Seed dispersal, seed entrapment, and seedling recruitment in a temporally variable desert playa

Natural establishment of seedlings in desert playas with temporally variable precipitation hinges on many factors, including seed production, seed dispersal, seed entrapment, seed germination, and seedling survival. We investigated natural seed dispersal patterns and the effects of surface texture, wind barriers, resource availability (water and nutrients), and protection from herbivory on seedling establishment in a desert playa. We hypothesized that the seed rain would be consistent throughout the year and that seedling establishment would improve in resource-amended plots with barriers to wind dispersal. Contrary to our hypotheses, seed flux peaked seasonally during the winter, and fertilization had no consistent effect on seedling establishment. Seed availability for dispersal correlated with precipitation in the previous water year, whereas seedling recruitment was greatest when current-year precipitation during the spring germination and summer seedling growth periods was high. Gravel and barrier surface treatments contained more surviving seedlings than other surface treatments. Comparison with plots located outside of herbivore exclosures, however, showed that greater seedling presence in gravel plots may be due somewhat to protection from herbivory provided by the gravel, rather than simply to the greater seed-trapping quality of the gravel. With abundant seed availability, application of surface treatments, like coarse gravel, combined with increased seasonal water availability could lead to improved shrub establishment from seed in desert playas.     

Seedling size and survival for Chrysothamnus nauseosus

Seedling size and survival in relation to summer drought were examined for Chrysothamnus nauseosus growing under field and greenhouse conditions. In the field, summer survival rates were less than 2% annually for the three years monitored. The effect of initial seedling height on subsequent survival was examined in both the field and greenhouse by grouping seedlings into live and dead categories on each census date and comparing initial heights for seedlings in these categories. For a majority of the census dates, the initial height of surviving seedlings was greater than the initial height of those that subsequently died (significant differences ranged from 1 to 8 mm), indicating that seedlings that were taller at the initiation of the drought period had a higher probability of survival. In the greenhouse, taller seedlings had greater shoot and root biomass and rooting depth. Seedlings that are larger (i.e., taller and have greater aboveground biomass) in late spring appear to have a higher probability of surviving summer drought due to greater rooting depth and hence increased access to moisture in deeper soil layers. Seed availability and safe sites for germination were probably not limiting since large numbers of seedlings successfully germinated in a patchy pattern during the study period. Seedling size and probability of survival were not related to either seedling density or the distance to nearest seedling neighbor. Survival through summer drought appears to be the main limitation to seedling recruitment in this population.     

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.     

Establishment, growth, and early survival of woody riparian apecies at a Colorado gravel pit

Presence of a wetted edge during the period of seedfall was an effective predictor of suitable establishment (defined as germination and survival to the 1st autumn) locations for Populus deltoides subsp. monilifera, Salix amygdaloides, S. exigua, and Tamarix ramosissima seedlings during 3 successive years of a gravel pit revegetation project in Fort Collins, Colorado. At locations predicted to be suitable for establishment, position within the pit (possibly reflecting additional moisture provided by seepage) was a significant factor in determining whether establishment actually occurred. Cover of herbaceous species, which became established at the same time as, or after, woody seedlings, was positively related to probability of establishment for 8 of 11 species-year combinations, probably reflecting more favorable moisture conditions at certain locations. Herbaceous cover also was positively related to seedling height at the end of the 1st summer of growth for 9 of 11 species-year combinations. Neither establishment nor 1st-summer growth was consistently related to overall decline in the water table as estimated by the drop in surface-water level during the growing season. Flooding in the 1st spring after establishment was negatively related to subsequent survival for 5 of 8 species-year combinations. The 4 species established at different elevations in the pit, depending on location of the wetted edge during the period of seedfall, and there was no evidence that differential mortality subsequently altered their distribution along the elevation gradient. However, the primary objective in this study was to restore native woody species, and we attempted to maintain conditions conducive to meeting this objective. Differential postestablishment mortality may be more important in structuring the riparian community in more rigorous riverine environments.     

