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1.
Creosote bushes (Larrea tridentata) form islands of fertility that influence annual plant abundances in desert ecosystems, and the distribution of native and non-native annuals with respect to creosote may differ. We established plots on the north and south facing sides of L. tridentata in the Mojave and Sonoran Deserts within four different microhabitats spanning from under the shrub canopy into the open. We counted native and non-native annuals twice during the winter growing season to determine the effects of L. tridentata on the spatial distribution of native and non-native annuals. In both deserts, annual plant abundances were higher on the north side of L. tridentata and in open areas. Native annuals in the Mojave were most abundant near the edge of the shrub canopy, whereas native annuals in the Sonoran were most abundant in open areas. The effects of L. tridentata fertility islands on annual abundances were not consistent between the two deserts we studied. Our study emphasizes the importance of research on native and non-native annuals in multiple regions before generalizations can be made about the effects of L. tridentata on annual plant abundances in desert scrub ecosystems. 相似文献
2.
A perennial forage grass, Agropyron desertorum was sown under the canopies of four shrub species or in open areas to test for facilitation during seedling establishment in an arid rangeland in Karnakh, Northeast Iran. Height, area and volume of shrubs were measured. Microclimate conditions and seedling establishment were assessed three times within two consecutive growing seasons. Near surface light intensity and air temperature were lower under shrubs, which led to initially higher soil moisture and grass establishment under the canopy of some shrub species. The leguminous shrub (Astragalus gossypinus) showed facilitation during moderate stress (summer 2009), but shifted to a negative effect during the severe drought (summer 2010). Competition, possibly for light, reduced the establishment of Agropyron seedlings under the cushion-like shrub (Acantholimon prostegium). Salsola arbusculiformis and Artemisia kopetdaghensis respectively showed a neutral and a facilitation effect in the first season, but a combined effect of allelopathy and drought led to the high mortality of Agropyron seedlings under their canopy. In conclusion in this arid rangeland, shrubs may facilitate establishment of understory plants under moderate drought stress, and for non-resource factors (light and temperature), but these positive effects are suppressed due to competition under severe drought conditions. 相似文献
3.
Anthropogenic environmental degradation transforms mature vegetation into sites in succession, and actions to restore these altered environments must be based on ecological theories. Nucleation, promoted by facilitation, is an ecological process that can be applied to the restoration of altered environments. The original vegetation of many semi-arid regions has been profoundly altered, and is difficult to recuperate due to rigorous climates. Observations of secondary succession sites raise the following question: do some semi-arid plant species promote nucleation processes and can they therefore be considered nurse species? To address this question, vegetation surveys were undertaken in different environments: under the canopy of the shrub Combretum leprosum and in adjacent open areas. Shrubs in different stages were classified by canopy size: small, intermediate and large. Diversity and number of seedlings increased as the size of the C. leprosum canopies increased. Some of the environmental variables examined supported the role of C. leprosum as a facilitator species, such as the improvement in soil conditions under its canopy. Thus C. leprosum could be of significant importance in restoring degraded areas of the semi-arid region where it is present, by allowing the establishment of other plant species. 相似文献
4.
We examined large-scale climatic and small-scale biotic and abiotic factors affecting Pinus jeffreyi (Pinaceae) seedling establishment at its low-elevation boundary along the conifer forest-sagebrush steppe ecotone in eastern California, USA. In three successive growing seasons, P. jeffreyi seedlings were planted in three microhabitats (under Artemisia tridentata or Purshia tridentata canopies, and in open intershrub spaces) at a site within the ecotone (2300 m) and at a lower-elevation site (2200 m). We measured Photosynthetically Active Radiation (PAR; 400–700 nm), soil moisture and temperature, and air temperature in the three microhabitats, and monitored seedling survival, drought stress, and herbivory. At both sites, soil moisture decreased two- to six-fold between May and August of each summer. Maximum and minimum daily air temperatures were higher and lower, respectively, in intershrub areas, as were maximum daily soil temperatures and maximum daily PAR. At the ecotone site, mean P. jeffreyi seedling survival was longest following a very wet winter and shortest following a dry winter. For both elevations, seedlings had shorter survival times in intershrub microsites as compared to shrub microsites. Drought and herbivory both influenced seedling mortality within each microhabitat, but drought was the major determinant of seedling mortality by the end of each summer. 相似文献
5.
