Blog - Flora & Fauna
Woodlands web updates : 27
Tree survival and drought. Researchers at the University of California have been working on a method that helps predict whether forests / woodlands can survive periods of drought. As climate change is altering patterns of snow and rainfall, so periods of drought are likely to become more common. Forests are important in terms of carbon sequestration, that is, they take up carbon dioxide from the air and convert it into sugars, starches etc that are stored in the leaves, branches, stems and roots. However, in order to assimilate and convert carbon dioxide (in photosynthesis), trees (indeed all plants) need a supply of water. When water is limited, trees need to make use of their reserve materials. Just as we make use of body reserves of fat and glycogen when food / diet in inadequate. However, reserves can only sustain a tree for a finite period of time. If drought persists, the tree reaches a ’tipping point’ and it will die. The researchers studied a forest in the Sierra Nevada that experienced a period of drought between 2012 and 2015. During this period, millions of trees died. The team recorded rainfall, soil moisture and temperature in the forest AND the amount of carbon dioxide that the trees absorbed, and their reserve materials. They found that the trees were able to maintain function / health after the onset of the drought but with the passing of time, the trees exhausted their reserves and were unable to use / convert carbon dioxide into food. They had reached the tipping point and died. The methodology of this study was called CARDAMON (carbon data assimilation with a model of carbon assimilation); it is hoped that it can be used to evolve strategies to enhance forest and woodland resilience in the face of climate change. Pollinators. [caption id="attachment_35902" align="aligncenter" width="675"] hoverfly[/caption] University researchers from the UK and Finland have been trying to determine the most effective pollinators of crop plants, like strawberries (and other fruits). Plentiful and effective pollinators are needed to ensure a good harvest of the fruits. The researchers studied the pollinators at three strawberry farms through the (long) growing season for the fruit. They adopted two approaches : They caught the insects that visited the strawberry flowers and analysed the pollen they carried in detail (pollen load and type). They also counted the number of flower visits by the different insects, (a quick way to identify key local pollinators). Many insects were identified, including :- European drone fly : Eristalis arbustorum Honeybee : Apis mellifera Levels drone fly : Eristalis abusivus Buff tailed bumblebee : Bombus terrestris White tailed bumblebee : Bombus lucorum Common drone fly : Eristalis tenax Red tailed bumblebee : Bombus lapidarius Early bumblebee : Bombus pratorum Bent-shinned Morellia : Morellia aenescens Hoverflies are true flies, that is, they belong to the order Diptera or true flies, as they have a pair of wings and a pair of halteres (balancing / orienteering organs used when in flight). Several of the flies in the genus Eristalsis are known as Drone Flies (due to their resemblance to honey bee drones). The larvae of Eristalis species are commonly found in putrid / stagnant water and sometimes referred to as “rat-tailed maggots”. It was noted that pollinators also made use of the wild plants to supplement their diets, as strawberries alone cannot meet the nutritional needs of pollinators. ‘Elsanta’ strawberries have a relatively low sucrose and protein content in both their nectar and pollen. The precise order of importance of pollinators varied between farms. Bee (Apis and Bombus) species and hoverfly (Eristalis) emerged as key pollinators. The European drone fly was the most important pollinator at two of the three farms studied, evidence that hoverflies can be effective pollinators. One farm had commercial hives of the honey bee but they were less significant than the activities of of the hoverflies and bumblebees. The abundance of a particular insect, coupled with its active period were / are important determinants of pollinator importance. Sawdust and plastics - a possible use?. Plastics represent a relatively new, but persistent and major form of pollution (on land, in the sea, indeed everywhere). Whilst many plastic objects are instantly visible in the form of discarded bottles, fast food containers, many plastic pollutants are in the form of very small particles of plastics - nano and microplastics. The concern is that we and other organisms are taking these microscopic particles into our bodies from our food / drinking water. However, it is possible that plant materials may offer some ‘solutions’. Water that contains micro and nano plastics can be filtered through sawdust that has been treated with tannic acid. Tannic acid is large molecule, its molecular formula is C72H52O46 . Tannic Acid is found in certain plant galls (swelling of trees caused by parasitic wasps) and in the twigs of certain trees, such as Chestnut and Oak. The wood sawdust contains fibres of cellulose, combined with hemicelluloses and lignin. Water can flow through this material by capillary action. This plant-based filtration (known as bioCap) of plastic-laden water is capable of dealing with a wide range of nanoplastics (PVC, PET, polyethylene etc), and tests with mice suggest that the filtered water may be sufficiently free of plastic to pose little risk.
