Effects of Competition and Temporal Variation on the Evolutionary Potential of Two Native Bunchgrass Species

The capacity of restored plant populations to adapt to new environmental challenges depends on within‐population genetic variation. We examined how much genetic and environmentally based variation for fitness‐associated traits exists within populations of two native grasses commonly used for restoration in California. We were also interested in understanding how phenotypic expression of genetic variation for these traits varies with growth environment. Thirty maternal families of Elymus glaucus (Blue wild rye) and Nassella pulchra (Purple needlegrass) were sampled from both coastal and interior populations and reciprocally transplanted into three replicated common gardens with and without interspecific competition at each site. Reproductive output of families differed both among years and with competition treatments. Phenotypic expression of genetic variation in culm production differed among populations and was very low when families were grown with interspecific competition. Without interspecific competition, the degree of genetic determination peaked in year two in both species (8.4 and 15.1% in E. glaucus and N. pulchra, respectively). Significant genetic differences in reproduction and phenotypic plasticity were found among N. pulchra subpopulations sampled less than 3 km apart, further highlighting the importance of thoroughly sampling available genetic variation in populations used for restoration. The variable and generally low expression of genetic variation indicates that rates of adaptation in restored populations of these native grasses may vary temporally and may be especially slow within competitive environments.     

Local Adaptation and Effects of Grazing among Seedlings of Two Native California Bunchgrass Species: Implications for Restoration

Adaptation to environmental factors may influence the germination and establishment of focal species in ecological restoration. Reciprocal transplants remain one of the best methods to detect local adaptation, but long‐term studies are often not feasible. We conducted reciprocal transplants of the native California bunchgrasses Elymus glaucus and Bromus carinatus between two central California locations to seek evidence of adaptation to local environmental conditions in a single growing season. Experimental plots at one location included grazed and ungrazed sites. The combination of locations and grazing treatments allowed us to determine whether the ability to detect evidence for adaptation depended on grazing regime. In addition, we measured the direct effects of grazing on seedling growth and survival concurrent with our investigation of local adaptation. We detected a homesite advantage for seedling growth or survival in both species, but the factors contributing to adaptive differentiation were species specific. Evidence of local adaptation was detected for seedling biomass in Bromus and for survivorship in Elymus. The homesite advantage observed in both species was greatly reduced under grazed conditions and in Elymus was significant only in the ungrazed plots. Climate and soil analyses detected significant differences between locations in five soil attributes and two climate variables. In particular, differences in exchangeable magnesium indicated that one of the two transplant locations consisted of serpentine soil, which is widely known to drive adaptation in plant populations. Together, these results suggest that it is possible to investigate the scale and factors involved in local adaptation with short‐term transplant studies.     

Intra and interspecific competition among invasive and native species during early stages of plant growth

Plant competition is a primary ecological process limiting grassland restoration success. Appropriate restoration techniques require an understanding of the degree to which intra and interspecific competition control invasive and native plant growth. The objective of this study was to determine how the intensity of intra and interspecific competition changes during early stages of plant growth. Two invasive (Bromus tectorum and Taeniatherum caput-medusae) and two native (Pseudoroegneria spicata and Poa secunda) species were grown in a diallel competition experiment, either alone or in 1:1 binary combinations and exposed to two levels of N (no N or 400 mg N kg⁻¹ soil added) in a greenhouse. Total biomass for each species was quantified over four harvests and competitive effects were calculated. Our results show that the relative magnitude of intra and interspecific competition changes through time. Intraspecific competition was intense for native species at the initial harvests and therefore important in contributing to the outcome of final size of native species seedlings. Interestingly, bluebunch wheatgrass imposed interspecific competition on annual grasses at the first two harvests and appeared to be a better competitor than Sandberg’s bluegrass. We found that fast growing invasive species became more competitive compared to slow growing native species with increasing N and appear to establish a positive feedback mechanism between size and resource uptake. Opportunities to improve restoration success exist from determining the optimum combination of density, species proportion, and their spatial arrangement in various ecosystems and environments.     

