Investigation of the ‘canopy effect’ in the isotope ecology of temperate woodlands

Anomalously ¹³C-depleted carbon stable isotope values from closed forest ecosystems have been termed the ‘canopy effect’. Originally this was ascribed to recycling of depleted carbon from forest floor decomposition of organic material, although others have suggested that it is equally likely to be due to variations in leaf-level processes in response to increased shade. This depletion in the heavier carbon isotope is passed on to woodland herbivores feeding within the forest environments. A similar isotopic depletion has also been reported in the archaeological literature from more open temperate woodland settings, but no measurements have been made on the plants at the base of the food chain in order to quantify the effect. In this study we attempt this by examining the carbon and nitrogen stable isotopic values of different species of grasses from a range of open to closed habitat settings within Wytham Wood, Oxfordshire, UK. We find a strong relationship between carbon isotopic depletion of plant tissue and lowered light intensity with an up to 5‰ shift between grass grown in open and closed locations. In order to follow this up the food chain, we also report data on wool from sheep grazing in open pastures near the Wood, and on fallow deer living within the woodland, but which turn out not to show a strong canopy effect, probably related to their feeding strategies. We conclude that there is indeed a strong ‘canopy effect’ in temperate woodland, probably related to differential light levels, but that not all apparently woodland-dwelling mammals show such an effect. We also show considerable isotopic variation at the base of the food chain, which should counsel caution when attempting to interpret dietary isotopes using mixing models.     

Combining pollen and charcoal: evaluating Holocene vegetation composition and dynamics

Pollen analysis reveals vegetation change on a local and regional level over (ideally) continuous time spans but with complications when it comes to the reconstruction of spatially precise species occurrence and composition. Wood charcoal analysis provides site-related information on species occurrence and woodland composition, especially when sediments containing pollen are lacking, but is normally discontinuously available and has the danger of human bias due to potential selection of certain species for e.g. fire making, house construction or charcoal making. Combining the two methods – assuming that suitable palaeoarchives are neighbouring – might compensate for their disadvantages and strengthen the interpretations on vegetation composition and dynamics on a local, extra-local and regional scale. Three examples are given where pollen analysis from “classical” archives – mires and lake sediments – was carried out, and where additionally species identification of wood charcoals was done on samples extracted from close-by archives. The first example (Kugelstattmoos, Bavarian Forest, Germany) combines a Holocene pollen diagram derived from a small mire with late medieval/early modern times charcoal spectra from charcoal production sites which are situated on the margin of the mire. Results show the local occurrence of the main woodland species Fagus sylvatica and Abies alba, and raises new questions on the montane species composition of the formerly natural forests in the Bavarian Forest. The second example (Bargstedter Moor, Schleswig-Holstein, Germany) combines Holocene pollen diagrams from a peat bog with late medieval/early modern times charcoal spectra from charcoal production sites in a woodland close to the bog. It shows the role of Quercus, Fagus, Alnus and Carpinus in medieval and early modern times woodlands and its use by man. The third example (Kleiner Tornowsee, Brandenburg, Germany) combines pollen data from sediments of a small lake with charcoal data derived from Holocene colluvisols and soils. The examples show that anthracological and palynological data complement one another to create a more detailed picture of woodland history and landscape change than one method alone would provide.     

Estimates of Historical Tree Densities in the North Lachlan River Catchment,New South Wales,Australia

Vegetation structure and composition during the pre‐ and early‐European period in Australia is often proposed as a benchmark against which to measure the effects of modern landscape management practices. However, little quantifiable information exists for that historical period. One potential source of information is the land survey record, used in this study to provide estimates of tree densities that existed during the period 1870 to 1900 in part of central New South Wales. Estimates were obtained for 23 parishes in the north Lachlan River valley, between the towns of Condobolin and Tottenham. Densities were calculated from portion corner to nearest tree data obtained from historical survey plans, using the plotless, closest individual method. Densities ranged from 6.0 to 31.4 trees per hectare for parishes with enough data points for reliable calculation. These figures would account for trees of a diameter large enough to be blazed with a survey mark. Any smaller trees would be additional and, although no quantitative data existed for this, qualitative evidence suggested that the figures obtained were more likely to be underestimates due to the presence of such small trees. The study area was predominantly covered by woodlands which varied in density across the landscape, in comparison to the modern landscape where much of the native vegetation has been cleared for agriculture. Although the tree density figures should be considered to be indicative rather than precise, they provide quantifiable historical figures for past conditions in an area where the historical record is otherwise sparse. Such information is valuable for interpreting ecological changes brought about by modern land management.     

OSL dating as an alternative tool for age determination of relic charcoal kilns

Chronometric studies of charcoal production remains are largely based on ¹⁴C-dating of associated charcoal. Owing to intrinsic limitations, however, this method provides no meaningful time resolution for post-1650 CE features. We investigate the potential of optically stimulated luminescence (OSL) dating of heated sandy sediments as an alternative and complementary tool for dating charcoal kiln remains. Seven samples from five relic charcoal kilns and 11 complementary samples from the underlying sandy substrate are used. Through a range of procedural tests, we demonstrate that the single-aliquot, regenerative-dose procedure in combination with OSL signals from quartz allows determining equivalent doses both accurately and precisely. For four of the five investigated kilns, OSL ages are consistent with independent age information from ¹⁴C-dating and written sources. Especially for post-1650 CE features, the precision can be significantly better than that of ¹⁴C-dating, and we highlight the potential of OSL dating for distinguishing, relatively, between charcoal production phases with an unprecedented time resolution. We conclude that the approach is a promising alternative to ¹⁴C.