Surface Soil Salinity Research Articles

Quantifying soil salinity in areas invaded by Tamarix spp.

Elevated soil salinity is often associated with Tamarix invasion; however, it's unclear whether soils are more saline because of Tamarix or other environmental factors. Surface soil salinity was investigated along a flow-regulated, arid river with dense Tamarix of varying age to determine which factors best explain soil salinity. Flooding was the most important predictor, reducing salinity by nearly 70%. Soils under Tamarix had lower salinity than adjacent areas without woody cover in non-flooded areas suggesting that evaporation in arid environments may contribute more surface salts than Tamarix or may exacerbate plant inputs. Under most conditions, higher salinities were found under Tamarix than natives. An exception to this pattern was that soils under the smallest trees were more saline for natives. Relationships between soil salinity and stem size suggest that salts increase over time under Tamarix unless they are removed by flooding. However, the most mature stands had lower salinity than expected, reflecting some additional mechanism. Soil texture and distance from the river were important, but interacted strongly with other factors. The observed relationships between surface soil salinity and Tamarix stem size, a predictor of aboveground age, suggest Tamarix plays an active role in floodplain salinization within the sampled area.

Spatial Variation of Soil Salinity in the Mexicali Valley, Mexico: Application of a Practical Method for Agricultural Monitoring

The degradation of irrigated lands through the process of soil salinization, or the buildup of salts in the soil, has hampered recent increases in agricultural productivity and threatens the sustainability of large-scale cultivation in critical agricultural regions of the world. Rapid detection of soil salinity on a regional basis has been identified as key for effective mitigation of such land degradation. The ability to detect regional patterns of soil salinity at an accuracy sufficient for regional-scale resource management is demonstrated using Landsat 5 Thematic Mapper (TM) imagery. A case study of the Mexicali Valley of Baja California, Mexico was selected due to the region's agricultural significance and concern for future soil salinity increases. Surface soil salinity was mapped using georeferenced field measurements of electrical conductivity (EC), collected concurrently with Landsat 5 TM imagery. Correlations between EC measurements and common indices derived from the satellite imagery were used to produce a model of soil salinity through regression analysis. Landsat band 7, TNDVI, PCA 1, Tasseled Cap 3 and Tasseled Cap 5 were found to offer the most promising correlations with surface soil salinity. Generally low levels of soil salinity were detected, however, distinct areas of elevated surface salinity were detected at levels potentially impacting sensitive crops cultivated within the region. The difficulty detecting low levels of salinity and the mid-range spatial resolution of Landsat 5 TM imagery restrict the applicability of this methodology to the study of broad regional patterns of degradation most appropriate for use by regional resource managers.

Short-term and diurnal patterns of salt secretion by Tamarix ramosissima and their relationships with climatic factors

Tamarisk (Tamarix spp.) is an invasive shrub or tree species in the United States. Glands within Tamarix leaves secrete salt that may increase surface soil salinity and consequently result in an unviable ecosystem for native riparian tree species. To increase our understanding of the effects of invasive Tamarix on the ecosystem and its native riparian vegetation, we investigated salt secretion of a stand of Tamarix ramosissima in the Las Vegas Wash, Nevada. Short-term and diurnal salt secretion rates were monitored in the stand from September to October 2009. Soil samples were collected beneath the canopy so that water and salinity conditions could be determined. We used weather data from a Las Vegas database in conjunction with soil temperature data collected for this study. The major cation secreted by salt glands was sodium. Diurnal variation in the salt secretion rate had a maximum during the day, suggesting that salt secretion is dependent on salt transportation to leaves. Variations in the salt secretion rates were mostly influenced by solar radiation, suggesting that salt secretion in T. ramosissima would be enhanced by light in natural conditions.