Local Adaptation and the Effects of Grazing on the Performance of Nassella pulchra: Implications for Seed Sourcing in Restoration

The use of local seed sources for revegetation is accepted practice to reduce the potential that propagules will be poorly adapted to site conditions. However, data are often lacking to determine the distance within which seed sources represent local genotypes. Short‐term reciprocal transplant studies represent a class of tools to detect local adaptation of target species. We conducted a reciprocal transplant of Nassella pulchra between two central California locations to test for adaptation to local environmental conditions over a 3‐year period. Experimental plots at one location were split between grazed and ungrazed sites to evaluate the potential influence of livestock grazing on the detection or magnitude of local adaptation. During each year of the study, evidence of a home‐site advantage depended on the location, traits studied, and population. At the end of the 3‐year study period, however, we detected consistent evidence of a home‐site advantage for seedling biomass among grazed sites at one location and ungrazed plots at the other location. In effect, local adaptation was only apparent in the final year of the study. Short‐term reciprocal transplant studies are an effective tool to guide the selection of seed sources most likely to germinate and to become established at a restoration site, but such studies cannot rule out local adaptation, which may not be immediately detectable.     

Role of biotic interactions and physical factors in determining the distribution of marsh species along an estuarine salinity gradient

Understanding the mechanisms that shape plant distribution patterns is a major goal in ecology. We investigated the role of biotic interactions (competition and facilitation) and abiotic factors in creating horizontal plant zonation along salinity gradients in the Elbe estuary. We conducted reciprocal transplant experiments with four dominant species from salt and tidal freshwater marshes at two tidal elevations. Ten individuals of each species were transplanted as sods to the opposing marsh type and within their native marsh (two sites each). Transplants were placed at the centre of 9‐m² plots along a line parallel to the river bank. In order to disentangle abiotic and biotic influences, we set up plots with and without neighbouring vegetation, resulting in five replicates per site. Freshwater species (Bolboschoenus maritimus and Phragmites australis) transplanted to salt marshes performed poorly regardless of whether neighbouring vegetation was present or not, although 50–70% of the transplants did survive. Growth of Phragmites transplants was impaired also by competition in freshwater marshes. Salt marsh species (Spartina anglica and Puccinellia maritima) had extremely low biomass when transplanted to freshwater marshes and 80–100% died in the presence of neighbours. Without neighbours, biomass of salt marsh species in freshwater marshes was similar to or higher than that in salt marshes. Our results indicate that salt marsh species are precluded from freshwater marshes by competition, whereas freshwater species are excluded from salt marshes by physical stress. Thus, our study provides the first experimental evidence from a European estuary for the general theory that species boundaries along environmental gradients are determined by physical factors towards the harsh end and by competitive ability towards the benign end of the gradient. We generally found no significant impact of competition in salt marshes, indicating a shift in the importance of competition along the estuarine gradient.     

Assessing the effect of genetic diversity on the early establishment of the threatened seagrass Posidonia australis using a reciprocal‐transplant experiment

Two common goals for restoration are rapid plant establishment and long‐term plant persistence. The success of transplanted populations may be jeopardized if the donor transplants are not genetically diverse, and/or poorly matched to their new environment. Here, we test the effects of local adaptation and plot‐level genetic diversity on the early establishment phase of a threatened seagrass species, Posidonia australis, by performing a reciprocal transplant experiment across two genetically and geographically distinct populations in southeastern Australia. Posidonia australis is a long‐lived, slow‐growing species that has no seed bank, and the successful transplantation of live shoots and seedlings is the only available restoration method. Our results show a strong effect of local adaptation and genetic diversity on P. australis survivorship and performance over the first 6 months following transplantation. High‐genetic diversity plots displayed higher survival rates and exhibited reduced productivity and increased carbohydrate reserves within the rhizome. This suggests that high‐diversity plots included shoots that were conserving energy stores by actively reducing growth rates during the early stages of transplantation. The lowest diversity plots exhibited high leaf and root productivity and corresponding low carbohydrate reserves. This may be a sign of stress in the low‐diversity transplants, potentially explaining the very low survival rate. We suggest that future restoration efforts source donor transplants from multiple local sources to ensure both local adaptation and sufficient genetic diversity to increase the likelihood of early establishment success.     

