水蚀风蚀交错带黑沙蒿灌丛斑块种群结构及动态特征研究

黑沙蒿(Artemisia ordosica)作为黄土高原水蚀风蚀交错带沙地植被演替的先锋植物,对维护该地区生态系统的稳定性起到重要的作用。该研究以黄土高原水蚀风蚀交错带黑沙蒿灌丛斑块作为研究对象,通过对灌丛斑块的样地调查和数据统计,编制了黑沙蒿灌丛斑块的静态生命表,绘制其存活曲线和生存分析函数的曲线,并采用时间序列模型对种群的数量动态进行预测,以揭示水蚀风蚀交错带黑沙蒿灌丛斑块种群结构和动态特征。结果表明:(1)黑沙蒿灌丛斑块数量结构属于增长型,但Ⅰ、Ⅱ级灌丛数目少于Ⅲ级数目。(2)数量变化动态指数(V′pi)趋于零,说明灌丛斑块对外界干扰比较敏感。(3)存活曲线趋于Deevey-Ⅱ,表明黑沙蒿斑块种群各大小级的死亡率基本接近。(4)生存分析和时间序列预测表明,黑沙蒿小灌丛斑块个体的缺乏是未来中灌丛斑块数量衰减的主要原因。研究认为,黄土高原黑沙蒿灌丛斑块在研究区内生长更新良好,能够适应水蚀风蚀交错带的气候环境,种群处于稳定增长状态,为其他植物种群的定居和恢复创造了水养生境条件。     

北京东灵山地区景观格局及破碎化评价

 人类活动已将北京东灵山地区地带性原生暖温带落叶阔叶林破坏殆尽。本项研究基于植被图,在地理信息系统(GIS)支持下,选择多种景观格局指数,从斑块面积、斑块数、斑块周长、分形维数和多样性的角度进行了格局分析和破碎化评价。结果表明,该区景观类型以次生林和灌丛为主,面积分布极不均衡,且多呈小面积零散分布。各森林类型的分布面积、周长和斑块数一般小于灌丛、草地和农田,且边界密度和斑块密度较高,显示出较高程度的破碎化。各景观类型的平均斑块面积和周长遵从分形规律,分形维数1.30,景观整体的斑块边界的褶皱程度较低。景观多样性的Shannon指数,景观组分的类型面积(2.262)<类型周长(2.435)<类型斑块数(2.675),均小于等概率情形(2.940)。上述结果均显示,该区森林景观破碎化程度较高。     

中国东北样带生物群区及其对全球气候变化响应的初步探讨

 本文应用室内模拟与野外实地调查相结合的方法,对中国东北森林草原样带上生物群区及其过渡带的位置与面积进行了判定,并预测了未来全球变化条件下生物群区及过渡带可能的变化趋势。结果表明：中国东北样带内的生物群区和过渡带可由基于气候数据和Ho1dridge方法的数量模型较确切地判定出,与实际调查的结果吻合较好。在未来气候变化下,过渡带面积呈扩大化趋势;森林区对降水量的变化反应敏感;草原区地理分布位置变动很大,但其相对面积变化较小;荒漠灌丛对全球变化的反应最为剧烈。     

落叶松毛虫对大兴安岭呼中林区森林的景观长期影响模拟

应用空间直观景观模型(LANDIS)模拟了落叶松毛虫对呼中林区森林景观的长期影响,利用统计软件APACK计算了落叶松毛虫、代表性树种的分布面积以及反映物种分布格局的聚集度指数和森林斑块的平均面积,模拟了300年(1990—2290年)内有无落叶松毛虫干扰预案下大兴安岭呼中林区森林景观的动态变化.结果表明:研究区落叶松毛虫的分布面积呈先增加后降低的趋势;在落叶松毛虫干扰预案下,落叶松在模拟前150年的分布面积、平均斑块面积均低于无干扰预案,聚集度指数在前190年低于无干扰预案;干扰预案下白桦的分布面积和平均斑块面积百分比均高于无干扰预案,聚集度指数只在模拟的80～190年高于无干扰预案;樟子松的分布面积、聚集度指数和平均斑块面积在干扰预案下略低于无干扰预案.落叶松毛虫在一定程度上导致森林景观的破碎化.     

