Clearance of Holm Oak and plant diversity on the Undercliffs

 Holm Oak is deemed the most serious of the invasive weeds on the Axmouth-Lyme Regis Undercliffs National Nature Reserve, and it continues to spread, especially on Pinhay Cliffs. The previous Site Manager, Albert Knott, reckoned that it has a profound effect on plant communities (and, therefore, on the insects and other animals that depend on them). The shade that its dense evergreen canopy casts means that there is virtually no herb layer, and the tree’s litter is acidic and decomposes slowly. Management of Holm Oak is most needed on those precious remaining patches of flowery chalk grassland, and perhaps in areas where this could be restored. One such area is a shoreline cliff just east of Humble Point, which we know from old aerial photos to have been open in the past. Here, contractors employed by Natural England supplemented by volunteers from the Axe Vale & District Conservation Society cleared Holm Oak during the winter of 2007/08.

This conservation work has provided an opportunity to see if chalk grassland will reappear on this area,
With the encouragement of Tom Sunderland (Natural England’s site manager), Joan Millard, Marjorie Waters and I laid out five marked squares (quadrats) each 1 m x 1 m, at random on a slope from where Holm Oak had been removed. A sixth quadrat was placed at the edge of remaining Holm Oak woodland as a “control”.

The relative proportion of bare ground, litter and vegetation cover in each plot was assessed and the plant species present were noted, and their abundance recorded. Plots were rated on four occasions, on 30th April, 26th June and 22nd September 2008, and on 6th May 2009.

At the start, in April 2008, 85% of the surface of the plots was bare and only 15% vegetation-covered. By May 2009 these figures had almost reversed, with only 25% of the surface uncovered by plants and 75% vegetated. The control plot remained virtually unchanged.

The most striking change was the increase in diversity. Plots that contained just one or two species of flowering plants at the start of our observations contained up to 16 a year later. This is about half the diversity level (number of species per unit area) found in chalk grassland within the Reserve. Not surprisingly, many of the flowering plants that appeared have wind-dispersed seeds – species like Sowthistle, Groundsel, Creeping Thistle and species of Willowherb. These are species that would often be described as weeds and which appear on many recently-cleared sites. However, species like Yellow-wort, Hairy Violet, Hawkweed Ox-tongue, Ploughman’s Spikenard, Glaucous Sedge and Bird’s-foot Trefoil suggest that the seed of at least some typical chalk grassland plants may have persisted in the soil.

So what hope is there of restoring more chalk grassland to supplement the remaining patches within the Undercliffs National Nature Reserve? Of the 32 plant species found, 12 are typical components of scrub and woodland. The two most densely vegetated quadrats (3 and 5) were each dominated by Giant Fescue which is likely to exclude many other species by its sheer vigour. Chalk grassland relies for its persistence on grazing and/or mowing. Without these it is quickly colonised by scrub and thicket – as happened in many parts of southern England after rabbits were removed by the myxomatosis epidemic of the 1950s. If this patch is to continue as open ground bearing chalk grassland, continuing management will be needed – more work for AVDCS work parties!