Changes in Soil Composition and Floral Coverage on a Glacial Foreland Chronosequence in Southern Iceland

The land surface in front of the Skaftafellsj?kull in southern Iceland, exposed by ice recession commencing about the start of the twentieth century, constitutes a foreland with a maximum age of about 100 years and a more distal outwash plain. The ages of different surfaces within this sequence are constrained by moraines of known or estimated ages. Across this chronosequence, we measured at various sites the extent of floral coverage of the surface, the soil carbon and nitrogen contents of the substrate and the soil CO2 flux rate. All measured parameters exhibit values increasing with distance from the ice front, which correlates approximately with age. The strongest correlations are seen between distance and the carbon and nitrogen concentrations of the soil. Marked horizonation of the soil is observed only on the oldest surfaces (100+ years).

End-Triassic nonmarine biotic events

The Late Triassic was a prolonged interval of elevated extinction rates and low origination rates that manifested themselves in a series of extinctions during Carnian, Norian and Rhaetian time. Most of these extinctions took place in the marine realm, particularly affecting radiolarians, conodonts, bivalves, ammonoids and reef-building organisms. On land, the case for a Late Triassic mass extinction is much more tenuous and has largely focused on tetrapod vertebrates（amphibians and reptiles）, though some workers advocate a sudden endTriassic（TJB） extinction of land plants. Nevertheless, an extensive literature does not identify a major extinction of land plants at the TJB, and a comprehensive review of palynological records concluded that TJB vegetation changes were non-uniform（different changes in different places）, not synchronous and not indicative of a mass extinction of land plants. Claims of a substantial perturbation of plant ecology and diversity at the TJB in East Greenland are indicative of a local change in the paleoflora largely driven by lithofacies changes resulting in changing taphonomic filters. Plant extinctions at the TJB were palaeogeographically localized events, not global in extent. With new and more detailed stratigraphic data, the perceived TJB tetrapod extinction is mostly an artifact of coarse temporal resolution, the compiled correlation effect. The amphibian, archosaur and synapsid extinctions of the Late Triassic are not concentrated at the TJB, but instead occur stepwise, beginning in the Norian and extending into the Hettangian. There was a disruption of the terrestrial ecosystem across the TJB, but it was more modest than generally claimed. The ecological severity of the end-Triassic nonmarine biotic events are relatively low on the global scale. Biotic turnover at the end of the Triassic was likely driven by the CAMP（Central Atlantic Magmatic Province） eruptions, which caused significant environmental perturbations（cooling, warming, acidification） through outgassing, but the effects on the nonmarine biota appear to have been localized, transient and not catastrophic. Long-term changes in the terrestrial biota across the TJB are complex,diachronous and likely climate driven evolutionary changes in the context of fluctuating background extinction rates, not a single, sudden or mass extinction.

Spatial and temporal dynamics of growth of woody plant species (birch and willows) on the foreland of a retreating glacier in southern Iceland

Background:The forelands of retreating glaciers are invaluable natural laboratories in which to explore the processes of primary succession.Numerous studies have been conducted on foreland chronosequences to identify temporal and spatial trends of the successional communities.This study focused on the spatio-temporal distribution of three woody plant species on the foreland of a retreating glacier in southern Iceland where historical observations provide precise age control of the moraines.To evaluate colonization and successional trends,we examined which species increase in abundance with time and tested the role of proximity to a seed source in colonization.Additionally,we quantified the rate at which biomass carbon is added to the landscape.Results:The density of stems of Betula pubescens increases with moraine age across the foreland chronosequence while the density of stems of both Salix lanata and Salix phylicifolia decreases.We found low statistical significance to the relationship between the density of B.pubescens and distance from a forested ridge nor did we find a relationship between the lengths of the stems and the moraine ages.Woody biomass increased fastest during early successional stages and reached a maximum of 28.5 g C m^(−2) on the oldest moraine.Conclusions:Early colonization of moraines was controlled by environmental filters which favored both Salix species.Colonization by B.pubescens followed as environmental factors,e.g.,favorable soil properties,improved.We found no conclusive evidence that proximity to a potential source of B.pubescens propagules was a significant factor in controlling colonization.The assumption that the abundance of individuals increased with time through later successional stages proved valid for B.pubescens,but not for either species of Salix.These findings are consistent with the classical spatial successional model of community homogenization.Thus,general successional processes at the landscape scale control the temporal dynamics of individual species.

Successional trends and processes on a glacial foreland in Southern Iceland studied by repeated species counts

Introduction:Primary succession on glacial forelands is increasingly relevant as rapid glacial retreat is exposing growing land areas to plant colonization.We investigated temporal trends,controls,and outcomes in floral succession on a subarctic glacial foreland.Specifically,we examined changes in community composition(mosses,low shrubs,forbs,trees,and graminoids)over long-term(decadal)and short-term(<10 years)scales and attempted to identify the underlying processes responsible for the observed successional patterns.Methods:The study area was the foreland of the Skaftafellsjӧkull,located in Vatnajӧkull National Park near the south coast of Iceland.We established nine transect lines at varying distances from the ice front representing surfaces of age ranging from less than one decade to over 100 years.Each transect consisted of five measurement stations of 1 m2 where we measured vegetative cover(VC),species richness(SR),and species density(SD)and calculated species evenness(SE).Measurements were made initially in 2007 and repeated at the same geographic coordinates in 2014.Results:VC increased with distance from the ice front from 16%to over 90%.SR and SD increased from the youngest pioneer community through a mid-successional stage corresponding to an age of over 60 but less than 100 years.Increased VC but declining SR,SD,and SE characterized the oldest(over 100 years)bryophyte-dominated surfaces.Species turnover,which involved forbs almost exclusively,increased moderately from early through midsuccessional sites and declined on older sites.Comparison of the measurements made in 2014 to those made in 2007 demonstrates increased SR at mid-successional sites while SD remained relatively constant.Conclusion:At a small scale,colonization is controlled by local factors such as microtopography and aspect,particularly in proximity to the glacier.At the landscape level,changes in VC and community structure are controlled by time and nutrient availability.Low nutrient levels and limited site availability favor bryophyte dominance on the oldest surfaces.The greatest community-level changes observed over the 7-year interval were increases in surface cover by mosses and low shrubs,particularly in mid-successional and older sites.These changes suggest that the community on the oldest surfaces has not yet reached equilibrium.