Woodland Flowers

There are a few plants, such as Mistletoe Viscum album, that live on trees directly. The rest have to have something to cover their roots, even if this is just a thin skim on a branch fork that allows a Polypody fern Polypodium vulgare to establish. Soil does not just provide mechanical support for the plants but is also the source of the mineral nutrients and water needed for growth.

Woodland soil is a complex mix of organic material (leaf litter, dead roots, faeces, living invertebrates and microbes) and mineral particles derived from the weathering of rocks locally or brought in by wind and water. The proportions of the mineral and organic components and how they are structured vary considerably (Kennedy 2002). Ainsdale National Nature Reserve, north of Liverpool, has large areas of pine planted on old sand dunes; apart from the surface litter the soil is almost entirely mineral. Conversely the Birch wood at Holme Fen in Cambridgeshire is growing on the drying peat left after the nearby Whittlesey Mere was drained in the middle of the 19th century, so it has an almost entirely organic soil. Drying, shrinkage and oxidation of the peat has lowered the soil surface by several metres over the last century and a half.

Below the soil surface, plants may have specialised roots and stems that spread out, budding-off new individuals that allow the original plant to occupy new ground. Other types of modified stem act as storage organs, allowing the plants to survive unfavourable periods, such as summer droughts or winter cold. In some species these below-ground structures can survive for several years without producing aerial leaves and shoots. Seeds can also get buried in the soil and provide a way for plants to re-colonise areas after periods when they may have been absent above ground.

Our geology is extremely varied, as shown in the wonderful map produced by William Smith (1769–1839) at the beginning of the 19th century with diagonal stripes of colour across the country for the different rock types. The modern version of this is the British Geological Society’s interactive website, which allows you to look at the geology in your area. In Great Britain there is a major split between the younger rocks, which generally produce more fertile soils of the south-east ‘lowlands’, and the ‘upland’ soils of the north-west, which are underlain by older, harder rocks. There are however many exceptions. Sherwood Forest in the lowland grows on acidic sands; Marle Hall Woods near Llandudno and Rassal Ashwood in Wester Ross are in the uplands of Great Britain but are on limestone, giving base-rich, fertile soils.

The effect of the underlying geology on the soil may be masked by deposits of surface drift material. During the Ice Ages as the glaciers moved down valleys they scraped off and carried with them pre-existing soil, stones and sometimes huge boulders. In the process stones might be ground down, first into fine gravel and then to dust. When the ice melted, this material was left behind. In the post-glacial period rock-dust and sand was blown around and some eventually accumulated in small hollows.

The characteristics of the mineral component of the soil are strongly determined by the size of the particles present: from sands (the largest, particles greater than 0.06mm in diameter) through silts (0.06–0.002mm) to clays (<0.002mm). If you take a piece of soil and rub it between your fingers, then a soil with a high sand content feels gritty. If the soil is moistened it does not stick together well and you cannot easily mould it into a ball between your hands. If it does stick together and you can roll the ball out into a thin worm and bend the worm into a circle without breaking it, there is a lot of clay in the soil. If the soil does stick together, but will not form a worm, and feels soapy or slippery, the dominant particles are probably silts. Clay and silt soils are often described as ‘heavy’, those with lots of sand as ‘light’. These terms derive from the effort generally needed to plough them in the past.

The large size of sand particles is usually matched by equally large gaps between them, meaning that water tends to drain through them quickly. There is usually plenty of oxygen in these big spaces, allowing the plant roots to go down deep. The smaller the particles, the tighter they are likely to pack down, leaving smaller spaces between them. Silt and clay soils tend to hold more water and there may be less oxygen in the soil spaces. This in turn may mean that the plant roots cannot go down very far.

Most of the nutrients needed for woodland plant growth (potassium, phosphoros, calcium and magnesium plus smaller quantities of other elements) come from soil minerals. Soils derived from different rocks vary in the amount of these elements that is available for plants to take up. As a broad generalisation, sandy soils tend to have fewer available nutrients. They are therefore less fertile than clays or silts.

