Characterizing safe sites for seedling establishment of endangered Arctomecon humilis Coville in a warm desert gypsum environment

Plant species endemic to gypsum occur in arid and semiarid regions worldwide and are an important component of biodiversity conservation, but gypsophile habitat selection within more arid gypsum environments remains largely unexplored. Our goal in this study was to inform efforts to introduce new populations of the endangered Mojave Desert gypsophile Arctomecon humilis Coville (dwarf bear poppy) through direct seeding at more remote locations that are less impacted by threats associated with urban expansion. We characterized approximately 600 ‘safe sites’ where seedlings of this species had recently established and examined distribution of these safe sites across habitats on the gypsum cuesta landscape. Using drone imagery at the landscape scale, we found that juveniles showed an even stronger preference for the white surface cover class than adults and were also markedly under-represented in the dark crust cover class. Two-thirds of the juveniles were found on knoll tops and upper back slopes, while only 3% were found on bottoms. At the plot scale, most juveniles were associated with surfaces where lichen crust and white gravel occurred in a fine-scale mosaic. At the emergence microsite scale, two-thirds of the seedlings had emerged from narrow gaps between two or more contact substrates, most often lichen crust and gravel. Even when seedlings emerged from lichen crust alone, emergence was usually from a gap and not from the surface of intact lichen thallus. On bottoms, toe slopes, and lower back slopes where dark lichen crust is often heavy and continuous, very few juveniles were found. Fewer than 7% of juveniles were found in direct association with living adults, likely because of adaptations for both enhanced seed dispersal and a persistent seed bank. There was also no evidence for establishment facilitation by nurse plants of other species. These results are contrasted with gypsophile establishment ecology in less arid environments. They also enable drone-based screening at the landscape scale as an efficient first step in selection of introduction sites.     

Gypsum Physical Soil Crusts and the Existence of Gypsophytes in Semi-arid Central Spain

Probably gypsophytes are the most interesting set of edaphic specialists of arid and semiarid climates. Despite they conform a global biodiversity priority, there are almost no information about those adaptive traits that confer such a specialised behaviour. Our broad hypothesis is that gypsophytes are “refuge-endemics” that are able to grow on gypsum soils due to their ability to surpass extremely hard gypsum soil physical crust during emergence. With this in mind we have conducted an experimental approach combining field and greenhouse assays. Seeds from two gypsophytes, genuine and widely distributed in the Iberian Peninsula gypsophytes (Helianthemum squamatum and Lepidium subulatum) and one gypsovag (Teucrium capitatum), a generalist plant that can also grow on gypsum soils were used in our experiments. Two complementary experimental approaches were conducted. The first involved a field experiment in which the presence or absence of the physical crust together with the sowing date were manipulated and a greenhouse experiment in which the irrigation amount and the types of soil were controlled. Variables of interest were the percentage of germination, growth and survival. In the field experiment we found a significant decrease in the final germination of the gypsovag in the plots with intact crusts. On the other hand, H. squamatum is able to grow in the three tested soils, despite higher survival and growth on genuine gypsum soils. Our results confirm the hypothesis that gypsum edaphic specialists base their behaviour to a great extent on the ability to surpass extremely hard gypsum surface crusts, although this seems a marginal adaptive trait as shown by the capability to grow on a complete array of soils and the negative effect of the crust along the earlier development life stages of gypsophytes. Furthermore, a gypsovag such as Teucrium capitatum presents extreme difficulties to emerge on non-disturbed gypsum physical crusts but once surpassed its growth and survival is not limited.     

Gypsophile vegetation patterns under a range of soil properties induced by topographical position

