Using Precision Prairie Reconstruction to Drive the Native Seeded Species Colonization Process

Restoring forb richness to the northern tallgrass prairie (U.S.A.) is often problematic. A potential solution is the establishment of native forb‐seeded patches that can serve as colonization sites. This study was designed to determine the following: (1) the success at which native forbs sown in small patches can colonize the surrounding vegetation matrix and (2) whether soil amendments (C additions, P fertilization, and seed bank reduction) applied to the seeded sites can facilitate such colonization (patch quality). Colonization was investigated at (1) the immediate surrounding (1 m belt transect, BT) of the seeded sites and (2) in disturbed but not seeded patches located 3 m from the seeded site (out‐plots). Soil amendments did not affect colonization, but native forb density in the BT and out‐plots was correlated to the density of forbs in the seeded sites (r² = 0.3, p < 0.01). Seeded native forb density in the out‐plots was higher than in the adjacent BT (19 vs. 5 plants/m²), suggesting that competition from the matrix vegetation may be more important in forb colonization than distance from the source. Taller forbs with larger seeds showed advantages in colonization success. This study showed that the colonization of matrix vegetation with native forbs from seeded patches is a viable method for prairie reconstruction but requires the availability of disturbed sites. In most prairies, animal‐generated soil disturbances are common. In their absence, disturbances of the vegetation matrix will need to be added to the management plan.     

COLONIZATION AND ESTABLISHMENT OF MISSOURI PRAIRIE PLANTS ON ARTIFICIAL SOIL DISTURBANCES. II. DETECTING SMALL-SCALE PLANT-TO-PLANT INTERACTIONS AND SEPARATING DISTURBANCE FROM RESOURCE PROVISION

We investigated whether seedlings and shoots interacted with one another while colonizing small soil disturbances in a tall-grass prairie in Missouri. Specifically, we tested for density dependence of longevity and whether seedlings responded to the creation of a disturbance and/or the provision of an uncolonized resource. There was little evidence of density-dependent mortality among either seedlings or vegetative shoots. There were essentially no differences in either colonization or mortality of seedlings between disturbances with a new, competition-free resource and disturbances without a new resource. Excluding vegetative invaders from a site by means of a polyvinylchloride pipe did not prompt a greater number of seedlings to arise or survive. These results substantiated a noninteractive portrayal of colonization. Individuals of seedlings and shoots appeared and disappeared in a fashion largely independent of one another.     

Vegetation responses to different spatial patterns of soil disturbance in burned and unburned tallgrass prairie

Pocket gopher (Geomyidae) disturbances are created in spatiallypredictable patterns. This may influence resource heterogeneity and affectgrassland vegetation in a unique manner. We attempt to determine the extent towhich density and spatial pattern of soil disturbances influence tallgrassprairie plant community structure and determine how these disturbances interactwith fire. To investigate the effects of explicit disturbance patterns we createdsimulated pocket gopher burrows and mounds in various spatial patterns.Simulated burrows were drilled into the soil at different densities inreplicated plots of burned and unburned prairie. Separate plots of simulatedmounds were created in burned and unburned prairie at low, medium, or high mounddensities in clumped, uniform, or random spatial dispersions. In both burned and unburned plots, increased burrow density decreasedgraminoid biomass and increased forb biomass. Total-plant and graminoid biomasswere higher in burned than unburned plots while forb biomass was higher inunburned plots. Total-plant species richness was not significantly affected byburrow density or burning treatments, but graminoid species richness increasedin unburned plots and forb species richness increased in burned plots. Plant species richness was temporarily reduced directly on mounddisturbances compared to undisturbed prairie. Over time and at larger samplingscales, the interaction of fire and mound disturbance patterns significantlyaffected total-plant and graminoid species richness. The principal effect inburned and unburned prairie was decreased total-plant and graminoid speciesrichness with increased mound disturbance intensity. Although species richness at small patch scales was not increased by anyintensity of disturbance and species composition was not altered by theestablishment of a unique guild of disturbance colonizing plants, our studyrevealed that interactions between soil disturbances and fire alter the plantcommunity dominance structure of North American tallgrass prairie primarily viachanges to graminoids. Moreover, these effects become increasingly pronouncedover time and at larger spatial sampling scales.     