The other major element plants need for growth is nitrogen. This ultimately comes from the atmosphere but has to be converted first into compounds that can be absorbed by the plant roots. Nitrogen fixation is carried out by bacteria, which may be free-living in the soil or in nodules on the roots of plants of the pea family, such as the woodland vetches, and on Alder roots. Subsequently, the nitrogen compounds may be released into the soil, for example when the roots die, where they may be taken up by other species. Nitrogen oxides and ammonia may also be released into the atmosphere, but then be washed back into the soil. Additional wet and dry deposition of nitrogen compounds comes from human sources, such as farming or the exhaust gases of cars.

The elements that become available as the rocks weather also affect how acidic or alkaline the soil is. If there is a high concentration of calcium, for example from the breakdown of chalk and limestone, then the soil will be alkaline, with a pH (the scale used to measure soil acidity/alkalinity) higher than 7. Other rocks, such as slates, tend to give rise to acid soils, with a pH less than 7, perhaps going as low as 3 or 4. In very acid soils aluminium ions, which are toxic to plants, start to be released into the soil water.

The ground flora composition is affected by the available water in the soil, the nutrients that their roots can take up, and the soil acidity. Heinz Ellenberg, as well as assigning scores for shade tolerance (discussed in the previous chapter), classified species according to their ability to cope with different degrees of soil wetness, soil nutrients and soil acidity. The scores indicate where a species is most likely to be found, in the presence of competition from other plants – most will grow best in fertile, well-watered soils if growing on their own.

There are few woodland species associated with either very low or very high soil moisture scores compared to the rest of the British flora: in very dry conditions woodland tends to give way to scrub and savanna and there are not many floating woods! However, Royal Fern Osmunda regalis, Common Marsh Bedstraw and Lesser Spearwort Ranunculus flammula are placed at 9 (needing wet conditions) on the moisture score, contrasting with Wood Sage, Harebell, or Old-man’s Beard (score 4), which grow better in free-draining soils.

There are also relatively few woodland species with very low Ellenberg nutrient scores compared to those in non-woodland habitats. Species can withstand low-fertility levels in high light conditions but not many are able to cope with the double stress of both low light and low fertility. Species tolerating low-nitrogen conditions are usually also associated with acid soils, for example Purple Moor-grass and Tormentil (score 2 on the nutrient score), while at the fertile end of the nutrient spectrum (score 8) are Cleavers, Hedge Woundwort Stachys sylvatica and Hedge Garlic.

Water and nutrient availability are also affected by the nature and distribution of the organic component of the soil. In a wood, organic material is most obviously added to the soil surface in the autumn in broadleaved woods, through falling tree leaves, or dying Bracken fronds. There are other, less noticeable, contributions through the rest of the year: the bud scales as the tree leaves emerge in spring, the dieback of the Bluebell leaves in early summer, the petals from the Bramble flowers. Sometimes you can hear it happening: the gentle pitter-patter of caterpillar frass (their faeces) falling from the canopy in summer. There are occasional more dramatic additions as when a deer dies, or a tree falls. Organic matter is also being added below ground as plant roots die and decay.

In the ground, bacteria, fungi and invertebrates, from earthworms to tiny springtails, break down the material by physical and chemical means. The relative importance of different species groups in the decomposition process can be observed by placing a fixed amount of litter into mesh bags, where the size of the mesh allows access to small organisms but excludes larger ones such as worms. Some of the carbon and nutrients is incorporated in the cells of the decomposer organisms themselves, some is incorporated into the soil, where it may remain largely inert for periods from a few years to centuries or more.

Elm, Alder, and particularly Ash, leaves tend to break down rapidly; most have gone by the following summer. Other tree leaves such as those of Sycamore and Lime may take two years to disappear; Oak and Beech three years, and some of the conifers even longer (Ellenberg 1988). The woodland ground flora produces a smaller mass of leaves and stems each year, but this is generally broken down more easily than the tree leaves, so the nutrients in them are recycled more quickly. Where there is Bracken, though, and, to a lesser extent, grasses growing in open glades, the dead leaves and fronds can build up as a thick mat. Bramble thickets also accumulate large amounts of standing dead stems that remain separate from the soil for some months because they are held up by the subsequent year’s living stems. Such stems may make up a third of the mass of dense thickets.

As leaves, stems and twigs decay, some of the carbon is added to the upper soil layers. Provided the soils are not too acid, earthworms will drag leaves and other organic material down into the soil so that the carbon is quite deeply distributed through the soil profile, leaving perhaps only a thin layer of surface litter. These are called mull humus soils, and the richest vascular plant communities tend to be associated with such soils.

