H. J. M. Nelissen

Stratospheric ozone reduction and ecosystem processes: enhanced UV-B radiation affects chemical quality and decomposition of leaves of the dune grassland species Calamagrostis epigeios

This study reports changes in the plant’s chemical composition and the decomposition of this plant material under enhanced solar UV-B radiation. Calamagrostis epigeios, a dominant grass species in the dune grassland in The Netherlands, was grown outdoor on an experimental field under ambient and enhanced solar UV-B (5 and 7.5 kJ m-2 day-1 UV-BBE, respectively), corresponding to about 15% stratospheric ozone depletion. After one growing season aerial plant parts were harvested. The decomposition of this harvested leaf material was studied in a dune grassland and on the above mentioned experimental field under ambient (5 kJ m-2 day-1 UV-BBE) and enhanced (7.5 kJ m-2 day-1 UV-BBE) radiation, using litter bags. The chemical quality of the leaves grown under enhanced solar UV-B changed. There was an increase in the leaf content of lignin, while no significant changes occurred for the content of α-cellulose, hemicellulose and tannins under enhanced UV-B. In the field, the rate of decomposition of leaf material grown under enhanced UV-B (with an increased content of lignin) was reduced. The content of lignin of the decomposing leaf material increased, but less under exposure to enhanced UV-B. The latter may be explained by photodegradation of the lignin. The consequences of enhanced UV-B radiation for carbon fluxes in the dune grassland ecosystem are discussed.

Combination toxicology of metal-enriched soils : Physiological responses of a Zn- and Cd-resistant ecotype of Silene vulgaris on polymetallic soils

Plants of an ecotype of Silene vulgaris (Caryophyllaceae) originating from a Cd-Pb-Zn mine at Plombiere (Belgium) were grown on 15 polymetallic soils for a full life-cycle to investigate physiological responses which can help explain previously reported disorders in plants. The degree of regulation of the metal concentration in the young seedlings was a very reliable indicator of the subsequent plant performance. Uptake of Zn could be regulated up to 200 nmol water-soluble Zn g−1 dry soil without surpassing 7 μmol Zn g−1 dry leaf tissue supporting the hypothesis of a high regulating potential of Zn-resistant ecotypes. As soon as a certain ecotype-specific threshold was surpassed the Zn concentration in all plant tissues strongly increased. The Cu concentration in roots and shoots had no threshold and showed the tendency to increase near linearly with the external Cu soil concentration. Similar behaviours were found for Cd, Mn and Pb. The metal concentration of seeds was the lowest of all plant parts; nevertheless it increased linearly with increasing concentration of Fe and Zn in the soil. From visible symptoms the degree of chlorosis was positively related with the concentration of Zn, but not with that of Cu, and interrelated with Fe availability. High cyanidin concentrations in leaves were not indicating a surplus of heavy metals, but deficiency in phosphorus and to a lesser degree in nitrogen. Phytochelatins (PCs) were only present in measurable amounts in leaves of plants grown on soils rich in Cu; but PCs amounts in the early vegetative phase could not be related to vegetative and seed biomass at the reproductive stage. Therefore, it is concluded that PCs are a less reliable indicator of metal toxicity during a full life-cycle than the metal concentration of young seedlings.

Life-cycle phases of a zinc- and cadmium-resistant ecotype of Silene vulgaris in risk assessment of polymetallic mine soils

Short-term exposure of plants to heavy metals is often used for risk assessment of metal-enriched soils (OECD guideline 208) without considering the reliability of the assessment for long-term exposure, i.e. for the completion of a plants life-cycle. In the present study with 15 orogenic soils three phases of the life-cycle of a Zn-Cd-resistant ecotype of Silene vulgaris were studied to improve risk assessment of metal-enriched soils. The first phase, i.e. emergence of seedlings was not related to the water-soluble or total metal concentration of the soils. Seedling mortality was low as long as the water-soluble metal concentration did not surpass 0.15 micromol Zn and 0.04 micromol Cu g(-1) dry soil. Curtailment of the life-cycle prior to flowering, i.e. the vegetative growth as second phase, occurred on those soils where roots and shoots were heavily enriched by Zn already in the seedling phase. In the third phase, i.e. the generative phase, time to flowering and yield differences between orogenic soils were substantial, but soil metal concentrations could not be directly related to timing of reproduction or biomass. Ranking of data showed a high inconsistency of the responses to metal exposure during the first phases of the life-cycle. It is concluded that total plant mass and seed mass are the only realistic endpoints of life-cycle bioassays in risk assessment as long as ranks are inconsistent between two successive early phases of the life-cycle.

