Distribution and abundance of butterflies in a mountain area in the northern Iberian peninsula

The effect of alutudinal range, distance to the latitudinal boundary of geographical range, body size and larval food plant on both the distribution and abundance of butterflies have been studied in a mountain area along a marked altitudinal gradient Multiple regression analysis revealed that distribution was positively related to altitudinal range and abundance Altitudinal range accounted for a great part of vanance in species distribution Altitudinal range increased as both distance to latitudinal boundary of geographical range and body size increased Abundance was not affected by altitudinal range, body size or distance to latitudinal boundary Larval food plants family was related to the abundance of butterflies, but not to distribution or altitudinal range These results suggest that regional distributions of butterflies are likely to be limited by climatic tolerances of species, while local abundance might be influenced by local resource levels     

Spatial and temporal variation of photosynthesis in intertidal Mazzaella laminarioides (Bory)Fredericq (Rhodophyta, Gigartinales)

The red alga Mazzaella laminarioides is an economically important species with an extended latitudinal distribution along the Chilean coast. Its populations form mid-intertidal stands, several meters wide, and therefore are differentially exposed to environmental variables that result in temporal and spatial variability in productivity. We evaluated the effect of latitude and intertidal height on productivity by in situ measurement of photosynthetic performance. Daily and seasonal variations of O₂-evolution rate and maximal quantum yield (F _v/F _m) were determined in plants from the upper and lower intertidal zone at two localities 1500,km apart. Results suggest that plant responses were mainly affected by irradiation, temperature and desiccation. At local level, upper intertidal plants showed a reduced photosynthetic rate and quantum efficiency as compared to those displayed by plants from the lower intertidal, indicating their higher level of excitation energy acclimation. Stronger acclimation differences between upper and lower intertidal plants were observed in spring and summer. Differences in photosynthetic parameters between reproductive phases were recorded in autumn and winter, regardless of the position of the individuals in the intertidal zone. The effects of tidal elevation on seasonal patterns of photosynthesis were also influenced by latitude. Seasonal variation in photosynthetic efficiency was observed in plants from the northern population at both intertidal elevations, but only at the upper intertidal level in the southern population. This study shows that production variability in M. laminarioides results from differences in the intensity of environmental factors observed seasonally at local (intertidal) and latitudinal scales.     

Interdomain ecological networks between plants and microbes

While macroscopic interkingdom relationships have been intensively investigated in various ecosystems, the above–belowground ecology in natural ecosystems has been poorly understood, especially for the plant–microbe associations at a regional scale. In this study, we proposed a workflow to construct interdomain ecological networks (IDEN) between multiple plants and various microbes (bacteria and archaea in this study). Across 30 latitudinal forests in China, the regional IDEN showed particular topological features, including high connectance, nested structure, asymmetric specialization and modularity. Also, plant species exhibited strong preference to specific microbial groups, and the observed network was significantly different from randomly rewired networks. Network module analysis indicated that a majority of microbes associated with plants within modules rather than across modules, suggesting specialized associations between plants and microorganisms. Consistent plant–microbe associations were captured via IDENs constructed within individual forest locations, which reinforced the validity of IDEN analysis. In addition, the plant–forest link distribution showed the geographical distribution of plants had higher endemicity than that of microorganisms. With cautious experimental design and data processing, this study shows interdomain species associations between plants and microbes in natural forest ecosystems and provides new insights into our understanding of meta‐communities across different domain species.     

民勤绿洲荒漠过渡带植物群落数量分类和排序研究

绿洲-荒漠过渡带作为绿洲与荒漠的连接区域,在干旱区生态环境退化与恢复研究中具有重要意义。该研究以民勤绿洲荒漠过渡带为例,在47个植物群落调查样地的基础上,应用双向指示种分类法(TWINSPAN)和除趋势对应分析法(DCA)对研究区典型植物群落进行数量分类和排序,并讨论了影响植物群落类型及空间分布的环境因子。结果表明:(1)研究区植物群落结构简单,植物种以旱生耐盐植物为主。(2)TWINSPAN分类将研究区的植物群落划分为11个群丛类型,即白刺+盐生草群丛、白刺+芦苇群丛、白刺+盐爪爪群丛、盐爪爪+黑果枸杞群丛等,分类结果在空间上反映了民勤绿洲荒漠过渡带植物群落的演替变化趋势。(3)DCA排序结果验证了TWINSPAN分类结果的合理性,同时指出制约民勤绿洲荒漠过渡带植物群落类型及空间分布的重要环境因子是地下水埋深和地貌形态。     

