The influence of local elevation on the effects of secondary salinity in remnant eucalypt woodlands: Changes in understorey communities

Increasing land salinization in Australia is predicted to lead to severe declines in species diversity in affected areas, and perhaps significant numbers of species extinctions. Much of the diversity that will be lost consists of understorey and mid-storey species, yet the overwhelming majority of research has focussed on salinity tolerance in tree species. We investigated how the presence of a shallow, saline water table affected the understorey species composition, species richness and species diversity in two remnant Eucalyptus wandoo Blakely woodlands in the Western Australian wheatbelt. Species richness and species diversity were significantly lower in areas with a shallow water table at elevations < 0.5 m above the lowest local elevation, compared with both higher elevations and with areas of low elevation without a shallow water table. Species composition (Bray-Curtis similarities) was also significantly different in low elevation, saline areas. At one site, saline areas were colonized by native and alien species that were not present in the surrounding vegetation, yet the community that has developed does not contain either the species or structural diversity of the surrounding system. At the other site, no colonisation of saline areas by new species occurred. Even though small differences in elevation (< 0.5 m) at our study sites were important in moderating the impacts of salinity in areas with a shallow water table, the loss of species diversity, species richness and structural complexity in low-lying elevations indicated that the ecological risk from secondary salinity to species associated only with drainage lines, seasonally wet flats and other low-lying areas is severe. The priority is to identify those vegetation communities that are restricted to only low relative elevations within the landscape and that only occur in remnants predicted to be at a high risk of developing a shallow and saline water table.     

The effects of tree establishment on water and salt dynamics in naturally salt-affected grasslands

Plants, by influencing water fluxes across the ecosystem–vadose zone–aquifer continuum, can leave an imprint on salt accumulation and distribution patterns. We explored how the conversion of native grasslands to oak plantations affected the abundance and distribution of salts on soils and groundwater through changes in the water balance in naturally salt-affected landscapes of Hortobagy (Hungary), a region where artificial drainage performed ∼150 years ago lowered the water table (from −2 to −5 m) decoupling it from the surface ecosystem. Paired soil sampling and detailed soil conductivity transects revealed consistently different salt distribution patterns between grasslands and plantations, with shallow salinity losses and deep salinity gains accompanying tree establishment. Salts accumulated in the upper soil layers during pre-drainage times have remained in drained grasslands but have been flushed away under tree plantations (65 and 83% loss of chloride and sodium, respectively, in the 0 to −0.5 m depth range) as a result of a five- to 25-fold increase in infiltration rates detected under plantations. At greater depth, closer to the current water table level, the salt balance was reversed, with tree plantations gaining 2.5 kg sodium chloride m⁻² down to 6 m depth, resulting from groundwater uptake and salt exclusion by tree roots in the capillary fringe. Diurnal water table fluctuations, detected in a plantation stand but not in the neighbouring grasslands, together with salt mass balances suggest that trees consumed ∼380 mm groundwater per year, re-establishing the discharge regime and leading to higher salt accumulation rates than those interrupted by regional drainage practices more than a century ago. The strong influences of vegetation changes on water dynamics can have cascading consequences on salt accumulation and distribution, and a broad ecohydrological perspective that explicitly considers vegetation–groundwater links is needed to anticipate and manage them.     

