Annual plants in arid and semi-arid desert regions

Annual plants are the main vegetation in arid and semi-arid desert regions.Because of their unique traits,they are the optimal experimental subjects for eco-logical studies.In this article,we summarize annual plants' seed germination strategies,seedling adaptability mechanism to environments,seed dispersal,and soil seed banks.We also discuss the biotic and abiotic factors affecting the composition and dynamics of annual plant populations and communities.Because annual plants have important ecological functions in desert vegetation systems,this study on annual plants will be of great bene-fit to the conservation and restoration of desert ecosys-tems,the rational utilization of resources,and the sustainable development of desert regions.     

干旱半干旱荒漠地区一年生植物研究综述

1年生植物是干旱半干旱地区植被的重要组成部分,并以其独有的植物特性成为许多植物生态学关键问题研究的最佳试验材料。本文综述了干旱半干旱地区1年生植物的种子萌发对策、幼苗对环境的适应机制、种子传播和种子库等方面的研究成果,并探讨了影响1年生植物种群、群落组成和动态的生物与非生物因素。基于1年生植物类群在荒漠植被系统中重要的生态功能,有关1年生植物的研究成果将有利于荒漠生态系统的保护和恢复、资源的合理利用以及区域的可持续发展。     

几种沙漠地区野生药用植物资源及其引种栽培

几种沙漠地区野生药用植物资源及其引种栽培尹林克（中国科学院吐鲁番沙漠植物园,新疆吐鲁番８３８００８）Ｒｅｓｏｕｒｃｅｓｏｆｓｅｖｅｒａｌｓｐｅｃｉｅｓｏｆｗｉｌｄｍｅｄｉｃｉｎａｌｐｌａｎｔｓｉｎｄｅｓｅｒｔｒｅｇｉｏｎａｎｄｔｈｅｉｒｉｎｔｒｏｄｕ...     

Dormancy cycling and persistence of seeds in soil of a cold desert halophyte shrub

Background and Aims

Formation of seed banks and dormancy cycling are well known in annual species, but not in woody species. In this study it was hypothesized that the long-lived halophytic cold desert shrub Kalidium gracile has a seed bank and dormancy cycling, which help restrict germination to a favourable time for seedling survival.

Methods

Fresh seeds were buried in November 2009 and exhumed and tested for germination monthly from May 2010 to December 2011 over a range of temperatures and salinities. Germination recovery and viability were determined after exposure to salinity and water stress. Seedling emergence and dynamics of the soil seed bank were investigated in the field.

Key Results

Seeds of K. gracile had a soil seed bank of 7030 seeds m⁻² at the beginning of the growing season. About 72 % of the seeds were depleted from the soil seed bank during a growing season, and only 1·4 % of them gave rise to seedlings that germinated early enough to reach a stage of growth at which they could survive to overwinter. About 28 % of the seeds became part of a persistent soil seed bank. Buried seeds exhibited an annual non-dormancy/conditional dormancy (ND/CD) cycle, and germination varied in sensitivity to salinity during the cycle. Dormancy cycling is coordinated with seasonal environmental conditions in such a way that the seeds germinate in summer, when there is sufficient precipitation for seedling establishment.

Conclusions

Kalidium gracile has three life history traits that help ensure persistence at a site: a polycarpic perennial life cycle, a persistent seed bank and dormancy cycling. The annual ND/CD cycle in seeds of K. gracile contributes to seedling establishment of this species in the unpredictable desert environment and to maintenance of a persistent soil seed bank. This is the first report of a seed dormancy cycle in a cold desert shrub.     

Diversity and Coexistence of Sonoran Desert Winter Annuals

Abstract Annual plants make up ca. 50% of local floras in the Sonoran Desert. As with most plant communities, there is no shortage of potential coexistence generating mechanisms, and several mechanisms are likely contributors to coexistence at different spatial scales in the Sonoran Desert, e.g. spatial heterogeneity and the behaviors of predators and grazers. We explore one mechanism of likely importance for desert annuals: temporal environmental variation. It is widely recognized that coexistence is promoted by temporal variation if species such as desert annuals have "temporal niches" in the sense that each has years in which it out-performs the others. It is usually suggested that some resistent life-history stage, such as a seed bank, is also necessary to buffer each species from the negative population dynamic impact of unfavorable years. Using ten years of demographic data, we document the large year-to-year variation in population dynamics of desert annuals and show that ten species respond differently to temporal variation. Competition experiments document reversals in competitive superiority. Also, all species have a between-year seed bank, such that only a proportion of the seed bank germinates in any given year. Thus this system meets our intuitive requirements for variance-based coexistence. Dynamic models of this system demonstrate that subtle aspects of the species biology determine whether coexistence criteria are actually met. Specifically, variable germination fractions are required and coexistence is most readily favored with "predictive" germination. Germination fractions in this system do vary among years in a species specific fashion. Also, for the three years of available data, germination was predictive, in that each species had greater germination fractions in year of greater demographic success. Thus all of the population dynamic elements necessary for temporal variance mediated coexistence seem to be present in this system.     

