over 60% of individuals were found at relatively high density in riffle habitats, which compose c. 16% of the total study area. Mean monthly temperature in the region increased over the past 100 years, and mean monthly discharge of the Black River declined over the past 65 years. With no significant trends in total monthly precipitation, declines in discharge suggest that reduction of stream flow is likely due to lowering of the water table and decreased groundwater recharge. Significant changes in climate and hydrological regimes, and increases in anthropogenic threats (increased water demand, degraded water quality) in the region, may induce significant declines in population size of this imperilled mussel. We demonstrated the importance of considering habitat heterogeneity and hydrological cycles over time to examine population dynamics. Survival of benthic invertebrates in desert streams is sensitive to hydrological cycles, which are expected to be altered via climate change and extensive water use. Species recovery plans need to incorporate knowledge of spatial distributions when designing strategies for habitat assessment and making conservation decisions.

Exploring the use of stable carbon isotope ratios in short-lived leporids for local paleoecological reconstruction

Most ecological proxies used in archaeological research operate at scales that are too coarse-grained for consideration of huntergatherer adaptive decisions. Hunter-gatherers adapt to local ecological conditions and short (&lt;em&gt;e.g.&lt;/em&gt; seasonal, yearly) time frames. Our goal is to develop proxies to identify ecological shifts at fine-grained temporal and spatial scales for archaeological research. We use stable carbon isotope ratios (δ13C) in bone collagen from 58 modern leporids from two distinct ecological areas in the American Southwest as a proxy to reconstruct vegetation and climate patterns at fine-grained scales. Higher δ13C values in collagen of cottontail (&lt;em&gt;Sylvilagus&lt;/em&gt; sp.) and jackrabbit (&lt;em&gt;Lepus&lt;/em&gt; sp.) collected in the northern Chihuahuan Desert of New Mexico and West Texas suggest a more C4/crassulacean acid metabolism (CAM) dominated local vegetation when contrasted to samples from Kerr County in Central Texas. Comparisons of temperature and precipitation patterns between the two areas, along with vegetation data, suggest that key ecological differences, reflected in the collagen isotopic compositions, are likely related to rainfall amounts and the type of green forage available to leporids, especially during winter months. Leporids in dry areas may be relying on CAM plants, including prickly pear, which has a C4 isotopic signature. Alternative resources are likely to be available in wetter areas such as Central Texas.

Differential sensitivities of grassland structural components to changes in precipitation mediate productivity response in a desert ecosystem

Summary In arid and semi‐arid ecosystems, there are legacies of previous‐year precipitation on current‐year above‐ground net primary production. We hypothesized that legacies of past precipitation occur through changes in tiller density, stolon density, tiller growth, axillary bud density and percentage of viable axillary buds. We examined the sensitivity to current‐ and previous‐year precipitation of these grassland structural components in Bouteloua eriopoda, the dominant grass in the northern Chihuahuan Desert. We conducted a rainfall manipulation experiment consisting in −80% reduced precipitation, ambient, +80% increased precipitation treatments that were subjected to one of five precipitation levels in the previous two years (−80% and −50% reduced precipitation, ambient, +50% and +80% increased precipitation). The first two years preconditioned the experimental plots for year three, in which we created wet‐to‐dry and dry‐to‐wet transitions. Measurements were taken in year 3. We found that stolon density was the most sensitive to changes in precipitation and that percent‐active buds were insensitive. We also found that past precipitation had a significant legacy on grassland structural components regardless of the precipitation received in the current year, and that the legacy occurs mostly through changes in stolon density. Here, we showed that there is a differential sensitivity of structural components to current and past precipitation and supported previous findings that vegetation structure is one of the controls of productivity during precipitation transitions.

Fire effects in the northern Chihuahuan Desert derived from Landsat-5 Thematic Mapper spectral indices