Iberian gypsophile plant communities are considered a priority for conservation by the European Community because of their highly specialized flora in gypsum outcrops in arid and semiarid regions. Despite the ecological importance of these ecosystems, the edaphic factors that constrain plant communities on gypsiferous soils remain unclear. It has been proposed that both the chemical and physical restrictive conditions of gypsum soils determine gypsophily in plants. Here we hypothesize that the rigors of the gypsum soil environment depends on topography, decreasing from flat areas on hilltops to south-oriented slopes and finally to slopes oriented to the north. We also hypothesized that the relaxation of the rigors of the gypsum soil environment with topography affects both to individual plant and community characteristics of gypsophile vegetation: we expect a reduction of gypsophyte abundance, an increase of diversity and the amelioration of facilitative interactions of plant species. We analysed the physical and chemical properties of gypsum soils that have been proposed that determine the rigors of the gypsum soil environment (i.e.: unbalanced ion concentrations and superficial soil crust). The predicted rigor gradient along topographical locations was confirmed and was mainly caused by superficial soil crust. The decreasing rigor gradient was accompanied by a fall in the abundance of gypsophytes. However, when gypsophytes were considered separately, several patterns were observed, indicating distinct tolerance to relaxation of rigor of the gypsum soil conditions and different competition abilities between gypsophytes. Plant species were more clumped, and gypsophile communities presented higher diversity, evenness and richness values where rigor of gypsum soil conditions were maximum (flat hilltop positions). Relaxation of rigor (north-oriented slopes) was characterized by loss of facilitative interaction between species and the dominance of the gypsovag Rosmarinus officinalis L., although richness was still very high, which can be attributed to the coexistence of gypsophytes and gypsovags. We conclude that the rigor of gypsum soil environment gradient with topography is mainly determined by superficial soil crust, and it is a crucial determinant of gypsophile plant communities.     

Small-scale Environmental Heterogeneity and Spatiotemporal Dynamics of Seedling Establishment in a Semiarid Degraded Ecosystem

In semiarid environments, surface soil properties play a major role in ecosystem dynamics, through their influence on processes such as runoff, infiltration, seed germination, and seedling establishment. Surface soil properties usually show a high degree of spatial heterogeneity in semiarid areas, but direct tests to evaluate the consequences of this heterogeneity on seedling establishment are limited. Using a combination of spatial analysis by distance indices (SADIE) and principal components analysis (PCA) we quantified the spatiotemporal patterns of seedling survival of a Mediterranean native shrub (Pistacia lentiscus) during the first 3 years after planting on a semiarid degraded site in southeastern Spain. We used a variation partitioning method to identify environmental variables associated with seedling survival patterns. Three years after planting, only 36% of the seedlings survived. During the first summer, one-third of the seedlings died, with secondary major mortality in the 3rd summer after planting. The spatial pattern of survival became strongly clumped by the end of the first summer, with clearly defined patches (areas of high survival) and gaps (areas of low survival). The intensity of this pattern increased after subsequent high-mortality periods. Of the 14 variables, the ones most strongly coupled to seedling survival were bare soil cover, sand content, and soil compaction. These findings contribute to our understanding of the linkages between the spatial heterogeneity of abiotic factors and the response of plant populations in semiarid degraded ecosystems and can be used to optimize restoration practices in these areas.     

Sulphur phytoaccumulation in plant species characteristic of Gypsiferous soils

The SO4= toxicity in gypsiferous soils, which represent more than 100 million hectares worldwide, constitutes one of the major problems limiting world agricultural output. Currently, phytoremediation of SO4= is regarded, from agricultural and environmental standpoints, as one of the most effective alternatives for the decontamination and recovery of these soils. In this study, we analyzed the behavior of five plant species characteristic of gypsiferous soils (Gypsophila struthium, Helianthemun alypoides, H. squamatum, H. syriacum, and Lepidium subulatum) in order to establish the variability of these plants in terms of S-extraction mechanisms. Our results indicate that the species best adapted and with the densest population was Lepidium subulatum. This plant showed the highest concentration of total S and the greatest organic S, as well as the highest levels of amino acids and proteins. In this study, we propose that the incorporation of S into organic compounds is the essential metabolic process determining the tolerance of Lepidium subulatum to gypsiferous soils. In conclusion, the foliar accumulation of organic S could be defined a priori as one of the key characteristics in developing plants with a potential for regenerating zones with high SO4= concentrations.     

