Trees are remarkably resilient.  In various forms they have been arounds for millions of years.  They have survived asteroid impact (66 million years ago), and a series of successive ice ages.  However, in more recent times they have faced a new challenge, the relentless march of humankind. Early societies felled trees for timber for dwellings, boats*, wood for fire, making tools, as part of ‘flash and burn’ agriculture to create a ‘swidden’ to grow food.  The material felled to create a swidden was allowed to dry and then burnt, the ash released mineral nutrients into the soil for the crops.  As more complex civilisations / societies evolved, there were attempts to restore degraded forests / bare land, and  to protect forests.  The Zhou (Chou) dynasty established a ‘forestry service’ over two thousand years ago, and in India the emperor Ashoka (268 -232 BCE) ordered wide scale reforestation.  Much later in the Middles Ages there were efforts to restore degraded areas, for example, around Nuremberg in the C12th.  Most of these early efforts were concerned with increasing timber production or the mitigation of natural disasters.  In the last two centuries, significant areas of natural forest and woodland have been lost, increasing the risk of soil erosion, flooding and disease (as animals are displaced). In the last century, the percentage cover by forest / woodland in some countries (for example, the UK) was low so vast areas were planted with a single species. Fast growing species (conifers such as pines, spruce and larch) were often grown on what was regarded as marginal land, creating plantations.   In some parts of the Mediterranean, Eucalyptus was planted.  These were species that could cope with the challenging nature of the soil and / or the  topography.   Many of the European initiatives met with some success in terms of timber production and / or the stabilisation of degraded areas (for example, reducing erosion).  Such schemes also created jobs, contributing to the local economy.  But when grassland or heathland were used to create single species plantations, this was often accompanied by a loss in biodiversity. In places, the introduction of non-native species has been challenging as they have become invasive, for example, black cherry. The success of any scheme is dependent on Sound planting techniques and aftercare Selection of the right species for the area The co-operation / involvement of local peoples. Mass planting of a single species can also contribute to the rapid spread of disease / pests (e.g.bark beetles, pine procesionary moth).  Vast swathes of coniferous forest were affected by acid rain (associated with sulphur dioxide from the burning of fossil fuels) in the last century.  This was termed Waldsterben [Wald=forest + sterben=to die].   Problems such as these, coupled with increasing environmental awareness contributed to a rethink of the aims and objectives of forest management / renewal / restoration. However, there were examples where ecological recovery was good, for example, some spruce and black pine ‘monocultures’ were diversified through the planting of a understorey of broadleaved trees, as in Slovenia. In recent times, timber production, control of erosion and reduction of disaster risk remain relevant still, but the importance of biodiversity, resilience and ecosystem services are now uppermost.  There is a move from ‘quantity to quality’ of forest and woodland.  In many countries, a growing interest in recreation and tourism (recognising the importance of green space for mental and physical well being), coupled with growing environmental concerns and recognition of climate change has emerged.   Forest and woodlands across the world, from the boreal regions to the Equator are under threat.  Many have been lost or badly degraded, and are in desperate need of restoration.  Forests are no longer regarded as sources of timber, but are important providers of ecosystem services, such as the mitigation of flooding. So, in more recent times they have been efforts to restore and repair forests and woodlands.  Homogenous and dense plantations / forests in boreal regions had clearings created to allow light demanding species to establish.  Limitations have been placed on clear cutting, and the use of fencing, tree protectors have [caption id="attachment_41889" align="alignleft" width="300"] squirrel[/caption] helped to reduce browsing pressure (by deer / squirrels etc).  One means of promoting biodiversity is ensuring that the woodland / forests offer deadwood.  This provides a ‘home’ to species as varied as woodpeckers to saproxylic beetles. These beetles help break down wood so that it can be further broken down by fungi and bacteria, returning nutrients to the soil. Tree girdling was a technique used in Finland to create deadwood, it severs the conducting tissue (phloem) so that the supply of sugars is interrupted.  Thought is now given to the selection and introduction of tree species that are adapted and resilient to anticipated climate change impacts.   Though countries have adopted a variety of techniques in recent times, the extent of forest and woodland restoration has been largely limited by the funding available.  Restoration does not come cheap, funding over significant period of time is needed, and time itself for the effects of  the measures to become apparent. For detailed information on forest restoration see - https://link.springer.com/article/10.1007/s40725-024-00235-3 Intereesting facts : Henry V111’s flag ship, the Mary Rose , was built using oak and elm. It was the first big ship of the Tudor naval fleet and it is estimated that over 600 trees were needed for its construction,. That is equivalent to about 16 hectares of forest / woodland.  And Cver 370 species are supported in the territories of the Karen swidden farmers in northern Thailand.  