Trees – come in all shapes and sizes
Trees come in many shapes and sizes. Some are tall and thin, like Poplars, others have a ‘rounded’ canopy, like oak and horse chestnut. Sometimes we ‘persuade’ trees to assume a particular shape or form, perhaps through pollarding or coppicing - or something more extreme - like topiary or bonsai. However, sometimes nature itself has unusual or dramatic effects on trees. Wind can leave trees on cliff tops or exposed places distorted and growing almost horizontally along the direction of the prevailing wind. Occasionally, something very strange is seen. For example, at Gryfino in Western Poland, there is a forest with some very weird looking trees. There are about 400 trees that are bent at the base. At first, the trunk lies more or less parallel with the ground, then it bends upwards and the stem is erect. Consequently each trunk of these pines trees has a pronounced bend in it (see photo below). The rest of the trees in this forest are quite normal, growing upright and straight - like most pines.It is thought that the pines were planted back in the 1930’s though the local town was forsaken by the residents during the second world war (and only repopulated in relatively recent times). The trees are sometime referred to as the Crooked Forest. There has been much speculation as to how the trees came to be so mis-shapen. The theories run from The landing of alien space craft! This crushed / flattened the trees when young and tender The trees were damaged by German tanks during the war (but why only a select number of trees?) Genetic mutation(s) which resulted in abnormal growth Fungal infection(s) which resulted in abnormal growth The young trees were flattened by a heavy fall of snow, which perhaps persisted for some time. The trees were able to right themselves in the Spring, through a normal geotropic response. The trees were part of plantation / forest, in which some were deliberately cut at a young / sapling stage. The area was a tree farm, where some of the pines were cut / bent for later use in furniture or frames. By bending a young tree down to the ground in this manner (for some time), compression wood is formed. Such wood has higher lignin and lower cellulose content and it is stronger than wood that is bent after a straight tree is felled (for example, by a steaming process). Indeed, English ‘hedgerow oak’ was known to be the best for the curved timbers needed to internally strengthen a sailing ship. Trees were even deliberately bent in certain ways so as to " grow" a required set of curved timbers. Such curved timbers were known as “compass timbers”. In Gryfino, it is likely that the war interrupted the activities of local foresters / woodworkers, they left and these trees were left to grow on in their rather unusual form. Thanks to Kalasancjusz at Pixabay for the image of the crooked forest.
The short lives of many urban trees
The woodlands blog has reported on urban forests, the trees in our cities, lining our roads and in our gardens. This green infra-structure in our towns and cities provides a range of economic, environmental, and social benefits. The importance of green, leafy spaces was emphasised during the early days of the Covid pandemic, helping with mental and physical wellbeing of many people. Urban trees offer Valuable habitats for wildlife and can provide biological corridors / stepping stones that enable birds and other animals to move through the urban environment. Shade and cooling in streets and parks. They can help reduce the risk of flooding, allowing more water to enter the soil rather than running off hard surfaces of tarmac and concrete. The capture of pollutants, improving local air quality by capturing fine particles from the air (mainly through deposition on leaf surfaces). Trees and shrubs seem particularly effective in removing ozone. Through their photosynthetic capacity, trees can take up carbon dioxide into organic form - carbon sequestration. The amount of carbon taken up by London’s urban forest each year has been estimated at 77,200 tonnes. However, recent studies suggest that many urban trees are under threat : Trees are subject to heat stress as many cities experience the heat island effect, the ambient urban temperature is significantly above the surrounding countryside. Many struggle to get sufficient water as they are planted in small square of soil and surrounded by tarmac, concrete or paving stones. Soil compaction is often an issue, affecting water permeability. They may experience an ‘excess of nutrients’ - due to dog’s urine, this is a source of urea and other nitrogen compounds. Once planted, young trees may not receive after-care / management. This point is significant. Many trees die within the first few years of planting. In Boston (USA), some 40% of trees are dead within seven years of planting. Similar figures are true for New York. Both rural and urban trees suffer significant mortality when young but whereas the death rate of rural trees tend to decrease after a few years - urban trees are more likely to die as they age. [caption id="attachment_40541" align="alignleft" width="300"] Young urban tree[/caption] There is a struggle to reach maturity. Most trees need two or three decades to offset the carbon emissions associated with their planting / maintenance etc, and they then sequester carbon at a significant rate. Work at Boston University (in Professor Lucy Hutyra’s lab) and Harvard has focused on the problems that urban trees are facing, and another issue (apart from those mentioned above) has been identified - the microbiome of the root [that is the variety of micro-organisms that surround / inhabit the root tissues]. Urban trees seem to have fewer symbiotic fungi in their root systems when compared to rural trees. Roots often develop mycorrhizal associations with fungi. Such systems allow the roots to access more water / minerals and in return the tree ‘offers’ the fungal network a supply of carbohydrates. Jenny Bhatnagar (Harvard) has investigated the soil microbiome in eight different plots, some urban and some rural in Massachusetts. Interestingly, the investigation found that whilst there were more fungi in urban plots, they ‘seemed more reluctant’ to establish symbiotic associations with the roots of the trees. This failure could be due to the excess nitrogen / nitrates in the soil (from animal urine / faeces?). When there is an excess of nitrogen available, trees tend to dispense with their fungal partners. The hotter temperature of urban soils might 'favour' a bacterial population (some bacteria ‘fix’ nitrogen). [caption id="attachment_40526" align="alignright" width="300"] Once, there was a cherry tree ...