Flood flows and population dynamics of Arizona sycamore ( Platanus wrightii )

Platanus wrightii is a pioneer tree species of warm-temperate riparian deciduous forests in southwestern United States and northern Mexico. Dendrochronological analysis of populations in central and southern Arizona indicated that P. wrightii seedlings establish episodically. Long intervals (10-40 years) elapsed with no apparent establishment. Seedling establishment years were positively associated with winter flood size and annual flow rate, and weakly negatively associated with summer flood size. Large floods sometimes preceded multi-year establishment waves. During the past 2 decades an abundance of winter floods and very wet springs has allowed frequent establishment of P. wrightii . For example, plants established at many sites during the winter flood years of 1993 and 1995, particularly along channels scoured and widened by flood waters. Platanus wrightii also reproduces asexually. Ramets were more abundant than genets in all populations, and ramets established more frequently than seedlings. Ramet density (mean number per genet) varied widely between populations, from 2 (Huachuca Canyon) to 9 (Haunted Canyon), and increased with tree size within 4 of 9 populations. Population size structure varied across a gradient of watershed area. Populations along streams draining the largest watersheds had an abundance of small trees (mean trunk diameter of P. wrightii is influenced by temporal as well as spatial differences in stream flow regimes.     

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.     

Human trampling effects on regeneration and age structures of Pinus edulis and Juniperus monosperma

We examined effects of human foot traffic on age structures and densities of seedlings and saplings of Pinus edulis (Colorado pinyon) and Junisperus monosperma (one-seed juniper) in a heavily used urban park, Garden of the Gods, Colorado. Age structures show no stand-destroying disturbances, but they do contain small peaks 85-95 yr ago (minimum age), which have been interpreted as responses to heavy grazing. For Pinus edulis reverse J-shaped age structures indicate a strongly reproducing population, while flat age structures of J. monosperma show low present reproduction. Young trees showed strong preferences for establishing under existing trees and shrubs and not among herbs or on bare soil. Pinus edulis seedling density was reduced by 73% in heavily trampled areas compared to lightly trampled areas. However, there were no differences in density when only the area protected by rocks, shrubs, or trees was considered. This indicates that direct effects such as physical damage and soil erosion kill young trees, and indirect effects, such as lower seed production, do not cause the lower densities in heavily trampled areas. Pinus edulis saplings and J. monosperma seedlings and saplings showed no differences in density across trampling intensities. In heavily trampled areas of Garden of the Gods Park, recent increases in use have apparently reduced Pinus edulis seedling establishment enough that long-term regeneration is threatened. Managers of all pinyon-juniper woodlands must recognize that in areas strongly impacted by foot traffic, and also presumably by similar disturbances such as vehicle traffic, sufficient regeneration likely does not occur to replace trees. The areal extent of severely disturbed areas should be limited, and managers should seek to avoid further degradation of less damaged areas.     

Desert rodents reduce seedling recruitment of Salsola paulsenii

Heteromyid rodents in the deserts of North America have been shown to harvest large quantities of seeds of both native and introduced plants from soil seed banks, but rarely has the impact of this seed removal been demonstrated experimentally. I used a series of fenced plots (some of which excluded rodents) to demonstrate that heteromyids at a western Nevada study site can measurably reduce seed banks and subsequent seedling establishment of Salsola paulsenii , an introduced invasive weed that has become a significant problem over much of the desert Southwest. The frequency of S. paulsenii seedlings in both 2004 and 2005 was significantly greater around the interior perimeters of plots that permitted access by rodents than in plots that excluded rodents. Density of S. paulsenii seedlings was significantly greater inside than outside rodent exclusion plots, but there was no such difference in seedling density inside versus outside plots that permitted rodent access. Salsola paulsenii has such a conspicuous presence in many desert environments that the effect of rodents in reducing its abundance may not be visually apparent; however, rodents may still ameliorate competitive effects of this weed on coexisting plants. Heteromyid rodents disperse seeds through caching, and they also consume them. Caching may enhance establishment of native plant seedlings, but is unlikely to benefit exotics such as S. paulsenii .     