Forests, woodlands, trees are vital to life.  They absorb carbon dioxide, they release oxygen, they offer food and shelter to countless species (including us).  The global forests (equatorial to boreal) play an important role in mitigating climate change due to fossil fuel emissions.  However, many forests and their particular tree species are being  threatened by the world’s warming climate.  Recent years have seen catastrophic fires in many parts of the world, from Canada, Siberia, Sweden to Australia. In 2019/20, intense fires caused extensive damage to the Eucalypt forests in Australia.  Eucalypt rich woodland / forest is likely candidate for fire because the leaves of Eucalypts produce volatile and highly combustible oils.  The litter underneath such trees is rich in organic compounds such as phenols, which slow down the microbial breakdown of the dead leaves.   Consequently,  a layer of dry, eminently burnable material builds up. In Eastern Australia, some 40+% of the native eucalypt forests suffered severe canopy damage.   Trees on the west coast of America have also been subject to intense fires.  Their susceptibility to fire has been accentuated by drought across the region.  Analysis of the growth rings of trees, such as the Red Cedar (in areas such as Oregon) show that trees suffered reduced growth in the years prior to their death.  Drought stress increases the probability of attack by bark beetles and pathogens.  In California, many native species such as white fir, red fir and ponderosa pine have died and provided material for the fires that were to follow.  Fires in 2020/21 swept across the region, destroying vast swathes of forest.  The fires were of such an intensity that even Giant Sequoias were killed.   [caption id="attachment_40596" align="aligncenter" width="675"] Forest Fire in Canada[/caption] Sequoias had been thought ‘indestructible’ as they have a thick bark, which protects the inner living tissue, plus their canopy is usually well above the flames on the forest floor.  In the past, the fires burned litter on the ground, removing competitors, and releasing nutrients.  The heat would also open up the cones of the Sequoias releasing their seeds, so young trees could establish. Some of the Sequoias that died in these recent fires had stood for centuries and survived many wildfires.   In the past, the amount of litter / dead material was limited.  Indigenous people managed these forests (reducing the fuel load) to create forage for game animals, so that wildfires were of mild to moderate intensity.  Now, the fires are different - they are intense. There is more material to burn - including the trees that have already died from drought and disease. The fires can now reach into the canopies of the Sequoias. One of the Sequoias that died was the King Arthur tree - the 8th largest giant redwood in the world; it died in the Castle Fire of 2020. The drought driven deaths of many tree species is probably the start of a longer lasting shift in the growing range of the affected trees.  Temperature and water availability are two of the major determinants of the range of a given species.  It is possible that trees may ‘move’ northward and upward (grow at higher elevations).  Trees will begin to ‘die off’ at the edge of their range / lower elevations as drought / warming increases.  Die offs may also affect commercial plantations of species such as Douglas fir.