西藏色季拉山土壤物理性质垂直地带性

以色季拉山为代表的藏东南高原山地的水土流失已成为区域生态环境的重要问题。对色季拉山不同海拔梯度土壤物理性质进行分析,结果表明:(1)除海拔3600、4200m外,土壤容重随土层深度增加而增大;总孔隙度、毛管孔隙度随土层深度增加而减小;非毛管孔隙度随土层变化无明显规律;饱和含水率、毛管含水率、田间持水率均随土层深度增加而减小。(2)不同海拔梯度,土壤容重总均值、总孔隙度、毛管孔隙度和非毛管孔隙度变化范围分别为:0.58—1.10g/cm~3、57.00%—72.47%、53.33%—67.59%和3.20%—4.87%。饱和含水率、毛管含水率、渗透性具有相同规律,均为3800、3400m处最大,3200m和3600m处最小,4000—4600m居中,田间持水率随海拔梯度变化呈M型波动性趋势。(3)土壤物理性质具有较强的空间异质性,各指标间有明显的空间自相关现象。土壤物理性质各指标在不同土层和海拔间有较明显的差异性,人为干扰也是导致土壤物理性质空间异质性的重要原因。(4)总体上,色季拉山表层土壤(0—10cm)物理结构优于深层次(10—30cm)土壤;3200m和3600m处最差,4000—4600m居中,3400—3800m最佳。研究结果提示,以色季拉山为代表的藏东南原始森林地带,土壤结构脆弱,为保持水土,应防止旅游和森林生产经营的过度开发。     

黑河下游河岸绿洲区包气带土壤水分与植被生长状况的研究

黑河下游荒漠平原区近年来由于河水流入量的持续减小,致使依赖河水补给来源的地下水呈减少趋势,造成区域性地下水位下降,从而引起一系列生态环境问题。通过研究黑河下游河岸绿洲区非饱和带的含水量、地下潜水埋深、土壤类型结构等与河岸绿洲植被生长状况之间的关系,发现土壤结构、地下水位埋深和土壤盐分是决定研究区植被生长状况的3个主要因素,而土壤结构是不可改变的,土壤盐分又与地下水关系密切,因此,地下水是极端干旱区植被生长状况的决定因素。     

Ecological consequences of altered hydrological regimes in fragmented ecosystems in southern Australia: Impacts and possible management responses

Abstract Although the potential impacts of rising water tables and secondary salinization on agricultural land in southern Australia have been recognized for some time, it is only recently that the impacts on native vegetation have been considered. Despite the likely extent and severity of the problem, no comprehensive approach to assessing the impact of salinity upon native vegetation has been attempted to date. In the present paper, we discuss the causes and impacts of rising water tables and dryland salinity, assess the levels of risk in different ecosystem types and consider the possibilities for the maintenance of biodiversity and ecosystem function in vegetation at risk. We examine the salinity risk to woodland vegetation in the Western Australian (WA) wheatbelt, and consider both broad‐scale context and finer‐scale variation within individual patches of vegetation. From this information, we develop a set of conceptual models of the potential impacts of shallow saline water tables on ecosystem structure and processes in remnant vegetation in agricultural areas, particularly in the WA wheatbelt. First, we suggest that fine‐scale variability in surface topography and soil characteristics may play an important role in limiting the impacts of rising saline water tables. The outcome will depend on the interaction of the heterogeneity of the impact, species distribution in relation to small‐scale environmental heterogeneity and variation in species response to hydrological change. Second, we suggest that shallow saline water tables can be considered to cause an ‘edge effect’, which moves inwards from the edge of remnants of native vegetation. Finally, we consider how saline surface flows exacerbate the effects of shallow saline water tables and hasten vegetation decline in remnant areas. We put these models forward as hypotheses to be tested in different situations. We contrast the situation of secondary salinization in Australian vegetation with that of naturally saline systems in Australia and elsewhere, and suggest that these systems may provide important signposts toward developing management approaches for vegetation at risk. In conclusion, we consider the need to set priorities for the protection and restoration of natural vegetation at risk from altered hydrology, based on an assessment of relative threat and probability of persistence or recovery. We highlight the urgency for action that protects native vegetation from the increasing risks of rising water tables.     