In drylands, environmental conditions under shrub canopy differ from those found in open sites. We should expect that microclimate conditions under shrubs with distinct canopy architecture should also be different. Plant Area Index (PAI) of the three most abundant shrubs species (Porlieria chilensis, Adesmia bedwellii and Proustia cuneifolia) in Bosque Fray Jorge National Park, north-central Chile was measured using a Plant Canopy Analyzer. During two years (2004-2005), we recorded the Relative Humidity and Air Temperature underneath and away from the canopy of the shrubs.The three shrub species showed significant differences in PAI. Microclimate at 30 cm and 2 m above the soil in the open conditions were drier and warmer than underneath shrub canopies. Vegetation patches generate moderate microclimate conditions.Canopy structure can buffer climatic variability, contributing to high herbaceous productivity as well as shrub recruitment. Reflecting shrub architecture and observed PAI values, the lowest microclimate variations were observed under the canopies of P. chilensis, followed by P. cuneifolia and finally A. bedwellii. We bring a novel approach quantifying the Plant Area Index instead of the Plant cover and using a low cost method that integrates the distribution of leaves and may be derived from remote sensing products. 相似文献
6.
Shrubs in semi-arid ecosystems promote micro-environmental variation in a variety of soil properties and site characteristics. However, little is known regarding post-fire seedling performance and its association with environmental variation in former shrub canopy and interspace microsites. We compared post-fire seeding success and various soil properties important for seedling establishment between shrub canopy and interspace microsites in Wyoming big sagebrush (Artemisia tridentata) plant communities in southeast Oregon, U.S.A. We burned 5, 20 × 20 m sites and established paired canopy and interspace micro-transects seeded with bluebunch wheatgrass (Pseudoroegneria spicata, 193 seeds/m) or crested wheatgrass (Agropyron cristatum, 177 seeds/m). At one year post-fire, seedling density was 69% higher (p = 0.012) for crested wheatgrass (compared to bluebunch wheatgrass) and 75% higher (p = 0.019) for interspace microsites (compared to canopy). However, tiller and leaf area production were over twice as high (p < 0.05) in canopy microsites. Soil color and soil temperature, explained 19–32% of variation in seedling performance metrics. Shrub effects on seeding success are complex and interact with abiotic disturbances, but patterns of increased seedling performance in canopy microsites and their relationships to soil variables may suggest tactics for increasing success of restoration practices. 相似文献
7.
灌丛对流动沙地土壤特性和草本植物的影响 总被引：28，自引：16，他引：12
通过对流动沙地灌丛内外土壤特性、土壤养分含量、土壤种子库和草本植物群落特征的差异性调查,分析了灌丛对沙地土壤特性和林下草本植被的影响。结果表明,在流动沙地0—20 cm土壤中细沙、极细沙、粘粉粒、有机质、总氮和总磷、有效磷和土壤水分含量,小叶锦鸡儿灌丛下分别较灌丛外高17.3%、4.4%、 49.5%、43.8%、40.0%、23.1%、16.3%和10.8%,黄柳灌丛下较灌丛外分别高3.5%、21.3%、0.0%、20.0%、16.7%、8.3%、10.6%和28.1%。小叶锦鸡儿、差不嘎蒿和黄柳灌丛下凋落物蓄积量要比灌丛外分别高18.3倍、365.2倍和15.5倍。差不嘎蒿灌丛下土壤种子库密度较灌丛外高10.9倍。原为半固定、半流动沙地优势种的多年生草本植物白草,不仅能在流动沙地灌丛下存活,而且具有较高的密度、高度、盖度和地上生物量。结果还表明,从灌丛中心到灌丛边缘,凋落物产量、土壤种子库密度、草本植物密度、盖度、生物量均存在明显的递减梯度,在灌丛外不远处消失。这些结果说明,在流动沙地,灌丛具有明显的“肥岛”效应和“保种”作用。 相似文献
8.