Plant responses to rising water tables and nutrient management in calcareous dune slacks

Plant species of oligotrophic wet dune slacks have dramatically decreased as a result of desiccation and eutrophication. The aim of this study was to test in a field experiment the effects of restoration management in oligotrophic, wet dune slacks (groundwater level rise in combination with topsoil removal or mowing) on abiotic variables and on survival and biomass of four plant species. The effect of groundwater level rise on abiotic variables strongly differed between mown sampling locations and those with topsoil removal. At locations with a mowing treatment, a large rise in water tables led to increased N availability and higher reduced iron concentrations than at other locations. Such effects were absent at locations with recent topsoil removal. No effect of groundwater level rise on P-availability was found. Topsoil removal on average lowered N availability by 13%, P availability by 65% and Fe2+ by 56%. All phytometer species survived better in mown dune slacks than in dune slacks that had received topsoil removal. Survival of all species was negatively related to groundwater level rise. On the short term local extinction risks of small populations may be enhanced by rewetting and topsoil removal. On the long-term, however, such measures are crucial to maintain vegetation of oligotrophic wet dune slacks in a degraded dune landscape.

Reaction of savanna plants from Botswana on UV-B radiation

The annual savanna grasses Chloris virgata (C4) and Tragus berteronianus (C3) and the tree Acacia tortilis were exposed in a greenhouse to elevated UV-B radiation (16.8 kJ m-2 d-1 UV-BBe) and to no UV-B and grown on a poor and a rich soil for one life-cycle (grasses) and one growing season (Acacia). UV-B radiation had no effect on biomass production and caryopses mass of both annual grasses. The longevity of the cotyledons of A. tortilis was shortened by 4 to 10 days under enhanced UV-B radiation, which also hampered the translocation of Fe, Mg and Mn from the cotyledons to the seedling and the retranslocation of Mn on both soil types and that of P on fertile soil out of senescent leaves. At the end of the growth period (190 days after germination), photosynthesis of UV-B radiated leaves of A. tortilis was significantly decreased and supported the tendency of decreased biomass of UV-B radiated plants. It is concluded that from the investigated savanna species the grasses are relatively well adapted to increased UV-B due to their actual exposure to high UV-B radiation under Botswana conditions, whereas saplings of A. tortilis are more sensitive to UV-B radiation.

NUTRITIONAL STATUS OF SOIL AND PLANT SPECIES IN SEVERAL CLEARINGS IN CONIFEROUS WOODS COMPARED TO THAT IN TWO RELATED HABITATS

The nutritional status of the soils of a series of coniferous woodland clearings in the Netherlands was found to be intermediate between that of the soil in a dune area with a climax population of Chamaenerion angustifolium and that of the edge (and undergrowth) of a Quercus robur stand. In the dune area the greater part of the NPK was present as an internal pool in the plant tissues of C. angustifolium enabling the population of this species to persist and inhibiting further succession through limiting the availability of NPK. It is suggested that succession in clearings might stagnate in the same way in the absence of new tree growth. Because C. angustifolium is capable of persisting as a population under poor N conditions, it is unsuitable as an indicator species for nitrate-rich soils. The same may be true for Senecio sylvaticus, the other character species of the association Epilobio-Senecionetum sylvatici. A comparison of species in clearings with those occurring on the edge of a wood seems to indicate that, when using field measurements only (i.c. Ntotal and NO3-N in the soil and in the plant tissues, and the activity of nitrate reductase in the leaves), conclusions may indeed be drawn concerning the adaptability of the species to a particular habitat factor (i.c. the availability of nitrate), but not concerning their requirements for that factor or how the species utilize it.

