Depletion of soil moisture by two cold-desert bunchgrasses and effects on photoshythetic performance

This study compared the abilities of two cool-season bunchgrasses to extract moisture from a drying soil and compared photosynthetic and stomatal responses of the two species as soil moisture supplies were depleted. When grown in 49-L pots in a greenhouse, Leymus cinereus extracted more water from the soil and maintained higher gas exchange rates to lower absolute amounts of soil water than did Agropyron desertorum . The soil water content at the lower limit of extraction was 10.3% for L. cinereus and 13.3% for A. desertorum . When soil moisture was expressed as extractable soil water, there was little difference between the species in pattern of water use. Both species maintained high stomatal conductances (g w ) and photosynthetic rates (A) until extractable soil moisture was reduced to about 15%. For field-grown plants under severe water stress, A was higher in L. cinereus than in A. desertorum at comparable leaf water potentials. The relationship between A and g w was similar for the two species; higher A in L. cinereus was a consequence of higher g w . Thus, higher A in L. cinereus is achieved through some sacrifice of water-use efficiency.     

Improving seed germination of Salicornia rubra (Chenopodiaceae) under saline conditions using germination-regulating chemicals

Salicornia rubra Nels. (Chenopodiaceae) is a salt-tolerant annual species occurring in salt playas of the Great Basin desert in western United States. It forms pioneer communities on the most saline location of an inland salt playa at Goshen, Utah. Seeds of Salicornia rubra are capable of germinating at 1000 mM NaCl at 25-35° C alternating temperature regime. Dormancy-regulating chemicals were evaluated for their ability to alleviate the innate and salinityenforced dormancy in seeds of Salicornia rubra . Betaine, GA3, kinetin, fusicoccin, ethephon, thiourea, proline, and nitrate had no effect in alleviating primary dormancy. Increases in NaCl concentration progressively inhibited germination of Salicornia rubra seeds. Ethephon, fusicoccin, GA3, kinetin, thiourea, and nitrate promoted germination under low saline conditions. At high salinity fusicoccin had no effect, whereas GA3, kinetin, and ethephon substantially alleviated salinity effects. Application of these dormancy-regulating compounds could be of practical value in seeding a saline area for restoration purposes, particularly under high saline conditions.     

Ecological impacts of seed harvester ants on soil attributes in a Larrea -dominated shrubland

The influence of seed harvester ant ( Pogonomyrmex rugosus ) colonies on soil properties and soil surface and moisture characteristics was investigated through comparison of adjacent, nonnest (reference, 4 m beyond ant colony) areas in Las Vegas, Nevada. Effects of ant colonies on both terrace and slope sites were investigated. Soil moisture content and soil bulk density in a creosote bush ( Larrea tridentata )--dominated shrubland were significantly lower, while soil temperature, soil organic matter, and percent pore space were significantly higher in soils with ant nests relative to adjacent reference soils. Soil pH and texture did not differ significantly between nest and reference soils. Among soil surface characteristics, percent bare soil and rock (gravel, cobble, and boulder) cover were not significantly different between nest and reference soils. In evaluating soil moisture characteristics, soils with ant nests had a significantly higher water infiltrability and greater depth of water penetration, but a significantly lower area of water spread (surface-water runoff) at both terrace and slope sites. Between the 2 geomorphic surfaces, water infiltrability and depth of water penetration were significantly greater at the terrace than at the slope. Water-borne soil movement (fluvial erosion) was significantly greater at the slope than terrace but did not differ significantly between nest and reference soils. The presence of active P. rugosus colonies in the L. tridentata -dominated shrubland altered certain soil properties and appeared to have a protective influence on the soil by fostering more infiltration and less runoff of surface water in southern Nevada.     

Effect of irrigation on survival of third-stage Haemonchus contortus larvae (Nematoda: Trichostrongylidae)