Fragmentation,vegetation change and irruptive competitors affect recruitment of woodland birds

Climate change may amplify the adverse effects of fragmentation by also affecting interspecific interactions. Increased competition may reduce the ability of already stressed species to acquire resources (breeding sites and food), reducing recruitment and the long‐term viability of species. We assessed how measures of recruitment of native birds were influenced by the area of native vegetation, vegetation characteristics, vegetation change as an indication of degradation, and the occurrence of an increasingly prevalent native competitor (the noisy miner Manorina melanocephala). We recorded avian breeding behavior on 120 forest transects in the box‐ironbark forests of south‐eastern Australia, in 2010–2011. On the same transects, we measured vegetation characteristics that had previously been measured in 1995–1997 to assess vegetation change during a 13‐yr drought. Vegetation area and the abundance of the noisy miner had a greater effect on species’ breeding behavior than did local vegetation characteristics and vegetation degradation. Greater abundances of the noisy miner reduced breeding activities of species with a body mass smaller than the noisy miner (< 63 g), while breeding increased in some larger (> 63 g) species. Recruitment measures for the noisy miner were positively associated with smaller fragments and greater vegetation change indicating that fragmentation and vegetation degradation have facilitated the colonization or recruitment by the noisy miner. The interaction between climate change, fragmentation and vegetation degradation appears to have led to increased effects of interspecific competition in fragments of native vegetation, with potential adverse effects on the viability of many bird species. The spread and increasing abundance of a hyperaggressive native species suggests that species assemblages will be increasingly disrupted by the interacting effects of climate change, fragmentation, degradation and interspecific interactions.     

Test of local adaptation to biotic interactions and soil abiotic conditions in the ant-tended <Emphasis Type="Italic">Chamaecrista fasciculata</Emphasis> (Fabaceae)

Few previous studies have assessed the role of herbivores and the third trophic level in the evolution of local adaptation in plants. The overall objectives of this study were to determine (1) whether local adaptation is present in the ant-defended plant, Chamaecrista fasciculata, and (2) the contribution of ant-plant-herbivore interactions and soil source to such adaptation. We used three C. fasciculata populations and performed both a field and a greenhouse experiment. The first involved reciprocally transplanting C. fasciculata seedlings from each population-source to each site, and subsequently applying one of three treatments to one-third of the seedlings of each population-source at each site: control, reduced ant density and reduced folivory. The greenhouse experiment involved reciprocal transplants of population-sources with soil sources to test for a soil-source effect on flower production and local adaptation to soil conditions. Field results showed that ant and herbivore treatments reduced ant density (increasing folivory) and herbivore damage relative to controls, respectively; however, these manipulations did not impact C. fasciculata reproduction or the likelihood of survival. In contrast, greenhouse results showed that soil source significantly affected flower production. Overall, plants in both experiments, regardless of population-source, always had higher reproductive output at one specific site. Native populations did not outperform nonnative ones, causing us to reject the hypothesis of local adaptation. The absence of treatment effects on plant reproduction and the likelihood of survival suggest a limited effect of ants and folivores on C. fasciculata fitness and local adaptation during the study year. Temporally inconsistent effects of biotic forces across years, coupled with the young age of populations, relative proximity of populations and possible counter effects of seed predators may reduce the likelihood of local adaptation in the populations studied.     