内蒙古典型草原小叶锦鸡儿灌丛化对水分再分配和利用的影响

灌丛化是全球草原地区存在的主要环境问题。通过对内蒙古典型草原区小叶锦鸡儿灌丛和草地斑块冠层降雨再分配、地表径流、土壤含水量的对比观测,研究了小叶锦鸡儿灌丛化对该区水分再分配和利用的影响。结果表明,灌丛和草地斑块的冠层截留量分别占降雨量的20.86%和7.88%,灌丛和草地斑块的平均地表径流系数分别为5.95%和17.19%。土壤含水量观测结果显示,0—60 cm土层中,降雨事件过程中,灌丛斑块较草地斑块能捕获更多水分,灌丛斑块植被冠层下方土壤含水量高于草地斑块;而在雨后无有效降水补充土壤水分的前提下,0—60 cm土层中,灌丛斑块土壤水分蒸散发量高于草地斑块,其中0—10cm土层中灌丛斑块土壤水分蒸散发速率低于草地斑块,10—60 cm土层中灌丛斑块土壤水分蒸散发速率高于草地斑块。研究认为,在水分为关键性限制因子的干旱半干旱区,小叶锦鸡儿灌丛化过程增加草原生态系统中水分分布的空间异质性,灌丛斑块能捕获、利用更多水分以维持更多的生物量。     

黑河中游湿地景观破碎化与气候变化的关系

在遥感和GIS技术支持下,基于1975-2010年长时间序列遥感影像和气象数据资料,结合小波分析、趋势面模拟与空间插值法,选取斑块密度(PD)、斑块平均面积(MPS)、斑块形状破碎化指数(FS)等景观指数,分析了黑河中游的气候空间分布和变化特征,以及该区域湿地的景观破碎化;通过相关分析和多元逐步回归分析,探讨了黑河中游湿地景观破碎化与气候变化的关系.结果表明:1975-2010年,研究区降水量和气温的整体分布格局为高气温区低降水量、低气温区高降水量,且干转湿和冷转暖为其主要变化特征;内陆干旱地区气温对湿地动态变化的影响大于降水,降水对湿地的生态作用甚微;研究区湿地景观破碎化主要表现为MPS减小、PD上升以及FS增大,研究期间,湿地MPS减少48.95 hm2,PD上升0.006个·hm-2.     

西藏拉萨河流域中下游不同植被类型表土花粉组合特征与年均降水量关系的研究

为定量恢复西藏拉萨河流域全新世的古气候环境,作者对拉萨河流域中下游表土孢粉组合特征与植被以及降雨量分布的相关性进行了研究。表土花粉研究结果显示,拉萨河流域中下游表土花粉组合能较好反映样方植被群落特征。根据研究区域孢粉组合特征,拉萨河流域中下游主要划分为8个孢粉组合带,即高山沼泽草甸带、高山灌丛草甸带、亚高山灌丛带、桦木林带、河谷沙棘林带、亚高山草甸带、亚高山半荒漠灌丛带和亚高山荒漠带。花粉类型与年均降水量的相关性分析结果表明,乔灌木花粉含量、Cy/G+A比值、莎草科、蓼属均与年均降水量呈显著正相关,而草本花粉含量、禾本科、蒿属、藜科与年均降水量呈显著负相关。因此,年均降水量是影响拉萨河中下游植被类型和表土花粉组合特征的重要因素。     

气候变化驱动下黄土高原刺槐林气候适宜性和脆弱性

刺槐是黄土高原乡土树种,具有优良的水土保持和固碳功能。黄土高原生态恢复实践中实施了大规模的植树造林,刺槐林面积占沟壑丘陵区人工植树造林面积90%以上。由于种植时没有考虑刺槐的气候适宜性,一些地区的刺槐林出现了退化现象。采用最大熵模型,在0.5km×0.5km空间精度上分别模拟了1961—1990、1966—1995、1971—2000、1976—2005、1981—2010,以及2100年(典型浓度路径RCP4.5和RCP8.5气候情景下)黄土高原刺槐的气候适宜性和敏感性。模拟结果显示：黄土高原刺槐分布及其动态变化主要受到最冷月温度、极端低温、降水量、年辐射量等气候因子影响,低温(最冷月温度、极端低温)是影响刺槐的最关键因子。黄土高原西北和北部广大地区,自然环境条件不适合刺槐林生长;黄土高原东南部(关中平原和山西南部)比较适合刺槐生长。相对1961—1990年,1961—2010年期间刺槐林适宜区分布格局基本没有改变,RCP4.5和RCP8.5气候情景下刺槐林适宜区分布格局也没有显著改变。图层叠加分析发现,刺槐的气候适宜度(即存在概率)发生了明显改变。黄土高原西部和北部属于不适宜刺槐生...     

