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.

Plants need to protect themselves from the vagaries of the environment - they need to prevent or resist the entry of disease pathogens (bacteria, fungi and viruses) and also restrict the loss of water from their tissues.   The flow of water through a plant (the transpiration stream) is important in the supply of minerals to the growing regions and the ’control' of leaf temperature (as water evaporates it uses heat energy -  exerting a cooling effect). However, uncontrolled water loss would soon result in wilting and death, so the outer layer of plants - the epidermis, has a special layer, the cuticle.  The cuticle is made from waxes, which often form complex and intricate patterns (when viewed with an electron microscope).   Read more...