Regeneration of Nothofagus dombeyi(Mirb.)Ørst.in little to moderately disturbed southern beech forests in the Andes of Patagonia(Argentina)

Natural forests and stands subjected to little to moderate human impact are continuously declining worldwide and with these,their biodiversity and ecosystem services.Many Nothofagus forests in the south of the South American continent are in a pristine state or only moderately impacted by humans.Forest grazing by livestock,in the past and still today often practiced in a non-sustainable way is,however,increasingly under discussion to meet current environmental and socio-economic challenges.Accordingly,we investigate the regeneration of Nothofagus dombeyi,a keystone species of the Patagonian Andes,in secondary forests in Argentinian northern Patagonia,particularly addressing the role of disturbance through grazing by livestock at various intensities.We test the hypothesis that the regeneration of this tree species is favored by grazing impact and,thus,disturbance of the herb layer and soil surface.In support of our hypothesis,Nothofagus dombeyi regeneration was significantly higher in terms of individuals and height classes in moderately grazed forests.Multivariate analysis shows significant positive effects of moderate grazing pressure,herb layer cover,and the occurrence of bare soil on the regeneration of Nothofagus dombeyi.Our results show that an integration of livestock grazing and forest regeneration is possible and that agroforestry systems can be an adequate management option for stakeholders in the region.A grazing management can also be part of a forest fire prevention strategy.However,the regeneration success and grazing pressure should be continuously monitored.

Altitudinal patterns of stand structure and herb layer diversity of Picea schrenkiana forests in the central Tianshan Mountains,Northwest China

Altitude is a useful indicator to examine patterns of forest structure and species diversity in relation to environmental factors.In this study,the altitude patterns of forest stand structure and species diversity were analyzed across 20 plots in the Tianchi Nature Reserve,Northwest China.The results showed that mean stem height（Hm）,maximum stem height（Hmax） and mean stem diameter at breast height（Dm） of Picea schrenkiana trees all decreased significantly with increasing altitude.Potential tree height（H＊） decreased while stem taper increased significantly as altitude increased,suggesting remarkable altitudinal changes in biomass allocation between the diameter and height growth of Picea schrenkiana.Understory herbaceous richness increased significantly with increasing altitude,or with decreasing total basal area（TBA）,Hm and stand volume（Volume）.High light availability for understory herbs might account for the higher species richness at high altitude.Sorensen Index decreased significantly with the increase in altitude intervals,while the Cody Index demonstrated a converse pattern,suggesting greater differences in species composition with larger distances.

Cutting of Phragmites australis as a lake restoration technique： Productivity calculation and nutrient removal in Wuliangsuhai Lake, northern China

Reed is one of the most frequent and dominant species in wetlands all over the world, with common reed （Phragmites australis （Cav.） Trin. ex Steud.） as the most widely distributed species. In many wetlands, P. australis plays a highly ambivalent role. On the one hand, in many wetlands it purifies wastewater, provides habitat for numerous species, and is a potentially valuable raw material, while on the other hand it is an invasive species which expands aggressively, prevents fishing, blocks ditches and waterways, and builds monospecies stands. This paper uses the eutrophic reed-swamp of Wuliangsuhai Lake in Inner Mongolia, northern China, as a case to present the multiple benefits of regular reed cutting. The reed area and aboveground biomass production are calculated based on field data. Combined with data about water and reed nutrient content, the impact of reed cutting on the lake nutrient budget （N and P） is investigated. Currently, at this lake around 100,000 tons of reed are harvested in winter annually, removing 16% and 8% of the total nitrogen and phosphorus influx, respectively. Harvesting all available winter reed could increase the nutrient removal rates to 48% and 24%, respectively. We also consider the effects of summer harvesting, in which reed biomass removal could overcompensate for the nutrient influx but could potentially reduce reed regrowth.

