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.  

In 1820, John Keats famous poem “To Autumn’ was published.  Its opening line often serves to describe autumn as ‘The season of mists and mellow fruitfulness’.  Indeed, Autumn has arrived here.  Woodlands have been transformed into a 'tableau' of red, yellows, and oranges as the leaves are shed and the woodland floor has become a veritable fungal jungle, (as Jasper has described).  Hawthorn and other bushes are laden with berries, conkers and acorns are generously strewn across woodland floors, squirrels are eating hazelnuts (and hiding them as a winter food store).  But this cornucopia of fruits and seeds may be in response to the long, hot and very dry summer we have experienced.  Trees and shrubs have been stressed by the heat and drought. Some have responded by mobilising their reserves / efforts into producing more fruits and seeds, to ensure that they pass on their genes to the next generation.   Different trees are responding in other ways. Some are ‘holding on’ to their leaves for longer, whilst others (like some birches and rowans) have already shed theirs - in order to limit the damage from wilting and water loss during the intense heat and drought of summer.  [caption id="attachment_39130" align="aligncenter" width="675"] Autumnal colour[/caption] Blackberries appeared early this Summer and few are to be seen this Autumn, some animals (like the dormouse) will search in vain. [caption id="attachment_39026" align="aligncenter" width="675"] Black Bryony berries (photo by Art Symons)[/caption] Will this Autumn be long and mild? It may extend into the traditional winter months. There is also the threat of extreme weather events (like Storm Arwen that brought severe winds across the UK last November).  Whilst we might welcome mild weather (with the high price of gas and electricity), it could be mean an increase in bacterial and fungal infections, not only of plants but also affecting overwintering insects - tucked up in cocoons and pupal cases. Prolonged cold periods, coupled with frosts limit the spread of such infections.  The cold of winter is also the traditional signal for animals like hedgehogs to hibernate.  If they are still active in winter when food is scarce, then they will lose condition and possibly starve to death.  Milder weather in Autumn and Winter also affects the migratory behaviour of birds, some birds may choose to overwinter here.  Some seeds need to experience cold temperatures before they will germinate in the following Spring. Climate change and severe weather events are here to stay until there is a coordinated and concerted effort to reduce greenhouse emissions on a global scale.  On a local scale, our gardens (which represent some 400,000 hectares of land) can make a contribution by encouraging wildlife / pollinators and promoting biodiversity.  Gardens can also help to some extent with extreme rainfall.  During heavy rain, water runs off hard surfaces and into the drains, these may also deal with sewage.  When the drains are overwhelmed by storm water, sewage is discharged into our rivers (and the sea).   Gardens can help by  reducing hard landscaping, so that rain can soak into the soil instead of running off into the drains making use of water butts.  They capture water, so it does not enter the drainage system.  It is also there to use when the garden needs water (and there is a hose pipe ban) mulching the soil with a layer of plant material. It is an effective method of conserving water in the soil and it reduces surface runoff, increasing the infiltration / penetration of water into the soil.  It helps keep the soil moist in times of drought, and helps reduce run off during heavy rainfall.  Particularly important when planting young trees / shrubs. [caption id="attachment_38957" align="aligncenter" width="675"] a light mulch (in Art's garden)[/caption]