During May through October 1976, samples of sheep pellets containing laboratory - reared, thirdstage Haemonchus contortus larvae were placed on irrigated and nonirrigated pasture plots at Provo, Utah. Periodically thereafter, grass clippings, soil scrapings, and remaining pellets were collected and baermannized to determine their comparative survival from the two environments. Water was added to the irrigated plots in accordance with a weekly sprinkling regime designed to furnish sufficient moisture to maintain pasture grass in this semiarid region. Meteorologic measurements were collected daily from both irrigated and nonirrigated sections.   During the year the nonirrigated section received a total of 131 mm of precipitation, whereas an additional 979 mm of water were added via sprinkling to the irrigated section. The monthly mean maximum temperature at soil surface under grass cover for the six - month study period on the irrigated section averaged 17.7 C less than on the nonirrigated section, and the corresponding soil moisture content remained 14.4 percent higher. A bioclimatograph of conditions on the nonirrigated section showed that none of the months during the year had levels of temperature and moisture which fell within the prescribed limits for optimum pasture transmission of H. contortus; on the irrigated section only October of the six - month study period failed to have suitable conditions for optimum pasture transmission. Larvae placed on the plots survived significantly longer and also in significantly greater numbers on the irrigated section, and irrigation enhanced the ability of larvae to migrate from pellets onto vegetation.     

Monoterpene concentrations in litter and soil of singleleaf pinyon woodlands of the western Great Basin

Mean monoterpene content of singleleaf pinyon ( Pinus monophylla [Torr.] Frem.) litter from stands growing on two soil series was 340 ± 310 ug/g air dry weight (adw). Individual monoterpene hydrocarbons suggested as potential allelopathic compounds occur in extremely small amounts, 0.5-1.10 ug/g adw. Mineral soils contained 50 times less total terpene, 6.6 ug/g ± 4.8 adw, than the litter immediately above. Results suggest that allelopathic effects would more likely occur in litter than mineral soil. These findings are substantiated by previous reports of decreased emergence and growth of herbaceous species in pinyon litter but not mineral soil.     

Soil-vegetation relations of recovering subalpine range of the Wasatch Plateau

Vegetation and soils inside and outside an abandoned sheep corral on degraded subalpine range of the Wasatch Plateau were studied to determine the influence of approximately 37 years' use of the corral on soil and plant development. Vegetal and surface cover were estimated. Herbage, litter, and soils were sampled inside and outside the corral and analyzed for C org , N, P, and S. Soil pH, bulk density, and C0 3 -C also were measured. Storage (mass/unit area) of C org , N, P, and S was determined for each component. Yield and vegetal composition were significantly affected inside the corral boundary. Herbage yield was 2.2 times greater, litter mass 16 times greater, foliar cover of grasses 2 times greater, and forb cover 70% lower inside than outside the corral. Cover of meadow barley ( Hordeum brachyantherum ), a component of the predisturbance vegetation of the Wasatch Plateau, was nearly 12 times greater inside than outside the corral. These and other vegetal and cover differences reflect inside-outside differences in concentration, storage, and availability of soil C org , N, P, and S. Concentrations of C org and total and available N, P, and S were greater in the surface 5 cm of soil inside the corral. Available P inside the corral was much higher in all soil layers. Because of bulk density differences, storage was greater inside the corral only for C org and N at 0-5 cm and for P at 5-15 cm. Lower soil pH inside the corral appears related to soil P distribution and C0 3 -C storage. Results suggest a need to reexamine earlier conclusions that tall forbs are the climax dominants of the Wasatch summer range.     

Soil and vegetation development in an abandoned sheep corral on degraded subalpine rangeland

Vegetation and soils inside and outside an abandoned sheep corral on degraded subalpine range of the Wasatch Plateau were studied to determine the influence of approximately 37 years' use of the corral on soil and plant development. Vegetal and surface cover were estimated. Herbage, litter, and soils were sampled inside and outside the corral and analyzed for C org , N, P, and S. Soil pH, bulk density, and C0 3 -C also were measured. Storage (mass/unit area) of C org , N, P, and S was determined for each component. Yield and vegetal composition were significantly affected inside the corral boundary. Herbage yield was 2.2 times greater, litter mass 16 times greater, foliar cover of grasses 2 times greater, and forb cover 70% lower inside than outside the corral. Cover of meadow barley ( Hordeum brachyantherum ), a component of the predisturbance vegetation of the Wasatch Plateau, was nearly 12 times greater inside than outside the corral. These and other vegetal and cover differences reflect inside-outside differences in concentration, storage, and availability of soil C org , N, P, and S. Concentrations of C org and total and available N, P, and S were greater in the surface 5 cm of soil inside the corral. Available P inside the corral was much higher in all soil layers. Because of bulk density differences, storage was greater inside the corral only for C org and N at 0-5 cm and for P at 5-15 cm. Lower soil pH inside the corral appears related to soil P distribution and C0 3 -C storage. Results suggest a need to reexamine earlier conclusions that tall forbs are the climax dominants of the Wasatch summer range.     

