In the southern parts of Britain, beech is a dominant woodland/forest tree, further north, oak is prominent.  Beech trees are often large with smooth, silvery grey bark.  They can grow to a height of 150 feet, with a stout trunk (perhaps 10 feet in diameter) and an impressive canopy. The leaves, certainly on younger trees, may persist throughout winter in a brown and withered state — a phenomenon known as marcescence.  The root system of the beech is shallow but extensive.  The large roots spread out in all directions, and establish mycorrhizal connections, often with fungi such as Russula and Laccaria.  The mycorrhizae help the trees by supplying mineral nutrients (like phosphate) and water.  In return, the trees provide various organic nutrients to the fungus. Despite these associations, beech trees are susceptible to drought.  After the drought of the summer of 1976, many beech trees died. It is not surprising that people are concerned about the ‘health’ of beech trees in light of climate change — higher temperatures, extreme weather,  specifically periods of drought.  It was thought that climate change would reduce growth of trees like beech through the increasing frequency and intensity of summer droughts. Recently, a study conducted by researchers at the University of Liverpool looked at tree growth data (annual growth ring and masting data) accumulated over more than forty  years and found that growth was indeed reduced (by some 28%).   However, the reason was that the trees were investing more energy into reproduction than into growth.  Beech trees are known for their mast years - see previous blog on masting. In a mast year, a tree will produce enormous quantities of seeds (beech nuts✝︎). However, it seems that the changing climate is causing a ‘breakdown’ in the masting process, and whilst the trees now reproduce more more frequently.  Total seed production and seed viability is reduced.   It may be that the diminished reproductive capacity of beech trees as a result of climate change will affect their ability to regenerate woodlands and forests in the UK and indeed across Europe in the coming years.   [caption id="attachment_41997" align="aligncenter" width="675"] Marcescence[/caption] ✝︎ Masting means that so many seeds that even the most voracious squirrels cannot consume all of them * After the summer of 1976, drought damaged trees were still dying some 15 years later.

City temperatures are rising across the Earth.  Heat stress causes illness and even death. Between March and June last year, India experienced record-breaking temperatures, with 37 cities surpassing 45oC. Hundreds of people died from heat death and over 40,000 heatstroke cases in 17 states were recorded.  Increasing urban temperatures also mean greater use of air conditioning (and associated energy use).  In an attempt to mitigate these rising temperatures, many cities have implemented tree planting programmes, e.g. One Million Tree campaigns. During the day, trees cool cities by: Blocking the sunlight from reaching certain areas The loss of water vapour (transpiration) via their stomates - tiny pores on the underside of leaves Changing the flow of air in and around trees and buildings. These are the positive aspects of trees in cities, but there can be a downside to their presence. The heat build-up  stored in roads and buildings during the day can irradiate back into the atmosphere at night, but it can be ‘trapped’ by a dense leaf tree canopy. Cambridge researchers have now analysed the results of some 182 recent studies of trees in different cities and countries / climates, and have reported that : Urban trees can result in an air temperature reduction of some 12oC as experienced at the pedestrian level Trees reduced the peak monthly temperatures in most of the cities studied The cooling effect varied significantly, according to the tree species present, the urban layout and the climate the trees experience. Seemingly city trees exert a greater cooling effect in hot and dry climates, and less in hot and humid climates.  For example, in tropical rainforest climates [which experience very high humidity] then the day time cooling effect may be as low as 2oC, and the warming effect at night +0.8oC.  The study also noted that cities with an ‘open’ urban layout with a mix of evergreen and deciduous trees (of different sizes) were more likely to experience a good cooling effect.  However, in cities with a dense or compact layout, such as Cairo or Dubai, planting mainly evergreen species was found to be more effective. The study concludes that whilst urban planners need to provide more green space in cities,  they must also carefully consider the ‘mix’ of trees that are planted.  The study also emphasises that trees alone will not provide a solution to rising temperatures, but the use of solar shading and reflective materials will play a part, as will the nature and structure of the urban layout. Full details of this study can be found here :  https://www.nature.com/articles/s43247-024-01908-4  

