克氏针茅草原的土壤种子库特征与植被恢复

近几十年来，由于过度放牧和不合理的农业开垦，内蒙古的天然草原退化严重。自2000年起，中国政府制定了“退耕还林还草”政策，并在草原地区实行了“禁牧、轮牧、休牧”等草原管理措施，期望依靠自然力来恢复退化的草原。然而，这些政策究竟能够带来什么样的后果还是一个未知数。土壤种子库和种子雨的资料对于全面理解植物种群和群落的动态具有重要价值。退化生态系统的恢复评价应该包括土壤种子库和种子雨的资料。因此，我调查了不同利用方式下克氏针茅（Stipa krylovii）草原的土壤种子库、种子雨和地上植被组成，来探讨政府的恢复政策对于该地区草原生态系统的影响。 研究地点位于中国科学院多伦恢复生态学试验示范研究站的永久实验样地，该实验样地在内蒙古自治区锡林郭勒盟的多伦县境内，该地区属于典型的农牧交错区。 采用幼苗萌发法，本研究测定了三种具有不同土地利用历史的样地的土壤种子库密度和物种组成。它们分别是：放牧样地、围封样地和弃耕样地。主要的研究结果有： （1）弃耕样地的土壤种子库密度最大，且主要是一年生和杂草类植物的种子，弃耕地由于其开垦的历史导致种子库的物种组成与放牧样地和围封样地的种子库有明显区别，其种子库缺少合适的多年生植物的种子是限制弃耕地自然恢复为天然草原的重要因素。 （2）放牧虽然会降低土壤种子库的密度，但是其中并不缺少多年生植物的种子。恢复由过度放牧引起的退化草原比恢复由农业开垦导致的退化草原要容易一些。围封对于由过度放牧引起的草原退化有明显的积极作用。 （3）采用Sorensen指数来计算地上植被与土壤种子库的相似性，这三个样地土壤种子库与地上植被的物种相似性均较高。在地上植被生物多样性的调查中，不能将草原中的一些早春植物遗漏。 采用网筛分离法，本研究测定了四种具有不同土地利用历史的样地的种子雨密度和物种组成。它们分别是：放牧样地、围封样地、割草样地和弃耕样地。主要的研究结果有： （1）不同样地间种子雨的密度差异显著（p < 0.05）。围封样地的种子雨密度最大，弃耕样地和放牧样地的种子雨密度居中，割草样地的种子雨密度最小。 （2）弃耕样地的地上植被和种子雨中均存在大量的一年生植物，这可能需要放牧或者割草等其它的一些管理措施来去除一年生植物的优势，从而加快弃耕地向天然草原的恢复速度。 （3）围封样地和割草样地的种子雨和地上植被中的物种数目均比放牧样地的要少，且围封样地中多年生非禾草类植物的比例大大增加，说明禁牧会改变草原植被的物种组成，这些究竟给草原植被的恢复带来怎样的影响，需要进一步的研究。 （4）克氏针茅草原整个取样期间种子雨的降落是连续的，因此在一些长期土壤种子库的研究中应该结合种子雨的数据，不能简单的将7月份的土壤样品作为长期土壤种子库。 采用网筛分离法，本研究测定了不同留茬高度的刈割实验样地的土壤种子库密度和物种组成。主要研究结果有： （1）无割草处理的对照样地的土壤种子库密度与其它割草处理样地的土壤种子库密度之间差异不显著。可能的原因是刈割的实验小区过小，植物种子的散布能够从不割草样地传播到割草处理的样地，或者动物的采食以及搬运降低了对照样地中种子的数量。 （2）虽然多年生禾草类植物在地上植被中占优势地位，但其在土壤种子库中的密度很小，均不到土壤种子库总量的10%。可见无性繁殖对于保持多年生禾草类植物在植被中的优势地位具有重要的意义，而相反的是非禾草类植物在土壤中保存大量的种子作为种群扩大的重要手段。