[/caption] It is not clear as yet why so many urban trees fail. It could be that the loss of the symbiotic fungi renders the trees more susceptible to certain disease-causing microbes. The hotter and drier soils at the edges of fragmented forests have more pathogens and not so many symbiotic fungi. A number of simple aftercare / management measures would help young trees to establish : Watering the trees in their early years Preventing soil compaction to allow water to percolate, and oxygen to diffuse to the roots. Mulching around the tree base (helps water availability and slows nutrient input from urine etc.) An interesting article on mycorrhizae and urban trees may be found here. [caption id="attachment_40537" align="alignleft" width="220"] Olive[/caption] The importance of soil micro-organisms is also indicated by research in Australia, where shrublands / woodlands have been invaded by African olive trees. The olives have disrupted the partnerships between the Acacia trees (hickory wattle) and symbiotic soil bacteria (Rhizobia ssp). This is another symbiotic association, where the partners exchange materials for mutual benefit, Where the Olives have grown, the Acacia have problems establishing root nodules with the bacteria. To restore these scrublands, a full understanding of the soil / root microbiome will be important. Full details of this work can be found here. Postscript : In today’s Guardian (03/11/2023), Helena Horton’s article “Ministers should target tree survival ‘rather than planting’” reinforces the points made in the blog about the early mortality of young trees - urban or rural. Increasing woodland cover will only occur if young saplings survive.
Plastics and tree guards
Plastic is a problem, plastic is universal. A class from Ramsbury Primary School went on a walk round their village, looking for signs of plastic pollution. When they looked in the hedgerows (lining the paths and fields), they found old plastic tree guards (and hedge guards). Some were breaking up into pieces, some growing growing into the bark of the trees. In addition, there were plastic bottles, face masks, dog poo bags, sweet wrappers, plastic ropes, plastic bags, and plastic wrappers from hay bales. Plastic litters our world. Each year, hundreds of million tonnes are produced. It is used but often it is not recycled - it is discarded. It litters the land, rivers and oceans. It is now almost impossible to walk in the countryside or on a beach without encountering plastic in one form or another. Discarded plastic can kill or injure. Mammals, reptiles, birds can be harmed through eating plastic or becoming entangled in it. Plastics are made up of repeating units (monomers) that join together to form long chains (polymers). There are six major polymer types, PET, HDPE, PVC, LDPE, PP and PS. Many are derived from petrochemicals. Additives are incorporated into plastics and these can gradually leach back out either during normal use, or when in landfills, or following improper disposal in the environment. Whilst plastics serve many different functions, their makeup means that they do not easily break down, they persist. Consequently, a lot of plastic goes to landfill or it may be burnt (to generate energy) - which in turn can release greenhouse gases and pollutants. Ideally plastics would be reused, like glass bottles were recycled in the dairy industry for over a century. Polyethylene is used widely for plastic bottles and food packaging, PVC is used to make pipes (for water / sewage), coating for electrical cables, uPVC windows and fascia boards. Recycled PVC can be used to make certain types of tree guards, for example :Spiral guards. Such guards offer protection to young trees and hedgerow so that they can establish themselves, avoiding being chomped by rabbits, deer or sheep. The guards also offer a micro-climate that helps growth. UV stabilised polyethylene is used to make netting / mesh to protect young trees. [caption id="attachment_34477" align="alignright" width="300"] Tree guards, to protect young trees on moorland[/caption] Tree failure can be an expensive process, so it is important to give young trees a ‘good start in life. A ‘weed’ free area around the planted tree reduces competition for water, light etc. In theory, it should be possible to reuse plastic guards, but they are often damaged, degraded or have to be cut to remove them from the young tree. As they are not biodegradable, it is important that they are collected and removed. Ideally this material should be recycled. If many trees are being planted, it may be simpler / more cost effective to fence off the planted area to protect young trees from browsing activity. Because of the problems associated with plastic tree guards, there are now a number of alternatives available. For example, wool-based tree guards / shelters (eg. Next Gen) are fully biodegradable being made from wool A biodegradable polyol made from ethically sourced cashew nutshell liquid and castor oil A polymer that breaks down over time Other biodegradable forms of tree protection make use of a polymer made from sugar cane (eg. HyTex products). Such guards decompose slowly through the action of microbes (bacteria and fungi), temperature and humidity, gradually forming a compost - so their removal is not needed.