Climate warming could increase recruitment success in glacier foreland plants

Background and Aims Glacier foreland plants are highly threatened by global warming. Regeneration from seeds on deglaciated terrain will be crucial for successful migration and survival of these species, and hence a better understanding of the impacts of climate change on seedling recruitment is urgently needed to predict future plant persistence in these environments. This study presents the first field evidence of the impact of climate change on recruitment success of glacier foreland plants.Methods Seeds of eight foreland species were sown on a foreland site at 2500 m a.s.l., and at a site 400 m lower in altitude to simulate a 2·7 °C increase in mean annual temperature. Soil from the site of origin was used to reproduce the natural germination substrate. Recruitment success, temperature and water potential were monitored for 2 years. The response of seed germination to warming was further investigated in the laboratory.Key Results At the glacier foreland site, seedling emergence was low (0 to approx. 40 %) and occurred in summer in all species after seeds had experienced autumn and winter seasons. However, at the warmer site there was a shift from summer to autumn emergence in two species and a significant increase of summer emergence (13–35 % higher) in all species except two. Survival and establishment was possible for 60–75 % of autumn-emerged seedlings and was generally greater under warmer conditions. Early snowmelt in spring caused the main ecological factors enhancing the recruitment success.Conclusions The results suggest that warming will influence the recruitment of glacier foreland species primarily via the extension of the snow-free period in spring, which increases seedling establishment and results in a greater resistance to summer drought and winter extremes. The changes in recruitment success observed here imply that range shifts or changes in abundance are possible in a future warmer climate, but overall success may be dependent on interactions with shifts in other components of the plant community.     

Different ecological strategies during the seed germination phase of nine grasses inhabiting the hyper‐arid Arabian desert

The effect of photoperiod, thermoperiod and their interaction on seed germination and its rate was studied for nine grasses growing in the Arabian desert. Germination tests were conducted under two photoperiods and three thermoperiods. Germination of light‐incubated seeds of Hyparrhenia hirta, Pennisetum divisum, Stipacapensis, Centropodia forsskaolii, Stipagrostis plumosa, Cymbopogon parkeri and Panicum turgidum was significantly higher than that detected in the darkness. Photoperiod did not influence the seed germination of Aristida adscensionis and Imperata cylindrica. Seeds of all species, except P. divisum, C. parkeri and I. cylindrica, germinated indepen dently of the tested temperature. Our results indicate that species belonging to the same family and growing in the same hyper‐arid desert habitat may influence and act in different ways during the seed germination phase, causing species‐specific effects, and therefore might affect the ecology of each taxon in this harsh environment.     

Fruit and seed heteromorphism in the cold desert annual ephemeral Diptychocarpus strictus (Brassicaceae) and possible adaptive significance

Background and Aims

Diptychocarpus strictus is an annual ephemeral in the cold desert of northwest China that produces heteromorphic fruits and seeds. The primary aims of this study were to characterize the morphology and anatomy of fruits and seeds of this species and compare the role of fruit and seed hetermorphism in dispersal and germination.

Methods

Shape, size, mass and dispersal of siliques and seeds and the thickness of the mucilage layer on seeds were measured, and the anatomy of siliques and seeds, the role of seed mucilage in water absorption/dehydration, germination and adherence of seeds to soil particles, the role of pericarp of lower siliques in seed dormancy and seed after-ripening and germination phenology were studied using standard procedures.

Key Results

Plants produce dehiscent upper siliques with a thin pericarp containing seeds with large wings and a thick mucilage layer and indehiscent lower siliques with a thick pericarp containing nearly wingless seeds with a thin mucilage layer. The dispersal ability of seeds from the upper siliques was much greater than that of intact lower siliques. Mucilage increased the amount of water absorbed by seeds and decreased the rate of dehydration. Seeds with a thick mucilage layer adhered to soil particles much better than those with a thin mucilage layer or those from which mucilage had been removed. Fresh seeds were physiologically dormant and after-ripened during summer. Non-dormant seeds germinated to high percentages in light and in darkness. Germination of seeds from upper siliques is delayed until spring primarily by drought in summer and autumn, whereas the thick, indehiscent pericarp prevents germination for >1 year of seeds retained in lower siliques.