草地植被恢复对次降雨土壤水分动态的影响分析

通过对宁南典型草原植被恢复过程次降雨土壤水分动态的研究,阐明植被恢复对次降雨后土壤水分的影响及机理。结果表明,次降雨提高了土壤含水率和贮水量,均表现出1 d>3 d>7 d。草地封育能够提高次降雨资源化效率,随封育时间延长,次降雨后0-60 cm土壤含水率和0-100 cm土壤贮水量不断提高。降雨对封育草地土壤水分的影响范围在100 cm土层内,100 cm以下不能得到降雨的补充。封育时间延长土壤水分活跃层加深,坡耕地仅为40 cm,封育17 a后达到60 cm。土壤持水能力越强,表层土壤饱和导水率越大,雨后1 d在0-100 cm土壤贮水量越大。地上生物量愈大,雨后1~7 d在0-200 cm土壤耗水量越大。

半干旱典型草原区封育草地土壤结构特征研究

以宁夏固原云雾山自然保护区典型草原为研究对象,采用空间序列代替时间序列的方法,以坡耕地为对照,对封育演替草地百里香(Thymus mongolicus Ronn.)、铁杆蒿(Artemisia sacrorum Ledeb.)、大针茅(Stipa grandis P.Smirn.)和本氏针茅(Stipa bungeana Trin.)群落0~10 cm表层土壤水稳性团聚体分布、孔隙度及土壤结构评价指标进行了研究和分析。结果表明:草地实施封育措施能明显改善土壤结构特征,随着草地植被自然演替,土壤的结构稳定性和孔隙状况逐步得到提高;在演替过程中,封育草地土壤的>0.25 mm水稳性团聚体含量(WSAC)、平均重量直径(MWD)、几何平均直径(GMD)和孔隙分形维数(Dp)逐渐增加,团聚体分形维数(Da)逐渐减少,说明植被演替能促进形成良好的土壤结构;同时,土壤结构影响因素随着草地植被演替过程表现出有机碳含量显著增加,容重显著降低,毛管孔隙度逐渐增大,非毛管孔隙度逐渐降低。本研究还比较了多项土壤结构评价指标,表明与MWD和GMD相比,指标WSAC(>0.25 mm)、Da及Dp能更好地反映出各封育草地群落之间土壤结...

Distribution of respiratory droplets in enclosed environments under different air distribution methods

Dispersion characteristics of respiratory droplets in indoor environments are of special interest in controlling transmission of airborne diseases. This study adopts an Eulerian method to investigate the spatial concentration distribution and temporal evolution of exhaled and sneezed/coughed droplets within the range of 1.0~10.0μm in an office room with three air distribution methods, i.e. mixing ventilation (MV), displacement ventilation (DV), and under-floor air distribution (UFAD). The diffusion, gravitational settling, and deposition mechanism of particulate matters are well accounted in the one-way coupling Eulerian approach. The simulation results find that exhaled droplets with diameters up to 10.0μm from normal respiration process are uniformly distributed in MV, while they are trapped in the breathing height by thermal stratifications in DV and UFAD, resulting in a high droplet concentration and a high exposure risk to other occupants. Sneezed/coughed droplets are diluted much slower in DV/UFAD than in MV. Low air speed in the breathing zone in DV/UFAD can lead to prolonged residence of droplets in the breathing zone.

Conduction-radiation effect on natural convection flow in a fluid-saturated non-Darcy porous medium enclosed by non-isothermal walls

The effect of conduction-convection-radiation on natural convection flow of Newtonian optically thick gray fluid, confined in a non-Darcian porous media square cavity is numerically studied. For the gray fluid consideration is given to Rosseland diffusion approximation. Further assuming that (i) the temperature of the left vertical wall is varying linearly with height, (ii) cooled right vertical and top walls and (iii) the bottom wall is uniformly-heated. The governing equations are solved using the Alternate Direct Implicit method together with the Successive Over Relaxation technique. The investigation of the effect of governing parameters namely the Forschheimer resistance (Γ), the Planck constant (Rd), and the temperature difference (Δ), on flow pattern and heat transfer characteristics has been carried out. It was seen that the reduction of flow and heat transfer occurs as the Forschheimer resistance is increased. On the other hand both the strength of flow and heat transfer increases as the temperature ratio, Δ, is increased.

