Mosses and lichens are often overlooked as part of the woodland flora.  The same is true in towns and cities.  They can be found on the trunks of trees, on walls and rooftops.  They are epiphytes, that is,  plants which grow on the surface of another plant. They derive moisture and nutrients from the air, rain, and debris that is deposited on and around them. They cling to these surfaces, come rain or shine.  Climate change and pollution affects these small, often unnoticed plants in both woodland and urban settings.    As global temperatures continue to rise, many places face extremes of temperature.   Heat waves are becoming more common, as we have recently experienced.  Some 1500+ heat-associated deaths were recorded in England during the heat waves of  last summer [2025].    The highest rates were among elderly individuals [aged 85+] with the impacts felt particularly in care homes and hospitals.   London and the East of England recorded the highest number of heat-associated fatalities. Cities and urban environments often face the brunt of these climate extremes.  The concrete, brick and tarmac of cities absorb and hold onto the sun’s energy creating ‘urban heat islands’. Urban temperatures are often several degrees higher than surrounding rural areas. Urban temperatures have soared in recent times, for example in New Delhi, a record high of 126.1oF (52.3oC) was recorded. Such temperatures claim lives. This year parts of Europe have recorded temperatures of 40oC. Urban trees suffer in this heat as do their epiphytic flora.  The mosses and lichens in our cities fall into this category.  Researchers at Leiden University have investigated the effect of changing temperatures on the mosses and lichens growing on Lime (Linden) trees in Dutch cities (Amsterdam, Leiden, and Rotterdam).  They sampled numerous sites, measuring the microclimate for each tree and recording the species present.  They found, perhaps unsurprisingly, that urban heat islands were reshaping the moss and lichen communities.  Species that were adapted to drier, brighter and nitrogen-rich environments were favoured; for example, a type of wreath lichen [Phaeophyscia orbicularis]. When wetted this turns from a grey / brown colour to green, and its symbiotic green algae can begin to photosynthesise producing sugars. Those species that were associated with moister and shady conditions were found to be ‘losing out’.  They noted that the larger lime trees supported more diverse epiphyte communities; this might be associated with the older bark offering more micro-habitats for the mosses and lichens to ‘gain a foothold’.   [caption id="attachment_41261" align="aligncenter" width="675"] Lichens on an old stone wall.[/caption] The changing climate and urban heat islands are altering the species that can survive in our towns and cities, on trees and artificial surfaces.

The base of a tree trunk is often covered with a ‘carpet’ of moss and lichens and the branches of the tree may bear ‘decorations’ of different mosses.  In shady areas, there may be a soft, spongey layer of moss underneath the tree.  Mosses are ‘simple’, non-vascular plants. They lack the sophisticated transporting tissues (phloem and xylem).  Nor do they have true roots, instead they have small structures called rhizoids, which help them attach to a surface. They ‘like’ moist places and are dependent on water for their reproduction.   The ancestors of mosses were probably some of the first plants to colonise land (previously there were only algae in the seas), possibly in the Ordovician Period. Mosses rarely grow to any great height, due to the absence of supporting mechanical tissue (lignified tissues, like xylem) but they can form extensive ‘mats’ in damp, shady places (as can liverworts).  Exposed to direct sunlight, they lose water rapidly.  Such mossy mats can, in some situations, help reduce soil erosion. The mats may also allow for the accumulation of humus and soil formation.  A particular example of the accumulation of mossy material is seen with the moss Sphagnum. Sphagnum grows in acidic, marshy conditions, often forming a bog. Low fertility and a cool climate result in slow growth of the Sphagnum (and other plants). The subsequent decay of dead plant material is even slower (due low oxygen levels). Hence, peat forms and accumulates.  Large areas of land can be covered to a depth of several metres with peat.  Bogs are a very effective means of locking up carbon for hundreds, if not thousands of years.   Sadly, many wetlands have been drained and allowed to dry out, then the peat cut from them as a form of fuel.  When peat areas are drained (channels are cut through the peat), they degrade and dry out. They are then at risk of  catching fire as has been seem in recent times:for example, the burning of  Slieve Beagh in Ireland and Langdale Moor in Yorkshire. The Interesting fact : Sphagnum has an interesting property in that can hold water  many times its own weight.  Because of this absorbency, Sphagnum was used as a wound dressing in WW1. https://earthobservatory.nasa.gov/images/150780/irelands-cutaway-peatlands https://www.bbc.co.uk/news/articles/cvg9n0j07epo .