Microhabitat affinities of Gambel oak seedlings

Previous work suggested that Gambel oak seedlings are rare in the northern parts of its range in Utah where summer rainfall is relatively low but should be abundant in southern parts of the range where summer rainfall is usually high. Gambel oak grades from a relatively minor component of a ponderosa pine/mixed conifer assemblage in the south to a virtually monotypic formation in the north, where it exists as long-lived clones.   Quadrat analysis in Arizona and New Mexico, within the oak zone, revealed a seedling density ranging from 120 to 1320 per hectare. We found a significant tendency of seedlings to be located on the NE (cool, shady) side of sheltering objects in the environment. Mature ponderosa pine ranged in density from ca 40 to 500 stems per hectare, whereas mature Gambel oak ranged from ca 10 to 20 genets per hectare with ca 1 to 7 ramets per clone. These results support our previous conclusion that Gambel oak in northern Utah probably became established as a minor component of a mixed pine/oak woodland at a time in mid-Holocene when summer rainfall was much higher than today.     

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.     

Cone and seed production in Pinus ponderosa : a review

Factors associated with seed cone production in Pinus ponderosa were reviewed to identify broad patterns and potential effectiveness of restoration activities. Cone and seed production are quite variable, with differences between (1) years, (2) sites, and (3) individual trees. Between-year, population-wide crop failures suggest large-scale triggers for cone and seed production, perhaps high temperatures and dry weather. Stem diameter is the most important determinant for cone production at the tree level, with other factors such as genetic disposition, moisture, soil nutrients, and insect pests and disease playing a smaller role. Some extrinsic factors affect growth rate, indirectly affecting cone production. For example, less competition and lower stand densities result in P. ponderosa trees that increase in diameter more quickly, possibly because of more light, and produce seeds earlier. This literature suggests that restoration activities, especially thinning, will result in trees better able to produce larger seed crops. The effect of prescribed fire is less clear, with contradictory effects depending on site conditions, burn severity, and nutrient status of the site.     

Tamarix aphylla : a newly invasive tree in southern Nevada

In the southwestern United States, the nonnative athel pine ( Tamarix aphylla ) was presumed to be sterile and therefore not as likely to spread as its widely distributed, nonnative congener, T. ramosissima . However, at Lake Mead National Recreation Area (LMNRA) in southern Nevada, populations of T. aphylla have recently spread beyond their limited pre-1990 distribution and now form extensive monospecific stands. Over a 3-year period, we quantified seed production and germination from 60 T. aphylla trees at LMNRA. The annual mean seed production period was 50.6 days, and the mean potential germination (under laboratory conditions) was 22%, indicating that T. aphylla trees at LMNRA are capable of sexual reproduction in southern Nevada. No seeds germinated in field experiments, apparently because of high soil salt levels. However, seedling regeneration is becoming increasingly common at LMNRA. Tamarix aphylla trees occupied a distinct zone along the shoreline of Lake Mead, above T. ramosissima in elevation and below native Larrea tridentata communities, suggesting either competitive exclusion or differential resource utilization. The T. aphylla zone tended to have the highest mean values for total vegetation cover, leaf litter depth, soil salinity, soil moisture, pH, total Kjeldahl nitrogen, and soil organic matter. The capacity for sexual reproduction of this alien plant, combined with a suite of characteristics shared with the invasive T. ramosissima (e.g., drought tolerance and copious saline leaf litter), makes T. aphylla a potentially invasive species along the shores of LMNRA and other mesic areas in the desert Southwest.     

A sagebrush wilt disease of unknown cause

In a rare 1969 epidemic, spruce cone rust caused by Chrysomyxa pirolata infected 40 – 100 percent of trees and 20 – 67 percent of cones on riparian Colorado blue spruce on plots located in a 2200 – 2400 m elevational zone in Huntington Canyon of central Utah. Uredinial and telial sporulation on Pyrola spp. began in mid - June, a time closely correlated with opening of pistillate spruce cones. Come phenology and host habitat, as influenced by elevation, are apparently important factors in the restricted niche of the cone rust fungus in Utah. Several preceding consecutive years with extended periods of spring and fall moisture were associated with occurrence of the epidemic, although no cause - and - effect relationship was established. Weather records indicate that these events are infrequent in this climatic zone, and there was no detectable recurrence of cone rust for at least 9 years following 1969. Outwardly normal seeds developed in diseased cones, but seed germinability was reduced by 25 percent. Aecial spore masses between cone scales, cone resinosis, and distortion of cone scales prevented seed dispersal to the extent that the seed crop was effectually destroyed.     