The woodlands blog has previously reported on the damage being wreaked by bark beetles.  These beetles may be small (less than a centimetre in length) but their effects on the western forests of North America has been immense. Some areas have lost 90% of their conifers.   Outbreaks of these beetles have been increasing in size and severity.  Indeed, across Europe, the eurasian bark beetle (Its typographus) has killed millions of conifers. Whilst bark is broadly protective, it can also offer a home to certain insects.  Bark beetles lay their eggs just below the bark so that when the larvae hatch, they can feed on the nutrient-rich living tissue of the cambium and phloem. Consequently, the tree's transport systems begin to fail.  The beetles may also introduce disease-causing fungi and bacteria. Ageing stands of trees coupled with warmer winters, which help the overwintering stage of the insect, have contributed to the spread of bark beetles. .  Conifers, by their nature, are not defenceless.  When a pine tree is cut / wounded, it produces a pale yellow and sticky fluid - RESIN to seal the cut or wound [see above image]. This material helps prevent the entry of pests or pathogens, and can stem water loss. The resin may trap insect invaders as  witnessed by those trapped in time capsules of amber.  Resin is rich in terpenes, these are used in the building of many complex organic molecules and contribute to the make-up of the volatile oils, produced by many plants.  Terpenes are made from units of isoprene, which has the formula C5H8. So the basic formula of a terpene is (C5H8)n, where n is the number of isoprene units that have been joined together.  Terpenes are also readily available in coniferous oils, which contribute to the unique smell of a pine forest or a burning log. [caption id="attachment_40973" align="alignleft" width="300"] Old and dying tree[/caption] These ‘chemical defences’ should trap, poison or deter an insect invader, such as the bark beetle.  But it would seem that bark beetles ‘don’t mind’ these defences. Research suggests that the eurasian bark beetle might have an ally.  Certain fungi (from the genus - Grosmannia) are found in association with these beetles. When the  Grosmannia fungi infect spruce trees they alter the chemical profile the trees, so that infected trees produce different volatile chemicals - ie they smell different.  The bark beetles are able to detect these differences and exploit this ‘breach’ of the trees natural defences.  The unique chemical profile of infected trees and the pheromones produced by the beetles probably help explain the swarming behaviour of the bark beetles.  A heavy beetle infestation results in the death of a tree. However, there is a possible positive in this rather sad tale.  At present, traps for bark beetles rely solely on using pheromones but if the pheromones can be combined with the chemicals produced by the fungi then it opens the door to more effective beetle traps.  

As stated in the previous post, bark is a mixture of living and dead cells.  Cork cells abound, especially in trees like the cork oak, where the cork may be regularly harvested.  This may be used for flooring, insulation and yes - corks for wine bottles. However, bark is a source of many things.  From early times, bark from trees like alder, buckthorn, oak, birch etc were used to make dyes for clothing.  Material from the inner bark of some trees (e.g. lime, willow, mulberry) was a source of fibres for clothing and cordage (string / yarn). Herbalists also found a use for the bark of certain trees.  Infusions of willow were used to treat fevers, the ‘ague’, rheumatic aches and pain.  It contains salicin, which the body converts to salicylic acid, an early 'form' of 'aspirin'. Interestingly, Nicholas Culpeper, in The Complete Herbal (of 1653) gives a number of uses for willow, including to staunch wounds, but does not mention pain control. The bark of the Cinchona [Jesuit’s bark] gave quinine - a treatment for malaria [caption id="attachment_39935" align="alignleft" width="300"] Amber[/caption] When a tree, like a pine, is injured (through storm damage or insect attack), the bark can produce resin - a sticky and viscous liquid.   The resin is produced in resin ducts present within the bark tissue, though the ducts may be present in deeper tissues. Research has shown that ponderosa pine trees that had more (and wider) resin ducts survived drought and bark beetle attack better.  The resin can harden and help seal wounds . Many resins contain terpenes, such as alpha-pinene and limoneme.  The resin from pine and other conifers can under special circumstances be converted into amber.  Sometimes, the resins produced can be fragrant. Trees of the genus Boswellia and Commiphora produce a aromatic resin that gives frankincense and myrrh respectively.  Both are produced by the wounding of a tree so that its resin seeps out.  Both may be used in the making of incense. Another bark exudate comes from certain species of Acacia - Gum arabic, which forms from the hardened sap (adjacent image).  Acacia species belong to the ‘Bean’ family (Fabaceae).  The gum is collected from trees, mostly in Sudan and the Sahel.  Gum arabic is a mixture of glycoproteins and polysaccharides.  The polysaccharides are constructed from the sugars arabinose and galactose. It is soluble in water and edible, and has a number of uses in the food and pharmaceutical industries. Tapping or wounding the bark of different trees can result in various fluids being released, for example, latex.    White or yellow latex is produced by the rubber tree (Hevea brasiliensis).  The latex is found in special vessels within the bark - laticifers. The process of tapping rubber trees is outlined in some detail here.   Latex production is not confined to woody trees, small herbaceous plants like dandelion and spurge can produce a white, milky latex (as can the opium poppy). [caption id="attachment_39984" align="alignleft" width="300"] Euphorbia latex[/caption] The latex produced by some members of the Spurge family can cause burning pain, inflammation or even blistering - for example that of the Pencil tree.  