Land‐use and isolation interact to affect wetland plant assemblages

Different management regimes imposed on similar habitat types provide opportunities to investigate mechanisms driving community assembly and changes in species composition. We investigated the effect of pasture management on vegetation composition in wetlands with varying spatial isolation on a Florida cattle ranch. We hypothesized that increased pasture management intensity would dampen the expected negative effect of wetland isolation on native species richness due to a change from dispersal‐driven community assembly to niche‐driven assembly by accentuated environmental tolerance. We used native plant richness, exotic plant richness and mean coefficient of conservatism (CC) to assess wetland plant assemblage composition. Sixty wetlands were sampled, stratified by three levels of isolation across two pasture management intensities; semi‐native (less intensely managed; mostly native grasses, never fertilized) and agronomically improved (intensely managed, planted with exotic grasses, and fertilized). Improved pasture wetlands had lower native richness and CC scores, and greater total soil phosphorus and exotic species coverage compared to semi‐native pasture wetlands. Increased wetland isolation was significantly associated with decreases in native species richness in semi‐native pasture wetlands but not in improved pasture wetlands. Additionally, the species–area relationship was stronger in wetlands in improved pastures than semi‐native pastures. Our results indicate that a) native species switch from dispersal‐based community assembly in semi‐native pastures to a species‐sorting process in improved pastures, and b) recently‐introduced exotic species already sorted for more intensive management conditions are primarily undergoing dispersal‐based community assembly. That land‐use may alter the relative importance of assembly processes and that different processes drive native and exotic richness has implications for both ecosystem management and restoration planning.     

Succession and fluctuation in Artemisia dominated grassland

Vegetation dynamics were studied from 1940 to 1978 in two grazed pastures and associated exclosures in sand sagebrush (Artemisia filifolia) dominated grassland, western Oklahoma, USA. In both pastures and one exclosure, pattern of vegetation change reflected fluctuation rather than succession. In the other exclosure, the grassland exhibited a directional change from annual grasses and forbs to dominance by perennial grasses. Rate of change was consistent during the 39 year period. Cover of grasses increased more in grazed than ungrazed areas. Grass cover was negatively correlated with high air temperatures early in the growing season. Forb cover remained relatively constant over time and shrub cover peaked during the 1960s. Abundance of annuals and cool season species was positively correlated with rainfall early in the growing season.Species diversity and richness were lowest in the ungrazed areas, as a result of increased dominance by perennial grasses such as Schizachyrium scoparium. In pastures and exclosures, richness was positively correlated with growing season precipitation. Cover of the common species differed among sample areas within years and fluctuated between years. Few general patterns emerged from correlations of environmental variables with cover of individual species. In general, vegetation dynamics in these sand sagebrush grasslands reflect a tradeoff in that total cover changes little over time because the loss of some species is compensated for by increased growth of others. Such trade-offs reflect the individualistic response of the component species within each pasture or exclosure. Although changes in growth form composition were related to climatic fluctuation, broad-scale climatic variables could not successfully predict small-scale patterns of change by individual species over time.     

Isolated Trees and Grass Removal Improve Performance of Transplanted Trema micrantha (L.) Blume (Ulmaceae) Saplings in Tropical Pastures

Cattle pastures established in areas previously covered by tropical rainforest can be abandoned in unproductive and degraded conditions. Transplanting native tree species to pastures is one common practice among rainforest restoration techniques, but several environmental barriers compromise transplant success. We assessed whether the presence of isolated trees and the removal of pasture grasses affect survival, growth, and allometry of transplanted saplings of the pioneer tree Trema micrantha (L.) Blume (Ulmaceae) into abandoned pastures in southeast Mexico. An isolated tree was selected in the center of each of four pasture sites of one hectare, and grass treatments were applied under the tree's canopy (0–10 m from the trunk) and in open pasture (15–48 m from the trunk). Grass removal treatments were control (grass present), cut with machete, herbicide application, and total grass removal with a gardening hoe. After 1 year, sapling survival was not different between the canopy and pasture areas (53%). Saplings showed higher survival probability (p < 0.05) in the hoe treatment (63 ± 9%) than in the control treatment (38 ± 9%). Height and crown cover growth rates were faster under the canopy of isolated trees compared with the open pasture. Saplings showed significantly greater crown area/height ratios under the canopy of isolated trees. Stressful environmental conditions restricted sapling growth in the open pasture. We conclude that complete removal of grasses and transplanting T. micrantha saplings in the vicinity of isolated trees can improve transplant success.     