Microclimate and substrate quality controls on nitrogen mineralization in a New England high salt marsh

New England high salt marsh primary productivity is limited by N, but variation in plant N availability across salt marsh vegetation zones has not been quantified. To investigate this, we measured in situ net N mineralization rates throughout the growing season in three zones of a Maine high salt marsh, Juncus gerardii, Spartina patens, and mixed perennial forb. We also measured microclimate factors (soil temperatures and moistures) and substrate quality parameters (soil organic matter, soil total nitrogen, soil C:N ratio) to see if either related to differences in net N mineralization. To determine the relative importance of substrate quality and microclimate, we measured N mineralization of the different soil types in the laboratory, holding microclimate parameters constant. We also investigated the relative importance of microclimate and substrate statistically, with principal components analysis and multiple regression. In situ net N mineralization rates were significantly higher in the forb zone than in graminoid zones, but graminoid zone N mineralization rates did not vary significantly from each other. Soil temperatures, moistures, carbon, and nitrogen were all significantly higher in the forb zone than in graminoid zones, but C:N ratio did not vary significantly across zones. Principal components analysis and multiple regression revealed that microclimate was a more significant predictor of total N mineralized over the course of the growing season than was substrate quality. In contrast, when microclimate conditions were held constant, forb zone N mineralization was still significantly higher than that of graminoid zones, suggesting that substrate quality does exert some control on this process. Thus, both microclimate and substrate quality appear to influence N mineralization rates across vegetation zones of this Maine salt marsh.     

Climate and soil texture influence patterns of forb species richness and composition in big sagebrush plant communities across their spatial extent in the western U.S.

Big sagebrush (Artemisia tridentata Nutt.) plant communities are widespread in western North America and, similar to all shrub steppe ecosystems worldwide, are composed of a shrub overstory layer and a forb and graminoid understory layer. Forbs account for the majority of plant species diversity in big sagebrush plant communities and are important for ecosystem function. Few studies have explored geographic patterns of forb species richness and composition and their relationships with environmental variables in these communities. Our objectives were to examine the fine and broad-scale spatial patterns in forb species richness and composition and the influence of environmental variables. We sampled forb species richness and composition along transects at 15 field sites in Colorado, Idaho, Montana, Nevada, Oregon, Utah, and Wyoming, built species-area relationships to quantify differences in forb species richness at sites, and used Principal Components Analysis, non-metric multidimensional scaling, and redundancy analysis to identify relationships among environmental variables and forb species richness and composition. We found that species richness was most strongly correlated with soil texture, while species composition was most related to climate. The combination of climate and soil texture influences water availability, which our results indicate has important consequences for forb species richness and composition, and suggests that climate change-induced modification of soil water availability may have important implications for plant species diversity in the future.     

Drought alters carbon fluxes in alpine snowbed ecosystems through contrasting impacts on graminoids and forbs

? Climate change is predicted to increase the frequency of drought events in alpine ecosystems with the potential to affect carbon turnover. ? We removed intact turfs from a Nardus stricta alpine snowbed community and subjected half of them to two drought events of 8 d duration under controlled conditions. Leachate dissolved organic carbon (DOC) was measured throughout the 6 wk study period, and a (13)CO(2) pulse enabled quantification of fluxes of recent assimilate into shoots, roots and leachate and ecosystem CO(2) exchange. ? The amount of DOC in leachate from droughted cores was 62% less than in controls. Drought reduced graminoid biomass, increased forb biomass, had no effect on bryophytes, and led to an overall decrease in total above-ground biomass compared with controls. Net CO(2) exchange, gross photosynthesis and the amount of (13)CO(2) fixed were all significantly less in droughted turfs. These turfs also retained proportionally more (13)C in shoots, allocated less (13)C to roots, and the amount of dissolved organic (13)C recovered in leachate was 57% less than in controls. ? Our data show that drought events can have significant impacts on ecosystem carbon fluxes, and that the principal mechanism behind this is probably changes in the relative abundance of forbs and grasses.     