Environmental site characteristics and incidence of chokecherry black knot in Utah

Normal 0 false false false EN-US X-NONE X-NONE MicrosoftInternetExplorer4 st1\:*{behavior:url(#ieooui) } /* Style Definitions */ table.MsoNormalTable {mso-style-name:"Table Normal"; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-priority:99; mso-style-qformat:yes; mso-style-parent:""; mso-padding-alt:0in 5.4pt 0in 5.4pt; mso-para-margin:0in; mso-para-margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:11.0pt; font-family:"Calibri","sans-serif"; mso-ascii-font-family:Calibri; mso-ascii-theme-font:minor-latin; mso-fareast-font-family:"Times New Roman"; mso-fareast-theme-font:minor-fareast; mso-hansi-font-family:Calibri; mso-hansi-theme-font:minor-latin; mso-bidi-font-family:"Times New Roman"; mso-bidi-theme-font:minor-bidi;} Black knot disease of chokecherries, induced by Dibotryon morbosum (Schw.) Th. & Syd., is widely distributed in Utah. The incidence of black knot was measured by determining the ratio of total black knot gall length to total stem length of plants and then expressing that value as a percentage of diseased stems in the sample plot. The environmental site factors measured were elevation, exposure, slope, soil pH, soil depth, distance to surface water, plant moisture stress, and associated vegetation. Numerical values were determined for each of these variables at each of 18 randomly located plots. Correlation coefficients for plant moisture stress and soil temperature were –.439 (p = .065) and –.440 (p = .055). Multiple regression analyses using plant moisture stress and soil temperature gave a regression coefficient of –.641 (p = .05). As plant moisture stress and soil temperature decreased, incidence of black knot increased.        

Vegetation zones and soil characteristics in vernal pools in the Channeled Scabland of eastern Washington

Vernal pools are seasonal pools occurring in Mediterranean-type climates within which grow concentric zones of vegetation. We studied two vernal pools that lie within an Artemisia tridentata/Festuca idahoensis shrub-steppe landscape in the Channeled Scabland of eastern Washington to determine the relationship between vegetation zonation and soil characteristics. Abundant plant species in the pools include Elymus cinereus, Poa scabrella, Lomatium grayi, Allium geyeri, Eleocharis palustris, Epilobium minutum, Myosurus aristatis, Deschampsia danthonioides, and Psilocarphus oregonus . We surveyed topography, measured plant species frequency and cover to describe the vegetation zones, and used Sorenson's index of percent to similarity to verify our designation of plant zones as communities. In one pool we described soil profiles and sampled soils throughout the growing season according to plant communities. We analyzed soils for pH; electrical conductivity; sodium, calcium, and magnesium ions; sodium absorption ratio; particle size; organic carbon; and water matric potential. ANOVA tests of soil characteristics and topography among plant communities showed that only differences in topography are statistically significant. There are, however, trends in particle size, some soil chemical parameters, and soil moisture potential among plant communities along the topographic gradient. Electrical conductivity decreased with increasing dryness of the soil through spring and summer. Seasonal changes in soil moisture potential showed that shallower soils in the centers of pools were wetter during the wet season and drier during the dry season than are deeper soils. These changes in moisture may be the most important influence on vegetation distribution within the vernal pools.     

Does the presence of Wyethia mollis affect growth of Pinus jeffreyi seedlings?

