Woodland Flowers

In the 1960s Bob Dylan sang about ‘changing times’, and this certainly applies to our woods. Previous chapters have considered how differences in woodland management and the increases in deer pressure are affecting the ground flora. This chapter looks at another set of changes, the effects of pollutants in the form of sulphur and nitrogen compounds added to the atmosphere, the fertilisers that may be spreading into our woods from adjacent fields, and the potentially all-pervading effects of climate change.

Trees are better than shorter growing vegetation such as Heather or grasses at capturing gases and particles in the atmosphere – a process known as ‘scavenging’. This is beneficial when trees along roads in towns scavenge the pollutants coming from cars and lorries and improve the air quality in nearby houses and schools. Shelter belts around intensive livestock units on farms help capture some of the ammonium compounds that are produced by the units, stopping them spreading any further.

However, pollutants from the atmosphere collected on the foliage may wash down the trunks and into the soil. There they contribute to the release of hydrogen ions, which acidify the soil and streams or lakes into which they are subsequently washed (Ormerod et al. 1989). In very acid conditions aluminium ions are released, which can be toxic to plants. On the Continent, researchers compared records of plants growing in the woods from the 1920s and 1940s with 1980s surveys from the same areas. They showed that there were links between changes in soil acidity and which species had increased or decreased over this period (Falkengren-Grerup 1995, Brunet et al. 1996). Studies in British woods suggested some similar trends (Ling 2003, Kirby et al. 2005).

Since the 1980s, atmospheric sulphur emissions have been reduced and woodland soils in Great Britain are generally becoming less acid. There are now regulations to control pollution, such as The National Emissions Ceiling Directive, which sets commitments for EU member states, including for nitrogen and sulphur compounds. (These commitments are likely to remain now that Britain has left the European Union).

Concentrations of nitrogen oxides in the atmosphere have declined, but there has been little change in the emissions of ammonium compounds (RoTAP 2012). The net result is that the total deposition of nitrogen has changed little since the 1980s. Over much of Great Britain, the levels are still above those at which significant adverse effects on pollution-sensitive species may occur (Matejko et al. 2009). An eminent scientist at the Centre for Ecology and Hydrology once calculated that an annual deposition of nitrogen about 20kg/ha/yr was the equivalent of that found in 2,000 standard-sized cowpats! The excess nitrogen deposition from past emissions has led to changes to the diversity and composition of open habitats such as grassland, heaths, moors and dunes (Field et al. 2014), but the evidence for changes in the woodland flora has been less clear-cut until recently.

In experiments in greenhouses and gardens, woodland plants grow better with more nutrients, which may help them survive better under dense shade. With extra nitrogen the plants can produce larger leaves and capture more sunlight. In the field, however, a greater availability of nutrients can be a disadvantage for the woodland flora. The plants may be lusher and more attractive to herbivores. Additional nutrients may favour tall, more competitive species that outgrow smaller, stress-tolerant woodland specialists. Extra nutrients may change the nature of the below-ground relationships between fungi and plant roots.

Continental researchers found a link between the nitrogen oxides and ammonium compounds taken up by the trees, greater growth and increased carbon storage in the forests – a good thing in relation to climate change. However, as the levels of nitrogen compounds scavenged by the trees increased, so more of the nitrogen accumulated in the soils and, at the highest levels, started to be washed into streams (de Vries et al. 2009, Dise & Wright 1995). In Swedish Oak forests the extra nitrogen in regions with high pollution encouraged species such as Broad Buckler-fern, Rosebay Willowherb, Raspberry, Nettle and Wavy Hair-grass (Brunet et al. 1998).

In 2008, Sally Keith from Bournemouth University revisited woods in Dorset that had first been recorded by Professor Ronald Good, a local botanist, in the 1930s when nitrogen pollution levels were lower. The mean number of species in each wood was much the same, but there were fewer differences between sites. Species typical of fertile soils were more common in 2008 than in the 1930s (Keith et al. 2009). Katy Ling at the University of the West of England compared records from the 1960s for beechwoods in the Cotswolds with what she found in the 1990s, and again nitrogen-loving species tended to have increased, whereas stress-tolerating species such as Sanicle decreased (Ling 2003). The Countryside Survey 2007, which covers the whole of Great Britain, found an increase in competitive species, which are favoured by high nitrogen, in woodland compared to the 1990s survey results (Norton et al. 2012).

