Dan Yeloff

The disappearance of Sphagnum imbricatum from Butterburn Flow, UK

The disappearance of the previously abundant moss species Sphagnum imbricatum has been investigated at Butterburn Flow, northern England, using organic geochemical, elemental, macrofossil, pollen and testate amoebae analyses. Variations in the assemblage of peat-forming plants were tracked using the macrofossil distributions as well as the relative chain lengths of n-alkanes and concentrations of 5-n-alkylresorcinols and triterpenols. No significant changes to the vegetation assemblage could be detected prior to the loss of S. imbricatum. Variations in water depth were reconstructed using a testate amoebae transfer function and inferred qualitatively using bulk elemental composition and biomarkers for changing redox conditions in the bog subsurface: the degree of isomerization in the C31 hopanes, and the concentrations of bishomohopanol and archaeol. Pollen analysis reconstructed the landscape surrounding the mire and revealed evidence for human disturbance. The results suggest that bog surface wetness increased with the transition from Sphagnum imbricatum to Sphagnum magellanicum, but the increase was not large and S. imbricatum had previously survived similar periods of wetness. However, the loss of S. imbricatum coincides with increasing human disturbance surrounding the bog, which may have altered nutrient inputs to the bog surface from agriculturally derived dust, to the detriment of S. imbricatum but to the benefit of S. magellanicum and Eriophorum vaginatum. It is proposed here that the stresses imposed by the combination of changing nutrient inputs and a rapidly rising water-table drove the disappearance of S. imbricatum from Butterburn Flow at c. cal. AD 1300.

Volcanic ash deposition and long-term vegetation change on subantarctic Marion Island.

ABSTRACT A c. 5500 year record of peatland development and vegetation change was generated from a core recovered from an Agrostis magellanica peat bog on subantarctic Marion Island, using palynomorph, plant macrofossil, and tephra analyses. Two tephra horizons (both 17 cm thick) were identified and dated to ca. 2900 cal. BP and ca. 1700 cal. BP. Succession of the vegetation as a consequence of tephra deposition, particularly by the pioneer Azorella selago, appears to have been very slow, lasting as long as c. 700 yr. The slow pace of vegetation succession highlights the sensitivity of the indigenous Marion Island flora to environmental change, and the vulnerability to the spread of alien invasive species.

Is pollen morphology of Salix polaris affected by enhanced UV-B irradiation? Results from a field experiment in High Arctic tundra

This study tested the hypothesis that the thickness of the pollen wall will increase in response to enhanced UV-B irradiation, by examining the effect of enhanced UV-B irradiance on the pollen morphology of Salix polaris Wahlem. grown in a field experiment on the Arctic tundra of Svalbard. Measurements of pollen morphology were conducted by light microscopy on plants grown at two sites, Adventdalen and Isdammen. Salix vegetation was grown under control, enhanced UV-A, and two enhanced UV-B (simulating 15 and 30% reduction in the thickness of the stratospheric ozone layer) treatments. At the Adventdalen site, pollen wall thickness significantly increased under enhanced UV-A and UV-B treatments compared with the control. A thicker pollen wall helps to prevent damage by UV-B radiation of the DNA of the pollen. In contrast, plants at the Isdammen site did not exhibit any significant pollen morphological response to the enhanced UV treatments. The inconsistency in plant response to enhanced UV treatments between the two sites may be explained by greater habitat heterogeneity at the Isdammen site; abiotic soil conditions including nutrient and water availability may also have an influence on pollen morphology.

The disappearance of Sphagnum imbricatum from Butterburn Flow, UK: a reply to comments by Bjorn Robroek et al

We welcome the comments by Bjorn Robroek et al. (The Holocene 19 (2009) 1093—1094, this issue) on our paper (McClymont et al., The Holocene 18 (2008) 991-1002) and the opportunity to discuss further the complexities that surround the disappearance of Sphagnum imbricatum from Butterburn Flow (our study), and the implications for understanding the disappearance of this species in northwest Europe. We also wish to clarify our site of study; we presented data only from Butterburn Flow, northern England. Although we note that the disappearance of S. imbricatum here is part of a wider European decline in the late Holocene, we did not present data from Wales or Ireland as suggested in the opening paragraph of Robroek’s comment. We also noted that the replacement of S. imbricatum by S. magellanicum occurred over c. 44 years, but proposed that it may have been longer owing to evidence for reduced peat accumulation across the transition.