Herbaceous Biomass Response to Prescribed Fire in Juniper-Encroached Sagebrush Steppe

Western juniper (Juniperus occidentalis Hook.) has expanded into sagebrush steppe plant communities the past 130 ? 150 yr in the northern Great Basin. The increase in juniper reduces herbage and browse for livestock and big game. Information on herbaceous yield response to juniper control with fire is limited. We measured herbaceous standing crop and yield by life form in two mountain big sagebrush communities (MTN1, MTN2) and a Wyoming/basin big sagebrush (WYOBAS) community for 6 yrs following prescribed fire treatments to control western juniper. MTN1 and WYOBAS communities were early-successional (phase 1) and MTN2 communities were midsuccessional (phase 2) woodlands before treatment. Prescribed fires killed all juniper and sagebrush in the burn units. Total herbaceous and perennial bunchgrass yields increased 2 to 2.5-fold in burn treatments compared with unburned controls. Total perennial forb yield did not differ between burns and controls in all three plant communities. However, tall perennial forb yield was 1.6- and 2.5-fold greater in the WYOBAS and MTN2 burned sites than controls. Mat-forming perennial forb yields declined by 80 ? 90% after burning compared with controls. Cheatgrass yield increased in burned WYOBAS and MTN2 communities and at the end of the study represented 10% and 22% of total yield, respectively. Annual forbs increased with burning and were mainly composed of native species in MTN1 and MTN2 communities and non-natives in WYOBAS communities. Forage availability for livestock and wild ungulates more than doubled after burning. The additional forage provided on burned areas affords managers greater flexibility to rest and treat additional sagebrush steppe where juniper is expanding, as well as rest or defer critical seasonal habitat for wildlife.     

Root Biomass and Distribution Patterns in a Semi-Arid Mesquite Savanna: Responses to Long-Term Rainfall Manipulation

Expansion of woody plants in North American grasslands and savannas is facilitated in part by root system adaptation to climatic extremes. Climatic extremes are predicted to become more common with global climate change and, as such, may accelerate woody expansion and/or infilling rates. We quantified root biomass and distribution patterns of the invasive woody legume, honey mesquite (Prosopis glandulosa), and associated grasses following a long-term rainfall manipulation experiment in a mixed grass savanna in the southern Great Plains (United States). Root systems of mature trees were containerized with vertical barriers installed to a depth of 270 cm, and soil moisture was manipulated with irrigation (Irrigated) or rainout shelters (Rainout). Other treatments included containerized, precipitation-only (Control) and noncontainerized, precipitation-only (Natural) trees. After 4 yr of treatment, soil cores to 270 cm depth were obtained, and mesquite root length density (RLD) and root mass, and grass root mass were quantified. Mesquite in the Rainout treatment increased coarse-root ( &spigt; 2 mm diameter) RLD and root mass at soil depths between 90 cm and 270 cm. In contrast, mesquite in the Irrigated treatment increased fine-root ( &spilt; 2 mm diameter) RLD and root mass between 30 cm and 270 cm depths, but did not increase total root mass (fine + coarse) compared to the Control. Mesquite root-to-shoot mass ratio was 2.8 to 4.6 times greater in Rainout than the other treatments. Leaf water stress was greatest in the Rainout treatment in the first year, but not in subsequent years, possibly the result of increased root growth. Leaf water use efficiency was lowest in the Irrigated treatment. The increase in coarse root growth during extended drought substantially increased mesquite belowground biomass and suggests an important mechanism by which woody plant encroachment into grasslands may alter below ground carbon stocks under climate change scenarios predicted for this region.     

Short- and Long-Term Vegetation Change Related to Grazing Systems,Precipitation, and Mesquite Cover