河北小五台山主要植被类型的分布与地形的关系：基于遥感信息的分析

小五台山是河北省最高峰,具有较完整的暖温带植被垂直带谱。本文通过野外群落植被调查,结合由TM遥感影像解译1：50000地形图制作的DEM数据而得到的植被图,分析了小五台山主要植被群落的分布状况及其与地形的关系。结果表明,小五台山从低海拔到高海拔,依次出现灌丛、阔叶林、针阔混交林、暗针叶林、矮林、灌丛、草甸等植被类型。森林景观的面积最大,占总面积的60．7％;平均斑块面积最大的是阔叶林和亚高山草甸,斑块破碎化最严重的是水体和针阔混交林。不同的植被类型在不同的地形条件下分布特点不同,林地在北坡所占面积比例(占总面积的75％)远大于南坡(46%),说明水分条件是小五台山森林分布的限制因子之一;灌丛和草甸与林地相反。随海拔高度的变化,不同植被类型的分布面积比例发生变化,出现替代现象,形成植被垂直带谱。由于坡度的影响,增加了带谱内植被组成的复杂性。     

基于MaxEnt模型不同气候变化情景下的豆梨潜在地理分布

针对豆梨的原生境保护和资源利用问题,本研究基于豆梨全球236个分布点和19个环境因子,利用最大熵模型(MaxEnt)和地理信息系统(GIS)预测了豆梨在不同气候条件下的全球生态适宜区.结果表明: 豆梨的生态适宜区主要集中在北美洲、亚洲等地区,面积共约1.6×10⁷ km².其中,中国生态适宜度较高的地区主要分布在湖南省、湖北省、安徽省、江西省、江苏省、浙江省、福建省等地.影响豆梨地理分布的主要气候因子是年平均气温和年降水量,气温季节性变化次之.由模型预测可知,在不同的气候背景下,豆梨适宜生境和低适宜生境的面积有所不同.在空间分布上,豆梨适宜生境和低适宜生境的范围和几何中心都由东部向西部地区扩散,北美洲的适宜生境增长较快,而欧洲地区的低适宜生境增长较快.     

Shrub encroachment effect on the evapotranspiration and its component-A numerical simulation study of a shrub encroachment grassland in Nei Mongol,China北大核心CSCD

Aims Shrub encroachment is a common global change phenomenon occurring in arid and semi-arid regions. Due to the difficulty of partitioning evapotranspiration into shrub plants, grass plants and soil in the field, there are few studies focusing on shrub encroachment effect on the evapotranspiration and its component in China. This study aims to illustrate shrub encroachment effect on evapotranspiration by the numerical modeling method. Methods A two-source model was applied and calibrated with the measured evapotranspiration (ET) by the Bowen ratio system to simulate evapotranspiration and its component in a shrub encroachment grassland in Nei Mongol, China. Based on the calibrated model and previous shrub encroachment investigation, we set three scenarios of shrub encroachment characterized by relative shrub coverage of 5%, 15% and 30%, respectively, and quantified their effects caused by shrub encroachment through localized and calibrated two-source model. Important findings The two-source model can well reconstruct the evapotranspiration characteristics of a shrub encroachment grassland. Sensitivity analysis of the model shows that errors for the input variables and parameters have small influence on the result of partitioning evapotranspiration. The result shows that shrub encroachment has relatively small influence on the total amount of ET, but it has clear influence on the proportion of the components of evapotranspiration (E/ET). With shrub coverage increasing from 5% to 15% and then 30%, the evapotranspiration decreased from 182.97 to 180.38 and 176.72 W·m-2, decreasing amplitude values of 0.34% and 0.44%, respectively. On average, E/ET rises from 52.9% to 53.9% and 55.5%, increasing amplitude values to 2.04% and 3.25%. Data analysis indicates that shrub encroachment results in smaller soil moisture changes, but clear changes of ecosystem structure (decreasing ecosystem leaf area index while increasing vegetation height) which lead to the decrease of transpiration fraction through decreasing canopy conductance. The research highlights that, with the shrub encroachment, more water will be consumed as soil evaporation which is often regarded as invalid part of evapotranspiration and thus resulting in the decrease of water use efficiency.     