In acid soils, there are fewer deep-burrowing earthworms to mix the surface layers with the mineral soil below. Litter builds up at the soil surface and fungi play a bigger role in breaking down the material. There can develop a sharp line between an upper organic zone and the mineral soil beneath – this is known as mor humus type. Iron compounds are leached out of the upper mineral layers by organic acids formed by the breakdown of the litter, leaving a greyish-white sandy layer. Sometimes this iron is re-deposited lower down as an impervious iron-pan that limits the rate at which water can drain away.

In British forests the action of earthworms in incorporating litter to the mineral soil is seen as positive, to be encouraged in both gardening and farming generally. It was a surprise to me, on a trip to the north-eastern United States, to discover that, there, deep-burrowing earthworms are invasive species: fishermen use worms as live bait and may discard any unused ones at the end of the day (Bohlen et al. 2004). These abandoned worms are changing the nature of some American forest soils. In the natural absence of deep-burrowing worms many American hardwood forests had developed deep surface organic layers. The invasive worms change the decomposition processes, eating their way through these layers and dragging some remains down into their burrows. The greater mixing of organic and mineral layers facilitated by the worms can lead to less carbon being stored in the soil, changes to the patterns of nutrient cycling, and also in the diversity of both the aboveground vegetation and the buried seed flora. Rich herb communities can be reduced down to just one or two species. Some of the smaller trees are left on little pedestals of twisted roots as the worms eat the forest floor out from underneath them. Earthworms also change the fungal communities that thread their way through the forest floor, decomposing the litter (Dempsey et al. 2013).

Some fungal threads (hyphae) invade the roots of plants to form what are called mycorrhizae – plant–fungus interactions. The host plant (usually) acts as a source of carbon compounds, derived from photosynthesis, for the fungus, but the host benefits from increased availability of water and nutrients such as phosphate absorbed initially by the fungal hyphae system. It has long been recognised that most trees have mycorrhizal roots; on species such as Oak, for example, the fungus forms a distinct sheath around the fine roots, from which the hyphae then spread out through the forest floor, connecting with other trees. However, more recent research has shown that under natural conditions most woodland ground flora plants also have mycorrhizae (Harley & Harley 1987). The links are not as obvious as with the fungal sheaths on many tree roots, because the fungus grows into the roots and forms structures within the host-plant cells themselves.

The movements between the components of the system are more complex than just the transfer of sugars from the plant to fungus, and water and mineral nutrients from the fungus back to the plant. Some of the sugars moving into the fungus from one plant may end up in other plants connected via the fungal hyphae, meaning that there is a transfer of energy from plant to plant, possibly from one species to another (Wohlleben 2016). The dynamics of the relationship between the fungus and the host-plant can change over time. If the host-plant is weakened the fungus may end up killing the host, but the so-called host can be the freeloader as well. The early growth of most orchids relies on carbon from the fungus as well as mineral nutrients, because the dust-like seeds of orchids contain too few reserves to grow a stem and leaves on their own. For several years the orchid is in effect a parasite on the fungus. The fungus in turn draws on carbon from its connections to adjacent trees, so a three-way relationship is formed.

If the fungal partner is adversely affected by a change in environmental conditions, this could have knock-on effects for the host-plant’s growth. Fungi are more sensitive than most vascular plants to increases in heavy metals and nitrogen deposition to British woodland soils from air pollution, and so some of the pollution impacts that we see may be indirect responses to the disruption of the below-ground relationships.

I dug up some Bramble plants in Oxfordshire as part of my doctoral research, tracing their roots for about a metre, but in Australia, where introduced brambles are a major weed, their roots have been followed down for over four metres. Developing a deep-root system might initially seem like a good strategy for a plant as it opens up a larger volume of soil for extraction of water and nutrients. However, deep roots are at risk from waterlogging and they may be costly to produce in terms of the plant’s photosynthesis output. Other woodland plants (Wood Anemone, Wood-sorrel, Germander Speedwell Veronica chamaedrys, Hairy Violet Viola hirta) have only relatively shallow roots but this makes them vulnerable to summer droughts.

