Trees relax me
What is it about trees? For some reason they relax me, especially in this month of May. It is a month when the weather is not too hot, and the trees have mostly turned green. A few species are still deciding what to do, larch in particular, but May is when the pattern of a woodland has largely become established for the year. It was why I selected Skelghyll Woods, just outside the Lake District town of Ambleside, and its Champion Tree Trail[i] in particular. The trail is short, a paltry 1.28 kilometres (0.8 miles), with a tiny area of steep climbing and should theoretically be over in a jiffy. Yet it is not, as whenever I have walked the trail, I have discovered something new. This occasion was no exception.
Trees are known to produce various volatile terpenes and emit them into the atmosphere. Terpenes are naturally occurring chemical compounds found in plants and some animals. They are responsible for the aromas, flavours, and even colours associated with various types of vegetation. The terpenes from trees have higher levels in summer than in winter as well as being higher in the light than the dark. Terpenes in forest air refresh and relax the mind, effects that have been demonstrated by measuring brain waves and blood pressure[ii]. Perhaps that is why forest bathing is so good[iii]. There are massive advantages to human health of being in the vicinity of trees[iv] and the Champion Tree Trail is no exception.
Champion Tree Trail
The Trail starts immediately outside the entrance to Stagshaw Garden[v], or Gardens (plural), depending on which sign is displayed. Stagshaw, an eight-acre woodland garden, was created by Cubby Acland, a former National Trust land agent who lived in the cottages adjacent to the garden. The cottages, plus the land where the garden now stands, and the woodland above, were passed from the Wansfell Estate to the National Trust in 1957, and Cubby created the garden from scratch between 1959 and his death in 1979. His garden, Stagshaw Garden, does not officially form part of the Champion Tree Trail, but is so near that not to visit seems pointless. Helpfully and importantly, entrance is free, while there is limited parking nearby.
There are few better ways of seeing the effects of climate change than to walk through woodland. Skelghyll Woods lie immediately to the east of Windermere lake. Sadly, this stretch of water is a national icon in trouble as not all is well beneath its picturesque surface. Nutrient pollution and ghastly water quality are the main reasons for Windermere’s deteriorating health.
Polluted Windermere
The key pollutant of concern is phosphorus, although others such as nitrogen may also have damaging effects. The quantity of phosphorus entering the lake from the land around has increased dramatically in recent times thanks to human activities. Mankind has brought in large quantities of nutrients from outside - food grown elsewhere, fertiliser, polluted rainfall, visitors - while some nutrients have been released from within by land management and sewage outfall. Whatever the origin, these surplus nutrients have stressed the lake’s ecological capacity to deal with them.
Phosphorus is vital for all living organisms and is primarily derived from rocks and soils. In excess, it becomes a polluting nutrient. Excess phosphorus in Windermere comes from two main sources - wastewater and land management:
1. Wastewater
A significant portion of Windermere’s phosphorus is derived from treated and untreated human sewage and washing products such as soaps and detergents. There are five sewage treatment plants in the Windermere catchment area, and approximately 1900 private systems, mainly septic tanks, that are found in off-grid and remote locations.
Current water treatment processes do not remove all phosphorus, leaving residual amounts. An estimated 25-35% of Windermere’s phosphorus originates from this source, even with investment in phosphorus removal technologies. During heavy rainfall, untreated sewage is routinely released from public wastewater treatment works. In 2022, raw sewage was dumped into Windermere for approximately 5904 hours over 246 days. This accounted for approximately 5% of the phosphorus load at the time, because the heavy rain had a diluting effect. Leaking or poorly maintained private systems, including septic tanks, discharge significant amounts of phosphorus and are estimated to contribute about 30% of Windermere's total phosphorus load. Overall, 60-70% of the total phosphorus load in Windermere is attributed to human wastewater sources.
2. Land management
Approximately 30% of Windermere’s remaining phosphorus burden comes from land-use activities in the wider catchment. These include farming activities, forestry operations, and urban run-off. There are minor sources of phosphorus, too, such as faulty pipe connections and wildfowl droppings, for example Canada geese.
