Soil pH and species diversity in coastal dunes

Soil pH was measured at two different spatial scales in coastal dunes on Norderney, North Sea, and in Mecklenburg-Vorpommern, Baltic Sea, Germany. Relationships between the variability in soil pH, species richness and species diversity are presented. Species richness and diversity were highest in grey dunes, where soil pH was at intermediate levels; both variables were lower in yellow and brown dunes. The variability in pH increased with increasing species diversity and also with scale. Overall, soil pH variability decreased with increasing vegetation cover. The lowest pH heterogeneity was found in heath dominated by Empetrum nigrum L. and grey dunes dominated by Campylopus introflexus (Hedw.) Brid. Increasing abundance of dominant species and decreasing species diversity of vegetation apparently reduces soil heterogeneity. Decreasing species diversity of vegetation is likely to explain decreasing variability in soil pH.     

Germination responses to abiotic stress shape species distributions on coastal dunes

Plant species occupy distinct zones on coastal dunes, but the mechanisms limiting their distributions have not been fully explained. We combined field surveys of plant distributions and abiotic conditions with controlled germination experiments to assess the contribution of germination requirements to plant zonation. Species presence and abiotic conditions were measured in ten transects across the barrier dune at Waquoit Bay, Massachusetts. Germinating seeds of six species were exposed to four fully crossed treatments: pre-treatment (soaked in fresh water, soaked in salt water, or not soaked), temperature (low, moderate, or high), soil salinity (none, moderate, or high), and light (full light, shade, dark). Species distributions in the field were affected by both distance from the shore and presence of dominant shrubs. Germination tolerance of soil salinity reflected species zonation: species found on the front slope of the dune tolerated salinity, while germination of other species was limited by salinity alone or by salinity in combination with high temperature. Shrubs reduced soil surface temperature and decreased light, but these conditions had limited effects on germination. These results indicate that limitations to germination can contribute to explaining species distributions on coastal dunes.     

Effects of the expansion by Hippophaë rhamnoides on plant species richness in coastal dunes

Question: Is the expansion of Hippophaë rhamnoides in coastal dunes associated with a decline in plant species richness, and is this decline best described by a hump‐backed relationship between species number and shrub cover? Location: Grey and yellow dunes on the East Frisian islands Spiekeroog and Norderney. Methods: Total plant species richness as well as the number of herbaceous and cryptogam species were determined in 2001 using plots of 16 m² size. We compared shrubland plots with varying cover of Hippophaë with neighbouring dune grassland plots without shrubs as reference sites. Soil samples were collected to determine the values of some important edaphic variables (pH, organic matter, nitrogen). Results: The shrubland plots with Hippophaë had or tended to have lower soil pH and C/N ratios and higher contents of organic matter and nitrogen than the grassland plots. Total species richness was marginally significantly related to the cover of Hippophaë in a hump‐backed manner on both islands. The pattern was more pronounced for mosses and lichens than for herbaceous species. For all species groups on Spiekeroog and for the herbaceous species on Norderney, the hump‐backed relationship was much improved when using the difference in species number between shrubland and grassland plot as a dependent variable. Relationships could be improved by including the soil parameters as co‐variables. Species richness was highest at moderate levels of shrub expansion, while it was much reduced in very dense shrubland. The decrease in species number is caused by the decline in grassland species typical of the open dunes, including some rare taxa. Conclusions: The expansion of Hippophaë rhamnoides is a serious threat to the plant species richness of open coastal dunes, and needs to be counteracted by management measures.     

Movement and modelling of buried seed of the invasive perennial Chrysanthemoides monilifera in coastal dunes and biological control

Movement in the soil of seeds of the invasive shrub Chrysanthemoides monilifera was studied in relation to the success of proposed biological control treatments. Marked seeds were buried at depths from 0 to 6 cm and recovered after periods of 3–24 months. Survival declined with time but was higher for more deeply buried seeds. The average movement of seeds was 0.7 cm per year downwards which enhances their survival but also tends to remove them from the zone of highest likelihood of germination. A model was constructed to predict the size of the pool of potentially germinable seed under treatments including pre-dispersal predation and burning. It indicated that satisfactory control with a pre-dispersal seed predator will be achieved only if there is a < 95% reduction in seed-fall to the soil surface over the entire year. A control agent that is 100% effective for most of the year will probably be unsatisfactory if its rate of predation drops to > 90% for a part of the year (e.g. during winter when seed-fall is abundant). The effectiveness of the control could be enhanced by using intense fires to reduce the soil seed-pool and to kill many of the mature shrubs. However, the feasibility of this combination of treatments will depend on the reaction of the biological control agent to fire.     

