Evaluating heavy metal concentration of plants on a serpentine site for phytoremediation applications

The chemical analysis of plants and soils is a frequently used approach in understanding a serpentine ecosystem. Studies on vegetation growth in serpentine soils focused on various plant species for remediation purposes of soil contamination with heavy metals, emphasizing their role in the metal extraction or stabilization in the soil. The aims of this study were to measure the concentrations of Cr, Mn, and Ni in the soils and plants and to elucidate the phytoremediation potential of the studied plants. This study was performed at an abandoned site of serpentine mining in eastern Taiwan. Seven plant species were collected for analysis of Cr, Mn, and Ni, including Crotalaria micans, Miscanthus floridulus, Leucaena leucocephala, Bidens pilosa, Pueraria lobata, Melilotus indicus, and Conyza canadensis. The Cr and Ni concentrations in all studied plants were higher than those in general plants. In all species, the mean concentrations of Cr, Mn, and Ni in the shoots were lower than those in the root. None of the collected specimens exhibited hyperaccumulation of Cr, Mn, and Ni. All studied species may be used to remediate contaminated soils through phytostabilization of Cr and Mn, whereas M. floridulus and M. indicus are appropriate plants for phytostabilization of Ni. However, C. micans, L. leucocephala, B. pilosa, P. lobata, and C. canadensis have the potential to remove Ni from contaminated soils for the purpose of phytoextraction.     

Correlations between metals in tree-rings of Prosopis julifora as indicators of sources of heavy metal contamination

Heavy-metals (Cu, Pb and Zn) in tree-ring sequences of Prosopis juliflora, a tree species native to arid environments, were analyzed by ICP-MS. The tree-ring sequences were obtained from three specimens growing in an urban area previously reported as contaminated by the activity of a Cu smelter facility. The metal found in highest concentration in the wood was Zn, with concentrations up to 120 mg/kg and an enrichment factor up to 26; followed by Cu (up to 9.6 mg/kg, enrichment factor up to 8.6) and Pb (up to 1.4 mg/kg, enrichment factor up to 3). By assessing the correlation between different metal concentration trends, it was possible to infer two main pollution sources: vehicle traffic and Cu smelter emissions. Vehicle traffic is indicated by a correlation between Pb and Zn over time within individual trees, whereas contamination from the Cu-smelting facility is indicated by a correlation of Cu over time between trees. In tree A there was a significant within-tree correlation between Pb and Zn concentration trends (r = 0.856, P < 0.001), whereas Cu showed no correlation with the other metals. For tree B, there were no within-tree correlations between these metals, but when comparing the concentration–time trends between trees A and B, there was a significant correlation for Cu (r = 0.768, P < 0.01). The tree-ring sequence from tree C showed significant within-tree correlation for Cu:Zn (r = 0.430, P < 0.01) and for Pb:Zn (r = 0.753, P < 0.001). The highest enrichment values were found in tree A, located along the path of the growing-season dominant wind direction from the smelter facility, and not in the tree growing closer to the smelter (tree C), suggesting that the smelter’s emissions are dispersed to longer distances through the tall chimneys, attenuating the impact to the area directly closest to the smelter facility. It is concluded that Prosopis juliflora appears as a good bioindicator based on its metal accumulation capacity and lack of metal mobility among tree rings, thus providing information on the chronology and sources of heavy-metal pollution in urban and industrial areas.     

Effects of chromium on seed germination, root elongation and coleoptile growth in six pulses

The ecotoxicological effects of Cr²⁺ on germination and early seedling growth of six pulses were investigated. Seeds of these plants were exposed to seven different concentrations of Cr (0–3.2 mM). The results indicated that root elongation and coleoptile growth of six pulse plants were more sensitive than seed germination for measurement of the toxic of Cr²⁺ pollutions. Different species show different levels of tolerance to Cr²⁺ pollution. Lablab purpureus and Glycine max are the most sensitive to Cr²⁺, their germination percentage, root and coleoptile length were significantly lower than other tested species, by contrast, Lathyrus odoratus and Dumasia villosa are the most resist species, their germination and seedling growth almost were not influenced by Cr²⁺ pollution significantly comparing the control. There were significantly negatively correlations between seedling growth and increasing concentration of Cr²⁺ for Glycine max, Vigna radiate and Lablab purpureus. The significantly negative correlations between germination and increasing concentration of Cr²⁺ were shown for Glycine max, Vigna radiate and V angularis.     