Characteristics of desert flora and vegetation in lower reach of Shiyang River论Basin

石羊河下游位于干草原向荒漠过渡区域,有天然种子植物23个科,73个属,123个种,不仅有典型的戈壁荒漠成分,也渗入一些草原化荒漠植物种类.其荒漠植被特征更明显,群落层片结构简单,群丛建群种的74.4%是灌木和半灌木,构成了灌木、半灌木旱生群落;1年生草本植物随季节性降雨可形成＂灌草丛＂,成为荒漠植被的重要组成部分.在植物地理区系成分上,温带成分属占优势,其中地中海至中亚分布型及其变型最多,其次为北温带分布及其变型.整个区域内植物的分布集中在生境较适宜的区域,空间结构的成群性高,但在小区域内植物种的分布则较单一.植被可划分为4个植被型,5个植被亚型,13个群系组,23个群系,38个群丛.地域性植被以绿洲为中心,成环状分布,可分为3个带.     

Reproductive effort in desert versus mediterranean crucifers: the allogamous Erucaria rostrata and E. hispanica and the autogamous Erophila minima

Components of the total sexual investment of plants growing under mediterranean climatic and edaphic conditions were compared with those of plants growing in the desert, in the closely related allogamous species pair Erucaria hispanica and E. rostrata and in populations of the autogamous species Erophila minima. In all cases lower total investment was evident in the desert plants. At the prezygotic phase it was expressed by (1) reduced production of flowers; (2) lower allocation to the production of male gametophytes and some floral organs; and (3) packaging of more ovules per ovary. The ratio of reproductive: vegetative biomass which was found to be greater in the desert plants and their lower pollen: ovule ratio are perhaps indicative of greater efficiency. Their smaller investment at the postzygotic phase was expressed in: (1) reduction in total numbers of fruits and seeds; (2) decrease in seed size and weight. Yet in the desert plants the number of seeds per total biomass was found to be significantly larger and fertility rates (seed-set per ovule, fruit-set per flower per plant) were equal to or greater than those in the mediterranean plants. The trends observed in this study in desert plants, which may result in more efficient exploitation of resources, are similar in the species involved, regardless of their breeding system-autogamous or allogamous.     

Morphological variation in Puya (Bromeliaceae): an allometric study

A group of representative species of the genus Puya was studied to determine if there are allometric relationships between vegetative and floral parts, whether these relationships correlate with their pollination system and if plant size is correlated with elevation and latitude. Fifty-three species representing the morphological variation and distribution of the genus were studied. Total plant height, as well as leaf, inflorescence, petal and sepal length were measured and these data subjected to univariate and multivariate analyses. To test for correlation between plant size and altitudinal and latitudinal distribution, ANOVAs were performed. When the pollination system of a species is known, additional multivariate and univariate analyses were also performed. The results indicate that the characters studied are correlated with a size component, exhibiting positive allometry for sepal and petal length and negative allometry for leaf length. Inflorescence length is an isometric character. There was no significant correlation between plant size and altitudinal or latitudinal distribution. The ANOVAs show that the only character correlated to pollinator type was petal length. Small plants with small flowers are correlated to pollinators such as insects, while medium to large plants with medium to large-sized flowers are correlated with pollinators such as birds and bats. Large plants have small flowers, that are more evident and attract more pollinators.     