Dune slack restoration in Dutch mainland coastal dunes

The 45,000 ha Dutch dunes are relatively well protected but they suffer from severe desiccation. During the last decade a lot of knowledge has been obtained about the steering processes of eco-hydrology in dunes resulting in well-founded schemes to restore wet dune slack communities. In two case studies a large-scale restoration of the landscape was necessary. Since 1987 in the North-Holland Dune Reserve the groundwater catchment has been reduced (from 8 million m³ to 2 million m³ per year) and the resulting development monitored. In another case, large-scale intervention on 35 ha of the Amsterdam Water Supply Dunes in 1995 resulted in the restoration of active sand drift with opportunities for the establishment of pioneer vegetation. In small parts of the area, the surface has stabilised as a result of vegetation growth, but elsewhere drifting sand has spread and covered previously stable surfaces. Many wet to moist dune slacks developed and results relating to geomorphology, hydrology and vegetation are presented. Finally, in 1996 yet another project of 50 ha was carried out in the Meijendel dunes, in which infiltration ponds were removed to allow vegetation succession to develop from scratch. Results of these three case studies will be presented. If only hydrological restoration is carried out, sod-cutting was found to be necessary to restore the whole series of vegetation types of dune slacks. In two cases an area was denuded. The seed bank and surrounding slacks seem to play an important role in the success of restoration, this is especially true for the pioneer vegetation types. Future monitoring will prove whether critical (Red List) plant species will return.     

Fire indirectly benefits fitness in two invasive species

Ecosystems perturbed from their natural disturbance regimes are more vulnerable to establishment and dominance of exotic plant species. Restoration efforts that reintroduce fire have achieved mixed success in reducing the abundance of exotic plants. The responses of many native species to fire are well known; fire-adapted species respond directly (heat and smoke cue germination) and indirectly (post-fire environment benefits seedling survivorship and growth) to fire. However, the direct and indirect effects of fire are unknown for most exotic plant species. We tested the direct and indirect effects of fire on two exotic invaders of Asian origin, Ailanthus altissima and Lonicera maackii, in North American woodlands. To quantify the direct effects of fire, we compared germination rates of seeds exposed to varying levels of heat and smoke in a laboratory and placed at different soil depths during a prescribed fire in the field. We examined the indirect effects of fire by comparing seedling recruitment in burned and unburned woodland plots. Results indicate that neither A. altissima nor L. maackii have germination cues associated with fire. However, both species have greater seedling recruitment in burned as compared to unburned areas in the field. Although seeds of these invasive species are not specifically adapted to fire, they still benefit from post-fire environments and pose a challenge to restoration of fire-maintained ecosystems. Future studies using our approach will allow land managers to better predict how communities will respond to restoration efforts and to understand variability observed in past restoration projects.     

Seeds redistribution in sand dunes: a basis for coexistence of two rodent species

Spatial and temporal heterogeneity is a major factor structuring communities and contributing to coexistence of the species they contain. In this study we examine a critical aspect of environmental heterogeneity that is assumed to promote coexistence in two gerbil species of the Western Negev Desert. Previous studies assumed that temporal partitioning, in activity time, is the result of daily redistribution of seeds that the dominant species is the first to utilize while the sub-ordinate and efficient species is being pushed to use the later and poorer part of the night. We tested the assumption that daily afternoon winds generating spatial and temporal heterogeneity in seed availability by the redistribution of sand and seeds. This was done by comparing plots experiencing normal wind condition with manipulated plots where wind action was diminished by a shade-cloth fence. Our results show that considerable amount of sand and seeds are redistributing regularly on a time scale of a single day. Our results also show that gerbil foraging behavior is strongly related to the pattern of the redistribution dynamics of the seeds. When we prevented redistribution of seeds, gerbil foraging activity was reduced considerably. However, both seed redistribution and gerbil activity did not change much on control plots. Furthermore, the two gerbil species responded differently to the reduction in seed redistribution. The larger Gerbillus pyramidum was shown to be more sensitive to the reduction than the smaller G. a. allenbyi . Daily variability in the availability of seed resources is probably the niche axis which, together with the trade-off in foraging efficiency of the species, forms the mechanism for the coexistence of the two gerbil species in the semi-stabilized sands.     

Genetic diversity of rhizobia associated with Acacia longifolia in two stages of invasion of coastal sand dunes

We examined the genetic diversity of root nodule bacteria associated with the Australian legume Acacia longifolia in two stages of invasion of a coastal sand dune system. All isolates belonged to the genus Bradyrhizobium. A higher diversity was found in the long-established trees. The results suggest the introduction of exotic bradyrhizobia with the plant.     