Evaluation of Restoration Techniques for the Succulent Karoo, South Africa

Abstract Possible constraints on the passive recovery of bare areas in the Karoo, a semiarid region in South Africa, include inadequate supply of seed, availability of suitable microsites for plant establishment, altered soil properties, and the truncation of key soil biotic processes. Here we investigate the possibility of initiating the restoration of bare areas by soil surface treatments with gypsum (CaSO₄) and/or organic mulch. We also apply an exogenous seed source to test the hypothesis that seed availability limits autogenic recovery. Both gypsum and mulch improved rain water infiltration, gypsum more so than mulch, and both treatments resulted in significantly higher numbers of reseeded seedlings compared with controls. Gypsum also improved the survival of the cohorts of seedlings of the larger seeded Tripteris sinuata. Tripteris showed the highest number of seedlings (maximum count of 150 seedlings/1,000 viable seeds sown) and surviving plants of the three reseeded species, which included two small‐seeded species, Ruschia spinosa and Chaetobromus dregeanus. Throughout the study period significantly higher plant volumes of naturally seeded annuals and perennials were recorded in the gypsum and/or mulch treatments compared with the controls. Germination and emergence of reseeded and naturally seeded plants appears to be determined by the availability of cool season (autumn to spring ) soil moisture, whereas follow‐up rainfall during this time is important for plant survival. Mulching of bare areas in the Succulent Karoo has the potential to re‐create vegetated areas that will further capture and conserve water, soil, and nutrients. Gypsum also showed positive results but might not be a cost‐effective option because of transport costs to these remote arid areas.     

Reintroduction of Wild Lupine (Lupinus perennis L.) Depends on Variation in Canopy,Vegetation, and Litter Cover

We experimentally examined the effects of canopy, vegetation, and leaf litter cover on the demography of Wild lupines (Lupinus perennis) in a central North American oak savanna spanning 9 years. We also compared the distribution of Wild lupine across the landscape to results predicted by the demographic experiments. With less canopy cover, soil temperatures were warmer and seedlings emerged earlier. Seedling survival increased 14% with each additional leaf grown. Seedling survival was four times greater in openings and partial shade than in dense shade. Seedling survival was also influenced by interactions between canopy cover and vegetation cover, between canopy cover and leaf litter, and among canopy cover, vegetation cover, and litter cover. In openings, seedlings had higher survival when vegetative cover was present, suggesting a positive shading effect on survival, but with greater canopy cover vegetative cover reduced survival. Seedling survival was greater for plants that experienced herbivory, a result that was probably related to plant size and quality rather than having been eaten. Survival of lupines to 9 years after seed planting was greatest in the partial shade, moderate in openings, and least in dense shade. Wild lupine cover across the landscape was greatest when litter cover was low and canopy cover and ground layer cover were moderate. Reduction of canopy cover by burning or cutting, and reduction of leaf litter by prescribed burning will benefit the reintroduction of Wild lupine by increasing light, reducing litter cover, and creating disturbances; however, the reduction of vegetation cover in openings may hinder lupine reintroduction.     

Inhibitory effects of Artemisia herba-alba on the germination of the gypsophyte Helianthemum squamatum

The potential allelopathic role of Artemisia herba-alba has been evaluated in order to explain the community pattern of the gypsum semiarid environments of central Spain. This pattern shows a sharp ecotone between a gypsophile sparse shrubby community dominated by Helianthemum squamatum, which grows on slopes with gypsum surface crusts, and a nitrohalophilous community on the gypsum alluvial soils of piedmont dominated by Artemisia herba-alba. In order to explain this pattern, resource limitation was discarded because no significant differences in several soil parametrers, but fine earth fraction and organic matter content, had been detected in a previous study. Results confirm the inhibitory effect of aqueous extracts on the final germination percentage of scarified seeds of Helianthemum squamatum and also on the shape of the germination curves, which indicate delay of germination for the aqueous extracts. Assays with soil of the Artemisiacommunity (three types) showed that germination was strongly inhibited in soils obtained below the canopy of mature plants (litter maintained) and retarded in the other treatments. We suggest that the spatial community pattern detected in this gypsum environment and characterised by a sharp ecotone could be at least partially controlled by interference through allelopathy. This determines the excliusion of Helianthemum squamatum plants from alluvial soils.     