The Big Windermere Survey[vi] in 2023 found that only 5% of in-lake samples had high compliance with ecological standards for total phosphorus, while 79% were moderately or poorly compliant. Windermere is not doing well thanks to its increased pollution. Samples from the shoreline of Windermere and rivers and lakes in its catchment revealed higher levels of phosphorus in August 2023 compared with 2022. Just 3% of samples collected from the lake's shoreline met minimum standards under UK legislation. In June 2022, it was 58%[vii].
There are also evident long-term trends taking place in Windermere. A review[viii] by the Centre for Ecology and Hydrology (2023) examined these trends and found that recent surface temperatures in the North and South Basins of Windermere were 0.8°C higher than the 1981-2010 average, with the warmest water being found in July and August. Meanwhile, oxygen levels in the North Basin had decreased by 9.1% compared with the 30-year average.
In addition, there is evidence of significant sediment and sewage accumulation in Windermere over the past 170 years. Surveys suggest that discharges have ended up as loose sludge covering over 90% of the lakebed. This sludge is a potential source of phosphorus, which could promote the blue-green algal growth that Windermere shows so clearly.
As I looked at Windermere from my vantage point of Skelghyll Woods, knowing what I now know about the lake, somehow it felt different. There was nothing scenic about it.
Diseased trees
The Champion Tree Trail has plenty of huge trees, and many smaller ones as well. My first stop was an ash tree that was not looking healthy, thanks to likely ash dieback[ix]. This is a disease caused by the fungus Hymenoscyphus fraxineus, although there is some resistance to the disease now being shown by up to 25% of affected ash trees. I could see some wilting leaves, diamond-shaped lesions on bark, and dieback of shoots and branches. The spread of ash dieback has been linked to warmer temperatures and changing precipitation patterns, which favour the fungus's lifecycle. Climate change was having its effect.
There were also a few oak trees that seemed to be struggling. This was possible oak decline, characterised even at this early stage by reduced leaf size. One can expect early leaf drop, and pest infestations such as the oak processionary moth (OPM) to follow[x]. That said, OPM is only known to be established in a relatively small geographical area of the country across London and surrounding areas, but its further spread is a possibility[xi]. Prolonged droughts and extreme weather events, both linked to climate change, have increased the susceptibility of oaks to these pests and pathogens. Stress from environmental changes weakens oak trees, making them more vulnerable to decline.
In Lakeland, it is also important not to forget Phytophthora, a water mould pathogen that is becoming more frequent because of climate change[xii] and especially when conditions are wet. There is plenty of rain in Lakeland. Symptoms include root rot, bleeding cankers, and dieback in several tree species, notably larch, oak, and beech. Both Stagshaw Garden and the adjacent Skelghyll Woods have a history of Phytophthora. Sadly, root rot will not be disappearing in a hurry.
Spindle Ermine Moth
As I walked, I made a double-take when I saw a mass of white web tightly binding the leaves of a deciduous branch. I instantly thought of the Spindle Ermine Moth but also of spider mite, although I was not expecting either.
The Spindle Ermine Moth (Yponomeuta cagnagella) is a notable species as it is known for its strikingly intricate silk webs and its propensity to defoliate spindle trees (Euonymus europaeus). Spindle trees are not common to find in Lakeland and yet I was faced by at least two, each bound down with webs. The Spindle Ermine Moth is native to Europe and plays a significant role in the ecosystem that surrounds the spindle tree. Sadly, climate change appears to be altering this. The moth is characterised by white wings embellished with black spots. Its larvae, which spin the communal webs that sometimes envelop entire bushes, feed voraciously on spindle leaves, leading to complete defoliation. Although this may not be lethal to the host plant, it can certainly reduce its vitality. All parts of the spindle tree are poisonous to mankind, the tree receiving its name because the Dutch once made spindles from its wood. Its leaves turn red later in the year.
The development of the Spindle Ermine Moth is extremely sensitive to environmental conditions, especially temperature and humidity[xiii]. Warmer temperatures can accelerate larval growth, reduce the duration of the life cycle, and lead to multiple generations each year in regions where previously only one generation occurred[xiv]. This phenological shift can enhance population sizes and increase the frequency and severity of defoliation. Changes in rainfall, especially when Lakeland has so much rain, can drown eggs and larvae. Meanwhile drought conditions can reduce the availability of high-quality foliage necessary for larval development[xv]. Climate change can also lead to a timing mismatch between the moth’s life stages and the availability of spindle tree leaves, so that larvae can hatch when food resources are reduced[xvi]. There are thus huge direct effects on the Spindle Ermine Month created by climate change.