库姆塔格沙漠典型植物种群特征与自然环境要素的因子分析

通过对库姆塔格沙漠区的典型植物梭梭、胡杨、沙生柽柳的种群特征及其地理、土壤、水分三类要素的7种因子进行实地样方调查,并采用因子分析法,对影响三种群生长与分布的环境因子进行分析。结果表明:(1)土壤养分是区域三种群生存和发展的最主要影响因子,表明区域环境严酷、土壤贫瘠,对植物生长发育的限制作用明显。(2)海拔影响较小,并均与其它因子共同产生作用,表明地理因子对植物或植物种群的影响主要是通过对其它因子的作用而产生的间接影响。(3)水分对三种群的影响明显,且各不相同,水分与海拔高度变化具有一定的相关性,共同影响了三种群的分布和种群发展。(4)水分是影响梭梭和胡杨种群分布的主要因子,水分对沙生柽柳的影响反映了其生存环境的地下水位及其矿化度特征。研究认为,由于库姆塔格沙漠区域长期地质及气候环境变化形成的土壤基质在短期内难以得到改善,土壤养分匮乏的影响将长期存在,植物种群的保护与恢复存在较大难度,因此防治区域沙漠化发展、保护植物现有生境将是维持库姆塔格沙漠区域植物生长和种群发展的有效手段。     

Opposing clines for high and low temperature resistance in Drosophila melanogaster

In insects, species comparisons suggest a weak association between upper thermal limits and latitude in contrast to a stronger association for lower limits. To compare this to latitudinal patterns of thermal responses within species, we considered latitudinal variation in heat and cold resistance in Drosophila melanogaster. We found opposing clines in resistance to these temperature extremes in comparisons of 17–24 populations from coastal eastern Australia. Knockdown time following heat shock increased towards the tropics, whereas recovery time following cold shock decreased towards temperate latitudes. Mortality following cold shock also showed a clinal pattern. Clinal associations with latitude were linear and related to minimum temperatures in the coldest month (for cold resistance) and maximum temperatures in the warmest month (for heat resistance). This suggests that within species both high and low temperature responses can vary with latitude as a consequence of direct or indirect effects of selection.     

Rodent middens reveal episodic,long‐distance plant colonizations across the hyperarid Atacama Desert over the last 34,000 years

Aim To document the impact of late Quaternary pluvial events on plant movements between the coast and the Andes across the Atacama Desert, northern Chile. Location Sites are located along the lower and upper fringes of absolute desert (1100–2800 m a.s.l.), between the western slope of the Andes and the Coastal Ranges of northern Chile (24–26° S). Methods We collected and individually radiocarbon dated 21 rodent middens. Plant macrofossils (fruits, seeds, flowers and leaves) were identified and pollen content analysed. Midden assemblages afford brief snapshots of local plant communities that existed within the rodents’ limited foraging range during the several years to decades that it took the midden to accumulate. These assemblages were then compared with modern floras to determine the presence of extralocal species and species provenance. Results Five middens span the last glacial period (34–21 ka) and three middens are from the last glacial–interglacial transition (19–11 ka). The remaining 13 middens span the last 7000 years. Coastal hyperarid sites exhibit low taxonomic richness in middens at 19.3, 1.1, 1.0, 0.9, 0.5 ka and a modern sample. Middens are also dominated by the same plants that occur today. In contrast, middens dated to 28.1, 21.3, 17.3, 3.7 and 0.5 ka contain more species, including Andean extralocals. Precordillera middens (c. 2700 m) show a prominent increase in plant macrofossil richness, along with the appearance of Andean extralocals and sedges at 34.5 and 18.9 ka. Six younger middens dated to 6.1–0.1 ka are similar to the modern local vegetation. Main conclusions Increased species richness and Andean extralocal plants occurred along the current lower fringes of absolute desert during the last glacial–interglacial transition and late Holocene. The absence of soil carbonates indicates the persistence of absolute desert throughout the Quaternary. Colonization by Andean plants could have been accomplished through long‐distance seed dispersal either by animals or floods that originated in the Andes. We postulate that dispersal would have been most frequent during regional pluvial events and associated increases in groundwater levels, forming local wetlands in the absolute desert, and generating large floods capable of crossing the Central Depression.     