Spatio-Temporal Variability of Soil Salinity in Alluvial Plain of the Lower Reaches of the Yellow River—A Case Study

Soil salinity and hydrologic datasets were assembled to analyze the spatio-temporal variability of salinization in Fengqiu County, Henan Province, China, in the alluvial plain of the lower reaches of the Yellow River. The saline soil and groundwater depth data of the county in 1981 were obtained to serve as a historical reference. Electrical conductivity (EC) of 293 surface soil samples taken from 2 km × 2 km grids in 2007 and 40 soil profiles acquired in 2008 was analyzed and used for comparative mapping. Ordinary kriging was applied to predict EC at unobserved locations to derive the horizontal and vertical distribution patterns and variation of soil salinity. Groundwater table data from 22 observation wells in 2008 were collected and used as input for regression kriging to predict the maximum groundwater depth of the county in 2008. Changes in the groundwater level of Fengqiu County in 27 years from 1981 to 2008 was calculated. Two quantitative criteria, the mean error or bias (ME) and the mean squared error (MSE), were computed to assess the estimation accuracy of the kriging predictions. The results demonstrated that the soil salinity in the upper soil layers decreased dramatically and the taxonomically defined saline soils were present only in a few micro-landscapes after 27 years. Presently, the soils with relatively elevated salt content were mainly distributed in depressions along the Yellow River bed. The reduction in surface soil salinity corresponded to the locations with deepened maximum groundwater depth. It could be concluded that groundwater table recession allowed water to move deeper into the soil profile, transporting salts with it, and thus played an important role in reducing soil salinity in this region. Accumulation of salts in the soil profiles at various depths below the surface indicated that secondary soil salinization would occur when the groundwater was not controlled at a safe depth.

Detection of salt affected soil in rice-wheat area using satellite image

Pakistan’s agricultural system is predominately irrigated, which consumes 90% of fresh water resources and contributes 80% to the national production. It is ironic that the irrigated areas are the ones that are afflicted most by the twin menaces of waterlogging and salinity. Climate change is likely to exacerbate this soil degradation process by way of heavy rainfalls and by increased evaporation, respectively. Detection of soil salinity is usually done by laborious soil sampling. A study, to delineate surface soil salinity in the prime rice-wheat cropping area of Pakistan, was conducted. The study employed an index-based approach of using optical remote sensing data in combination with geographic information system. The effectiveness of different satellite imagery indicators was examined. Several combinations of the ratio of signals received in different spectral bands were used for development of this index. Near infrared and thermal IR spectral bands proved to be most effective as this combination helped easy detection of salt affected area from the non saline area. Results showed that 19% of the rice-wheat cropping area of Gujranwala district in Rachna Doab of central Punjab province of Pakistan is salt affected. These results are in agreement with the published survey data. Seasonal dependency of salinity was also analyzed so as to obtain correct classification.

Effect of Water Quality and Different Planting Methods on Wheat Yield

To study the effects of water quality and planting method on wheat yield, a field experiment was conducted. The study indicated that grain yield and yield components decreased with the utilization of saline irrigation water. However, salinity had the least effect in the system using furrow-irrigated raised wavy beds with 60 cm furrow to furrow width (FIRWB60) because it had less surface soil salinity compared to conventional flat planting (FP). For the FIRWB60 system, 1000-grain weight, grain protein, and leaf chlorophyll content were greater than FP. This resulted in grain yield increases of 5.3 and 12.5%, for FIRWB60 in the saline irrigation water of 8 and 12 dS m –1, respectively, compared with FP. Based on the grain yield and yield components, it can be concluded that FIRWB is a suitable planting system for reducing salinity defects. Therefore, it can provide sustainable agriculture in arid and semi-arid regions.

Trends in Soil Quality Under High Tunnels

Growers have indicated that changes in soil quality under production in high tunnels is an important problem, but these have not yet been quantified or critically assessed in the central Great Plains of the United States. We conducted surveys of grower perceptions of soil quality in their tunnels (n = 81) and compared selected soil quality indicators (salinity and particulate organic matter carbon) under high tunnels of varying ages with those of adjacent fields at sites in Kansas, Missouri, Nebraska, and Iowa in the United States. Fourteen percent of growers surveyed considered soil quality to be a problem in their high tunnels, and there were significant correlations between grower perceptions of soil quality problems and reported observations of clod formation and surface crusting and to a lesser extent surface mineral deposition. Grower perception of soil quality and grower observation of soil characteristics were not related to high tunnel age. Soil surface salinity was elevated in some high tunnels compared with adjacent fields but was not related to time under the high tunnel. In the soil upper 5 cm, salinity in fields did not exceed 2 dS·m−1 and was less than 2 dS·m−1 under 74% of high tunnels and less than 4 dS·m−1 in 97% of high tunnels. The particulate organic matter carbon fraction was higher in high tunnels than adjacent fields at 73% of locations sampled. Particulate organic matter carbon measured 0.11 to 0.67 g particulate organic matter per g of the total carbon under high tunnels sampled. Particulate organic matter carbon in the soil was also not correlated to age of high tunnel. Soil quality as measured in this study was not negatively impacted by use of high tunnel structures over time.

