Shade facilitates an invasive stem succulent in a chenopod shrubland in South Australia

Abstract The invasive stem succulent Orbea variegata (L) Haw. (Asclepiadaceae) tends to be more abundant underneath shrubs than in open areas in chenopod shrublands near Whyalla, South Australia. To assess the role of facilitation in the life cycle of O. variegata, we investigated the effect of chenopod shrubs on different life stages of the species by experimentally manipulating temperature, light, soil moisture and nutrient levels. Experimental results suggest that the reduction in light and temperature under shrubs, but not increased nutrient levels, are the main facilitative mechanisms for O. variegata. Temperatures above 30°C, which are more likely to occur on the soil surface of open areas than under shrubs, inhibited seed germination. Seedling survival at low watering frequency and the growth of established ramets were increased by 75–80% shade cloth. Ramets growing in full light contained a high concentration of anthocyanin pigments. One of the functions of these pigments is to absorb excess radiation, suggesting that O. variegata experiences radiation stress in full light. In the field O. variegata performed considerably better under Atriplex vesicaria Heward ex Benth. (Chenopodiaceae) or under 75–80% shade cloth than in full light. Monthly irrigation of 20 mm did not reduce this positive effect of the A. vesicaria canopy on O. variegata, suggesting that O. variegata is inhibited by high light intensities or temperatures, independent of water availability. In conclusion, whereas shrub canopies do not seem to be required for the establishment or survival of O. variegata, shrubs improve adult growth and can improve establishment. The possibility of exotic plants being facilitated by other plants has to be taken into account when assessing the probability and rate of invasion.     

Shrub canopies influence soil temperatures but not nutrient dynamics: An experimental test of tundra snow–shrub interactions

Shrubs are the largest plant life form in tundra ecosystems; therefore, any changes in the abundance of shrubs will feedback to influence biodiversity, ecosystem function, and climate. The snow–shrub hypothesis asserts that shrub canopies trap snow and insulate soils in winter, increasing the rates of nutrient cycling to create a positive feedback to shrub expansion. However, previous work has not been able to separate the abiotic from the biotic influences of shrub canopies. We conducted a 3‐year factorial experiment to determine the influences of canopies on soil temperatures and nutrient cycling parameters by removing ~0.5 m high willow (Salix spp.) and birch (Betula glandulosa) shrubs, creating artificial shrub canopies and comparing these manipulations to nearby open tundra and shrub patches. Soil temperatures were 4–5°C warmer in January, and 2°C cooler in July under shrub cover. Natural shrub plots had 14–33 cm more snow in January than adjacent open tundra plots. Snow cover and soil temperatures were similar in the manipulated plots when compared with the respective unmanipulated treatments, indicating that shrub canopy cover was a dominant factor influencing the soil thermal regime. Conversely, we found no strong evidence of increased soil decomposition, CO₂ fluxes, or nitrate or ammonia adsorbtion under artificial shrub canopy treatments when compared with unmanipulated open tundra. Our results suggest that the abiotic influences of shrub canopy cover alone on nutrient dynamics are weaker than previously asserted.     

Spatial relations between shrubs and Prosopis glandulosa canopies

Abstract. Prosopis glandulosa, an arborescent legume, may act as a nurse plant that facilitates the establishment of other woody species. We hypothesized that attenuation of radiant energy and increased soil nutrients beneath P. glandulosa canopies facilitate establishment of subordinate shrubs and shrub cluster development. We determined the spatial distribution pattern of shrubs under P. glandulosa at three locations in southern Texas. Density of Celtis pallida, Zanthoxylum fagara, and total woody plants were comparable among the four cardinal directions at each location, which countered the prediction that shrub density would be greater on the north side of P. glandulosa canopies if attenuation of solar energy was a factor in cluster development. Total woody plant density increased with increasing P. glandulosa basal diameter, canopy radius, and height only at one location. Total woody plant density decreased with increasing total N in the upper 15 cm of soil at two of the three locations. Late in shrub cluster development, extraction of N from the soil and incorporation of N into plant tissue in dense shrub clusters may operate to inhibit further increases in subordinate shrub density.     