Growth and survival of seedlings of native plants in an impoverished and highly disturbed soil following inoculation with arbuscular mycorrhizal fungi

We investigated whether arbuscular mycorrhizas influenced growth and survival of seedlings in an extremely impoverished and highly disturbed soil. Seedlings of four plants species native to the site were either inoculated with native sporocarpic arbuscular mycorrhizal (AM) fungi or fertilised prior to transplanting, and followed over 86 weeks at the site. One treatment was also irrigated with N-rich leachate from the site. In a laboratory experiment, seedlings were fertilised with excess P for 6 weeks, and location of the P store determined. Growth and survival of AM and fertilised seedlings were similar at the site. Inoculated mycorrhizal fungi and roots appeared to extend into the surrounding soil together. P concentration in leaves of all plants was extremely low. Irrigation with leachate increased growth of seedlings. In the laboratory experiment, significantly more P was stored in roots than shoots. We suggest that successful revegetation of extremely disturbed and impoverished sites requires selection of mycorrhizal fungi and plants to suit the edaphic conditions and methods of out-planting.     

Variation in nutrient availability and plant species diversity across forb and graminoid zones of a Northern New England high salt marsh

Plant zonation patterns across New England salt marshes have been investigated for years, but how nutrient availability differs between zones has received little attention. We investigated how N availability, P availability, and plant N status varied across Juncus gerardii, Spartina patens, and mixed forb zones of a Northern New England high salt marsh. We also investigated relationships between several edaphic factors and community production and diversity across the high marsh. P availability, soil salinity, and soil moisture were higher in the mixed forb zone than in the two graminoid zones. NH⁺ ₄-N availability was highest in the J. gerardii zone, but NO^– ₃-N availability and mid season net N mineralization rates did not vary among zones. Plant tissue N concentrations were highest in the mixed forb zone and lowest in the S. patens zone, reflecting plant physiologies more so than soil N availability. Community production was highest in the J. gerardii zone and was positively correlated with N availability and negatively correlated with soil moisture. Plant species diversity was highest in the mixed forb zone and was positively correlated with P availability and soil salinity. Thus, nutrient availability, plant N status, and plant species diversity varied across zones of this high marsh. Further investigation is needed to ascertain if soil nutrient availability influences or is a result of the production and diversity differences that exist between vegetation zones of New England high salt marshes.     

Effects of herbaceous plant functional groups on the dynamics and structure of an alpine lichen heath: the results of a removal experiment

Alpine lichen heaths are polydominant, low-productive communities where lichens and, at some localities, the dwarf shrub Vaccinium vitis-idaea prevail. To analyse the role of herbaceous dominants in the structure of alpine heaths, we established a removal experiment. We tested (1) whether dominant graminoids and forbs differ in their effects on species richness, phytomass, and abundance of individual species; and (2) which remaining species are able to replace the removed dominants. Permanent plots were established in 1996 in the Teberda Reserve (the Northwestern Caucasus, Russia) at 2750 m a.s.l. with four treatments: (1) control, (2) forb removal, (3) graminoid removal, (4) graminoid and forb removal. Target species were clipped every year to avoid regrowth. We counted the shoot numbers of all vascular plants yearly from 1996 to 2009. In 2010, we harvested the aboveground phytomass. We used mixed-effects models to assess the responses of the remaining species and phytomass fractions to the removal. The biomass of the removed dominants was almost compensated for by the remaining species, but due to diffuse competition none of the subordinate species became a new dominant. Graminoid removal reduced community weighted mean leaf dry matter content and litter accumulation. Species number per plot completely recovered after graminoid, not forb removal. Shoot numbers of Antennaria dioica, Arenaria lychnidea, Campanula tridentata, Ranunculus oreophilus, and Trifolium polyphyllum showed positive responses to graminoid removal. Anemone speciosa shoot numbers decreased after graminoid removal. In alpine heath, graminoid dominants contribute more to community structure than forb dominants.     

Litter decomposition in moist acidic and non-acidic tundra with different glacial histories