The effect of plant litter on ecosystem properties in a Mediterranean semi‐arid shrubland

Abstract. We studied the distribution of litter in a shrubland of the Negev with a semi‐arid Mediterranean climate of less than 200 mm of rainfall per year. Our focus was on the effects of litter on properties of landscape patches relevant to ecosystem processes (water runoff and soil erosion), annual plant community responses (seedling density, biomass production and species richness), and animal activity (soil disturbance by termites). Three 60‐m transects, extending across a pair of opposing north‐ and south‐facing slopes and their drainage channel, showed that litter accumulates not only under shrubs, but to a lesser extent also on the crusted inter‐shrub open areas. We used 35 experimental units (‘cells’, 0.5m × 1 m), each containing a crust and a shrub patch. Because runoff flows from crusted patches and is intercepted by shrub patches, the latter were in the lower third of the cells. Leaf litter was added in single and double amounts providing ca. 0.5 and 1.0 cm litter depth, to either, both, or none of the patches. Litter addition significantly decreased the amount of runoff, regardless of the location and amount of litter applied. Litter on the crust increased species number and seedling density of species with low abundance. Adding a double litter layer increased annual plant biomass production, while a single amount had no effect. Litter addition to the shrub patch affected neither biomass nor species richness. Litter addition to both patches at both quantities caused a large increase in termite activity. Termites caused disturbance by disrupting the crust, which may contribute to the reduction in runoff amounts. In the open, flat crust patches, annual plant communities are limited in their productivity and species richness, as there are few structures stopping the outflow of water, soil and seeds. Litter adds such structures, but affects the plant communities only when added to litter‐free crust. Litter accumulation and its patchy distribution have large impacts on landscape patch properties affecting resource distribution, plant productivity and diversity, and animal activity. Therefore, understanding litter distribution in relation to the patchy structure of the landscape of semi‐arid shrubland should be viewed as an important component of shrubland management.     

The use of branch piles to assist in the restoration of degraded semiarid steppes

Desertification is a major environmental problem in arid and semiarid regions. Tree plantation has been commonly employed to foster the recovery of degraded areas. However, this technique is costly, and their outcomes are often uncertain. Therefore, we evaluated an alternative method for the restoration of degraded semiarid steppes that involved the construction of branch piles to attract frugivores as potential seed‐dispersing birds, promoting seed rain, and fostering the formation of woody patches. We measured the success of branch piles in terms of the number of bird visits and seed input compared to naturally occurring shrub patches. Generally, frugivorous birds visited branch piles less frequently than shrub patches. Yet, branch piles accumulated seeds of patch‐forming shrub species. Seed rain was higher under patches of the dominant shrub Rhamnus lycioides than under branch piles. In contrast, woody patches and branch piles did not differ in seed input of the less abundant Pistacia lentiscus shrub. Our study demonstrates that branch piles are used by frugivorous birds and accumulate seeds of patch‐forming shrubs. Branch piles may be a suitable method to promote the expansion of bird‐dispersed plant species and restore semiarid wooded steppes. However, their efficiency largely depends on pile persistence and economic cost.     