Seasonality of mycorrhizae in coastal sand dunes of Baja California

 Populations of arbuscular mycorrhizal fungi were estimated from spores associated with seven plant species in coastal dunes at El Socorro, near Ensenada, Baja California, during six months in 1992. The seasonal patterns of percent root colonization were also described in the same species during the wet season (January–March) and the dry season (April–July). Comparisons were made between the pioneer species (Abronia maritima) in the mobile dunes and six species (Abronia umbellata, Atriplex julacea, Camissonia californica, Haplopappus venetus, Helianthus niveus and Lotus spp.) in the fixed dunes. Mycorrhizal colonization in Abronia maritima was slight (<1%) and we observed few spores (<1/g soil). All of the species in the fixed dune formed mycorrhizae with up to 80% colonization in early summer, and no more than 4 spores/g soil by late summer. The highest percentages of total colonization and abundance of spores did not coincide temporally for any of the seven species, but the percentages were higher in summer than in spring. Arbuscules were more abundant when the soil was moist, and vesicles more abundant when it was dry. Accepted: 16 August 1995     

Competition between two species for two complementary or substitutable resources

The classical models of interspecific competition are phenomenological, that is, they purport to describe the trajectories followed by the abundances of both competitors, without specifying either mechanisms or the dynamics of what the competition is for, i.e. resources. Yet the conditions for the different outcomes of competition inferred from these models are most often interpreted in terms of resources. Here it is contended that the dynamics of resource supply and exploitation must be explicitly taken into account if these conditions are to be obtained in any accurate form. First, the distinction between perfectly substitutable, imperfectly substitutable and perfectly complementary resources is examined. This leads to an array of models intended to mirror interspecific exploitative competition for two resources for each category considered. Then, the conditions for the coexistence of the two competitors in each case are derived and presented with direct reference to the modes of resource use.     

Responses of two invasive macrophyte species to salt

Aquatic ecosystems are particularly sensitive to the introduction of species due to human activities. Increasing salinity in freshwaters due to sea intrusion or to human activities (road salt, industry, etc.) is known to have a negative impact on aquatic organisms and could play a role in the dynamics of invaders. This study compares salt tolerance levels of two introduced aquatic plants Myriophyllum aquaticum (Velloso) Verdcourt and Ludwigia grandiflora (Michaux) Greuter and Burdet. Plants were collected in spring, summer and autumn 2010, and were exposed to a range of salt concentrations (1, 3 and 6 g/L). Plant growth was determined by measuring seven morphological traits and their photosynthetic activity. Increased salt levels induced a decline in growth and photosynthetic activity in L. grandiflora, while photosynthetic activity in M. aquaticum remained constant at all salt concentrations. The response of both species to salt varied according to the season. M. aquaticum allocated its energy to growth in autumn, whereas the growth of L. grandiflora remained constant whatever the season. Our results suggested that M. aquaticum stimulated root and leaf production at the end of summer, which conferred resistance to salt stress and allowed this species to become invasive by overcompensating for this stress. Conversely, L. grandiflora induced premature senescence and lost its leaves. In the context of worldwide salinisation of freshwaters, M. aquaticum could colonize brackish water and other water bodies, whereas L. grandiflora invasion could be limited.     

Displacement in two invasive species of leafminer fly in different localities

Liriomyza sativae was displaced by L. trifolii in the USA probably due to the lower insecticide susceptibility of L. trifolii, but L. trifolii was displaced by L. sativae in Japan. To know the possible causes of the species displacement observed in Japan, differences in the insecticide susceptibility, fecundity, effects of natural enemies, and reproductive interference between L. trifolii and L. sativae were evaluated. As a result, the higher fecundity of L. sativae and differential effects of parasitoids on the two Liriomyza species are considered to be contributing factors.     

Interactions between nematodes and their microbial enemies in coastal sand dunes

European foredunes are almost exclusively colonised by Ammophila arenaria, and both the natural succession and the die-out of this plant have been linked to populations of plant-parasitic nematodes (PPN). The overarching aim of this study was to investigate top-down control processes of PPN in these natural ecosystems through comparative analyses of the diversity and dynamics of PPN and their microbial enemies. Our specific aims were, first, to identify and quantify PPN microbial enemies in European sand dunes; second, to assess their life history traits, their spatial and temporal variation in these ecosystems, and third, to evaluate their control potential of PPN populations. This was done by seasonal sampling of a range of sites and making observations on both the nematode and the microbial enemy communities in rhizosphere sand. Nine different nematode microbial enemies belonging to different functional groups were detected in European sand dunes. Their high diversity in these low productivity ecosystems could both result from or lead to the lack of dominance of a particular nematode genus. The distribution of microbial enemies was spatially and temporally variable, both among and within sampling sites. Obligate parasites, either with low host-specificity or having the ability to form an environmentally resistant propagule, are favoured in these ecosystems and are more frequent and abundant than facultative parasites. Three microbial enemies correlated, either positively or negatively, with PPN population size: Catenaria spp., Hirsutella rhossiliensis and Pasteuria penetrans. Microbial-enemy supported links in the food-web may be involved in the control of PPN populations through indirect effects. The endospore-forming P. penetrans was the most successful top-down control agent, and was implicated in the direct control of Meloidogyne spp. and indirect facilitation of Pratylenchus spp. Overall, our findings suggest strong and diverse top-down control effects on the nematode community in these natural ecosystems.     