Occurence and effect of Chrysomyxa pirolata cone rust on Picea pungens in Utah

In a rare 1969 epidemic, spruce cone rust caused by Chrysomyxa pirolata infected 40 – 100 percent of trees and 20 – 67 percent of cones on riparian Colorado blue spruce on plots located in a 2200 – 2400 m elevational zone in Huntington Canyon of central Utah. Uredinial and telial sporulation on Pyrola spp. began in mid - June, a time closely correlated with opening of pistillate spruce cones. Come phenology and host habitat, as influenced by elevation, are apparently important factors in the restricted niche of the cone rust fungus in Utah. Several preceding consecutive years with extended periods of spring and fall moisture were associated with occurrence of the epidemic, although no cause - and - effect relationship was established. Weather records indicate that these events are infrequent in this climatic zone, and there was no detectable recurrence of cone rust for at least 9 years following 1969. Outwardly normal seeds developed in diseased cones, but seed germinability was reduced by 25 percent. Aecial spore masses between cone scales, cone resinosis, and distortion of cone scales prevented seed dispersal to the extent that the seed crop was effectually destroyed.     

Bird community composition in relation to habitat and season in Betatakin Canyon, Navajo National Monument, Arizona

Bird species density, diversity, and species richness in relation to habitat and seasonal variations were studied in the Betatakin Canyon area of Navajo National Monument, Arizona. The two most prominent habitat types are a riparian forest deep in the canyon bottom and a mature pinyon-juniper woodland on the mesas and slopes above the canyon. One hundred thirty-five species of birds were encountered during the study and recorded by season and habitat. The avifauna assemblages demonstrate definite habitat selection into groups associated with the riparian and pinyon-juniper woodland communities. Diversity, density, and species richness were greatest in the riparian habitat during the spring and summer months. The differences in bird community composition were greater between seasons than between habitats. Pinyon pine and Gambel oak were highly selected as perch sites in the pinyon-juniper and riparian areas, respectively.     

Poisonous plants of Utah

Bird species density, diversity, and species richness in relation to habitat and seasonal variations were studied in the Betatakin Canyon area of Navajo National Monument, Arizona. The two most prominent habitat types are a riparian forest deep in the canyon bottom and a mature pinyon-juniper woodland on the mesas and slopes above the canyon. One hundred thirty-five species of birds were encountered during the study and recorded by season and habitat. The avifauna assemblages demonstrate definite habitat selection into groups associated with the riparian and pinyon-juniper woodland communities. Diversity, density, and species richness were greatest in the riparian habitat during the spring and summer months. The differences in bird community composition were greater between seasons than between habitats. Pinyon pine and Gambel oak were highly selected as perch sites in the pinyon-juniper and riparian areas, respectively.     

Persistence of subalpine forest--meadow ecotones in the Gunnison Basin, Colorado

Forests of the southern Rocky Mountains are punctuated by persistent meadows called parks that are dominated by grasses and forbs. In an attempt to elucidate the maintenance of subalpine parks in the Gunnison Basin, Colorado, soil texture and tree morphology differences along 60-m transects spanning the forest-park ecotone were studied in 6 representative parks. Seedling survivorship, percent seed germination, and soil moisture available to plants were also studied along one of the transects in Willow Park. Soil analyses revealed 40% more silt and significantly less sand and clay in all 6 parks ( P P = 0.011), with 54% more water in forest than in park soils. Measures of growth rate obtained from tree height, dbh, and age were significantly higher nearer the ecotone ( P P = 0.002). These results suggest that stress of mature Engelmann spruce ( Picea engelmannii ) and lodgepole pine ( Pinus contorta ) is an unlikely explanation of park maintenance. P. engelmanii percent seed germination and seedling survivorship were significantly higher in the forest than in the park ( P < 0.001). This may be largely due to the more severe seedling microclimate observed in the park. Results indicate that limited seedling establishment is primarily responsible for maintenance of subalpine parks in the Gunnison Basin.     