Other studies looking at change across Europe over the last 20–40 years have not shown such clear evidence of a nitrogen impact (Verheyen et al. 2012). In some of these studies the baseline for the comparison was after 1970, so the main changes in plant species caused by increased nitrogen might already have happened. Elsewhere the nitrogen levels may still be building up in the soil, but the effects on the vegetation have not yet come through because the plant growth is still more limited by light than by nutrients. This raises the risk that the nitrogen time-bomb may be triggered when woods are opened up by coppicing or thinning, which allows more light to reach the ground.

In addition to the general effects of atmospheric pollution, woodland edges are particularly affected by nitrogen coming off farmland. This can lead to increases in ‘weedy’ species at woodland edges, sometimes well into the woodland interior. For example, there is a dairy farm on one side of Wytham Woods, and if the wind is in the right direction I get a strong whiff of slurry and dung. Hedge Garlic and Cow Parsley are now common where the wood-edge abuts the farmland.

Elsewhere in Great Britain the impacts of ammonia compounds on woodland downwind of more intensive livestock units can be very marked, leading to fewer species overall as well as increases to some high-nutrient species. In one study, Carole Pitcairn and co-workers found more Yorkshire-fog, Raspberry and nettles close to livestock units (Pitcairn et al. 2002). Other species that increase where nitrogen levels are raised include Rosebay and other willowherbs, Lady and Broad Buckler-fern, Three-nerved Sandwort and Common Hemp-nettle. Pesticides may also drift off arable crops and into the woodland, leading to reductions in the abundance of woodland specialists such as Primrose, Dog’s Mercury and Wood Dog-violet in the outer ten metres of the wood (Gove et al. 2007).

Hedge Garlic, for better or worse?

Hedge Garlic Alliaria petiolata is common in hedges and wood-edges on moist, relatively fertile soils, where there is some disturbance. People who collect wild plants for food value its leaves, which do taste of garlic when crushed. In America it has been reported that the leaves can have a higher concentration of vitamin C than oranges, and higher levels of vitamin A than spinach (Cavers et al. 1979).

Hedge Garlic is widespread through Great Britain and much of the temperate zone in Europe. It grows from an overwintering rosette of leaves up to about a metre high the following summer, with small white flowers (Grime et al. 2007). In Great Britain, Green-veined White butterfly caterpillars feed on the leaves of Hedge Garlic, and it is also one of the food plants for the Orange-tip butterfly, which feeds mainly on the flowers and developing seed-pods. Another food plant for the Orange-tip is Lady’s Smock. In the early 1990s a study of the butterfly at Monks Wood in Cambridgeshire commented that Lady’s Smock was then the only food plant present in the wood. By 2016 Hedge Garlic was also common towards the edges of the wood as at Wytham, presumably because of increased nitrogen coming off the adjacent fields.

Hedge Garlic is one of several woodland plants (Purple-loosestrife and False Brome are others) that have become invasive in woodland in parts of the mid-western and north-eastern United States and Canada, displacing the indigenous ground flora. Once established, Hedge Garlic becomes a permanent part of the community, slowly increasing and taking advantage of any disturbance that happens. When railing against introductions to Great Britain such as Himalayan Balsam or Japanese Knotweed it is easy to forget that our woodland plants can be just as troublesome elsewhere.

Hedge Garlic and nettles are species that benefit from nutrient enrichment at wood-edges.

Over long timescales the climate has changed dramatically: Ice Ages and warmer interglacial periods have come and gone. In this interglacial there have been shorter-term fluctuations in some regions. There was a warm period in early medieval times in Europe followed by the Little Ice Age that lasted from the 16th to mid-19th century. Since then, mean temperatures have been rising and have gone above that of the medieval warm period. Records for the warmest days, months and years ever recorded are regularly being broken (Beebee 2018). Human-generated emissions of greenhouse gases are the main cause of this current rising trend in temperatures, and most projections suggest that temperatures will continue to rise for much of this century.

All areas of the UK are likely to get warmer, more so in summer than in winter, with the greatest increase in summer mean temperatures in southern England. This should lead to longer growing seasons, but also to an increased risk of heatwaves, so the woodland flora may suffer from more frequent droughts. Oxlips, weakened by deer grazing, died following the hot dry summers of 1976, 1990 and 1995 in Hayley Wood (Rackham 2003). Dog’s Mercury showed severe wilting by the end of the hot summers of 1995, 2003 and 2018 in Wytham Woods. The vegetation subsequently recovered, but this recovery may be less complete if droughts become more common (Morecroft & Taylor 2010).

Total annual rainfall may not change very much, but average winter rainfall is likely to increase, particularly in western Great Britain, while summer rainfall is likely to decrease. Rainfall may be more intense when storms do occur, increasing the risks of severe flooding.