Fire effects on desert ecosystems may be long-lasting based on ecological impact of fire in these environments which potentially is detected from multispectral sensors. To assess this, we analyzed changes in spectral characteristics from 1986 to 2010 of pixels associated with the location of fires that occurred between 1986 and 1999 in Big Bend National Park, USA, located in the northern Chihuahuan Desert. Using Landsat-5 Thematic Mapper (TM) data, we derived spectral indices including the simple ratio (SR), normalized difference vegetation index (NDVI), soil-adjusted vegetation index (SAVI), and normalized burn ratio (NBR) from 1989, 1999, and 2010 from the TM data and compared changes in spectral index values for sites with and without observed fire. We found that the NDVI and SAVI had significantly different values over the time for burned sites of different fire sizes. When differences of the spectral indices were calculated from each time period, time since fire was correlated with the SR and NBR indices. These results showed that large fires potentially had a persistent and long-term change in vegetation cover and soil characteristics which were detected by the extraordinary long-data collection period of the Landsat-5 TM sensor. © The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or repro- duction of this work in whole or in part requires full attribution of the original publication, including its DOI. (DOI: 10.1117/1.JRS.8.083667)

Regional trends and local variability in monsoon precipitation in the northern Chihuahuan Desert, USA

In the southwestern United States, monsoon precipitation can affect changes to the land surface, vegetation communities and ecosystem services. To better understand monsoon precipitation, we quantified change in precipitation properties from 1910 to 2010 at 22 sites across the northern Chihuahuan Desert using United States Historical Climatology Network daily data. We also assessed precipitation variability at the Sevilleta National Wildlife Refuge (SNWR) – located at the desert's ecological boundary – using daily data from a recent 10-year period. Evaluating precipitation at these locations allows for comparison of precipitation variability between ecologically-stable ecoregions and the less-stable boundaries where ecological change may be more likely to occur. Regional data from 1910 to 2010 show an increase in the number of precipitation events, a decrease in their magnitude, and also an increase in the length of extreme periods with and without precipitation. At the SNWR, total precipitation is influenced by a small number of large events, while the majority of events (65%) have an insignificant effect. These analyses suggest that local variability in precipitation may be greater than is often attributed to the summer monsoon, and the difference between wet and dry monsoons depends on the occurrence of a small number of large events.

No Long-Term Effects of Prescribed Fire on Lehmann Lovegrass (Eragrostis lehmanniana)–Invaded Desert Grassland

AbstractDesert grasslands of the southwestern United States have experienced an increase in the abundance and distribution of woody plant species over the past century. Shrub encroachment has caused a substantial loss of grasslands in the northern Chihuahuan Desert. The Chihuahuan Desert has also been invaded by Lehmann lovegrass, a fire-adapted species from southern Africa. In 1999, the U.S. Department of Agriculture–Agricultural Research Service burned a remnant desert grassland to determine the effects of prescribed fire on shrub–perennial grass dynamics. The grassland also contained the nonnative perennial grass Lehmann lovegrass. I am reporting on a study that was initiated to determine whether prescribed burning would alter the vegetative community within and proximal to a Lehmann lovegrass–dominated patch. Cover of Lehmann lovegrass showed no significant response to the burn treatment. Of the dominant native species, only black grama and broom snakeweed had a significant year by treatment interaction. No species or growth form had a significant vegetation type by year by treatment interaction. After 6 yr, differences between burned and unburned transects were not significant for any species or growth form.

Soil moisture correlates with shrub–grass association in the Chihuahuan Desert

Abstract The current ongoing expansion of shrubs at the expense of grasses in the Chihuahuan Desert is largely believed to result from a shift in competition between shrubs and grasses. Both groups, shrubs and grasses, occupy a gently sloping silty (loessial) soil within the Jornada Experimental Range (JER), northern Chihuahuan Desert, NM, USA. At the sites dominated by tarbush (Flourensia cernua), the shrubs seem to be randomly distributed in between barren surfaces, surfaces covered by biocrusts, and dense stands of burrograss (Scleropogon brevifolius), which are mainly confined to 2–4 m diameter and 0.02–0.1 m deep depressions. Often, the shrubs are accompanied by a dense stand of tobosa grass (Pleuraphis mutica) that occupies the under-canopy. In order to study shrub–grass interactions, four habitats were defined: playa habitats with bare silty material (PL), habitats with mature biocrust (CR), habitats with burrograss mainly confined to depressions (DP) and tarbush shrubs with a dense population of tobosa grass under the shrub canopy (UC). Periodic moisture measurements at the 0–30 cm soil and potential evaporation took place during the summers of 1999 and 2009. In addition, the grass biomass in these habitats was monitored. Moisture content and grass biomass followed the pattern PL DP > UC. The findings point to a close link between grass biomass and the soil moisture content, with the high grass biomass at DP explained by runoff contribution, while that of UC by the higher moisture content mainly attributed to the reduced evaporation under shading conditions. The co-existence of shrubs and under-canopy grass does not support the notion of high competition between both groups of plants. It may rather indicate facilitation. The close link between soil moisture content and the grass habitats also suggests that lower water availability during summertime may explain current grass scarcity in JER.

