When you step into your garden during the Spring or Summer months, you might be aware of the scent of honeysuckle, lavender or indeed an old fashioned rose.  Similarly, in a woodland the scent of pine or other conifers may permeate the air.  These various scents often involve a group of compounds known as the terpenes.  The resin that exudes from pine bark is rich in terpenes. Terpenes are used in the building of many complex organic molecules and contribute to the make-up of volatile, organic compounds [VOCs] - produced by many plants.  The scents released from nectaries or other parts of flowering plants are often associated with attracting pollinating insects, but they can serve other functions.  For example, some VOCs released by a plant may warn surrounding plants of attack by insects. However, as you walk down the street, the air is rarely filled with these pleasant, natural scents. Instead the fumes of traffic, factories and industry assail us.  They are the exhaust emissions from cars, lorries, buses etc, plus the various bits and pieces [particulates] that are released from the ‘rubber’ of car tyres, brake pads etc.  The damaging effects of particulates are now well documented.  The effects include lung disease such as chronic obstructive pulmonary disease (COPD) and reduced lung function.   Particulates help create the concoction of substances that we inhale; sadly our air is often polluted with many different compounds. Scientists from Oxford University and Kew recently investigated air quality in different locations around Oxford.  The locations varied from the centre of the town, where the Botanic Garden is found, out to the University’s Wytham Woods.  Wytham is one of the most intensively studied woodland areas in the world.  The wood ‘holds’ 500 plus plant species and 800 or more butterfly and moth species. Records of bird populations go back some sixty or more years (largely due to the work of David Lack and others).   The air samples that the scientists collected were subjected to detailed chemical analysis using the technique of gas chromatography-mass spectrometry. Some 245 different compounds were identified in the various air samples, with each of the six different sites haveing their own signature collection of chemicals.  Sites near to busy roads contained high levels of benzene and toluene, which are traffic-related chemicals.  The tree rich areas naturally showed a greater presence of plant chemicals (VOCs).  Unsurprisingly, when they sampled the air in the Botanic Garden (over the course of a year), they found the highest levels of these plant compounds.  Warmer weather increased the levels of these VOCs / plant derived chemicals, which have been associated with improved mood and stress reduction. Full details of the Oxford study here : https://nph.onlinelibrary.wiley.com/doi/10.1002/ppp3.70191

The honey bee, Apis mellifera, stores food in the form of bee bread. Bee bread is formed through the fermentation of a mixture of pollen, nectar and bee saliva.   It is 'inoculated' with a range of bacteria and yeasts that ferment the material after storage in the comb cells of a hive.  Bee bread is the chief protein resource for bees, particularly for the feeding of larvae [and adults].  As it is a nutrient-rich material, it ‘supports’ various microorganisms, despite its acidic nature and low water content. Bee bread is also coated with propolis.  Propolis (sometimes called ‘bee glue’) is a resinous substance collected by bees from tree bark and leaf buds. This resin is ‘chewed’, mixed with salivary enzymes and the partially digested material is mixed with beeswax.  It is an antimicrobial substance.  It is used by bees to seal holes in their honeycombs and help in the construction of the hive. The very nature of bee bread and the coating of propolis create a ‘challenging environment’ for microbes to grow and survive.   However, despite the ‘unwelcoming’ nature of bee bread, several species of fungi and bacteria form a microbiome within a hive, and are thought to play an rôle in the life of the bees. Recent studies have revealed that the fungus Aspergillus flavus is well adapted to survive in bee colonies.  A strain extracted from a hive was found not only tolerate low pH (which other strains of the fungus could not cope with) but could also deal with the low water content of bee bread, and with the propolis - which is thought to have anti-fungal properties.  Further work demonstrated that this strain of the fungus had mutations that allowed it to develop within the ‘bee bread environment’.  That this fungus can live with the bees suggests that there might be some form of mutual benefit to both fungus and bee, but the relationship (if there is one) is not as yet understood. Full details of this study can be found here  

Many natural ecosystems are periodically exposed to fire.  After a fire, there is often reduced competition and increased nutrient availability (from ash etc.).  The plants and flowers that grow after a fire are visited more often by pollinators, such as bees and other insects.  This can result in increased production of fruits and seeds. Bushfires have been part of certain australian ecosystems for thousands of years and some native species are ‘fire adapted’.  They have come to 'rely' on fires as a means of reproduction and / or  dispersal. Whilst no one fire can be attributed to climate change alone, rising temperatures and aridity, lengthening of the ‘fire season’, combined with bursts of extreme ‘fire weather’, all combine to suggest climate change is implicated. As the frequency of fires increases, the possible benefits of fire to such ecosystems / species are being lost. Fire can help with the physical dispersal of seeds from the parent plant.  In some parts of the world, such as South Africa and Australia, fire and / or smoke can be the stimulus for seed dispersal and subsequent germination.  Plants such as some species of Protea, Banksia, certain members of the myrtle family (e.g. some Eucalypts), and some Pines and Sequoias 'make use' of fire to disperse their seeds. Seed dispersal involving fire is termed serotiny.  Many of these plants produce woody fruits or cones in which the seeds are held.  The mechanism underlying seed release varies but can be due to a resin that ‘seals’ the seeds inside the fruit or cone.  The resin ‘melts’ / liquefies on exposure to heat releasing the seed or there may be a structure called a seed separator (as in Banksia).  Serotinous conifers (like lodgepole pine), have mature cones in which the cone scales are naturally sealed shut with resin.   Most of the seeds stay in the canopy until the cones reach 122-140o F  (i.e 50 to 60oC).  At these temperatures, heat / fire  melts the resin and  the cone scales open to expose the seed. The seed can then drop or drift to a burned but cooling ash-rich soil bed. The seeds do well on the burnt soil available to them as the site offers reduced competition, more light, warmth plus the nutrients from the burning of leaves and litter.  Some species align their germination to immediate post-fire conditions - stimulated by chemicals present in the smoke.  The organic compounds karrikins,  products of the degradation of cellulose are  a germination ‘cue’ for some species.  Karrikins are thought to be present on the soil surface after a fire.  When it rains,  the karrikins are 'washed' into the soil, and seeds present in the soil seed bank are then stimulated to germinate. Thanks to Steve Sangster and John Cameron for images of woodland fire.  