In 1991 Plantlife published a booklet called Death knell for the Bluebells, a first attempt to suggest what the implications of climate change might be for this most-loved flower. With hindsight the analysis was perhaps somewhat naïve, and a more recent report noted that the fate of the native Bluebell in a warmer climate remains uncertain (Plantlife 2004). However, it set the scene for further studies ranging from analyses of which species are potentially at most risk, to ideas on adapting the management of habitats and landscapes to cope with the expected future conditions (Natural England & RSPB 2014, Morecroft & Speakman 2015, Pearce-Higgins et al. 2017).

These studies suggest that by the end of the 21st century, and earlier in some cases, temperatures and rainfall levels in some areas of Great Britain may no longer be suitable for certain species currently growing there. Other places that are currently not climatically suitable may become so. Recent decades have seen marked northerly expansions in the ranges of some butterflies and birds, for example the Speckled Wood butterfly and Nuthatch (Beebee 2018). Conversely some northern mountain butterflies are showing a decline.

There is so far little evidence that woodland plant distributions are changing. However, this may be because the microclimate changes within woodland have not been as great as outside the woods. Many woodland plants are also long-lived, with stored reserves that allow them to cope with several unfavourable years, and those reserves have not yet been exhausted (Carey 2015). Change will also be slow for the many woodland plants that only disperse over a short distance in any one year, meaning that it may take decades for distribution changes to be detected. We may however see signs of change happening by comparing what we find in woods today with what was recorded in the same spots 30, 40 and 50 years ago (De Frenne et al. 2013, Kirby et al. 2005).

As individual species grow faster or slower, spread to new sites or die out from old localities, the composition of the ground flora communities will gradually change. Over the longer term, we expect that some plants currently limited to southern Great Britain might spread further north, while more plants from southern climates that are currently grown in gardens may escape into nearby woodland. Scattered plants of Honey Garlic Nectaroscordum siculum, a native of southern Europe with showy clusters of bell-shaped blossoms, have started to appear in Wytham Woods several hundred metres away from where it had been planted in a garden.

While plants may be slow to move to new areas to escape climate change, they can adapt to new conditions through varying the time of year when they flower, fruit, or lose their leaves. Tim Sparks showed in 1994 that Oak trees were coming into leaf significantly earlier than in the past, helping to make phenology – the study of the timing of natural events – respectable science again. Nature’s Calendar, a recording scheme managed by the Woodland Trust, allows anyone to record the times of first flowering of Bluebell, Lesser Celandine, Lady’s Smock, Hedge Garlic and Ivy, or the first sightings of ripe blackberries. You can follow the records from the south-west peninsula up to north Scotland.

Another champion of phenology was the naturalist Richard Fitter. He and his son Alastair, Professor in the Department of Biology at the University of York, showed that the average first flowering dates for 385 British species since 1990 were 4.5 days earlier than in the previous four decades (Fitter & Fitter 2002, Fitter et al. 1995). There were differences between species: White Dead-nettle used to flower in winter only occasionally, but now does so regularly; the flowering of Lady’s Smock advanced by over a week, but Wood Spurge hardly advanced its flowering at all. Some plants only flower after they have experienced cold periods in winter; warmer autumns could interfere with this process, which means early flowering species, such as Green Hellebore Helleborus viride and Moschatel, may flower later under a warming climate. The order of spring events might thus change (Roberts et al. 2015).

Warmer spring temperatures mean earlier starts to leaf growth. Early emergence of leaves – provided they are not blighted by frosts – should give plants a longer growing season, enabling the plants to produce more seed or bigger bulbs. Species such as Wood Anemone, Lady’s Smock and Cock’s-foot seem able to track these temperature patterns well (Tansey et al. 2017). The flowers that bloom in the spring, however, might not enjoy a longer season. Oak, and other trees and shrubs, have also started to come into leaf earlier. If the advance of tree canopy green-up is more than the advance in growth of the spring flowers, then the period of high light availability, and therefore high growth, for the ground flora plants will be reduced by climate warming.

In future the flora may change faster, as the tree and shrub layers respond to climatic variations. Extreme droughts are a threat to shallow rooting trees (Peterken and Mountford 1996, Cavin et al. 2013); many Beech in Lady Park Wood on the Gloucestershire/Monmouthshire border, and in the New Forest, died after the 1976 drought. More frequent storms and the spread of new pests and diseases may create more canopy gaps. Alternatively, dense-canopied trees such as Sweet Chestnut and Small-leaved Lime with more of a southerly distribution in Europe may do better under a warmer climate and replace Oak or Ash, creating more shade and cooler conditions at ground level.