Mulching and water quality effects on soil salinity and sodicity dynamics and cotton productivity in Central Asia

Water scarcity and the predicted impact of climate change will necessitate the use of alternate available water resources in agriculture, such as saline water, to narrow the gap between demand and supply of freshwater. Saline water, in combination with freshwater or alone, is used to irrigate cotton ( Gossypium hirsutum L.) in Central Asia in summer when there are often severe freshwater shortages. The use of saline water without appropriate management can result in the accumulation of salts in the root zone with associated negative impacts on crop productivity. The accumulation of salts in surface soil layers can be managed by reducing evaporation from the soil surface. A 3-year field study on a saline soil (EC e = 13.9 dS m −1; SAR = 3.1) in the Syr-Darya River Basin of Uzbekistan was undertaken to evaluate the effects of wheat straw mulching on alternate irrigation furrows (1.5 t ha −1) and different levels of irrigation water salinity (4.0, 6.2, and 8.3 dS m −1) on soil salinity and sodicity dynamics, cotton yield, and crop water productivity. Compared to the pre-experiment status in 2005, the average increase in salinity in the upper 0.15 m layer of post-cotton 2007 soil under mulching treatments was significantly less than the non-mulching treatments. On average, there was a 20% increase in surface soil salinity of the non-mulching treatments compared to the mulching treatments. These treatment differences were less with increasing soil depth. Similar trends were observed with respect to changes in soil SAR in the top soil and across the soil profile. Cotton yield and water productivity under mulching treatments were significantly greater than non-mulched treatments at a given irrigation water salinity level. In addition, cotton yields were up to 800 kg ha −1 higher and crop water productivity (lint + seed) up to 0.47 kg m −3 greater in the mulching treatments than the farmers’ managed fields with conventional practices in the same region. These results suggest that by using appropriate combinations of water quality and mulching, there could be substantial increase in crop yield and water productivity resulting in water savings of up to 0.5 m 3 for each kg of cotton produced. When translated on a broader scale, such water savings are significant in a region where freshwater supplies are constrained and salt-induced water quality deterioration is widespread.

Medium‐term vegetation dynamics and their association with edaphic conditions in two Hungarian saline grassland communities

AbstractMedium‐term (5.5 years) changes in the cover of major species in “Artemisia saline puszta” (Ass) and “Pannonic Puccinellia limosa hollow” (PPlh) grassland communities in the Kiskunság region, Hungary, were monitored and analyzed in relation to abiotic factors (e.g. air temperature, precipitation, soil moisture, salinity and alkalinity). Soil salinity varied considerably, indicating leaching and desalinization of surface layers as the most typical process occurring in the region. Yearly average covers of Artemisia santonicum and Plantago maritima were negatively and positively related to surface soil salinity, respectively, in accordance with their salt tolerance. Multiple regression analysis showed soil pH and salinity to be the most important factors determining yearly average cover of plants at Ass. Increasing pH increased the cover of A. santonicum and P. maritima, but decreased the cover of Podospermum canum. Increasing salinity decreased the cover of A. santonicum and P. canum. At PPlh, pH of groundwater had a positive effect and the lakewater level had a negative effect on the cover of Puccinellia limosa. The results provide information on the ongoing changes in the soil properties and the resulting changes in plant composition in these Hungarian salt‐affected grasslands.

