Blog - Climate Change
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.
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.
Losing woodlands and forests.
Across the world forests and woodlands are under threat, suffering fragmentation and shrinkage. This is not good news for the plants and animals that rely on these habitats for their survival. In a large forest or woodland, animals can move around over considerable distances in search of food / partners - without having to leave the area that supports them. Similarly, plant seeds when dispersed are more likely to find the micro-climate that they need for germination and subsequent growth (humidity, shade, soil type etc). Some species have very ‘exacting’ requirements that can only be met in the heart of a forest or woodland. For example, there is a frog that is restricted to undisturbed mountainous forests in Borneo. Sadly, recent surveys suggest that many forests continue to suffer from fragmentation / loss of area. The two main reasons for this loss are : clearance for agriculture (palm oil plantations etc) - this has mainly affected tropical and sub tropical area. Sometimes fires are used as a deliberate ‘tool’ to clear an area of forest so that the area can then be used for agriculture. wild fires - these have affected the Boreal Forests but also regions of the Amazon Basin. In areas like Siberia and Canada, drought and high temperatures have lead to extensive fires. (Zombie fires are underground peat fires that smoulder in the winter months but reignite when the ground dries in the Spring or Summer.) Recent times have seen extensive fires across Siberia, Canada, parts of the West Coast of America and Australia. Woodlands have experienced fragmentation due to the expansion of agriculture, the building of motorways & roads, and the expansion of housing. Wild flower meadows have suffered even more dramatically - with some 90+% lost in relatively recent times. Obviously fires are devastating locally, killing vast numbers of animals and plants. Fire also destroys the organic content of the soil and its complex microbial population. The plumes of smoke released by fires (such as those seen in Canada and Australia) spread extensively. The Canadian fires (883 fires raging at one point) left mile after mile of blackened forest, and forced hundreds of people from their homes. The smoke spread far beyond Canada’s borders, as far away as parts of Europe. New York City was ‘bathed’ in an ‘orange haze’ and experienced a hazardous level of air pollution. The plumes from such fires are rich in black carbon soot. The soot particles absorbs solar radiation, keeping heat in the atmosphere. Recent analysis of smoke plumes indicates that there is also ‘dark brown carbon’. This consists of a previously unknown type of particle and whilst these particles absorb less light per particle than black carbon, they are approximately four times as many brown carbon particles in wildfire smoke (compared to black soot particles). There is also the suggestion that these brown particles retain capacity to absorb solar radiation for longer.
The DiversiTree Project and Woodland Diversity
Rapid onset climate change, and the spread of new pests and diseases are creating unprecedented challenges to the long-term survivability of UK woodlands. This looming threat is becoming ever more tangible, and the need for strategies of resilience building is urgent. Promoting diversification within and amongst woodlands has been identified as one such strategy with the potential for significant, positive impact. DiversiTree is a UKRI-funded project led by the James Hutton Institute which is measuring the impact a more diverse mixture of tree species has on building resilient woodland ecosystems, as well as how woodland managers and others understand woodland diversity, and what they are CURRENTLY doing to promote resilient woodlands. The project also hopes to generate practical advice and results which managers can use to make better informed decisions regarding the species mix of their woodlands, especially with regard to conifers. A key question which often accompanies discussions of woodland diversity is the planting of non-native species within British woodlands. The DiversiTree project is taking an evidence-focussed approach to its assessment and are investigating how ecological resilience interacts with woodlands with different priorities or objectives and what this might mean for the longer-term ecological sustainability of the forests of the UK. In actuality, many native woodlands are rather species poor, and could potentially benefit from a period of managed diversification with native species, non-natives, or a mixture depending on local objectives and context. What is critical here, is understanding the ecological role ANY tree can serve in a complex landscape, and planting in a manner which enhances and strengthens a woodland’s biodiversity. If you’d like to learn more about our work and keep updated with our progress, please follow us on Twitter @DiversiTree_UK (https://twitter.com/DiversiTree_UK?s=20) or email [email protected] with any questions. Seumas Bates (Environmental Anthropologist, Bangor University)
Where do butterflies come from?