新疆北部草地典型灌木的碳氮特征

灌木在维持干旱半干旱区生态系统稳定性方面发挥着重要的作用。该研究调查了新疆北部草地典型灌木物种, 并分析了这些灌木叶、枝、茎的碳(C)、氮(N)含量特征, 可为新疆草地植被碳氮储量的准确估算以及碳氮循环过程提供基础数据。结果表明: 北疆地区草地的典型灌木有白刺(Nitraria spp.)、刺旋花(Convolvulus tragacanthoides)、红砂(Reaumuria soongarica)、假木贼(Anabasis spp.)、 锦鸡儿(Caragana spp.)、麻黄(Ephedra spp.)、沙拐枣(Calligonum mongolicum)、梭梭(Haloxylon spp.)、金丝桃叶绣线菊(Spiraea hypericifolia)、驼绒藜(Krascheninnikovia spp.)、小蓬(Nanophyton erinaceum)、盐爪爪(Kalidium spp.)、猪毛菜(Salsola spp.)等, 归属于蔷薇科、豆科、麻黄科、柽柳科、蒺藜科、蓼科、旋花科、藜科。北疆草地典型灌木各器官C含量为茎(45.76 ± 3.43)% >枝(44.27 ± 4.51)% >叶(39.15 ± 5.91)%, N含量为叶(2.21 ± 0.59)% >枝(1.55 ± 0.44)% >茎(1.34 ± 0.35)%, C:N为茎(36.74 ± 10.80) >枝(31.07 ± 10.43) >叶(18.94 ± 5.82)。灌木地上部分C含量为(43.77 ± 4.43)%, N含量为(1.56 ± 0.45)%, C:N为(31.78 ± 10.12); C含量变异程度较小, 变异系数为10%, N含量的变异系数为27%, C:N的变异系数为32%。灌木同一器官的C、N含量及C:N在不同科间有显著差异, 蔷薇科各器官的C含量显著大于其他科(p < 0.05); 豆科叶和茎的N含量显著大于其他科(p < 0.05), 麻黄科枝的N含量显著大于其他科(p < 0.05); 旋花科叶和枝的C:N显著大于其他科(p < 0.05), 蔷薇科茎的C:N显著大于其他科(p < 0.05)。     

Carbon and nitrogen traits of typical shrubs in grassland of northern Xinjiang,China

灌木在维持干旱半干旱区生态系统稳定性方面发挥着重要的作用。该研究调查了新疆北部草地典型灌木物种, 并分析了这些灌木叶、枝、茎的碳(C)、氮(N)含量特征, 可为新疆草地植被碳氮储量的准确估算以及碳氮循环过程提供基础数据。结果表明: 北疆地区草地的典型灌木有白刺(Nitraria spp.)、刺旋花(Convolvulus tragacanthoides)、红砂(Reaumuria soongarica)、假木贼(Anabasis spp.)、 锦鸡儿(Caragana spp.)、麻黄(Ephedra spp.)、沙拐枣(Calligonum mongolicum)、梭梭(Haloxylon spp.)、金丝桃叶绣线菊(Spiraea hypericifolia)、驼绒藜(Krascheninnikovia spp.)、小蓬(Nanophyton erinaceum)、盐爪爪(Kalidium spp.)、猪毛菜(Salsola spp.)等, 归属于蔷薇科、豆科、麻黄科、柽柳科、蒺藜科、蓼科、旋花科、藜科。北疆草地典型灌木各器官C含量为茎(45.76 ± 3.43)% >枝(44.27 ± 4.51)% >叶(39.15 ± 5.91)%, N含量为叶(2.21 ± 0.59)% >枝(1.55 ± 0.44)% >茎(1.34 ± 0.35)%, C:N为茎(36.74 ± 10.80) >枝(31.07 ± 10.43) >叶(18.94 ± 5.82)。灌木地上部分C含量为(43.77 ± 4.43)%, N含量为(1.56 ± 0.45)%, C:N为(31.78 ± 10.12); C含量变异程度较小, 变异系数为10%, N含量的变异系数为27%, C:N的变异系数为32%。灌木同一器官的C、N含量及C:N在不同科间有显著差异, 蔷薇科各器官的C含量显著大于其他科(p < 0.05); 豆科叶和茎的N含量显著大于其他科(p < 0.05), 麻黄科枝的N含量显著大于其他科(p < 0.05); 旋花科叶和枝的C:N显著大于其他科(p < 0.05), 蔷薇科茎的C:N显著大于其他科(p < 0.05)。     

Harvester ant response to spatial and temporal heterogeneity in seed availability: pattern in the process of granivory