Spatiotemporal changes in soil salinity in a drip-irrigated field

We explored the spatiotemporal variability in soil salinity using in situ data of soil electrical conductivity (EC) sampled on six dates in 2007 in a drip-irrigated cotton field. Descriptive statistics and geostatistics were used to analyze the data, and the kriging method was applied to map the spatial patterns of EC. The results showed that EC was negatively correlated with cotton yield ( p < 0.01) and was the key limiting factor for cotton growth under the drip irrigation system. Mean EC increased during the pre-irrigation period and decreased in the drip-irrigation stages. The coefficient of variation (CV i ) showed an increasing trend from 45.2 to 64.5%. In the cotton growth stages, the effective ranges of spatial correlation generally decreased from 138.84 to 102.76 m. The six smoothed contour maps displayed quite similar patterns, with high salinity in the eastern and southern sections and low salinity in the western and northern sections of the study area. The topography and climate highly influenced the spatiotemporal variability of soil salinity. The widespread practices of plastic film mulching, organic matter and drip irrigation also affect soil salinity. The use of saline groundwater for irrigation requires that surface soil salinity be controlled by leaching and draining excess water and salt. While cotton can be safely planted in this region, other crops with low thresholds of salinity tolerance cannot be planted in early spring until the surface salinity level decreases.

Salt tolerance and avoidance mechanisms at germination of annual pasture legumes: importance for adaptation to saline environments

Mechanisms for tolerance and avoidance of salinity at germination were examined in five self-regenerating annual pasture legumes of Mediterranean origin (Medicago polymorpha, Melilotus siculus, Trifolium subterraneum, T. michelianum and T. tomentosum). The maximum NaCl concentrations, for which no reduction in germination percentage occurred, were 300 mM for M. siculus, 240 mM for M. polymorpha and 120 mM for T. michelianum, T. subterraneum and T. tomentosum. Following 21 days exposure to 300 mM NaCl, imbibed seeds of T. subterraneum showed reduced germinability upon transfer to non-saline solution, whereas those of the four other species recovered full germinability. Following exposure to 600 mM NaCl, however, only T. michelianum and Melilotus siculus had some degree of recovery, with 38% and 31% germinability, respectively. Na+ in imbibed seed tissues of all species increased markedly with increasing NaCl concentration, while K+ decreased in all but Melilotus siculus. Seed coat impermeability (‘hard seeds’) acted as protection against the toxic effects of salinity. Melilotus siculus, Medicago polymorpha and T. tomentosum showed a delay in hard-seed breakdown (‘seed softening’) over the summer–autumn period, compared to T. subterraneum and T. michelianum, which acts to defer germination until soil surface salinity levels are likely to be lower. These three species also had higher levels of residual hard seeds. The results from this study support field observations that Melilotus siculus and Medicago polymorpha are the best adapted annual pasture legumes for highly saline soils in southern Australia, T. tomentosum and T. michelianum have some adaptation to moderately and mildly saline soils, respectively, whereas T. subterraneum has no adaptive traits for even mildly saline environments. A model for annual legume germination on saline land in southern Australia is also presented.

Effect of different mulch materials on winter wheat production in desalinized soil in Heilonggang region of North China

Freshwater shortage is the main problem in Heilonggang lower-lying plain, while a considerable amount of underground saline water is available. We wanted to find an effective way to use the brackish water in winter wheat production. Surface mulch has significant effect in reducing evaporation and decreasing soil salinity level. This research was aimed at comparing the effect of different mulch materials on winter wheat production. The experiment was conducted during 2002~2003 and 2003~2004. Four treatments were setup: (1) no mulch, (2) mulch with plastic film, (3) mulch with corn straw, (4) mulch with concrete slab between the rows. The result indicated that concrete mulch and straw mulch was effective in conserving soil water compared to plastic film mulch which increased soil temperature. Concrete mulch decreases surface soil salinity better in comparison to other mulches used. Straw mulch conserved more soil water but decreased wheat grain yield probably due to low temperature. Concrete mulch had similar effect with plastic film mulch on promoting winter wheat development and growth.