High Sulfide Intrusion in Five Temperate Seagrasses Growing Under Contrasting Sediment Conditions

Five temperate seagrasses (Amphibolis antartica, Halophila ovalis, Posidonia australis, Posidonia sinuosa and Zostera nigricaulis) were surveyed along the south-west coast of Western Australia. These morphological different seagrasses grow in contrasting sediments with large variations in sedimentary organic matter, carbonate and iron contents. We tested if sulfur composition in the plants responded to sulfur dynamics in the sediments and if plant morphology affected the sulfur composition of the plants. The sediments were characterized by low sulfate reduction rates (<9 mmol m^?2day^?1), low concentrations of dissolved sulfides in the pore waters (<74 μM) and low burial of sulfides (total reducible sulfur <0.8 mol m^?2) in the sediments. However, all seagrasses showed high intrusion in the below-ground parts with up to 84 % of the sulfur derived from sedimentary sulfides. There were no direct links between sulfur in the plants and sulfur dynamics in the sediments, probably due to low iron contents in the sediments limiting the buffering capacity of the sediments and exposing the plants to sulfides despite low rates of production and low pools of sulfides. The intrusion was linked between plant compartments (roots, rhizomes and leaves) for the two small species (H. ovalis and Z. nigricaulis), whereas the intrusion into the leaves was limited for the larger species (P. australis and P. sinuosa) and for A. antarctica, where extensive rhizomes and roots and the long stem for A. antarctica separate the leaves from the sediment compartment. Elevated intrusion was observed at two study locations, where natural deposition of organic matter or nutrient enrichment may be contributing factors to enhanced sulfide pressure.     

Assessment of the potential of semi-arid plants to reduce soil erosion in the Konartakhteh watershed,Iran

Soil erosion is a major environmental problem in arid and semi-arid areas. Although bioengineering is important in preventing soil erosion, plant architecture and mechanical properties in these areas are rarely studied. In this study, in order to evaluate the potential of native plants to reduce soil erosion in semi-arid regions, their above-ground (e.g., stem density, radius of the stem, etc.) and below-ground (e.g., root area ratio, root tensile strength, etc.) characteristics were measured in the field and laboratory. Five indicators, namely, stem density (SD), sediment obstruction potential (SOP), plant stiffness (MEI), relative soil detachment (RSD), and root cohesion (Cr), were taken into account. Each indicator was scored according to a five-point scale (0?=?low, 4?=?high), and then, the score of each indicator was represented on an ameba diagram. Finally, for understanding traits of plants and evaluating their potential to control rill and gully erosion, the area occupied by the ameba diagram was studied. The results indicated that the shrub Ziziphus spina-christi (MEI?=?108.35 N, RSD?=?0.398, Cr?=?8.34 kPa, SOP?=?0.097, and SD?=?0.00270) is a very suitable native plant species for controlling both the gully and rill erosion. In addition, Scariola orientalis is effective for sediment obstruction, but its low scores on the MEI and RSD indicators imply that it is not able to control gully development. Furthermore, Noaea mucronata, Platychaete glaucescens, Astragalus gummifer, Alhagi persarum, Lycium shawii, and Prosopis farcta have a distinct potential to reduce the rate of gully erosion. These results have wide applicability for adopting soil conservation measures to other semi-arid environments.     