The UN Biodiversity meeting COP16 has announced that 38% of the earth’s trees are facing extinction in ‘the wild’.  The biggest threats to trees are Clearing land / forests for farming Fires and climate change Logging tree for timber The spread of pests The spread of disease One example of a tree under threat is the monkey puzzle tree.  In Chile, climate change has resulted in the country’s Araucanía (Monkey Puzzle) region becoming hotter and drier, and as a result forest fires have become more frequent. It is thought that a million of these trees have been lost in a recent fire in the National Park, and many of these trees were mature specimens - hundreds of years old. Apart from their intrinsic beauty, trees, woodlands and forests are important.  They Absorb carbon dioxide from the atmosphere, helping mitigate global warming Produce oxygen vital for us (for respiration) and so many living organisms on this planet Provides ‘homes’ / habitats for many hundreds of species of birds, mammals, insects, arachnids, lichens, mosses and other epiphytes. Help offset some of the effects of pollution. If the trees are lost, then many of the species that live on or in the trees will also be lost, so the world is at risk of a major biodiversity crisis.  The list of endangered species grows longer.   Populations of the hedgehog have shrunk across Europe, as farming expands, and cities & roadways grow so its natural habitat shrinks. In the UK, other species such as the Red Squirrel, the Water Vole, the Scottish wildcat (an elusive and rare animal), the hazel dormouse and the grey long eared bat maintain a precarious hold on life. A delegation of scientists from Kew Gardens, was present at COP 16 in Colombia, talking about ‘the dark spots of biodiversity knowledge’, that is, identifying those regions of the world where collecting efforts need to be prioritised in order to document the plant species currently unknown to science (before they are lost).    

In 2023 the forest fires in Western Canada were so extensive that they burnt 37 million acres of forest lands, which is ten times the total area of forestry in the UK.  The carbon emissions were so great that they alone had a carbon footprint bigger than every country apart from the US, China and India.  Canada does not include emissions from wildfires in its carbon budget so the impact of its forest fires is in addition to its industrial and domestic carbon footprint. Although the total area burnt the next year - in 2024 - was smaller, the 2024 fires were particularly intense and destroyed 50% of the buildings in the city of Jasper. Forest fires affected infrastructure so badly that tourists were excluded from the whole of Jasper National Park in Alberta for many weeks.  This new intensity can also destroy trees that have been growing for hundreds of years and may have survived many other other fires.  This would have been the case for the King Arthur tree in California which had been the 9th largest Giant Redwood in the world: this huge and ancient Sequoia was destroyed in the 2020 Castle Fire. It is uncertain what is causing these intense infernos beyond climate change, to which the fires are themselves contributing. Many people believe that allowing too much dry and dead wood to build up creates the conditions for particularly hot fires which will easily jump fire breaks and natural circuit breakers, such as rivers   It may be that the First Nations [as Canadians call the native peoples who inhabited the country for thousands of years before European settlers arrived] were better at managing the land and would have reduced the fuel load in the forests, and there would have been a greater variety of tree species. Another paradox is that when the National Park authorities are most effective at putting out fires this allows a greater fuel load to build up so that there can be fewer fires but much bigger ones. To counter this, some Canadian Park managers are deliberately starting fires which they believe will be controllable - this is evident in the forests next to Lake Minnewanka. These are done in the off-season when large conflagrations are unlikely. In the past, forestry companies tried to extinguish fires by using float planes, such as the Hawaii Martin Mars dropping water.  This was put in an air museum in August 2024. The seaplane's water deposits were less well-targeted than today's helicopter drops.   Nowadays, forest blazes are usually left to run their natural course and this can take a long time.  Sometimes a fire will continue smouldering underground and it can be 18 months before a forest fire is confirmed to be finished. In cases where buildings, main roads or railway lines are threatened there will often be efforts to extinguish the fire using helicopters.  Drones are also used to survey the progress of fires. One measure which might reduce the issue is to replant burnt-out areas with  species that occur naturally and are  less flammable and better as surviving such as Eucalyptus.  However, the hotter, drier summers are creating conditions where we are in a new age of more intense forest fires and the Canadians are adjusting as fast as they can.  This means short term measures such as evacuation plans and more firefighting equipment, as well as longer term plans such as working on a better mix of trees and combating climate change more widely.