Conclusions

The life cycle of D. strictus is morphologically and physiologically adapted to the cold desert environment in time and space via a combination of characters associated with fruit and seed heteromorphism.     

Trade-offs between seed dispersal and dormancy in an amphi-basicarpic cold desert annual

Background and Aims

Several studies have demonstrated trade-offs between depth of seed dormancy and dispersal ability for diaspore-dimorphic species. However, relatively little is known about trade-offs between these two life history traits for a species that produces more than two diaspore morphs. The aim of this study was to investigate the relationship between seed dormancy and dispersal in Ceratocarpus arenarius, an amphi-basicarpic cold desert annual that produces a continuum of dispersal unit morphs.

Methods

A comparison was made of dispersal and dormancy breaking/germination responses of dispersal units from ground level (a), the middle of the plant canopy (c) and the top of the plant canopy (f). Various features of the morphology and mass of dispersal units and fruits (utricles) were measured. The role of bracteoles in diaspore dispersal by wind, settlement onto the soil surface and dormancy/germination was determined by comparing responses of intact dispersal units and fruits. Movement of dispersal units by wind and animals, seed after-ripening, germination phenology and the presence of water-soluble germination inhibitors in bracteoles were tested using standard procedures.

Key Results

Dispersal units a, c and f differed in morphology and mass; in the majority of cases, extremes were exhibited by a and f, with c being intermediate. Overall, relative dispersal ability was f > c > a, whereas relative intensity of dormancy was a > c > f. Bracteoles increased dispersal distance by wind, enhanced settlement of diaspores onto the soil surface and mechanically inhibited germination.

Conclusions

The results provide evidence for a model in which there is a continuous inverse-linear relationship between diaspore dispersal ability and depth of dormancy. Thus, dispersal unit heteromorphism of C. arenarius results in a continuum, from no dispersal ability/high dormancy (dispersal unit a) to high dispersal ability/low dormancy (unit f), which may be a bet-hedging strategy in the cold desert environment.     

Leaf temperature of desert sand dune plants: perspectives on the adaptability of leaf morphology

The leaf temperature of six annual and six perennial plant species was monitored during spring and summer on a sand dune ecosystem in the delta Mediterranean coast of Egypt. During winter, leaves of all tested perennial species attained temperatures higher than the air temperature at night and shortly after sunrise, with maximum leaf–air temperature differences reaching up to 8°C. The lowest differences were less than 1°C. Around noon, the leaves of several species attained temperatures lower than that of the air whereas others showed higher temperatures. The opposite was true during summer, when leaf temperatures were lower than air temperature. The maximum leaf–air temperature differences occurred after midnight towards sunrise and reached up to 10°C. The lowest differences were found around noon and were of less than 5°C. The annual plant species have more pronounced variations than perennials in their leaf temperatures during the night and for most of the day. The leaves were heated or cooled a few degrees above or below the air temperature. The results are discussed in relation to the morphological characters of the leaves. The variation in leaf temperature at different times of the day was significantly related to leaf morphology, specific leaf area, thickness, volume, leaf area index and the surrounding environment.     

Seed dispersal by ants in the semi-arid Caatinga of North-East Brazil

BACKGROUND AND AIMS: Myrmecochory is a conspicuous feature of several sclerophyll ecosystems around the world but it has received little attention in the semi-arid areas of South America. This study addresses the importance of seed dispersal by ants in a 2500-km(2) area of the Caatinga ecosystem (north-east Brazil) and investigates ant-derived benefits to the plant through myrmecochory. METHODS: Seed manipulation and dispersal by ants was investigated during a 3-year period in the Xingó region. Both plant and ant assemblages involved in seed dispersal were described and ant behaviour was characterized. True myrmecochorous seeds of seven Euphorbiaceae species (i.e. elaiosome-bearing seeds) were used in experiments designed to: (1) quantify the rates of seed cleaning/removal and the influence of both seed size and elaiosome presence on seed removal; (2) identify the fate of seeds dispersed by ants; and (3) document the benefits of seed dispersal by ants in terms of seed germination and seedling growth. KEY RESULTS: Seed dispersal by ants involved one-quarter of the woody flora inhabiting the Xingó region, but true myrmecochory was restricted to 12.8 % of the woody plant species. Myrmecochorous seeds manipulated by ants faced high levels of seed removal (38-84 %) and 83 % of removed seeds were discarded on ant nests. Moreover, seed removal positively correlated with the presence of elaiosome, and elaiosome removal increased germination success by at least 30 %. Finally, some Euphorbiaceae species presented both increased germination and seedling growth on ant-nest soils. CONCLUSIONS: Myrmecochory is a relevant seed dispersal mode in the Caatinga ecosystem, and is particularly frequent among Euphorbiaceae trees and shrubs. The fact that seeds reach micro-sites suitable for establishment (ant nests) supports the directed dispersal hypothesis as a possible force favouring myrmecochory in this ecosystem. Ecosystems with a high frequency of myrmecochorous plants appear not to be restricted to regions of nutrient-impoverished soil or to fire-prone regions.     