An obvious answer to this question would be - caterpillars. But when did butterflies first appear? There are now some 160,000 species of moths and butterflies -worldwide. Seemingly, they appeared some 100 million years ago - in North America. They evolved from nocturnal moths in the period when flowering plants were undergoing a major expansion (in the Cretaceous period). Butterflies may have become diurnal to avoid predation by bats, or it may have been to take advantage of nectar production and availability [using the proboscis]. The butterflies and their caterpillars were able exploit the diverse range of food resources that these ‘new’ plants offered. Butterflies moved out from North America to South America and then on to other parts of the world, though they probably did not arrive in Europe until some 17 million years ago. The evolutionary expansion of the butterflies has been investigated by researchers at the University of Florida; they analysed the genetic makeup of many species (from 90 countries). They were able to build up a picture of the relationships between the various groups of butterflies and also determined their evolutionary point of origin. They also catalogued the plants eaten by the caterpillars and it was found that some two thirds of butterfly caterpillars feed on plants from the legume family (the Fabaceae - peas and beans). It is probable that the first butterfly caterpillars also fed on these plants. Research at the Georgetown University in Washington DC suggests that larger species of butterfly are ‘coping’ better with higher temperatures, associated with global warming. Bigger wings seem to offer a greater range of movement and the opportunity to find new and suitable habitats. Smaller butterflies are not faring so well. The study involved an analysis of the range of some 90 North American species between 1970 and 2010, during which period the monthly minimum temperature increased by 1.5oF. Others have analysed the butterfly collections at the Natural History Museum, using digital technology. The Natural History Museum’s British and Irish butterfly (and moth) collection is probably the oldest, largest, and most diverse of its kind in the world; some of the specimens date back over a hundred years The measurements of the various specimens were paired with the temperature that the species would have experienced in its caterpillar stage. It was found that for several species that the adult butterfly size increased as the temperature increased (during late larval stage). So, it may be that we will see a gradual increase in butterfly size as temperatures increase with global warming. Join the Big Butterfly Count ? Between Friday 14th July and Sunday 6th August , the big butterfly count will take place. For full details visit : https://bigbutterflycount.butterfly-conservation.org/about Thanks to Angus for images.
Bumblebee pollen collecting
Bumblebees (and bees) collect nectar and pollen. Pollen is a vital food, used in the various stages of a bumblebee’s life. In Spring, newly emerged queens feed on pollen, then it is used to feed its their sister workers. The workers, in turn, take over the feeding of the colony (the larvae and future queens). If not enough pollen is collected, then the colony will not thrive, which can have significant long term effects. Bumblebees are already facing many threats (from habitat fragmentation, agrochemicals and disease). The collection of pollen is a demanding process, and bumblebees will forage over a wide area. They start their pollen collecting activities earlier than many insects as they can warm themselves up by ‘shivering’, that is, rapid muscle contractions which generate heat, warming the insects up ready for flight. Bumblebees can fly in colder conditions and at higher elevations than many other insects. However, research at North Carolina State University has shown that the North American bumblebee (Bombus impatiens) can overheat when exposed to high temperatures (circa 42oC plus). So, if a bee is carrying a significant load of pollen and it is a hot day, its muscles have to work harder and the bee is at risk of overheating. A bumblebee loaded with pollen may be 2oC hotter than an unladen bee; it may be reaching its ‘thermal limit’ - a temperature at which its organs are damaged. Climate change means that many parts of the world are now experiencing extreme weather events, when temperatures can reach into the forties. [caption id="attachment_39978" align="aligncenter" width="675"] Bumblee leaving foxglove[/caption] Increasing temperatures could affect the foraging activities of bumblebees in a significant way - affecting how much pollen is collected and how much pollination takes place. If pollen collection is reduced then colony development is affected and so population numbers will be affected. Bumblebees are key pollinators in natural and agricultural systems, and if their numbers decline there will be ecological and agricultural consequences.