VEGETATION, SOILS, AND HYDROGEOMORPHOLOGY OF RIPARIAN PATCH TYPES OF A DRYLAND RIVER

From a landscape perspective, riparian corridors can be viewed as mosaics of vegetation patches. We delineated 10 patch types within the floodplain of the San Pedro River (Arizona) on the basis of physiognomy, dominant overstory species, and tree size class; and we assessed differences in hydrogeomorphology, vegetation structure, plant species richness, and soil chemistry and texture. Patches of tamarisk (Tamarix), an introduced species, fell within the continuum of variation shown by other patch types in the landscape mosaic. Among the tree-dominated types, cottonwood-willow (Populus-Salix) and tamarisk patches were inundated more frequently than mesquite (Prosopis) patches, while cottonwood-willow patches had shallower groundwater than tamarisk or mesquite patches. Due to the wetter conditions, cottonwood-willow patches had a high relative abundance of wetland and exotic species in the understory. Tamarisk patches and wet shrublands (Baccharis salicifolia–Salix exigua) had high woody stem densities while cottonwood-willow patches had dense canopy cover. In association with differences in canopy cover, cottonwood-willow patches had low herbaceous species richness but high woody species richness, while tamarisk patches had high herbaceous and low woody species richness. Soil electrical conductivity, silt content, organic matter content, and available phosphorus increased from young to old stands of both tamarisk and cottonwood-willow, often resulting in greater differences between patches of different size/age class than between patches with different dominant species. Surface soil salinity (electrical conductivity) was low in all patches, including those dominated by tamarisk. Nitrate was abundant in soils of tamarisk patches (perhaps reflecting their high clay content) and wet shrubland patches. Dry shrublands (Hymenoclea-Ericameria) and wet shrublands were similar to young forest patches in having coarse soils with little organic matter.

Naturally Saline Boreal Communities as Models for Reclamation of Saline Oil Sand Tailings

Abstract Reclaimed landscapes after oil sands mining have saline soils; yet, they are required to have similar biodiversity and productivity as the predisturbance nonsaline landscape. Given that many species in the boreal forest are not tolerant of salinity, we studied the effects of soil salinity on plant communities in natural saline landscapes to understand potential plant responses during the reclamation process. Vegetation–soil relationships were measured along transects from flooded wetlands to upland forest vegetation in strongly saline, slightly saline, nonsaline, and reclaimed boreal landscapes. In strongly saline landscapes, surface soil salinity was high (&gt;10 dS/m) in flooded, wet‐meadow, and dry‐meadow vegetation zones as compared to slightly saline (&lt;5 dS/m) and nonsaline (&lt;2 dS/m) landscapes. Plant communities in these vegetation zones were quite different from nonsaline boreal landscapes and were dominated by halophytes common to saline habitats of the Great Plains. In the shrub and forest vegetation zones, surface soil salinity was similar between saline and nonsaline landscapes, resulting in similar plant communities. In strongly saline landscapes, soils remained saline at depth through the shrub and forest vegetation zones (&gt;10 dS/m), suggesting that forest vegetation can establish over saline soils as long as the salts are below the rooting zone. The reclaimed landscape was intermediate between slightly saline and nonsaline landscapes in terms of soil salinity but more similar to nonsaline habitats with respect to species composition. Results from this study suggest it may be unrealistic to expect that plant communities similar to those found on the predisturbance landscape can be established on all reclaimed landscapes after oil sands mining.

Abiotic edaphic factors affecting the growth of a threatened North American Sunflower, Helianthus paradoxus (Asteraceae)

This study evaluated the relationships among soil moisture, soil salinity, and soil oxygen on the growth of Helianthus paradoxus (Asteraceae), a threatened inland salt marsh species of western North America. The study was conducted in large growth boxes (1×2×0.3 m) tilted at an angle to achieve a saturated to dry water gradient similar to that found in the marsh. This experimental design allowed the evaluation of major abiotic factors (soil moisture and soil salinity) which have been shown to be potentially important for this species, while removing major biotic factors, such as competition from other community dominants. Maximum aboveground biomass occurred in the middle rows of the boxes, where surface soil water was reduced and subsurface soil water was intermediate in the gradient. Regression analyses indicated that H. paradoxus would grow best where surface soil water is approximately 5%, subsurface soil water ranges from 20 to 30%, and where surface soil salinity is less than 0.5 g kg−1. Edaphic variables, particularly soil moisture and soil salinity, affect the growth of H. paradoxus. Data presented here suggest that the survival of this species depends on maintenance of the hydrologic regime.