A California grasslands alkali specialist,Hemizonia pungens ssp. pungens,prefers non‐alkali soil

Question: Edaphically severe habitats commonly support edaphic endemics, specialized plant species that do not occur elsewhere. The endemism of native plant species in edaphically specialized habitats suggests either (a) that these native endemic species are uniquely specialized to survive and grow better under the conditions prevalent in these harsh areas, or (b) that these areas represent refuges from competition with other (often exotic) species. Location: Central Valley, California, USA. Methods: We surveyed the vegetation distribution in alkali sinks and carried out a reciprocal transplant greenhouse experiment crossed with a competition treatment to examine the interplay between interspecific competition and edaphic factors in determining relative performance of an alkali endemic forb (Hemizonia pungens ssp. pungens) and its exotic grass competitor (Lolium multiflorum). Results: Lolium consistently performed better in non‐alkali soil. In contrast, Hemizonia, in the absence of competition, performed significantly better on the non‐alkali soils, but in competition with Lolium, performed equally well on alkali and non‐alkali soils. Conclusions: These results suggest that Hemizonia does not inherently prefer harsh alkali soil, but is better able to tolerate alkali soil and may be excluded from more moderate soils by competition. Therefore, edaphic and biotic effects may interact to determine the spatial distribution of this edaphic endemic. Our data suggest a mechanism by which competition between native and exotic species on impoverished soils leads to dominance of native species and creation of refugia for native species where exotic species are unable to thrive.     

Response of endemic and exotic earthworm communities to ecological restoration

Land conversion and environmental changes associated with agronomic practices are believed to have led to the disappearance of New Zealand endemic earthworms from agricultural land. Introduced European earthworms have since largely replaced endemic species in farming systems. We investigated the impact of vegetation restoration on earthworm communities. Recolonization by endemic earthworms increased with time after restoration at two studied sites in the South Island of New Zealand. However, exotic species did not disappear with restoration of native vegetation, even after 30 years. The persistence of exotic species leads to the cohabitation of the two communities and potential for interspecific competition.     

Spacing and Competition Between Planted Grass Plugs and Preexisting Perennial Grasses in a Restoration Site in Oregon

Planting native species into restoration settings where other natives already occur is a common practice. However, the competitive consequences of such plantings are rarely studied. Planting density also affects restoration costs. Here we examined the effects of established individuals of Lemmon's needlegrass (Achnatherum lemmonii) on plugs of bluebunch wheatgrass (Pseudoroegneria spicata) and Idaho fescue (Festuca idahoensis) in a restoration site in Oregon. All three of these grasses are local native perennials. Plugs were planted at 6, 12, and 18 cm from established A. lemmonii bunchgrasses and also in plots without A. lemmonii neighbors. Plug survival was uniformly high, averaging more than 98%. Plugs planted at 6 cm from established grasses showed significantly lower growth and reproduction than plugs planted at 18 cm, which had similar values to plugs not planted in the vicinity of A. lemmonii. These results suggest that interplanting distances of as little as 18 cm were sufficient to greatly reduce competitive effects on newly planted plugs, at least in early establishment at this site.     

Subtle differences in environmental stress along a flooding gradient affect the importance of inter-specific competition in an annual plant community

Empirical evidence suggests that the direction and intensity of plant–plant interactions may depend on the favourability of the environment. Previous studies have mainly focused on steep gradients of environmental stress or disturbance, while the interplay of competition and environment has not been tested for subtle environmental differences. Here, we present results from a study on plant communities of temporary wetlands in East-German farmland. Due to yearly ploughing in autumn, the vegetation is composed of annual species. Flooding does not affect adult plants and the elevation on the gradient expresses differences in the length of the growing season rather than in disturbance intensity or severe environmental stress. We tested whether such subtle differences in environmental stress may affect the importance of interspecific competition by the dominant species. Two treatments were applied at two elevations: removal of the dominant species (Matricaria maritima ssp. inodora) and reciprocal transplants of the seed-bank of the two elevations. At both elevations, removal of Matricaria inodora led to an increase in total species richness and number of wetland species, but the effects were substantially stronger at high elevations. Removal and the elevation on the flooding gradient significantly influenced the plant community composition. In particular, the weed communities became more similar to the wetland communities after the removal. Transplanted weed species did not emerge at low elevations. While two of four target species had significantly higher densities after the removal at high elevations, none of them was influenced by removal at low elevations. This indicates that, consistent with previous studies from other habitat types, competition by the dominant species was more intense under conditions of low environmental stress. The overall results suggest that both flooding as well as interspecific competition are important in structuring the plant communities along the freshwater gradient studied.     

