Can Regeneration of Green Ash (Fraxinus pennsylvanica) be Restored in Declining Woodlands in Eastern Montana?

Green ash (Fraxinus pennsylvanica Marsh.) dominates many deciduous woodlands at the western margin of its range in eastern Montana. Evidence suggests that the majority of green ash woodlands are in degraded condition with declining tree canopies and ground layers dominated by exotic grasses. The dense sod formed by these perennial grasses was hypothesized to interfere with green ash regeneration from seed. The purpose of this study was to test potential methods of restoring green ash regeneration in these declining woodlands. The effects of preseeding grazing and herbicide treatment and postgermination fertilizer on the recruitment, survival, and growth of green ash seedlings at each of four study sites typical of declining green ash woodlands in southeastern Montana were assessed. Six green ash trees at each of three sites were cut to examine the relationship of age, size, and health to sprouting ability and growth. Herbicide application had a positive effect on green ash recruitment and survival of green ash seedlings in woodlands with a dense sod of exotic grasses; seedling survival after 3–4 yr was ca. 10 times greater in herbicide-treated plots compared to controls or grazed plots. Seedlings grew slowly although fertilizer had a small positive effect on growth at one site. All coppiced trees produced basal sprouts, but sprout growth was severely curtailed at two of the three sites by deer browsing, suggesting that coppicing could increase tree canopy cover by replacing weakened trees with new and more vigorous boles and branches, but only where browsing by cattle and deer is reduced. Maintaining eastern Montana green ash woodlands in good condition should be given priority because restoration will be difficult.     

The long-term effect of fire and grazing by wildlife on range condition in moist and arid savannas in the Kruger National Park

Few assessments of the effects of fire and grazing on the herbaceous components of savannas have been reported for Africa. In the Kruger National Park, South Africa, range condition was monitored at three savanna sites spanning a rainfall gradient of 450 to 700 mm and subject to grazing by wildlife. The sites were burnt at 1-, 2- and 3-year intervals, in different seasons and included a control treatment excluding fire for >40 years. Initial surveys of the herbaceous vegetation were conducted between 1954 and 1957 and repeated between 1998 and 2001. Grass species were categorised as either decreaser or increaser species, which decline or increase with over- or under-utilisation, respectively, and their potential to provide forage and fuel for fires indicative of range condition were derived. Range condition improved with regular burning in the moist sites, but deteriorated in the arid sites over all seasons and frequencies of burning. It is concluded that the current homogenous approach to burning across the Park needs to be adapted by reducing the frequency of fires at more arid sites and can be achieved by burning according to the proportions of decreaser and increaser species reflecting the ecological status of the herbaceous vegetation.     

放牧对天山北坡中段草原群落结构和功能群生产力的影响

通过对天山北坡中段典型羊茅草原的动态监测,分析了放牧对草原群落结构和功能群生产力的影响。结果表明：放牧不能改变草原植物生活型功能群的组成结构,且多年生禾草占据主导地位,放牧与不放牧条件下所占比重依次为30．74％～44．999,5、30．819／6～35．71％;中度放牧能够增加草原植物种数,但不能改变羊茅、针茅、草原苔草的优势地位,其优势度分别为11．64％～19．40％,11．05％～19．70％和11．65％～23．79％;植物生活型功能群组成中,半灌木、灌木与多年生禾草、杂类草,多年生豆科草与杂类草间植物量显著正相关,生态互补效应显著。     

Responses of Chaparral and Oak Woodland Plant Communities to Fuel-Reduction Thinning in Southwestern Oregon

