Chihuahuan Desert Shrubland Research Articles

Sensitivity of root production to long‐term aridity under environmental perturbations in Chihuahuan Desert ecosystems

Abstract Root production influences carbon and nutrient cycles and subsidizes soil biodiversity. However, the long‐term dynamics and drivers of belowground production are poorly understood for most ecosystems. In drylands, fire, eutrophication, and precipitation regimes could affect not only root production but also how roots track interannual variability in climate. We manipulated the intra‐annual precipitation regime, soil nitrogen, and fire in four common Chihuahuan Desert ecosystem types (three grasslands and one shrubland) in New Mexico, USA, where the 100‐year record indicates both long‐term drying and increasing interannual variability in aridity. First, we evaluated how root production tracked aridity over 10–17 years using climate sensitivity functions, which quantify long‐term, nonlinear relationships between biological processes and climate. Next, we determined the degree to which perturbations by fire, nitrogen addition or intra‐annual rainfall altered the sensitivity of root production to both mean and interannual variability in aridity. All ecosystems had nonlinear climate sensitivities that predicted declines in production with increases in the interannual variance of aridity. However, root production was the most sensitive to aridity in Chihuahuan Desert shrubland, with reduced production under drier and more variable aridity. Among the perturbations, only fire altered the sensitivity of root production to aridity. Root production was more than twice as sensitive to declines with aridity following prescribed fire than in unburned conditions. Neither the intra‐annual seasonal rainfall regime nor chronic nitrogen fertilization altered the sensitivity of roots to aridity. Synthesis. Our results yield new insight into how dryland plant roots respond to climate change. Our comparison of dryland ecosystems of the northern Chihuahuan Desert predicted that root production in shrublands would be more sensitive to future climates that are drier and more variable than root production in dry grasslands. Field manipulations revealed that fire could amplify the climate sensitivity of dry grassland root production, but in contrast, the climate sensitivity of root production was largely resistant to changes in the seasonal rainfall regime or increased soil fertilization.

As above, not so below: Long-term dynamics of net primary production across a dryland transition zone.

Drylands are key contributors to interannual variation in the terrestrial carbon sink, which has been attributed primarily to broad-scale climatic anomalies that disproportionately affect net primary production (NPP) in these ecosystems. Current knowledge around the patterns and controls of NPP is based largely on measurements of aboveground NPP (ANPP), particularly in the context of altered precipitation regimes. Limited evidence suggests belowground NPP (BNPP), a major input to the terrestrial carbon pool, may respond differently than ANPP to precipitation, as well as other drivers of environmental change, such as nitrogen deposition and fire. Yet long-term measurements of BNPP are rare, contributing to uncertainty in carbon cycle assessments. Here, we used sixteen years of annual NPP measurements to investigate responses of ANPP and BNPP to several environmental change drivers across a grassland-shrubland transition zone in the northern Chihuahuan Desert. ANPP was positively correlated with annual precipitation across this landscape; however, this relationship was weaker within sites. BNPP, on the other hand, was weakly correlated with precipitation only in Chihuahuan Desert shrubland. Although NPP generally exhibited similar trends among sites, temporal correlations between ANPP and BNPP within sites were weak. We found chronic nitrogen enrichment stimulated ANPP, whereas a one-time prescribed burn reduced ANPP for nearly a decade. Surprisingly, BNPP was largely unaffected by these factors. Together, our results suggest that BNPP is driven by a different set of controls than ANPP. Furthermore, our findings imply belowground production cannot be inferred from aboveground measurements in dryland ecosystems. Improving understanding around the patterns and controls of dryland NPP at interannual to decadal scales is fundamentally important because of their measurable impact on the global carbon cycle. This study underscores the need for more long-term measurements of BNPP to improve assessments of the terrestrial carbon sink, particularly in the context of ongoing environmental change.

