by Dr Astrid Biddle, joint BSBI VC recorder for Hertfordshire
I am writing this in response to a request to explain the project's background and the biological details of the sedge. This is an unapologetically botany–orientated version, and readers are encouraged to Google unfamiliar terms, but hopefully their meaning is clear within the context of the account. Herts and Middlesex Wildlife Trust will write an account for general readership describing the Natural England Species Recovery Programme project, which will become available on their website.
This is the story of Scarce Tufted-sedge, a fabulous habitat engineer, how it was discovered in the UK, how it’s doing, and what we are doing to secure its future in the UK.
What’s a sedge? I could say it’s not a rush and not a grass but that’s not a great start. Sedges are often elegant and beautiful plants and, along with grasses and rushes, belong to a group of plants called the monocots. Easily dismissed as dull or uninteresting, the family of sedges (Cyperaceae) show an amazing diversity. Studying their structure reveals a subtle complexity. Many of us will learn the rhyme, "Sedges have edges, rushes are round; Grasses have knees that bend to the ground", which tells these monocots apart. There are many exceptions I can think of in the rhyme, but the sedge edges spoken about are on the stem, the vertices of the triangular cross-sections forming the edge, particularly towards the base. Looking from above, the leaves surrounding the stem fall away in these three directions. The stems are solid and have no joints or nodes, like grasses.
What is Carex cespitosa? The scientific name is very appropriate, as cespitose means growing as a clump, but it’s more than that; it’s a tussock-forming plant. Tussock-forming sedges avoid the problem of water fluctuation by evolving to grow as tussocks. Vertical root growth raises the plants away from the water table, allowing oxygen to reach the roots during high water levels. That adaptation towards tussock formation allows it to exploit a niche that is more competition-free. The other plants around it drown, creating a very distinctive and strange-looking landscape.
In his book "The Day of the Triffids", John Wyndham apparently based the Triffids on Greater Tussock-sedge. I haven’t found a source for this claim, but if true, he certainly misjudged. To my eyes, they are friendly beings, the botanical version of “Cousin It”. Like little botanical volcanoes, they are habitat engineers creating new vegetation islands. The waters rise and fall, helping the height of the tussocks form according to that hydroperiod, the height and duration of the water. One tussock is composed of a single genet, a genetically distinct individual formed through the integration of both flowering and vegetative ramets (shoots).
To my eyes, they are friendly beings, the botanical version of “Cousin It”.
The older shoots die, and silt is trapped. The tussock pedestal forms a necromass, a mass of vertical roots, decaying material, and silt. The tallest reported in the Czech Republic is an incredible 1.5 m tall. It is home to numerous invertebrates, and between the tussocks, a maze and hideaway for waterfowl and other fauna. Seeds from nearby plants fall into the growing tussocks, generating a non-soil seedbank. Lidia Borkowska in Poland has sliced a tussock into sections, and like archaeology, by cultivating the sleeping seedbank, she could see the history of vegetation development around the tussock.
The tussock pedestal forms a necromass, a mass of vertical roots, decaying material, and silt.
Global distribution maps (GBIF) show that C. cespitosa has been recorded from 26 countries globally. It has a Palearctic distribution, with records heavily weighted towards the eastern end of the range in the Russian Federation, Poland, and Japan. It’s a critical species, both variable and challenging to identify. So, across its range, it has acquired new names as botanists try to define their local flora.
It is most closely related to the Nigra group of sedges, a group which has similar characteristics because they are closely related, and with which they can sometimes hybridise.
So, did we know where it grows in Europe? Pedro Jiménez-Mejías and his research group reappraised the European distribution in 2014. He used an herbarium revision of specimens and a bibliographic survey to confirm records of the species in 19 European countries. In European countries for which assessments have been made following the IUCN criteria, it is classed as threatened in several. Looking at the map, it appears to be more widespread to the east of the range, but the threat to it is increased to the south and west.
European distribution of Carex cespitosa. Dense populations indicated by shading and incidence recording by symbols: white shapes before 1968 & black after, triangles bibliographic references & circles herbarium specimens (Jiménez-Mejías et al, 2014).
How was it found in the UK? During his work, Pedro found a herbarium sheet bearing the identification of Tufted Sedge (Carex elata), that was submitted by Dony in 1960. He redetermined the scrap of material as C. cespitosa. Looking at the herbarium sheet, among other features, there was a reddish leaf sheath, which is not characteristic of Carex elata. BSBI County Recorder Trevor James was contacted and reviewed a site at a spring in Braughing, Hertfordshire, where he had previously recorded Carex elata. He revised this determination to C. cespitosa (James et al. 2014). The sites recorded by James and Dony were not the same but 2.5 km apart. Dony’s site was also revisited, but the present conditions no longer supported sedge species. Braughing is presently the only known UK site, and together with populations in Spain, represents the western limit of its global distribution.
A herbarium sheet found at the Natural History Museum.