Such toxic saps most likely evolved to deter animals from grazing. Sometimes, a watery sap may be collected from the bark.  This is the case with Birch.  Sap may be collected (tapped) in early Spring, when sugars and other materials are being mobilised for growth, leaf production etc.  Sap may be collected later but is said to then have a bitter taste.  The sap is an interesting ‘cocktail’ of amino acids, protein, sugars (glucose & fructose), betulinic acid, proteins, vitamins C & B, and minerals.  It is used to make a much favoured drink in Northern Europe and should be consumed within days of collection.  Birch trees are quite sensitive to tapping. Not watery, but very sugary is Maple Syrup. Maple trees are tapped by drilling holes through their bark and into their trunks. Starch is stored in the trunks and roots before winter, it is then converted and mobilised in late winter / early Spring. The collected  sap (through tapping) is then heated to produce a concentrated syrup. Thanks to Montemari at Pixabay for image of gum arabic

Bark exists to protect a tree from ‘attack’ by the elements, pests, ‘predators’ (animals who would eat it) and disease causing organisms.  There is no easy definition of what constitutes bark,   a slightly technical definition might be ‘the tissues that lie outside the vascular cambium'.  The vascular cambium is a layer of dividing cells that gives rise to xylem tissue and phloem tissue.  The cells nearer the centre form the xylem, those towards the outside form the phloem.    The inner part of the bark contains various types of living cells, for example, glands that produce latex (as in natural rubber), oils and resins.  Moving outwards, there lies the rhytidome or outer bark, an amalgam of living and dead material - notably cork cells.  The cork cells fill with a waxy material - Suberin. Eventually, these cells die and form much of the bulk of the bark.  The nature of bark is immensely variable. Wind, fire and frost can seriously damage or kill trees but bark helps  to protect them.   Trees are eminently combustible as is evidenced by the recent forest fires in Australia and California. However, some trees have a very thick bark that can protect them against fire.  The cork oak has a bark that can be up to 30 cm thick, it is so thick that it can be harvested periodically without killing the trees.  Cork oak is grown extensively in the mediterranean region. Giant Redwoods too are noted for having an extremely thick bark. Their bark is very fibrous and can be up to three feet thick, it offers protection against fire (and rock fall which is also a hazard in their home habitat). In contrast to cork oak and redwoods, some trees like the eucalypts have a bark that is rich in oils and very flammable.  The bark also ‘peels’, strips are shed onto the forest floor. There are many species of Eucalyptus and several different types of bark are recognised.  [caption id="attachment_35352" align="alignleft" width="300"] Woodland recovering from a fire[/caption] If and when this oil rich bark builds up on the forest floor, it will contribute significantly to the intensity and ferocity of any fire. Indeed, it has been likened to adding petrol to a fire ’3 centimetres of leaf litter can cause a conflagration equivalent to one fuelled by a centimetre of refined gasoline’.  The leaves are also rich in oil so the crowns of the trees can also contribute to / exacerbate any fire.  The peeling or exfoliation of bark is not restricted to Eucalypts, it can be seen in trees much closer to home - such as the birch.  Its bark can be removed in long strips and has been used in covering a canoe or roofing material. Whilst bark can protect against fire, it can also deter animals - large or small from inflicting damage.  For example, there is an African species of Acacia known as knobthorn that has a bark covered with thorn-like structures.  These 'thorns' deter elephants from eating the bark.  Elephants can consume a lot of vegetation in a day and tree bark is much favoured.  A variety of animals may feed on bark material, for example deer, squirrels, and beavers, but the list could also include orang-utans, rhinos, bush babies and porcupines. North American porcupines use their large front teeth to eat bark and stems. Bushbabies generally feed on insects during the wet seasons, but during drought / dry periods - they feed on the resins / gum that flows from the trees in their woodlands. In the UK, a lot of bark damage is done by deer, especially during the winter months when other food sources are limited.  In the summer months, male deer rub their heads / antlers against the trunks of trees - inflicting damage.  Such activity can prevent regeneration in natural woodlands.  Tree guards may be needed to allow young trees to establish themselves (or fencing to create a ‘deer free’ zone).  Guards also protect against rabbit damage.  Grey squirrels can also cause damage to trees as they gnaw stems to reach the ‘sweet’, sap-filled tissues just below the bark, this activity is usually seen in late Spring and early Summer. [caption id="attachment_5312" align="alignleft" width="300"] xylem vessels[/caption] Whilst bark is broadly protective, it can also offer a home to certain pests.  Bark beetles lay their eggs below the bark so that when the larvae hatch, they can feed on the nutrient rich tissue of the cambium and phloem.  Bark beetles have been responsible for the loss of millions of trees in the United States and Canada.  The scale of the loss is much greater than in the past, when cycles of beetle infestation and fire created a mosaic across the countryside of young and old trees.  Ageing stands of trees coupled with warmer winters (which have helped the overwintering stage of the insect)  have contributed to the spread of bark beetles.  