Nitrogen and phosphorus removal and physiological response in aquatic plants under aeration conditions

To analyze the variation of physiological responses between Nymphaea tetragona Georgi. and Pontederia cordata L. and the water qualities under aeration conditions, the selected plants were cultivated in 12 purifying-tanks (aeration, non aeration), to treat heavily polluted river water. The characteristics of both plants were investigated, which included contents of chlorophyll and soluble protein, activities of peroxidase and catalase, accumulations of nitrogen and phosphorus, densities of tillers and roots, lengths of roots, culms and leaves, biomass of roots and shoots. The water qualities were analyzed correspondingly. Results indicated that aeration affected morphological and physiological characteristics of the plants and the water qualities and effects became more significant on N. tetragona than P. cordata. Biomass and length of roots, culms and leaves under the non aeration conditions exceeded that under the aeration conditions. Aeration contributed to the activities increase of peroxidase and catalase of the roots and the contents decrease of chlorophyll and soluble protein of the leaves. Nitrogen and phosphorus contents of the roots, culms and leaves increased under the non aeration conditions. Aeration resulted in tillers and roots densities of N. tetragona decreased, while they increased for P. cordata. Total phosphorus and soluble phosphorus removals decreased 8.42 % and 8.05 % in the tank with N. tetragona under the aeration conditions. In the tank with P. cordata, total nitrogen and NH₄+ ?N removals increased 14.44 % and 16.06 % under the aeration conditions. This work provided valuable data for optimizing the plants allocation in the ecological restoration project of the polluted water.     

Evaluation of the use of legumes for biodegradation of petroleum hydrocarbons in soil

The aim of this research was to evaluate the potential of six legumes: Medicago sativa L., Glycine max, Arachis hypogea, Lablab purpureus, Pheseolus vulgaris and Cajanus cajan to restore within a short period of time soil contaminated with 3% crude oil. The legumes in five replications were grown in crude oil-contaminated and crude oil-uncontaminated soil in a completely randomized design. Plants were assessed for seedling emergence, plant height and leaf number. GC–MS was used to analyze the residual crude oil from the rhizosphere of the legumes. Plant growth parameters were reduced significantly (P < 0.05) for legumes in contaminated soil compared to their controls. In the 4th week after planting (WAP), shoot height increased across the species up to the 8th WAP. However, in the 12 WAP no significant increase in the shoot of all species was observed. Two WAP legumes planted in contaminated soil had significantly (P < 0.05) higher leaf number than these planted in uncontaminated soil with the exception of M. sativa. In the 4th WAP, only A. hypogea and P. vulgaris had increased leaf number, while in the 6th WAP, only L. purpureus had increased leaf number and survived up to the 12th WAP while most of the legumes species died. Chromatographic profiles indicated 100% degradation of the oil fractions in C. cajan and L. purpureus after 90 days. For other legumes however, greater losses of crude oil fractions C₁–C₁₀ and C₁₀–C₂₀ were indicated in rhizosphere soil of P. vulgaris and G. max, respectively. The most effective removal (93.66%) of C₂₁–C₃₀ components was observed in G. max-planted soil even though vegetation was not established. The legumes especially C. cajan, L. purpureus and A. hypogea are promising candidates for phytoremediation of petroleum hydrocarbon-impacted soil.     

The elemental and isotopic composition of sulfur and nitrogen in Chinese coals

Coal combustion is an important atmospheric pollution source in most Chinese cities, so systematic studies on sulfur and nitrogen in Chinese coals are needed. The sulfur contents in Chinese coals average 0.9 ± 1.0%, indicating that most Chinese coals are low in sulfur. A nearly constant mean δ³⁴S value is observed in low sulfur (TS < 1) Chinese coals of different ages (D, P₁, T₃ and J₃). High sulfur Chinese coals (OS > 0.8%), often found at late Carboniferous (C₃) and late Permian (P₂) in southern China, had two main sulfur sources (original plant sulfur and secondary sulfur). The wide variety of δ³⁴S values of Chinese coals (−15‰ to +50‰) is a result of a complex sulfur origin. The δ¹⁵N values of Chinese coals ranged from −6‰ to +4‰, showing a lack of correlation with coal ages, whereas nitrogen contents are higher in Paleozoic coals than in Mesozoic coals. This may be related to their original precursor plant species: high nitrogen pteridophytes for the Paleozoic coals and low nitrogen gymnosperms for the Mesozoic coals. Different to δ³⁴S values, Chinese coals showed higher δ¹⁵N values in marine environments than in freshwater environments.     