Rangeland scientists struggle with how long rangeland experiments must continue in order to detect treatment effects, particularly in semiarid ecosystems characterized by slow responses and high spatiotemporal variability. We compared changes in eight grass and three shrub categories to grazing systems (yearlong vs. seasonal rotation with equivalent long-term stocking rates), and covariates (precipitation and mesquite [Prosopis velutina] gradients) over 12 yr (1972–1984) and 34 yr (1972–2006) on the Santa Rita Experimental Range, Arizona. We used split-plot analysis of variance, with year as the split, to make these comparisons. Grazing systems did not influence plant dynamics as shown by the lack of grazing system by year effect on all response variables in either time period. The absence of a detectable grazing effect on vegetation changes may be due to overriding influences of grazing intensity, pasture size, precipitation variability, and few replicates. Also, more time may be needed to detect the small accumulating and potentially temporary effects from grazing systems. The grazing system main effects present at the beginning and throughout the study suggest that pastures assigned to each grazing system had different potentials to support vegetation. Nearly twice the number of response variables were related to the precipitation covariate than to mesquite cover, but only about half of all the relationships were consistent between time periods. The struggle to know how long to observe before detecting a grazing system effect was not resolved with the additional 22 yr of observation because we cannot definitively reject that either more time is needed to detect small but cumulative effects or that the two grazing systems are not different.     

紫花苜蓿生物量分配对植物-土壤化学计量的响应

C∶N∶P化学计量学在植物-土壤养分分配研究中应用广泛,但如何运用化学计量学方法来探究植物地上地下生物量分配还需要进一步研究。本研究选取榆林地区种植1 a, 3 a, 5 a, 7 a和9 a的紫花苜蓿(Medicago sativa L.)人工草地,测定了紫花苜蓿的地上、地下生物量,土壤理化性质及碳氮磷含量,分析了植物-土壤C∶N∶P化学计量特征及其对紫花苜蓿地上、地下生物量和根冠比变化的响应。结果表明,紫花苜蓿人工草地土壤C∶N,C∶P和N∶P随种植年限延长均而增加,紫花苜蓿植株C∶N,C∶P和N∶P随种植年限延长均呈现先增加后降低变化趋势。种植1～5 a紫花苜蓿种植受N限制,种植7～9 a紫花苜蓿受P限制,紫花苜蓿通过增加自身根冠比来应对日益增强的N∶P,同时庞大的根系使得土壤肥力得到改善。     

污水胁迫下7种草本地被植物耐污性比较研究

本研究选取7种草本地被植物为试验材料,采用双套盆法,研究7种植物在污水胁迫下的耐污性。通过观测植株胁迫期间形态表现,并测定植物叶片中的细胞膜透性变化、叶绿素含量、丙二醛含量和脯氨酸含量等生理指标,运用隶属函数法对7种草本地被植物的耐污性进行综合评价。研究结果表明：随着胁迫天数的增加,7种植物的外部形态存在明显差异性,马蔺（Iris lactea）胁迫前后形态变化最大,基本整株褐化,耐污性最弱。葱兰（Zephyranthes candida）、黄菖蒲（Iris pseudacorus）和波斯菊（Cosmos bipinnata）形态变化很小,其耐污性较强。根据隶属函数综合评价得出马蔺的耐污性最弱,并得出7种草本地被植物耐污性顺序为：波斯菊（Cosmos bipinnata） > 黄菖蒲（Iris pseudacorus） > 葱兰（Zephyranthes candida） > 狼尾草（Pennisetum alopecuroides） > 萱草（Hemerocallis fulva） > 玉簪（Hosta plantaginea） > 马蔺（Iris lactea）,与形态观测记录基本一致。该研究结果可为园林绿地设计和海绵城市建设中植物的选择提供理论依据。     

挖沙废弃地草本能源植物生物质品质对氮肥的响应

在京郊挖沙废弃地上开展了柳枝稷、荻和芦竹的生物质品质氮肥效应研究,结果表明:施用氮肥对柳枝稷的纤维素、半纤维素、木质素含量影响均不显著;对荻和芦竹的纤维素、木质素含量影响不显著,但对二者的半纤维素含量影响显著;适量施用氮肥对柳枝稷、荻和芦竹的生物质产量、纤维素和半纤维素产量、纤维素类物质含量和产量均具有明显的促进作用.在边际土地上,可以通过施用适量氮肥对草本能源植物的生物质品质进行一定的调控.     