Testing the Home‐Site Advantage in Forest Trees on Disturbed and Undisturbed Sites

Restoration of plant populations is often undertaken using seed or plants from local sources because it is assumed they will be best adapted to the prevailing conditions. However, the effect of site disturbance on local adaptation has rarely been examined. We assessed local adaptation in three southwestern Australian forest tree species (Eucalyptus marginata, Corymbia calophylla, and Allocasuarina fraseriana) using reciprocal transplant trials at disturbed and undisturbed sites. Performance of plants within the trials was assessed over 2 years. Planting location accounted for the majority of the variation in most measures of performance, although significant variation of percent emergence among source populations was also detected. In all species, percent emergence and survival of plants sourced from Darling Range populations was significantly higher than that of plants from the Swan Coastal Plain, regions of contrasting edaphic and climatic environment. Survival of E. marginata over the first 18 months and emergence of C. calophylla were both higher in local plants, providing at least weak evidence for local adaptation. Where a local advantage was observed, the relative performance of local and nonlocal seed did not vary among disturbed and undisturbed sites. Evidence for enhanced establishment from local seed in at least one species leads us to recommend that where sufficient high‐quality seed supplies exist locally, these should be used in restoration. We also recommend longer‐term studies to include the possibility of local adaptation becoming evident at later life history stages.     

Tamarix as Habitat for Birds: Implications for Riparian Restoration in the Southwestern United States

Exotic vegetation has become a major habitat component in many ecosystems around the world, sometimes dramatically changing the vegetation community structure and composition. In the southwestern United States, riparian ecosystems are undergoing major changes in part due to the establishment and spread of the exotic Tamarix (saltcedar, tamarisk). There are concerns about the suitability of Tamarix as habitat for birds. Although Tamarix habitats tend to support fewer species and individuals than native habitats, Arizona Breeding Bird Atlas data and Birds of North America accounts show that 49 species use Tamarix as breeding habitat. Importantly, the relative use of Tamarix and its quality as habitat vary substantially by geographic location and bird species. Few studies have examined how breeding in Tamarix actually affects bird survivorship and productivity; recent research on Southwestern Willow Flycatchers has found no negative effects from breeding in Tamarix habitats. Therefore, the ecological benefits and costs of Tamarix control are difficult to predict and are likely to be species specific and site specific. Given the likelihood that high‐quality native riparian vegetation will not develop at all Tamarix control sites, restoration projects that remove Tamarix but do not assure replacement by high‐quality native habitat have the potential to reduce the net riparian habitat value for some local or regional bird populations. Therefore, an assessment of potential negative impacts is important in deciding if exotic control should be conducted. In addition, measurable project objectives, appropriate control and restoration techniques, and robust monitoring are all critical to effective restoration planning and execution.     