Since its formation, the earth has undergone change.  Life forms have come and gone.  There have been five major extinctions, the last being at the end of the Cretaceous Period; it killed off the dinosaurs and many other species.  This particular extinction event is thought to have been particularly rapid, due to an asteroid impact.  It caused a series of cataclysmic events and a rapid cooling of the Earth’s climate. Other changes, such as intense volcanic activity and tectonic uplift, may have pre-dated the asteroid impact but the event saw the elimination of many, many life forms. We are witnessing significant global change, that is also rapid in geological terms. Changes in the Earth’s climate and species composition usually take place over millennia, indeed over millions of years.  However, recent years have been very warm.  Global temperatures have changed noticeably. The warming that has been recorded “is exceptional relative to any period since before the last ice age, about 125,000 years ago”.  This warming has resulted in extreme and severe weather events in this country and across the world.  This year a record breaking January temperature of 19.9oC was recorded at Achfary, with storms Henk, Isha, and Jocelyn in the same month. The Earth’s warmest year on record (between 1850 to 2023) was 2023.  In early September 2023, the UK experienced a significant heatwave when daily maximum temperatures exceeded 30°C [somewhere in the UK] for seven consecutive days. [caption id="attachment_35526" align="aligncenter" width="675"] Drought![/caption] Such changes are not without effect. Phenology observations indicate that trees are producing their leaves earlier, woodland plants are coming into flower earlier. See the woodlands blog “Spring is on the move”.  A concern with these changing phenologies is that ‘mismatches’ can occur.  When trees come into leaf determines when caterpillars can feed and that, in turn, affects when birds can feed on the caterpillars and raise their young.  If these events do not occur in synchrony then the ‘functioning of the ecosystem’ is disturbed.  [caption id="attachment_25123" align="alignleft" width="300"] Leaf 'unfolding'[/caption] The agricultural and horticultural ecosystems that we have created are also affected by climate change. This year, heavy rainfall has meant that farmers in many parts of the UK have been unable to plant certain crops [such as potatoes, wheat and vegetables] during the key spring months. Some crops have rotted in the soil. In April, there was 111.4mm of rain, [the average for April is 71.9mm]; the sixth wettest April of the last 189 years.  Persistent wet weather also affects lambing, and can mean it is not possible to turn dairy cattle out onto grass / pasture, which in turn affects milk production. Monthly temperatures are more likely to be above average than below as climate change take effect.  This was true for the first three months of the year.  Warmer air holds more moisture and it can evaporate more water from the seas / oceans. A one degree (Celsius) rise in temperature adds 7% more moisture in the air.  Woodlands are affected by heavy rain as soil becomes waterlogged, which affects woodland flowers, and wet winters do no favours for animals that hibernate. The UK is not the only place to be affected by extremes of weather, be it rainfall and flooding, or high temperatures and drought.  India has recently experienced a period of extreme temperature, with temperatures approaching 50oC.  Such temperatures push human physiology to its limits.  Just as extreme rain is a problem for farmers, so is extreme heat and / or drought.  Brazil has been the main exporter of oranges for producing orange juice, but its recent crop has been substantially reduced as a result of flooding and drought; resulting in the worst harvest in decades. Spanish orange production has also been reduced due to drought. Like California, large parts of Florida ‘the Sunshine State’’ has seen its once-famous citrus industry reduced over the past two decades. Two diseases, greening and citrus canker have taken their toll, and then Hurricane Ian in September 2023, hit the citrus industry at the beginning of its growing season.  