《The Professional geographer》2013,65(4):505-512
Abstract The saguaro cactus, an icon of the American southwestern desert, defines the extent of the Sonoran Desert through portions of its range. Saguaros rely upon nurse plants for establishment and survival in the harsh temperature and moisture regime of the desert. Freezing temperatures limit the range of the saguaro to the north in Arizona. However, nurse plants extend the range of saguaros by providing more moderate conditions beneath their canopies. This study focuses on precisely these small, highly localized temperature variations, which, we suggest, are linked to regional-scale patterns of distribution of the species, particularly in marginal, cold-limited locales. We sampled 291 saguaros over thirty populations in Arizona, in the northern portion of their range. Saguaros establish under the south side of their nurse's canopy in the coldest parts of their range more often than expected, while a significant distributional bias is not observed in those in warmer, more moderate plots. To further investigate air temperature as a possible factor in the directional bias of saguaro establishment, we collected microclimate data under the north and south sides of the canopies of two common nurse plants, Cercidium microphyllum (a tree) and Ambrosia deltoidea (a shrub) in Phoenix, Arizona. South sides of nurses have significantly higher minimum temperatures than do the north sides. Survivorship in the coldest areas appears to be greatest under the south side of a nurse's canopy, where warmer wintertime microclimatic conditions occur. We suggest that within-canopy microclimatic variability under a single nurse plant may be reflected in regional distributional and in the survivorship patterns of saguaros throughout Arizona. 相似文献
9.
The saguaro cactus, an icon of the American southwestern desert, defines the extent of the Sonoran Desert through portions of its range. Saguaros rely upon nurse plants for establishment and survival in the harsh temperature and moisture regime of the desert. Freezing temperatures limit the range of the saguaro to the north in Arizona. However, nurse plants extend the range of saguaros by providing more moderate conditions beneath their canopies. This study focuses on precisely these small, highly localized temperature variations, which, we suggest, are linked to regional‐scale patterns of distribution of the species, particularly in marginal, cold‐limited locales. We sampled 291 saguaros over thirty populations in Arizona, in the northern portion of their range. Saguaros establish under the south side of their nurse's canopy in the coldest parts of their range more often than expected, while a significant distributional bias is not observed in those in warmer, more moderate plots. To further investigate air temperature as a possible factor in the directional bias of saguaro establishment, we collected microclimate data under the north and south sides of the canopies of two common nurse plants, Cercidium microphyllum (a tree) and Ambrosia deltoidea (a shrub) in Phoenix, Arizona. South sides of nurses have significantly higher minimum temperatures than do the north sides. Survivorship in the coldest areas appears to be greatest under the south side of a nurse's canopy, where warmer wintertime microclimatic conditions occur. We suggest that within‐canopy microclimatic variability under a single nurse plant may be reflected in regional distributional and in the survivorship patterns of saguaros throughout Arizona. 相似文献
10.
Shrub encroachment is the advance of woody plants over grasslands. For some shrub species this process is favoured by fire and high precipitation. In northwestern Patagonia spring precipitation is related with the El Niño Southern Oscillation (ENSO). Over the past years, ENSO has shown an increase in both amplitude and frequency. Our study focuses on the population dynamics of the shrub Fabiana imbricata Ruiz & Pav. (Solanaceae) using stochastic models which involve different fire frequencies and variations in spring precipitation. Our results indicate that F. imbricata's population grows at a wide range of fire frequencies, reaching its maximum every 6–10 years. The elasticity analysis show that, as fire frequency diminishes, population growth rate depends almost exclusively on adult survival, recruitment events are rare and the presence of seedlings or juveniles is seldom observed. In this case, the landscape would be formed by mature F. imbricata shrublands, and population persistence would depend on adult survival, seed production and their accumulation in the soil bank. However, in the case of very high fire frequencies, more recurrent recruitment pulses would be observed, with a landscape dominated by young shrublands composed of individuals of different ages and reproductive status. 相似文献
11.