Plant species composition is a potentially important source of variation in soil processes, including decomposition rates. We compared litter decomposition in two common and compositionally distinct tundra vegetation types in the northern foothills of the Brooks Range, Alaska: moist acidic tundra (soil pH 3–4), which occurs primarily on older landscapes, and moist non-acidic tundra (soil pH 6–7), which occurs primarily on landscapes with a more recent history of glaciation and has higher graminoid and forb abundance and lower woody shrub abundance than acidic tundra. To separate the influence of plant community composition from that of the soil environment, we decomposed the same nine substrates at a moist acidic and a moist non-acidic site located less than 2 km apart. Substrates included leaf litter of the dominant species in each growth form (graminoid, deciduous shrub, evergreen shrub, forb, moss) as well as woody stems of the deciduous shrub Betula nana. Then, we estimated above-ground community-level decomposition by weighting the decay rate of each species in the community by its proportional contribution to overall above-ground net primary production (ANPP). In contrast to our expectations, community-level decomposition rates estimated using the site-average decay rate for each substrate were similar between the two sites, likely because growth forms differed little in their leaf litter decay. By contrast, when site-specific decay rates were used to estimate community-level decomposition, it was nearly twice as fast at the older, moist acidic tundra site because most substrates decayed faster at that site, indicating a more favorable environment for decomposition in acidic tundra. Site differences in soil moisture and temperature could not explain site differences in decomposition. However, higher soil N availability at the moist acidic tundra may have contributed to faster decomposition since, in a separate experiment, fertilization with N stimulated decomposition of a common substrate at both sites. In addition, lower pH in acidic tundra may promote greater abundance of soil fungi, perhaps explaining faster decomposition rates at that site. In summary, the large differences in plant species composition between moist acidic and non-acidic tundra are likely to not contribute to site differences in decomposition. Nevertheless, decomposition is much more rapid in moist acidic tundra. Thus, landscape age and associated differences in soil pH and nutrient availability are important sources of variation in decomposition rate in upland Alaskan tundra.     

Soil diaspore reserves above the timberline in the Austrian Alps

Soil diaspore reserves are considered to support self-healing processes after vegetation disturbances. Therefore, the stratified reserves of viable diaspores in superimposed soil layers of four sites above the timberline in the Austrian Alps were assessed. At each site, a semi-natural (“undisturbed”) extensive alpine pasture and the disturbed vegetation on hiking trails were investigated. Eighty soil cores in total (corresponding to 400 slices, each representing a 1-cm layer between 0 and 5 cm depth) were taken in autumn and subjected to germination tests after vernalization. The total diaspore numbers in disturbed and undisturbed plots did not differ significantly, but all undisturbed soils contained higher species numbers than disturbed ones. Seed shape and size clearly influenced the vertical distribution. Intact soils showed a significant decrease in big/long diaspores with increasing soil depth. Disturbances influenced the aboveground species composition and therefore the distribution of seeds of different size. In case of disturbances, the restriction of most big seeds to superficial layers means a threat for small populations of rare and protected species such as Viola lutea subsp. sudetica with relatively big seeds near the soil surface. When the disturbances stop, the diaspore communities might initiate a first, but with respect of landscape protection and preservation of species diversity insufficient step of vegetation restoration.     

INFLUENCE OF ENVIRONMENT AND POPULATION ORIGIN ON SURVIVORSHIP AND REPRODUCTION IN RECIPROCAL TRANSPLANTS OF AMPHICARPIC PEANUTGRASS (AMPHICARPUM PURSHII)

Reciprocal transplants of both seeds and seedlings were utilized to determine whether populations of the annual grass Amphicarpum purshii have become locally adapted to specific habitats due to the consistent production of cleistogamous subterranean seeds from year to year. The hypothesis was that subterranean seeds placed in the same habitat as the parents will produce seedlings of greater vigor and adults of higher reproductive capacity than plants from seeds transplanted to a different habitat far removed from the parents. For both seed and seedling transplant experiments involving three sites in the Pine Barrens of New Jersey, the effects of site on shoot dry weight and production of aerial spikelets, subterranean spikelets, and seeds were generally much more significant than the effects of population origin. With one exception, there was no tendency for seedlings (or plants from seeds) replanted into their home sites to outperform alien seedlings (or plants from seeds) transplanted into these same sites. The overriding importance of environmental factors (relative to genetic differences among populations) in determining the phenotypic expression of life history characters, and selection occurring during succession at a site may retard the evolution of genetic adaptation to local habitat conditions in this species.     