Large parts of the one famous citrus industry (oranges and grapefruit) have been lost and farmers are turning to the PONGAMIA tree to repurpose fallow land. [caption id="attachment_41381" align="alignleft" width="650"] Pongamia  : image thanks to Sarangib on Pixabay[/caption] This is a climate-resilient tree from India. They do not need fertiliser or pesticides.   It has been grown as a shade tree. As a member of the Fabaceae, it produces small, brown beans.  These are so bitter than not even wild hogs will eat them.  However, the beans are easily harvested by a machine that shakes the tree.  A San Francisco based company has found a way to remove the bitter tasting chemicals and use the beans in food production, as they yield a high quality protein and also an oil.  The bean (a legume) has been used to make a table oil, protein bars and a biofuel.   Orange juice production is not the only drink to be affected by changing climate.  Drought affects coffee plants and damages the quality of the soil, and excessive rainfall ‘favours’ fungal disease [e.g.coffee leaf rust and cherry rot], all of which will impact the yield and quality of the beans harvested.  Similarly, chocolate production is threatened. Cacao trees are impacted by global warming,  they can only grow and thrive within 10 degrees of the Equator, needing stable temperatures, high humidity, and ample rain.  However, temperatures are rising while rainfall has decreased. These changes lower the humidity. The trees are also under attack by a virus - cacao swollen shoot virus disease (CSSVD). Changing temperatures and rainfall patterns will influence what crops can be grown and where, it will also influence their cultivation and the working patterns associated with those crops.  Climate change is thus a factor contributing to food inflation and insecurity across the world.    

There are eleven thousand trees in Kew Gardens.  Each year, a few trees are lost due to natural causes, old age, disease etc.   In 2002, a drought resulted in the loss of  some 400 trees.  Such a prolonged dry spell is  likely to occur again and again as global temperatures rise, and climate change takes a hold. Modelling of future climate scenarios by Kew scientists suggests that towards the end of this century between a third and a half of Kew’s trees could be lost.  Trees like the English oak, beech, birch and holly could be vulnerable to warmer temperatures and extended dry spells.  There is a plan at Kew to replace gradually trees with species currently found in warmer areas, such the Mediterranean, Asia and Central America. Examples might include species such as the iberian alder, cherry hackberry and Montezuma’s pine.  Many of the plants in the gardens will survive, [including Kew’s ‘Old Lions’] as they were collected from in and around the Mediterranean; some of these date back to the victorian era or earlier. The ‘old lions’ of Kew are trees from the original grounds / garden that still survive. Examples include : Japanese pagoda tree (Styphnolobium japonica) Maidenhair tree (Ginkgo biloba) Oriental plane (Platanus orientalis) Caucasian elm (Zelkova carpinifolia) Black locust (Robinia pseudoacacia) The Caucasian elm dates from 1762, when an arboretum was planted.  It is thought that it might have been in a batch of plants from the Caucasus, planted in what is now the herbarium paddock.  In 1905, the height of the tree was recorded as 60 feet (18M), though they can grow to 100 feet.  A larger caucasian elm can be seen at  Tortworth. One species of oak that is common at Kew is the holly or holm oak (Quercus ilex).  This is a common, naturalised oak that was probably introduced into the country in the sixteenth century.  It is a hardy, slow growing tree and many new holm oaks were planted in 2008 to redefine the Syon Vista.  The wood of the tree is strong and, in the past, it was used in carts and farming equipment. Its acorns start off green in colour but turn a reddish brown; they are a tasty treat for pigs. The threat to Kew's trees is not unique, parks and urban spaces across the country need to plan for the future, to ensure that their trees can offer some resilience to changing weather patterns. Full details of Kew's planning here.  