Bees, agrochemicals and the microbiome
Mason bees and agrochemicals The blog has reported many times on the threats to bees - money bees, bumblebees and ‘wild bees, such as mason bees / solitary bees. The threat to bees from neonicotinoids has been well documented, now there is a report that suggests that certain other agrochemicals may be harmful to bees. Researchers at the Julius Maximilians University at Würburg have been investigation the effect of a fungicide (Fenbuconazole) on the reproductive behaviour of horned mason bees (Osmia cornuta). A number of Osmia species are used to improve pollination in fruit and nut crops. They are efficient pollinators having a special pollen collecting / carrying structure called a scopa. Mason bees are solitary bees. Each female is fertile and makes her own nest and no worker bees for these species exist. In the Spring, male and female bees emerge from a nest. The males generally exit first and remain near the nest, ready to mate with the females. A female bee selects a mate on their ‘smell’ / odour and the ‘quality’ of their thoracic vibrations (achieved through muscle contractions). After mating the males soon die. The females search for and select a nest site, visiting flowers to collect pollen and nectar for their nests. Once a certain amount of food has been collected within the nest, the females lay their eggs on top of this material (in a series of cells) and then seal off the nest. The eggs hatch to form larvae which feed upon the food and within weeks forms a cocoon, in which it continues to develop to an adult. Though the fungicide (Fenbuconazole) is considered to be of low toxicity and the bees were exposed to a sub-lethal dose, nevertheless the Fenbuconazole had significant effects on the bees. Pesticide exposed males were more likely to rejected by the females, compared to ‘control’ bees that were not exposed to the fungicide. The thoracic vibrations of the exposed males were less powerful / noticeable and the composition of their odour or smell was different. The smell of the bees is dependent on particular hydrocarbon compounds in their cuticle - their exoskeleton. It is possible, therefore, that the mating behaviour and reproductive success of these bees is being affected by agrochemicals. Carpenter bees. The microbiome refers to the collection of micro-organisms that lives on or in us, particularly within within the gut. Whilst these micro-organisms are small, they contribute to our health and ‘well being’. They offer protection against pathogens, help our immune system develop, and enable us to digest. Just as we have a microbiome so do bees. Scientists as York University (Canada) have been investigating the microbiome of three species of carpenter bees (from North America, Asia and Australia). The term "carpenter bee" comes from their nesting behaviour, most species burrow into plant material such as dead wood or stems, though a few create tunnels in soil. Social bees (like honeybees and bumblebees) acquire their microbiome by interacting with their hive or nest ‘mates’. Solitary bees, like the carpenter bees, get their microbiome from the environment as they forage for food. The researchers found that: The bees’ microbiome contained Lactobacilli, which are important for good gut health, helping protect against fungal pathogens and facilitating nutrient uptake. They also discovered crop pathogens in the microbiomes of the carpenter bees which were previously only found in honeybees. Whilst these pathogens are not necessarily harmful, it is possible that the wild bees could be vectors for spreading disease. With thanks to Pixabay (Umsiedlungen and Sabinem34) for the above images of bees Finding flowers. Research at the University of Exeter has shown that bees can distinguish between various flowers through a combination of colour and pattern. This selectivity is achieved despite the ‘acuity’ of a bee’s vision being quite low (about a 100 times lower than ours) - this means they can only see the pattern of a flower when they are quite close (a matter of centimetres). The researchers analysed a significant amount of data on plants and visiting bee behaviour, and they used experiments involving artificial shapes and colours. One particular finding was the importance of the contrast between the outside of the flower and the plant’s foliage. This seemed to help beesfind their way to the flowers quickly .
Trees and the vagaries of climate.
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.
After-effects of forest fires.