Response of conifer seedlings to nitrate and ammonium sources of nitrogen

Summary Differences in growth responses of Douglas fir, western hemlock, Sitka spruce, and white spruce to nitrate and ammonium N sources were examined in sand culture and artificial soil culture. Effects of the two forms of N on growth, needle area, and N uptake of three Douglas fir halb-sib progenies were examined in a second sand culture. Response of Douglas fir to the two forms of N was followed over two years in nursery soil of different pH levels. In sand culture 1 mean seedling dry weight of all species, except hemlock, was greatest when ammonium N and nitrate N were provided in equal amounts. In all species, except Sitka spruce, ammonium alone resulted in greater growth than nitrate alone. Use of ammonium N resulted in greater growth of all species, than was obtained with nitrate N, at pH values in the region 5.4 and 7.5 in artificial soil culture. Only Douglas fir showed substantial differences due to N source below pH 5. Growth of all species was greater at pH 5.4 than at 7.5 in each N source treatment. Growth of Douglas fir seedlings was greatest with ammonium N and least with nitrate N in sand culture 2. Supply of nitrate and ammonium in equal proportions resulted in intermediate growth. Leaf area/plant weight ratio was unaffected by N source. Analysis of nutrient solutions showed appreciable nitrification of ammonium N during the 7 days between solution changes. In the three greenhouse experiments, with little exception, increase in proportion of ammonium in N supply resulted in increase of seedling tissue N concentration. This effect was more pronounced in roots than shoots. Total N uptake by ammonium fed seedlings was about double the N uptake of nitrate fed seedlings in sand culture 2. Nursery grown Douglas fir seedlings showed greater growth response to ammonium sulphate than to calcium nitrate, and this appeared due entirely to form of N supply in the first year. A similar response in the second year was partly due to greater soil acidification by ammonium sulphate. Compared with calcium nitrate, ammonium sulphate increased N concentration of one-year old shoots, but this difference was not detected by foliar analysis of two-year old seedlings.     

土壤结构与光照水平对秋茄某些生长和生理参数的影响

在香港的３个红树森样地即黄竹湾（沙土）、西径（沙壤土）和米埔（粘壤土）进行了土壤结构对秋茄（Kandelia candel(L.)Durce)生长和生理影响的研究,并在米埔比较了林内和林外秋茄幼苗的生长和生理参数以观察光照水平的效应。在沙土和沙壤土生长的１．５年秋茄幼苗比粘壤土具有较粗的基径的较高的总生物量,说明秋茄幼苗在沙土和沙壤土中比在粘壤土中生长更好。沙土１．５ 茄幼苗的叶片厚度分别为沙壤土和粘壤土的１．７５和２．０５倍,表明沙土中的秋茄幼苗具有旱生结构以维持体内水分。然而,沙土和沙壤土４．５年秋茄幼树的叶片厚度无显著差异,沙土和沙壤土中１．５年秋茄幼苗分配于根系的生物量比例约为５０％,高于粘壤土的值（约４０％）。沙土和沙壤土中１．５年的秋茄比粘壤土具有较低的叶绿素含量、根系活力、硝酸盐还原酶活性、过氧化物酶（ＰＯＸ）活性、超氧化物歧化酶（ＳＯＤ）活性及较高的丙二醛（ＭＤＡ）含量。米埔１．５年秋茄幼苗在红树林外比林内有更好的长势,具有更大的叶面积、特殊叶面积、叶片数量及生物量。林内幼苗具有较高叶绿素含量,较低叶绿素a/b比值,较高硝酸盐还原酶活性和较强的根系活力,林外幼苗的叶片ＰＯＸ和ＳＯＤ活性比林内的值稍高,ＭＤＡ含量比林内显著要高。     

SOME EXPERIMENTS AND FIELD OBSERVATIONS ON THE GERMINATION OF WILD OAT (AVENA FATUA AND A. LUDOVICIANA) SEEDS IN SOIL AND THE EMERGENCE OF SEEDLINGS

The distinguishing characters of Avena fatua and A. ludoviciana are described.
Pot experiments and field observations showed that most seeds of A. fatua germinated in spring and a few in autumn; hardly any germinated in summer or winter. Seeds of A. ludoviciana germinated in winter only. The greatest depth of sowing from which seedlings of either species reached the surface was 9 in., but seedlings from this depth were weak and yellow when they first appeared. A. ludoviciana gave more and sturdier seedlings than A. fatua from 6 and 9 in. There was no evidence of induced dormancy in seeds of A. fatua buried at depths down to 20 in. Germination of this species was hastened by monthly cultivation of the soil. The maximum survival of A. fatua was 3 years in pots and slightly longer in the field; seeds of A. ludoviciana in pots survived only 2 years. The two or three seeds of each spikelet of A. ludoviciana germinated in turn, starting with the largest, but the interval between germination of successive seeds varied.
Seedlings from freshly sown seeds of both species were more vigorous than seedlings from seeds which had been buried for a year or more. Germination and subsequent growth of both species took place in soil of pH 4.5 to 7.0 approx.     