Latitudinal diversity gradients in bryophytes and woody plants: Roles of temperature and water availability

It remains unclear whether the latitudinal diversity gradients of micro- and macro-organisms are driven by the same macro-environmental variables. We used the newly completed species catalog and distribution information of bryophytes in China to explore their spatial species richness patterns, and to investigate the underlying roles of energy availability, climatic seasonality, and environmental heterogeneity in shaping these patterns. We then compared these patterns to those found for woody plants. We found that, unlike woody plants, mosses and liverworts showed only weakly negative latitudinal trends in species richness. The spatial patterns of liverwort richness and moss richness were overwhelmingly explained by contemporary environmental variables, although explained variation was lower than that for woody plants. Similar to woody plants, energy and climatic seasonality hypotheses dominate as explanatory variables but show high redundancy in shaping the distribution of bryophytes. Water variables, that is, the annual availability, intra-annual variability and spatial heterogeneity in precipitation, played a predominant role in explaining spatial variation of species richness of bryophytes, especially for liverworts, whereas woody plant richness was affected most by temperature variables. We suggest that further research on spatial patterns of bryophytes should incorporate the knowledge on their ecophysiology and evolution.     

Resource Islands Predict the Distribution of Heterotrophic Bacteria in Chihuahuan Desert Soils

The resource island hypothesis predicts that soil resources such as nitrogen, phosphorus, and water will be distributed evenly in grasslands but have a patchy distribution focused around plants in shrublands. This hypothesis predicts that microorganism numbers will follow resources and be (i) evenly distributed in grasslands, (ii) concentrated around individual plants in shrublands, and (iii) higher where resources are higher when comparing the same vegetation type. This study enumerated total heterotrophic bacteria and a subset of these, the nitrogen-efficient guild (NEG), in three shrublands (playa fringe mesquite, tar bush, and creosote) and two grasslands (playa and bajada). Both heterotrophs and NEG members followed the distribution pattern predicted by the resource island hypothesis. There were no significant differences in heterotroph or NEG numbers comparing at-plant and interplant samples for both the playa and bajada grasslands. Furthermore, populations were generally higher in nutrient-rich playa grasslands than nutrient-poor bajada grasslands. In contrast, both heterotroph and NEG numbers were higher at shrubs than between shrubs in all three shrub sites. These results suggest that resource abundance in resource islands predicts the distribution of heterotrophic bacterial numbers in desert soils.     

Demographic trends in the Cactaceae

Although our biological knowledge regarding cactus species is thorough in many areas, only in recent years have ecologists addressed their demographic behavior. Here we attempt a first review of the present knowledge on cactus demography, including an analysis of the published information on species with different growth forms and life-history traits. Our review shows that cactus distribution ranges are determined by environmental heterogeneity and by species-specific physiological requirements. Temperature extremes may pose latitudinal and altitudinal distribution limits. At a more local scale, soil properties dramatically affect cactus distribution. Most cacti show a clumped spatial distribution pattern, which may be the reflection of a patchy resource distribution within their heterogeneous environments. The association of cacti with nurse plants is another factor that may account for this aggregated distribution. Many cacti grow in association with these perennial nurse plants, particularly during early life-cycle phases. The shade provided by nurse plants results in reduced evapotranspiration and buffered temperatures, which enhance cactus germination and establishment. In some cases a certain degree of specificity has been detected between particular cactus species and certain nurse plants. Yet some globose cacti may establish in the absence of nurse plants. In these cases, rocks and other soil irregularities may facilitate germination and establishment. Cacti are slow-growing species. Several abiotic factors, such as water and nutrient availability, may affect their growth rate. Competition and positive associations (i.e., mycorrhizae and nurse-cacti association) may also affect growth rate. Age at first reproduction varies greatly in relation to plant longevity. In general, cactus reproductive capacity increases with plant size. Populations are often composed of an uneven number of individuals distributed in the different size categories. This type of population structure reflects massive but infrequent recruitment events, apparently associated with benign periods of abundant rainfall. A few cactus species have been analyzed through the use of population-projection matrices. A total of 17 matrices were compiled and compared. Most of them reflect populations that are close to the numerical equilibrium (λ = close to unity). Elasticity analyses revealed that the persistence of individuals in their current size category (“stasis”) is the demographic process that contributes the most to population growth rate. Also, adult categories (rather than juveniles or seedlings) show the largest contributions to λ. No differences were apparent regarding this matter between cacti with different life-forms. This review shows that our knowledge of cactus population ecology is still incipient and rather unevenly distributed: some topics are well developed; for others the available information is still very limited. Our ability to preserve the great number of cactus species that are now endangered depends on our capacity to deepen our ecological understanding of their population processes.     