香果树(Emmenopterys henryi)种群种子雨、种子库及 实生苗数量的海拔梯度变化

香果树为我国Ⅱ级野生保护植物,由于原生境中香果树种群的数量迅速减少,目前已濒临灭绝,种群亟待恢复。研究了武夷山不同海拔的香果树种群种子雨、种子库及其幼苗存活现状,目的是阐明不同海拔地区的香果树种群自然更新的动态过程,寻找其更新脆弱的环节及其影响因素,为香果树自然种群的恢复提供理论依据。以分布于不同海拔高度（819、980、1 140及1 301 m）的香果树母树为中心,在其周围（东、南、西、北四个方位）布设种子雨收集框和土壤种子库样方,连续观测种子雨和种子库的动态,并对不同地表覆盖物中种子萌发及幼苗数量进行观测和记录。结果表明,武夷山香果树种群种子雨持续时间近2个月,尤其是11月底至12月中旬为种子雨高峰期;随着海拔的上升,香果树种子总密度以及虫蛀种子密度均显著降低,但千粒重以及饱满种子密度则有所增加;香果树土壤种子库为瞬时种子库,其水平分布以母树南侧和西侧种子密度较大,母树北侧种子密度最小。香果树种子的垂直分布主要集中于枯落物和苔藓层;3月时香果树土壤种子库中存留的饱满种子仅占种子散布后1.80%,而1年实生苗存活率为6.18%,土壤种子库的损耗是其种群自然更新困难的主要原因;不同地表覆盖物对香果树幼苗的存活产生显著影响,枯落物及苔藓层的幼苗死亡率显著高于土壤表面。建议清理林内枯落物及苔藓,增加林内裸露土壤的面积,减小地上草本的盖度,以减少种子霉烂、虫蛀的几率,增加香果树实生苗胚根与土壤接触的几率以及改善光照条件,促进香果树自然更新。     

内蒙古荒漠草原亚洲小车蝗的种群动态

2010—2011年于内蒙古乌兰察布市四子王旗格根塔拉草原对亚洲小车蝗Oedaleus asiaticus(B.-Bienko)种群动态进行了研究。结果表明,亚洲小车蝗种群空间格局为随机分布;亚洲小车蝗于6月中旬开始孵化出土,1～3龄蝗蝻高峰期在6月中下旬至7月初,终见期在7月下旬;4～5龄蝗蝻于6月下旬始见,高峰期在7月上中旬,终见期在7月末;成虫于7月上旬始见,高峰期在7月中旬至8月下旬,终见期在9月上旬。应用最优分割法将亚洲小车蝗种群动态划分为3个阶段:(1)6月中旬,为蝗蝻开始出土期,数量稀少,空间格局为聚集分布或随机分布;(2)6月下旬至7月上中旬,为蝗蝻发生盛期,密度低时为随机分布,密度高时为聚集分布;(3)7月中下旬至9月上旬,为成虫发生期,空间格局为随机分布。     

生物结皮对5种不同形态的荒漠植物种子萌发的影响

生物结皮广泛分布于干旱、半干旱区, 强烈影响着土壤表层理化特性, 进而对种子散布、萌发和定居产生影响。目前关于生物结皮与植物种子萌发关系的研究结论存在争议。该文通过室内人工控制实验, 研究了生物结皮对古尔班通古特沙漠5种具不同种子形态特征的荒漠植物白梭梭(Haloxylon persicum)、蛇麻黄(Ephedra distachya)、角果藜(Ceratocarpus arenaarius)、涩芥(Malcolmia africana)和狭果鹤虱(Lappula semiglabra)的种子萌发的影响。结果表明, 在干燥和湿润两种条件下, 生物结皮对不同形态植物种子萌发均具有不同的作用。在干燥条件下, 生物结皮显著抑制了角果藜和涩芥种子的萌发(p<0.05), 对其它3种植物无显著影响; 而湿润条件下, 生物结皮显著抑制了白梭梭、角果藜和狭果鹤虱种子的萌发(p<0.05), 对蛇麻黄、涩芥则无显著影响。