Small-scale spatial soil-plant relationship in semi-arid gypsum environments

Studies on soil patterning on a small scale in arid and semi-arid regions have rarely been conducted. Many papers implicitly assume that plant distribution is controlled by some soil variables acting at small scales. We have directly tackled the relationships between soil and some biotic variables including plant distribution at small scales in an Iberian semi-arid gypsum environment. This has been carried out by means of Canonical Correspondence Analysis as a hypothesis-testing tool. CCA models show that the spatial data matrix is able to explain a relevant fraction of the soil data set (P < 0.001). The most important variable, as firstly selected in the CCA stepwise selection procedure, suggests the existence of a vegetation-elevation gradient in relation to soil physical properties; the rest of selected variables indicates the existence of other spatial trends which may be related to certain microgeomorphological features. On the other hand, only the cover of annuals and the cover of litter are selected in the case of the biotic data set as constraining matrix, but not the cover of any perennial plant. Partial CCA models indicated that the remaining information explained by the spatial data set after adjusting the biotic set as covariables is also significant (p < 0.001). This variability is not related to the existence of vegetation bands as shown by the two selected variables in the case of the partial CCA models. The primary source of spatial soil variation is related to the existence of three community bands and these differences are able to explain even the change of plant life forms in vegetated band. The soil parameters controlling the changes are mainly related to texture and surface features. However, we detected other sources of spatial soil variation out of this primary model. This hierarchical spatial pattern seems to be related to some geomorphological traits of the landscape, such as soil crust strength, presence of gypsum crystals or bare zones, and not to the presence of mature gypsophytes (at least the five most frequent) which might ameliorate the soil environment. Furthermore, the biotic data set is not able to explain any new fraction of soil variability out of that already explained by the spatial data set. This revised version was published online in June 2006 with corrections to the Cover Date.     

Plant life on gypsum: a review of its multiple facets

The adaptation of plants to particular soil types has long intrigued biologists. Gypsum soils occupy large areas in many regions of the world and host a striking biological diversity, but their vegetation has been much less studied than that developing over serpentine or saline soils. Herein, we review all aspects of plant life on gypsum ecosystems, discuss the main processes driving their structure and functioning, and highlight the main conservation threats that they face. Plant communities in gypsum habitats typically show distinctive bands at very small spatial scales, which are mainly determined by topography. Plants living on gypsum soils can be classified into three categories: (i) wide gypsophiles are specialists that can penetrate the physical soil crust during early life stages and have physiological adjustments to cope with the chemical limitations imposed by gypsum soils; (ii) narrow gypsophiles are refugee plants which successfully deal with the physical soil crust and can tolerate these chemical limitations but do not show specific adaptations for this type of soils; and (iii) gypsovags are non‐specialist gypsum plants that can only thrive in gypsum soils when the physical crust is absent or reduced. Their ability to survive in gypsum soils may also be mediated by below‐ground interactions with soil microorganisms. Gypsophiles and gypsovags show efficient germination at low temperatures, seed and fruit heteromorphism within and among populations, and variation in seed dormancy among plants and populations. In gypsum ecosystems, spatio‐temporal changes in the composition and structure of above‐ground vegetation are closely related to those of the soil seed bank. Biological soil crusts (BSCs) dominated by cyanobacteria, lichens and mosses are conspicuous in gypsum environments worldwide, and are important drivers of ecosystem processes such as carbon and nitrogen cycling, water infiltration and run‐off and soil stability. These organisms are also important determinants of the structure of annual plant communities living on gypsum soils. The short‐distance seed dispersal of gypsophiles is responsible for the high number of very narrow endemisms typically found in gypsum outcrops, and suggests that these species are evolutionarily old taxa due to the time they need to colonize isolated gypsum outcrops by chance. Climate change and habitat fragmentation negatively affect both plants and BSCs in gypsum habitats, and are among the major threats to these ecosystems. Gypsum habitats and specialists offer the chance to advance our knowledge on restrictive soils, and are ideal models not only to test important evolutionary questions such as tolerance to low Ca/Mg proportions in soils, but also to improve the theoretical framework of community ecology and ecosystem functioning.     