According to the United Nations, a forest is anywhere that is at least 20% trees. As 21% of our capital city, London, lies under the canopy of trees - it is an urban forest*. It is estimated that there are some 8 million plus trees - nearly as many trees as people. London is not alone, Johannesburg is a densely wooded city with some 6 million trees, planted throughout the streets and private properties. Tree Cities of the World is a programme that recognises cities and towns committed to ensuring that their urban forests and trees are properly maintained and sustainably managed. Urban environments can create difficult conditions for tree growth and development. The trees may be exposed to pollutants, high temperatures (heat island effect), drought and/or flooding, and challenging conditions for growth. . Whilst trees may be planted, their subsequent care / nurturing may be limited due to insufficient resources (money / care etc). There needs to be long term maintenance to sustain not just healthy trees but also to make sure that the trees do not damage pavements / roads etc (for example, through root penetration). Trees for Streets is a new national tree sponsorship scheme that some councils have partnered with, which gives local residents the chance to have a tree near them or in a local park. It is a project run by the charity Trees for Cities which aims to support local communities in revitalising forgotten spaces, planting trees and improving the local environment. [caption id="attachment_39418" align="aligncenter" width="675"] Greenery in SE London. View towards St.Helier's hospital.[/caption] In the past, London was a much smaller city surrounded by countryside and woodland, but there are still areas of ancient woodland within it. Some of this woodland remains such as the Great North Wood in South London (hence Norwood and Forest Hill). Other place names - Wood Green, Forest Gate, Nine Elms and Burnt Oak bear witness to the wooded landscape that was once prevalent across London. In fact, some 8% of London’s area is still woodland, and some of it is even defined as ancient woodland (e.g. Epping Forest). [caption id="attachment_39421" align="aligncenter" width="675"] Dulwich Park[/caption] There are also the many parks of London - Hyde Park, Regent’s Park, Richmond Park, Dulwich Park etc. Add to these the trees found in school fields, private gardens, squares (like Berkeley and Portman Squares), plus the trees that line so many streets (estimated at 900,000). Trees (like sycamore and buddleia) have also colonised areas of the built environment, like railway lines / cuttings. The most common London trees are sycamore (7.8%), oaks (7.3%) and birch (6.2%). However, the urban forest has a wide spectrum of species that includes native species, such as ash, hawthorn, hornbeam, field maple and holly, but there is a wide variety of exotics and cultivars in parks, streets and private gardens. In some parts of the capital, the London Plane is a noticeable presence, due to its resistance to pollution and tolerance of root compaction. It sheds 'large flakes' or sections of its bark exposing new material of a variety of colours (brown, grey, yellow), and is sometimes described as ‘self cleaning’. The London Plane is thought to be a hybrid of the American sycamore and Oriental plane. So the urban forest is quite diverse in terms of species when viewed across the capital, but there are parts of the city where species diversity is poor and the age profile of the trees is sometimes limited. This homogeneity can favour pests and disease. Diversity generally favours to resilience. Currently, trees face diseases such as acute oak decline, Chalara ash dieback, horse chestnut leaf miner, Massaria disease of plane and oak processionary moth. London’s urban forest faces an increasing human population and the challenges of climate change. The latter may bring substantial warming and changing rainfall patterns. Wetter, milder winters and drier, hotter summers may be more common in the coming decades. Some trees will be better able to cope with these changing conditions. Future planting will have to follow the maxim of “right tree, right place”. The value of London’s forest is difficult to quantify or to put a figure on. It is a major part of the ‘green infrastructure’ – that is the matrix of green spaces, parks, recreation grounds, lakes, canals, and rivers plus the street trees , green roofs and allotments that provides a range of economic, environmental, and social benefits. The importance of green, leafy spaces came to the fore during the early days of the Covid pandemic, helping with mental and physical wellbeing of Londoners. [caption id="attachment_27166" align="alignleft" width="300"] Mature oak in park.[/caption] The components of the forest offer valuable habitats for wildlife and also provide biological corridors / stepping stones that enable birds and various animals to move through the urban environment. The ancient woodlands and veteran trees offer a home to a variety of wildlife such as bats, stag beetles, orchids etc. In recent heatwaves, people have appreciated that trees also provide shade and cooling in streets and parks. Another aspect of extreme weather is very heavy rainfall, trees and green areas can help reduce the risk of flooding, allowing more water to enter the soil rather than running off hard surfaces of tarmac and concrete. Trees also help capture pollutants, improving local air quality by capturing fine particles from the air (much of this is through deposition on leaf surfaces). One source suggest that trees remove some 2241 tonnes of pollutants each year. Trees and shrubs seem particularly effective in removing ozone, and through its photosynthetic capacity the urban forest can take up carbon dioxide into organic form. The amount of carbon taken up by London’s urban forest each year has been estimated at 77,200 tonnes. To maintain and augment this urban forest, it is important in the coming years that the threats of pests and diseases are fully assessed and controlled The threats arising from climate change are recognised / mitigated Woodlands are properly managed (eg. coppicing); this may include the training of personnel. Create opportunities for planting of trees, hedgerows and woodland. [caption id="attachment_39422" align="aligncenter" width="675"] Tree nursery - 'ready for planting'.[/caption] * https://cdn.forestresearch.gov.uk/2022/04/21_0024_Leaflet-CC-factsheet-Urban-forests_wip06_Acc.pdf