The influence of temporal and spatial heterogeneity in seed availability on the foraging behaviour of the harvester ant Messor arenarius was studied in an arid shrubland in the Negev Desert, Israel. The study investigated the implications of behavioural responses to heterogeneity in seed availability for the seed predation process and the potential for feedback effects on vegetation. Vegetation and seed rain were monitored across two landscape patch types (shrub patches and inter-shrub patches) in 1997. Shrub patches were shown to have higher plant and seed-rain density than inter-shrub patches. Patch use and seed selection by M. arenarius foragers were monitored through the spring, summer and autumn of 1997. After a pulse of seed production in the spring, the ants exhibited very narrow diet breadth, specialising on a single annual grass species, Stipa capensis. At this time, ants were foraging and collecting seeds mainly from inter-shrub patches. In the summer, diet breadth broadened and use of shrub patches increased, although the rate of seed collection per unit area was approximately equal in the two patch types. The increase in the use of shrub patches was due to colony-level selection of foraging areas with relatively high shrub cover and an increase in the use of shrub patches by individual foragers. In the autumn, a pulse of seed production by the shrub species Atractylis serratuloides and Noaea mucronata led to a reduction in diet breadth as foragers specialised on these species. During this period, foragers exhibited a large increase in the proportion of time spent in shrub patches and in the proportion of food items collected from shrub patches. The seasonal patterns in foraging behaviour showed linked changes in seed selection and patch use resulting in important differences in the seed predation process between the two landscape patch types. For much of the study period, there was higher seed predation pressure on the inter-shrub patches, which were of relatively low productivity compared with the shrub patches. This suggests that the seed predation process may help maintain the spatial heterogeneity in the density of ephemeral plants in the landscape.     

Role of Salix reinii patches in spatio-temporal patterns of cohabitants on a Japanese volcano

深山柳斑块在日本火山共栖物种时空格局中的作用 灌木斑块通常能为定植物种提供支持，尤其是在具有压力和/或干扰的环境中，这种作用更明显。山地上的压力和干扰强度通常与海拔梯度有关，并会因灌木斑块(树冠和凋落物)而降低。本研究旨在利用火山上不同海拔高度的灌木斑块来阐明这些关系。从2005年早春到2006年夏季，我们在日本北部高丽山(海拔1131 m)的3个海拔高度监测了深山柳(Salix reinii)斑块内外所有地块中的所有嫩枝及其相关 环境，例如温度、光照、湿度和化学成分。我们通过面积、分枝密度和凋落物厚度对斑块结构进行评价。研究结果表明，灌木斑块的结构在不同海拔之间并无差异。通过在整体上增加植物物种的丰富度，这些斑块提高了稀有物种的多样性。斑块内的水分、氮和磷含量均要高于斑块外。灌木斑块对于共栖物种的影响总体上是正向的，但对幼苗丰度、嫩枝存活率和开花的影响则是负面的。这些结果表明灌木斑块的生长格局决定了植物群落结构，并促成了植物的多样性。     

腾格里沙漠东南缘沙质草地灌丛化对地表径流及氮流失的影响

以腾格里沙漠东南缘沙质草地和灌丛生境为研究对象,采用人工模拟降雨实验对草地样方、灌丛间裸地样方及含灌丛斑块样方的产流及氮流失过程进行观测,揭示了地表径流及其引起的氮流失对沙质草地灌丛化的响应.结果表明:(1)草地样方出现表面积水和地表径流的时间及开始产流需要的降雨量均大于含灌丛斑块样方和灌丛间裸地样方,裸地样方最小;灌丛生境径流系数为34.46％,显著小于裸地样方,大于含灌丛样方,产流量是沙质草地生境的2.26倍;表明灌木入侵造成的植被盖度下降引起了土壤水分入渗率的减小和地表产流的增加.(2)含灌丛样方单位体积径流含氮量瞬时值大于裸地样方,小于草地样方,三类样方瞬时值与单位时间径流量均呈线性负相关;草地样方单位时间氮流失量略小于含灌丛样方,两者均显著小于灌丛间裸地样方;灌丛生境氮流失总量为0.23 g/m2,是草地生境的2.09倍,灌丛和草地生境单位体积径流含氮量总体平均值分别为0.011 g/L、0.012 g/L;表明沙质草地灌丛化引起了养分流失的显著增加.     

半干旱地区天然草地灌丛化与土壤异质性关系研究进展

 回顾了半干旱地区天然草地灌丛化的成因和机理、灌丛化导致草地土壤水分和养分空间异质性及其对生态系统生物地球化学过程的影响,以及土壤异质性与土地退化关系等方面的研究进展,周期性气候干旱和过度放牧是天然草地灌丛化的主要原因,伴随灌木入侵而出现的草地土壤水分和养分的空间异质性,是造成生态系统水分和养分流失,以及土壤加速侵蚀的原因之一。因此,半干旱地区天然草地的灌丛化应得到一定的控制,使群落中灌丛保持适宜的密度,以避免生态系统水分和养分的损失。