Effectiveness of Low‐Cost Planting Techniques for Improving Water Availability to Olea europaea Seedlings in Degraded Drylands

Reforestation projects in semiarid lands often yield poor results. Water scarcity, poor soil fertility, and structure strongly limit the survival and growth of planted seedlings in these areas. At two experimental semiarid sites, we evaluated a variety of low‐cost planting techniques in order to increase water availability to plants. Treatments included various combinations of traditional planting holes; water‐harvesting microcatchments; stone or plastic mulches; small waterproof sheets to increase water harvesting; dry wells; buried clay pots; and deep irrigation. Some of these treatments were also combined with addition of composted biosolids. Waterproof sheets significantly enhanced water harvesting (43%) and soil moisture in the planting hole (40%), especially for low‐intensity rainfall events. Treatment effects on the survival and growth of Olea europaea seedlings varied between experimental sites. At the most water‐limited site, clay pots, and dry wells improved seedling survival, while no treatment enhanced seedling growth. At the least water‐stressed site, the application of composted sludge significantly improved seedling growth. We conclude that nutrient‐mediated stress is subordinate to water stress in arid and semiarid environments, and we suggest modifications on the microsite scale to address these limiting conditions in Mediterranean drylands .     

Relationships Between Plant Spatial Patterns, Water Infiltration Capacity, and Plant Community Composition in Semi-arid Mediterranean Ecosystems Along Stress Gradients

Water redistribution from bare soil to vegetation patches is a key feature of semi-arid ecosystems, and is responsible for their patchy vegetation patterns. The magnitude of water redistribution depends on the properties of the bare soil (which determine the amount of water run-off) and the capacity of vegetation patches to trap water run-on. We examined the relationships between plant spatial patterns, water infiltration into bare soil, and plant community composition in semi-arid sites with different hydro-physical properties (silty and gypseous soils) in NE Spain. We also studied the effect of two stressors, aridity and grazing, on water infiltration and plant spatial patterns. Our results indicate a negative correlation of bare soil sorptivity (the capacity to absorb water by capillarity) and vegetation aggregation. There was a strong positive correlation between perennial grass cover and the spatial aggregation of vegetation, but aggregation was not associated with positive associations of different plant types. The aggregation of vegetation was positively correlated with species richness and the overall extent of vegetation cover. Grazing reduced water infiltration into silty soils, which are prone to compaction. In contrast, soil crust affected the hydrology of gypseous soils, especially in the most arid sites, where grazing increased infiltration, reducing surface sealing due to breaking of the soil crust. Together, our results suggest that biotic and abiotic factors affect the hydro-physical properties of soils in the semi-arid ecosystems of NE Spain, which is linked to the plant communities through the spatial distribution of plants.     

The Importance of Soil Characteristics in Determining Survival of First‐Year Cottonwood Seedlings in Altered Riparian Habitats

Most major rivers in the southwestern United States have been hydrologically altered to meet human needs. Altered hydrological regimes have been associated with declines in native riparian forests. Today, many riparian areas have little or no regeneration of native riparian species and are now dominated by exotic Saltcedar (Tamarix chinensis Lour.). Success of riparian restoration efforts at least partially depends on the number of seedlings surviving the first growing season. Seedling survival is influenced by many abiotic and biotic factors including competition from other plants and available soil moisture, which is partially dependent on soil texture. In this study, we evaluated the relative importance of four soil categories (sandy loam, loam, silt, and clay), rate of soil moisture decline, salinity, beginning‐ and end‐season Saltcedar density, initial Cottonwood (Populus deltoides Marshall subsp. wislizenii (Wats.) Eckenw.) seedling density, percent vegetation cover by potential dominant competitors Pigweed (Amaranthus L.) and Barnyard grass (Echinochloa crusgalli L., Beauv.), and average total vegetation height to Cottonwood seedling survival. Factors influencing seedling survival differed among the four soil types. Rate of moisture decline was important in sandy soils, whereas vegetation height influenced seedling survival in loamy soils. Overall, models of seedling survival in all the four soil types indicated rate of moisture decline as the single most important variable influencing Cottonwood survival. High initial densities of Saltcedar were correlated to higher survival in Cottonwood seedlings. Therefore, it is important to identify soil texture and understand soil moisture decline rates when proposing riparian Cottonwood restoration.     

Crown development in tropical rain forest trees in gaps and understorey

Emergence and establishment of Tsuga sieboldii seedlings were monitored for six years in an old-growth mixed forest in Japan, to clarify the reason why adult T. sieboldii trees are distributed only on convex slopes. T. sieboldii seedlings germinated on exposed soils without broad-leaved litter cover on both convex and concave slopes, but they died out from concave slope during the first two years after germination. Along a gradient from convex to concave slope, litter cover became higher and surface soil erosion became severer, and T. sieboldii seedlings died at the plots with severe surface soil erosion. Other environmental variables, such as relative illuminance, vegetation cover of seedling layer and soil water potential were not significantly different between convex and concave slopes. We concluded that the restricted seedling establishment of T. sieboldii to convex slope is determined by stability of surface soils and broad-leaved litter cover.     