VARIABILITY IN GROWTH AND REPRODUCTION IN F1-FAMILIES OF AN EROPHILA-VERNA POPULATION

SummarySeeds from two morphologically different groups in an Erophila verna dune population were used in a growth experiment to analyse and quantify flexibility and plasticity of growth and reproductive characteristics.A strong correlation between plant morphology and individual seed weight was shown in parents and progeny. Seeds of plants with narrow leaves were twice as heavy (H-plants) as those from plants with broad leaves (L-plants) Families of H-plants allocated ca. 10% more dry matter to the roots. These characters were fixed, presumably due to selfing. Plants of both types are characterized by a graded control function between the vegetative and the reproductive phase. Most of the families showed a single switch from vegetative to reproductive growth, two families showed even two switch periods.Between-family differentiation is interpreted as a result of multiniche selection in populations of Erophila verna. Within-family variation is disadvantageous due to low fecundity.

Growth Response to Different Levels of Soil Moisture and Soil Fertility in Four Winter Annual Species during Their Life-Cycle

Summary The effect of three different soil moisture levels and two soil fertility levels on the growth, biomass allocation and reproductive effort of four winter annual plant species from the open dune habitat i.e. Aira praecox, Cerastium arvense, Erophila verna , and Veronica arvensis , has been investigated. Soil moisture has a considerable effect on the growth and reproduction of all species except Veronica arvensis , in which an increasing soil fertility seems to be of overriding importance. In the experimental period (27-30weeks), only Cerastium semidecandrum and Erophila verna completed their life-cycle and produced seeds. In these species absolute seed biomass and number per plant are markedly depressed by a low soil moisture level. In contrast to Erophila verna , the values of these parameters increase in Cerastium semidecandrum when the soil fertility is high. Mean seed weight was not affected by the treatments. Regarding the different niches of these winter annual species in the dune habitat, we discuss in how far the niches of these annuals represent different strategies or may be considered one particular strategy sensu G rime (1979).

Bleeding sap and leaves of silver birch (Betula pendula) as bioindicators of metal contaminated soils

Bleeding sap of Betula pendula contained high levels of manganese on soils near ore outcrops and on acid soils, whereas high zinc levels were restricted to metal-mining and metal-smelting areas. The relationships between metal contents of bleeding sap and those of the soils were dependent on the kind of chemical soil extraction. Copper and zinc sap levels had a significantly positive correlation with the aqua regia soil extract, iron and manganese with the CaCl 2 -extract, and cadmium with the NH 4 -acetate soil extract. Metal contents of mature leaves were highest in autumn and all, except cadmium, were related in a similar metal-specific way to the same chemical soil extracts as the bleeding sap. Lead levels of sap and leaves had no consistent correlation with type of soil extract. The timing of collection and the advantages of the analysis of bleeding sap versus mature leaves are discussed.

The calcium demand of the calcicole sedge Schoenus nigricans

Summary Growth and reproduction of the perennial sedge Schoenus nigricans from calcareous wet dune-slacks was investigated in response to increasing proportion of calcium carbonate in dune sand to test the hypothesis that a high calcium supply is necessary to ensure a full life cycle of this calcicole species and counteract the population decrease in wet dune slacks. Plant growth and nutlet quality, but nor reproduction per se were significantly lower in plants grown for 30 months at 0.03% Ca compared to all other concentrations from 0.25 to 2% Ca. Plants at 0.03% Ca had the lowest nutlet mass. With increasing Ca supply, the Ca concentration in the roots increased more than that in leaves and shoots, where Ca was associated with the fractions being soluble in water and acetic acid. Ca-oxalate was not present. The insoluble Ca fraction was not affected by the external Ca supply, but was only related to the age of the tissue. A quotient of water-soluble calcium/water-soluble total cations of 0.15 seems to be sufficient for a good performance of S. nigricans in soils with 0.25% Ca or more, thus indicating a relatively low calcium demand of this calcicole species which has a high degree of Ca-tolerance.