火烧迹地柽柳灌丛资源岛特征及植被的自然恢复

干旱半干旱地区灌丛资源岛特征及形成机制多有报道,但资源岛土壤对群落稳定性与火烧迹地植被的自然恢复作用尚不明确。以酒泉盐碱地柽柳灌丛地火烧3年后,自然恢复的柽柳(Tamarix ramosissima)及其冠下草本群落为研究对象,构建柽柳枯立株体量指数(SSI,Shrub Size Index)和恢复力综合指数,探讨了资源岛特征与植被恢复、冠下草本群落多样性的关系,量化不同大小柽柳灌丛的恢复力稳定性。结果表明:(1)在不同SSI的柽柳枯立株下土壤有机质和含水率明均显高于灌丛间地,形成了明显的资源岛特征。土壤有机质最大值出现在0—10 cm土层,中灌丛的肥力积聚效果最明显。(2)土壤主要以中性盐为主,在0—40 cm土层,灌丛区域土壤可溶性盐低于冠外,呈明显的盐谷特征,中灌丛的盐谷分布最为明显。(3)随着枯立株SSI的增大,柽柳新生枝条的数量及其高度均有所增大,冠下植物的高度、盖度、密度和地上植物量也明显高于灌丛间地,且灌丛越大恢复效果越明显。(4)灌下植物Simpson指数、Shannon⁃Wiener指数、Margalef指数和Pielou指数随着枯立株SSI的增大均先降后升,多样性指数低,群落结构简单。(5)恢复力综合指数随着枯立株SSI的增大呈先升后降的趋势,当SSI=52.17时灌丛群落的稳定性最强。柽柳较小时主要是冠下草本植物的恢复维持群落稳定性,随着灌丛的增大逐渐以柽柳灌木的恢复来维持群落稳定性。柽柳灌丛形成的高养低盐的土壤环境对植物群落稳定性的维持和植被的恢复有促进作用,对荒漠生态系统火烧迹地植被恢复和生态保护具有重要意义。     

锦鸡儿属灌丛化对草原化荒漠区土壤线虫群落组成和代谢足迹的影响

调查了内蒙古草原化荒漠区3种锦鸡儿[垫状锦鸡儿(Caragana tibetica)、荒漠锦鸡儿(C.roborovskyi)和狭叶锦鸡儿(C.stenophylla)]灌丛内、外土壤线虫群落多样性、组成和代谢足迹,以及相关的土壤理化性质和植物群落特征,旨在探讨锦鸡儿属灌丛对土壤线虫群落的影响,并分析这种影响是否具有灌木种间差异,同时从线虫功能团水平上探究灌丛如何通过非生物因素和生物因素影响线虫群落组成。结果表明：锦鸡儿属灌丛对土壤线虫多度、丰富度和多样性无显著影响,但却显著影响土壤线虫群落组成。原因是灌丛对土壤线虫群落具有物种选择性;灌丛可能主要是通过根系分泌物、凋落物质量等因素,而不是通过土壤理化性质和林下植物影响线虫群落组成。土壤线虫组成的变化引起线虫代谢足迹发生明显变化。灌丛内土壤线虫功能代谢足迹大于灌丛外(除狭叶锦鸡儿外),说明灌丛内土壤线虫群落对碳的利用率更高。3种锦鸡儿属灌木中,狭叶锦鸡儿的结构代谢足迹最大,表明狭叶锦鸡儿对捕食杂食类线虫代谢活性的促进作用更强。土壤线虫组成的变化通过线虫代谢足迹导致土壤食物网结构发生显著性变化。狭叶锦鸡儿灌丛土壤干扰程度低,营养富集状况好...     