As we move through Autumn,  the leaves of deciduous trees have ‘done their job’.  Their capacity for photosynthesis dwindles (as does the light and temperature.  In fact, the leaves become a ‘liability’ to a tree, in that they would make use of the reserves that have been stored away. Also, the leaves offer resistance to the winds of winter so a tree is likely to sustain damage. If branches are lost from the stem,  then bacteria and / or fungi could enter.  The leaves are ‘discarded’ as Autumn progresses.  The green colour of the chlorophyll is lost and other colours emerge - reds, oranges and yellows. These colours are associated with different pigments, the carotenes, xanthophylls and the anthocyanins.   As the leaves are lost so the trunk and branches becomes more apparent, but they are not necessarily ‘naked’ - devoid of cover.  Indeed, walking through a woodland at this time of year, one is often struck by the abundance of mosses. Tree trunks are often laden with ‘carpets’ of moss and their branches bear ‘decorations’ of many different mosses.  Beneath the tree, there is often a soft, spongey layer of mosses. Mosses are bryophytes.  They are ‘simple’, non-vascular plants ; that is to say they do not have sophisticated transporting tissues (of phloem and xylem).  Nor do they have true roots, instead they have structures called rhizoids (which may be uni or multicellular). They live in moist places, indeed they are dependent on water for their reproduction. Mosses or rather their ancestors were some of the first plants to colonise land (which was the rather inhospitable rock of the Ordovician period), previously there were only algae in the seas.  Mosses develop sex organs, The male organ is called is called an antheridium, it produces male gametes that can move by means of a flagellum.  These gametes swim in a film of water towards the female organ (an archegonium).  When the male and female gamete fuse, the structure grows to form a sporophyte.  This eventually produces a capsule, which releases spores that grow on to form a new generation.   Mosses seldom grow to any great height, mainly due to the absence of supporting mechanical tissue but they can form extensive mats in damp, shady places (as can liverworts).  These mossy mats can in some situations prevent soil erosion, and in others allow the accumulation of humus and soil enrichment / formation.  An extreme example of the accumulation of mossy material I can be seen in the case of the moss Sphagnum.  Sphagnum is unusual in that can hold many times its own weight in water; because of this absorbency, Sphagnum was used as a wound dressing in WW1. It grows in acidic, marshy conditions, often forming a sphagnum bog. The low fertility and cool climate result in slow growth of the Sphagnum (and other plants). The decay of dead plant material is even slower (due low oxygen levels). Hence, peat forms and accumulates.  Large areas of the land can be covered to a depth of several metres with peat.  Peat bogs are a very effective means of carbon sequestration, locking away carbon for hundreds iff not thousands of years.  Unfortunately, many have been drained and allowed to dry out  and / or the peat cut from them as a form of fuel.  When peat areas are drained (channels are cut through the peat), they degrade and dry out.  Then, they are then at risk of burning as has been seem in recent years -for example, in the U.K (Saddleworth Moor). [caption id="attachment_39795" align="aligncenter" width="675"] Moss (Dicranum?) growing with Lichen (Cladonia sp)[/caption] 'Mossy ball', 'autumn leaves ', moss and lichen photos - thanks to Art Symons.

Images of deer in a woodland clearing may seem charming, but quite how deer and woodlands interact is not fully understood. It is certainly the case that deer affect tree regeneration;  they browse on saplings and can cause extensive damage to bark (through territorial behaviour / antler rubbing) to larger trees.  The lower levels of the plant communities are also affected by deer.  By changing these, deer also can change the animals to be found in this lower layer of a woodland. Research by Dr. Simon Chollet and colleagues showed that deer interaction can influence soil water availability,  and soil fertility. Their study looked at the role of Sitka black-tailed deer on the plant communities in forests in parts of western Canada. Chollet et al. examined twenty areas where deer were excluded for some two decades. An exclosure is a limited area from which browsing animals, such as deer, are excluded by fencing or other means. Unsurprisingly, they found that deer did have a significant effect on the plant communities. The exclusion of deer resulted in an increase in vascular plant richness and cover, though bryophyte (mosses etc) cover declined. They also found that with the deer gone,  the exclosures started to develop broadly similar plant communities.  There was no increase in beta diversity - that is the development small, localised communities of varying character. It was suggested that the dominance of certain species within the exclosures was linked to historic over-browsing.   Consequently, those plants that are able to withstand the presence of deer over time are best placed to achieve ‘dominance’ once this grazing pressure is removed.  This might explain why the different enclosures developed similar vegetation and plant species.    Biotic homogenisation has been seen in the UK woodlands, as was discussed in a previous blog.  It can be the result of introduced species, pollution (eutrophication - increased nitrate and phosphate levels), which allows botanical thugs (like nettles) to dominate, or poor woodland management.