So, could it have been more widely overlooked in the UK? C. cespitosa was also missed in Spain until Pedro et al. recognised it in the Pyrenees in 2007. Again, mistaken for Carex elata. An alert urging people to look for it was published in the BSBI News, and I’ve posted alerts on social media, but nothing else has come forward (Denholm & Lansdown 2021, Biddle 2023). There are few rare plants you can search for in winter, but this is exactly what has worked in the past. Cosson & Morcrette in 1999, identified new locations for C. cespitosa in the Jura massif, increasing recognised colonies from 10 to 43 by employing a winter prospecting protocol. Anyone fighting through waist-high Hemlock knows how difficult it is to find anything within it, so that was a clever strategy.
Winter tussocks of Carex cespitosa.
How’s our C. cespitosa population doing at Braughing? It’s difficult to say because there was a lack of monitoring in the past. Turning to what evidence we have. Trevor James described about 200 large tussocks in the main colony in 2011, which may have reduced somewhat. The main observational difference was, bizarrely, in the choice of footwear to access the site. Summer photos from 1987 show what appeared to be floating islands of sedge surrounded by water, but I can now wear trainers on the site, at any time of year. The squelch is going! Groundwater abstraction across Hertfordshire is responsible for many of the drying wetlands.
A quick analysis of vegetation lists for the satellite area between 1985 and 2021 revealed a significant decrease in moisture-loving plants, an increase in nutrient-loving plants, and a reduction in light-demanding species. This is what would be expected given the combination of natural succession, higher abstraction levels, unknown (and delayed) results of hydrological modification from the nearby pond creation, higher background nutrients and a Himalayan Balsam invasion.
The historic construction of the railway embankment additionally disconnected the population from the river, a source of seed dispersal. After flowering, peduncle elongation is observed in the UK population (Biddle, unpublished 2023) and the Pyrenean (Jiménez-Mejías, pers. com.), a feature seen in other Phacocystis sedges. Its purpose is to drop seed into the water, not within the tussock. Autochory and nautochor, self- or water-mediated dispersal, are the mechanisms described in the literature. I would, however, feel certain that it also benefitted from avian vectors.
The positioning of the colonies in dappled shade on the pasture's edge is also significant; they were on the edge before the pond was dug. In the past, cattle would have gathered here to cool off in summer, hooves poaching the ground, creating wet hollows. The shade added to the competitive release of the vegetation and limited the soil surface desiccation. Now, there are lower water levels, and grazing no longer occurs. This parallels a population described and monitored in the Netherlands by de Boer 1974. It has since substantially declined. He observed that the sedge grew in river meanders, which, through natural processes, became cut off and infilled over many years. He perceived the process as sped up by unfavourable artificial changes in the hydrological conditions. C. cespitosa in cut-off locations can lose the ability to reconnect with the river course, and on the edge of the population range can more easily lead to extinction. It looks likely that the constancy of the spring at Braughing has held the north population stable so far.
Please don’t visit this site. It’s on private land, and you need permission to access it. Too much footfall will not help preserve the population here.
It’s risky to have one population, and at this point Herts and Middlesex Wildlife Trust created a new flagship project, which was made possible by Natural England’s Species Recovery Programme. It is being reintroduced to four sites in Hertfordshire.
Plant translocations and reintroductions are a relatively new science. 85% of European reintroductions have occurred since 2001, most in the last decade. Godefroid et al. recently analysed the reasons for success and failure. A combination of these factors is required for success. These are a firm understanding of the species' autecology and pressures, careful site selection and preparation, appropriate genetic diversity, mycorrhizal associates, a proper propagation protocol, monitoring, aftercare, appropriate site management, risk mitigation for factors like weather, adequate project length funding, and long-term outcomes monitoring. Unsurprisingly, reported success rates for establishing populations were low.
Finding out these things was a tricky task for a really rare plant with only one U.K. site. So, I started with germination trials, cultivation, and, over one Christmas holiday, an extensive literature review to be sure of understanding the species. So, what conditions do we need? The answer to this was not straightforward as there was not a lot of directly relevant data out there. I did, however, find some great work by Emilia Brzosko in Poland, which charted the succession in a wet meadow where C. cespitosa was already dominant.
Tussock size is usually related to the development stage and biological age. Ramet (shoot) senescence increases with later developmental stages, and when this exceeds ramet generation, the genet senesces.
Health, which is perceived by vigour, height, colour, and a lack of dieback during the active growing season, determines the longevity of individual ramets. Both abiotic factors and the genet developmental stage determine that health.
C. cespitosa is a long-lived perennial, that can journey through the seral stages, colonising the open ground habitat of lake shores, river margins, and long-term inundations. It passes through the stages of swamp, meadows and scrub, eventually ending in over-shaded willow woodland, dying as the habitat dries and organic material accumulates. Tussock size is usually related to the development stage, biological age, and the hydroperiod, so population success may not be directly related to tussock size. The tussocks may be defined according to age/ stage classes, broadly following the classification by Brzosko (2001). The distribution of these classes allows an assessment of the population dynamics and its health. Populations can also be held longer without progressing through these stages where hydrology allows, such as at a spring.
C. cespitosa age/ stage classes:
- Upon germination in open conditions, the early stages progress rapidly. The classes are summarised below.