Predicting changes in bee assemblages following state transitions at North American dryland ecotones

Drylands worldwide are experiencing ecosystem state transitions: the expansion of some ecosystem types at the expense of others. Bees in drylands are particularly abundant and diverse, with potential for large compositional differences and seasonal turnover across ecotones. To better understand how future ecosystem state transitions may influence bees, we compared bee assemblages and their seasonality among sites at the Sevilleta National Wildlife Refuge (NM, USA) that represent three dryland ecosystem types (and two ecotones) of the southwestern U.S. (Plains grassland, Chihuahuan Desert grassland, and Chihuahuan Desert shrubland). Using passive traps, we caught bees during two-week intervals from March–October, 2002–2014. The resulting dataset included 302 bee species and 56 genera. Bee abundance, composition, and diversity differed among ecosystems, indicating that future state transitions could alter bee assemblage composition in our system. We found strong seasonal bee species turnover, suggesting that bee phenological shifts may accompany state transitions. Common species drove the observed trends, and both specialist and generalist bee species were indicators of ecosystem types or months; these species could be sentinels of community-wide responses to future shifts. Our work suggests that predicting the consequences of global change for bee assemblages requires accounting for both within-year and among-ecosystem variation.

Nocturnal soil CO2 uptake and its relationship to subsurface soil and ecosystem carbon fluxes in a Chihuahuan Desert shrubland

AbstractDespite their prevalence, little attention has been given to quantifying arid land soil and ecosystem carbon fluxes over prolonged, annually occurring dry periods. We measured soil [CO2] profiles and fluxes (Fs) along with volumetric soil moisture and temperature in bare interplant canopy soils and in soils under plant canopies over a three‐month hot and dry period in a Chihuahuan Desert shrubland. Nocturnal Fs was frequently negative (from the atmosphere into the soil), a form of inorganic carbon exchange infrequently observed in other deserts. Negative Fs depended on air‐soil temperature gradients and were more frequent and stronger in intercanopy soils. Daily integrated ecosystem‐level Fs was always positive despite lower daily Fs in intercanopy soils due to nocturnal uptake and more limited positive response to isolated rains. Subsurface [CO2] profiles associated with negative Fs indicated that sustained carbonate dissolution lowered shallow‐soil [CO2] below atmospheric levels. In the morning, positive surface Fs started earlier and increased faster than shallow‐soil Fs, which was bidirectional, with upward flux toward the surface and downward flux into deeper soils. These dynamics are consistent with carbonate precipitation in conjunction with convection‐assisted CO2 outgassing from warming air and soil temperatures and produced a pronounced diurnal Fs temperature hysteresis. We concluded that abiotic nocturnal soil CO2 uptake, through a small carbon sink, modulates dry season ecosystem‐level carbon dynamics. Moreover, these abiotic carbon dynamics may be affected by future higher atmospheric carbon dioxide levels and predictions of more prolonged and regular hot and dry periods.

Modeling evapotranspiration and its partitioning over a semiarid shrub ecosystem from satellite imagery: a multiple validation

Numerous modeling approaches have been proposed to estimate evapotranspiration (ET) and its partitioning between evaporation from soil (E) and transpiration from vegetation (T) over the last several decades. Although these ET models claimed to give reasonable E and T partitioning, few studies have compared their modeling results with direct E and T observations. In this study, a hybrid dual source scheme and trapezoid framework based evapotranspiration model (HTEM) fed with MODIS data was applied in a Chihuahuan Desert shrubland during the growing season of 2003 and validated with direct ET measurement using the Bowen-ratio technique and T measurement using scaled-up sap-flow measurements. Results show that the HTEM is capable of decomposing the remotely sensed land surface temperature into temperature components (soil and canopy temperatures) and providing accurate E and T estimates. At satellite overpass time, the root-mean-square error (RMSE) of estimated latent heat flux (LE) is 47.7 W/m 2 . The agreement between estimated and simulated LE was largely improved when observed net radiation and ground heat flux were used (35.1 W/m 2 ). At daily scale, the RMSE of estimated daily ET, E, and T are 0.52, 0.36, and 0.41 mm/day , respectively.