Some years back, the Woodlands blog posted various articles about hedgerows, noting the loss of many - due to the increased mechanisation of farming in the mid C20th. Now, there is greater recognition of the importance of hedgerows, and there are initiatives to promote the maintenance and expansion of hedgerows. But what is a hedgerow? Natural England offers a definition as follows : A hedgerow is defined as any boundary line of trees or shrubs over 20m long and less than 5m wide, and where any gaps between the trees or shrub species are less that 20m wide (Bickmore, 2002). Any bank, wall, ditch or tree within 2m of the centre of the hedgerow is considered to be part of the hedgerow habitat, as is the herbaceous vegetation within 2m of the centre of the hedgerow. This differs from the definition in the Biodiversity Action Plan, which included references to ancient hedges / species-rich hedges. The definition now includes all hedgerows consisting of at least one native woody species of tree or shrub (mainly), but it does exclude bramble and honeysuckle as ‘woody species’. According to one source, there are some 550,000km of hedgerow in England, with over 400,000 km being actively managed. Hedgerows are an important semi-natural habitat in what is otherwise a managed agricultural landscape. They are found across the country but there are more in lowland regions. Hedgerows in the south east are associated with large fields and fewer trees, the proportion of trees in hedgerows increases as one goes north and west. The nature of hedgerows varies across the country but all are important as : They provide a habitat, shelter (micro-climate provision) and resources for many different species (from plants to insects, birds and mammals). Hedgerows are particularly important as nesting sites for birds. They support animals that have key roles within the broader ecosystem, for example pollinators and predators of pests. They offer an important source of nectar that helps support wild bees - adjacent farmland can be a poor source of nectar Hedgerows are known to support threatened (red listed) species Hedgerows capture and store carbon (above and below ground) Hedgerows offer ecosystem services eg. mitigation of water flux and availability, landscape connectivity, soil conservation / stabilisation. A number of studies indicate that increasing the number of hedgerows would help with landscape connectivity (for example, for hedgehogs) and that planting of blackthorn and hawthorn in association with later flowering species would help support a number of wild bee species. Expanding the number of hedgerows could have some negative effects as they might offer a home to invasive species and / or pathogens; but one study has indicated that ash trees in hedgerows suffer less impact from ash dieback than trees in forests. To date there does not appear to be any detailed research on whether increasing hedgerow coverage would have any impact on tree disease / pathogen spread. Hedgerows, like woodlands themselves, face a number of challenges due to climate change. Warmer winters may mean that the ‘winter chill’ requirements of some shrubs / trees will not be met; this may mean flowers and fruits fail to form properly which in turn means less food for birds, small mammals etc. Drier summers may stress some species, trees like Beech are susceptible to drought. Extreme weather events (like Storm Arwen) can inflict damage on hedgerow trees. If a hedgerow is next to farmland, then it may experience drift from pesticide and / or herbicide spraying nutrient enrichment (eutrophication) due to the use of fertilisers. Hedgerows with a diverse structure, with plants, shrubs and trees of varying ages and heights provide the widest range of niches / microhabitats for wildlife. The inclusion of dead / decaying wood offers opportunities for various fungi, saproxylic beetles, woodlice etc. Some hedgerows are managed / reduced with a mechanical flail (see above !!!). If this is done annually, it can result in a loss of biodiversity. Trimming should be done on a 2 or 3 year cycle; and some sections of the hedge might be left for longer " see (https://www.hedgelink.org.uk/cms/cms_content/files/76_ne_hedgecutting.pdf). Thousands of tree and hedgerow plants are being planted to create a flood defence project at Castlehill, East Hull. The plan is to create some seven hectares of woodland and over five kilometres of new hedgerow, as part of a flood defence project (to store floodwater east of the city). Trees such as field maple, downy birch, English oak, and black alder are being planted along with species of willow, dog rose, guelder rose and blackthorn and hawthorn to create hedgerows and scrubland. Other species will be allowed to naturally develop in the area and the habitat is expected to reach ‘maturity in some fifteen to twenty years. There is a citizen science project that involves surveying hedgerows. It is organised by the People’s Trust for Endangered Species [PTES]. The Great British Hedgerow Survey guidelines can be found here : https://hedgerowsurvey.ptes.org/survey-guidelines Some times hedges offer a home to other things