Land use modifies Odonata diversity in streams of the Brazilian Cerrado

The presence of riparian vegetation provides microclimatic regulation of stream conditions [e.g. luminosity (lux), air temperature (°C) and relative humidity (%)], which varies naturally throughout the day. These variables explain the diurnal behaviour patterns of ectotherms such as Odonata in natural areas. However, human land uses (e.g. pastures) modify the abiotic conditions of riparian environments, favouring the presence of disturbance-tolerant species. In this context, we assess relationships between riparian land use (control streams-natural areas and pasture), abiotic conditions habitat integrity index (in control and pastures streams), (air temperature, luminosity and humidity in control streams), and Odonata diversity (between pasture and control streams and throughout the time of day) in Brazilian savannah (Cerrado) streams. First, the control streams had higher habitat integrity index than pasture. Higher abundance and difference in composition of Odonata species were observed in streams surrounded by pasture relative to natural areas. The conversion of natural areas to pasture near streams was also accompanied by an increase in overall body size of Odonata species. Odonata species richness and abundance in natural areas varied throughout the day, but peaked around 12:00 h, coinciding with changes in air temperature and luminosity. Our study highlights that changes in the physical characteristics of streams through conversion of natural habitats to pasture will change environmental conditions and act as a filter on the distribution and persistence of Odonata species in Cerrado streams.     

Tropical Montane Forest Restoration in Costa Rica: Overcoming Barriers to Dispersal and Establishment

Tropical forests are being cleared at an alarming rate although our understanding of their ecology is limited. It is therefore essential to design restoration experiments that both further our basic knowledge of tropical ecology and inform management strategies to facilitate recovery of these ecosystems. Here we synthesize the results of research on tropical montane forest recovery in abandoned pasture in Costa Rica to address the following questions: (1) What factors limit tropical forest recovery in abandoned pasture? and (2) How can we use this information to design strategies to facilitate ecosystem recovery? Our results indicate that a number of factors impede tropical forest recovery in abandoned pasture land. The most important barriers are lack of dispersal of forest seeds and seedling competition with pasture grasses. High seed predation, low seed germination, lack of nutrients, high light intensity, and rabbit herbivory also affect recovery. Successful strategies to facilitate recovery in abandoned pastures must simultaneously overcome numerous obstacles. Our research shows that establishment of woody species, either native tree seedlings or early‐successional shrubs, can be successful in facilitating recovery, by enhancing seed dispersal and shading out pasture grasses. On the contrary, bird perching structures alone are not an effective strategy, because they only serve to enhance seed dispersal but do not reduce grass cover. Remnant pasture trees can serve as foci of natural recovery and may enhance growth of planted seedlings. Our results highlight the importance of: (1) understanding the basic biology of an ecosystem to design effective restoration strategies; (2) comparing results across a range of sites to determine which restoration strategies are most generally useful; and (3) considering where best to allocate efforts in large‐scale restoration projects.     