Land Application of Treated Industrial Wastewater on a Chihuahuan Desert Shrubland: Implications for Water Quality and Mineral Deposition

Land application of wastewater conserves fresh water and recycles nutrients, but little is known of its impacts on arid and semiarid landscapes. Lagoon treated saline−sodic industrial effluent was applied from 2002–2005 to a 0.4-ha Chihuahuan Desert shrubland to assess the deposition and recovery of effluent components and changes in soil quality vis-à-vis a non-irrigated area. Effluent irrigation supplied 26% of the average annual nonstressed evapotranspiration (ET) of the native shrubs [Larrea tridentata (DC.) Coville and Prosopis glandulosa Torr. var. glandulosa], and increased both soil stress factors (sodicity, salinity, and pH) and soil fertility (N, P, and K). After 3 yr, the soil saturation extract electrical conductivity (ECe) reached as high as 6.1 dS m−1 and Cl− 76 molc m−3 at 105 cm depth under irrigated L. tridentata. After 4 yr, saturation extract sodium adsorption ratio (SARe) reached 25–35 at 7.5 cm under the irrigated shrubs and intershrub spaces. There were 27 Mg ha−1 of cumulative ionic deposits to the site comprised mostly of Na+, Cl−, and CaCO3 equivalent alkalinity, with soil analysis recovering most of the deposits (>57%) except for K+ and Na+ (8% to 13%). Subsurface leaching of and P was detected within the upper 1 m soil depth after 4 yr, and a high downward mobility of Cl− revealed the potential for deeper leaching at higher N loading rates. While long-term effects on the natural vegetation are unknown, results contribute to a limited scientific database for sustainable wastewater land application in semiarid regions.

Land Application of Treated Industrial Wastewater on a Chihuahuan Desert Shrubland: Impacts on the Natural Vegetation

Impacts of wastewater land application on desert vegetation are not adequately known. In a 4-yr field study, we evaluated the effects of treated, saline−sodic industrial effluent application on the aboveground vegetation biomass and mineral accumulation of a Chihuahuan Desert shrubland. The vegetation included two shrubs, Larrea tridentata (DC.) Coville and Prosopis glandulosa Torr. var. glandulosa, and seven herbaceous species in the intershrub spaces. Early summer fruit dry weights on terminal branches of the irrigated shrubs were 3 to 14 times higher than those on the non-irrigated shrubs. The combined irrigated vegetation produced 2 Mg of additional dried biomass per ha above that of the non-irrigated plot, and contained excesses of total Kjeldahl-N (TKN) and Ca2+ equivalent to 18% of effluent N deposition and 12% of effluent Ca2+ deposition. Under irrigated and highly sodic conditions (soil sodium adsorption ratio up to 35), the herbaceous Lepidium alyssoides A. Gray var. alyssoides produced the highest biomass of all plant species. However, there was a decline in plant species diversity as L. alyssoides became dominant in the irrigated intershrub spaces. While the findings demonstrate the natural attenuation of effluent minerals by native Chihuahuan Desert vegetation, highly sodic conditions may alter the composition of the vegetation community.

Soil heterogeneity and the distribution of desert and steppe plant species across a desert-grassland ecotone

Bouteloua gracilis (Kunth) Lag. ex Griffiths (blue grama), Bouteloua eriopoda (Torr.) Torr. (black grama), and Larrea tridentata Coville (creosotebush) are dominant plants on the McKenzie Flats portion of the Llano de Manzano landform within Sevilleta National Wildlife Refuge in central New Mexico, part of the biome ecotone from the Colorado Shortgrass Steppe to the Chihuahuan Desert. In this study, we examine the hypothesis that soil heterogeneity, determined by variation in surface soil depth, carbonate accumulation, and fine-textured fraction, controls relative dominance of the three species. The area is flat, generally <1% slope; however, abrupt soil differences exist even within the flattest parts of the landscape that correspond to the pattern of buried channels incised in a petrocalcic horizon (caliche) formed in a 0.5–1.2 million year-old paleosol beneath the current surface soil. Multivariate analyses of soil-moisture-related variables suggest that B. gracilis, a Colorado Shortgrass Steppe indicator, dominates the buried paleochannels where Holocene surface deposits are deepest and the argillic (clay-rich) B-horizon is thickest. B. eriopoda, dominant in Chihuahuan Desert grasslands, is most abundant where the buried petrocalcic horizon lies within 40–60 cm of the surface and the argillic horizon is thinner and weakly developed. L. tridentata, an indicator of desertified Chihuahuan Desert shrubland, is dominant where the petrocalcic horizon is exposed or near the surface. This study illustrates the strong relationship between geomorphology, soil development and vegetation patterns in arid and semi-arid environments.