Relationships between vegetation zonation and environmental factors in newly formed tidal marshes of the Yangtze River estuary

The Yangtze River delta is characterized by rapidly accreting sediments that form tidal flats that are quickly colonized by emergent vegetation including Scirpus mariqueter and the invasive species Spartina alterniflora. We measured soil surface elevation, water table depth, soil salinity, water content and compaction in the tidal flat, the Scirpus and Spartina zones and their borders to identify relationships between environmental factors and colonization by Scirpus and Spartina. With increasing elevation from tidal flat to Spartina, inundation frequency and duration, moisture and depth to water table decreased whereas soil salinity, temperature and compaction increased. High soil moisture and groundwater and low salinity were the characteristics of the tidal flat and its border with Scirpus. The Spartina zone and its border with Scirpus were characterized by greater salinity and elevation relative to the other zones. Our findings suggest that soil salinity controls patterns of plant zonation in the newly formed tidal salt marshes whereas elevation is of secondary importance. Our results suggest that patterns of vegetation zonation in tidal marshes of the Yangtze River delta are controlled by environmental factors, especially (low) salinity that favors colonization by Scirpus in the lower elevations of the marsh.     

青藏高原高寒草甸不同海拔梯度下土壤微生物群落碳代谢多样性

土壤微生物群落功能多样性对维持生态系统功能和稳定性具有非常重要的意义。为探究青藏高原高寒草甸不同海拔梯度下土壤微生物碳源利用差异以及影响机制,运用Biolog微平板技术,研究了西藏当雄县草原站4300—5100 m的6个不同海拔梯度下土壤微生物群落碳源代谢多样性。研究结果表明:(1)不同海拔下高寒草甸土壤微生物碳源的利用程度均随培养时间的延长而升高;微生物代谢活性和群落多样性指数均随海拔升高呈现先上升后下降的单峰变化趋势,整体表现4800 m4950 m4400 m4650 m5100 m4300 m;(2)主成分分析表明不同海拔显著影响了土壤微生物群落碳源代谢多样性,其中碳水化合物类、氨基酸类和胺类碳源是各海拔土壤微生物的偏好碳源;碳水化合物类、羧酸类、氨基酸类和胺类碳源的利用强度受海拔影响较大;(3)分类变异分析表明,土壤、植物和气候因素是影响不同海拔碳源利用变异的主要影响因子,可解释不同海拔的碳源利用差异的79.0%;排除环境因子之间的多重及交互作用,偏曼特尔检验表明土壤含水量、植被丰富度和年均降水量是影响不同海拔微生物碳源利用多样性的最重要的环境因子。综上,研究表明青藏高寒草甸不同海拔土壤微生物碳源代谢多样性呈现显著的海拔差异趋势,其海拔差异主要受到土壤含水量、植被丰富度和年均降水量的影响。     

Water relations and growth responses of Uniola paniculata (sea oats) to soil moisture and water-table depth

Summary This study examined the water relations and growth responses of Uniola paniculata (sea oats) to (1) three watering regimes and (2) four controlled water-table depths. Uniola paniculata is frequently the dominant foredune grass along much of the southeastern Atlantic and Gulf coasts of the United States, but its distribution is limited in Louisiana. Throughout most of its range, U. paniculata tends to dominate and be well adapted to the most exposed areas of the dune where soil moisture is low. Dune elevations in Louisiana, however, rarely exceed 2 m, and as a result the depth to the water table is generally shallow. We hypothesized that if U. paniculata grows very near the water-table, as it may in Louisiana, it will display signs of water-logging stress. This study demonstrated that excessive soil moisture resulting from inundation or shallow water-table depth has a greater negative effect on plant growth than do low soil moisture conditions. Uniola paniculata's initial response to either drought or inundation was a reduction of leaf (stomatal) conductance and a concomitant decrease in leaf elongation. However, plants could recover from drought-induced leaf xylem pressures of less than-3.3 MPa, but prolonged inundation killed the plants. Waterlogging stress (manifested in significantly reduced leaf stomatal conductances and reduced biomass production) was observed in plants grown at 0.3 m above the water table. This stress was relieved, however, at an elevation of 0.9 m above the water table. As the elevation was increased from 0.9 to 2.7 m, there were no signs of drought stress nor a stimulation in growth due to lower soil moisture. We concluded that although U. paniculata's moisture-conserving traits adapt it well to the dune environment, this species can grow very well at an elevation of only 0.9 m above the water table. Field measurements of water-table depth in three Louisiana populations averaged about 1.3 m. Therefore, the observed limited distribution of U. paniculata along the Louisiana coast apparently cannot be explained by water-logging stress induced by the low dune elevations and the corresponding shallow water-table depth.     

