The grey—brown skeletal branches of trees are being cloaked in fresh green leaves as they unfurl from the buds that protected them through the winter months.  Their bright green colour is due to large amounts of chlorophyll.  The chlorophylls are pigments that can absorb many of the wavelengths of visible light, but not green.  Green wavelengths are reflected back into the environment, which is why our eyes perceive both young and mature leaves as green. Each leaf is made up of a variety of cells and tissues.  The top and bottom of the leaf are covered with a layer of cells  termed the epidermis.  It consists of many interlocking cells (rather like jigsaw pieces), sometimes called pavement cells.  Their function is to protect the underlying cells and also produce the waxy, waterproofing layer — the cuticle.   The lower epidermis is ‘pierced’ by the stomates.  These are the ‘breathing pores’ of the leaf, allowing the exchange of gases and water vapour.   The epidermis may also bear trichomes.  These are small ‘hair—like’ projections.  If there are many of them they can give the leaf a white or silvery appearance, helping to trap moist air near to the leaf surface to reduce water loss.  They  may also help to reflect sunlight, so that the leaf does not get too hot and on cold days can serve to protect the leaf from frost damage. Some trichomes have a protective function in that they may physically restrict the feeding of insects and other herbivores, and some contain a cocktail of toxic chemicals [e.g. nettles]. Under the upper epidermis and within the leaf, there is one or more layers of cells packed with chloroplasts - the palisade layer.  This is the principal site of photosynthesis within the leaf, where carbon dioxide is fixed into sugars and other vital nutrients.  The ‘by-product’ of photosynthesis is oxygen, which is not only essential for plant respiration but needed by the vast majority of animals on this planet.  It diffuses out of the leaf through the intercellular spaces of the next layer of the leaf - the mesophyll layer. The stomates allow gases in and out, but can close through the movement of their guard cells.  Stomates tend to close up at night or when the leaf experiences water stress.  Running throughout the body of the leaf is the xylem and phloem tissues, which conduct water, minerals and sugars etc around the plant. The sheer abundance of chlorophyll in many leaves masks the presence of other pigments, which only become visible when the leaf begins to senesce and the chlorophylls break down.  The leaf turns a yellow / orange colour due to the presence of carotenoid pigments.  Autumnal leaves can display a variety of colours due to other pigments such as the anthocyanins and xanthophylls.  Some leaves take protection very seriously   Curious fact : the leaf with the largest surface area is that of the Amazonian water lily, which can be 10 feet in diameter.

Trees and shrubs that grow in temperate regions, where the seasons alternate (warm / cold, dry /wet) create annual rings.  The rings formed in a deciduous tree (like beech, oak, lime) are generally quite noticeable when the tree is felled.  They may be counted to give an indication of the age of the tree.  Annual rings are formed because there is a difference in the creation of ‘wood’ / xylem tissue when growth is fast in the Spring and slow as Autumn progresses.  The thickness of the rings from year to year reflects the changing climate and environment that the tree experiences during its life. Xylem tissue is one component of a tree’s vascular tissue.  The xylem tissue conducts water and minerals around the plant, whereas phloem tissue transports sugars and other organic molecules.  Lying between these two tissues is the cambium.  This is a layer of dividing cells, which becomes active in the Spring forming new cells some of which will form new phloem tissue and others develop into xylem tissue. The cells that will form the xylem tissue undergo a series of dramatic changes.   The walls of the cells that will form the long tubes of the xylem are made of cellulose to begin with, but then they are strengthened with lignin.  Lignin is the ‘stuff of wood’.  It is a complex material - made from polyphenols and other substances such as pectins and hemicelluloses.  It is a waterproofing material that is highly resistance to decay.  It lines the tubes of the xylem so that water can be transported from the roots, up the trunk / stem to the leaves etc.  The xylem vessels that form in the Spring [early wood] have a greater diameter than those formed later in the year [late wood].  It is this size difference in the vessels that results in the visible ‘rings’ when a tree is felled. Careful study of tree rings can reveal information about climate, sometimes extending back through the centuries   using species such as the long lived Bristlecones. It has given rise to the discipline of dendrochronology [link opens / downloads a PDF].  This information can then be ‘combined’ with tree ring data from intact remains in cold, dry (and often high altitude) environments and material from archaeological sites.   Apart from measuring the ‘width’ of the annual rings by creating thin section of the wood that can be examined under microscope, it is also possible to use staining techniques to reveal which xylem tissue has a higher / lower, lignin / cellulose content.  By using a double staining technique with the dyes Safranin and Astra Blue, it is possible to identify which xylem vessels are rich in lignin, and which have more cellulose.  Tree rings which stain largely blue are formed from cells which have not lignified properly.  Lignin stains red.  A recent study of blue rings in Pine trees and Juniper shrubs suggests that blue rings are indicators of cold summers. These two species are typical of the upper tree line in Northern Norway. Furthermore, blue rings have the potential to weaken the pine trees, leaving them more susceptible to mechanical damage and / or disease.  This study has identified blue rings associated with the cold summers of 1877 and 1902, which might have been caused by the eruptions of volcanoes as far away as Ecuador and Martinique. Note : The xylem tissue in conifers is different to that of broad leaved deciduous trees.  It is made up of shorter structures called tracheids, which pass water from one to the next via pits - ‘pores’ in their lignified walls. For more information on Blue rings in Black Pine, click here  