Even trees have much of their feeding root-system in the upper part of the soil because that is where most of the available nutrients tend to be concentrated. This is often very apparent when trees blow over: the uplifted root plate is seldom more than a metre deep. This means there is a lot of below-ground competition in the humus and upper mineral layers. If this competition from the tree roots is reduced, for example by digging a trench around a plot, severing the tree roots from their parent stem, there is usually an increase in the ground flora growth within the trenched area.

Roots provide the water and nutrients from the soil needed for plant growth, which is usually fastest in the spring and early summer. At this time, there is a surge in nutrients in the soil as rising temperatures increase the microbial activity. Deciduous trees have not developed their full canopy and therefore are not so active, at least in the early spring. There can be a second, smaller, peak in autumn as the trees lose their leaves. The ground flora may play an important role at these times in reducing the amount of soluble nutrients such as nitrates that would otherwise be washed out of the soils. Capturing nutrients that might otherwise be lost is also an important role for the ground flora after the trees have been felled and so no longer have active root systems. Foresters may therefore create strips of dense ground vegetation along streamsides to help reduce run-off from felled areas.

Roots are not the only part of a plant that may be hidden in the soil: there are seeds as well. If you spread out soil from a woodland as a thin layer on a tray with a good supply of light and water, the seeds it contains start to grow. Stir it around from time to time to expose any seeds that may be at the bottom and more plants will appear.

Usually what comes up is a very different mix to the species that are growing above ground (Brown & Warr 1992). In one study less than 40% of the seedlings grown from the soil under neglected coppices in East Anglia were of species found in the surface vegetation. Many species typical of the shaded conditions in woods, such as Wild Garlic, Wood Anemone, Enchanter’s-nightshade, Bluebell, Sanicle and Dog’s Mercury, are rarely found in the buried seed bank. Either they do not produce much seed because their flowering is suppressed under shade, or else their seed does not survive for long in the soil. There may be small amounts of some specialist species such as Wood-sedge, Hairy Wood-rush Luzula pilosa and Yellow Pimpernel Lysimachia nemorum, but the buried flora largely consists of the more light-demanding non-woodland species and woodland generalists such as Soft Rush Juncus effusus and Bramble.

Some of the buried seed may germinate each year, but under shade the seedlings rapidly die. Only when conditions are more favourable do they survive long enough to grow and be noticed. Disturbance of the soil surface during coppicing brings other buried seed to the surface where increased warmth, light or a change in the ratio of red/far-red light wavelengths may stimulate them to sprout. Equally, new seeds are added to the seed bank each year and gradually buried as leaf litter builds up on top of them. Some seeds may be dragged down along with leaves by earthworms; small seeds may get washed down to deeper layers in the tunnels created by the worms.

Once buried, the clock starts ticking; the seed numbers start to decline as they rot or get eaten. The longer the time before the soil is disturbed again, the fewer the seeds that are likely to still be alive, to germinate and appear above ground. For much of the buried flora the critical period seems to be about 40–50 years (Brown & Warr 1992). When most ancient woods were managed as coppice the rotation lengths were almost always less than 30 years, which allowed for a strong showing from the species in the soil seed bank after each cut.

Some rare species such as Tintern Spurge Euphorbia serrulata may rely on buried seed to tide them over difficult shady periods. Tintern Spurge had been recorded from a South Wales wood, Coed Wen, but declined there in the post-Second World War period. When coppicing was resumed at the site in the 1980s, the plant reappeared in abundance (Marren 1992). Its seeds had been buried, lying dormant, until the soil disturbance and increased light brought them to the surface and encouraged their growth. Its more common relative, Wood Spurge, behaves similarly.

If the woodland ground flora has sometimes been overlooked in woodland conservation thinking, this applies even more strongly to the woodland soil. Yet it literally underlies many aspects of current conservation concern. Our ancestors were aware of, and exploited, different soil types. The free-draining chalk and sandy lands of southern England were extensively used by Mesolithic, Neolithic and Bronze Age peoples (chapter 12). Many such areas were cleared of trees early in prehistory; some heavily-used regions on the Wiltshire chalk around Stonehenge may never have developed much of a tree cover at all (Allen 2017). The wetter clay soils presented a more difficult challenge to our ancestors and these were often where woodland survived longest, or which recolonised first, whenever human pressure lifted. This effect is still partly reflected in the current distribution of ancient woodland, which has generally survived better on soils that were difficult to farm or gave only poor yields.