Probable Origin of Laterally Coalesced Nabkas and Adjacent Bare Lanes at Dugway Proving Ground, Utah, USA

ABSTRACT Unvegetated soil surface patterns 1 to 3 m wide, between 10 to over 1,000 m long, slightly curved to strongly sinuous, and usually parallel, were found to cover about 400 km2 in and near Dugway Proving Ground, western Utah, USA. In the only previously published reference to these features, they were called “desert ripples” and described as “small transverse clay dunes with intervening caliche troughs.” Contemporary world-wide literature reviews of banded vegetation and desert geomorphology indicates that “nabka” is the preferred label for the mounded features. The adjacent bare lanes are apparently unique. That previous article asserted that the combined features were caused by wind erosion. We proposed eight possible mechanisms for the creation and maintenance of these laterally coalesced nabkas and adjacent bare lanes and then investigated the physical, chemical, and biological character of them at small (cm to meters) and large (km) spatial scales. Significantly higher surface soil salinity was found in the lanes compared to either the adjacent mounds or internabka areas. The lanes are not usually concave and do not consistently run down slope or converge. They also are not underlain by petrocalcic layers. The associated mounds are not composed of clay, but are dominated by silt and not consistently on the down slope side. These pieces of evidence discount a sheet flood origin or being created by wave action of retreating ancient Lake Bonneville. Most of the evidence points to post-lake wind erosion as the force building and maintaining the mound-lane pattern. The mounds are thin deposits of silt over clay-dominated exposed lake bed. Variation in the morphology of these features can be explained by interaction with differing species and sizes of shrubs that grow in particular locations. Shrubs probably facilitate mound formation by enhancing wind turbulence. If shrubs become dense enough, individual shrub-centered mounds laterally coalesce, and sinuously linear lanes develop and become self-maintaining if the shrubs are not destroyed.

Sensitivity of productivity and deep drainage of wheat cropping systems in a Mediterranean environment to changes in CO 2, temperature and precipitation

Both anticipated climate change and dryland salinity pose a strategic threat to the sustainability of about 6 Mha of agricultural land in Western Australia (WA). These phenomena require an integrated analysis to estimate their potential impacts and initiate strategic thinking about adaptation. Water loss below the root zone, i.e. deep drainage, is the primary cause of sub-soil salt mobilisation leading to surface soil salinity in areas cleared of natural vegetation in Australia; hence deep drainage is an important externality of agricultural production. The purpose of this paper is to show how changes in CO 2 concentration, temperature and precipitation may affect agricultural production and deep drainage. Results are presented of a simulation experiment with the Agricultural Production Systems Simulator (APSIM)-Nwheat model in which we explored sensitivity of (1) wheat production and quality, and (2) deep drainage, for three sites in WA differing in average precipitation, two soils, various nitrogenous fertiliser rates and present wheat cultivars. Since results of Global Circulation Models (GCM) are still largely inconsistent for this region, we have opted for a factorial approach in adapting 90 years of historical weather data. This set-up enabled separation of effects of CO 2, temperature, precipitation and their interactions, and unravelling of the complex interactions between water and nitrogen availability, phenological development and climate change factors, in the extremely variable Mediterranean climate in WA. Elevated CO 2 concentration increased yields, particularly if nitrogen fertilisation was sufficient and conditions were relatively dry. Higher temperatures had non-linear effects, with initial (up to 3 °C) benefits on clay soils, but not on sandy soils, and then substantial yield declines. Both elevated CO 2 concentrations and temperatures (+3 °C) decreased grain protein, but in financial terms this was more than offset by the increase in yield in most cases. If, in addition, precipitation was decreased, financial returns dropped below present levels, particularly in the low precipitation regions. Deep drainage tended to be slightly higher under elevated CO 2 concentrations but when higher temperatures were also simulated this was reversed. Deep drainage was greatly reduced in the low precipitation scenarios. Evidently climate change is not only likely to affect productivity, but also deep drainage and hence dryland salinity. The impact can vary in direction such that both ‘win–win’ and ‘lose–win’ outcomes may occur, particularly depending on the relative change in precipitation.