With global temperatures rising and many places facing extremes of temperature, cities and urban environments often face the brunt of these climate extremes.  Cities absorb and hold onto the energy of the sun, creating ‘urban heat islands’. Recently, the temperature in New Delhi soared to a record high of 126.1oF (52.3oC), and other areas of India also suffered from the heat wave that claimed lives.  At a personal level, the shade of a tree can offer a place of refuge on a blisteringly hot day but a neighbourhood can benefit from the careful and strategic planting of trees.  Greater tree cover can mean that neighbourhoods are measurably cooler than those with few trees. If a heat wave is prolonged, then the physiological stress that people experience builds, affecting the old and young particularly.  Extreme heat / temperatures can also result in elevated levels of ozone, which affects people with asthma.  High temperatures may also be accompanied by high humidity and if the air has a high level of water vapour this makes it difficult for people to lose heat through sweating.  As water evaporates from the skin, its change of state (liquid to vapour) takes heat from the body. Researchers at UCLA analysed the ‘effects’ of four heat waves that occurred in the early years of the 21st century in Los Angeles, they focused on areas that varied in tree cover and pavements and road cover (essentially impermeable surfaces).  They also gathered information on ‘heat related’ visits to medical facilities.  They found that greater tree cover (and more reflective surfaces) reduced the number of heat-related medical interventions.   Whilst it might be agreed that increasing tree cover in urban settings is a good idea, there are practical problems.   Firstly, which trees to plant?  Ideally, the trees planted should be able to cope with the changing climate.  We don’t know what the climate will be like in 20 or 50 years but ideally the trees planted now should be able to cope with what nature might ‘throw at them’. Secondly, caring for the trees.  After planting, trees are vulnerable.  They need care and protection.  They need water - which is becoming an increasingly scarce resource in some parts of the world. Planting more trees needs to be coupled with increasing ‘green areas’ where water can permeate after rainfall into natural aquifers or water storage systems. Community involvement is also needed so that the trees are not only planted in areas where they will give the greatest benefit, but where people want them and will nurture them.   Los Angeles now has an Urban Forest Management Plan.  It aims to increase tree canopy in particular areas, locating areas to plant trees and collaborating with the residents of the areas.

At present, our forests and many across much of Europe have a medley of different species, and this has been the case for many hundreds of years.  They have survived minor fluctuations in climate and weather.  However, now climate and weather are changing in significant ways.  There are more extreme weather events, ranging from unprecedented rainfall to drought and periods of very high temperatures.  Winters seem to be be warmer and wetter, summers hotter and drier. Consequently, there is concern that many tree species being planted today will not be able to survive in the conditions that they are likely to experience in 50 or a 100 years time.  Species like the European Beech (Fagus sylvatica) are likely to struggle (like many did in the heat wave of 1976).  The root system of the beech is shallow, and though it has large roots spreading out in many directions, it cannot access water that may be present at deeper levels in the soil.   Though it is not known how native trees might adapt or be able to respond to a changing climate, it is possible that the number of tree species per km2 able to survive through to the next century may well fall by a third to a half in a warmer climate (depending on how quickly the warming occurs). Examination of some 60 plus European trees species at University of Vienna by Johannes Wessely et al suggested that the English or Pedunculate Oak (Quercus robur) may be a species that could cope with changing climatic conditions. It seems that native UK Oaks are genetically diverse, and this gives rise to variation and the potential to adapt to changing conditions.  Oak is wind pollinated and its light pollen can be dispersed over long distances, which promotes outbreeding and genetic diversity. Whilst the oak has always been valuable as a species for :- Timber production : it is used in furniture making and in the past thousands of oaks were used in the building of ships such as the Mary Rose. Carbon sequestration / storage - it is long lived and has a large above ground biomass Biodiversity : it provides a ‘home’ for many species of animals, plants and fungi. It offers food and shelter for many invertebrate species, numerous insects and spiders); its leaves often show the ‘scars’ of their feeding activities. Its bark is an ideal substrate for many lichen and bryophyte species (epiphytes). The roots of the trees establish mycorrhizal associations with various fungi. Now, the Oak may prove to be valuable in a warmer world as a species for timber production and reforestation projects.  The Oak’s ability to support other plant, animal and fungal species would also be important in terms of biodiversity and resilience..   Forests with a smaller number of tree species are thought to be less resilient to climate change and less biodiverse.   [caption id="attachment_41217" align="aligncenter" width="675"] A solitary oak[/caption]