Adsorption of hexavalent chromium onto activated carbon derived from <Emphasis Type="Italic">Leucaena leucocephala</Emphasis> waste sawdust: kinetics,equilibrium and thermodynamics

The adsorptive removal of Cr(VI) was studied using activated carbon derived from Leucaena leucocephala (ACLL). The physico-chemical properties of ACLL were determined using proximate analysis and N₂ BET surface area analysis. The N₂ BET surface area of ACLL was determined to be 1131 m² g^?1. The point of zero charge (pH_pzc) of 5.42 indicated that ACLL surface was positively charged for pH below the pH_PZC, attracting anions. The effect of experimental operating parameters such as time of contact, ACLL dose, pH, initial concentration and temperature was investigated. The optimum values of parameters such as concentration of 100 mg L^?1, 300 mg of ACLL dose, time of contact of 60 min, pH of 4 indicated the maximum Cr(VI) uptake of 13.85 mg g^?1. The pseudo-second-order kinetic model best fitted with the Cr(VI) adsorption data. Adsorptive removal of Cr(VI) onto ACLL satisfactorily fitted in the order of Redlich–Peterson > Freundlich > Langmuir > Temkin adsorption isotherm model. The thermodynamic parameters showed the adsorption of Cr(VI) onto ACLL was an endothermic and spontaneously occurred process.     

Spatial patterns of soil δC reveal grassland-to-woodland successional processes

Many grasslands and savannas around the world have experienced woody plant encroachment in recent history. In the Rio Grande Plains of southern Texas, subtropical woodlands dominated by C₃ trees and shrubs have become significant components of landscapes once dominated almost exclusively by C₄ grasslands. In this study, spatial variation of soil δ¹³C to was used to investigate patterns of transformation. Previous research has shown that grassland-to-shrubland transitions are initiated when discrete, multi-species shrub clusters organized around a honey mesquite (Prosopis glandulosa) tree nucleus established in grassland. It is inferred from space-for-time substitution and modeling studies that as new shrub clusters are initiated and existing clusters enlarge, coalescence will occur, leading to the formation of groves; and that groves will eventually merge to form woodlands. The hypothesis that present-day mesquite groves represent areas where individual discrete shrub clusters have proliferated and coalesced was evaluated by comparing patterns of soil δ¹³C within isolated shrub clusters (n = 6) to those in nearby groves (n = 3). Mean soil δ¹³C within discrete clusters was lowest in the center (−23.3‰), increased exponentially toward the dripline (−20.1‰), and stabilized at a relatively high value approximately 15 cm beyond the dripline (−18.9‰). The spatial structure of soil δ¹³C in groves was consistent with that which would be expected to occur if present-day grove communities were a collection of what once were individual discrete clusters that had fused. As such, it provides direct evidence in support of conceptual and mathematical models derived from indirect assessments. However, spatial patterns of soil δ¹³C suggest that groves are not simply a collection of clusters with respect to primary production and SOC turnover. This study illustrates how soil δ¹³C values can be used to reconstruct successional processes accompanying vegetation compositional change, and its consequences for ecosystem function.     

Accumulation and phytostabilization of As,Pb and Cd in plants growing inside mine tailings reforested in Zacatecas,Mexico

The aim of this research was to identify plant species with potential to accumulate and stabilize arsenic (As), lead (Pb) and cadmium (Cd) in mine tailings reforested and naturally recolonized locations in a semiarid region of Zacatecas, Mexico. Plant shoots from 44 species and their rhizospheric soils were analyzed for As, Pb and Cd concentration using atomic absorption spectroscopy. Most represented plant families were Asteraceae, Poaceae, Fabaceae and Cactaceae. The highest concentrations in shoots were As, followed by Pb and Cd. Among herbaceous species, Bouteloua gracilis showed the highest bioconcentration factor (BCF) of As, while Plantago lanceolata showed the highest bioconcentration factor of Pb. The shrub species with highest concentration of As in the rhizospheric soil were Opuntia robusta, Melilotus alba, Baccharis neglecta and Arundo donax (near BCF to 1.0). Similar results were observed in trees Casuarina equisetifolia, Prosopis laevigata, Fraxinus uhdei and Eucalyptus globulus. In addition, Tillandsia recurvata showed a suitable indicator of atmospheric deposition to As. In general, the results suggest that these species can be effective for tailings reforestation with the possibility to enclose potentially toxic elements. Specially, C. equisetifolia which is abundant, having the potential for future applications in other contaminated sites with different types of mine tailings or abandoned mines from arid and semiarid zones.     