Are biotic factors significant in influencing the distribution of halophytes in saline habitats?

The influence of biotic factors on the distribution and establishment of halophytes is being considered in this review. Physicochemical factors, such as salinity and flooding, often are considered to be the determining factors controlling the establishment and zonational patterns of species in salt marsh and salt desert environments. Sharp boundaries commonly are found between halophyte communities even though there is a gradual change in the physicochemical environment, which indicates that biotic interactions may play a significant role in deterining the distribution pattern of species and the composition of zonal communities. Competition is hypothesized to play a key role in determining both the upper and lower limits of species distribution along a salinity gradient. Field and laboratory experiments indicate that the upper limits of distribution of halophytes into less saline or nonsaline habitats is often determined by competition. There appears to be a reciprocal relationship between the level of salt tolerance of species and their ability to compete with glycophytes in less saline habitats. Halophytes are not competitive in nonsaline habitats, but their competitive ability increases sharply in saline habitats. Allelopathic effects have been reported in salt desert habitats, but have not been reported along salinity gradients in salt marshes. Some species of halophytes that are salt accumulators have the ability to change soil chemistry. Chemical inhibition of intolerant species occurs when high concentrations of sodium are concentrated in the surface soils of salt desert plant communities that are dominated by salt-accumulating species. Establishment of less salt-tolerant species is inhibited in the vicinity of these salt-accumulating species. Herbivory is reported to cause both an increase and a decrease in plant diversity in salt marsh habitats. Heavy grazing is reported to eliminate sensitive species and produce a dense cover of graminoids in high marsh coastal habitats. However, in other marshes, grazing produced bare patches that allowed annuals and other low marsh species to invade upper marsh zonal communities. A retrogression in plant succession may occur in salt marshes and salt deserts because of heavy grazing. Intermediate levels of grazing by sheep, cattle, and horses could produce communities with the highest species richness and heterogeneity. Grazing by geese produced bare areas that had soils with higher salinity and lower soil moisture than vegetated areas, allowing only the more salt-tolerant species to persist. Removal of geese from areas by use of inclosures caused an increase in species richness in subarctic salt marshes. Invertebrate herbivores could also inhibit the survival of seeds and the ability of plants to establish in marshes. Parasites could play a significant role in determining the species composition of zonal communities, because uninfected rarer species are able to establish in the gaps produced by the death of parasitized species.     

Patterns of functional diversity across an extensive environmental gradient: vertebrate consumers, hidden treatments and latitudinal trends

Over the last two decades, although much has been learned regarding the multifaceted nature of biodiversity, relatively little is known regarding spatial variation in constituents other than species richness. This is particularly true along extensive environmental gradients such as latitude. Herein, we describe latitudinal gradients in the functional diversity of New World bat communities. Bat species from each of 32 communities were assigned to one of seven functional groups. Latitudinal gradients existed for the richness, diversity and scaled‐dominance of functional groups. No significant patterns were observed for evenness of functional groups. Measures of functional diversity were different in magnitude and increased towards the equator at a faster rate than expected given the underlying spatial variation in species richness. Thus, latitudinal gradient in species richness alone do not cause the latitudinal gradient in functional diversity. When variation in species composition of the regional fauna of each community was incorporated into analyses, many differences between observed and simulated patterns of functional diversity were not significant. This suggests that those processes that determine the composition of regional faunas strongly influence the latitudinal gradient in functional diversity at the local level. Nonetheless, functional diversity was lower than expected across observed sites. Community‐wide responses to variation in the quantity and quality of resources at the local level probably contribute to differences in functional diversity at local and regional scales and enhance beta diversity.     