Diet and activity patterns of Arsi geladas in low-elevation disturbed habitat south of the Rift Valley at Indetu,Ethiopia

Understanding the basic natural history of threatened primate taxa is crucial to developing and implementing successful conservation strategies for them. Data on feeding ecology and activity patterns are particularly important for identifying the strategies through which primates invest time and foraging effort towards survival and reproduction at a given locale. Here, we report the results of the first study of the diet and activity budget of Arsi geladas, a population of < 1000 individuals endemic to a heavily disturbed region of the southern Ethiopian Highlands and believed to represent a new taxon of geladas. We conducted our research on a band of 34 individuals belonging to five, one-male units at Indetu, eastern Arsi, Ethiopia, from August 2010 to May 2011 (excluding March 2011). Feeding accounted for 41.7% of total scans, followed by moving (20.3%), resting (19.0%), and social behavior (19.0%). Feeding and moving increased and resting and socializing decreased during the dry season when food availability was probably lower than during the wet season. Geladas ate mostly graminoid leaves (51.7% of feeding scans) though they also consumed graminoid rhizomes (24.4%), forb tubers (7.1%), forb leaves (7.1%), cactus, shrub, and tree fruits (3.6%), graminoid corms (1.7%), forb roots (1.6%), and unidentified items (3.0%). Underground foods (corms, rhizomes, roots, and tubers) accounted for 22–47% (mean = 35%) of the monthly (n = 9) diet and were eaten slightly more during the wet season than during the dry season. Contributions of human crops to the gelada diet could not be quantified without creating conflict between farmers and researchers, though we did note that geladas visited farms on 5–10% of study days. Threats from farmers, children, and dogs limited the geladas’ access to crops once they entered the farms. Further research involving questionnaire surveys of farmers, direct observation of crop damage by geladas, and DNA metabarcoding of gelada feces are crucial to the development of strategies to mitigate human-gelada conflict in the densely populated Arsi Zone of Ethiopia.     

Population dynamics of the perennial Plantago media in semi‐natural grasslands

Abstract. In this study we investigated population dynamics of a perennial forb, Plantago media, in semi‐natural grasslands in southeastern Sweden. Plantago media is a rather common inhabitant of grasslands in this region, but it has been shown to experience dispersal limitations both among managed grassland sites and at potential dispersal routes along road verges. The demography of P. media was studied in 22 permanent plots at two sites over five years. A seed sowing experiment was also performed at each site. The life‐cycle of P. media includes seedlings, juveniles, small vegetative rosettes, large vegetative rosettes and flowering rosettes. The population growth rates (λ‐values) were negative for both populations, with one exception: the first year transition matrix at one site. The projected life span for individuals surviving from seedlings to flowering rosettes varied between 19.6 yr and 227.8 yr. Elasticity analysis showed that remaining in the large rosette stages, both vegetative and flowering, was the most important factor influencing population growth rate. LTRE analysis gave similar results, indicating that spatial and temporal variation have similar effects on the population growth rate. The expected time to extinction in populations with an initial size of between 100 and 1000 individuals varied between 60 and 200 yr. The seed sowing experiment showed that seedling emergence was enhanced by both seed addition and disturbance, suggesting that recruitment is limited by seed and microsite availability. Recruitment was not found to have much impact on population growth rate. However, long‐term population maintenance may depend on occasional small‐scale disturbances that enhance pulses of recruitment. Fragmented landscape is likely to effect plant populations including rare and endangered species as well as more common species which have limited dispersal mechanisms.     