Allometric Estimates of Aboveground Biomass Using Cover and Height Are Improved by Increasing Specificity of Plant Functional Groups in Eastern Australian Rangelands

Plant aboveground biomass (AGB) is a useful metric to assess ecosystem functioning, and its sensitivity to changing environmental conditions provides insight into potential global change impacts. Allometric estimates of AGB using vegetation characteristics such as plant cover or height provide nondestructive biomass proxies for repeated measurements but can introduce uncertainty to estimates. We estimated the relationship between both plant cover and a cover·height index and AGB for 15 plant species from six sites to identify the most reliable approach to estimate biomass nondestructively in semiarid eastern Australian rangelands. Estimates were made by grouping species at four different levels of specificity, to test whether generic estimates were more robust than grouping species based on life history and morphological characteristics. Estimates were then tested on a 1.5-m² plot at each site for validation. In all cases, models were highly significant (P < 0.001) with adjusted R² values ranging from 0.42 to 0.96 for cover models and 0.38 to 0.98 for cover·height index models. We found the addition of height improved model fits in four groups while reducing model fits in two groups. The error around AGB estimates for cover·height index−based models ranged from −66.8 to 4% (absolute mean 35%). Cover-based models had errors between −13.4% and 53% (absolute mean 14.2%). For cover-based estimates of AGB in validation plots, grouping plants by plant functional types (PFTs) increased accuracy (absolute mean error 17.3%) compared with estimates using data from all 15 species (absolute mean of 65.2%). Overall cover was a useful surrogate to estimate AGB (with the exception of one site, accuracy ranged from −2.3% to 11.5%), while height (thought to be a surrogate for canopy characteristics) provided benefit in a few circumstances. We suggest that future research should test additional nondestructive proxies and group species based on PFTs to improve AGB estimates using allometry.     

Grazing History Effects on Rangeland Biomass,Cover and Diversity Responses to Fire and Grazing Utilization

Exclusion of large grazers from rangelands that evolved with significant grazing pressure can alter natural processes and may have legacy effects by changing magnitude or direction of community responses to subsequent disturbance. Three moderately grazed pastures were paired with 12-ha areas with 15 yr of livestock exclusion. Six treatments were assigned to each in a 2 x 3 factorial arrangement of fire (fall fire or no fire) and grazing utilization (0%, 50%, or 75% biomass removal) to determine grazing history effects on rangeland response to subsequent disturbance. Livestock exclusion increased C₃ perennial grass (1 232 vs. 980 ± 50 kg ? ha^-1) and forbs (173 vs. 62 ± 19 kg ? ha^-1) and reduced C₄ perennial grass (36 vs. 180 ± 25 kg ? ha^-1) with no effect on total current-year biomass. Diversity was greater in pastures than exclosures (H’ = 1.5400 vs. 1.3823 ± 0.0431). Every biomass, cover, and diversity measure, except subshrub biomass, was affected by fire, grazing utilization, or both. Contrary to expectations, grazing history only interacted with fire effects for old standing dead material and interactions with grazing utilization were limited to old dead, bare ground, richness and dominance. Fire by grazing utilization interaction was limited to bare ground. Fire reduced annual grass (64 vs. 137 ± 29 kg ? ha^-1), forbs (84 vs. 133 ± 29 kg ? ha^-1), and diversity (H’ = 1.3260 vs. 1.5005 ± 0.0537) with no difference in total current-year biomass (1 557 vs. 1 594 ± 66 kg ? ha^-1). Grazing to 75% utilization reduced total current-year biomass (1 467 vs. 1 656 ± 66 kg ? ha^-1) and dominance (0.4824 vs. 0.5584 ± 0.0279). Grazing history affected starting points for most variables, but changes caused by grazing utilization or fire were similar between pastures and exclosures, indicating management decisions can be made based on independent knowledge of grazing or fire effects.     

金马河温江段河岸带不同生境草本群落物种多样性和生物量变化研究

本研究以金马河温江段河岸带植被为研究对象，采用典型样地法对5种生境（河滩地、砾石地、芦苇地、斑茅地和沙坑地）进行调查，分析其3年来（2015-2018年）植被群落特征变化。结果表明：从时间尺度上看，2015年仅有草本层32科80属113种，2018年草本、灌木层共计57科118属161种，各生境草本层优势种变动较小，物种多样性指数值增大，地上、地下生物量变化情况为河滩地和沙坑地增加，砾石地和芦苇地变化不明显，斑茅地减少；从空间尺度上看，2015年各生境之间的物种多样性指数和地上生物量均不存在显著差异，而2018年各生境间变异程度复杂；该段河岸带3年间取得了一定程度的生态恢复效果，但芦苇（Phragmites australis）、葎草（Humulus scandens）呈扩张趋势，需要注意控制生长，维持生境物种多样性。     