The impact of shade on establishment of shrubs adapted to the high light irradiation of semi-arid environments 总被引：1，自引：0，他引：1
Light may be an important limiting resource that influences community structure of chenopod shrublands. As part of a larger study that aimed to determine the factors that influence chenopod community structure, the focus of this study was the influence of plant canopy on the growth and establishment of smaller plants. We therefore measured the height and cover of three chenopods (Enchylaena tomentosa, Maireana brevifolia and Maireana georgei) when growing within and outside of the canopy of Atriplex bunburyana under field conditions. All three chenopods had lower cover and E. tomentosa was taller when growing within the canopy of A. bunburyana in comparison to those growing outside of the canopy. The chenopods were then grown under three artificial shade regimes. Plant height, cover, biomass, relative leaf area and photosynthetic surface area measurements showed that each species responded differently to shade. E. tomentosa biomass was facilitated by shade. This was inferred by an increase in total plant biomass. M. brevifolia, in contrast, tolerated shade by increasing above-ground biomass allocation. M. georgei was adversely affected by the shade regimes: root biomass decreased in response to shade. Competition for light is, therefore, likely to influence chenopod community structure of semi-arid and arid environments. 相似文献
12.
Aerial photography from the 1930s serves as the earliest synoptic depiction of vegetation cover. We generated a spatially explicit database of shrub (Prosopis velutina) stand structure within two 1.8 ha field plots established in 1932 to address two questions: (1) What are the detection limits of panchromatic 1936 aerial photography?, and (2) How do these influence P. velutina biomass estimates? Shrub polygons were manually digitized on 1936 imagery and linked to 1932 field measurements of P. velutina canopy area. Aboveground 1932 P. velutina biomass was estimated using a site-specific allometric relationship for field-measured canopy area. Shrub canopy detection limits on the 1936 imagery were comparable to those reported for contemporary imagery. Based on a conservative shrub size detection threshold of 3.8 m2, 5.8% of P. velutina biomass was missed. Spatial resolution (0.6 vs. 1.0 m) did not influence detection limits, but the overall accuracy of shrub cover estimates was greater on 1.0 m images. Presence of the sub-shrub Isocoma tenuisecta may also have significantly influenced estimates of P. velutina canopy area. These analyses illustrate the importance of standardizing aerial photo interpretation protocols, accounting for uncertainty estimating shrub biomass, and caution species-specific interpretations for historic aerial photography. 相似文献
13.
The population status of an endemic cactus (Mammillaria gaumeri) was assessed in coastal Yucatán, México. Similar numbers of mature plants were found in open and full sun microhabitats. Plants in full sun had more ramets and were of larger diameter. There was a correlation between M. gaumeri abundance and the biomass of a disturbance indicator, Myrmecophila tibicinis (Orchidacea). Higher seedling survival in shade microhabitats and aggregated distribution of adults suggests that shrubs serve as nurse plants. Initial mortality rates of seedlings were many times higher in full sun microhabitats than in shade conditions. Herbivory had no effect on seedling survival. 相似文献
14.
In Mediterranean ecosystems, fire influences plant population dynamics and changes plant community structure by affecting germination and establishment of seeder shrubs. Fabiana imbricata is a long-lived seeder shrub with a broad distribution in South America. In Northwestern Patagonia grasslands there are many records of F. imbricata recruitment after fires. We hypothesized that recruitment is promoted by: 1) soil erosion that exposes deeply buried seeds; 2) direct fire effects such as heat that could break seed dormancy; 3) indirect fire effects (i.e. increase of light availability and elimination of allelopathic substances) and 4) water availability in spring. In field conditions, we tested the effects of shading and soil disturbance on F. imbricata recruitment and seed availability and distribution in the soil profile. Under controlled conditions, we investigated the influence of leaf leachate, light, heat and water on seed germination and seedling emergence. Seed germination was inhibited by leachate, and seedling emergence was inhibited by seed heated to 80 °C and 120° for 5 min. F. imbricata seedlings only emerged in germination treatments that simulated wet and very wet spring conditions. Fire and postfire wind that favor seed exposure, combined with high precipitation early in the growing season, would be necessary for successful shrub recruitment. When F. imbricata shrubland colonizes the grassland, it strongly modifies local vegetation structure, reduces species richness and increases fuel loads. F. imbricata is a key species in the ecosystem specially related to fire regime. Information on the postfire regeneration of this species would provide valuable knowledge of the changes in biomass accumulation in a fire-prone Mediterranean ecosystem. 相似文献
15.