Local soil,but not commercial AMF inoculum,increases native and non‐native grass growth at a mine restoration site

Soil communities are often degraded in mined sites, and facilitating the recovery of soil mutualists such as arbuscular mycorrhizal fungi (AMF) may assist with the restoration of native plants. At a grassland mine restoration site, I compared a commercial AMF inoculum with soil collected from beneath native grasses as a source of inoculum, as well as a control treatment. Field plots were broadcast‐inoculated and seeded with native grasses, and biomass of native and non‐native species was measured in three consecutive years. In addition, greenhouse‐grown seedlings of a native bunchgrass (Stipa pulchra) were inoculated with similar treatments, transplanted into the field, and assessed after 18 months. When broadcast inoculation was used, the local soil inoculum tended to increase non‐native grass biomass, and marginally decreased non‐native forb biomass in the second year of study, but did not significantly affect native grass biomass. Broadcast commercial inoculum had no detectable effects on biomass of any plant group. Stipa pulchra transplants had greater N content and mycorrhizal colonization, and marginally higher shoot mass and K content, when pre‐inoculated with local soil (relative to controls). Pre‐inoculation with commercial AMF increased AMF colonization of the S. pulchra transplants, but did not significantly affect biomass or nutrient content. The findings indicate that at this site, the use of local soil as an inoculum had greater effects on native and non‐native plants than the commercial product used. In order to substantially increase native grass performance, inoculation of transplanted plugs may be one potential strategy.     

Potential contribution of natural enemies to patterns of local adaptation in plants

Genetic differentiation among plant populations and adaptation to local environmental conditions are well documented. However, few studies have examined the potential contribution of plant antagonists, such as insect herbivores and pathogens, to the pattern of local adaptation. Here, a reciprocal transplant experiment was set up at three sites across Europe using two common plant species, Holcus lanatus and Plantago lanceolata. The amount of damage by the main above-ground plant antagonists was measured: a rust fungus infecting Holcus and a specialist beetle feeding on Plantago, both in low-density monoculture plots and in competition with interspecific neighbours. Strong genetic differentiation among provenances in the amount of damage by antagonists in both species was found. Local provenances of Holcus had significantly higher amounts of rust infection than foreign provenances, whereas local provenances of Plantago were significantly less damaged by the specialist beetle than the foreign provenances. The presence of surrounding vegetation affected the amount of damage but had little influence on the ranking of plant provenances. The opposite pattern of population differentiation in resistance to local antagonists in the two species suggests that it will be difficult to predict the consequences of plant translocations for interactions with organisms of higher trophic levels.     

Weak asymmetric interspecific aggression and divergent habitat preferences at an elevational contact zone between tropical songbirds

Closely related tropical bird species often occupy mutually exclusive elevational ranges, but the mechanisms generating and maintaining this pattern remain poorly understood. One hypothesis is that replacement species are segregated by interference competition (e.g. territorial aggression), but the extent to which competition combines with other key factors such as specialization to distinct habitats remains little studied. Using vegetation surveys and reciprocal playback experiments, we explored the dynamics of interspecific aggression between two Nightingale-Thrushes Catharus sp. in Central America. We show that lower-elevation Black-headed Nightingale-Thrushes Catharus mexicanus are aggressive towards higher-elevation Ruddy-capped Nightingale-Thrushes Catharus frantzii where they meet at contact zones. However, interspecific aggressive responses were weak and unidirectional, and the two species were associated with different habitats. We conclude that the contact zone is maintained and located primarily by habitat selection, and is probably reinforced by interspecific aggression. This has important implications for understanding how montane species will respond to climate change because the pace and extent of range shifts will not depend solely on habitat shifts or interspecific competition, but instead on interactions between these two factors.     

Competition between Native Hawaiian Plants and the Invasive Grass Megathyrsus maximus: Implications of Functional Diversity for Ecological Restoration