Comparative study of body composition of different fish species from brackish water pond

Seven species of fish (Catla catla, Cirrhinus mrigala, Cyprinus carpio, Hypophthalmicthys molitrix, Labeo rohita, Mori-Rahu Hybrid and Thalla-Rahu Hybrid) were collected from a brackish water pond near Muzaffargarh, Pakistan for the comparison of body composition. All the seven species were found relatively well adapted to the saline environment though some of them showed significant differences in body composition. Results obtained did not show any adverse effect of salinity on these fish species. The mean values of body constituents, except for protein content (dry and wet body weight) differed significantly (P<0.05) among various fish species. Minimum amount of water content and maximum amount of lipids, organic content and condition factor were observed in Cyprinus carpio indicating that Cyprinus carpio show overall better growth in brackish water as compared to other species. Cyprinus carpio may be recommended for culturing in such water bodies and farmers may be encouraged to farm this species on mass scale.     

Effect of exo-polysaccharides producing bacterial inoculation on growth of roots of wheat (Triticum aestivum L.) plants grown in a salt-affected soil

Effect of soil salinity on physico-chemical and biological properties renders the salt-affected soils unsuitable for soil microbial processes and growth of the crop plants. Soil aggregation around roots of the plants is a function of the bacterial exo-polysaccharides (EPS), however, such a role of the EPS-producing bacteria in the saline environments has rarely been investigated. Pot experiments were conducted to observe the effects of inoculating six strains of EPS-producing bacteria on growth of primary (seminal) roots and its relationship with saccharides, cations (Ca²⁺, Na⁺, K⁺) contents and mass of rhizosheath soils of roots of the wheat plants grown in a salt-affected soil. A strong positive relationship of RS with different root growth parameters indicated that an integrated influence of various biotic and abiotic RS factors would have controlled and promoted growth of roots of the inoculated wheat plants. The increase in root growth in turn could help inoculated wheat plants to withstand the negative effects of soil salinity through an enhanced soil water uptake, a restricted Na⁺ influx in the plants and the accelerated soil microbial process involved in cycling and availability of the soil nutrients to the plants. It was concluded that inoculation of the EPS-producing would be a valuable tool for amelioration and increasing crop productivity of the salt-affected soils.     

Early Wisconsinan (MIS 4) Arctic ground squirrel middens and a squirrel-eye-view of the mammoth-steppe

Fossil arctic ground squirrel (Spermophilus parryii) middens were recovered from ice-rich loess sediments in association with Sheep Creek-Klondike and Dominion Creek tephras (ca 80 ka) exposed in west-central Yukon. These middens provide plant and insect macrofossil evidence for a steppe-tundra ecosystem during the Early Wisconsinan (MIS 4) glacial interval. Midden plant and insect macrofossil data are compared with those previously published for Late Wisconsinan middens dating to ～25–29¹⁴C ka BP (MIS 3/2) from the region. Although multivariate statistical comparisons suggest differences between the relative abundances of plant macrofossils, the co-occurrence of steppe-tundra plants and insects (e.g., Elymus trachycaulus, Kobresia myosuroides, Artemisia frigida, Phlox hoodii, Connatichela artemisiae) provides evidence for successive reestablishment of the zonal steppe-tundra habitats during cold stages of the Late Pleistocene. Arctic ground squirrels were well adapted to the cold, arid climates, steppe-tundra vegetation and well-drained loessal soils that characterize cold stages of Late Pleistocene Beringia. These glacial conditions enabled arctic ground squirrel populations to expand their range to the interior regions of Alaska and Yukon, including the Klondike, where they are absent today. Arctic ground squirrels have endured numerous Quaternary climate oscillations by retracting populations to disjunct “interglacial refugia” during warm interglacial periods (e.g., south-facing steppe slopes, well-drained arctic and alpine tundra areas) and expanding their distribution across the mammoth-steppe biome during cold, arid glacial intervals.     