Root dynamics of Melaleuca halmaturorum in response to fluctuating saline groundwater

Melaleuca halmaturorum is a salt and waterlogging tolerant tree and thus often occurs in saline areas fringing permanent wetlands and in ephemeral swamps. The dominance of this tree in natural groundwater discharge areas may result in M. halmaturorum transpiration making a major contribution to groundwater discharge. To quantify this the seasonal changes in tree water sources in response to fluctuating soil salinity and waterlogging were examined. This study was conducted in a natural system where seasonally fluctuating saline groundwater (64 dS m^–1; 0.3–1.2 m deep) allowed the patterns of M. halmaturorum root water uptake to be followed over a 15 month period. Tree water sources were examined using the naturally occurring stable isotopes of water, while new root growth was examined using a field root observation window and from soil cores. The presence of isotopic fractionation of ²H under conditions of soil salinity and waterlogging was tested in a glasshouse experiment. Measurements of soil and leaf water potential were also made to examine the possible water sources and limits to water uptake. No isotopic fractionation was found by tree roots under conditions of salinity and waterlogging. M. halmaturorum trees were active in taking up groundwater at most times and combined this with a shallower soil water source replenished by rainfall in winter. Water uptake was concentrated in the deeper parts of the soil profile when the groundwater was at its deepest and salt had accumulated in the surface soils, at the end of summer. When groundwater rose, at the end of winter, roots responded by extracting water from near the soil surface (0–0.1 m), at the new watertable. This pattern of water uptake in response to groundwater fluctuations and salt accumulation in the surface soil was also reflected in new root tip appearance at the root observation window. Fluctuations in leaf water potential fallowed fluctuations in surface soil (0.1 m depth) water potential at all times. In winter leaf water potential reflected the absolute values of the surface soil water potential but in summer it was between surface soil and groundwater water potentials. We conclude that M. halmaturorum used groundwater in summer and a combination of rainfall and groundwater from the surface soils in winter. The ability to take up water from saline substrates through the maintenance of low leaf water potential, combined with this ability to rapidly alter root water uptake in response to changes in soil water availability contributed to the survival of M. halmaturorum in this saline swamp.     

Ecohydrological gradients and their restoration on the periphery of extracted peatlands

The moss layer transfer technique is effective at restoring extracted peatland surfaces. However, remnant peatlands persist on the periphery of extracted surfaces. These remnant peatlands drop steeply to extracted surfaces, producing artificial ecotones that are more challenging to restore. We asked to what degree natural ecotones at undisturbed reference fens can act as models for the restoration of artificial ecotones around an extracted peatland, and whether management actions can ameliorate conditions in artificial ecotones. We compared changes in elevation, water table, peat, and multiple vegetation characteristics between natural ecotones and unmanaged artificial ecotones. We then clear‐cut peripheral strips, completely filled perimeter canals, and smoothed peripheral slopes around sections of the extracted surfaces to assess whether hydrological conditions improved. Without management, artificial ecotones are not good models of natural ecotones. The elevation gradient is steep, and water tables drop steeply within 8 m of blocked perimeter canals, with possible effects at 25 m. The consequent vegetation had denser tree saplings, faster tree growth, almost no moss cover, and low moss species richness. After these management actions, water tables increased to within approximately 5 cm of those along natural ecotones. Future study is required to assess the extent of vegetation recovery, but these results hold promise for a more holistic rehabilitation of ecotones on the periphery of extracted peatland surfaces. We present recommendations to optimize the management actions on the periphery of extracted peatlands.