Effects of willow canopies on plant species performance in a low-alpine community

It is believed that abiotic, rather than biotic, factors are ofparamount importance to the performance of plants in alpine and arctichabitats.This study examines how Salix lapponum affects 15associated species by comparing individual growth and reproductive performanceof adult plants growing inside and outside shrub canopies. The study alsoincludes experimental removal of Salix lapponum shrubs,andmeasurements of shoot density of five species inside and outside intactcanopies. Mean above-ground plant weight of nine species was significantlyhigher inside canopies compared to outside. Mean leaf number inside canopieswassignificantly higher for two species, and total seed number was significantlyhigher inside for one species. Mean leaf number was significantly lower insidecanopies compared to outside for one species. Mean seed weight wassignificantlylower inside canopies for one species. Removal of Salixlapponum shrubs increased bulbil weight of Bistortavivipara and seed number and plant weight of Euphrasiafrigida. These results suggest that canopy removal relaxedcompetition. Shoot density was substantially higher outside canopies comparedtoinside for four species and slightly higher outside for one species. Soilmoisture, soil organic content, soil pH, and temperature inside canopies werenot significantly different from outside, whereas photosynthetically activeradiation (PAR) was markedly reduced inside canopies. Increased growth insidecanopies is likely a compensatory mechanism to decreased PAR, which enableplants inside canopies to achieve reproductive outputs that equals those onoutside plants. The lower plant density inside canopies is probably caused byreduced germination and establishment possibilities there, due to reduced PARand higher litter accumulation.     

Species‐specific influences of shrubs on the non‐dormant soil seed bank of native and exotic plant species in central‐northern Monte Desert

Deserts shrubs are well known to facilitate vegetation aggregation, mostly through seed trapping, and stress amelioration during and after plant establishment. Because vegetation aggregation effects are a by‐product of shrub presence, beneficiary species may not only be native, but also exotic. However, despite the high risk that exotic invasive species pose to ecosystem services, little is known of the role of desert shrubs on plant invasions. We assessed the influence of two shrub species on the non‐dormant soil seed bank (i.e. the number of seeds that readily germinate with sufficient water availability) of an invasive annual grass (Schismus barbatus) and of coexisting native species in a central‐northern Monte Desert (Argentina). Soil samples were collected beneath the canopies of two dominant shrub species (Bulnesia retama and Larrea divaricata) and in open spaces (i.e. intercanopies) in May 2001. Overall, the density of germinated seedlings of Schismus and that of the native species were negatively associated across microsite types. Schismus density was similar to that of all native species pooled together (mostly annuals), and was highest in Larrea samples (with no significant differences between Bulnesia and intercanopies). On the contrary, the density of all native species pooled together was highest in Bulnesia samples. Our results suggest that shrubs may contribute to plant invasions in our study system but, most importantly, they further illustrate that this influence can be species specific. Further research is needed to assess the relative importance of in situ seed production (and survival) and seed redistribution on soil seed bank spatial patterns.     

Effects of Prosopis flexuosa on soil properties and the spatial pattern of understorey species in arid Argentina

Abstract. In arid zones dominant woody plants are capable of causing changes in microclimate and soil properties likely to affect species composition, as well as the establishment and spatial distribution of plant species. In North American and European deserts species richness appears to be higher under the canopy of shrubs and trees, in contrast with Chilean deserts where it seems to be lower. Since Prosopis flexuosa (Fabaceae, Mimosoideae) is the most conspicuous tree in the central Monte desert, Argentina, we analysed the effect of this species on the composition and abundance of the shrub and herbaceous layers and on soil properties. We considered two mesohabitats: ‘under P. flexuosa canopy’ and ‘intercanopy areas’. In addition, we analysed the differences between two microhabitats under canopies: ‘northern part of the canopy’ and ‘southern part of the canopy’. Results indicate that species composition and soil properties are affected by both mesohabitats and microhabitats. We found a higher number of shrubs under canopies, whereas that of grasses and perennial forbs increased in intercanopy areas. Concentrations of organic matter, nitrogen, potassium and phosphorus, factors limiting biological productivity in Monte desert soils, were significantly higher under than outside P. flexuosa canopies. Electrical conductivity and concentrations of Na⁺, Ca⁺⁺, Mg⁺⁺ were higher in the northern than in the southern microhabitats. No differences in species richness, evenness or diversity were found between mesohabitats or between microhabitats. We conclude that P. flexuosa modifies the spatial pattern of plant species in the shrub and herbaceous layers and the chemical conditions of the soil, generating spatial heterogeneity on different scales.     