Effects of habitat type and degradation on avian species richness in Great Basin riparian habitats

The overwhelming majority of bird species in the Great Basin region are found in riparian habitats. However, most previous research on the impact of change in habitat condition through degradation on these bird communities failed to account for the large intersite differences, in both habitat type and extent of degradation. We examined songbird communities in 4 riparian habitat types (meadows, willow-birch-, and aspen-dominated forest stands) during summers 1994 (last year of a 7-yr drought) and 1995 (following the 6th wettest winter recorded) in the Toiyabe Mountain Range of central Nevada. Habitat degradation significantly influenced bird species richness in riparian areas, but the impact was dependent upon habitat type. While meadow bird communities were affected adversely by habitat degradation, with significant drops in species richness on degraded sites, bird species richness in forested riparian habitats was consistently greater on degraded sites. Data for the 6 most common species seen during our study indicated that degradation may have influenced distribution of American Robins ( Turdus migratorius ) and Yellow Warblers ( Dendroica petechia ), but habitat type was the best predictor of abundance for House Wrens ( Troglodytes aedon ), Red-naped Sapsuckers ( Sphyrapicus nuchalis ), Warbling Vircos ( Vireo gilvus ), and Brewer's Blackbirds ( Euphagus cyanocephalus ). Avian species diversity in meadow habitats may be linked to moisture levels during specific times of the year. Diversity increased during the pre-migratory period of the dry year (1994) when compared with that of the breeding season, but was unchanged in the wet year (1995).     

Woody riparian vegetation response to different alluvial water table regimes

Woody riparian vegetation in western North American riparian ecosystems is commonly dependent on alluvial groundwater. Various natural and anthropogenic mechanisms can cause groundwater declines that stress riparian vegetation, but little quantitative information exists on the nature of plant response to different magnitudes, rates, and durations of groundwater decline. We observed groundwater dynamics and the response of Populus fremontii , Salix gooddingii , and Tamarix ramosissima saplings at 3 sites between 1995 and 1997 along the Bill Williams River, Arizona. At a site where the lowest observed groundwater level in 1996 (-1.97 m) was 1.11 m lower than that in 1995 (-0.86 m), 92-100% of Populus and Salix saplings died, whereas 0-13% of Tamarix stems died. A site with greater absolute water table depths in 1996 (-2.55 m), but less change from the 1995 condition (0.55 m), showed less Populus and Salix mortality and increased basal area. Excavations of sapling roots suggest that root distribution is related to groundwater history. Therefore, a decline in water table relative to the condition under which roots developed may strand plant roots where they cannot obtain sufficient moisture. Plant response is likely mediated by other factors such as soil texture and stratigraphy, availability of precipitation-derived soil moisture, physiological and morphological adaptations to water stress, and tree age. An understanding of the relationships between water table declines and plant response may enable land and water managers to avoid activities that are likely to stress desirable riparian vegetation.     

A life history study of the Snake River plains endemic Lepidium papilliferum (Brassicaceae)

Lepidium papilliferum is an ephemeral species that occupies ""slick spot"" microhabitats in the matrix of sagebrush steppe vegetation of the southwestern Snake River plains, Idaho, USA. We related population demographic data collected from 1993 to 1996 to on-site precipitation data on the Orchard Training Area west of Boise. We also carried out field seed-retrieval and in situ seed bank studies. We found that L. papilliferum has a dual life history strategy. A fraction of each cohort sets seed as summer annuals, while the remaining plants remain vegetative and potentially biennial. Surviving biennials flower and set seed along with the annual cohort of the following year. The switch to flowering as an annual appears to be based on threshold rosette size. Probability of survival to flowering was much lower for biennials than for annuals of the same cohort, but surviving biennials sometimes had enhanced seed production. The summerdry environment of the Snake River plains combined with the slick spot habitat has apparently selected for a primarily summer annual life cycle for this species. Seeds were highly dormant at dispersal and were not responsive to dormancybreaking cues. Those from a given cohort of L. papilliferum remained viable in the soil for at least 11 years. This persistent seed bank provides a buffer against extinction in sequences of years when seed production is low or absent. Estimated seed bank size varied from near zero for a heavily disturbed site that formerly supported the species to 18 viable seeds · dm -2 for an extant population in high-quality habitat. Management for population preservation for L. papilliferum should focus on protecting the seed bank from destruction caused by livestock trampling and other anthropogenic disturbances.