Dioecy and its correlates in the flowering plants

Considerable effort has been spent documenting correlations between dioecy and various ecological and morphological traits for the purpose of testing hypotheses about conditions that favor dioecy. The data analyzed in these studies, with few exceptions, come from local floras, within which it was possible to contrast the subsets of dioecious and nondioecious taxa with regard to the traits in question. However, if there is a strong phylogenetic component to the presence or absence of dioecy, regional sampling may result in spurious associations. Here, we report results of a categorical multivariate analysis of the strengths of various associations of dioecy with other traits over all flowering plants. Families were scored for presence of absence of monoecy or dioecy, systematic position, numbers of species and genera, growth forms, modes of pollination and dispersal, geographic distribution, and trophic status. Seven percent of angiosperm genera (959 of 13,500) contain at least some dioecious species, and ≈6% of angiosperm species (14,620 of 240,000) are dioecious. The most consistent associations in the data set relate the presence of dioecy to monoecy, wind or water pollination, and climbing growth. At both the family and the genus level, insect pollination is underrepresented among dioecious plants. At the family level, a positive correlation between dioecy and woody growth results primarily from the association between dioecy and climbing growth (whether woody or herbaceous) because neither the tree nor the shrub growth forms alone are consistently correlated with a family's tendency to include dioecious members. Dioecy appears to have evolved most frequently via monoecy, perhaps through divergent adjustments of floral sex ratios between individual plants. Monoecy itself is related to abiotic pollination and climbing growth as revealed by multivariate analysis. Dioecy and monoecy are concentrated in the less advanced superorders of Thorne (1992) and subclasses of Cronquist (1988). The frequency of dioecy found in a local flora therefore reflects the level of dioecy in its particular pool of families as much as, or more than, local selective factors. The positive associations of dioecy with abiotic pollination and monoecy are related to floral developmental and morphological attributes, as is the negative association with bird and bat pollination; the positive association of dioecy with climbing growth is tentatively explained in terms of differential selection for optimal resource allocation to sexual function. If rapid upward growth is at a premium in climbers and if fruit set at least temporarily inhibits growth or requires the production of thicker, more slowly growing stems to support heavy fruits, it might be advantageous to postpone femaleness. If the effect is strong, this may favor male plants.     

Host-plant leaps versus host-plant shuffle: a global survey reveals contrasting patterns in an oligophagous insect group (Hemiptera,Psylloidea)

Global analyses of interspecific interactions are rapidly increasing our understanding of patterns and processes at large scales. Understanding how biodiversity assembles and functions on a global scale will increasingly require analyses of complex interactions at different ecological and phylogenetic levels. We present an analysis of host-plant associations in the sap-sucking Psylloidea (～3,800 species) using the most comprehensive assemblage of host data for this group compiled from 66?% of published records. Psyllids are known for high levels of host specificity and host switching between related plants at local scales, but a global survey implicates historical processes that are not entirely consistent with those at local scales. In particular, saltationary host switching events appear to have been a key factor explaining the wide but patchy distribution of psyllid host-plants throughout the angiosperm phylogeny. Alternative explanations involving co-diversification with subsequent extinction seem implausible. At the seed plant family level, we compare associations for psyllids with those of their relatives the aphids, but, despite notable differences in biogeographic distributions, find few plant families (2%) that host only psyllids but not aphids, while a much larger percentage (31%) host aphids but not psyllids, and 43% of plant families distributed throughout the plant phylogeny host neither group.     