Diversity and Abundance of Biological Soil Crust Taxa in Relation to Fine and Coarse-Scale Disturbances in a Grassy Eucalypt Woodland in Eastern Australia

Over the last 200 years the box woodlands of eastern Australia have been considerably altered by European farming practices. These changes have been accompanied by a reduction in the size and number of patches of vegetation as well as the quality of the understorey vegetation and underlying soil surface. We measured diversity and abundance of soil crust taxa in relation to habitat complexity, remnant area and width, diversity of vascular plants as well as the number, size and separation of patches of vegetation and grass butts (coarse-scale patchiness), and an index of surface stability derived from measures of seven soil surface features of small microsites (patches of bare/crusted, litter- or grass-covered soil; micro-scale) on both coarse- and fine-textured soils at 35 sites in south-eastern Australia. Fifty taxa were recorded from the 35 sites, and there were more taxa from sites with fine-textured soils (12.7) compared with coarse-textured soils (4.4). The soil crust community was dominated by a few relatively common species, with many species occurring at only a few sites. Half the number of species accounting for <1% of total abundance. Bare and crusted microsites supported more species and greater cover compared with grassy microsites. Crust diversity declined with increasing coarse-level disturbances (i.e. declines in habitat complexity, remnant area and width, and diversity of vascular plants) but the results were not consistent between soil types. No measures of fine-scale disturbance were related significantly to any of the crust diversity or abundance measures, and there was no evidence of a recent grazing effect on crust composition. The fact that few sites had many species (and visa versa) suggests to us that many sites are probably required to conserve soil crust taxa in these highly fragmented landscapes     

Seedling emergence and summer survival after direct seeding for woodland restoration on old fields in south‐western Australia

Restoration opportunities provided by an emerging carbon market have largely focused on large‐scale woodland restoration projects. Gondwana Link is one such project operating in a 1000‐km corridor in south‐western Australia. We identified environmental factors affecting the success of woody‐species restoration at a dry‐woodland Gondwana Link site, Peniup, by relating the emergence and survival of 1522 seedlings to abiotic and biotic variables, including soil conditions and weed cover. We found soil conditions were highly variable across the site and, together with the dry Mediterranean‐climate summer, affected seedling emergence and summer survival. Seedling emergence was higher in sandy soils, but summer survival was higher in clay soils. Most of the seedlings that emerged and survived the summer were in either the Fabales or Myrtaceae family. We concluded that attempts to analyse restoration outcomes that do not consider how the influence of primary abiotic and biotic factors changes over time may mask the mechanisms driving seedling establishment.     

Effects of biological soil crusts on emergence of desert vascular plants in North China

Biological soil crusts are a universal and common feature in arid and semi-arid regions and their appearance profoundly affects soil surface properties which may greatly change the seed dispersal, germination and establishment. To date, only a handful of experiments have exerted to investigate the effects of crusts on vascular plants and the conclusions from these studies are variable. In this study, we investigate the influences of two different crusts universally spreading in southeastern part of the Tengger Desert with four chronosequences (24, 41, 50 years old in artificial vegetation area and natural vegetation crusts) on vascular plants. Crusts were placed at three different sites to simulate different environmental factors (wind velocity and soil crust moisture), we set two soil moisture regimes for crusts to investigate how vascular plants respond under different moisture regimes in crusts. Emergence densities of vascular plants were significantly higher in moss crust than in algae crust. With the development of crusts, seed emergence increased in moss crust while decreased in algae crust. As for effects of moisture, our results showed that soil moisture had a significant effect on seed emergence in both types of crusts at all developing phases. Crusts with higher moisture had more seedlings than those with lower moisture. The above results indicated that the appearance of crusts changed the surface soil properties, which had greatly influenced the entrapment and lodgement of seeds in the study area, thus subsequently influence seed emergence through affecting natural factors.     