Fire suppression has led to large fuel accumulations in many regions of the United States. In response to concerns about associated wildfire hazards, land managers in the western United States are carrying out extensive fuel-reduction thinning programs. Although reductions in cover by woody vegetation seem likely to cause changes in herbaceous communities, few published studies have reported on consequences of such treatments for native or exotic plant species. We compared vegetation and abiotic characteristics between paired thinned and unthinned chaparral and oak woodland communities of southwestern Oregon 4–7 yr posttreatment and contrasted impacts of manual vs. mechanical treatments. Herbaceous cover increased on thinned sites, but species richness did not change. Herbaceous communities at thinned sites had an early postdisturbance type of composition dominated by native annual forbs and exotic annual grasses; cover by annuals was nearly twice as high on treated as on untreated sites. Absolute and proportional cover of native annual forbs increased more than any other trait group, whereas exotic annual forbs and native perennial forbs declined. Exotic annual grass cover (absolute and proportional) increased, whereas cover by native perennial grasses did not. Shrub reestablishment was sparse after thinning, probably because of a lack of fire-stimulated germination. Manual and mechanical treatment impacts on abiotic site conditions differed, but differences in vegetation impacts were not statistically significant. Fuel-reduction thinning may have some unintended negative impacts, including expansion of exotic grasses, reductions in native perennial species cover, persistent domination by annuals, and increased surface fuels. Coupled with sparse tree or shrub regeneration, these alterations suggest that ecological-state changes may occur in treated communities. Such changes might be mitigated by retaining more woody cover than is currently retained, seeding with native perennials after treatment, or other practices; further research is needed to inform management in these ecosystems.     

科尔沁沙质草地禾本科植物主要功能类型对连续放牧的反应

为了正确评价连续放牧对禾本科牧草的影响,在科尔沁典型沙质草地,设置不同放牧强度的4块样区,连续放牧4年后对各样区植被中禾本科植物的盖度和高度进行了样方调查,分析了3个禾本科植物功能类型.结果表明:1)禾本科植物的平均盖度对重牧比较敏感,而在中牧区、低牧区和对照区差异较小;2)3个功能类型植物的高度随放牧强度的增加,呈有规律的降低.而且多年生牧草的高度对放牧强度最敏感,一年生优质牧草次之,一年生劣性牧草最不敏感;3)多年生牧草在各样区的相对盖度随放牧强度的增加,逐渐减少,在重牧区仅为0.69%;一年生优质牧草在各样区的相对盖度表现为中牧区＞低牧区＞重牧区＞封育对照区,说明放牧干扰有提高此相对盖度的影响;一年生劣性牧草在各样区的相对盖度随放牧强度的增加,逐渐增大.     

Herbicide Protection Pods (HPPs) Facilitate Sagebrush and Bunchgrass Establishment under Imazapic Control of Exotic Annual Grasses

Revegetation of exotic annual grass−invaded rangelands is a primary objective of land managers following wildfires. Controlling invasive annual grasses is essential to increasing revegetation success; however, preemergent herbicides used to control annual grasses prohibit immediate seeding due to nontarget herbicide damage. Thus, seeding is often delayed 1 yr following herbicide application. This delay frequently allows for reinvasion of annual grasses, decreasing the success of revegetation efforts. Incorporating seeds into herbicide protection pods (HPPs) containing activated carbon (AC) permits concurrent high preemergent herbicide application and seeding because AC adsorbs and renders herbicides inactive. While HPPs have, largely in greenhouse studies, facilitated perennial bunchgrass emergence and early growth, their effectiveness in improving establishment of multiple species and functional groups in the field has not been assessed. Five bunchgrass species and two shrub species were seeded at two field sites with high imazapic application rates as bare seed and seed incorporated into HPPs. HPPs significantly improved establishment of sagebrush (Artemesia tridentata Nutt. Spp. wyomingensis Beetle & Young) and crested wheatgrass (Agropyron cristatum [L.] Gaertn.) over the 2-yr study. Three native perennial grass species were protected from herbicide damage by HPPs but had low establishment in both treatments. The two remaining shrub and grass species did not establish sufficiently to determine treatment effects. While establishment of native perennial bunchgrasses was low, this study demonstrates that HPPs can be used to protect seeded bunchgrasses and sagebrush from imazapic, prolonging establishment time in the absence of competition with annual grasses.     