Partly because of these uncertainties, woodland has generally been classed as at medium to low risk from climate change from a conservation perspective (Natural England & RSPB 2014). Climate change effects on woodland plants are currently small compared to other factors such as deer browsing or emerging tree diseases. However, while the impacts on the woodland flora may be delayed, they cannot be avoided altogether: what then for the banks of sweet primroses?

Models have been developed to suggest how species will cope with future climates, based on the temperature and rainfall patterns where a species currently occurs across Europe. Projections of climate change across Great Britain are then used to see where suitable conditions might occur in future (Pearce-Higgins et al. 2015). Woodland generalists are less likely to be at risk than woodland specialists; northern species are more at risk than those with a southerly distribution. The results are not precise predictions of where Primrose, Bluebell or Twinflower will grow in future, but help us to think about what future changes to look for. The models may also help in identifying ways we might offset some of the more undesirable changes.

The outputs from the models raise challenges for conservation policy and practice. If there are suitable conditions beyond the current range of a species and the chances of natural colonisation are small, should we assist their migration? Which species currently in woods on the near Continent, not native to Great Britain but grown in gardens, might be accepted as part of our woodland flora under future climatic conditions?

It is to be hoped that further pollution controls will eventually lead to falls in the overall emissions of nitrogen compounds. At the wood level it may be possible to offset some of the potential effects of increased nitrogen entering the system by avoiding opening up the tree canopy through large fellings. This may allow the more shade-tolerant woodland flora species to maintain themselves alongside more light-demanding, competitive species, that are favoured by high nitrogen levels. Developing grassy strips between woods and arable fields can help to reduce the spread of nitrogen from agricultural fertilisers into the wood and also allows for a more

A future native for our woods?

Liverleaf Hepatica nobilis, gets its name from the shape of the three-lobed leaf which was thought to resemble a liver, and is one of the plants I look out for on visits to Continental woods. It is a member of the buttercup family but with (usually) blue flowers that grow from a basal rosette of leaves, and is found in a wide range of conditions from deeply shaded beechwoods to more open grassy places, often associated with limestone but sometimes on sandy and clay-rich soils. In a Swedish study, individual plants of Liverleaf and Sanicle were followed over the course of several decades with little mortality, suggesting that these herbs may live as long as some of the trees and shrubs above them (Inghe & Tamm 1985).

The seeds are ant-dispersed and the plant is often associated with ancient woodland on the Continent. Another Swedish study looked at the colonisation of new woodland next to an ancient wood and found a pattern not dissimilar to that shown by Rackham (2003) for Oxlip at Hayley Wood: Liverleaf declined in abundance with increasing distance from the ancient woodland boundary, with scattered outliers well ahead of the main invasion front (Brunet & Von Oheimb 1998).

Liverleaf is rather intolerant of frost, which may explain why it did not establish in Great Britain or western France naturally. However, it is now grown in gardens and has naturalised in scattered places across Great Britain. It is an attractive little plant that seems unlikely to prove too competitive for other members of our woodland flora.

Liverleaf Hepatica nobilis – a possible future addition to our flora?

scrubby, flowery wood-edge vegetation to develop. This will buffer the woodland edge, keeping it cooler and more humid.

Our projections of the real impact of long-term climate change on the woodland flora remain somewhat sketchy, and ideas as to what we might do about them are necessarily generalised. Woodland management recommendations for conservation might need to be altered. Reducing the size of gaps created by felling, as well as possibly helping with the nitrogen problem mentioned above, should also help maintain a cooler microclimate for the ground flora. We may need to give higher conservation priority to woods where the topography allows potentially vulnerable species to survive for longer, for example on north-facing slopes or areas close to the water table (Suggitt et al. 2018).

We must reduce other pressures on our woodland flora, because individuals and populations that are growing well are more likely to be able to cope with whatever the future climate holds. Climate change then becomes another reason for managing deer, for example, because there is little hope for plants to spread if the deer are eating the flowers and so stopping seed production. Maintaining and restoring mixed landscapes with trees and hedges between woods may increase the potential for species to spread out through the landscape.

Almost inevitably, there will be unexpected side effects for the woodland flora, with climate change altering the range or severity of pest and disease impacts on our forest trees. The way that land is managed, and the balance between farm and forest, may also alter. Some of these effects could be positive. A substantial increase in woodland cover could form part of the nation’s climate change adaptation and mitigation strategies (Read et al. 2009), meaning more potential habitat for woodland plants, provided the plants can get to this new woodland.

Above all, it means agreeing what sorts of woodland we want in future and what we want it to provide, not just from the point of view of wood production or wildlife conservation, but also for the enjoyment that people get from visiting woods (chapter 17). This is now seen as an increasingly valuable service that woodland provides.