Evaluation of oxidative stress in Vigna radiata L. in response to chlorpyrifos

This study aims to evaluate the effect of chlorpyrifos on several metabolic and stress related parameters of Vigna radiata L. Twenty-day-old plants were exposed to several concentrations of chlorpyrifos, ranging from 0 to 1.5?mM through foliar spray in the field condition. Analyses were done at pre-flowering (Day 5), flowering (Day 10), and post-flowering (Day 20) stages after the treatment. Lipid peroxidation rate, proline, dehydroascorbate, oxidized and total glutathione were all ascended. Chlorpyrifos enhanced lipid peroxidation rate and proline content with 1.5?mM at Day 20 whereas dehydroascorbate, oxidized and total glutathione were increased in 1.5?mM at Day 10. However, dose dependence significantly declined in content of ascorbate and reduced glutathione levels were observed at all growth stages. Among the enzymatic antioxidants, activities of superoxide dismutase, ascorbate peroxidase and glutathione reductase enhanced significantly in all the concentrations at Day 10. Maximum catalase activity was observed at Day 10 in control and it declined thereafter. The above results clearly depicted the provoked state of oxidative stress under chlorpyrifos exposure in Vigna radiata L. and therefore can be used to evaluate the degree of insecticide contamination to plant, which may be serving as biomarker in Vigna radiata L.     

Mechanisms of expansion for an introduced species of cordgrass, Spartina densiflora, in Humboldt Bay, California

The dominant plant in Humboldt Bay salt marshes in Spartina densiflora, a species of cordgrass apparently introduced from South America. At several salt marshes and restoration sites around Humboldt Bay, distribution of this plant has increased significantly. We investigated the relative contributions of vegetative tiller production and seed germination to the establishment and expansion of S. densiflora. Lateral spread of plants surrounded by competitors were compared to areas without competing plant species. Plants growing in areas without competitors had significantly higher rates of vegetative expansion (p<0.0001). Viable seed production, germination rates, seedling survivorship, and growth of adult plants were measured in six salinity treatments. Approximately 1,977±80 viable seeds are produced per plant (0.25–0.5 m²). The number of germinating seeds was inversely related to increases in salinity. Salinity treatments between 19‰ and 35‰ produced significantly lower germination rates than salinities of 0–18‰ (p<0.0001). Seedling survivorship was 50% at ≤4‰ and 8–14% at ≥11‰. Lateral expansion of adult, greenhouse-grown plants occurred in all salinity treatments, with modest decreases in the highest salinity treatments (p<0.05). Our findings indicate that S. densiflora expands primarily by vegetative expansion, and lateral tillers are produced by throughout the year. Spartina densiflora produces prolific amounts of seed, but recruitment in mature salt marshes may be limited by competitors and higher salinities. At restoration sites, planting of native species such as Salicornia virginica, Distichlis spicata, or Jaumea carnosa may prevent monospecific stands of S. densiflora from developing.     

Late quaternary climatic variations in Northwest Africa deduced from East Atlantic sediment cores

Characteristic variations in glacial and interglacial periods are shown by the CaCO₃ contents, amounts and grain size of terrigenous material (>40 μm), quartz/mica ratios, and desert quartz numbers in East Atlantic sediment cores, collected during Meteor cruise 25 off Sahara and Senegal, 15–27°N. The following results were obtained. During Holocene an arid climate with eolian supply from the continent prevailed throughout the region (fine grained, slight terrigenous input; high CaCO₃ values; high desert quartz numbers) except in the Senegal area. Here terrigenous muds indicate river supply and hence humid climate. During upper and lower Würn the climate was humid in the present day Sahara north of 20°N (low desert quartz numbers: input of large quantities of coarse terrigenous material; low CaCO₃ values). South of 20°N the climate was arid, the Senegal river input disappeared during upper Würm, desert quartz numbers are as high as in Sahara dune samples, and wind strength was stronger than in the Holocene. Climate during middle Würm resembled Holocene climatic conditions. Climatic conditions during Eem (Riss) were probably similar to those during Holocene (Würm).     