Yew woodland (in Southern England) tends to develop on the thin soils that form over chalk, often on the sides of dry valleys . They can be found along the North Downs, and in West Sussex on parts of the South Downs. Yew woodlands can also be found in the Cotswolds and in the Wye Valley (on limestone). In Ireland, there is a significant Yew wood near Killarney, again growing on top of limestone. Yew woodland may develop from chalk grassland, when grazing by sheep or rabbits is removed. With no grazing, Juniper and / or Hawthorn start to grow.  Then, between these, shade-tolerant Yew seedlings begin to grow.   Eventually, the Yew will overtake the Juniper / Hawthorn, and shade them out. The dead remains of juniper and hawthorn sometimes may be seen on the floor of a Yew woodland. Read more...

There are some rare but beautiful woodlands around the coast of Britain nick-named the "temperate rainforest".  Yesterday, I visited one of these woodlands in North Wales and was mesmerised by it - ancient oaks and birch which had little timber value but immense ecological value - trees covered in different mosses, a carpet of soft and varied bilberries and ferns.  The wood had a damp and humid feel that encouraged liverworts, lichen and fungus and there was no obvious trace of human intervention - it felt just right for dinosaurs! Apparently these woods have evolved as a result of the Gulf Stream that keeps the area warm but also wet: this creates woodlands quite unlike any others in Britain.  From the top of this woodland you can see the sea.  Temperate rainforest woods like this must once have covered the Atlantic fringe of Western Europe from North West Scotland all the way round to the South of Portugal - they are rich in biodiversity and some species here are not found anywhere else in the world.  Bats love these woods, especially the greater and lesser horseshoe bats which enjoy the rich food source of invertebrates that thrive here. So the challenge for the owner is how to protect and manage these rare woodlands but it's not just a case of leaving them alone.  In the case of this particular woodland the plan is to graze it in the winter to protect the mosses, bryophytes and lichens and to ensure that natural regeneration takes place.  If the woodland isn't grazed the bramble and holly will create impenetrable conditions which will shade out the bryophytes and lichens as well as discourage birch and oak seedlings from emerging.  Even so, the intensity of grazing is important and Natural Resources Wales (NRW) suggests one sheep per hectare for the winter months.  Alternatively ponies can be used for such grazing but they need more management and attention than sheep. The damp and humid conditions are helped by the streams and waterfalls of these woods and the abundance of small plants means that there is constant evaporation.  In fact it is probably the difficult access and rugged terrain which has preserved these woodlands for so long - historically they were not suitable for agriculture or industrial forestry and so never got converted from their ancient state which goes back to the last ice age.  The sessile oaks are very important for wildlife but not very productive of good timber so they have often been left to grow quite mature.  Similarly the Birch trees have relatively low timber value which has been their salvation.   [caption id="attachment_20968" align="aligncenter" width="600"] Asymmetric distribution of moss on birch[/caption]  

Nitrogen pollution, in the form of nitrates, ammonia and various oxides of nitrogen, is a threat to ecosystems, ecosystem services and biodiversity.   Monitoring and measuring such pollutants in rain and air borne particles is expensive and, ideally, needs frequent samples. It has been estimated that some 400,000 tonnes of airborne pollution are deposited over Britain each year.  Research by Dr H Harmens et al at the Centre for Ecology and Hydrology (Bangor) has indicated that there may be a simpler and cheaper way of gauging nitrogen pollution  - through the assessment of the state of local moss populations. Read more...

Throughout the twentieth century, stations like that at Manua Loa have monitored the level of carbon dioxide in the air.  It is clear that atmospheric concentrations of carbon dioxide have risen from about from about 300 ppm in the 1960’s to nearly 400 ppm by 2010. However, if we look back some 480 million years (to the Ordivician Period) the level of carbon dioxide was some sixteen times higher than the present level and the average global temperature was about 25 oC, that is, about 10 oC higher than today’s average.  So how and why has global temperature fallen ? Read more...