Plant Spatial Pattern Predicts Hillslope Runoff and Erosion in a Semiarid Mediterranean Landscape

Abstract The importance of the spatial pattern of vegetation for hydrological behavior in semiarid environments is widely acknowledged. However, there is little empirical work testing the hypothetical covariation between vegetation spatial structure and hillslope water and sediment fluxes. We evaluated the relationships between vegetation structural attributes (spatial pattern, functional diversity), soil surface properties (crust, stone, plant, and ground cover, and particle size distribution) and hillslope hydrologic functioning in a semiarid Mediterranean landscape; in particular, we tested whether decreasing patch density or coarsening plant spatial pattern would increase runoff and sediment yield at the hillslope scale. Runoff and sediment yield were measured over a 45-month period on nine 8 × 2-m plots that varied in vegetation type and spatial pattern. We grouped vegetation into functional types and derived plant spatial pattern attributes from field plot maps processed through a GIS system. We found that there was an inverse relationship between patch density and runoff, and that both runoff and sediment yields increased as the spatial pattern of vegetation coarsened. Vegetation pattern attributes and plant functional diversity were better related to runoff and sediment yield than soil surface properties. However, a significant relationship was found between physical crust cover and plant spatial pattern. Our results present empirical evidence for the direct relationship between the hydrologic functioning of semiarid lands and both the spatial pattern and the functional diversity of perennial vegetation, and suggest that plant spatial pattern, physical crust cover, and functional diversity may be linked through feedback mechanisms.     

黄土丘陵区生物土壤结皮表面糙度特征及影响因素

地表糙度是影响地表径流和侵蚀过程的重要属性.生物结皮在干旱半干旱区广泛分布,是地表糙度的影响因子之一.本文采用链条法测定了黄土丘陵区不同发育阶段生物结皮表面糙度特征,分析了不同发育阶段生物结皮表面糙度对坡向、土壤含水量和冻融作用的响应及其与各理化性质的相关性,初步探索了生物结皮对地表糙度的影响及其相关因素.结果表明: 生物结皮显著改变地表糙度,随着生物结皮从藻结皮向藓结皮演替,其糙度先降低后增加,生物结皮发育形成10年以后,其表面糙度基本趋于稳定;研究区早期形成的藻结皮表面糙度较裸土降低47.0%,深色藻结皮(藓类盖度<20%)较裸土降低20.4%,混生结皮(藓类盖度为20%～60%)和苔藓结皮(藓类盖度>70%)表面糙度与深色藻结皮基本一致;坡向对发育10年以上的生物结皮表面糙度的影响不显著;土壤含水量影响地表糙度特征.研究区浅色藻结皮表面糙度随水分变化较为剧烈;随着生物结皮发育,深色藻结皮、混生结皮和苔藓结皮表面糙度随水分的变化趋于平缓.冻融增加了生物结皮表面糙度.浅色藻结皮经两次冻融后表面糙度增加29.7%;深色藻结皮、混生结皮和藓结皮表面糙度的影响需经过反复冻融才有所体现.生物结皮表面糙度与藓结皮盖度呈显著正相关(P＜0.1).     

Effects of Biological Soil Crusts on Seed Bank, Germination and Establishment of Two Annual Plant Species in the Tengger Desert (N China)

The presence of biological soil crusts can affect the germination and survival of vascular plants, but the reasons are not well investigated. We have conducted a field investigation and greenhouse experiments to test the effect of crusts on two desert annual plants, which occur on the stabilized dunes of the Tengger Desert in N China. The results showed that biological soil crusts negatively influenced the seed bank of Eragrostis poaeoides and Bassia dasyphylla. The important effect of biological soil crusts on seed germination and establishment were performed indirectly through reducing the amount of germinating seeds. Field investigation and experimental results with regard to the seed bank indicated that higher seedling density was found in disturbed crust soil and bare soil surface than in intact crust soils. Greenhouse experiments showed that the effects of biological soil crusts on germination and establishment of the two plants are not obvious in moist condition, while disturbed crusts are more favorable to seed germination in dry treatment. Significant differences in biomass were found between disturbed crust soil and bare soil. Survival and growth of the two annual plants were enhanced in both algal and moss crusts during the season of rainfall or in moist environment, but crusts did not affect seedling survival in the dry period. The small seeded E. poaeoides has higher germination than larger-seeded B. dasyphylla in crust soils, but B. dasyphylla has a relatively higher survival rate than E. poaeoides in crust soils.