高寒牧区村域生态足迹——以甘南州合作市为例

高寒牧区是一类特殊的地域,其生态环境本底和压力明显区别于其它地域类型。从自然本底上讲,"高寒"特征造成生态系统结构单一、节律缓慢、功能稳定性差,因此,脆弱性强,而修复难度大;从压力上讲,"高寒"又一定程度上屏蔽了外界要素的过分介入,导致人口分布的高度离散性和稀疏性,人地矛盾转化为更直观的草畜矛盾。论文结合高寒牧区的特质,对生态足迹模型和生态承载力模型进行了修正,强调了高寒牧区生态承载力的主控因素———牲畜特征、人口特征、生境特征和地域特征,以甘南州合作市为案例,具体测算和分析了41个行政单元的生态足迹和生态承载力。结果表明:(1)人口生态足迹和牲畜生态足迹分布格局大致呈反向态势,说明在高寒牧区,人口与牲畜对环境造成的压力存在着显著的空间差异。(2)从人口生态足迹和牲畜生态足迹对总生态足迹的贡献度上看,总生态足迹的高低主要取决于牲畜生态足迹,说明牲畜是主控因素。(3)从地均生态承载力分布态势看,低承载区主要分布于北部海拔较高的广大牧区,这里生态结构非常单一,主要以高寒草甸草原和沼泽草原生态类型为主,寒冻风化作用异常强烈,生物有效生长期非常短。而高承载区分布在市区周围和南部洮河谷地沿岸,生态系统除草地生态系统外,还有森林生态系统。从而说明人为条件的改善、自然生态系统结构的复杂程度和自然环境的严酷程度是决定高寒牧区承载力高低的主要因素。(4)从生态盈余与赤字的空间分布态势看,赤字区域主要集中分布在广大的纯牧区,超载过牧是生态状况的关键性致因。因此提高生态承载力、减少生态赤字的关键集结于草畜关系上。有鉴于此,需要采取以下措施:一是根据天然草场的承载能力,建立适宜的草畜平衡关系,将超载的牲畜转移出去;二是科学合理的提高草场的生产能力和承载能力,这就需要加强草原生态保护建设;三是调整经济结构,减少社会经济发展对天然放牧业的高度依赖性,推进传统草地畜牧业向现代畜牧业转型。     

Factors Limiting Tropical Rain Forest Regeneration in Abandoned Pasture: Seed Rain,Seed Germination,Microclimate, and Soil1

Vegetation, seed rain, seed germination, microclimate, and soil physical and chemical parameters were measured in a recently abandoned pasture and adjacent primary rain forest in southern Costa Rica. The goal of this study was to assess the importance of these factors in limiting forest regeneration in abandoned pastures. Seed rain of animal dispersed species decreased dramatically in the pasture >5 m from the forest/pasture edge; fewer wind dispersed seeds fell in the pasture than in the forest, but the difference was much less than for animal dispersed seeds. Percent seed germination of most species studied was similar in the forest and in pasture with grasses; seed germination was lower during the dry season in areas of pasture cleared of grasses. Air temperature, vapor pressure deficit (VPD), and photon flux density (PFD) were much higher in the pasture than in the forest at 1 m above the ground. VPD and PFD at ground level and soil temperature were similar in the pasture and the forest, indicating that pasture grasses strongly modify microclimatic conditions near the soil surface. The lowest gravimetric water content recorded in the pasture during the dry season was 0.5 and leaf relative water contents of the two species measured in the forest and pasture were identical, suggesting that plants in the pasture were not water stressed. Levels of most soil nutrients were lower in the pasture as compared to the forest; however, aboveground and root biomass for seedlings grown in pasture and forest soils did not differ significantly. Although a number of factors impede forest recovery in abandoned pastures, these results suggest that the most imporrant limitation is lack of seed dispersal.     

Forest-to-pasture conversion influences on soil organic carbon dynamics in a tropical deciduous forest

On a global basis, nearly 42% of tropical land area is classified as tropical deciduous forest (TDF) (Murphy and Lugo 1986). Currently, this ecosystem has very high deforestation rates; and its conversion to cattle pasture may result in losses of soil organic matter, decreases in soil fertility, and increases in CO₂ flux to the atmosphere. The soil organic matter turnover rate in a TDF after pasture conversion was estimated in Mexico by determining natural abundances of¹³C. Changes in these values would be induced by vegetation changes from the C₃ (forest) to the C₄ (pasture) photosynthetic pathway. The rate of loss of remnant forest-soil organic matter (fSOM) was 2.9 t ha^–1 year^–1 in 7-year-old pasture and decreased to 0.66 t ha^–1 year^–1 by year 11. For up to 3 years, net fSOM level increased in pastures; this increment can be attributed to decomposition of remnant forest roots. The sand-associated SOM fraction was the most and the silt-associated fraction the least depleted. TDF conversion to pasture results in extremely high rates of loss of remnant fSOM that are higher than any reported for any tropical forest.     