Analysis of the specificity of three root-feeders towards grasses in coastal dunes

Among the root-feeding nematodes that accumulate in the rhizosphere of grasses in European dunes, the genus Pratylenchus is of special relevance given its diversity and distribution. Although different species of Pratylenchus have been reported in dune grasses, the specificity towards dune plants, a fundamental aspect of the biology of the species, has hitherto not been studied. Two inoculation experiments using different combinations of grasses and nematodes were performed. The multiplication and the effect on plant growth of P. dunensis and P. brzeskii, two species which only occur in dune areas was compared with that of P. penetrans, a broad host-range species. The three Pratylenchus spp. could multiply under all hosts; however, there was a clear host-dependent response. The species-specific response observed might account for the shift of Pratylenchus spp. detected in the field. Although, a negative effect on the growth of A. arenaria was demonstrated for the three nematode species, different densities were needed to observe the same effects in plant biomass which point at nematode-specific tolerance. While the typical dune species needed very high densities to produce damage, P. penetrans needed very few specimens. The results obtained indicate that species with similar feeding adaptations show very different multiplication abilities on co-occurring hosts, an aspect that is usually overlooked for belowground herbivores in natural systems. The obtained results might suggest a coevolutionary relationship between specific nematode species and Ammophila arenaria.     

Vegetation dynamics of coastal sand dunes near Malindi, Kenya

This study describes the composition, structure and distribution of coastal dune vegetation near Malindi, Kenya. The vegetation was made up of 156 plant species. Sixty families were recorded with Gramineae (seventeen species) and Papilionaceae (sixteen species) being the most widely represented. Fifteen plant communities were described in the different geomorphologic units. The plant communities exhibited a distinct zonal distribution. Halopyrum mucronatum and Ipomoea pes‐caprae plant communities were common in the unstable geomorphologic units. Cordia somaliensis and Pluchea dioscoridis plant communities were common in the more stable geomorphologic units. Most of the plants in the more stable geomorphologic units were shrubs and trees. Succulent herbs were identified in the slacks and the drowned valley, which have moist and damp environments.     

The vegetation of road verges in the coastal dunes of the Netherlands

The vegetation of road verges in the coastal dune area of the Netherlands was surveyed phytosociologically. Nine plant communities have been distinguished. They belong to theAmmophiletea, Koelerio-Corynephoretea, Molinio-Arrhenatheretea, Plantaginetea majoris andArtemisietea vulgaris. Most plant communities appear to be, fragmentarily developed and have thus been described by the so-called deductive method. Vegetation in road verges often appears to be heterogeneous. Species composition is influenced by man, distance from the sea, fertility and moisture content of the soil. It is clearly correlated with total nitrogen, magnesium and organic matter. Therophytes are preponderant; these and sclerophytes indicate the dry environment. The majority of the species are apophytes.     

Differential salt tolerance of two Artemisia species growing in contrasting coastal habitats

To investigate factors determining the differences in their salt tolerance, growth and germination, experiments were conducted on two plant species belonging to genus Artemisia: Artemisia fukudo Makino, a biennial salt marsh plant and Artemisia stelleriana Bess, a perennial coastal hind dune plant. Growth experiments revealed that salinity (100 and 300 m m NaCl) inhibited the relative growth rate (RGR) in A. stelleriana significantly but not in A. fukudo. These specific differences in salt tolerance were mainly attributed to differential responses of net assimilation rate (NAR). That is, the reduction in RGR in A. stelleriana was mainly due to the reduction in NAR, whereas no significant reduction in NAR was observed in A. fukudo. The reduction in RGR in A. stelleriana in the salt treatment was also attributable to a reduced leaf area ratio (LAR). Specific leaf area (SLA) in the two species decreased in the 300 m m treatment. The decrease in SLA in A. fukudo was, however, compensated for partly by an increase in leaf weight ratio (LWR). Germination experiments also showed that A. fukudo has a higher salt tolerance than does A. stelleriana. These results are consistent with the differences in the salinity conditions between the native habitats of the two species.