Coexistence through mutualist‐dependent reversal of competitive hierarchies

Mechanisms that allow for the coexistence of two competing species that share a trophic level can be broadly divided into those that prevent competitive exclusion of one species within a local area, and those that allow for coexistence only at a regional level. While the presence of aphid‐tending ants can change the distribution of aphids among host plants, the role of mutualistic ants has not been fully explored to understand coexistence of multiple aphid species in a community. The tansy plant (Tanacetum vulgare) hosts three common and specialized aphid species, with only one being tended by ants. Often, these aphids species will not coexist on the same plant but will coexist across multiple plant hosts in a field. In this study, we aim to understand how interactions with mutualistic ants and predators affect the coexistence of multiple species of aphid herbivores on tansy. We show that the presence of ants drives community assembly at the level of individual plant, that is, the local community, by favoring one ant‐tended species, Metopeurum fuscoviride, while preying on the untended Macrosiphoniella tanacetaria and, to a lesser extent, Uroleucon tanaceti. Competitive hierarchies without ants were very different from those with ants. At the regional level, multiple tansy plants provide a habitat across which all aphid species can coexist at the larger spatial scale, while being competitively excluded at the local scale. In this case, ant mutualist‐dependent reversal of the competitive hierarchy can drive community dynamics in a plant–aphid system.     

Positive interactions between alpine plant species and the nurse cushion plant Laretia acaulis do not increase with elevation in the Andes of central Chile

In alpine habitats, positive interactions among plants tend to increase with elevation as a result of altitudinal increase in environmental harshness. However, in mountains located in arid zones, lower elevations are also stressful because of scarce availability of water, suggesting that positive interactions may not necessarily increase with elevation. Here we analysed the spatial association of plant species with the nurse cushion plant Laretia acaulis at two contrasting elevations, and monitored the survival of seedlings of two species experimentally planted within and outside cushions in the semiarid Andes of central Chile. Positive spatial associations with cushions were more frequent at lower elevations. Species growing at the two elevations changed the nature of their association with cushions from neutral or negative at higher elevations to positive at lower elevations. Survival of seedlings was higher within cushions, particularly at lower elevations. The increased facilitation by cushions at lower elevations seems to be related to provision of moisture. This result suggests that cushion plants play a critical role in structuring alpine plant communities at lower elevations, and that climatic changes in rainfall could be very relevant for persistence of plant communities.     

豚草和三裂叶豚草不同植株部位种子萌发与入侵扩散关系

豚草(Ambrosia artemisiifoliaLinn.)和三裂叶豚草(A. trifidaLinn.)是我国恶性入侵物种,分布常呈高密度单一种群,且结实量庞大。探讨二者不同植株部位种子萌发(休眠)与扩散特点,对了解二者入侵机制具有重要意义。以新疆伊犁新源县发生的豚草和三裂叶豚草为材料,在种子成熟期,根据植株高度、枝条长度,按比例从上到下分为9个部位,对不同植株部位种子的形态特征、数量和萌发特性进行比较,分析这两种植物不同植株部位种子萌发与扩散的共性和差异性,研究二者种群密度调节和入侵的关系。结果表明:1)两个物种内不同植株部位间种子的长、宽、百粒重无显著差异,但三裂叶豚草种子的长度和宽度分别是豚草的2—3倍,百粒重高7倍。结合两个物种在伊犁地区分布差异,认为种子大小是两个物种分布区域性差异的原因之一。2)豚草和三裂叶豚草植株外部的上顶、中顶、上中部位种子数占植株总种子数量的50%,中中、下顶占比约23%,而下部的上基、中基、下中、下基的种子数占比约27%,表明当年生产的种子有近73%的比例具有远距离扩散的潜力。3)豚草和三裂叶豚草不同植株部位种子的萌发率具有上端中端下端的趋势;初始萌发时间为下端中端上端;萌发持续时间为上端中端下端。这种萌发方式避免了同一生长季大批种子同时萌发有可能导致高密度死亡的风险。基于上述研究分析,认为豚草和三裂叶豚草不同植株部位种子具有不同的适应功能。其中,上部所产生的种子具较强的扩散能力和低休眠性,有利于两物种快速占据新生境并扩大种群;而中、下部位的种子在母株周围就近扩散,翌年萌发率低,缓解了种群竞争。豚草和三裂叶豚草不同植株部位生产的种子特性和萌发差异是两个物种进行种群密度调节和扩散入侵的重要原因。