Vegetation, microclimate, seedling frequency, freezing tolerance, and cold acclimation were compared for seedlings of Artemisia tridentata collected from 1775, 2175, and 2575 m elevation in the eastern Sierra Nevada, California. Data were used to test the hypothesis that ecotypic differences in stress physiology are important for seedling survival along gradients from desert to montane ecosystems. The vegetation canopy cover and A. tridentata seedling frequency were greatest at 2575 m, compared to 1775 and 2175 m. Snow cover ameliorated temperatures near the soil surface for part of the winter and depth varied across elevations. Freezing tolerance was compared for seedlings maintained in growth chambers at day/night air temperatures of 25°C/15°C. The temperature at which electrolyte leakage and Photosystem II function (FV/FM) from leaves were half-maximum was approximately −13·5°C for leaves of seedlings from all three elevations. Shifting day/night air temperatures from 25°C/15°C to 15°C/5°C initiated about 1·5° of acclimation by plants from all three altitudes, with seedlings from the highest elevation exhibiting the greatest acclimation change. Measurements of ambient air and canopy temperatures at the three elevations indicated that wintertime average low temperatures were consistent with the measured degree of freezing tolerance. At small spatial scales used in this study, pollen and seed dispersal between study sites may have precluded resolution of ecotypic differences. Patterns of freezing tolerance and cold acclimation may depend on a combination of mesoclimate and microclimate temperatures, canopy cover, snow depth, and snow melt patterns. 相似文献
16.
Recent drought has led to unprecedented levels of plant mortality across the Southwestern US. An unaddressed feature of this drought's impact is how soil characteristics and soil hydrological behavior affect desert plant canopy die-back and mortality. Here, we present a multi-year study in the Mojave Desert assessing canopy die-back and whole-plant mortality of white bursage (Ambrosia dumosa) and creosotebush (Larrea tridentata) in soils varying in surface and sub-surface horizon development, and topographic (hillslope vs. channel) positions. Canopy die-back and mortality was more widespread A. dumosa than in L. tridentata, and dead plants tended to be smaller than surviving plants, especially in channel and hillslope locations. This suggests that juveniles were particularly vulnerable where plants depended heavily on augmentation of incident precipitation by runoff. Canopy die-back was greater in young, weakly developed soils that fostered extensive plant growth, while plants growing in older, well-developed soils showed markedly lower branch and plant mortality, especially in A. dumosa. We attributed these differences in plant response in part to variation in distributions of large rocks within soil profiles, which might affect soil hydrological heterogeneity and intensity of plant competition for water. 相似文献
17.
Spatial and temporal variability of soil properties under Caragana microphylla shrubs in the northwestern Shanxi Loess Plateau, China 总被引：1，自引：0，他引：1
Shrub-induced spatial and temporal heterogeneity of soil properties is common in arid and semiarid ecosystems, and it facilitates the development of plant species diversity. We selected 5-, 10-, 20-, 30-, and 40-year-old Caragana microphylla shrubs in the Shanxi Loess Plateau to evaluate the spatial and temporal heterogeneity of soil properties under and outside the shrub canopy. In addition, the presence of adventitious plant species was investigated to assess the development of herbaceous species diversity. Soil samples were collected from two depths (0-5 cm and 5-10 cm). The establishment and development of shrubs promoted temporal variation, improved soil texture, enhanced soil organic matter (SOM), total nitrogen (TN), and cation exchange capacity (CEC), and decreased pH, bulk density (BD), and soil water content (SWC). The results further confirmed that SOM, TN, and CEC were significantly higher at the center than at the outside of the shrub canopies (P < 0.05) and were higher at the 0-5 cm depth than at the 5-10 cm depth. Moreover, the differences in SOM, TN, and CEC from the center to the outside of shrub canopies were greater under 30- and 40-year-old shrubs than under 10- and 5-year-old shrubs. Furthermore, the spatiotemporal heterogeneity of the soil properties facilitated the development of herbaceous species diversity. 相似文献
18.