不同管理措施对川西北草地土壤氮和碳特征的影响

川西北地区是我国的主要牧区之一。草地承包后, 草地建设中出现了各种草地经营管理方式———围栏、翻耕和完全的人工建设。采用常规化学分析和气压过程分离 (BarometricProcessSeparation, BaPS) 法, 对不同类型草地 (天然放牧草地、围栏草地、翻耕草地和人工草地 ) 的土壤氮、碳库以及反硝化速率、总硝化速率、N2 O和CO2 排放速率进行了研究。结果表明 :研究地点土壤有机质、全氮含量分别为 10 1.8和 5.1g·kg-1, 比典型的亚高山土壤有机质和全氮含量 (分别是 181.3和 7.4 g·kg-1) 明显低, 而且, 与通常的观念不同的是, 土壤NO3 - N含量是NH4+ N含量的 3~11倍。这可能是由于研究地过度的人为干扰造成的。研究还发现, 不同管理措施对土壤氮、碳库, 氮转化速率和土壤呼吸有显著影响。天然放牧草地围栏后, 土壤有机质和全氮含量明显升高, 比如, 围栏草地和翻耕草地的有机质、全氮含量分别比天然放牧草地高 6 1%、5 8%和 4 6 %、5 1%。氮转化速率和土壤呼吸大大加快, 尤其是在翻耕草地, 比如, 翻耕草地的总硝化速率和N2 O排放速率分别是天然放牧草地的 5.1和 2.4倍。因此, 虽然春季翻耕可能提高作物 (包括牧草 ) 产量, 但它同时也承担了巨大的生态学风险, 包括增加排放到大气中的CO2 和N2 O的量以及淋溶到地下水的NO3 -的量。因此, 建议在川西北亚高山区的人工草地建设中慎重选择翻耕措施。研究还发现, 研究地土壤的总硝化速率是净硝化速率的 2 0~ 93倍, 净硝化速率不能反映高海拔地区土壤硝化的准确状况。     

Grassland management intensification affects the soil fauna in a subtropical highland

The conversion of natural grasslands to cultivated pastures can have a significant impact on the composition and structure of soil macro- and mesofauna groups, compromising the resilience of these organisms and the ecosystem services they provide. We studied the responses of these groups to increasing levels of soil disturbance across a gradient of four land management practices: Natural grassland (NG), Improved-natural grassland (IG), Perennial-cultivated pasture (PP), and Annual-cultivated pasture (AP). The NG area had a higher abundance of macrofauna, greater dominance of certain groups, particularly Isoptera, and a lower abundance of mesofauna. On the other hand, the IG area had a higher abundance and diversity of macro- and mesofauna, with a lower dominance of specific groups. The PP area had a higher abundance of Coleoptera, Oligochaeta, and Hemiptera, whereas the AP area, despite soil disturbance, showed a higher abundance of mesofauna, particularly mites, Collembola, and Enchytraeidae, but lower diversity. Different grassland management practices have significantly altered the composition and structure of macro- and mesofauna groups, resulting in high dissimilarity between communities. We recommend IG as a more productive and sustainable alternative to the total replacement of natural vegetation (NG) with intensified converted pastures (PP and AP), as it maintains soil fauna diversity and ecosystem services.     