Restoration of Dry Afromontane Forest Using Pioneer Shrubs as Nurse-Plants for Olea europaea ssp. cuspidata

Shrubs are often considered competitive barriers for seedlings planted in reforestation programs, although they can facilitate tree recruitment, especially in ecosystems under high abiotic stress. An alternative reforestation technique using pioneer shrubs as nurse‐plants for Olea europaea ssp. cuspidata was tested in exclosures in northern Ethiopia. Seedlings were planted in three different microhabitats, and their survival was monitored. The microhabitats were bare soil patches between shrubs, patches under the dominant shrub Acacia etbaica, and patches under Euclea racemosa, an evergreen shrub, which supports the majority of naturally established Olea recruits. The ability of shrubs to offer protection against browsing goats was tested experimentally. Controlled shading was used to determine whether solar irradiation causes seedling mortality in environments without water stress. Data were analyzed using Kaplan–Meier survival analysis, Kruskal–Wallis analysis of variance (ANOVA), and one‐way ANOVA. Olea survival was significantly higher and shoot damage by goats was lower when planted under shrub cover compared to bare soil patches, particularly under Euclea canopies, although high shade levels reduced seedling performance. Reduction of solar radiation by shrub canopies and thus control of soil–water evaporation and seedling transpiration most likely controlled the observed facilitation. Planting under shrubs may increase seedling survival and assist regeneration of dry Afromontane vegetation. Preserving pioneers also reduces soil erosion and conserves biodiversity. Excluding livestock is essential for Olea woodland restoration and allows persistent but morphologically modified Olea shrubs to develop vigorous regrowth. Facilitative processes are guiding principles for assisted forest restoration, but above‐average rains may be critical to restore higher biomass levels in semiarid areas.     

Resource heterogeneity generated by shrubs and topography on coastal sand dunes

In early stages of primary succession, colonizing plants can create resource patches that influence the abundance and distribution of other species. To test whether different colonizing shrubs generate contrasting patches on coastal sand dunes, we compared soil characteristics and light availability under the nitrogen-fixing shrub Lupinus arboreus, under the non-nitrogen-fixing shrub Artemisia pycnocephala, and between shrubs on dunes at a site in northern California. Concentrations of inorganic nitrogen and net nitrogen mineralization rates were generally 1–10 times greater in soil under Lupinus than under Artemisia or between shrubs. Soil water content was mostly lower under shrubs. Mean photon flux density near ground level was reduced by at least 80% at 35 cm inside shrub canopies. Topography appeared to have more effect on soil moisture but less direct effect on nitrogen availability than did Lupinus. However, Lupinus probably increases nitrogen levels more on higher, drier dunes. Microhabitats under and between nitrogen-fixing shrubs constitute a mosaic of individually poor but complementary patches in which high levels of light and moderate levels of soil nitrogen are present but tend not to occur together.     

Direct and indirect interactions among plants explain counterintuitive positive drought effects on an eastern Mediterranean shrub species

Coexistence of woody and herbaceous plants may be governed by a complex set of direct and indirect interactions, whose relative importance have been rarely assessed. We experimentally studied woody species establishment in a mixed plant community by disentangling the potential role of such biotic interactions and the effect of environmental variations on them. Seedling establishment of the common eastern Mediterranean shrub species Sarcopoterium spinosum was investigated under different rainfall and light conditions, combined with the effect of the presence of adult shrubs and annual neighbors. We predicted that seedlings will be directly affected by competition with annuals with increasing water availability, while direct effects of adult shrubs will be positive via amelioration of water stress. Indirect effects were expected beneath shrub canopies due to reduced water stressed and light availability for both annuals and shrub seedlings, which may intensify competition between annuals and shrub seedlings. To test these predictions we performed field and garden experiments in which we combined manipulation of shrub and annual presence with manipulations of water availability and light conditions to simulate the effect of shrub canopy. In contrast to our prediction, shrub seedling establishment was not facilitated but inhibited by adult shrubs because of light limitation. As expected, annuals had direct negative effects on shrub seedlings under wet conditions, which shifted to neutral or positive effects under dry conditions. Thus, interactions among shrubs and annuals, and in particular the release from competition during drought years, leads to a counterintuitive positive effect of drought on shrub seedling establishment. Our findings point to the importance of experimentally studying multidimensional interactions for coexistence of different life forms and to the underestimated role of light for success in water‐limited ecosystems.     