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.  

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]

According to some interpretations of the bible, the Magi or ‘wise men’ travelled from afar bearing gifts of gold, frankincense and myrrh to present to the infant Jesus. The meaning and significance of these gifts has been debated over the years.  One things is clear - all were valuable materials as might have been presented to a king or deity.  Together with spices, frankincense and myrrh moved through ancient trade routes for thousands of years. Gold is a relatively rare element and as such is a precious metal that has been used for jewellery and coinage throughout recorded history.  It is a noble metal, that is it is relatively unreactive, resisting attack by most acids - with the exception of aqua regia, a mixture of nitric and hydrochloric acids. But what of frankincense and myrrh?   Their origins are not geological as both are plant products.  They come from a group of plants known as the Torchwood family or the Burseraceae.  These are trees or shrubs that have prominent resin ducts / canals.  The resin ducts are tubes, surrounded by cells which produce and secrete a resin into the canals / ducts.  The resin is viscous, ’antiseptic’ and aromatic (often smelling of almonds) and helps to prevents microbial attack (and may deter wood boring insects). Frankincense comes from trees of the genus Boswellia.  Nearly all species of this genus have a bark that produces an aromatic sap but it is B. sacra, (also known as the olibanum tree) that is the main source of frankincense (from its papery, peeling bark).  It is found in Somalia, Yemen and Oman, often growing in relatively inhospitable places.  To obtain the resin, the bark of the tree is cut and resin seeps out and is collected, rather like the tapping of a rubber tree.  The trees do not produce resin until they have reached a certain maturity and over-exploitation of the trees can lead to their death.  The seeds from heavily tapped trees are less likely to germinate than those from trees that have not been ‘drained’ of resin.  All Boswellia species are threatened by habitat loss, over-exploitation, and damage by long horn beetles. What is Frankincense used for?  The word derives from the Old French ‘franc encens’ meaning high quality incense.  Many tonnes of frankincense are traded each year and are used in religious ceremonies, and in the making of perfumes and natural medicines.  In ancient times, the Egyptians used it in the process of mummification, it was added to the body cavities together with natron (a mixture of sodium salts).  The resin has also been used in traditional Chinese medicine for its antibacterial properties and ‘blood moving’ properties. Like Frankincense, myrrh is a resin harvested by wounding the bark of a tree. The bark is a silvery grey, and the twigs are quite spiny (see image).  The resin that exudes is ‘waxy’ and quickly congeals becoming hard and glossy, darkening as it ages. The tree in question is Commiphora myrrha.  It is found in north east Africa - Somalia, Yemen, Eritrea and parts of Saudi Arabia.   The related Commiphora gileadensis, native to Israel, Palestine and Jordan, is also accepted as an alternate source of myrrh.  Myrrh has been used as an antiseptic in mouthwashes, and as a constituent in salve / ointment for skin abrasions, bruises and sprains.  It  has also been used in perfumes and  as a special flavouring for wine.  Like frankincense, it was used in making incense❋ and in the preparation of bodies for mummification / embalming.  In Exodus [30:22-24], God said to Moses to take the best spices and liquid myrrh to make a holy anointing oil.  Anointing oil is still used in certain ceremonies / rituals of both Eastern Orthodox and Western Churches.  In some cultures, it can be used to ‘fumigate’ or refresh a house, giving a warm, earthy and balsamic odour.  It is also said that myrrh is a powerful detoxifier, can lower cholesterol and stabilise blood sugar levels.  Both frankincense and myrrh were extensively traded in ancient and more recent historic times, along with various spices (such as cinnamon, ginger and nutmeg) across the Mediterranean and Arabian peninsula, through to India.  Interestingly, in Ancient Rome, myrrh was priced at five times the cost of frankincense. ❋ Incense can be made from various aromatic plant materials that produce a scent. The actual ingredients used vary by region / area. Apart from frankincense and myrrh, incense may contain cinnamon musk patchouli (from a plant of the mint family) sandalwood. Many thanks to Pixabay for images of frankincense and myrrh  (Leo_65, xbqs42  et al))   .

Pine trees are found across the world, there are over one hundred different species.  Many are native to the coniferous forests  (Taiga) of the Northern Hemisphere.  Their evergreen needles (leaves) offer shade in summer, and the trees may offer a degree of shelter from the winds of autumn and winter.  Gardeners and foresters 'like' Pines as they generally tolerate nutrient poor and somewhat dry soils.   In the period after WW2,  considerable areas of ‘low grade’ land were pressed into service (in the U.K.).   Areas around Thetford and Kielder were used, as were some sandy coastal sites (for example,  Holkham in Norfolk) and many large tracts of land in Scotland.  Pines are central to the business of agroforestry in places like the U.K,  New Zealand and Brazil, providing a source of lumber.    Read more...

Each year, a variety of conifers are sold as Christmas trees, for example, the Norway Spruce Picea abies Silver Fir Abies alba  Nordmann Fir Abies normanniana Scots Pine Pinus sylvestris and in North America Douglas Fir Pseudotuga menziesii  and Balsam Fir Abies balsamea. Read more...