Photosynthetic responses of native and introduced C4 grasses from Venezuelan savannas

Summary Introduced African grasses are invading the grasslands of the Venezuelan savannas and displacing the native grasses. This work, which is part of a program to understand the reasons for the success of the African grasses, specifically investigates whether introduced and native grasses differ in some photosynthetic characteristics.The responses to photon flux density, leaf temperature, leaf-air vapour pressure difference and leaf water potential of leaf photosynthetic rate of two introduced African C₄ grasses (Hyparrhenia rufa and Melinis minutiflora) and of a lowland and a highland population of a native Venezuelan grass (Trachypogon plumosus) grown under controlled conditions were compared. These responses in all three species were typical of tropical C₄ pasture grasses. The introduced grasses had higher maximum leaf conductance, net photosynthetic rates, and optimum temperature (H. rufa only) for photosynthesis than T. plumosus. However, T. plumosus was able to continue photosynthesis to lower leaf water potentials than the two introduced grasses, and the efficiency which it utilized water, light and mineral nutrients to fix carbon were similar to those of the introduced grasses.The higher rates of leaf photosynthesis of the introduced grasses contributed to, but only partially explained, the higher growth rates compared to T. plumosus. The higher growth rates and nutrient concentration of the introduced grasses are consistent with their ability to establish rapidly, compete successfully for resources, and displace T. plumosus from moist, fertile sites. Conversely, the slower growth rate, lower nutrient concentrations, and superior water relations characteristics are consistent with the capacity of T. plumosus to resist invasion by introduced grasses in poorer sites.     

Net nitrogen mineralization and net nitrification rates in soils following deforestation for pasture across the southwestern Brazilian Amazon Basin landscape

Previous studies of the effect of tropical forest conversion to cattle pasture on soil N dynamics showed that rates of net N mineralization and net nitrification were lower in pastures compared with the original forest. In this study, we sought to determine the generality of these patterns by examining soil inorganic N concentrations, net mineralization and nitrification rates in 6 forests and 11 pastures 3 years old or older on ultisols and oxisols that encompassed a wide variety of soil textures and spanned a 700-km geographical range in the southwestern Brazilian Amazon Basin state of Rondônia. We sampled each site during October-November and April-May. Forest soils had higher extractable NO₃ ^?-N and total inorganic N concentrations than pasture soils, but substantial NO₃ ^?-N occurred in both forest and pasture soils. Rates of net N mineralization and net nitrification were higher in forest soils. Greater concentrations of soil organic matter in finer textured soils were associated with greater rates of net N mineralization and net nitrification, but this relationship was true only under native forest vegetation; rates were uniformly low in pastures, regardless of soil type or texture. Net N mineralization and net nitrification rates per unit of total soil organic matter showed no pattern across the different forest sites, suggesting that controls of net N mineralization may be broadly similar across a wide range of soil types. Similar reductions in rates of net N transformations in pastures 3 years old or older across a range of textures on these soils suggest that changes to soil N cycling caused by deforestation for pasture may be Basin-wide in extent. Lower net N mineralization and net nitrification rates in established pastures suggest that annual N losses from largely deforested landscapes may be lower than losses from the original forest. Total ecosystem N losses since deforestation are likely to depend on the balance between lower N loss rates from established pastures and the magnitude and duration of N losses that occur in the years immediately following forest clearing.     

Developing strategies and methods for rehabilitating degraded pastures using native grasses

Summary   The purpose of this comment paper is to highlight the need for more research that aims to develop strategies and methods for establishing and maintaining native grasses in degraded pastures. Clues to finding the best strategies are linked to the vigorous debate concerning the relationship between biodiversity and ecosystem function, which has resonated in ecological research for more than 20 years. Studies aimed at investigating the operational validity of the 'sampling effect' hypothesis vs. the 'niche complementarity' hypothesis could provide valuable direction on how to successfully re-establish healthy and stable native pasture communities, including the number of species and functional groups needed. Once strategies have been developed, then practical methods on grazing management and sowing are also needed. In the long run, research initiatives in this direction could act to benefit the conservation of native grass diversity, while at the same time, sustain important ecological (and economic) functions such as production, nutrient cycling and resistance to invasion.     