Why do some species in arid lands increase under grazing? Mechanisms that favour increased abundance of Maireana pyramidata in overgrazed chenopod shrublands of South Australia

While the abundance of some plant species decreases under high grazing intensity, others become more abundant. Release from competition by decreaser species contributes to this pattern in mesic systems, but this may not be the case in xeric systems where competition may be less intense. Here we examine three mechanisms that may be involved: (i) increased recruitment and growth because of soil changes produced by grazing, for example, increased soil nutrient availability through dung accumulation; (ii) increased recruitment favoured by the breaking up of the lichen crust; and (iii) reduced competition because of the decline of decreaser species. We used field and glasshouse experiments to determine the possible contribution of these mechanisms to the increase of the chenopod Maireana pyramidata around a watering point in a chenopod shrubland of South Australia. There was no evidence of nutrient accumulation close to the watering point, and while seedlings of M. pyramidata responded to nutrient addition, their growth was the same in soil collected from areas with different grazing intensity. While a broken lichen crust increased the emergence of both M. pyramidata and the decreaser Atriplex vesicaria, the effect was larger for the former. We found no competition between seedlings of the two species or between juveniles of A. vesicaria and seedlings of M. pyramidata in glasshouse experiments. Adult plants of both A. vesicaria and M. pyramidata produced similar growth reduction in seedlings of M. pyramidata. Furthermore, a field removal experiment failed to detect any competitive effect of A. vesicaria on M. pyramidata. Our data indicate that the disintegration of the soil crust by grazer activities can be a major factor controlling floristic changes in overgrazed rangelands. These results imply that factors that control establishment may be more important than competition in shaping shrub population dynamics in these systems. Ground surface itself can affect establishment opportunities, and this should be taken into account in management and restoration efforts in arid lands.     

毛乌素沙地优势灌丛对两种松树幼苗的保育作用

选择毛乌素沙地优势灌丛柠条(Caragana intermedia)、紫穗槐(Amorpha fruticosa)和沙柳(Salix psammophila)作为保育植物,选择2年生油松(Pinus tabulaeformis)和樟子松(Pinus sylvestris)幼苗作为目标植物,将其分别种植在开阔地、柠条、紫穗槐和沙柳4种微生境下,连续3a监测油松和樟子松幼苗的存活和生长动态,从而确定毛乌素沙地优势灌丛是否对植物幼苗的定居有促进作用。结果表明:柠条、紫穗槐和沙柳冠层下光照强度、大气温度和土壤温度都显著低于开阔地,致使土壤湿度增高,油松幼苗总存活率均显著高于开阔地,而樟子松幼苗总存活率仅紫穗槐冠层下显著高于开阔地(P0.05)。紫穗槐冠层下油松和樟子松幼苗总存活率均最高(P0.05),外来种紫穗槐可选为保育植物。保育灌丛下松树幼苗的主枝生长良好,并未受到抑制,但第1年主枝生长长度最小。光照强度和土壤湿度是决定主枝生长长度的重要因素。保育灌丛高度和冠幅面积与油松和樟子松幼苗存活率呈显著正相关(P0.05)。固氮灌丛(紫穗槐和柠条)对松树幼苗存活有持续的促进作用,但柠条冠幅结构紧密,可能因为对光的竞争,减弱了促进作用。沙柳灌丛下松树幼苗的存活率较低。油松幼苗具有一定程度的耐阴性,樟子松幼苗喜光性强,保育灌丛下油松幼苗存活率显著高于樟子松幼苗(P0.05)。总的来说,保育灌丛对松树幼苗的定居有促进作用,保育植物技术可作为一种有效的恢复措施,应用在毛乌素沙地的植被恢复中来。但是,保育灌丛的选择要考虑是本土种或外来种,以及灌丛的高度、冠幅大小、冠幅结构和化学效应,恢复物种的选择要考虑其自身的生物学特性及其与保育植物的相互作用。     