Effects of experimental rainfall manipulations on Chihuahuan Desert grassland and shrubland plant communities

Aridland ecosystems are predicted to be responsive to both increases and decreases in precipitation. In addition, chronic droughts may contribute to encroachment of native C3 shrubs into C4-dominated grasslands. We conducted a long-term rainfall manipulation experiment in native grassland, shrubland and the grass-shrub ecotone in the northern Chihuahuan Desert, USA. We evaluated the effects of 5 years of experimental drought and 4 years of water addition on plant community structure and dynamics. We assessed the effects of altered rainfall regimes on the abundance of dominant species as well as on species richness and subdominant grasses, forbs and shrubs. Nonmetric multidimensional scaling and MANOVA were used to quantify changes in species composition in response to chronic addition or reduction of rainfall. We found that drought consistently and strongly decreased cover of Bouteloua eriopoda, the dominant C4 grass in this system, whereas water addition slightly increased cover, with little variation between years. In contrast, neither chronic drought nor increased rainfall had consistent effects on the cover of Larrea tridentata, the dominant C3 shrub. Species richness declined in shrub-dominated vegetation in response to drought whereas richness increased or was unaffected by water addition or drought in mixed- and grass-dominated vegetation. Cover of subdominant shrubs, grasses and forbs changed significantly over time, primarily in response to interannual rainfall variability more so than to our experimental rainfall treatments. Nevertheless, drought and water addition shifted the species composition of plant communities in all three vegetation types. Overall, we found that B. eriopoda responded strongly to drought and less so to irrigation, whereas L. tridentata showed limited response to either treatment. The strong decline in grass cover and the resistance of shrub cover to rainfall reduction suggest that chronic drought may be a key factor promoting shrub dominance during encroachment into desert grassland.

Geomorphic-vegetation relationships using a geopedological classification system, northern Chihuahuan Desert, USA

Drylands typically have distinctive vegetative patterns that reflect the underlying physical landscape. We use a geopedological classification to organize the landscape into five categories from broad scale to fine scale: (1) physiographic divisions which describe regional topography, such as mountains and basin floors; (2) regolith type which identifies residuum versus transported sediments; (3) parent materials which describe mineralogy; (4) landforms which provide meso-scale descriptions of topography; and (5) soil texture, a fine-scale variable important for infiltration, erodibility, and available water holding capacity. The study was conducted in a 1753 km2 area of the White Sands Missile Range and easternmost Jornada Experimental Range and revealed the following. (1) At the broadest scale, mesquite is dominant on the basin floor when the parent material is siliceous sand, but (2) biological soil crust becomes dominant in the basin floor when the parent material is gypseous. (3) Creosotebush is dominant on rocky soils of the piedmont slopes regardless of parent material, but (4) grasslands become dominant on the bajadas upslope in the semiarid zone and into the semiarid mountain uplands regardless of bedrock type. This method provides a way of supplementing ecologic-edaphic studies and provides a framework within which mechanisms can be explored.

Landform influences on the resistance of grasslands to shrub encroachment, Northern Chihuahuan Desert, USA

In arid and semiarid regions, vegetative boundaries are often strikingly similar to landform boundaries. However, it is not well documented whether landforms exert an influence on the resistance of desert grassland to shrub encroachment. Dominant grassland communities have been displaced by woody shrubs over the last 150 years in the Jornada Basin, southern New Mexico. Digital vegetation maps from 1858, 1915–1916, 1928–1929, 1938, and 1998, in conjunction with a detailed landform map, were analyzed in a Geographical Information System. The generated time series maps and spatial data compiled from these datasets were used to quantify the extent and rate that grasslands were replaced by shrubs on eight contiguous landforms. From this assessment, we generated a resistance index that revealed desert grasslands were least resistant (most susceptible) to shrub expansion on sandy landforms and bajadas and most resistant to shrub invasion on ephemerally flooded playas. This study demonstrates that landforms both provide the broad-scale background for detailed mechanistic studies and affect the sensitivity of grasslands to shrub encroachment.