Allelopathic Cover Crop Prior to Seeding Is More Important Than Subsequent Grazing/Mowing in Grassland Establishment

The effects of grazing, mowing, and type of cover crop were evaluated in a previous winter wheat–fallow cropland seeded to grassland under the Conservation Reserve Program in eastern Colorado. Prior to seeding, the fallow strips were planted to forage sorghum or wheat in alternating strips (cover crops), with no grazing, moderate to heavy grazing, and mowing (grazing treatments) superimposed 4 yr after planting and studied for 3 yr. Plots previously in wheat had more annual and exotic species than sorghum plots. Concomitantly, there were much greater abundances of perennial native grass and all native species in sorghum than wheat cropped areas. The competitive advantage gained by seeded species in sorghum plots resulted in large increases in rhizomatous western wheatgrass. Sorghum is known to be allelopathic and is used in crop agriculture rotations to suppress weeds and increase crop yields, consistent with the responses of weed and desired native species in this study. Grazing treatment had relatively minor effects on basal and canopy cover composition of annual or exotic species versus perennial native grass or native species. Although grazing treatment never was a significant main effect, it occasionally modified cover crop or year effects. Opportunistic grazing reduced exotic cheatgrass by year 3 but also decreased the native palatable western wheatgrass. Mowing was a less effective weed control practice than grazing. Vegetative basal cover and aboveground primary production varied primarily with year. Common management practices for revegetation/restoration currently use herbicides and mowing as weed control practices and restrict grazing in all stages of development. Results suggest that allelopathic cover crop selection and opportunistic grazing can be effective alternative grass establishment and weed control practices. Susceptibility, resistance, and interactions of weed and seeded species to allelopathic cover species/cultivars may be a fruitful area of research.     

Vegetation Response to Mowing Dense Mountain Big Sagebrush Stands

A decrease in fire frequency and past grazing practices has led to dense mountain big sagebrush (Artemisia tridentata Nutt. subsp. vaseyana [Rydb.] Beetle) stands with reduced herbaceous understories. To reverse this trend, sagebrush-reducing treatments often are applied with the goal of increasing herbaceous vegetation. Mechanical mowing is a sagebrush-reducing treatment that commonly is applied; however, information detailing vegetation responses to mowing treatments generally are lacking. Specifically, information is needed to determine whether projected increases in perennial grasses and forbs are realized and how exotic annual grasses respond to mowing treatments. To answer these questions, we evaluated vegetation responses to mowing treatments in mountain big sagebrush plant communities at eight sites. Mowing was implemented in the fall of 2007 and vegetation characteristics were measured for 3 yr post-treatment. In the first growing season post-treatment, there were few vegetation differences between the mowed treatment and untreated control (P > 0.05), other than sagebrush cover being reduced from 28% to 3% with mowing (P < 0.001). By the second growing season post-treatment, perennial grass, annual forb, and total herbaceous vegetation were generally greater in the mowed than control treatment (P < 0.05). Total herbaceous vegetation production was increased 1.7-fold and 1.5-fold with mowing in the second and third growing seasons, respectively (P < 0.001). However, not all plant functional groups increased with mowing. Perennial forbs and exotic annual grasses did not respond to the mowing treatment (P > 0.05). These results suggest that the abundance of sagebrush might not be the factor limiting some herbaceous plant functional groups, or they respond slowly to sagebrush-removing disturbances. However, this study suggests that mowing can be used to increase herbaceous vegetation and decrease sagebrush in some mountain big sagebrush plant communities without promoting exotic annual grass invasion.     

不同放牧强度对多年生黑麦草—白三叶草地群体密度的影响

在中、高放牧强下黑麦草分蘖密度略有下降，而在低放牧强度下则有所上升。白三叶枝条密度受放牧强度影响较小。秋季的重牧使黑麦草分蘖密度极显著下降（P＜0．01）。6月份随着放牧强度的增强黑麦草案死亡数显著减少（P＜0．05），草地群体密度亦随之减少（P＜0.05）。在秋季随着放牧强度的增大黑麦草单蘖重明显减少（P＜0．05）。黑麦草和白三叶的单菜（枝）重与其密度均呈强负相关（P＜0．01）。白三叶的单枝重与生物量呈正相关（P＜0．05）。以上分析表明，中等放牧强度（70％利用率）有利于保持合理的草地群体密度和较高的生物量。     