During a drought, the trees in a woodland or forest become 'stressed' and may die.  The  reason for their death is not immediately obvious (beyond lack of water), and  it is not possible to ‘transplant’ a mature tree and its complete root system to a lab for detailed investigations.  However, recently, researchers at the University of Innsbruck have taken ‘the lab’ to a set of mature pine and pine trees. The trees were fitted with rugged and waterproof ultra-sound detectors.  Some of the trees had their canopies covered by a ‘roof’ so that the summer rain was denied to the trees, and they essentially experienced a ‘drought’.   Drought stressed trees produce ultrasound ‘clicks’ (faint acoustic waves that bounce off of air bubbles) that can be picked up by the detectors.  Air bubbles or emboli form in the vascular system of the trees when they are struggling for water.  Water is drawn up the xylem vessels by the evaporation of water (via the stomata) from the leaves, there is a continuous column of water.  When the column of water breaks, bubbles form with the xylem vessels and the transport of water to the leaves is reduced.  If the flow of water is substantially reduced the tree will die. The sound detectors found that the spruces produced more clicks than the beeches when water stressed, suggesting more emboli were formed within their xylem tissues.  It may be that the beeches were able to access the deeper reserves of water in the soil, whereas the spruces had a shallower root system. Trees can, of course, reduce water loss from their leaves by closing down their stomates.  But when their stomates are closed, they cannot take in carbon dioxide for photosynthesis and make the sugars / starch that they need for their metabolism.  At the end of the experiment, the trees that experienced ‘drought’ were drenched with water and most recovered well, and their rates of photosynthesis caught up with the ‘control’ groups of trees (those with summer rain).  However, the spruces’ water reserves were somewhat depleted; this was determined by measuring the resistance the tissues offered to an electrical current. The ability to withstand / recover from drought could over time affect the make up of woodlands and forests,  particularly if the trend for hotter and drier summers continues. Interestingly, some work in the United States (at University of Wisconsin–Madison) suggests that young tree saplings that have experienced drought or heat are more likely to survive when transplanted into more challenging areas.  It seems that the soil microbes that young saplings experience can help young trees establish themselves.  Saplings grown in soil (and microbes) that have experienced drought / cold / heat are more likely to survive when later transplanted and faced with similar conditions.  Trees with ‘cold-adapted’ microbes survived better when experiencing Wisconsin’s winter temperatures. The work was conducted with different species of tree in a variety of locations in Wisconsin and Illinois. The transplant locations varied in temperature and rainfall.  It may be that fungi that inhabit the roots of the saplings are involved in these ‘responses’, though the microbial population of the soil is diverse. For more details of this work, follow the link here.

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]

The woodlands blog has previously reported on the danger that the Tree of Heaven presents.  Whilst its foliage and fruits are attractive, it can be a highly invasive plant.  it can overwhelm natural vegetation / species because It grows extremely rapidly It can produces thousands and thousands of seeds in any given year It can spread by underground suckers It produces chemicals that inhibit the germination and growth of other species - a phenomenon known as allelopathy. Read more...

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...

Bark is the term that is often applied to the outer covering of tree stems and other woody plants. It serves to protect a tree from  Water loss Insect attack Infection by bacteria and fungi Physical damage (by fire, animals, rock fall) The nature of bark is immensely variable.  In some trees, the bark is extremely rough, corrugated and thick.  In others it is is thinner and appears to peel off in strips.   Redwoods are noted for having an extremely thick bark (see featured image above). Their bark is very fibrous and can be up to three feet thick. Read more...

Forests and woodlands are important in the global ecosystem; they have taken up some 20 to 30% of the carbon dioxide released from fossil fuels in recent times. It had been assumed that the dense and biodiverse tropical forest ecosystems (close to the equator) were particularly effective in ‘soaking up this carbon dioxide’.   However, there is doubt that this will continue to be the case as forests shrink in size.  Plus, recent work at the University of Birmingham (Dr Tom Pugh) has shown that where forests were re-growing,  they took up large amounts of carbon partly because more carbon dioxide was available,  but also as a result of the younger age of the trees. This ‘youthful’ carbon uptake was not associated with tropical areas, but with regenerating forests of more temperate regions. Read more...