Regeneration of Pinus jeffreyi in the Sierra Nevada is often limited on sites dominated by Wyethia mollis . Allelopathic chemicals and competition for soil moisture have been suggested as possible mechanisms for limiting regeneration. We tested the hypothesis that soil chemical and microbial properties from sites in different stages of succession influence seedling growth of Pinus jeffreyi . Soil was collected from an early-seral site dominated by Wyethia mollis , a mid-seral site dominated by the shrubs Arctostaphylos patula, Ceanothus prostratus, C. velutinus , and Purshia tridentata , and a late-seral site dominated by mature Pinus . These sites were compared for nutrient content, Pinus seedling growth capacity, and microbial population size. Soil (0-33 cm) from the early-seral site had the lowest C, microbial biomass, and fungal and bacterial populations. There were no consistent trends in soil nutrient content among sites. The early-seral site had the lowest soil Ca and Mg contents but also had a lower C/N ratio and more than twofold greater P content than either the mid- or late-seral site. Pinus seedling growth and foliar nutrient concentrations were compared at 3 harvest dates (220, 314, and 417 days after germination) in a greenhouse bioassay. The treatment design was a 3 × 2 factorial with soil from each of the 3 sites either with or without Pinus seedlings. Pots without seedlings were used as controls to assess the effects of seedlings on microbial biomass. Seedling growth in the early-seral soil was initially suppressed in comparison to growth in the mid-seral soil, but by the final harvest total seedling weight was similar between these 2 treatments. The most obvious treatment effect was a reduction in growth for seedlings planted in lateseral soil, probably due to a nutrient imbalance in the soil. Seedlings grown in late-seral soil had Fe and Al levels that were nearly twice those of seedlings grown in early- and mid-seral soils. Microbial biomass followed a temporal pattern similar to that found for seedling growth. Differences in microbial biomass between the early- and mid-seral soils, although initially large, were not detected by the final harvest. We interpret these results to indicate that allelopathy or soil nutrient deficiencies resulting from the presence of Wyethia are unlikely to be responsible for limited growth of Pinus seedlings in Wyethia -dominated stands.     

Habitat and distribution of pygmy rabbits ( Sylvilagus idahoensis ) in Oregon

Normal 0 false false false EN-US X-NONE X-NONE MicrosoftInternetExplorer4 st1\:*{behavior:url(#ieooui) } /* Style Definitions */ table.MsoNormalTable {mso-style-name:"Table Normal"; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-priority:99; mso-style-qformat:yes; mso-style-parent:""; mso-padding-alt:0in 5.4pt 0in 5.4pt; mso-para-margin:0in; mso-para-margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:11.0pt; font-family:"Calibri","sans-serif"; mso-ascii-font-family:Calibri; mso-ascii-theme-font:minor-latin; mso-fareast-font-family:"Times New Roman"; mso-fareast-theme-font:minor-fareast; mso-hansi-font-family:Calibri; mso-hansi-theme-font:minor-latin; mso-bidi-font-family:"Times New Roman"; mso-bidi-theme-font:minor-bidi;} An investigation to determine the location and extent of populations of pygmy rabbits ( Sylvilagus idahoensis ) in Oregon, and to describe several biotic and physical components within communities that include pygmy rabbits, was conducted from October 1981 to September 1983. Of 211 sites suspected of supporting the species based on interpretation of museum records, aerial photographs, soil maps, and interviews with biologists and area residents, 51 exhibited evidence of being inhabited when examined in summer 1982. Soil and vegetation components were sampled at 15 sites occupied by pygmy rabbits and 21 sites adjacent thereto. At inhabited sites, mean soil depth (51.0 ± 2.3 cm), mean soil strength of surface (0.8 ± 0.2 kg/cm 2 ) and subsurface (3.8 ± 0.3 kg/cm 2 ) horizons, shrub height (84.4 ± 5.8 cm), and shrub cover (28.8 ± 1.4%) were significantly greater ( P 2 ), density of forbs (3.4 ± 0.6/1,000 cm 2 ), and cryptogam cover (2.4 ± 0.5%) were not. Except for the clay component of subsurface soils, texture of surface and subsurface soils were not significantly different between sites occupied by pygmy rabbits and adjacent sites. The affinity of pygmy rabbits for areas with greater shrub cover, shrub height, soil strength, and soil depth, and, to a small degree, coarser soil texture possibly was related to availability of forage, security from predation, and ease of burrow construction. Analysis of 472 samples of sagebrush ( Artemisia tridentata ) collected at and near sites inhabited by pygmy rabbits indicated their distribution was not dependent upon the presence of specific subspecies of sagebrush. A marked decrese in evidence of occupancy of sample sites and of pygmy rabbit activity at occupied sites in 1983 indicated that populations of pygmy rabbits were susceptible to rapid declines and possible local extirpation. Fragmentation of sagebrush communities poses a potential threat to populations of pygmy rabbits, but the severity of the threat presently is unknown.     

Habitat relationships of Glaux maritima in central Utah

Thirty-five study sites were established in the meadow communities surrounding Utah Lake in central Utah. The study sites ranged across several community types. Glaux maritima was found in all sites but varied as to its ecological importance in these communities. Sixteen soil factors were measured relative to the stands studied. Cover of Glaux maritima correlated with parts per million sodium and total soluble salts in the soil. No other factors correlated significantly with the cover of Glaux maritima. Glaux maritima occupied only those sites with high levels of moisture throughout the growing season. The high moisture levels came from springs, seeps, elevated water tables, and early seasonal inundation. High levels of Glaux maritima cover corresponded to low numbers of species in the habitat.        