In 2018, the blog reported on the extensive fires in Sweden, a country noted for its forests and woodlands, which cover approximately half of the country. Once the trees were mainly broad leaved species, but then oaks and alders began to decline. By the middle of the twentieth century, Spruces and Pines were dominant. This was mainly due to forestry management, to produce wood for fuel, charcoal [used in iron smelting], potash, tar and timber (for building). Fires burnt from the extreme north down to Malmo in the south. These fires affected some 20,000 hectares and destroyed woodlands valued at [circa] £50 million. Now work by scientists at Uppsala University, the Swedish University of Agricultural Sciences (SLU), and the Swedish Meteorological and Hydrological Institute (SMHI) have examined the effects of the fires (of 2014) in the Vastmänland province, where the fires were ferocious, burning down into the soils. They have found that the 'forested areas' continued to lose carbon for several years after the fire, and that nitrate and phosphate input to streams and rivers increased after the fires. This spring and summer have again witnessed intense and widespread fires across the Mediterranean region, Canada and the United States. Fires are a problem not only because of their immediate destructive potential, but because they result in the release of carbon dioxide - which further contributes to global warming and climate change. The United Nations Secretary-General said recently “The era of global warming has ended; the era of global boiling has arrived.” Data on these fires is not available as yet, but studies of the boreal fires in 2021 suggest those fires released some 1.76 billion tonnes of carbon dioxide into the atmosphere. The fires contributed nearly one quarter of world wide carbon dioxide emissions from fires in that year. [caption id="attachment_35352" align="aligncenter" width="650"] Woodland recovering from a fire[/caption] Boreal forests store roughly twice as much carbon in their trees and soil as tropical forests. These forests (often referred to as the Taiga) surround the Arctic Circle and research suggests the Taiga is warming faster than the global average, so areas like Northern Canada and Siberia now experience more heat and drought than in the past, and consequently are more likely to suffer from fires. Clearly, when there is a fire, carbon dioxide (and many other carbon compounds eg soot / small particles) are released by the burning of the trees but there is also the effect of fire on the soil and its organic content - the humus. Research indicates that during the fires in the boreal area some 150 tonnes of carbon dioxide may be released into the atmosphere per hectare. Furthermore, even after the fire, carbon continues to be lost from the soil. It may take some three years for carbon uptake by the soil to be recorded. Fires also lead to the rapid loss (leaching) of nutrients (e.g. phosphate) to local lakes and rivers - as there is little or no vegetation to absorb the nutrients. Rainfall is not intercepted by vegetation and so the flow of streams increases ( sometimes by 50%). A research paper produced by the Desert Research Institute (in Nevada) has indicated that smoke from the burning of pines has the effect of making soil particles more water-repellent. This repellency of smoke-affected soil particles could help explain the increased flooding, erosion, and surface runoff in fire damaged areas.
A sense of touch in plants.
Climbing plants like sweet peas can ‘feel’ their way around a support, a twig or fencing. Charles Darwin, who had an interest in climbing plants, described Clematis as a leaf climber. Clematis has compound leaves with three to five leaflets, It uses the stalks of the leaves or the leaflets to climb. Darwin noted that contact with another structure was enough for the leaf stalk / petiole to start bending around it. This ability to respond to touch is termed thigmotropism. It is a growth response. The growth rate on the side of the stem that touches the 'support' is slower than on the side opposite the point of touch. As a result the stem begins to curl around the support. The same response is seen in plants that climb using tendrils, such as White Bryony. Its tendrils are thin, wiry structures along the stem that ‘reach out’ into the space around a plant until they come into contact with something they can ‘grab’. Once contact is made, the tendril curls, forming a coil. This sense of ‘touch’ has recently been investigated using Thale or Mouse Ear Cress. It has been shown that the veins of the leaves respond to touch. The investigators used small glass beads to apply a small but distinct pressure, and recorded a series of rapid electrical signals (not dissimilar to those seen in nerves). Even when the veins were removed from the surrounding leaf tissue, they still showed electrical activity so the response was not reliant on surrounding cells. The electrical activity was also associated with proton pumps (moving hydrogen ions). The sensitivity of the veins to touch may be associated with the plant’s defence mechanism. Animals like aphids use their mouthparts (stylets) to penetrate the vascular tissue in the veins. The jasmonate system is involved in wound response. Another plant, the Venus fly trap (Dionaea muscipula), also responds to touch. It catches prey (insects and spiders) by means of touch. Dionaea catches its prey with a trap, formed from the terminal portion of each of the plant's leaves. The trap is activated by tiny hairs on the inner surfaces of the trap. When a hair is touched by an insect or spider crawling along the leaves, the trap prepares to close but it only snaps shut if the hairs are touched again, within approximately twenty seconds of the first stimulus.