Positive effects of shrubs on plant species diversity do not change along a gradient in grazing pressure in an arid shrubland

Facilitative or positive interactions among species are driven mainly by the environmental amelioration or protection from grazing provided by nurse plants. Some studies have suggested that protection from grazing is inconsequential in water-limited environments because of low herbivore densities and their grazing effects. Others, however, argue that herbivores have a major effect on semi-arid plant communities, and that protection from grazing is a significant factor driving positive plant–plant interactions in such environments. We identified a gradient in grazing pressure in a semi-arid shrubland in south-eastern Australia along which we compared soil condition, incident radiation and plant composition beneath two nurse shrub species with open (shrub-free) interspaces. Our aim was to assess the degree of microclimatic amelioration provided by both shrubs, and changes in the interactions (intensity, importance and frequency) between both nurse shrubs and their understorey species, and their effects on species richness at the community level. Both the relative interaction intensity (RII) and interaction importance (I_imp) indices of plant–plant interactions were generally positive and independent of grazing pressure. Soil beneath both nurse plants had significantly greater indices of nutrient cycling and infiltration, and contained more C and N than soil in the open. Almost twice as many species occurred under the canopies of both shrubs (44 species) than in the open (23 species), and the composition of species differed significantly among microsites. Fifty-four percent of all perennial plant species occurred exclusively under shrubs. Our results suggest that environmental amelioration is a stronger driver of the facilitatory effect of shrubs on their understorey species than protection from grazing. Our conclusions are based on the fact that the substantial effect of plant–plant interactions on plant species richness was largely independent of grazing pressure. Irrespective of the underlying mechanism for this effect, our study illustrates the ecological role of shrubs as refugia for understorey plants in semi-arid environments and cautions against management practices aimed at reducing shrub populations.     

Size-dependent foraging efficiency,cannibalism and zooplankton community structure

Only recently has the importance of positive interactions among plant species in structuring natural communities been supported by experimental evidence. Most studies have focused on interactions between a pair of species at a single life-history stage. In this study positive interactions between a woody nitrogen-fixing shrub (Myrica pensylvanica) and two herbaceous sand dune species (Solidago sempervirens, Ammophila breviligulata) which frequently grow beneath shrub canopies are examined throughout the life cycles of the herbaceous species. Comparisons of S. sempervirens and A. breviligulata growing beneath and outside M. pensylvanica shrubs show that plants growing in association with shrubs are larger, are more likely to flower, produce greater numbers of flowers and seeds, have higher midday xylem water potentials, have higher tissue nitrogen concentrations, and have higher photosynthetic efficiencies. Measurements of environmental conditions show that areas beneath shrubs are more shaded, have lower soil temperatures, and have higher soil nitrogen levels. The results from experimental manipulations designed to test the effects of Myrica shrubs on understory species suggest that the observed differences in plant performance are strongly influenced by canopy shading and soil nutrient enrichment associated with the shrubs. The results demonstrate that M. pensylvanica facilitates growth, reproduction, and recruitment of S. sempervirens and A. breviligulata growing beneath it. This study, one of the few to examine positive interactions at different life-history stages, supports previous predictions that positive interactions may be particularly important in plant communities characterized by physiologically stressful conditions. Received: 21 July 1999 / Accepted: 18 January 2000     

The importance of shrub cover for four cactus species differing in growth form in an Andean semi‐desert

