Under the veteran oak

During lockdown, I've been studying the vegetation in our various tree initiatives in the public park. There is an interesting story in what's appeared in the fenced area under the veteran oak in the two years since it was fenced off. So far, I have identified 97 species there, quite an impressive total for such a small plot. This richness is because the ground conditions vary from shade and competition for water close to the tree trunk, to ex-amenity grassland across much of the plot and distubed stony ground on the boundaries near to the paths. The area looks a bit untidy at this time of year because many species have finished flowering and are dying back in the drought conditions. The yellow flowers of Hawkweed and Bristly oxtongues, however, add colour.

The most abundant species are those of amenity grassland: Perennial ryegrass, Common bent, Red fescue and a little White clover. The weeds of amenity grassland are less abundant: Ribwort plantain and Daisy,  Other grassland plants have appeared in much smaller numbers: Yarrow, Sweet vernal grass, Creeping thistle, Spear thistle, Cock's-foot grass, Rough and Smooth meadow grasses and Broad-leaved dock. This sward is likely to last for some years before perennial woody species and sedges take over.

Growing on the stony soil near the path edges of the plot are a range of species that are common on road verges and pavements: Mugwort, False oat-grass, Spear-leaved orache, Buddleia, Soft brome, Shepherd's purse, Sticky mouse-ear, Couch grass, Cleavers, Herb robert, Pineappleweed, Buck's-horn plantain, Broad-leaved plantain, Annual meadow-grass, Bramble, Wild lettuce, Oxford ragwort, Groundsel, Sow thistles, Lesser chickweed, Wheat, Wall barley and Barley. An unusual species is Flax, the seed of which has probably originated in the "Wildflower meadow" plots elsewhere around the great field.  As the plot matures most of these will be displaced by perennial plants of Buddleia and Bramble and management of these may be required to direct the vegetation more appropriately.

Some trees and shrubs have arrived as seeds carried by birds or squirrels: there are many seedling Hawthorns, one horse chestnut, some Stinking iris and a few plums or cherries not yet big enough to identify to species. There are also a two plants typical of woodland edge or hedgerows: Jack-by-the-hedge and Cow parsley. It's these species and the introduced sedges that are likely to form the future vegetation

Across the whole of the plot Pendulous sedge, which was introduced as young plants last winter is surviving quite well and there's just the one clump of Wood sedge and Wild garlic, similarly introduced.

As the plot matures, the annual species will get shaded out by perennials and the number of species will decrease greatly. Eventually, the vegetation will need management so it doesn't compete too much with the veteran oak and it remains a disincentive for people to encroach and so risk harming the oak's trunk.

Blue-green bacteria in the lake

When checking the water clarity on the 30th July I saw a green scum on the surface near the jetties. It looked like blue-green bacteria, so I took a sample for microscopic examination. The scum proved to be mainly a coiled blue-green bacterium (first image). There were smaller amounts of Aphanizomenon flos-aquae (second image) and two or three other species (remaining images below). There are a large number of blue-green bacteria that occur in the UK and it's a specialist matter to identify most species, and beyond my resources. The Environment Agency takes a precautionary approach to blue-green scums, recognising the need to regard all cyanobacterial species and strains as capable of producing toxins, and potentially posing a threat to animal and human health and safety (A DECADE OF MONITORING AND MANAGEMENT OF FRESHWATER ALGAE, IN PARTICULAR CYANOBACTERIA, IN ENGLAND AND WALES. Krokowski, J. & Jamieson, J. 2002). Particular concerns with our lake will be the risk to those undertaking watersports, anglers and, to a lesser extent, the general public on the lakeside path. Some dog walkers encourage their animals into the water, which could put dogs at particular risk.

One can only speculate on the reasons why we have got a bloom this year. Blue-green bacteria thrive in nutrient-rich water in warm summer conditions and it's possible that waterweed clearance has stirred up nutirents normally locked away in the sediments of the lake bed. The lake is already rich in phosphates, but algal growth is probably limited by nitrogen availability. The ability of blue-greens to fix atmospheric nitrogen could be another factor leading to the bloom.