Age Structure and Growth of Degraded Populus euphratica Floodplain Forests in North-west China and Perspectives for Their Recovery

The Populus euphratica floodplain forests along the lower reaches of the Tarim River in continental-arid Xinjiang （northwest China） have been declining in recent decades as a result of dam constructions and the withdrawal of water from the river. Since 2000, controlled flooding has been carried out to counteract this process of degradation. However, the consequences of a changing water regime for the floodplain ecosystem are not yet sufficiently investigated. In 2004 we sampled a total of 150 P. euphratica trees along two transects at the middle and lower reaches of the Tarim River to compare the age structure and growth of these two tree populations. The anthropogenic drawdown of the groundwater table had no effect on the average growth of the trees at the lower reaches. However, tree growth abruptly increased after controlled flooding, thus showing distinct signs of recovery.

Influence of soil and microclimate on species composition and grass encroachment in heath succession

Aims and Methods Mostly due to land use changes,European heathlands have become increasingly rare.In addition,the increasing amount of atmospheric nitrogen deposition has resulted in an encroachment of grasses and a loss in species diversity.Despite many investigations,information about the precise environmental parameters that determine the development and maintenance of heathland vegetation is still insufficient.In order to determine the environmental factors that control heath succession and grass encroachment,and to develop appropriate management schemes,we studied the influence of several soil and microclimate parameters on species composition and vegetation characteristics in five successional stages in a coastal heathland on the island of Hiddensee,north-east Germany,where the encroachment of Carex arenaria has become a major problem.Important Findings We recorded the highest plant species richness in grey dune and birch forest plots,while the encroachment of C.arenaria let to a significant decline in plant species richness.The most important environmental factors influencing species richness and distribution of single species were microclimate,soil moisture,soil pH and the C/N ratio.While many studies reported the importance of differences in nutrient availability,we found no significant correlations between soil nutrient availability and vegetation pattern.Environmental conditions in dense C.arenaria stands,especially soil properties(e.g.soil pH),showed great differences in comparison to the other successional stages.However,no correlations between the encroachment of C.arenaria and single environmental factors were found.Our results show that not only soil nutrients are important abiotic factors in heaths but that also microclimate and soil moisture play an important role and that many factors are involved in heath succession and in the promotion of grass encroachment.Management plans for the conservation and restoration of heathlands should therefore focus on the specific site conditions and should take several abiotic and biotic factors into account.

Exotic tree seedlings are much more competitive than natives but show underyielding when growing together

Aims Invasive species continue to be a worldwide threat to ecosystems mainly as a cause for biodiversity loss.Forest ecosystems,for example,are subject to a change in species composition due to the invasion of exotic species.specifying the attributes that cause the strong competitiveness of several exotic species may improve the ability to understand and effectively manage plant invasions in the future.In this study the following hypotheses were tested:(1)biomass production of below-and aboveground plant compo-nents of the exotic tree species is higher than that of the natives,resulting in a higher competitiveness of the exotics;(2)the exclu-sion of root competition has a positive effect on the biomass pro-duction of the inferior native species;and(3)mixtures of native and exotic species yield a higher biomass production than the respective monocultures.Methods a pot experiment,containing about 2000 tree seedlings,was established.We investigated the biomass productivity and growth reactions of two native(Quercus robur l.,Carpinus betulus l.)and two exotic tree species(Prunus serotina Ehrh.,Robinia pseudoacacia l.)in different intra-and interspecific,competitive situations with and without the influence of root competition.Important Findingsthe biomass production of both exotic species was significantly higher and led to a strong competitive advantage,resulting in a biomass decrease of the less competitive native species.the high belowground biomass of both exotic species had a negative effect on the biomass production.the competitive pressure of exotic tree seedlings on the native ones was largely driven by root competition.Furthermore,mixtures of native and exotic tree species had a higher productivity than their growth in monocultures would have predicted.Competition was lower for exotic species in mixtures with the less productive native species compared to the competition in monocultures or in mixture with the other highly productive exotic species.accordingly,both highly competitive exotic species produced less biomass in mixture with each other compared to monocultures.Despite the significantly higher biomass of P.serotina in all mixtures and in monoculture,R.pseudoacacia seemed to be the dominating species.Due to its strong root competition,R.pseudoacacia significantly reduced the biomass production of P.serotina.