Quantifying soil surface change in degraded drylands: Shrub encroachment and effects of fire and vegetation removal in a desert grassland

Woody plant encroachment, a worldwide phenomenon, is a major driver of land degradation in desert grasslands. Woody plant encroachment by shrub functional types ultimately leads to the formation of a patchy landscape with fertile shrub patches interspaced with nutrient‐depleted bare soil patches. This is considered to be an irreversible process of land and soil degradation. Recent studies have indicated that in the early stages of shrub encroachment, when there is sufficient herbaceous connectivity, fires (prescribed or natural) might provide some reversibility to the shrub encroachment process by negatively affecting shrub demography and homogenizing soil resources across patches within weeks to months after burning. A comprehensive understanding of longer term changes in microtopography and spatial patterning of soil properties following fire in shrub‐encroached grasslands is desirable. Here, we investigate the changes in microtopography with LiDAR (light detection and ranging), vegetation recovery, and spatial pattering of soil properties in replicated burned, clipped, and control areas in a shrub‐grass transition zone in the northern Chihuahuan Desert four years after prescribed fire or clipping. Results indicate a greater homogeneity in soil, microtopography, and vegetation patterning on burned relative to clipped and control treatments. Findings provide further evidence that disturbance by prescribed fire may allow for reversal of the shrub encroachment process, if the event occurs in the early stages of the vegetation shift. Improved understanding of longer‐term effects of fire and associated changes in soil patterning can inform the use and role of fire in the context of changing disturbance regimes and climate.

Hierarchical analysis of vegetation dynamics over 71 years: soil–rainfall interactions in a Chihuahuan Desert ecosystem

Proliferation of woody plants in grasslands and savannas is a persistent problem globally. This widely observed shift from grass to shrub dominance in rangelands worldwide has been heterogeneous in space and time largely due to cross-scale interactions among soils, climate, and land-use history. Our objective was to use a hierarchical framework to evaluate the relationship between spatial patterns in soil properties and long-term shrub dynamics in the northern Chihuahuan Desert of New Mexico, USA. To meet this objective, shrub patch dynamics from 1937 to 2008 were characterized at patch and landscape scales using historical imagery and a recent digital soils map. Effects of annual precipitation on patch dynamics on two soils revealed strong correlations between shrub growth on deep sandy soils and above-average rainfall years (r = 0.671, P = 0.034) and shrub colonization and below-average rainfall years on shallow sandy soils (r = 0.705, P = 0.023). Patch-level analysis of demographic patterns revealed significant differences between shrub patches on deep and shallow sandy soils during periods of above- and below-average rainfall. Both deep and shallow sandy soils exhibited low shrub cover in 1937 (1.0% +/- 2.3% and 0.3% +/- 1.3%, respectively [mean +/- SD]) and were characterized by colonization or appearance of new patches until 1960. However, different demographic responses to the cessation of severe drought on the two soils and increased frequency of wet years after 1960 have resulted in very different endpoints. In 2008 a shrubland occupied the deep sandy soils with cover at 19.8% +/- 9.1%, while a shrub-dominated grassland occurred on the shallow sandy soils with cover at 9.3% +/- 7.2%. Present-day shrub vegetation constitutes a shifting mosaic marked by the coexistence of patches at different stages of development. Management implications of this long-term multi-scale assessment of vegetation dynamics support the notion that soil properties may constrain grassland remediation. Such efforts on sandy soils should be focused on sites characterized by near-surface water-holding capacity, as those lacking available water-holding capacity in the shallow root zone pose challenges to grass recovery and survival.

Associations of Small Migratory and Resident Birds with Two Scrub Habitats During Late Winter and Spring in the Northern Chihuahuan Desert, New Mexico

Abstract We used mist nets to survey small migrant and resident birds of scrub habitats in arroyos and adjacent uplands of the northern Chihuahuan Desert during late February–early May 1993–1997. We captured 723 individuals of 49 species. Species diversity was greater in arroyos than in adjacent uplands. Black-throated sparrows (Amphispiza bilineata), white-crowned sparrows (Zonotrichia leucophrys), and green-tailed towhees (Pipilo chlorurus) were captured most frequently. Rates of capture for residents, short-distance migrants, and Neotropical migrants were higher in arroyos than in adjacent shrubland and showed an increase after mid-April caused by an influx of Neotropical migrants heading north to their breeding grounds. Our results suggest that arroyos are important as stopover sites during spring for many migrants, especially Neotropical migrants, as they cross the northern Chihuahuan Desert.