There are indirect effects, too. For example, the community of predators and parasitoids that control Spindle Ermine Moth populations can be influenced by climate change. Warmer temperatures can benefit certain parasitoids, thereby enhancing biological control[xvii]. Parasitoids are small insects whose immature stages develop either within or attached to the outside of other insects. They eventually kill the host on which they feed, as opposed to parasites such as fleas and ticks, which generally feed on hosts without killing them[xviii].
Elevated atmospheric CO2 levels can alter plant physiology, affecting the nutritional quality of spindle leaves. Changes in leaf chemistry, such as increased carbon-to-nitrogen ratios, can impact larval growth and survival[xix]. As temperatures rise, the suitable habitat for the Spindle Ermine Moth slowly expands northward. Areas previously too cold become hospitable, leading to range expansion[xx].
The moth’s interactions with climate change are not unidirectional. For example, repeated defoliation can weaken spindle trees, and reduce their growth and vitality. This alters local vegetation dynamics and affects carbon sequestration. Healthy forests and shrublands act as carbon sinks, but if key species such as spindle trees decline, the capacity of these ecosystems to sequester carbon can be compromised[xxi].
Changes in moth populations can also affect biodiversity. For example, an increase in moth populations will support larger populations of their predators, thereby altering the dynamics of the local food web. Conversely, if moth populations decline because of climate-induced stressors, this may lead to a decline in the species that rely on them as a food source[xxii].
The Spindle Ermine Moth serves as a compelling example of how climate change can influence, and be influenced by, insect-plant dynamics. As global temperatures rise and weather patterns become more erratic, so the moth’s life cycle, distribution, and ecological impact can change. Watch that space.
Spider mites
I was worried, too, that the tight webs might not have been the Spindle Ermine Moth but the spider mite, although Lakeland was a fair way to the north for me to see this. Yet with the weather slowly warming and Windermere being plagued by high phosphorus levels, the spider mite was a distant possibility. It adores phosphorus.
Spider mites, belonging to the family Tetranychidae, are tiny arachnids and well-recognised agricultural pests that have a significant economic impact[xxiii] on a wide range of crops. They thrive in diverse environments, have a high reproductive rate[xxiv] and show resistance to many pesticides. This makes the spider mite a formidable adversary[xxv].
Spider mites thrive in hot, dry conditions and are found on the undersides of leaves where they feed on plant sap. They produce a fine webbing that protects them from predators and environmental factors. This webbing, which can cover entire plants during severe infestations, is a distinguishing characteristic of their presence[xxvi]. Spider mites are polyphagous, meaning they feed on a wide variety of plant species[xxvii]. Various management strategies have been proposed to fight the pest[xxviii],[xxix].
Much is said about the changes to Lakeland waters created by the surrounding land. This is so-called eutrophication. Less is said about changes to the land created by the waters[xxx]. For example, high nutrient levels in the water can enhanced plant growth. This can create an ideal environment for both Spindle Ermine Moth and spider mite populations to thrive[xxxi],[xxxii]. This relationship between aquatic nutrient levels and terrestrial pest populations is supported by the ecological principle that nutrient inputs can cross ecosystem boundaries, influencing both aquatic and terrestrial environments[xxxiii].
As climate change takes a tighter hold of the environment, so the spider mite is affected, as is the Spindle Ermine Moth.
Planting trees
The Champion Tree Trail is impressive for different reasons. First, its trees are massive and worth seeing just to witness their size. Yet second, and perhaps more importantly, they are a good example of how the work of earlier generations has benefitted those who follow. The Victorians were especially good at that. The Champion Tree Trail is the result of a Victorian arboretum planted around 1860. No one who planted the arboretum would have seen what was eventually achieved. Modern mankind, unquestionably selfish and self-centred, could learn much from these actions.