Long-Term Vegetation Trends on Grazed and Ungrazed Chihuahuan Desert Rangelands

Long-term information on the effects of managed grazing versus excluded grazing effects on vegetation composition of desert rangelands is limited. Our study objectives were to evaluate changes in frequency of vegetation components and ecological condition scores under managed livestock grazing and excluded livestock grazing over a 38-yr period at various locations in the Chihuahuan Desert of southwestern New Mexico. Sampling occurred in 1962, 1981, 1992, 1998, 1999, and 2000. Range sites of loamy (1), gravelly (2), sandy (2), and shallow sandy (2) soils were used as replications. Black grama (Bouteloua eriopoda Torr.) was the primary vegetation component at the seven locations. Dyksterhuis quantitative climax procedures were used to determine trends in plant frequency based on a 1.91-cm loop and rangeland ecological condition scores. Frequency measures of total perennial grass, black grama, tobosa (Hilaria mutica Buckley), total shrubs, honey mesquite (Prosopis glandulosa Torr.), and other vegetation components were similar on both grazed and ungrazed treatments (P > 0.1) at the beginning and end of the study. The amount of change in rangeland ecological condition scores was the same positive increase (14%) for both grazed and ungrazed treatments. Major changes (P < 0.1) occurred within this 38-yr study period in ecological condition scores and frequency of total perennial grasses and black grama in response to annual fluctuations in precipitation. Based on this research, managed livestock grazing and excluded livestock grazing had the same long-term effects on change in plant frequency and rangeland ecological condition; thus, it appears that managed livestock grazing is sustainable on Chihuahuan desert rangelands receiving over 25 cm annual precipitation.     

Appropriate Sample Sizes for Monitoring Burned Pastures in Sagebrush Steppe: How Many Plots are Enough,and Can One Size Fit All?

Statistically defensible information on vegetation conditions is needed to guide rangeland management decisions following disturbances such as wildfire, often for heterogeneous pastures. Here we evaluate sampling effort needed to achieve a robust statistical threshold using > 2 000 plots sampled on the 2015 Soda Fire that burned across 75 pastures and 113 000 ha in Idaho and Oregon. We predicted that the number of plots required to generate a threshold of standard error/mean ≤ 0.2 (TSR, threshold sampling requirement) for plant cover within pasture units would vary between sampling methods (rapid ocular versus grid-point intercept) and among plot sizes (1, 6, or 531 m²), as well as relative to topography, elevation, pasture size, spatial complexity of soils, vegetation treatments (herbicide or seeding), and dominance by exotic annual or perennial grasses. Sampling was adequate for determining exotic annual and perennial grass cover in about half of the pastures. A tradeoff in number versus size of plots sampled was apparent, whereby TSR was attainable with less area searched using smaller plot sizes (1 compared with 531 m²) in spite of less variability between larger plots. TSR for both grass types decreased as their dominance increased (0.5–1.5 plots per % cover increment). TSR decreased for perennial grass but increased for exotic annual grass with higher elevations. TSR increased with standard deviation of elevation for perennial grass sampled with grid-point intercept. Sampling effort could be more reliably predicted from landscape variables for the grid-point compared with the ocular sampling method. These findings suggest that adjusting the number and size of sample plots within a pasture or burn area using easily determined landscape variables could increase monitoring efficiency and effectiveness.     

Is the grazing tolerance of mesic decreaser and increaser grasses altered by soil nutrients and competition?

An ability to tolerate recurrent defoliation likely plays a role in the compositional shift from decreaser to increaser species with overgrazing of mesic grassland, but the grazing tolerance of local species has not been extensively studied. The growth response of two decreasers, three Increaser II grasses, and an Increaser III species to frequent, severe defoliation under three levels of competition from neighbours and two levels of soil nutrients was examined in a pot trial. The effects of competition and especially nutrients markedly modified the defoliation tolerance of different species, and grazing response groups varied in the manner in which their defoliation responses were mediated by these interactions. Contrary to expectation, defoliation constrained potential productivity most in nutrient-rich, competition-free environments, probably because it exacerbates a carbon limitation on growth. However, nutrient enrichment did enable Increaser II grasses, but not other species, to better tolerate intense defoliation, explaining why they can persist on intensively grazed fertile patches and in heavily stocked grassland, such as in communal areas, where nutrients are more available. It is concluded that grazing tolerance is not a fixed property of species or grazing response groups but depends on biotic and abiotic environmental factors.     