降水量的季节分配对羊草草原群落地上部生物量影响的数学模型

根据羊草草原群落地上部生物量连续１３ａ手定位观测资料，分析了降水量及其季节分析对群落地上部生物量年度波动的影响，并应用积分回归模型，计算出１－７月各旬降水量对群落地上部生物量的影响系数。     

三种冷地型草坪草地下部分对不同水分梯度的响应

对多年生黑麦草(Lolium perenne L.)、草地早熟禾(Poa pratensis L.)和苇状羊茅(Festuca arundinacea Schreb.)3种冷地型草坪草在30%～50%田间持水量(FC)、50%～70%FC、70%～90%FC三个水分梯度下的地下生物量进行了田间测定.结果表明,土壤水分状况直接影响草坪草根系的生长与构形分布.不同水分处理下,3种草坪草的73.4%～92.2%的根量分布在0～10 cm的土层中,5.1%～16.8%的根量分布在10～20 cm的土层中,1.7%～9.8%的根量分布在20～30 cm的土层中.水分梯度对根系的垂直空间分布有很大影响,在30%～50%FC处理下,0～10 cm层根量占0～30 cm总根量的73.4%～89.5%;50%～70%FC处理下,0～10 cm层根量占79.5%～91.6%;70%～90%FC处理下,0～10 cm层根量占78.3%～92.2%.10～20 cm、20～30 cm层根量比例随水分梯度的增加则逐渐减小.在30%～50%FC、50%～70%FC、70%～90%FC3个水分梯度下,3种草坪草8,9,10三个月根量累积值(g/m2)依次为,多年生黑麦草4 209.5、3 416.5、3 906.5;草地早熟禾3 716.6、3 452.9、3 447.8;苇状羊茅5 281.0、5 184.5、4 317.7,苇状羊茅根量最大,草地早熟禾根量最小.     

三种冷地型草坪草地下部分对不同水分梯度的响应

对多年生黑麦草(Lolium perenneL.)、草地早熟禾(PoapratensisL.)和苇状羊茅(Festuca arundinaceaSchreb.)3种冷地型草坪草在30%50%田间持水量(FC)、50%70%FC、70%90%FC三个水分梯度下的地下生物量进行了田间测定。结果表明,土壤水分状况直接影响草坪草根系的生长与构形分布。不同水分处理下,3种草坪草的73.4%￣92.2%的根量分布在0￣10cm的土层中,5.1%16.8%的根量分布在10￣20cm的土层中,1.7%9.8%的根量分布在20￣30cm的土层中。水分梯度对根系的垂直空间分布有很大影响,在30%￣50%FC处理下,0￣10cm层根量占0￣30cm总根量的73.4%￣89.5%;50%￣70%FC处理下,0￣10cm层根量占79.5%￣91.6%;70%￣90%FC处理下,0￣10cm层根量占78.3%￣92.2%。10￣20cm、20￣30cm层根量比例随水分梯度的增加则逐渐减小。在30%50%FC、50%70%FC、70%90%FC3个水分梯度下,3种草坪草8,9,10三个月根量累积值(g/m2)依次为,多年生黑麦草:4209.5、3416.5、3906.5;草地早熟禾:3716.6、3452.9、3447.8;苇状羊茅:5281.0、5184.5、4317.7,苇状羊茅根量最大,草地早熟禾根量最小。     

温带典型草原土壤理化性质及微生物量对放牧强度的响应

为了研究不同放牧强度对草地土壤理化性质及微生物量的影响,本文以内蒙古温带典型草原为研究对象,设置了4个放牧梯度：对照（围封）、28只羊每月放牧3天（轻度）、6天（中牧）、12天（重牧）,采用控制实验分析了不同放牧处理下草原土壤理化性质和土壤微生物量之间的相关关系。结果表明：放牧强度和土壤深度均对土壤pH值、硝态氮和速效磷含量产生极显著影响（P<0.01）,土壤总氮和总有机碳含量受放牧强度极显著影响（P<0.01）,轻牧处理含量显著高于其他处理样地。土壤微生物量碳、磷含量受放牧强度影响显著（P<0.05）。因子分析及相关分析结果表明,土壤MBP含量与土壤因子F1（TN,AN,NO₃^--N,TOC,AP,NH₄⁺-N）极显著正相关（P<0.01）,土壤MBN含量与土壤因子F1显著正相关（P<0.05）,说明MBN和MBP与土壤理化指标之间具有统一性,微生物量氮、磷可以灵敏地反映出放牧强度变化下的土壤理化性质和微生物生物量的变化。     

