Oxfordshire Recorders and Conservation Day, Saturday, 28 February 2015 Oxford University Museum of Natural History, Oxford OX1 3PW ______

Seed Bank Plants - presentation by Dr Judith A Webb All photos copyright J A Webb

Introduction

As long as I have been a botanist, I have been fascinated by seeds and growing plants from seeds.

The theme of this conference is ‘Recording the Invisible’ and my imagination was first caught by the account of the resurrection of the rare plant Starfruit Damasonium alisma - apparently ‘back from the dead’. This had disappeared (been lost) in the flora of ponds that had become silted up or choked with emergent vegetation of sedge and bulrush, losing the shallow open water it needed.

‘Cleaning-out’ such a pond – removing most of the choking vegetation and silt to return it to an earlier successional stage – activates long-dormant (invisible) Starfruit seed held in the seed bank in the pond sediment (as long as some of the original sediment with dormant seed has been left or actively returned to the rejuvenated pond).

It is important that we realise, as recorders, that a plant, though not seen as a flowering or vegetative organism, may not be extinct on a site. It may be still there but invisible in the form of viable but dormant seed in the soil or sediment. Such a seed bank might last for only a few years or for up to 100 years or longer depending on the species.

What follows are some examples of what I have found out from my own observations and research on plants with seeds with long dormancy and those with short dormancy.

1st Photo Set Port Meadow, SSSI, SAC rare seed bank species

I am Flora Guardian within the Ashmolean Natural History Society (ANHSO) for the very rare member of the Apiaceae locally known as creeping marshwort Apium repens. Its main site is Port Meadow here in Oxford, where it benefits from winter flooding and year-round grazing by cows and horses. It needs both winter flooding and fairly heavy grazing to thrive. Port Meadow is an ancient common- land grazing meadow, which is an SSSI and a SAC for this and other species.

Flooding lingers longest on the southern part of Port Meadow and when I first walk out in early summer to start surveying for creeping marshwort, I walk across the drying mud, churned by the feet of stock, and although I can see no creeping marshwort plants at all, I know I am walking over an invisible seed bank of this species in the soil. By July 2014 young, newly germinated, plants are visible, as shown here.

This last year another surprise was the abundant germination of another seed bank plant, the tiny mudwort Limosella aquatica, which had not been seen in this location in such large numbers for many years. It flowers and seeds very quickly and then dies, returning its population to the seed bank. This last summer the seedling creeping marshwort plants were slow off the mark.

They got as far as producing a few flowers, but no seed, by the time the floodwater started to rise again in October. Such plants do not survive the long flooding that lasts until the following June.

A worry here is that the seed bank is actually being used up by the germination of plants in July, but there is no time for replenishment of the seed bank before the young plants go under water again and are killed.

2014 was a remarkable year on Port Meadow for yet another seed bank plant - a dock noticed in early summer as being a little different from the normal range of docks on the meadow. It was subsequently identified by experts as the marsh dock Rumex palustris. Its large population here in 2014 was a new site record and the first example of this species recorded in Oxfordshire since the 1880s, when it was found on Otmoor. How could such an obvious and distinctive plant have been missed by the botanists who regularly visit Port Meadow? Most likely because it was present unseen in the seed bank. Long flooding events killed most green plants via anoxia in the late spring of 2013 and 2014. This left bare mud, which was churned by the feet of stock, exposing marsh dock seeds, which germinated and thrived due to the lack of competition from suffocating plants such as creeping bent. That’s my theory anyway.

2nd Photo Set Short-lived seed plants

I have been studying a couple of short-lived seed plants. All orchids (like the bee orchid pod and seeds seen here) have extremely short-lived seed. The bee orchid has virtually no seed reserve. Its seeds, which are tiny, like pepper dust, are dispersed by the wind and rely on finding a suitable soil and a symbiotic fungus to germinate within a few days of release. Although it has no seed bank in the soil, there may be a few years of ‘tuber-bank’ – bee orchid tubers can lie dormant underground for 4 years before you see the flowering plant emerging in the 5th year. After a few days in the wind or in an unsuitable habitat, orchid seed is dead.

Both the meadow thistle Cirsium dissectum and the quaking grass Briza media are perennial species that are declining in Oxfordshire, and quaking grass is on the new Vascular Plants Red List for England as ‘Near Threatened’. They have very short-lived seed, which may last only one year, i.e. no significant dormancy. Quaking grass seeds fall from the plant in summer (they have no dispersal to speak of) and germinate in the autumn near the parent plant. Almost none of the seeds in the soil are left alive by the next spring. The plant is a low-grower that has little competitive ability and is easily swamped by rank vegetation of stronger-growing grasses and herbs. This is why it survives in calcareous low- nutrient sites that are either grazed (chalk grassland) or cut for hay and grazed (Oxford floodplain meadows). It needs the reduction of competitor plants to keep its foothold.

Meadow thistle has a similar strategy. Its seeds have no dormancy and the pappus hairs fall off readily, so little spreads far from the parent plant. It cannot compete with rank vegetation and usually only one flower is produced per rosette. After flowering, that rosette dies, so the plant gets smaller; only non-flowering rosettes survive.