In Namaqualand, heavy grazing of low-lying areas has resulted in the reduction of palatable perennial shrubs and the dominance of the unpalatable shrub, Galenia africana. This study investigates the process of rehabilitation and how it can be used to gain an understanding of the ecological factors that may be sustaining altered vegetation states. We intervened to change the biophysical environment through the construction of microcatchments and brushpacks to act as traps for water, seed and organic material combined with the introduction of seeds and seedlings were implemented. Two years after implementation brushpacked microhabitats improved natural recruitment of herbaceous species compared to bare areas at two out of three sites. Microhabitats did not influence natural recruitment of perennial seedlings. Emergence of palatable perennial seed was not influenced by microhabitat manipulation. Seedlings transplanted under adult G. africana shrubs showed higher survival rates at two out of three sites compared to seedlings in unmodified areas. 相似文献
19.
Arbuscular mycorrhizal fungi (AMF) play important roles in ecosystem processes. However, little is known about AMF abundance in arid, nutrient-poor environments like the Mojave Desert of North America. We conducted two AMF studies: one examined AMF responses to elevated atmospheric CO2 at the Nevada Desert FACE (Free-Air-Carbon dioxide-Enrichment) Facility (NDFF), and the second examined seasonal dynamics at nearby sites. In both studies, AMF measurements (root colonization, extra-radical hyphal [ERH] length, and two measures of glomalin-related soil protein [GRSP]) and environmental factors (root length, soil water content, and precipitation) were measured across microsites (beneath shrubs, shrub interspaces) for two species (Larrea tridentata, Ambrosia dumosa). Elevated CO2 did not significantly affect AMF measurements. Other NDFF studies show no change in fine root production under elevated CO2 but show increased available nitrogen. We infer that additional fixed carbon under elevated CO2 is not allocated to soil resource foraging. However, AMF varied seasonally. ERH seasonally declined across species and microsites, but GRSP declined only beneath L. tridentata. Our results combined with previous results indicate that drought negatively affects AMF root colonization. Robust inter-relationship among AMF measurements only occurred between the two measures of GRSP, indicating that resources are independently allocated to different AMF structures. 相似文献
20.
Many studies have focused on positive-plant interactions such as nurse plants, which provide a sheltered subcanopy environment that benefits the nursed species. Most of these studies have focused on plant distribution and association patterns, while the microclimatic benefits are often assumed. This study quantifies 5 a.m. subcanopy temperatures as well as dew points beneath a common nurse tree of the Sonoran Desert, Cercidium microphyllum (palo verde, Fabaceae). Data are collected over 35 days in the winter (January and February) at six locations (at the base of the trunk, midway to the canopy edge, canopy edge, all to the north and to the south of the base of the tree) under each of two trees, as well as a control in the open.It is warmer beneath the tree than in the open, but also, it is warmer in the interior than at the canopy edge, and warmer to the south. Furthermore, differences in temperature between the subcanopy and the open site are greater on colder nights, and less pronounced on warmer winter nights, possibly due to the effect of cloud cover which often results in warmer overnight lows. In addition, variation in 5 a.m. temperature is greatest at the canopy edge and open, and temperature varies less in the interior where temperatures are also warmer. Subcanopy cover was quantified using fish-eye imagery. Results show that there is a significant relationship between 5 a.m. temperature and overlying cover.Dew point temperatures, surprisingly, were lower under the tree than at the canopy edge and in the open. That is, it is relatively dry under the nurse. This pattern can similarly be seen by distance and direction from the base of the tree (drier in the south). This may have to do with the nurse's roots and other vegetation growing beneath the nurse's canopy that compete for water in the ground, which leaves less available water to evaporate into the subcanopy air. 相似文献