Partitioning of evapotranspiration and its relation to carbon dioxide exchange in a Chihuahuan Desert shrubland

AbstractKey to evaluating the consequences of woody plant encroachment on water and carbon cycling in semiarid ecosystems is a mechanistic understanding of how biological and non‐biological processes influence water loss to the atmosphere. To better understand how precipitation is partitioned into the components of evapotranspiration (bare‐soil evaporation and plant transpiration) and their relationship to plant uptake of carbon dioxide (CO2) as well as ecosystem respiratory efflux, we measured whole plant transpiration, evapotranspiration, and CO2 fluxes over the course of a growing season at a semiarid Chihuahuan Desert shrubland site in south‐eastern Arizona. Whole plant transpiration was measured using the heat balance sap‐flow method, while evapotranspiration and net ecosystem exchange (NEE) of CO2 were quantified using the Bowen ratio technique.Before the summer rainy season began, all water and CO2 fluxes were small. At the onset of the rainy season, evapotranspiration was dominated by evaporation and CO2 fluxes were dominated by respiration as it took approximately 10 days for the shrubs to respond to the higher soil moisture content. During the growing season, periods immediately following rain events (&lt;2 days) were dominated by evaporation and respiration while transpiration and CO2 uptake peaked during the interstorm periods. The surface of the coarse, well‐drained soils dried quickly, rapidly reducing evaporation. Overall, the ratio of total transpiration to evapotranspiration was 58%, but it was around 70% during the months when the plants were active. Peak respiration responses following rain events generally lagged after the evaporation peak by a couple of days and were better correlated with transpiration. Transpiration and CO2 uptake also decayed rather quickly during interstorm periods, indicating that optimal plant soil moisture conditions were rarely encountered. NEE of CO2 was increasingly more negative as the growing season progressed, indicating a greater net uptake of CO2 and greater water use efficiency due mainly to decreases in respiration. Copyright © 2006 John Wiley &amp; Sons, Ltd.

Community response to removals of plant functional groups and species from a Chihuahuan Desert shrubland

Arid and semi‐arid ecosystems often exhibit diverse plant growth forms in water‐limited environments, but it is unclear whether resource competition (interference) is actually important in structuring communities. We chose a diverse Chihuahuan desert shrubland to examine the response of the plant community to experimental removals of selected perennial plant species or groups of species. Four treatments involved the removal of all individuals of all species of a single functional group (functional group removals: shrub removal, succulent removal, subshrub removal, perennial grass removal). Three other treatments involved removing species within functional groups. These seven treatments plus a control (no plants removed) were replicated six times each in 25×25 m experimental plots, in summer 1995. Permanent belt transects were surveyed for number and sizes of all vascular plants in spring and fall in 1997, 1999, 2000, and 2001. Those plots from which the dominant shrub, Larrea tridentata, was removed had not recovered in total plant cover or volume by 2001, but cover and volume in all other treatments were similar to those in control plots. Relatively few species demonstrated a positive response to the removal of other species or functional groups. The perennial grass group and forbs were the most responsive; perennial grass cover increased in the shrub removal treatment relative to the control but treatment differences diminished after dry growing seasons in 2000 and 2001. Results over the first five years suggest that either environmental conditions or intrinsic biological characteristics limit the ability of most plant species to respond to the removal of substantial fractions of community biomass and composition in the short term. Such slow response by both dominant and less abundant components of the community has implications for the recovery of semi‐arid systems after human disturbance or other events leading to the reduction of biological diversity.