The salinity during larval development affects the dispersion in adults of the tree-climbing crab Aratus pisonii

Adults of the tree-climbing crab Aratus pisonii occur in estuarine regions with a range of salinity of 0–35. However, the larvae cannot complete the development in a wide range of salinities. A study about the effect of salinity on the larval development was conducted in order to infer about adult dispersion and occurrence in a wide range of salinity. Two experiments were conducted: (1) from zoae I to megalopae stage and (2) from megalopa to juvenile stage. Larvae from eight females from Paranaguá Bay, Paraná, Brazil were obtained. After eclosion, for the first experiment, 100 zoea larvae from five females were acclimated, individualized and transferred to small aquariums with five distinct salinity treatments (S0, S5, S15, S25 and S35). For the second experiment, 50 megalopae from three females were individualized in aquariums in the same salinity treatments. Both experiments were conducted in incubator BOD with a constant temperature of 25°C and photoperiod of 12 hours (light/dark). Larvae were fed daily with eggs and naupli newly hatched of Artemia sp. The salinity affected the survival rate during the development in both experiments. Higher survival from zoea I to megalopa stage occurred in salinities 25 and 35 PSU, while from megalopa to juvenile stage in salinities 15 and 25 PSU. In both experiments the larvae did not tolerate low salinities as adults, with total mortality in S0 and S5. Our results suggest that the species have a larval export strategy of zoae larvae to oceanic regions (higher salinities) and megalopae needed to return to estuarine areas with intermediate salinities to complete the development. The occurrence of adults in estuarine areas with low (< 5 PSU) or high (> 25 PSU) salinities most likely occur after the metamorphosis to the juvenile stage by terrestrial dispersion or at the stage of megalopa (marine dispersion) during favourable weather events.     

Spring denitrification rates in soils of four eastside Sierra Nevada plant communities

Denitrification rates in soils of four subalpine plant communities in the Sierra Nevada were determined by the acetylene blockage method. The study area included riparian, meadow, forest, and barren sites. Data were collected during dawn-to-dusk measurements in April 1987. Soil atmosphere samples were analyzed for N 2 O content using gas chromatography. Generally, temporal variability in denitrification rate within each plant community was insubstantial. Denitrification rate and soil temperature were found to be significantly correlated only in the riparian and barren sites. Of the four communities, the riparian site was found to have the lowest rate of denitrification overall. However, differences among sites in denitrification rate could not be conclusively attributed to variation in soil temperature, moisture, organic matter, total C and N, C:N ratio, NO 3 - N, or pH.     

Cyanobacteria and cyanolichens: Can they enhance availability of essential minerals for higher plants?

In both field and greenhouse studies, cyanobacterial and cyanolichens of cold-temperature deserts often enhance growth and essential uptake by associated herbs. That effect is associated with better seedling establishment and larger seedlings. The following are possible mechanisms for these effects: (1) the microbiota concentrate essential elements in available forms in soil surface layers, (2) the microbial surface covers are usually darker colored than the soil itself and produce warmer soils during cool seasons when soil water is most available, (3) the gelatinous sheaths of several cyanobacterial genera common on alkaline deserts contain chelating compounds, and (4) conditions that favor persistent microbial growths on soil surfaces also favor maintenance of larger populations of microorganisms that form mycorrhizal and/or rhizosheath associations with seed plants. There is evidence that associated animals may be nutritionally benefited by the enhanced mineral content of forage plants growing in well-developed cyanobacterial crusts.     

