Vegetation succession and lichen diversity on dry coastal calcium-poor dunes and the impact of management experiments

The negative impact of grass and moss encroachment on the botanical diversity of West European coastal dunes attracted increasing attention in the 1990s. This paper focuses on moss encroachment during primary succession in the xeroseries. Until the mid-1970s, vegetation types rich in species of the lichen generaCladonia andCladina were found on the fixed,Corynephorus canescens-dominated, so-called grey dunes all over the island of Terschelling, The Netherlands. In addition, species ofHypogymnia, Parmelia andUsnea, which usually grow on trees, occurred here terrestrially on moss carpets or bare sand. These vegetation types are still present on the Noordsvaarder, a nature reserve in the western part of the island. They occur on parts of seven dune ridges parallel to the coast and form a chronosequence in which age increases with distance from the sea. Our study found the highest lichen diversity on the second and third dune ridges in a stage of primary succession that can be assigned to theViolo-Corynephoretum. The changes from lichen-rich to moss-dominated stadia were significantly related to soil development and acidification in connection with the ageing of the dune soil. The superficial cutting of sods in moss-encroached vegetation appeared to be unsuccessful as a management technique for restoring the biodiversity of cryptogams. Our findings suggest that the best option for maintaining lichen vegetation in theViolo-Corynephoretum is the blow-in of sand with a subneutral or neutral pH from reactivated and natural blowouts or from foredunes, with increasing lime content respectively.     

Causes and consequences of woody plant encroachment into western North American grasslands

As woody plants encroach into grasslands, grass biomass, density and cover decline as wood plant biomass, density and cover increase. There is also a shift in location of the biomass from mostly belowground in the grasslands to aboveground in the woodlands. In addition, species richness and diversity change as herbaceous species are replaced by woody species. This is not a new phenomenon, but has been going on continually as the climate of the Planet has changed. However, in the past 160 years the changes have been unparalleled. The process is encroachment not invasion because woody species that have been increasing in density are native species and have been present in these communities for thousands of years. These indigenous or native woody species have increased in density, cover and biomass because of changes in one or more abiotic or biotic factors or conditions. Woody species that have increased in density and cover are not the cause of the encroachment, but the result of changes of other factors. Globally, the orbit of the Earth is becoming more circular and less elliptical, causing moderation of the climate. Additional global climate changing factors including elevated levels of CO₂ and parallel increases in temperature are background factors and probably not the principal causes directing the current wave of encroachment. There is probably not a single reason for encroachment, but a combination of factors that are difficult to disentangle. The prime cause of the current and recent encroachment appears to be high and constant levels of grass herbivory by domestic animals. This herbivory reduces fine fuel with a concomitant reduction in fire frequency or in some cases a complete elimination of fire from these communities. Conditions would now favor the woody plants over the grasses. Reduced grass competition, woody plant seed dispersal and changes in animal populations seem to modify the rate of encroachment rather than being the cause. High concentrations of atmospheric CO₂ are not required to explain current woody plant encroachment. Changes in these grassland communities will continue into the future but the specifics are difficult to predict. Density, cover and species composition will fluctuate and will probably continue to change. Increased levels of anthropogenic soil nitrogen suggest replacement of many legumes by other woody species. Modification and perhaps reversal of the changes in these former grassland communities will be an arduous, continuing and perhaps impossible management task.     

地下水位升降和抽水对海水楔影响的实验研究

为掌握滨海地带无压含水层中的海水楔因地下水位升降及抽水引起的移动规律,采用室内实验的方法,考察了海水楔在不同的水面坡度和抽水位置下的移动特性.通过改变水面坡度的方法探讨了海水楔侵入和后退的规律.在入侵时,海水楔在形成明确的咸淡水界面的同时向着内陆移动.水面坡度越小海水楔内陆入侵的范围越大,达到平衡状态所需要的时间越长.海水楔后退时的速度比侵入时快,且海水楔后退后其入侵过的区域能完全恢复到入侵前的状态.在海水楔外部抽水初期,海水楔的长度略有减小.在此之后其长度急剧增加,但随着平衡状态的接近,这种增长率逐渐变小.抽水引起的咸淡水界面的隆起位置位于抽水位置和咸淡水界面的最短距离处.抽取的地下水中海水的比例随着抽水位置与海水楔的接近会急剧增大.在海水楔内部抽水时,海水楔的长度随着时间呈指数函数的关系减小,且较外部抽水时到达平衡状态时所需要的时间短.此外,还给出了能预测因地下水位的升降引起的海水楔前进和后退的数学模型.     