北京东灵山区两种生境条件下辽东栎幼苗补充与建立的比较

 北京东灵山区的人工油松（Pinus tabulaeformis）林中常常混生有萌生的辽东栎（Quercus wutaishanica）种群。局部地带辽东栎与油松形成混交林,在山脊及山坡上部一些特殊生境辽东栎甚至取代油松林成为优势种。为了探讨北京东灵山区辽东栎林的天然更新机制及辽东栎幼苗在人工油松林中的天然更新,在油松人工林的林下和林缘两种生境条件下将辽东栎种子播种于2 cm深的土中,对辽东栎幼苗的补充和建立进行了对照试验。结果显示,在辽东栎结实丰年,两种生境条件下播种后辽东栎种子丢失差异并不显著,而辽东栎幼苗补充和建立方面的差异极显著。由于森林动物特别是啮齿类动物的活动对辽东栎幼苗建立的影响具有二重性：消耗种子与促进萌发,本试验特别关注辽东栎幼苗子叶丢失现象及其丢失后果。由于辽东栎种子萌根较早,在幼苗出土前其粗大的主根长达10～20 cm,并已初步形成根系,表明已有相当比例的营养物质从子叶转移到根部。因此当幼苗出土后如果仅仅只是子叶的丢失对辽东栎幼苗的成活和生长均无明显的影响。通过研究发现辽东栎幼苗期子叶丢失有3种形式：1）仅丢失子叶,幼苗其它部位未受伤害,多发生在林缘生境中,占发芽坚果的29.69%;2）整个幼苗连根被拖出地面,多发生在林下生境,占62.43%;3）在子叶与根颈的连接处主干被咬断,林缘为50.88%,显著高于林下的8.41%。但只有后两种形式才导致幼苗死亡。导致这些现象的原因是由于林缘土壤腐殖质含量低,比较干燥,土质较坚实,以及幼苗出土前坚果主根生长得较长等缘故,当幼苗遭受动物捕食时不会轻易地被拖出地面,拖走的往往仅仅是子叶。至生长季节结束,林缘样方辽东栎幼苗的成活率接近20%,茎干平均高度7.94 cm,芽数11.24·株-1,叶片平均干重为81.14 mg·株-1,且几乎全部由丢失子叶的幼苗所构成;而林下样方辽东栎幼苗的成活率不足2%,茎干平均高度4.74 cm,芽数7.52·株-1,叶片平均干重仅为42.27 mg·株-1,以未丢失子叶的幼苗为主。试验结果表明,林缘及类似林缘的环境条件更有利于辽东栎的实生更新。     

Severely degraded dunes of the southern Kalahari: local extinction,persistence and natural re‐establishment of plants

This study aimed to quantify and understand the impact of severe land degradation on plant attributes and diversity on dunes of the southern Kalahari. Heavy grazing pressure in particular resulted in a significant decline of canopy cover and species number in annual and perennial life forms; forb and graminoid growth forms; erect and prostrate habits; and leafy stem, tussock and stoloniferous architectures. However, no significant change was found in shrub and tree forms which persisted without apparent new recruitment. Under these conditions, species diversity dropped sharply and a number of species, mainly graminoids, became apparently locally extinct. The perennial shrub, Crotalaria cf. spartioides, showed the converse with a high frequency of establishing seedlings on the degraded dunes. Changes in relative dominance show that the extreme treatment favours perennial over annual, woody over graminoid and forb, erect over prostrate and leafy stem over stoloniferous and tussock. Some of these results and certain species and soil responses differ from those reported from grazing studies elsewhere, and are possibly no longer directly related to the impact of the primary grazing pressure but to the secondary effect of subsequent instability of the dunes.     

Experimental analysis of patch dynamics and community heterogeneity in tallgrass prairie

Effects of fire and small-scale soil disturbances on species richness, community heterogeneity, and microsuccession were investigated in a central Oklahoma tallgrass prairie. In the fall of 1985, 0.2 m² soil disturbances were created on burned and unburned tallgrass prairie. Vegetation on and off disturbances was sampled at monthly intervals over two growing seasons. During the first growing season, the cover of forbs and annuals, and species richness were significantly greater on versus off disturbances, but these differences did not persist through the second year. The variation in species composition among disturbed plots (heterogeneity) was significantly greater compared to undisturbed areas throughout the study. Fire had no consistent effect on richness and heterogeneity of vegetation on soil disturbances but fire reduced heterogeneity on undisturbed vegetation. Rate of succession, based on an increase in cumulative cover of perennial grasses over time, did not differ among treatments during the first growing season. During the second year, rate of succession was significantly greater on burned soil disturbances compared to unburned soil disturbances. These results suggest that while small-scale soil disturbances have primarily short-lived effects on grassland community structure, disturbances do help to maintain spatial and temporal variation in tallgrass prairie communities. Unlike in undisturbed vegetation, however, species richness and heterogeneity on soil disturbances were little effected by fire, but the rate of colonization onto disturbances appeared to be enhanced by fire.