A study of animal production under different grazing regimes in the False Upper Karoo at Grootfontein

Three systems of animal production on the fragile Karoo veld, i.e. multi-paddock rotational grazing, pauci-paddock rotational grazing and continuous grazing were studied. Of these systems, multi-paddock rotational grazing gave the best results with continuous stocking resulting in the worst. All three systems studied had long histories of both continuous stocking- and of pauci-paddock rotational grazing. The veld, under both these previous systems of management has stabilised, but at different seral stages. The continuously stocked range stabilised in the early developmental seral stage, consisting mainly of annual grasses. The veld under pauci-paddock rotational management stabilised in the early to mid-developmental seral stage, consisting mainly of perennial grasses. Experimentation on veld in a stable condition is a prerequisite if valid conclusions in respect of rotational grazing and continuous stocking are to be drawn. Rotational grazing on overgrazed veld will result in poor animal performance, while it is generally accepted that continuous stocking on pristine veld will result in excellent animal performance for a number of years. Multi-paddock rotational grazing outperformed both pauci-paddock rotational grazing as well as continuous stocking on both types of veld. This is surprising taking into account the mid-seral stage condition of the vegetation. This supports the argument that with improved veld management, an improved production potential is possible on veld even in a degraded or current condition.     

不同放牧强度对多年生黑麦草─白三叶草地叶片数量特征的影响

不同放牧强度在同一季节对黑麦草和白三叶的叶片数及单位面积总叶量的影响均不显著,单枝叶片数也不显著。随着季节的变化黑麦草叶量减少而白三叶则增加,但草地总叶量呈减少趋势。在7～8月两种草的叶面积指数在中、低放牧强度下显著(P<005)高于高放牧强度。黑麦草的单叶面积受季节影响较大。叶面积指数与生物量的回归分析表明,白三叶在中、低放牧强度下呈强正相关(P<001)。在高放牧强度下则呈显著相关(P<005)。草地总叶面积指数与生物量在中等放牧强度下呈强正相关(P<0.01),这与此时黑麦草的叶面积指数与其生物量间相关显著(P<005)有关。     

Ventenata and Other Coexisting Exotic Annual Grass Control and Plant Community Response to Increasing Imazapic Application Rates

Ventenata (Ventenata dubia [Leers] Coss.) is an exotic annual grass that can invade intermountain rangeland plant communities, where it can form monotypic stands, degrade wildlife habitat, and reduce livestock forage. There is limited information on ventenata control in rangelands as it has only recently been identified as a substantial problem. Imazapic is a pre-emergent herbicide commonly used to control other exotic annual grasses and, therefore, is likely to control ventenata in rangelands. We evaluated five application rates of imazapic (0 − 175 g ae ∙ ha^− 1) on ventenata and other exotic annual grass control and plant community response at two rangeland sites in 2 yr (2014 and 2015). Imazapic reduced exotic annual grass (largely ventenata) cover and density, with greater control with increasing imazapic rates. Exotic annual grass density at the highest levels of control (82%−94%) was 184 − 299 plants ∙ m^− 2 the first yr after imazapic application. Exotic annual grasses fully recovered in the second or third yr after imazapic application. Bare ground generally increased with imazapic application. However, density of perennial vegetation (grasses and forbs) did not vary among treatments. Perennial vegetation cover generally did not increase with imazapic control of ventenata and other exotic annual grasses. Imazapic can control ventenata; however, even at the highest rates, control was not enough to shift the dominance from exotic annual species to perennial species. Integrating other treatments with imazapic application may be a strategy to improve ventenata control and increase perennial vegetation and will require further investigation. The difficulty and likely expense of achieving substantial and lasting control of ventenata suggest, similar to other exotic annual grasses, that preventing ventenata invasion and dominance should be a high management priority.     