The consequences and policy implications of urban encroachment into flood-risk areas: the case of Ibadan

Urban encroachment on floodplains has exacerbated flood disasters owing to the reduction in the floodplain’s capacity to mitigate flooding, thereby bringing more settlements and people at flood risk. This study examined the consequences and policy implications of urban encroachment into Ibadan’s flood-risk areas. The survey-based study relied on primary and secondary data, with multistage sampling procedures selecting 15% of the flood-affected buildings in 2011, from which a structured questionnaire was administered to 402 households. The study revealed that the institutional arrangements with respect to responsibilities, regulations and control of urban floodplains were fragmented and non-participatory. At least 62% of all the affected buildings encroached into the statutory setbacks to rivers. Household property lost/damaged included domestic goods (75%), domestic animals and pets (60%), working instruments/machines (31%), the source of domestic water (34%) and vehicles (29%). Strong direct correlations were found between the urban and peri-urban communities in (1) the number of damaged/lost property and (2) the number of households that suffered health-related challenges. The paper recommends that the overall co-ordination of flood management activities be entrusted to an institution that can assume responsibility for legal, technical, administrative and financial matters related to urban planning and flood-risk management.     

Cattle-rangeland management practices and perceptions of pastoralists towards rangeland degradation in the Borana zone of southern Ethiopia

A survey was conducted in the Borana pastoral areas of southern Ethiopia to assess current livestock production systems, rangeland management practices and the perceptions of the pastoralists towards rangeland degradation. This information is considered vital to future pastoral development planning and interventions. Data were collected from a total of 20 villages that were identified from 5 peasant associations, namely Did Yabello, Moyatte, Did Harra, Dubuluk and Melbana. The average household size in the study area was 7.23. The majority of the pastoralists relied on both livestock and crop farming. The average livestock holding per household was 14 cattle, 10 goats, 6 sheep and 2 camels. Livestock holdings, with the exception of camels, has shown a declining trend over time. The two most important traditional rangeland management strategies adopted by the pastoralists included burning and mobility, but since 1974/75 burning has no longer been practised. With regard to mobility, the livestock herding falls in two categories, namely: home based and satellite herding. The former involves the herding of milking cows, calves and immature animals (<2 years) close to the encampments. The latter constitutes the herding of bulls and immatures (>2 years) further away from the encampments. Based on the pastoralists' perceptions, the major constraints on livestock production in descending order, were recurrent drought, feed and water scarcity, animal diseases, predators and communal land ownership. All the respondents considered the condition of the rangelands to have declined dramatically over time. In the past most development policies were based on equilibrium theories that opposed the communal use of the rangelands and traditional range management practices. The way in which the pastoral system affects the rangeland ecosystem functioning is contentious to this theory and the 'tragedy of the commons'. There was also a perceived problem of bush encroachment and the ban on traditional burning practices and recurrent droughts were seen as aggravating factors to this invasion process. The increasing practice of crop cultivation on the rangelands was identified as a serious threat to future livestock production and traditional resource management practices. Suggestions for possible interventions to improve the productivity and sustainable use of these rangelands are made.     

Shrub communities as inhibitors of plant succession in southern Quebec

The purpose of our research was to identify shrub species growing in southern Quebec that inhibit ecological succession in power-line corridors. Results are presented in three parts. First, clonal characteristics that allowed the establishment of stable communities were identified. Second, successional vector analysis identified those species that have the potential to inhibit succession. In poorly drained sites those species wereCornus stolonifera, C. obliqua, Salix petiolaris, andSpiraea alba. In well-drained sites, those species wereZanthoxylum americanum, Rubus idaeus, Spiraea alba, Rhus typhina, andThuja occidentalis. Third, analysis of variance showed that there is a significantly larger number of tree seedlings found in adjacent herbaceous communities than found under the dense cover ofCornus stolonifera, C. obliqua, Salix petiolaris, Spiraea alba, Rhus typhina, Rubus idaeus, Thuya occidentalis, andZanthoxylum americanum. These results indicate that the planting of selected shrub species could, through biological control, delay reforestation.     

Influence of a Large Herbivore Reintroduction on Plant Invasions and Community Composition in a California Grassland

Abstract:  Despite many successful reintroductions of large mammalian herbivores throughout the world, remarkably little attention has focused on how these actions affect native and exotic vegetation at reintroduction sites. One such herbivore is tule elk ( Cervus elaphus nannodes ), which was on the brink of extinction in the mid 1800s, but now has numerous stable populations due to intensive reintroduction efforts. Here, we summarize results from a 5-year exclosure experiment that explored the effects of tule elk on a coastal grassland in northern California. Elk significantly altered the species composition of this community; the response of annual species (dominated heavily by exotic taxa) was dramatically different from perennial species. Elk herbivory increased the abundance and aboveground biomass of native and exotic annuals, whereas it either had no effect on or caused significant decreases in perennials. Elk also decreased the cover of native shrubs, suggesting that these herbivores play an important role in maintaining open grasslands. In addition, elk significantly reduced the abundance and biomass of a highly invasive exotic grass , Holcus lanatus, which is a major problem in mesic perennial grasslands. Our results demonstrate that the successful reintroduction of a charismatic and long-extirpated mammal had extremely complex effects on the plant community, giving rise to both desirable and undesirable outcomes from a management perspective. We suspect that these kinds of opposing effects are not unique to tule elk and that land managers will frequently encounter them when dealing with reintroduced mammals.