- Seedlings
- Juvenile
- Virginal tussock, either in the same year or the following year, and it is at this point that the first ramet (shoot) senescence starts to occur.
- Generative tussock where it begins to flower in the second year and does not possess many dying shoots.
- Generative senile where the tussock remains flowering but with dying shoots or a clearly decreasing density of shoots. Some large plants may now colonise the tussock, which may contribute to physical degradation.
- Subsenile is when it ceases to flower.
- Senile (dead) is when shoots no longer appear. This has value both in structure and is a form of non-soil seedbank.
My schematic on the age/ stage classes of Carex cespitosa and the pressures and drivers exerted on it.
People like simple stories, and our temptation is to think in binary terms: “It grows in this habitat,” not the other. We need to hold on to the ideas of gradation and longevity. As an analogy, it would be inappropriate to determine the habitat requirements of Willow by treating seedlings in open meadows the same as saplings in scrub and then trees in woodland.
Astrid Biddle with Lucy Biddle in Finland 2024
During a family holiday in Finland, I took the opportunity to examine C. cespitosa populations. And yes, they followed that pattern. It was fascinating to see the distribution of age/ stage classes in different habitats. It fitted the model.
Then we set about the task of raising young C. cespitosa. We were really fortunate in 2023, as our cool spring gave if a respite from the drying conditions, producing a vast abundance of viable seeds. Drier springs produce smaller and less viable seed. This was important as it meant that seeds could be collected from many individuals, giving the widest possible genetic diversity for future populations. Fantastically, they matched the cultivation techniques developed by Lawrence B & J Zedler (2011) of its close relative, Carex elata. Nosterfield Nursery had experience creating habitats using C. elata and using the method for pre-tussock formation, so they were commissioned to grow 800 young plants, achieving nearly 1,000!
Pre-formation of C. elata tussocks using the method of Lawrence B & J Zedler, 2011
The tussocks were preformed by a series of drowning and drying to encourage vertical root formation. This allowed the plants to be placed in water and grow away faster. At four years, C. elata when planted out, had an impressive 95% survival rate at Nosterfield Quarry, and that is what we wanted for C. cespitosa.
Nosterfield’s work, led by Martin Hammond at Ripon City Wetlands, a Yorkshire Wildlife Trust’s site for the new Carex elata swamp. Here, the young tussocks are in their first year after planting, and astonishingly, in just two years, with the right conditions, they had transformative results.
In Hertfordshire, we have been digging a set of scrapes at two different sites. Water Vole took over a third, so digging could no longer progress. A fourth site was found at Water End, Great Gaddesden, a particularly wet stretch of riverbank, recently left bare from a river restoration project. The predicted optimal conditions for the habitat are ‘squelchy’ areas with a water table at or near ground level, or sufficiently rain-filled, with bare soil. Unshaded expanses of sediment allow unimpeded germination and tussock formation. The hydroperiod should ensure that conditions never dry out in the summer and that it’s flooded ankle-deep in winter.
We will be planting nearly 1,000 sedges next month. It's exciting, and I have fantastic volunteer teams to help me, who will also help with the aftercare and monitoring. Of course, this is a process of solution trialling, and if we are successful, we hope to roll this out for C. cespitosa in other Hertfordshire places. A project like this doesn’t happen without a team. My first thanks are to Chloë Edwards and Team HMWT, but there are many more.
A selection of the references:
Borkowska, L., Lembicz, M., Kasprzykowski, Z., 2017. A Non-Soil Seed Bank Dependent on the Size of Clonal Plants: The Case of Carex cespitosa, a Guerrilla Species in an Unmown Meadow. pjoe 65, 258–268. https://doi.org/10.3161/15052249PJE2017.65.2.008
Brzosko, E., 2001. Changes in population structure of Carex cespitosa during 10 years of secondary succession in an abandoned meadow in Bialowieza, Poland 38.
Cosson, E., Morcrette, P., 1999. Statut de la laîche en touffe (Carex cespitosa L.) en Franche-Comté et en Suisse limitrophe. Le Journal de Botanique 9, 85–92. https://doi.org/10.3406/jobot.1999.1765
de Boer, H., 1974. De zodezegge, Carex cespitosa L., tenslotte toch in Nederland gevonden. Gorteria Dutch Botanical Archives 7, 57–63.
James, T.J., Jiménez-Mejías, P., Porter, M.S., 2012. The occurrence in Britain of Carex cespitosa, a Eurasian sedge rare in western Europe. New Journal of Botany 2, 20–25. https://doi.org/10.1179/2042349712Y.0000000002
Jiménez-Mejías, P., Hilpold, A., Frajman, B., Puşcaş, M., Koopman, J., Mesterházy, A., Grulich, V., Lye, K.A., Martín-Bravo, S., 2014. Carex cespitosa: reappraisal of its distribution in Europe. will 44, 327–343. https://doi.org/10.3372/wi.44.44303
Lawrence, B.A., Zedler, J.B., 2011. Formation of tussocks by sedges: effects of hydroperiod and nutrients. Ecological Applications 21, 1745–1759. https://www.jstor.org/stable/23023114