Linking ecological function to species composition in ecological restoration: Seed removal by ants in recreated woodland

Evaluations of ecological restoration typically focus on associating measures of structural properties of ecosystems (e.g. species diversity) with time since restoration efforts commenced. Such studies often conclude a failure to achieve restoration goals without examining functional performance of the organism assemblages in question. We compared diversity and composition of ant assemblages and the rates of seed removal by ants in pastures, 4‐ to 10‐year old revegetated areas and remnants of Cumberland Plain Woodland, and an endangered ecological community in Sydney, Australia. Ant assemblages of forest remnant sites had significantly higher species richness, significantly different species composition and a more complex functional group structure in comparison with ant assemblages of pasture and revegetated sites, which did not differ significantly. However, the rates of seed removal by ants in revegetated sites were similar to those in forest remnants, with the rates in pasture sites being significantly lower. Approximately, one‐third of all ant species were observed to remove seeds. Forest remnant sites had significantly different assemblages of seed removing ant species from those in pasture and revegetated sites. These results demonstrate that similar ant assemblages of unrestored and restored areas can function differently, depending on habitat context. Evaluation of restoration success by quantifying ecosystem structure and function offers more insights into ecosystem recovery than reliance on structural data alone.     

Plant invasion alters nitrogen cycling by modifying the soil nitrifying community

Plant invasions have dramatic aboveground effects on plant community composition, but their belowground effects remain largely uncharacterized. Soil microorganisms directly interact with plants and mediate many nutrient transformations in soil. We hypothesized that belowground changes to the soil microbial community provide a mechanistic link between exotic plant invasion and changes to ecosystem nutrient cycling. To examine this possible link, monocultures and mixtures of exotic and native species were maintained for 4 years in a California grassland. Gross rates of nitrogen (N) mineralization and nitrification were quantified with ¹⁵N pool dilution and soil microbial communities were characterized with DNA‐based methods. Exotic grasses doubled gross nitrification rates, in part by increasing the abundance and changing the composition of ammonia‐oxidizing bacteria in soil. These changes may translate into altered ecosystem N budgets after invasion. Altered soil microbial communities and their resulting effects on ecosystem processes may be an invisible legacy of exotic plant invasions.     

Estimates of seasonal nitrogen fixation of annual subterranean clover-based pastures using the15N natural abundance technique

Annual pasture legumes play a key role in ley farming systems of southern Australia, providing biologically fixed nitrogen (N) to drive the production of the pastures as well as subsequent crops grown in rotation. Seasonal inputs of biologically fixed N in shoot biomass of the subterranean clover (Trifolium subterraneum) component of grazed annual pastures were assessed using the¹⁵N natural abundance technique and appropriately timed sampling of herbage dry matter (DM) for N accumulation. At three study sites spanning a gradient across the Western Australian wheatbelt from 300 to 600 mm annual rainfall the performance of the clover and non-legume herbs and grasses was examined as paired comparisons involving two management treatments expected to give contrasting effects on pasture productivity, botanical composition and N₂ fixation. The proportion of clover N derived from atmospheric N₂ fixation (%Ndfa) ranged from 65 to 95% across sites, treatments and sampling times. Amounts of fixed N accumulated in clover shoot biomass ranged from 50 to 125 kg ha⁻¹, and paralleled trends in clover production. Substantial increases in pasture production in high yielding treatments generally occurred without decrease in %Ndfa, suggesting that N₂ fixation was essentially non-limiting to performance of the clover component. Seasonal profiles for accumulation of fixed N were skewed towards the late winter and spring period, particularly in low plant density pastures following a cereal crop. There were seasonal, site and treatment-specific effects on the proportion of clover and non-legume pasture components and consequently clover yield and N₂ fixation were variably affected by competition from non-legume species.     

生态恢复和经济发展悖论中毒害草的地位及悖论解决途径

探讨了毒害草存在的生态价值、利用价值及毒害草防治的弊端。认为过度放牧是畜牧业陷入困境的主要原因,毒害草的繁衍只是草原退化的直接反映,防除缺乏依据。现有的防治措施存在各种弊端,对环境和生物多样性保护不利;并指出毒害草可以覆盖地面、减少水土流失,对草原生态系统的保护和恢复有利,故认为毒害草的大量繁衍是草原生态系统与放牧系统协同进化的结果,是草原生态系统发展的防御机制,具有极大生态价值。同时提出草原生态系统可持续发展要从人的生存和经济发展着眼,要改变现有生活模式,发展多种经营,实行跨区域经济补偿;要控制人口数量,对现有人口要实现城镇化。     