Biodiversity loss is a global crisis, due primarily to habitat destruction and widespread nonnative invasions. Invasive grasses are particularly problematic in many tropical ecosystems, where they possess traits that promote their persistence and can drastically alter native plant communities. We explored the ecophysiological basis for restoring native Hawaiian dryland ecosystems currently dominated by the nonnative invasive grass Megathyrsus maximus (guinea grass) in a garden experiment. Three native species—Myoporum sandwicense (naio; canopy tree), Dodonaea viscosa (aalii; shrub), and Plumbago zeylanica (iliee; groundcover)—were grown with M. maximus at three levels of native functional diversity (one, two, or three species) while holding overall plant density constant. We tested which individual and functional combinations of native species were more productive and best suppressed M. maximus growth and reproduction. Megathyrsus maximus had 39–94% higher maximum photosynthetic rates (A_max) than native species and increasing native functional diversity did not affect M. maximus A_max. Aboveground, belowground, and total biomass of M. maximus varied with functional diversity, although intraspecific competition reduced growth as much as interspecific competition. Reproductive tiller production by M. maximus decreased significantly when planted with any of the native species and with increasing native functional diversity. These results indicate that high native functional diversity in an ecological restoration setting may aid in the control of a dominant invasive grass and the reintroduction of diverse native species. Recommendations for restoring degraded nonnative grasslands in Hawaii and throughout the tropics include selection of native species that are ecophysiologically competitive and have high functional diversity.     

Patterns of Differentiation in Wiregrass (Aristida beyrichiana): Implications for Restoration Efforts

Aristida beyrichiana (wiregrass) is increasingly being planted in restoration projects across the southeastern coastal plain, with little focus on genetic differences among populations across the region. Local and regional population differentiation for establishment and growth traits were examined in common garden and reciprocal transplant experiments. Seeds from up to 20 plants from each of seven populations were collected in northern and central Florida sites that encompassed gradients of soils, hydrology, and temperature. Reciprocal seed transplants using three of the common garden populations were conducted in two consecutive years. In the common garden, significant population differences were seen in seed weight, seedling emergence and survival, tiller height, number of tillers, the relationship between tiller number and tiller height, and flowering. Variation among maternal families was seen in tiller number and in the relationship between tiller number and tiller height. The reciprocal transplant study did not detect either local adaptation to sites of origin or consistent superiority of one source population or planting site in seedling establishment. These results suggest that the probability of seedling establishment is primarily dependent on environmental conditions rather than genetic differences. Genetic variation for traits related to fitness (e.g., tiller number) may be retained within populations because phenotypically plastic growth responses of seedlings to environmental variation buffer genetic variation against the action of selection. But despite the lack of evidence for genetic influences on initial establishment in wiregrass, our common garden study suggests genetic differences among populations. This result, when combined with previous results indicating local adaptation in later life stages of wiregrass, suggests that restoration efforts involving this species should use local seed sources from sites with similar soil and hydrological conditions.     

塔里木荒漠绿洲过渡带主要种群生态位与空间格局分析

于新疆塔里木盆地荒漠绿洲过渡带设置3条样带(长900 m×宽50 m)进行植物群落调查,运用生态位测度指标和点格局法对种群空间格局、空间关联性及生态位特征进行研究。结果显示,胡杨(Populus euphratica Oliv.)、多枝柽柳(Tamarix ramosissima Ledeb.)和甘草(Glycyrrhiza inflate Bat.)具有较大的重要值和生态位宽度,对其它生态位较窄的物种具有资源竞争和扩张优势,生态适应能力强,对群落结构和环境变化起决定性作用;生态过渡带植物的种间生态位重叠普遍较高,尤其是草本植物间更明显,此分配格局反映出荒漠植物长期适应旱化生境而发生趋同适应,生态位分化程度低,种间资源竞争激烈;优势种群空间格局呈明显的空间变异性,0~25 m尺度内多枝柽柳和甘草分别呈随机分布和聚集分布,胡杨种群在≤5 m尺度内呈聚集分布,并随尺度增大转为随机分布;胡杨与多枝柽柳、胡杨与甘草、多枝柽柳与甘草分别在4 m、≤16 m和≤25 m尺度内呈显著空间负关联,种间相互排斥。表明在匮乏环境资源条件下植物趋同适应与资源竞争是驱动荒漠植物群落演替和限制物种共存的主要原因。