Long-term vegetation monitoring

Introduction

Between 1973 and 1976 (hereafter referred to as the 1974 survey), 164 10x10m plots  were established at alternate points of a 100x100m grid by Dawkins and Field (1978).  Various measures of tree and shrub cover were made as follows: 

The occurrence of all woody species in the plot was noted

Diameters (breast height) of the four largest trees in the plot were measured;

Canopy cover (>2.5m) (split by species) and shrub cover (all species) (0.5-2.5m) was estimated across the plot diagonal;

The basal area of the trees in the plot and its surroundings was estimated by relascope sweeps;

Regeneration (seedlings and saplings up to 1m tall) were noted in 13 0.1m2 circlets positioned evenly along the two diagonals of the plot.

Ground flora species were listed for each plot and by rooted occurrence in the circlets; except in 1974 cover-abundance for each species across the whole plot has been estimated using the Domin scale.

A partial survey of these plots was carried out in the 1980s and a full re-survey was completed in 1991-92 (the ‘1991 resurvey’), 1999-2002 (the ‘1999 resurvey’) and 2011-12 (the ‘2012 resurvey’).  Between 2000 and 2010 there was also more regular recording (about every 2 years) of the 14 plots in Marley Wood.

DAWKINS, H. C. D. & FIELD, D. R. B. 1978. A long-term surveillance system for British woodland vegetation. Commonwealth Forestry Institute Occasional Paper 1. Oxford: Commonwealth Forestry Institute.

SAVILL, P. S., PERRINS, C., KIRBY, K. J. & FISHER, N. 2010. Wytham Woods, Oxford’s Ecological Laboratory, Oxford, Oxford University Press 

Current Research

The current emphasis is on analysing and writing up the results from the 2012 survey and trends over the last 40 years.

Aerial photographs taken in the 1940s show the canopy was very open in places, with large gaps, probably a consequence of wartime felling, and that large areas that have now become wooded, were rough grazing or fields.  By 1974 (when the plots were first surveyed), the average canopy cover was around 80% across the whole wood with a dense understorey as a consequence of both planting and natural regeneration.  Most trees were young; the largest tree in the plot was most likely to be in 11-20 cm diameter class.  Since 1974 there has been relatively little change in the canopy layer cover but the understorey cover has declined markedly.  The number of species records also dropped between 1974 and 1991.  The basal area of trees has more than doubled (shrubs were not included in this part of the recording process).  The increase in basal area is reflected also in the increase in the mean size of the largest trees in the plots and the shift in the median size class for the largest trees in the plots to 31-40 cm. This equates roughly to trees about 40-60 yrs old in 2012.  

Fraxinus excelsior has increased across all measures using all measures of abundance.   Acer pseudoplatanus, Quercus robur and Fagus sylvatica had slight declines or no change in occurrence, little change in their contribution to the canopy cover and slower basal area growth than F.excelsiorBetula spp and Ulmus spp. declined, particularly in terms of plot occurrences, but also in terms of their relative contributions to canopy cover and basal area.  There were fewer conifers in 2012 than in 1974, but their basal area had increased. Ilex aquifolium was unusual in showing a marked increase in plot occurrences, although as yet its contribution to canopy and basal area overall was very small.  Shrubs  other than Crataegus spp.  and Prunus spinosa declined after 1974, contributing to the overall loss of understorey cover, but there were indications of possible recovery by 2012 for Corylus avellana  and Euonymus europaeus.

Between 1974 and 1991 the main ground flora trends were a shift from bramble dominance to grasses over large parts of the Woods in response to increasing deer numbers and general canopy closure.  In the last decade the bramble cover has been increasing again.  The trends in other species have still to be analysed.

The plot data provide background information on woodland change for more detailed studies within the Woods, e.g Mihok et al. 2009.  The plot results have also been incorporated in a cross-European analysis of long-term changes in undisturbed woodland (Verheyen et al 2012; de  Frenne et al 2013).

MIHOK, B., KENDERES, K., KIRBY, K.J., PAVIOUR-SMITH, K. & ELBOURN, C.A.  (2009).  Forty-year changes in the canopy and the understorey in Wytham Woods.  Forestry 82 515-527.

VERHEYEN, K., BAETEN, L., DE FRENNE, P., BERNHARD-ROMERMANN, M, BRUNET, J., CORNELIS,  J., plus 17 other authors 2012.  Driving factors behind the eutrophication signal in understorey plant communities of deciduous temperate forests.  Journal of Ecology 100, 352-365.

DE FRENNE, P.,  RODRÍGUEZ-SÁNCHEZ, F., COOMES, D.A., BAETEN, L., plus  31 other authors  2013.  Microclimate moderates plant responses to macroclimate warming. Proceedings National Academy of Science USA 110, 18561-18565.

 

Current Papers

KIRBY, K.J., BAZELY, D.R.,  GOLDBERG, E.A.,  HALL, J.E., PERRY, S.C. & THOMAS, R.C. (under review) Changes in the tree and shrub layer of Wytham Woods (southern England): local and national trends compared.  Forestry.