A computer code for enclosed natural convection in the boundary layer regime

A computer code is developed for the numerical prediction of natural convection in rectangular two-dimensional cavities at high Rayleigh numbers. The governing equations are retained in the primitive variable form. The numerical method is based on finite differences and an ADI scheme. Convective terms may be approximated with either central or hybrid differencing for greater stability. A non-uniform grid distribution is possible for greater efficiency. The pressure is dealt with via a SIMPLE type algorithm and the use of a fast elliptic solver for the solenoidal velocity correction field significantly reduces computing times. Preliminary results indicate that the code is reasonably accurate, robust and fast compared with existing benchmarks and finite difference based codes, particularly at high Rayleigh numbers. Extension to three-dimensional problems and turbulence studies in similar geometries is readily possible and indicated.

A curve shortening flow rule for closed embedded plane curves with a prescribed rate of change in enclosed area

Motivated by a problem from fluid mechanics, we consider a generalization of the standard curve shortening flow problem for a closed embedded plane curve such that the area enclosed by the curve is forced to decrease at a prescribed rate. Using formal asymptotic and numerical techniques, we derive possible extinction shapes as the curve contracts to a point, dependent on the rate of decreasing area; we find there is a wider class of extinction shapes than for standard curve shortening, for which initially simple closed curves are always asymptotically circular. We also provide numerical evidence that self-intersection is possible for non-convex initial conditions, distinguishing between pinch-off and coalescence of the curve interior.

A symmetric solution for a cylindrical shell enclosed in an elastic casing

The problem of a long, thin circular cylindrical shell enclosed in an elastic casing and subjected to a ring of radial load on the inner rim is solved using the Love function for the casing in conjunction with Flügge shell theory. Numerical work has been done with a digital computer and the results for stress and displacement fields are given for various values of the shell geometry parameters and material constants.

Assessing resilience and state-transition models with historical records of cheatgrass Bromus tectorum invasion in North American sagebrush-steppe

1. Resilience-based approaches are increasingly being called upon to inform ecosystem management, particularly in arid and semi-arid regions. This requires management frameworks that can assess ecosystem dynamics, both within and between alternative states, at relevant time scales. 2. We analysed long-term vegetation records from two representative sites in the North American sagebrush-steppe ecosystem, spanning nine decades, to determine if empirical patterns were consistent with resilience theory, and to determine if cheatgrass Bromus tectorum invasion led to thresholds as currently envisioned by expert-based state-and-transition models (STM). These data span the entire history of cheatgrass invasion at these sites and provide a unique opportunity to assess the impacts of biotic invasion on ecosystem resilience. 3. We used univariate and multivariate statistical tools to identify unique plant communities and document the magnitude, frequency and directionality of community transitions through time. Community transitions were characterized by 37-47% dissimilarity in species composition, they were not evenly distributed through time, their frequency was not correlated with precipitation, and they could not be readily attributed to fire or grazing. Instead, at both sites, the majority of community transitions occurred within an 8-10year period of increasing cheatgrass density, became infrequent after cheatgrass density peaked, and thereafter transition frequency declined. 4. Greater cheatgrass density, replacement of native species and indication of asymmetry in community transitions suggest that thresholds may have been exceeded in response to cheatgrass invasion at one site (more arid), but not at the other site (less arid). Asymmetry in the direction of community transitions also identified communities that were at-risk' of cheatgrass invasion, as well as potential restoration pathways for recovery of pre-invasion states. 5. Synthesis and applications. These results illustrate the complexities associated with threshold identification, and indicate that criteria describing the frequency, magnitude, directionality and temporal scale of community transitions may provide greater insight into resilience theory and its application for ecosystem management. These criteria are likely to vary across biogeographic regions that are susceptible to cheatgrass invasion, and necessitate more in-depth assessments of thresholds and alternative states, than currently available.