Effects of slope angle and aspect on plant cover and species richness in a humid Mediterranean badland

Soil erosion is one of the most severe land degradation processes in the Mediterranean region. Although badlands occupy a relatively small fraction of the Mediterranean area, their erosion rates are very high. Many studies have investigated to what extent vegetation controls soil erosion rates. This study, however, deals with the impact of erosion on vegetation establishment. In semi‐arid badlands of the Mediterranean, soil water availability constitutes the main limiting factor for vegetation development. As a consequence, south‐facing slopes are typically less vegetated due to a very large water stress. However, these findings do not necessarily apply to humid badlands. The main objective of this paper is to determine the topographic thresholds for plant colonization in relation to slope aspect and to assess the spatial patterns of vegetation cover and species richness. We surveyed 179 plots on highly eroded badland slopes in the Central Pyrenees. We defined four aspect classes subdivided into slope angle classes. Colonization success was expressed in terms of vegetation cover and species richness. Slope angle thresholds for plant colonization were identified for each slope aspect class by means of binary logistic regressions. The results show that a critical slope angle exists below which plants colonize the badland slopes. Below this critical slope angle, plant cover and species richness increase with a decreasing slope angle. The largest critical slope angles in humid badlands are observed on south‐facing slopes, which contrasts with the results obtained in semi‐arid badlands. North‐facing slopes however are characterized by a reduced overall vegetation cover and species richness, and lower topographic threshold values. The possible underlying processes responsible for this slope‐aspect discrepancy in vegetation characteristics are discussed in terms of environmental variables that control regolith development, weathering and erosion processes. Moreover, possible restoration strategies through the use of vegetation in highly degraded environments are highlighted. Copyright © 2014 John Wiley & Sons, Ltd.     

Early Benthic Organism Colonization on a Caribbean Coral Reef (Barbados, West Indies): a Plate Experimental Approach

Abstract. Macrofaunal colonization was studied using coral plates fixed at 10 m depth on a fringing reef in Barbados, West Indies. Two sites were compared: Spring Garden, a site that is affected by eutrophication, and Six Men's Bay which is relatively free of this effect. Two sets of plates were immersed in winter and spring 1990 at each site and monitored for 28 and 19 weeks, respectively. Green and brown algae colonized first, followed 2 weeks later by ostracods, cumaceans, amphipods and polychaetes: terebellids, nereidids and syllids. Most colonizers either nestled or encrusted. A total of 11 017 invertebrates belonging to 145 species were collected. Polychaetes were the most abundant and diverse zoological group. Considering polychaetes, crustaceans and molluscs, the study sites shared around 75% of the species. Overall, densities were higher on lower (shaded) surfaces while diversities were higher on upper (illuminated) sides. Similarity coefficients and correspondence analysis showed different macrobenthic assemblages and colonization patterns in light and shaded and in polluted and unpolluted sites. Coral plates were associated according to their 'immersion time' and were closely linked to their stage of development. Spring Garden (polluted) presented a higher number of soft-bottom and deposit-feeder species ( e.g. , Capitella capitata ).     

A comparison of diatom colonization on natural and artificial substrata in seawater

Experiments to investigate diatom colonization on various substrata in seawater are described. No differences were found due to different methods of preparation of the substrata. Differences in the numbers of diatoms colonizing the different substrata emphasize the care needed in selecting a substratum on which to study the settlement of organisms, especially if the experiments are to be used for subsequent prediction.     

Temporal variability of disturbances: is this important for diversity and structure of marine fouling assemblages?

Natural communities are constantly changing due to a variety of interacting external processes and the temporal occurrence and intensity of these processes can have important implications for the diversity and structure of marine sessile assemblages. In this study, we investigated the effects of temporal variation in a disturbance regime, as well as the specific timing of events within different regimes, on the composition and diversity of marine subtidal fouling assemblages. We did this in a multi‐factorial experiment using artificial settlement tiles deployed at two sites on the North East coast of England. We found that although there were significant effects of disturbances on the composition of assemblages, there were no effects of either the variation in the disturbance regime or the specific timing of events on the diversity or assemblage composition at either site. In contrast to recent implications we conclude that in marine fouling assemblages, the variability in disturbance regimes (as a driving force) is unimportant, while disturbance itself is an important force for structuring robust ecosystems.     

