Diaspore characteristics and ecological adaptation of Bromus tectorum L. from different distribution regions

The invasion of nonnative plants is considered one of the main threats to the structure and function of North American ecosystems.Moreover,they can alter ecosystem processes and reduce biodiversity.In arid and semi-arid region of North America,the species of European annual grass Bromus tectorum L.is an outstanding example of these problems,which not only increase the fire density and change the fire regime,but replace native communities.Therefore,there are amount of researches on B.tectorum,including resource acquisition,water use efficiency and growth.Whereas the relevant research on the morphology of diaspore is scare.Diaspores have a fundamental role in seed germination and seedling establishment.Besides,as an important link between different generations,diaspores have a vital significance on individual reproduction and population extension.Hence,diaspores under selection for studying have an important implication.This study compares differences in seed morphology for Bromus tectorum collected from the United States,Kazakhstan,and Xinjiang of China.The following indices of B.tectorum diaspores were analyzed:size,thickness of covering layers,and micromorphological characteristics of the base,middle and transition area of diaspores as well as of the awn.Micromorphology of the lemma and the cross-section of the diaspore were observed by scanning electron microscopy.Results showed that thickness of the lemma and the palea of diaspores from B.tectorum-infested grasslands in the United States were reduced(P<0.05),likely because of environmental influences.This reduction facilitated the germination of diaspores and lowered the resistance of B.tectorum to adverse environmental conditions.The length of the awn also increased significantly(P<0.05),which helped in dispersal and anchoring of diaspores.Therefore,B.tectorum adapted ecologically to its new environment in the United States by strengthening its establishment ability.However,the defense capability of B.tectorum decreased.These results fit the evolution of increased competitive ability hypothesis(EICA)of invasive species.Analysis of various cells on the lemma revealed that prickle densities and collapsed,long epidermal cells were easily influenced by environmental factors such as temperature and moisture because of the physiologic function of these structures on silicon accumulation.However,the form and the position of silica cells,which were not greatly influenced by environmental factors,might be genetically controlled.Studying these structures at the microscopic level helps define the relationship between the diaspore and its environment.This study has a reference value for future studies on B.tectorum.     

Invasion syndromes：hypotheses on relationships among invasive species attributes and characteristics of invaded sites

As invasion science accepts that there is no single causal factor for biological invasion,the identification of groups of traits that are often associated,or "syndromes",is a logical move forward.Invasion syndromes are proposed to identify suites of site conditions(biotic and environmental)that render a site vulnerable to invasion by different types of invaders.This paper proposed four invasion syndromes which relate invader attributes(competitive ability,niche construction,phenotypic plasticity,and phenological niche separation)to the biotic characteristics(biodiversity and enemies)and environmental conditions(resource abundance and fluctuation)of invaded sites.The four invasion syndromes described in this paper are a development of hypotheses of how the many factors that influence species invasion might be associated.Invasion Syndrome 1 proposes that sites with relatively high resource abundance and high diversity should be vulnerable to invasion by species with high competitive ability.Invasion Syndrome 2 hypothesizes that sites with relatively low resource abundance and low diversity should be vulnerable to invasion by species with niche construction ability.Invasion Syndrome 3 postulates that sites with moderate or fluctuating resources and moderate diversity should be vulnerable to invasion by species with high phenotypic plasticity.Invasion Syndrome 4 hypothesizes that species introduced into a site where it has phenological niche separation from natives will not have to contend with interference from the biotic community at a site(diversity or natural enemies)and may invade where ever site environmental conditions suit its life history.Further work is needed to support,contradict,or refine these hypotheses and almost certainly will identify more invasion syndromes.     

Explaining patterns of species dominance in the shrub steppe systems of the Junggar Basin （China） and Great Basin （USA）

Natural scientists have long recognized that regions with similar climate tend to have similar vegetation.Preliminary observations suggest that shrub steppe communities of the western US and western China may be two such regions with similar annual precipitation,temperature,land use,and vegetation.These cold dry shrub steppes have traditionally been grazed.Despite these similarities,patterns of species dominance are different.Annual species that are rare in China become dominant when introduced to the United States.The objective of this study was to investigate how climate,land use and community structure may explain these patterns of species dominance.Community structure and grazing intensity were measured at 5 sites in each region.This information was combined with a broader review of the literature describing the history of grazing in both basins.Climate was analyzed based on a spatially-gridded,interpolated weather time series(monthly records) and climatological summary(1961-1990 mean conditions) data set from the Climate Research Unit.We found that differences in summer precipitation and winter minimum temperature,land use intensity,and shrub size may all contribute to the dominance of annual species in the Great Basin,particularly Bromus tectorum.In particular,previous work indicates that summer precipitation and winter temperature drive the distribution of Bromus tectorum in the Great Basin.As a result,sites with wet summers and cold springs,similar to the Chinese sites,would not be expected to be dominated by Bromus tectorum.A history of more intense grazing of the Chinese sites,as described in the literature,also is likely to decrease fire frequency,and decreases litter and shrub dominance,all of which have been demonstrated to be important in Bromus tectorum establishment and ultimate dominance.Further research is necessary to determine if other annuals that follow the same pattern of scarcity in the Junggar Basin and dominance in the Great Basin are responding to the same influences.     

Assessing forest understory biomass in northwest Florida (USA) and its potential to meet the state energy needs

A stratified random sampling approach was employed to quantify total biomass across prevalent non-commercial forest understory species found in six counties of northwest Florida, USA. The moisture content (wet basis) and calorific values of these species were also measured. Total green biomass from forest understory species was estimated to be around 12 million metric tons, mostly comprised of Cliftonia monophylla (titi, buckwheat tree) and Cyrilla racemiflora (white titi, swamp titi). This understory forest biomass would be sufficient to generate about 28.8 million GJ of electricity or 1589.25 million liters of ethanol. A need was identified to determine the inventory of forest understory biomass at the state level and assess the overall sustainability of utilizing forest understory biomass for bioenergy.     

Economic feasibility of and vegetation responses to biomass harvest in managed loblolly pine plantations

We evaluated two biomass harvest methods, (1) Whole Tree Thinning (WTT; third-row thinning), and (2) Whole Tree Thinning with Fuelchips (WTTF; third-row thinning plus remove all accessible hardwood stems >2.5 cm diameter at breast height and understory shrubs in thinned area of stand) in a 21-year-old loblolly pine (Pinus taeda L.) plantation in northwest Florida for their harvesting yields and productivities, costs, and effects on groundcover. Both WTT and WTTF produced similar quantities of roundwood (~70 Mg ha^?1) and chips (8.43–13.12 Mg ha^?1) without significantly added operational time (15.77–28.12 Mg h^?1). On-board costs of chip production (US$6.93 to 10.60 Mg^?1) and total cost of roundwood and chip production (~22.5 US$ Mg^?1) of the two harvest methods also did not differ significantly. Following either WTT or WTTF, overall percent groundcover recovered within 6 months. While shrub and grass cover were similar, forb cover in WTT increased significantly following six months of harvests. Importantly, the study suggests that biomass removal is an attractive option that could be integrated with traditional silvicultural thinning methods to manage vegetation and alleviate hazardous fuel and wildfire conditions, leading to improved forest health.