Plant ecology

History

After the glaciers in the UK retreated about 12000 years ago, a tundra-like vegetation developed on the glacial till and loess deposits covering the eroded limestone platforms. As climate improved, woodland cover became established on the newly formed soil. This was mainly birch and hazel to start with, followed by larger, longer-lived trees such as oak and ash.

About 4000 years ago, the effect of humans on the landscape started to outweigh the effect of natural processes. Tree felling to clear land for crops and grazing animals meant that areas which were originally wooded became less so. It is known that some areas of limestone pavement which are currently bare of woodland cover such as Malham, were once wooded. Other areas of limestone pavement were not as badly affected by grazing and soil loss and stayed wooded. The majority of wooded pavements are found at low elevation around Morecambe Bay, although there are some small areas of wooded pavement in the Yorkshire Dales, Durness in Scotland and in North Wales.

Grazing animals accelerated the process of soil loss from most upland pavement surfaces, and vegetation cover became increasingly fragmented. Woodland plants were unable to find a niche on the surface of the pavement where the soils were thin or non-existent, and retreated into the grikes between the blocky clints where there was deeper soil, shade, humidity and protection from grazing.

This is why it is possible to find typical woodland species such as honeysuckle (Lonicera periclymenum), dogs mercury (Mercurialis perennis), herb Paris (Paris quadrifolia) and angular Solomon’s seal (Polygonatum odoratum) in upland areas which are otherwise surrounded by tracts of grassland and moorland. This is particularly true for many woodland plants which have poor dispersal mechanisms where the seeds have no way of getting to where the plants are currently found. This woodland vegetation is a relic of the original plant cover, and one of the things that makes limestone pavement so unusual.

Current conditions

The clearance of woodland and the emergence of bare limestone clints created conditions suitable for a range of plants which are characteristic of open situations such as grassland or moorland. Where clints are covered by a thin layer of soil, plants typical of the surrounding limestone grassland will be found, for example, blue moor grass - limestone bedstraw (Seslaria caerulea-Galium sterneri) CG9 NVC communities. These plants are tolerant of drought and can grow on the most skeletal of soils and include thyme (Thymus polytrichus), sheeps fescue (Festuca ovina) and common bent (Agrostis capillaris).

Water collects in solution pans and in these fen and flush vegetation such as rushes (Juncus spp.) occur. Willow (Salix spp). has been known to grow in some of the larger solution features on limestone pavement.

Common plants found on limestone pavement

The diversity of plants found on limestone pavements has a direct relationship with the large number of ecological niches or habitats found on a pavement. There are a number of plants always found on limestone pavements throughout their geographic range in Britain. Ward and Evans (1975) and Natural England (1995) concentrated surveys on the plants associated with the grikes. These surveys showed that the most frequent species found in the grikes were herb Robert (Geranium robertium), maidenhair spleenwort (Asplenium trichomanes), dog’s mercury, harts-tongue fern (Phyllitis scolopendrium), wall-rue (Asplenium ruta-muraria), male fern (Dryopteris felix-mas), common dog violet (Viola riviniana) and wall lettuce (Mycelis muralis). The high proportion of ferns commonly found is down to the shady, humid conditions in grikes.

Rare plants

There are a number of nationally scarce plant species associated with limestone pavements which give them a high conservation value. These are listed in the table below.

Some of these plants are barely found anywhere else apart from limestone pavements. The best example of this is rigid buckler fern of which 85% of the UK population is found on limestone pavements.

A list of many of the plants associated with limestone pavements can be found here with an assessment of the types of pavements that they are found on.

Wooded limestone pavement

Wooded pavements generally have a covering of mosses and other bryophytes under a canopy of trees. The limestone pavement patterning is most often hidden by the growth of bryophytes and leaf litter. Trees are rooted in deep patches of soil or in the grikes, with natural clearings showing exposed areas of limestone pavement.

The main tree species found on wooded pavements are ash (Fraxinus excelsior) and sycamore (Acer pseudoplatanus) with some oak (Quercus spp.), and wych elm (Ulmus glabra). Some pavements have a significant amount of yew (Taxus baccata), and others occasionally, have small leaved lime (Tilia cordata). Understorey species include spindle (Euonymus europaeus), buckthorn (Rhamnus catharticus), hazel (Corylus avellana)and hawthorn (Crataegus monogyna). Many areas of wooded pavement have been coppiced in the past. Transition areas between woodland and clearing support the greatest diversity of plants with many of the species found on the open pavements also found here.

Some very old trees were once found within wooded pavement where they became established, but have now found themselves on open pavement as cover has decreased. Their establishment took place on deeper soils, their roots penetrated cracks and hollows in the limestone, and they now find themselves in a stressful environment. These trees, some of which are known to be hundreds of years old, may become ‘bonsai’ in form due to shoot die-back and browsing by deer and rabbits. Their contorted, dwarfed forms are frequently grazed back into the grikes. One particular tree on Gaitbarrows NNR has visibly shrunk in the 25 years between two monitoring photographs taken by Natural England staff.

A reasonably comprehensive list of plant species commonly found on limestone pavements can be found here

The ecology of grikes, runnels, pits and pans

Grikes

Grikes provide sheltered humid and shady conditions where climate is more moderate than in the open. Grikes also provide a habitat for species that are intolerant of grazing. The width and depth of grikes varies a lot. Most grikes are between 1 and 2m deep, but some have been measured as much as 6m in depth. Width can be anything from a few mm to up to 500mm. Deep grikes are considered to be those which are more than twice as deep as they are wide. It is only these deep grikes which provide the protection from grazing and extremes of climate which plants such as bloody crane’s-bill and angular Solomon’s-seal need to survive.

Woodland floor plants dominate the flora found in shallower grikes, and in the deepest grikes, ferns such as hart’s tongue fern can be found as these plants can tolerate the shadiness at depth.

Pits and pans

Pits and pans are the solution hollows associated with limestone pavement. Isolated islands of vegetation on clints will often have pits and pans developing underneath. The deeper soil here means that vegetation can survive for longer than in runnels. However, the isolated nature of the patch of vegetation means that eventually the soil and plants will shrink until there is a point where all will be lost through desiccation, wind erosion and disturbance. The final phase of this sequence is an open pan which holds water after rain with possibly an algal community being supported.

In pits and pans which still have soil, an ephemeral fen community can become established including plants such as Common Yellow sedge (Carex viridula), articulated rush (Juncus articulatus), butterwort (Pinguicula vulgaris) and birds-eye primrose (Primula farinosa). Pits and pans which have lost much of their soil depth will often hold a community of drought tolerant plants such as the stonecrops (Sedum spp). Much of the limestone pavement in England is suffering from the advanced stages of vegetation retreat with only isolated areas of soil cover.

Runnels

Most runnels have no vegetation, but where a soil cover is found, the plants supported are limestone grassland type plants. These include fescues, blue moor grass and wild thyme. Dropwort (Filipendula vulgaris), flea sedge (Carex pulicaris) and hairy violet (Viola hirta)can also be found growing in some runnels.

The runnels keep their soil and vegetation for longer than the clint tops as plant cover retreats, and the soil in the runnels is the last remnants of the post-glacial cover that would have been found right the way across the limestone. Soil in the runnels is prone to desiccation as roots of the plants cannot penetrate the limestone surface. When the soil dries out, it can be easily dislodged or blown away. This is one of the mechanisms by which the soil cover on limestone pavements is reduced.