Soil seed bank composition and distribution on eroded slopes in the hill‐gully Loess Plateau region (China): influence on natural vegetation colonization

On the Chinese Loess Plateau, serious slope and gully erosion have caused a decrease in soil water capacity and fertility, which has resulted in vegetation degradation and a reduction in agricultural productivity. Great efforts have been made to restore vegetation to control soil erosion, but the efficiency of artificial revegetation is not satisfactory. Natural revegetation is an alternative. However, while soil seed banks are an essential source for natural revegetation, their composition and distribution on eroded slopes remains unknown. In addition, whether or not seed loss during soil erosion limits vegetation colonization is also unknown. In this work, soil seed bank composition and distribution were studied in three situations. Specifically, three main microsites were selected as sampling plots: fish‐scale pits, as artificial deposited micro‐topography; under tussocks, as trap microsites; and open areas, as eroded areas. Soil samples were collected at depths of 0–2 cm, 2–5 cm and 5–10 cm. The soil seed bank was identified using germination experiments, and a total of 34 species were identified. The dominant species in the soil seed bank were annual/biennial herbs with an average proportion more than 90% and density reaching 19,000 seeds m^‐2. The pioneer species Artemisia scoparia was especially abundant. The dominant later successional species, such as Lespedeza davurica, Artemisia giraldii, Artemisia gmelinii, Stipa bungeana and Bothriochloa ischcemum, were present in the soil at a density that ranged from 38 to 1355 seeds m^‐2. Compared with the eroded open areas, the fish‐scale pits retained a higher density of seeds, and the tussocks retained a larger number of species. However, there was no serious reduction of the soil seed bank in the erosion areas. The present study indicates that, on these eroded slopes, the soil seed bank is not the key factor limiting the colonization of natural vegetation. Copyright © 2011 John Wiley & Sons, Ltd.     

Plant Colonization of Recent Lahar Deposits on Popocatepetl Volcano,Mexico

This work investigates the initial colonization on recent lahar deposits of the northeast slope of Popocatepetl volcano, Mexico, where 29 circular sample plots (1.57 m²) were established to determine the development stages of colonization in four types of habitats: 1997 lahar, 2001 lahar, margins, and terraces at the channel's bottom. Cluster analysis and the Sørensen Index were used to determine the floristic affinity of these lahars. Richness, frequency, percentage of species contribution, plant cover, density, and stem height were analyzed to determine the composition and structure of plant communities. The distribution of these variables reveals that the communities often have a simple internal structure; however, a relationship has already been established between changes in resilience and the age of the four lahars. Thus, floristic composition develops rapidly (1 sp. in terraces, 11 spp. in the margins, 29 spp. in the 2001 lahar, and 34 spp. in the 1997 lahar). Except for the 1997 lahar, however, structural characteristics are poorly developed in other incipient stages. As colonization advances, the affinity among the various components and sectors of the laharic deposits decreases, thus promoting the gradual incorporation of species found on the adjacent gorge slopes. The Principal Components Analysis used to identify other explanatory factors shows that of 15 variables studied, those associated with the morpho-sedimentology, the hydrovolcanic dynamics and stability of deposits (microtopography, thickness of the deposit, clast shapes, length of the deposit, depth of the gorge, and slope processes) explain a large percentage of variance. Only a few species (Lupinus campestris, Alchemilla procumbens and Penstemon gentianoides), are well adapted to poor soils and the effects of intense erosion caused by the flows.     

The distribution of algae and insects in hot spring thermal gradients at Waimangu,New Zealand

The distribution of algae and insects was examined in two hot spring outflow channels at Waimangu in the Rotorua thermal district. The upper temperature limit for algal growth was 68°c, with maximum biomass occurring at 59–62°c. Two species of Diptera were present on the algal mat, Ephydrella thermarum Dumbleton at 34–43°c and Chironomus cylindricus Freeman at 39–41°c. The composition and temperature relations of the community are similar to those found in thermal regions in other parts of the world.     