Question: Factors influencing seedling establishment are known to vary between open sites and those protected by plant cover. In many desert regions, protected microhabitats below shrubs are essential for establishment of many cactus species. Very little is known about these factors for Andean cacti and how the importance of vegetation cover varies with cactus species. Are Andean cacti associated more frequently to vegetation cover than to open ground? Are they associated to certain shrub species? Is the distributional pattern in relation to cover similar for different cactus species? In what microhabitat (below or away from shrubs) are cactus seeds more abundant? These questions are addressed for the case of an Andean semi‐desert. Location: Semi‐arid tropical Andes, La Paz department, Bolivia. Methods: We examined 132 isolated shrubs = 50 cm along a line across two microhabitats: areas below and away from shrubs/trees. Shrub crown size was measured. The among‐shrub samples were taken from open spaces contiguous to each of the sampled shrubs. In both microhabitats, all cactus species were recorded. The cardinal direction of the cacti was also registered. Correlation between canopy diameter and number of beneficiaries was evaluated for Prosopis flexuosa. The cactus seed bank in each microhabitat was also studied. Results and Conclusions: The four cactus species found behaved differently in relation to shrub canopies. These distributional differences could be due to differences in growth form. Columnar cacti apparently need the shade of shrubs. Only the columnar species is able to grow near the base of the tallest nurse species. The opuntioid cacti studied seem more facultative: although apparently preferring shrub un‐der‐canopies, they are able to establish in open ground. The globose cactus is the most indifferent to the presence of plant cover. These patterns parallel others found in North America. The capacity of different cacti to appear in open spaces could be related to vegetative propagation, and not necessarily to seedling tolerance of heat.     

Development of a new piosphere in arid chenopod shrubland grazed by sheep. 2. Changes to the vegetation

The initial development of a piosphere created by stocking a new watering trough with ca. 200 sheep at a near-pristine site in arid chenopod shrubland near Whyalla, South Australia, was studied. The following variables were monitored for up to 8 years using fenced plots as controls:shrub mortality; forage biomass and defoliation of the dominant shrubs and short-lived grasses; linear dimensions of Rhagodia ulicina; the density of short-lived species; and the invasion of exotics. Patterns in these variables were looked for which reflected the attenuation of stocking pressure with distance from water; that is, ‘piosphere patterns’. Piosphere patterns were detected for many variables. Those with values increasing towards the trough were mortality of the main forage shrub Atriplex vesicaria. the density of three short-lived taxa, viz. the bassias (Sclerolaena obliquicuspis, S. patenticuspis. Maireana sclerolaenoides), Tetragonia tetragonoides, and annual Zygophyllum spp. (Z. crenatum. Z. ovatum), and growth of short-lived grass species when sheep stocking pressure was low. Variables with values decreasing towards the trough were grass biomass, percentage of A. vesicaria individuals in flower, the percentage foliation of A. vesicaria and of the forage shrub Maireana sedifolia, and shrub forage biomass of A. vesicaria and M. sedifolia. The rate at which these patterns were registered varied. After only 3 months, a piosphere pattern was evident for grass biomass. Marrubium vulgare, an introduced weed, invaded the trough site within the first 6 months. Almost no A. vesicaria shrubs had died after 2 years, but after 8 years there was a marked piosphere pattern in their mortality. The piosphere patterns in forage biomass and defoliation of A. vesicaria and M. sedifolia became more pronounced with time. Some variables did not exhibit a piosphere pattern, such as the dimensions of the shrub R. ulicina and the densities of successive cohorts of the forb Erodium spp., even though there was an overall effect of grazing in reducing their values. There was no mortality of two dominant shrubs, M. sedifolia and R. ulicina, during the first 8 years.     