Response of Biomass and Seedbanks of Rangeland Functional Groups to Mechanical Control of Yellow Starthistle

Yellow starthistle (Centaurea solstitialis L.) is a nonnative pest of rangelands that decreases forage quality and yield. Mowing may control starthistle effectively and complement herbicide use in an integrated pest management strategy, but little research has investigated its effects on nontarget vegetation. We monitored biomass and seedbank size of annual and perennial species, in addition to starthistle, in response to 3 yr of mowing treatments, either mowing alone or in combination with solarization tarps or thatch removal. All mowing treatments were very effective at reducing starthistle biomass and seedbank: mowing alone reduced biomass 92 ± 2%, mowing with thatch removal 91 ± 1%, and mowing with solarization 95 ± 1%. Compared to seedbank sizes in the control plots, yellow starthistle seedbank decreased by 100% (mowing alone), 92% (mowing + thatch removal), and 100% (mowing with solarization) after 3 yr of treatment. Mowing also significantly improved perennial species' biomass. Annual species' biomass varied on a year-to-year basis but was not significantly affected by any treatment. Seedbank sizes of annuals and perennials also did not differ according to mowing treatment. This research indicates that late-season mowing can effectively reduce starthistle biomass without adverse effects on other vegetation and that mowing alone is sufficient to reduce starthistle seedbank size without additional methods of decreasing seed rain.     

早熟禾不同品种根系分布及生物量分配对干旱胁迫的响应

为探讨早熟禾(Poa L.)不同品种根系生长状况、生物量分配与水分胁迫的关系,采用PVC管栽法,对布鲁克(Poa pratansis L. cv. Brooklawn)、英派克(Poa pratansis L. cv. Impact)和苏比纳(Poa supine Schrad. cv. SupraNova)3个品种进行干旱胁迫,测定了生物量、根冠比、根系密度、相对生长率等指标,分析了根、冠生物量分配及根系生长动态规律.结果表明:干旱胁迫对早熟禾3个品种的根、冠生物量累积和分配影响不同, 3个品种通过提高根干物质重量的分配策略来提高其抗旱能力的顺序依次是布鲁克、英派克、苏比纳;轻度水分胁迫有助于根系生长;3个品种相对生长率随着干旱胁迫程度的增加而降低,其中苏比纳降低最多;干旱条件下,土壤深层根系密度、根冠比增加的顺序依次为布鲁克、英派克、苏比纳.     

两种结缕草品系对低温胁迫的生理响应研究

为探明不同生态条件下日本结缕草(Zoysia japonica Steud.)对低温胁迫的生理响应,以生态条件具明显差异的日本结缕草两品系室兰(Muroran)和表山北(Tawarayama Kita)为材料,采用电导法测定不同低温处理条件下其离体叶片及匍匐茎相对电导率的变化,拟合Logistic方程求得低温半致死温度(LT₅₀),并分析结缕草在低温胁迫下脯氨酸、可溶性糖含量、超氧化物歧化酶(SOD)活性及丙二醛(MDA)含量的变化规律。结果表明室兰叶片、匍匐茎的半致死温度均低于表山北,低温处理后两结缕草品系的脯氨酸、可溶性糖含量均有不同程度的升高,且随着温度的降低,室兰可溶性糖含量增幅比表山北大;两品系SOD活性随温度的降低呈"下降-升高-再下降"变化趋势,而MDA含量呈相反变化趋势。0℃时,SOD活性及MDA含量在两品系间表现出极显著差异(P<0.01)。综合各项生理指标评价,室兰的抗寒性强于表山北。     

贝加尔针茅草原植物多样性与地上生物量及其关系对刈割的响应

本研究以呼伦贝尔贝加尔针茅割草地为研究对象,探讨刈割干扰对植物群落多样性和地上生物量及其关系的影响。结果表明：刈割时间对植物的优势度和均匀度指数有显著影响,留茬高度对植物多样性影响不显著。随着留茬高度的增加,地上生物量显著下降;刈割时间对群落地上生物量影响不明显。植物多样性和地上生物量呈显著正相关关系。     

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.