The rate of infill and colonization by invertebrates of borrow pits in the Wash (S.E. England)

The outer borrow pit had a volume of 458 000 m³ when excavated and was further increased (by 22 500 m³) by the erosion of a lagoon, but was completely infilled in 3·5 years. The rate of sedimentation in the outer borrow pit, though declining with time, was considerably greater than previously recorded rates for intertidal banks and probably accounted for the paucity of fauna in the pit until completely infilled. Poorly sorted, unconsolidated, anaerobic, clay and silt sediments initially accumulated in the outer pit with up to 5 cm liquid mud at the surface, but after 2 years the predominant fraction was fine sand. Only Nepthys hombergii became established in the outer borrow pit before the pit was completely infilled, when the other pre-excavation, dominant species returned. The expectation that Capitella capitata and oligochaete species (opportunistic, ‘pollution indicator’ species) would be the first colonizers was not realized, though the oligochaete, Tubificoides benedeni was the most abundant species in the more consolidated, sandier sediments in the inner borrow pit 4 years after its excavation. The density of invertebrates in the inner pit was greater than the surrounding mud flats and was a focus for birds feeding in the area. The outer borrow pit after complete infill also had higher desities of bird food species than before excavation but was rarely visited by birds.     

Analysis and application of ichnofabrics

Bioturbation at all scales, which tends to replace the primary fabric of a sediment by the ichnofabric (the overall fabric of a sediment that has been bioturbated), is now recognised as playing a major role in facies interpretation. The manner in which the substrate may be colonized, and the physical, chemical and ecological controls (grainsize, sedimentation rate, oxygenation, nutrition, salinity, ethology, community structure and succession), together with the several ways in which the substrate is tiered by bioturbators, are the factors and processes that determine the nature of the ichnofabric. Eleven main styles of substrate tiering are described, ranging from single, pioneer colonization to complex tiering under equilibria, their modification under environmental deterioration and amelioration, and diagenetic enhancement or obscuration. Ichnofabrics may be assessed by four attributes: primary sedimentary factors, Bioturbation Index (BI), burrow size and frequency, and ichnological diversity. Construction of tier and ichnofabric constituent diagrams aid visualization and comparison. The breaks or changes in colonization and style of tiering at key stratal surfaces accentuate the surfaces, and many reflect a major environmental shift of the trace-forming biota due to change in hydrodynamic regime (leading to non-deposition and/or erosion and/or lithification), change in salinity regime, or subaerial exposure. The succession of gradational or abrupt changes in ichnofabric through genetically related successions, together with changes in colonization and tiering across event beds, may also be interpreted in terms of changes in environmental parameters. It is not the ichnotaxa per se that are important in discriminating between ichnofabrics, but rather the environmental conditions that determine the overall style of colonization. Fabrics composed of different ichnotaxa (and different taphonomies) but similar tier structure and ichnoguild may form in similar environments of different age or different latitude. Appreciation of colonization and tiering styles places ancient ichnofabrics on a sound process-related basis for environmental interpretation.     

Dendrochronology and lichenometry: colonization, growth rates and dating of geomorphological events on the east side of the North Patagonian Icefield, Chile

This paper highlights the importance for dating accuracy of initial studies of delay before colonization for both trees and lichens and tree age below core height, particularly in recently deglaciated terrain where colonization and growth rates may vary widely due to differences in micro-environment. It demonstrates, for the first time, how dendrochronology and lichenometry can be used together in an assessment of each other's colonization and growth rates, and then cross-correlated to provide a supportive dating framework. The method described for estimating tree age below core height is also new. The results show that on the east side of the North Patagonian Icefield in the Arco and Colonia valleys, Nothofagus age below a core height of 112 cm can vary from 5 to 41 years and delay before colonization may range from a maximum of 22 years near water to a minimum of 93 years on the exposed flanks of the Arenales and Colonia Glaciers. Tree age plus colonization delay supplied a maximum growth rate of 4.7 mm/year for the lichen Placopsis perrugosa and lichen colonization is estimated to take from 2.5 to approximately 13 years. A minimum lichenometric date of 1883 was estimated for an ice-formed trimline at the junction of the Arenales and Colonia glaciers and a maximum dendrochronological date of 1881 for a water-formed trimline in the Arco valley. Tree and lichen ages around the valley suggest that a glacial outburst drained the 1881 high level lake releasing approximately 265 million cubic metres of water. Repeated flooding, with a minimum of 38 high lake levels, is suggested by horizontal sediment lines on the Arco valley walls and moraine flanks. Dating confirmed diminishing flood levels with a last minor flood in 1963. The wider significance of the work is that it should produce more accurate dating of recent glacier fluctuations around the North Patagonia Icefield, an area where dated reference surfaces are extremely scarce.