Environmental attributes correlating with density of blackbrush ( Coleogyne ramosissima ) shurbs in the Spring Mountains of southern Nevada

Discovery of distinct mid-elevational bands of blackbrush ( Coleogyne ramosissima Torr.) shrublands on desert mountain slopes in the Mojave Desert caused an investigation of the relationships between environmental factors and Coleogyne distribution. Environmental factors were quantitatively examined to determine which were significant predictors of Coleogyne density at upper-elevational limits (ecotones) in the Spring Mountains of southern Nevada. Path analysis revealed significant, direct causal effects of air temperature, soil moisture, soil depth, and percent litter cover on the distribution of Coleogyne . Specifically, air temperature was a significant positive predictor, while soil moisture, soil depth, and percent litter cover were significant negative predictors of Coleogyne density, with the effects of other environmental variables parceled out. Path analysis also indicated that indirect effects of soil pH, bulk density, compaction, percent pore space, organic matter, soil temperature, salinity, cryptogam, and percent bare soil and rock cover on Coleogyne density were substantially more potent than their direct casual effects. Environmental attributes associated with elevational changes correlate with and may determine the density of Coleogyne shrubs at upper ecotones in southern Nevada.     

Ecological impacts of Pogonomyrmex on woody vegetation of a Larrea-Ambrosia shrubland

Discovery of distinct mid-elevational bands of blackbrush ( Coleogyne ramosissima Torr.) shrublands on desert mountain slopes in the Mojave Desert caused an investigation of the relationships between environmental factors and Coleogyne distribution. Environmental factors were quantitatively examined to determine which were significant predictors of Coleogyne density at upper-elevational limits (ecotones) in the Spring Mountains of southern Nevada. Path analysis revealed significant, direct causal effects of air temperature, soil moisture, soil depth, and percent litter cover on the distribution of Coleogyne . Specifically, air temperature was a significant positive predictor, while soil moisture, soil depth, and percent litter cover were significant negative predictors of Coleogyne density, with the effects of other environmental variables parceled out. Path analysis also indicated that indirect effects of soil pH, bulk density, compaction, percent pore space, organic matter, soil temperature, salinity, cryptogam, and percent bare soil and rock cover on Coleogyne density were substantially more potent than their direct casual effects. Environmental attributes associated with elevational changes correlate with and may determine the density of Coleogyne shrubs at upper ecotones in southern Nevada.     

Fertile island development around perennial shrubs across a Mojave Desert chronosequence

Spatial heterogeneity has been examined only recently as a factor in studies of ecosystem processes. The effect of this factor on desert organisms is exemplified in the ""fertile island"" a concentration of essential soil nutrients under perennial shrubs. In this study we examined fertile island patterns in undisturbed desert areas and on abandoned roadways to see at what rate and magnitude these fertile patches reappeared after disturbance. Small-scale examination of patterns of soil P, organic matter, moisture, and bulk density showed that soils beneath Larrea tridentata (DC.) Cov. shrubs on the roads lacked the tight circular gradient in these variables that was characteristic of soils beneath control shrubs, even 88 years after road abandonment. The nature of the initial soil disturbance altered both spatial patterns of soil N and temporal patterns of fertile island development. Fertile island patterns for total soil N, available P, and organic matter were more circular than patterns for bulk density, texture, or pH. We suggest that patterns of soil heterogeneity may develop first for elements that may be limiting to desert shrub growth (N, P, organic matter), followed by spatial development in other less limiting soil factors (bulk density, texture, pH).     

Experimental manipulations of precipitation seasonality: effects on oak ( Quercus ) seedling demography and physiology

Predicted changes in regional precipitation patterns and soil moisture caused by anthropogenic trace gas emissions may affect the distribution and abundance of woody plants in arid and semiarid regions. To test the response of woody plants to potential changes in precipitation regimes, we manipulated summer and winter precipitation on plots that contained seedlings of Quercus emoryi Torr. (Emory oak), the dominant tree in oak savannas of the southwestern United States. Throughout the growing season, we monitored seedling survival and physiology (predawn leaf water potential, midday instantaneous gas exchange, and leaf carbon isotope discrimination). Seedling survival and physiological performance differed little between treatments, which embodied 50% changes to quantities of summer and winter precipitation, and encompassed a continuum of precipitation from 359 mm · year -1 to 846 mm · year -1 . However, survival and physiological performance of seedlings were negatively impacted by seasonal environmental conditions common to all treatments, especially during the annual pre-'monsoon' drought. Seedling predawn leaf water potentials, net CO 2 assimilation, and stomatal conductance indicate that growing conditions for Q. emoryi seedlings at this site are generally restricted to periods with adequate soil moisture (i.e., April and August). Results contrast with an assumption implicit to the ""two-layer"" soil water resource partitioning hypothesis that woody plants in all life history stages are more dependent upon winter than summer precipitation. In fact, summer precipitation appears more important than winter precipitation for Q. emoryi seedling recruitment and growth.