Impacts of urban storm-water drainage channels on a northern Australian mangrove forest

A common practice in tropical mangrove forests is to widen, deepen or extend the natural channels, or create new channels, in order to increase drainage to limit mosquito breeding habitats. Such excavations were made around 1984 in the upper shore regions of a forest near Darwin in northern Australia. To determine whether changes near the channels appeared to be affecting the mangroves, we measured the survival of experimentally planted seedlings, vegetation structure and a set of environmental variables at several distances from the channels. We focused primarily on Ceriops australis, the most common species. Surface and sub-surface (30 cm) soil salinity, pH and moisture showed no consistent pattern with distance from channels but measurements before and after spring tides showed significantly greater changes in these soil parameters in the surface than in the sub-surface. The survival of planted C. australis seedlings was lower in the forests immediately adjacent to the channels than in forests further away. The growth of the seedlings, however, did not depend on location. Total basal area increased with distance from channels. The forests adjacent to some channels had a greater proportion of Lumnitzera racemosa and Avicennia marina. Although no consistent patterns were evident in soil variables, some patterns in the vegetation, and poorer survival of the seedlings, suggest that the channels might be negatively impacting C. australis. If this was the case, the effects on the forest were, however, relatively minor and apparently similar to those of natural channels.

Spatio-Temporal Variation of Salt Marsh Seedling Establishment in Relation to the Abiotic and Biotic Environment

Contrary to our expectations, soil salinity and moisture explained little of the spatial variation in plant establishment in the upper intertidal marsh of three southern California wetlands, but did explain the timing of germination. Seedlings of 27 species were identified in 1996 and 1997. The seedlings were abundant (maximum densities of 2143/m2 in 1996 and 1819/m2 in 1997) and predominantly annual species. CCAs quantified the spatial variation in seedling density that could be explained by three groups of predictor variables: (1) perennial plant cover, elevation and soil texture (16% of variation), (2) wetland identity (14% of variation) and (3) surface soil salinity and moisture (2% of variation). Increasing the spatial scale of analysis changed the variables that best predicted patterns of species densities. Timing of germination depended on surface soil salinity and, to a lesser extent, soil moisture. Germination occurred after salinity had dropped below a threshold or, in some cases, after moisture had increased above a critical level. Between 32% and 92% of the seedlings were exotic and most of these occurred at lower soil salinity than native species. However, Parapholis incurva and Mesembryanthemum nodiflorum were found in the same environments as the native species. In 1997, the year of a strong El Nino/Southem Oscillation event with high rainfall and sea levels, the elevation distribution of species narrowed and densities of P. incurva and other exotic species decreased but densities of native and rare species did not change. The 'regeneration niche' of wetland plant communities includes the effects of multiple abiotic and biotic factors on both the spatial and temporal variations in plant establishment.

Spatio‐temporal variation of salt marsh seedling establishment in relation to the abiotic and biotic environment

Contrary to our expectations, soil salinity and moisture explained little of the spatial variation in plant establishment in the upper intertidal marsh of three southern California wetlands, but did explain the timing of germination. Seedlings of 27 species were identified in 1996 and 1997. The seedlings were abundant (maximum densities of 2143/m2 in 1996 and 1819/m2 in 1997) and predominantly annual species. CCAs quantified the spatial variation in seedling density that could be explained by three groups of predictor variables: (1) perennial plant cover, elevation and soil texture (16% of variation), (2) wetland identity (14% of variation) and (3) surface soil salinity and moisture (2% of variation). Increasing the spatial scale of analysis changed the variables that best predicted patterns of species densities. Timing of germination depended on surface soil salinity and, to a lesser extent, soil moisture. Germination occurred after salinity had dropped below a threshold or, in some cases, after moisture had increased above a critical level. Between 32% and 92% of the seedlings were exotic and most of these occurred at lower soil salinity than native species. However, Parapholis incurva and Mesembryanthemum nodiflorum were found in the same environments as the native species. In 1997, the year of a strong El Niño/Southern Oscillation event with high rainfall and sea levels, the elevation distribution of species narrowed and densities of P. incurva and other exotic species decreased but densities of native and rare species did not change. The ‘regeneration niche’ of wetland plant communities includes the effects of multiple abiotic and biotic factors on both the spatial and temporal variations in plant establishment.