几种灌木、半灌木对沙地土壤肥力影响机制的研究

研究了科尔沁地广泛分布的几种灌木和半灌木的“肥岛”和根际效应。结果表明,在灌丛下土壤有机C、全N和全P分别比灌丛间地高56％、51％和37％,土壤电导率（EC）提高了56％,但pH值并无明显变化;灌木根际土壤较非根际土壤有机C和全N分别高76％和54％,根际土壤pH降低0．19个单位,EC提高了2－3倍,但土壤全P并无显著变化,灌丛下土壤发和根际中土壤养分含量显著相关,表明丛肥岛的利于根系的生长,导致更多的根际沉积;而根系的活动又促进了肥岛的发育,灌丛对沙地土壤肥力的影响机制在于：灌丛对土壤风蚀物质、降尘和凋落物等的截获,形成灌丛“肥岛”;并通过发达的根系以根际沉积的形式向土壤输入大量的有机物质,从而使周围土壤的肥力性关得以改善。     

Interactive effects of grazing and shrubs on the annual plant community in semi‐arid Mediterranean shrublands

Question: We studied the interactive effects of grazing and dwarf shrub cover on the structure of a highly diverse annual plant community. Location: Mediterranean, semi‐arid shrubland in the Northern Negev desert, Israel. Methods: Variation in the biomass and plant density of annual species in the shrub and open patches was monitored during four years, inside and outside exclosures protected from sheep grazing, in two contrasting topographic sites: north and south‐facing slopes that differed in their dominant dwarf shrubs species: Sarcopoterium spinosus and Corydothymus capitatus, respectively. Results: Above‐ground biomass, density and richness of annual species were lower under the canopy of both shrub species compared to the adjacent open patches in the absence of grazing. Grazing reduced the biomass of annuals in open patches of both topographic sites, but not in the shrub patches. On the north‐facing slope, grazing also reduced plant density and richness in the open patches, but increased plant density in the shrub patches. At the species level, various response patterns to the combined effects of grazing and patch type were exhibited by different annuals. Protection against the direct impacts of grazing by shrub cover as well as species‐specific interactions between shrubs and annuals were observed. A conceptual mechanistic model explaining these interactions is proposed. Conclusion: In semi‐arid Mediterranean shrublands grazing and dwarf shrub cover interact in shaping the structure of the annual plant community through (1) direct impacts of grazing restricted to the open patches, (2) species‐specific facilitation/ interference occurring in the shrub patches and (3) subsequent further processes occurring among the interconnected shrub and open patches mediated through variation in seed flows between patches.     

Species interactions at the level of fine roots in the field: influence of soil nutrient heterogeneity and plant size

Interference at the level of fine roots in the field was studied by detailed examination of fine root distribution in small soil patches. To capture roots as they occur in natural three-dimensional soil space, we used a freezing and slicing technique for microscale root mapping. The location of individual roots intersecting a sliced soil core surface was digitized and the identity of shrub and grass roots was established by a chemical technique. Soil patches were created midway between the shrub, Artemisia tridentata, and one of two tussock grasses, Pseudoroegneria spicata or Agropyron desertorum. Some soil patches were enriched with nutrients and others given only deionized water (control); in addition, patches were located between plants of different size combination (large shrubs with small tussock grasses and small shrubs with large tussock grasses). The abundance of shrub and grass roots sharing soil patches and the inter-root distances of individual fine roots were measured. Total average rooting density in patches varied among these different treatment combinations by only a factor of 2, but the proportion of shrub and grass roots in the patches varied sixfold. For the shrub, the species of grass roots sharing the patches had a pronounced influence on shrub root density; shrub roots were more abundant if the patch was shared with Pseudoroegneria roots than if shared with Agropyron roots. The relative size of plants whose roots shared the soil patches also influenced the proportion of shrub and grass roots; larger plants were able to place more roots in the patches than were the smaller plants. In the nutrient-enriched patches, these influences of grass species and size combination were amplified. At the millimeter- to centimeter-scale within patches, shrub and grass roots tended to segregate, i.e., avoid each other, based on nearest-neighbor distances. At this scale, there was no indication that the species-specific interactions were the result of resource competition, since there were no obvious patterns between the proportion of shrub and grass roots of the two species combinations with microsite nutrient concentrations. Other potential mechanisms are discussed. Interference at the fine-root level, and its species-specific character, is likely an influential component of competitive success, but one that is not easily assessed.