Good farming areas such as the Lleyn Peninsula, much of Essex, and Aberdeenshire tend to have few ancient woods compared to the steep Maentwrog Valley in Gwynedd, the stiff clay of the Weald or high cold ground of upper Deeside (Roberts et al. 1992, Spencer & Kirby 1992). At a local level around Wytham Hill, the ancient woodland survives in a band around the lower slopes of the hill on the hard clay soils. By contrast the fertile alluvial soils of the floodplain were used for grazing or hay-making. The upper slopes on free-draining sand and limestone provided common grazing until the 19th century; they had been cleared by the medieval period and have traces of earlier occupation.

Changes in soil nutrients occur all the time. In some places more nutrients are entering the soil than leaving it (through, for example, increased nitrogen compound deposition from car exhausts) and the soil is said to be becoming eutrophic. There are then likely to be changes in the flora with an increase in tall, competitive generalists such as nettles and brambles (see also chapter 16).

A splurge of Spurge

Wood Spurge Euphorbia amygdaloides has a low-density but persistent seed bank that results in a fine show in recently-cut coppice or where ride edges are cut back (Buckley et al. 1997). It is associated with ancient woodland, but in a study in Picardy the species was found more often in ancient woods that had been occupied in the Gallo-Roman period than forests which showed no such history (Plue et al. 2008). This may indicate that it does depend on some regular disturbance of the soil, such as comes with felling or coppicing, to persist over the very long term.

Once established, it can persist for some time, its thick stock producing tufts of stems. These bear only leaves in the first year, but in their second year are topped by a cluster of yellowish-green flowers. Like others of its family it produces a milky latex when cut and this may make it unpalatable to grazing animals, so it can be widespread in some wood-pastures such as the New Forest.

Wood Spurge is largely restricted to woods and hedges south of a line from the Wash to the Wirral, on dry, slightly acid to neutral soils. In Europe generally, it appears to be limited towards the north and east by winter temperatures and frost damage. This could mean it will benefit from warmer winters under climate change scenarios. Predicting the future distribution of Wood Spurge in Great Britain is however complicated because a vigorous sub-species that has been cultivated in gardens for a century or so has started to appear in the wild. Rather, as with the Welsh Poppy, we may see more occurrences but from a different gene pool.

Wood Spurge.

A mass of Wood Spurge in the New Forest.

There have also been times when more nutrients were removed each year than were added through the weathering of rock, and in this case the soils could become impoverished. High nutrient loss might occur where twigs and Bramble stems were collected and bound into faggots for firing ovens off-site, or where Bracken and leaf litter were gathered for use as animal bedding. (In the 1980s I saw pine needle litter being collected for this purpose from forests on the Canary Island of Tenerife). Oliver Rackham (2003) suggested that this nutrient depletion might have slowed the regrowth of the coppice over time, leading to longer rotations becoming necessary. Reduced soil fertility would tend to favour the more specialist woodland flora over competitive generalists: woodland management, through its effects on the soil, may therefore have biased the composition of the woodland flora that we have inherited (chapter 9).

It is, however, the variations in soil composition and structure across the country that have the more obvious effects on which species grow in any particular wood, with dramatic differences sometimes visible over the space of a few metres. At the north end of the Witherslack Woods in Cumbria, for example, a footpath by the roadside starts on acid soil. Here there are tall Oak and Birch, over Wavy Hair-grass, Bilberry, Heath Bedstraw, Wood-sorrel and Bracken. The ground is uneven and, in the hollows, wet Alder woodland has formed with Meadowsweet, Water Mint Mentha aquatica, a variety of sedges and rushes, and small herbs such as Yellow Pimpernel and Common Marsh Bedstraw. At the base of the steep limestone slope there is a sharp change to Ash-Hazel woodland with scattered Small-leaved Lime Tilia cordata and Elm, over Dog’s Mercury, False Brome, Lords-and-Ladies, Barren Strawberry, plus rarer species such as Wood Fescue Festuca altissima and Mezereon. Similar types of pattern can be found in lowland oakwoods, where Bracken and Bramble on free-draining slopes give way to dense sedge stands in wet valley bottoms. Finding a way to describe such variations in a consistent way was one of the reasons for developing the National Vegetation Classification, which is described in the next chapters 6 and 7.