Density-dependent grazing impacts of introduced European rabbits and sympatric kangaroos on Australian native pastures

Little information is available on relationships between pest animal density and damage in natural ecosystems. Introduced European rabbits, Oryctolagus cuniculus, cause severe damage to Australian native vegetation but density–damage relationships are largely unexplored. There are no recognized simple methods to estimate their impacts on native pastures, due in part to confusion with grazing impact of other herbivores. We tested simple quantitative sampling methods using multiple small quadrats to detect site differences in pasture cover, pasture species richness and dung pellet density of herbivores, from which rabbit density and relative abundance of larger herbivores were estimated. Native pasture cover and species richness declined exponentially with increasing rabbit density, within the range of 0–5 rabbits ha^?1, while cover of unpalatable exotic pasture species increased. By contrast, kangaroo abundance was positively related to palatable native pasture cover and negatively related to cover of unpalatable weeds, and had no negative effect on native pasture cover or species richness that was discernable against a background of low to moderate rabbit densities. Perennial native forbs and perennial grasses replaced invasive Wards weed as the dominant ground cover at low rabbit densities. We conclude that, regardless of previous grazing history, contemporary kangaroo grazing pressure and weed invasion, the severely degraded state of native pastures was perpetuated by rabbits. The effect of rabbits on native pasture can be recorded in a simple manner that is suitable for identifying density–damage relationships in the presence of other herbivores and changes over time. This method is seen as particularly useful in setting target densities below which rabbits must be managed to maintain native plant communities and ecosystem function in southern Australia. It may also be useful to demonstrate rabbits’ impacts in other regions, including optimum densities for plant biodiversity benefits in their native European range.     

Year‐to‐year changes in water level drive the invasion of Vochysia divergens in Pantanal grasslands

Abstract. In recent decades, cattle ranchers of the Pantanal of Mato Grosso, Brazil, have pointed to the accelerated spread of several herbaceous and woody plant species that invade natural and artificial pastures (campos). It has been speculated that overgrazing by an increasing number of cattle, lack of grazing in abandoned areas, or large‐scale changes in environmental conditions may be the reason for this invasion. This study focuses on ecological and ecophysiological aspects of Vochysia divergens (cambará), a flood‐tolerant tree that began spreading in the Pantanal during the last 30 years and is considered a very aggressive invasive plant. The study shows that the spread of cambará can be related to natural multi‐years wet periods. During multi‐years dry periods the species is reduced by the increasing impact of fires in the Pantanal. This points to the great importance of multi‐years climatic events on the vegetation cover of the Pantanal and indicates a very dynamic development in plant communities.     

Establishment of Fabaceae Tree Species in a Tropical Pasture: Influence of Seed Size and Weeding Methods

The successful establishment of trees in pastures may be related to species, seed size, and weed control treatments. This study experimentally evaluated the emergence, establishment after 2 years, and growth of individuals of 7 tree species with different seed sizes sowed under three weeding methods (no weeding, grasses clipped, and grasses removed). The experiment was carried out in a tropical abandoned pasture in Midwestern Brazil, dominated by the exotic invasive grass Urochloa brizantha. The effect of seed size on seedling emergence was significant and was more intense in the no weeding and weeding treatment plots than in grasses clipped. Furthermore, an increase in seed size resulted in an increase in the probability of establishment of tree species, although this pattern also differed among weeding treatments. The increase in seed size reflected negatively on tree growth in the absence of grasses, whereas in the presence of grasses there was an inverse relationship. The experimental results suggested that the use of seeds of different sizes for direct seeding in pastures is a possible strategy of Fabaceae tree species reintroduction in pastures within the Cerrado. Fabaceae species with seeds larger than 100 mg can establish in areas with no weeding, whereas species with seeds smaller than 50 mg can establish preferentially in areas with weeding, growing faster than larger ones. Thus, weeding strategies can determine the success of direct seeding of different species in restoration projects of tropical pastures.