Long-Term Effects of Weed Control With Picloram Along a Gradient of Spotted Knapweed Invasion

Broadleaf herbicides are commonly used in rangelands to suppress exotic weeds and release native communities from negative impacts of invasion. However, few studies have comprehensively evaluated treatment effects on differing community components across a gradient of initial invasion levels. We conducted a 6-yr experiment within grasslands of western Montana to measure local-scale effects of a broadcast application of picloram on 1) cover of the target invader, spotted knapweed (Centaurea stoebe L.), 2) prevalence of native functional groups, and 3) the secondary invader cheatgrass (Bromus tectorum L.) at differing initial levels of knapweed invasion. Treatment effectively suppressed knapweed, with cover in treated vs. control plots reduced by >60% in the sixth posttreatment year. Treatment also appeared to alleviate knapweed’s impacts on native perennial grasses, but only at the highest initial level of invasion, where cover of this group increased by >30% in treated vs. control plots to equal levels associated with noninvaded plots. In some cases, treatment appeared to exacerbate knapweed’s impacts on native forbs. At the no-invasion level, perennial forb cover declined by >20% in treated vs. control plots to match values associated with moderate or high levels of invasion, but these treatment effects were minimal at the latter invasion levels. Across initial invasion levels, species richness of perennial and/or annual forbs declined by >20% in treated vs. control plots. Treatment also promoted increases in cheatgrass cover, although differences between treated and control plots were relatively small by the sixth posttreatment year. Overall, effects of picloram application depended on initial levels of knapweed invasion, largely due to the varying strength of release effects. Selective treatment of invaded patches vs. broadcast applications would reduce side effects of broadleaf herbicide application and increase compatibility with other management measures designed to improve rangeland conditions and restore grassland communities.     

Soil Resources Influence Vegetation and Response to Fire and Fire-Surrogate Treatments in Sagebrush-Steppe Ecosystems

Current paradigm suggests that spatial and temporal competition for resources limit an exotic invader, cheatgrass (Bromus tectorum L.), which once established, alters fire regimes and can result in annual grass dominance in sagebrush steppe. Prescribed fire and fire surrogate treatments (mowing, tebuthiuron, and imazapic) are used to reduce woody fuels and increase resistance to exotic annuals, but may alter resource availability and inadvertently favor invasive species. We used four study sites within the Sagebrush Steppe Treatment Evaluation Project (SageSTEP) to evaluate 1) how vegetation and soil resources were affected by treatment, and 2) how soil resources influenced native herbaceous perennial and exotic annual grass cover before and following treatment. Treatments increased resin exchangeable NH₄⁺, NO₃^-, H₂PO₄^-, and K⁺, with the largest increases caused by prescribed fire and prolonged by application of imazapic. Burning with imazapic application also increased the number of wet growing degree days. Tebuthiuron and imazapic reduced exotic annual grass cover, but imazapic also reduced herbaceous perennial cover when used with prescribed fire. Native perennial herbaceous species cover was higher where mean annual precipitation and soil water resources were relatively high. Exotic annual grass cover was higher where resin exchangeable H₂PO₄^- was high and gaps between perennial plants were large. Prescribed fire, mowing, and tebuthiuron were successful at increasing perennial herbaceous cover, but the results were often ephemeral and inconsistent among sites. Locations with sandy soil, low mean annual precipitation, or low soil water holding capacity were more likely to experience increased exotic annual grass cover after treatment, and treatments that result in slow release of resources are needed on these sites. This is one of few studies that correlate abiotic variables to native and exotic species cover across a broad geographic setting, and that demonstrates how soil resources potentially influence the outcome of management treatments.     