Oxfordshire Recorders and Conservation Day, 28 February 2015 2 Oxford University Museum of Natural History, Oxford Seed Bank Plants - presentation by Dr Judith A Webb If the seeds cannot find suitable bare areas in the sward that autumn to allow them to successfully germinate the next spring, the population declines. Light grazing prevents flowering (thus retards rosette death) and encourages a small amount of vegetative growth sideways and the formation of new vegetative rosettes, so it helps the population increase slowly. A low-nutrient habitat again favours this plant by ensuring the surrounding vegetation does not become tall and rank.

Both of these species are adapted to ‘staying in one place’ under a regime of low nutrients and light extensive grazing. This is just the sort of management that seems to be impossible these days, which is one reason why the meadow thistle is on the Oxfordshire Rare Plants Register and why quaking grass has just been added to the England Red List.

rd 3 Photo Set Examples of seed bank species from different habitats

Foxgloves Digitalis purpurea are a common, long-lived, seed bank biennial species of woodlands, as are other typical ones such as red campion Silene dioica. These are the quick responders to the increased light and disturbance when a tree falls or in the first year after a coppicing management event. When things become shady again as the coppice re-grows, these species may disappear into the seed bank again and wait for the next disturbance and ‘increase in light’ event. In contrast, other woodland species such as wood anemone and bluebell have no persistent seed bank at all and rely quite a lot on vegetative reproduction.

Temporary shallow ponds are a habitat where seed bank species are common. One rare speciality of such habitat in Oxfordshire is the grass poly Lythrum hyssopifolia. This is a small annual that germinates on the bare mud of the drying out shallow pond in the ‘draw-down zone’. It has little competitive ability, so cannot survive if rank perennial grasses and other herbs invade the pond in a dry year. High water levels or prolonged flooding of the area may kill off the perennial competing vegetation that creates the bare conditions ideal for the plant to thrive once the water has dried up. Thus numbers of this plant vary erratically from thousands in a good year to none in a poor year. Its success therefore depends critically on the weather in the previous year and management. A long- lived seed bank ensures it can wait out the bad years of unfavourable water levels or inappropriate management.

4th Photo Set Seed bank plants of wet meadows and ditches New Marston Meadows SSSI, Oxford.

New Marston Meadows (hay meadows) are normally flooded by the Cherwell every winter. Silting- up of ditches led to their not draining down promptly in the spring and the flora was becoming sedge and meadowsweet dominated. A grant enabled some critical ditches to be cleaned out, with the aim of restoring MG4 community. The silt from the ditch was spread over a considerable area of the adjacent meadow. Two years from this activity, germination of seed bank plants from the ditch silt resulted in a spectacular display of the common perennial, ragged robin Silene flos cuculi and small numbers of the following annual plants, which are on the Oxfordshire Rare Plants Register: tasteless water pepper Persicaria mitis/laxiflora, bifid hemp nettle Galeopsis bifida and fig leaved goosefoot Chenopodium ficifolium.

Oxfordshire Recorders and Conservation Day, 28 February 2015 3 Oxford University Museum of Natural History, Oxford Seed Bank Plants - presentation by Dr Judith A Webb 5th Photo Set Fen plants and seed banks

A lot of my work recently has been overseeing restoration of short fen vegetation to some of Oxfordshire’s valuable SSSI calcareous alkaline fens to mimic the light grazing that had ceased. Areas that have been dominated by tall herb, sedge and reed are being cut and raked more frequently. Areas of scrub invasion are having the scrub cut or winched out.

Volunteers doing all this cutting and raking on a site inevitably cause a degree of poaching disturbance of the fen peat, bringing long buried seed to the surface.

Here are three rare fen species: bog pimpernel Anagallis tenella, marsh lousewort Pedicularis palustris and Grass of Parnassus Parnassia palustris*. So which ones might we expect to recur from the seed bank with all this renovation management? The answer from my observations is bog pimpernel (seed bank reputed to last more than 70 years) and marsh lousewort, but not Grass of Parnassus. The last species has short-lived seed, which, like orchid seed, must rely on moving on to new suitable sites by wind or water. So when the first two have ‘disappeared’ from the vegetation, they may not actually be ‘gone’. In the case of Grass of Parnassus, when it is noticed to have ‘disappeared’ from the vegetation, most likely it really has gone from the site for good, if more than a couple of years have passed since it was last seen. Remediation is more urgent for this last species than for the first two.

*The NatureGate website has a very interesting page for Parnassia palustris

6th Photo Set Long-lived propagules in other taxa, some liverworts and stoneworts, charophytes

Bryophytes do not have long-lived spores, except in a few cases. The Riccia liverworts have large, ornamented, thick-walled spores, which can persist in soil for some years, just waiting for enough disturbance, high light and lack of competing higher plant vegetation to germinate and thrive. The classic example is the common crystal wort Riccia sorocarpa, which is a species of bare soil in arable fields, gardens, waste ground, gravel tracks and footpaths. Dispersal may be on the feet of animals.

The stoneworts, Characeae, have resistant thick-walled oospores, which can persist in silt and mud in the bottom of a pond for many years. They also enable colonisation of new sites by being carried with earth particles blown on the wind from a dried-up pond to a newly-formed pond, or on the feet of water birds.

Rare species of stonewort such as Tassel stonewort Tolypella intricata and some Chara species have apparently ‘risen from the dead’ when a historic pond or ditch site has been cleaned out and the oospore containing sediment disturbed.

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Oxfordshire Recorders and Conservation Day, 28 February 2015 4 Oxford University Museum of Natural History, Oxford Seed Bank Plants - presentation by Dr Judith A Webb