Response of dominant grass and shrub species to water manipulation: an ecophysiological basis for shrub invasion in a Chihuahuan Desert Grassland

Increases in woody vegetation and declines in grasses in arid and semi-arid ecosystems have occurred globally since the 1800s, but the mechanisms driving this major land-cover change remain uncertain and controversial. Working in a shrub-encroached grassland in the northern Chihuahuan Desert where grasses and shrubs typically differ in leaf-level nitrogen allocation, photosynthetic pathway, and root distribution, we asked if differences in leaf-level ecophysiology could help explain shrub proliferation. We predicted that the relative performance of grasses and shrubs would vary with soil moisture due to the different morphological and physiological characteristics of the two life-forms. In a 2-year experiment with ambient, reduced, and enhanced precipitation during the monsoon season, respectively, the encroaching C(3) shrub (honey mesquite Prosopis glandulosa) consistently and substantially outperformed the historically dominant C(4) grass (black grama Bouteloua eriopoda) in terms of photosynthetic rates while also maintaining a more favorable leaf water status. These differences persisted across a wide range of soil moisture conditions, across which mesquite photosynthesis was decoupled from leaf water status and moisture in the upper 50 cm of the soil profile. Mesquite's ability to maintain physiologically active leaves for a greater fraction of the growing season than black grama potentially amplifies and extends the importance of physiological differences. These physiological and phenological differences may help account for grass displacement by shrubs in drylands. Furthermore, the greater sensitivity of the grass to low soil moisture suggests that grasslands may be increasingly susceptible to shrub encroachment in the face of the predicted increases in drought intensity and frequency in the desert of the southwestern USA.

Contrasting effects of microbiotic crusts on runoff in desert surfaces

Microbiotic crusts (MCs) play an important role in surface hydrology by altering runoff yield. In order to study the crust's role on water redistribution, rainfall and runoff were measured during 1998–2000 at three sites within the northern Chihuahuan Desert, New Mexico, USA: the Sevilleta National Wildlife Refuge (SEV), the White Sands National Monument (WS), and the Jornada Experimental Range (JER). Whereas quartz and gypsum sand characterize the SEV and WS sites, respectively, both of which have high infiltration rates, silty alluvial deposits characterize the JER site. Runoff was measured in four pairs of 1.8–6.4 m 2 plots having MCs, one of which was scalped in each pair. No runoff was generated at WS, whether on the crusted or the scalped plots. Runoff was however generated at SEV and JER, being higher on the crusted plots at SEV and lower on the JER plots. The results were explained by the combined effect of (a) parent material and (b) the crust properties, such as species composition, microrelief (surface roughness) and exopolysaccharide (EPS) content (reflected in the ratio of carbohydrates to chlorophyll). Whereas the effective rainfall, the fines and the EPS content were found to explain runoff initiation, the effective rainfall and the crust microrelief were found to explain the amount of runoff at SEV and JER where runoff generation took place. The findings attest to the fundamental role of the parent material and the crust's species composition and properties on runoff and hence to the complex interactions and the variable effects that MCs have on dryland hydrology.

Landscape and climatic effects on the evolutionary diversification of the Perognathus fasciatus species group

We evaluated evolutionary relationships of taxa within silky pocket mice of the Perognathus fasciatus species group, composed of 3 species, P. fasciatus, P. flavescens, and P. apache. These species are distributed throughout the Great Plains, Wyoming Basin, Colorado Plateau, and northern Chihuahuan Desert biogeographic regions in North America. We tested a previously postulated hypothesis of Pleistocene species divergence and introgression by analyzing mitochondrial DNA (mtDNA) and amplified fragment length polymorphisms (AFLPs). Both mtDNA and AFLP data support several genetic lineages in the fasciatus species group that are geographically structured. Molecular clock estimates reject a Pleistocene speciation hypothesis in favor of a deeper, more complex evolutionary history of initial divergence in the Miocene followed by secondary diversification beginning in the middle Pliocene and progressing through the Pleistocene. Results support recognition of an additional species within the group. Temporal and spatial congruence between the mtDNA clades in the fasciatus species group and other codistributed species of Perognathus appear to support a hypothesis of concerted diversification throughout the Chihuahuan Desert, Colorado Plateau, and Great Plains.