Temporal and Spatial Variation of Episodic Wind Erosion in Unburned and Burned Semiarid Shrubland

Redistribution of soil, nutrients, and contaminants is often driven by wind erosion in semiarid shrublands. Wind erosion depends on wind velocity (particularly during episodic, high-velocity winds) and on vegetation, which is generally sparse and spatially heterogeneous in semiarid ecosystems. Further, the vegetation cover can be rapidly and greatly altered due to disturbances, particularly fire. Few studies, however, have evaluated key temporal and spatial components of wind erosion with respect to (i) erosion rates on the scale of weeks as a function of episodic high-velocity winds, (ii) rates at unburned and burned sites, and (iii) within-site spatial heterogeneity in erosion. Measuring wind erosion in unburned and recently burned Chihuahuan desert shrubland, we found (i) weekly wind erosion was related more to daily peak wind velocities than to daily average velocities as consistent with our findings of a threshold wind velocity at approximately 7 m s(-1); (ii) greater erodibility in burned vs. unburned shrubland as indicated by erosion thresholds, aerodynamic roughness, and nearground soil movement; and (iii) burned shrubland lost soil from intercanopy and especially canopy patches in contrast to unburned shrubland, where soil accumulated in canopy patches. Our results are among the first to quantify post-fire wind erosion and highlight the importance of accounting for finer temporal and spatial variation in shrubland wind erosion. This finer-scale variation relates to semiarid land degradation, and is particularly relevant for predictions of contaminant resuspension and redistribution, both of which historically ignore finer-scale temporal and spatial variation in wind erosion.

MAMMAL MOUNDS STIMULATE MICROBIAL ACTIVITY IN A SEMIARID SHRUBLAND

This study was conducted to determine the influence of animal disturbance, in the form of banner-tailed kangaroo rat (Dipodomys spectabilis) mounds, on soil microbial abundance and activities in a Chihuahuan Desert shrubland. Total organic carbon (TOC), microbial biomass C (Cmic), and basal respiration were quantified in soils from beneath and between creosote bush (Larrea tridentata) in three zones: directly on the mounds, immediately surrounding the mounds, and between mounds. TOC, Cmic, and respiration were enhanced both in soils beneath the canopies of the shrubs and on the mammal mounds. Thus, mammals as well as shrubs contribute to spatial heterogeneity. Ratios of respiration to Cmic (metabolic quotient) were highest on the mammal mounds; however, the ratio of Cmic : TOC was not affected by shrub canopy cover or location relative to mammal mounds. The higher metabolic quotients in the vicinities of the mounds reflect physical disturbance by mammals and suggest higher proportions of relatively easily metabolized organic carbon than in soils more distant from mounds.

Mammal Mounds Stimulate Microbial Activity in a Semiarid Shrubland

This study was conducted to determine the influence of animal disturbance, in the form of banner-tailed kangaroo rat (Dipodomys spectabilis) mounds, on soil microbial abundance and activities in a Chihuahuan Desert shrubland. Total organic carbon (TOC), microbial biomass C (Cmic), and basal respiration were quantified in soils from beneath and between creosote bush (Larrea tridentata) in three zones: directly on the mounds, immediately surrounding the mounds, and between mounds. TOC, Cmic, and respiration were enhanced both in soils beneath the canopies of the shrubs and on the mammal mounds. Thus, mammals as well as shrubs contribute to spatial heterogeneity. Ratios of respiration to Cmic (metabolic quotient) were highest on the mammal mounds; however, the ratio of Cmic : TOC was not affected by shrub canopy cover or location relative to mammal mounds. The higher metabolic quotients in the vicinities of the mounds reflect physical disturbance by mammals and suggest higher proportions of relatively easily metabolized organic carbon than in soils more distant from mounds.