行间生草种类对苹果园春季土壤蒸发、空气湿度和土壤贮水的影响

为明确行间生草种类对华北平原苹果园春季土壤蒸发、果树冠层空气湿度、土壤贮水的影响特征,揭示果园生草对土壤有效水分的影响规律,于2019-2021年在河北省武邑县武罗农业综合示范园区开展苹果树行间生草试验,采用田间定位观测方法研究了苹果树行间套种越年生牧草（二月兰、毛叶苕子、小黑麦）和多年生牧草（鸭茅、白三叶）后牧草群体叶面积指数（LAI）、光合有效辐射（PAR）透射率、土壤温度、水分蒸发、冠层空气相对湿度、土壤含水量、土壤贮水量的变化特征及相互影响。结果表明,2021年4-5月苹果树行间套种越年生牧草时土壤蒸发量大于多年生牧草,平均增加3.14 mm。越年生牧草切碎覆盖还田前后对土壤蒸发的影响不同。还田前,越年生牧草对土壤蒸发的抑制效果大于多年生牧草,抑制率平均增加20.5%,这与越年生牧草生长速度快,群体LAI大、PAR透射率低,0~25 cm土壤温度低和土壤含水量下降相关。还田之后,对土壤蒸发的抑制效果小于多年生牧草,抑制率平均降低29.0%。越年生牧草对果树冠层1 m高度空气相对湿度的改善效果大于多年生牧草,空气相对湿度平均增加15.5%。冠层空气相对湿度变化与1 m土体内土壤贮水减少量呈显著线性正相关关系。越年生牧草对土壤贮水的消耗量大于多年生牧草,平均多消耗土壤贮水39.20 mm。行间生草后土壤蒸发量平均降低27.80 mm,土壤贮水消耗量平均增加52.50 mm。因此,华北平原苹果园行间生草后春季土壤蒸发减少量难以弥补牧草生长对土壤贮水的消耗量,降低了果园土壤有效水分。越年生牧草比多年生牧草更有利于改善春季果园空气湿度,但对土壤水分消耗较大,生产上应根据不同生草种类对土壤水分的影响特征优化果树春季灌溉制度。     

牧草如何生长--植物生长与放牧管理之关联

适当管理饲料饲草资源的基本原则是理解多年生牧草如何生长.长期以来,对根系生长有了基本的了解,对去叶和环境对根系生长的影响也知之甚多.然而,对去叶、干旱及其他环境因素对多年生牧草的影响以及这些牧草对去叶的反应缺乏应有的理解.从牧草返青到开花,植物处于短枝阶段,适度放牧并不对多年生牧草的健康和活力造成危害,因为植物会弥补其失去的叶片.然而,当牧草从短枝向长枝阶段(节间伸长和开花期)转移时,碳水化合物主要用于节间伸长和开花过程.在开花后和休眠前期,碳水化合物主要向茎基和植物基部转移,以供来年新枝条的发育.因此,在花后和休眠前期,放牧系统必须被调节到"无放牧"状态."无放牧"状态,如休息、轮牧、延迟轮牧和快速轮牧制,其价值在于这种定期的禁牧可提高多年生牧草的活力,改善草地的健康状况,并且在不利的环境条件下,保证在植物去叶后的后续生产能力.     

Effect of management on rangeland phytomass,cover and condition in two biomes in South Africa

In rangelands, grazing management is a main driver of rangeland condition. Due to masking effects of seasonal climate fluctuations, little is known about (dis)similarity of management effects on rangeland condition and forage provision across major dryland biomes. Taking a macro-ecological perspective, we analysed if management effects differed between South Africa’s central grassland and Kalahari savanna biomes. We recorded proxies of forage provision (phytomass, vegetation cover and their ratio) over five seasons, annual rainfall to account for seasonal climate fluctuations, and rangeland condition (through relative abundances of increaser and decreaser species). Regarding forage provision, we found effects of management for the savanna, where, irrespective of rainfall, rotational grazing management resulted in higher phytomass and phytomass–cover ratios than management with continuous grazing. In the grassland, however, this difference was only discernible for phytomass–cover ratio in two years with above-average antecedent rainfall. This suggests that management effects are biome-dependent and that modulating effects of annual rainfall are stronger in the grassland. In either biome, management effects on the dominance of increaser and decreaser species were negligible, i.e. rangeland condition did not differ across management types in either biome. We conclude that investigations on management effects should account for interactions with biome and rainfall.