The beetles breed and feed beneath the bark, damaging the phloem and cambium tissue.  Consequently, the tree's transport systems begin to fail and the beetles may also introduce disease-causing fungi and bacteria. To a certain extent, trees are able to repair damage to their bark but the response is varied according to the nature of the damage and the tree involved. Some trees can produce ‘callus tissue’ that heals over the ‘wound’, leaving a scar. Some trees, such as the pines, produce resins and antimicrobial compounds in response to injury.  This sticky resin may trap insect invaders as is witnessed by those trapped in time capsules of amber.   Apart from bark beetles, other animals and plants live in or on bark in a variety of associations, some parasitic as is the case with fungi (like the polypores), whilst lichens and mosses are epiphytes.  They use the bark as a substrate on which to live, grabbing nutrients and water from rainwater as it trickles down.   The many uses of bark tissue can be left for another woodlands post. [caption id="attachment_39940" align="aligncenter" width="620"] Section through bark[/caption]

Lichens losing ? Sitting on the bark of many trees and on the surfaces of fences and walls, there will be lichens.  They are there in summer, winter, spring and autumn.  Lichens come in an amazing variety of shapes, sizes and colours.  Some can grow in extreme environments such as the rocky summits of mountains. Such lichens grow slowly and may live for hundreds of years. Lichens are rather unusual in that they are an amalgam of two (or occasionally three) organisms : a fungus and algae. They are symbiotic systems, where the partners of the association work together for mutual benefit.  The fungus makes up the bulk of the lichen’s structure (known as the thallus), but the algae (green algae or cyanobacteria) are essential as they can photosynthesise and provide the organism with carbohydrates.   Lichen covered tree One of the most common algae found in lichens is a species known as Trebouxia.  It can exist in association with a fungus to form a lichen,  or as a free living organism.  If the Earth’s warming continues at the present rate, it may well be too hot for certain species of Trebouxia to survive (in their normal range). Dr M Nelson of the Field Museum (Chicago) has looked at the adaptability of Trebouxia species and suggests that it could take hundreds or thousands of years for Trebouxia species to cope with the temperature changes that we are currently experiencing.   These algae may well lose out in the evolutionary race to cope with climate change. This would, in turn, affect many different species of lichen. Lichens are important in arctic tundra ecosystems, where they together with mosses and liverworts make up the majority of the ground flora. They contribute to food chains, for example, reindeer moss is not a moss but a lichen.  Lichens are also pioneer species - they can colonise bare rock and contribute to its weathering (their exudates chemically degrade and physically disrupt the minerals).  Lichens may be used by birds as nesting material. Hedgehogs. Rural hedgehog populations are still in decline, dropping by 30 to 75%, this is in contrast to urban populations that are ‘steady’.  Though urban populations suffer mortalities on the roads, well managed urban areas, parks and wildlife-friendly gardens provide refuges for hedgehogs.  The loss of hedgerows and diminishing field margins is contributing to the decline of rural populations. Land of Plenty report The WWF-UK has produced a report entitled “Land of Plenty”, which addresses some of the problems that the UK faces now and in the coming decades. There are many reports relating to the loss of plant and animal species and the degradation of particular ecosystems (flower-rich meadows, peatlands, salt marshes etc).   Sadly, much of this  damage has been associated with the expansion of our farming / food production systems; indeed some 70% of the land is involved in agriculture.  The WWF report outlines how a move towards regenerative farming / agriculture can significantly reduce CO2 and methane emissions, reduce pollution (from fertilisers) and help with biodiversity and resilience.  Such changes would (in time) help limit farmers’ exposure to extreme weather events that affect crops / harvests.   One of the many suggestions in the report is the expansion of ‘woodland creation programmes, focussing on potential for broadleaf and native species’. The focus would be on natural regeneration in the first instance, but supported by active tree planting. Full details of the report available in PDF format here. Drought, bark Beetles and fires. Woodland recovering from a fire The Cameron Peak Fire in the Rocky Mountains of Colorado and the Creek Fire in the Sierra Nevada of California burned through forests where large number of the trees had been killed by bark beetles. Warmth favours the bark beetles.  Mountain pine beetles had killed millions of lodgepole pines.  A dead tree does not take up water, it dries out.  The drying out was ‘helped’ by the drought that the West Coast has experienced in recent years.  The fires burned with incredible ferocity.  In the case of the Creek Fire, the plume reached some 50,000 feet up into the air.  The fires were the result of Drought / climate change Bark beetle infestation Large numbers of dead, dry trees Consequently, large amounts of energy-rich dry biomass Full details of the factors behind the forest fires here. Drought is a major ‘stressor’ affecting many ecosystem across the globe.  To understand how drought affects different ecosystems, DroughtNet is working with a number of existing projects and the International Drought Experiment (IDE).  A recent experiment at the University of Florida demonstrated how drought-stressed pines did not grow as well, and when faced with an invasive species and fire - they were much likely to succumb than a healthy tree.