A late Wisconsin and Holocene vegetation history from the central brooks range: Implications for Alaskan palaeoecology

Five pollen diagrams reveal late Wisconsin and Holocene vegetation changes in the Walker Lake/Alatna Valley region of the central Brooks Range, approximately 100 km west of the area studied by D. A. Livingstone (1955, Ecology36, 587–600). New insights into the vegetation history of this region are provided by calculations of pollen influx and by the use of linear discriminant analysis to separate Picea glauca and P. mariana pollen. Three major pollen zones are identified: (1) a basal herb zone, characterized by high percentages of Cyperaceae, Gramineae, Salix, and Artemisia, and low total pollen influx; (2) a shrub Betula zone with increased total pollen influx and very high percentages of Betula pollen, predominantly in the size range of B. nana and B. glandulosa; and (3) and Alnus zone dominated by Alnus pollen. Lakes currently within the boreal forest or near tree line show relatively high percentages of Picea pollen in the Alnus zone. Several striking vegetation changes occurred between ca. 10,000 and 7000 yr B.P. Between ca. 11,000 and 10,000 yr B.P., Populus balsamifera pollen percentages as great as 30% indicate that this species was present at low-elevation sites near Walker Lake. These populations declined abruptly ca. 10,000 yr ago and have never regained prominence. About 8500 yr B.P., Picea glauca pollen reached 10–15%, indicating the arrival of P. glauca in or near the study area. P. glauca populations evidently decreased ca. 8000 yr ago, when Picea pollen percentages and influx fell to low values. About 7000 yr B.P., Alnus pollen percentages and influx rose sharply as alder shrubs became established widely. Picea once more expanded ca. 5000 yr ago, but these populations were dominated by P. mariana rather than P. glauca, which increased slowly at this time and may still be advancing northward. Some vegetation changes have been remarkably synchronous over wide areas of interior Alaska, and probably reflect responses of in situ vegetation to environmental changes, but others may reflect the lagged responses of species migrating into new areas.     

Patterns and implications of plant-soil δC and δN values in African savanna ecosystems

Southern African savannas are mixed plant communities where C₃ trees co-exist with C₄ grasses. Here foliar δ¹⁵N and δ¹³C were used as indicators of nitrogen uptake and of water use efficiency to investigate the effect of the rainfall regime on the use of nitrogen and water by herbaceous and woody plants in both dry and wet seasons. Foliar δ¹⁵N increased as aridity rose for both C₃ and C₄ plants for both seasons, although the magnitude of the increase was different for C₃ and C₄ plants and for two seasons. Soil δ¹⁵N also significantly increased with aridity. Foliar δ¹³C increased with aridity for C₃ plants in the wet season but not in the dry season, whereas in C₄ plants the relationship was more complex and non-linear. The consistently higher foliar δ¹⁵N for C₃ plants suggests that C₄ plants may be a superior competitor for nitrogen. The different foliar δ¹³C relationships with rainfall may indicate that the C₃ plants have an advantage when competing for water resources. The differences in water and nitrogen use likely collectively contribute to the tree-grass coexistence in savannas. Such differences facilitate interpretations of palaeo-vegetation composition variations and help predictions of vegetation composition changes under future climatic scenarios.     

Effect of irrigation water qualities on Leucaena leucocephala germination and early growth stage

The evaluation of nonconventional water resources on seed germination and seedling growth performance at early growth stages is still in progress, especially on multipurpose forest trees. This study was designed to test the effect of four water qualities [treated wastewater, industrial, gray and distilled water (control)] on germination and early seedling vigor of Leucaena leucocephala. The results showed that germination was not significantly affected by different water qualities. Seed germination reached a maximum after 17, 14, 14 and 21?days under gray, industrial, treated wastewater and control irrigation, respectively. The highest mean of shoot length was scored under gray water irrigation. Likewise, the highest mean of root length was scored under control, which was not significant from gray water. The means of shoot fresh and dry weight were the highest under treated wastewater. The means of root fresh weight and root dry weight were not significantly different under water treatments. The shoot/root ratios under all water qualities treatments were significantly higher than the control. Growth performance was in progress with no mortality during 21?days of growth. The best nonconventional water quality alternatives based on cleanness, nutrients and toxicity are the gray, treated wastewater and industrial water, respectively.