The fact that UK is an island has kept many potential pathogens and pests ‘at bay’.  However,  in recent times the growth volume of international trade has become a cause for concern.  Pests and pathogens can ‘hide’ in important plants and plant products (for example, timber that has not been debarked or suitably treated). The Great Spruce Bark Beetle is likely to have arrived here in wood that had not had its bark fully removed.  The beetle breeds under the bark of trees, creating tunnels resulting in the destruction of the cambium.  The cambium is a highly active tissue, producing new cells that will go on to form xylem and phloem tissue.  These tissues distribute nutrients and water around the tree.  With a damaged cambium, a tree is weakened and more susceptible to other pests or pathogens. In the case of the Great Spruce Bark Beetle, a bio-control measure was allowed; a natural predator of the bark beetle has been introduced (Rhizophagus grandis).  Consequently, the numbers of bark beetles have fallen.  Wherever the bark beetle goes, its predator sooner or later follows.  It is thought that its predator ‘finds’ the bark beetles due to the volatile chemicals released from the bark as a result of the beetles' burrowing activities. The U.K.’s control measures generally proceed by four steps : Try to prevents pests and pathogens arriving in the country eg. Inspection of plants etc at ports of entry If a foreign organism arrives then the authorities try to eradicate the pest / infected plants / trees, hopefully the organism does not become established If a pest or pathogen has become established then a containment policy is put in place If all of the above fail then the Forestry Commission and other bodies operate in a way that mitigates the effects of the pest or pathogen.   Phytophora ramorum is a disease of many plants that probably arrived here through the plant import trade.  It has persisted at a relatively low level for many years but from 2009 onwards it affected commercial stands of larch in South West England.  Felling of infected trees helps restrict the spread of this fungus-like pathogen; and a map is available to show where outbreaks of this disease have occurred. Clearly, these introduced species of pest / pathogen could significantly affect the make-up of our woodlands over the coming years, if diseases like acute oak decline / ash dieback are not controlled or contained.  Fortunately, the spread of disease-causing organisms and pests is monitored by both national and international organisations such as these : EU Plant Health Directive this requires nations to report new outbreaks or new pathogens, t the European Mediterranean Plant Protection Organisation is an intergovernmental organization responsible for cooperation in plant health within the Euro-Mediterranean region and  the International Plant Protection Convention, a plant health treaty signed by over 180 countries.  

Wind, fire and frost can seriously damage or kill trees.   Animals also wound them when they feed on bark tissues, and when they rub their bodies or antlers against tree trunks. Insects, like bark beetles can cause significant damage damage to woodlands and forests. The extent of damage to trunks and the bark of trees varies considerably in relation to the nature of the ‘attack’.  If the damage to the bark is severe and the vascular cambium is exposed then neither new water nor sugar conducting tissue can be formed.  Damage to the (outer) cork cambium (phellogen) will limit the trees ability to form the outer tissues of the bark - which protect the tree.  If the damage is restricted to the outermost bark layer then this will render the tree more susceptible to further damage (be it from herbivores or temperature extremes). Read more...