Planting trees during the Victorian era has provided numerous ecological and environmental benefits that we continue to enjoy today. The advantages are seen in many locations but are perhaps more evident in urban areas. These advantages include:
1. Urban Cooling and Air Quality Improvement: Victorian tree planting in cities has resulted in significant urban cooling and improved air quality. These trees provide shade, reduce temperatures, and filter pollutants. This is especially critical in urban areas where heat and pollution levels are high. For instance, trees planted along streets and in parks during the Victorian period in cities such as London and Manchester now contribute to lower urban temperatures[xxxiv] and cleaner air, improving public health and reducing heat-related illnesses[xxxv].
2. Biodiversity Support: Victorian-era trees have created habitats for various species, supporting biodiversity in urban and rural areas. Trees from this period are now mature and provide essential habitats for birds, insects, and other wildlife. Native woods and trees support a significant portion of the UK's priority species for conservation and contribute to the overall health of the environment[xxxvi].
3. Stormwater Management: Trees planted during the Victorian era play a crucial role in managing stormwater. Their extensive root systems help absorb and filter rainwater, reducing the risk of flooding and soil erosion. This is particularly evident in areas with large canopy trees, which intercept rainwater and improve soil permeability, thereby mitigating the impact of heavy rainfall.
4. Public Health Benefits: The presence of mature trees planted in the Victorian era has been linked to numerous public health benefits. Urban trees reduce the incidence of respiratory diseases by filtering airborne pollutants and provide psychological benefits by offering green spaces for recreation and relaxation. This enhances the quality of life for urban residents and supports physical and mental well-being[xxxvii].
5. Carbon Sequestration: Victorian trees have had decades to grow and now serve as significant carbon sinks. These trees capture and store carbon dioxide, helping to mitigate the effects of climate change. The long-term presence of these trees contributes to carbon sequestration, reducing the overall carbon footprint of urban areas and supporting global efforts to combat climate change.
Modern man could learn a thing or two from the Victorians.
The size of the trees of the Champion Tree Trail was impressive, especially its largest specimen, a Grand Fir (Abies grandis) that was measured in 2011 as being 58 metres (190 feet) high. It is even more impressive when looking at its roots, as many lie just under the surface, twisting this way and that, thanks to the shallow Lakeland soil. Bedrock is never far away in Lakeland, which is partly why the area can experience so much surface flooding. For many of the champion trees, their branches are also greatly reduced on the shaded side of the trunk, away from the sun. In addition, many of the trees showed very few branches sprouting from the lowermost reaches of the trunk, as barely any light was passing through the canopy overhead. The branches began very high up the trunk, as it was only there that sunlight was found.
Soil erosion
There was erosion, too, where the soil around the trees was being steadily worn away. I was saddened to see this as soil erosion is a significant environmental issue that entails the displacement of the upper layer of soil through natural forces. These include water and wind, as well as human activities such as agriculture and deforestation. There can be severe consequences for ecosystems, agricultural productivity, and water quality.
Soil erosion is primarily driven by water and wind, with various factors influencing the severity and rate of erosion.
1. Water Erosion: Water erosion occurs when rainfall, runoff, and river flows remove soil from the land. The intensity of rainfall, the slope of the land, soil type, and vegetation cover significantly impact water erosion rates. For instance, heavy rainfall on sloped land, well shown on the Champion Tree Trail, can lead to substantial soil loss. The process includes sheet erosion, where thin layers of soil are removed uniformly, and gully erosion, which creates deep channels in the soil[xxxviii],[xxxix].
2. Wind Erosion: Wind erosion is not so much of a problem on the Champion Tree Trail but is certainly prevalent in arid and semi-arid regions where strong winds and sparse vegetation cover exist. This type of erosion lifts and transports soil particles over long distances. Factors such as soil texture, moisture content, and wind speed play crucial roles in determining the extent of wind erosion. Sandy and dry soils are particularly susceptible[xl].
3. Human Activities: Human activities such as deforestation, overgrazing, and unsustainable agricultural practices can also exacerbate soil erosion. Deforestation removes the protective tree cover, exposing the soil to erosion. For this reason, forestry is taken very seriously in Skelghyll Woods. Beyond the woods, and although not seen on the Champion Tree Trail, overgrazing by livestock depletes vegetation, and can reduce the root structures that stabilise the soil. Additionally, practices such as inappropriate tillage disturb the soil structure, and make it more prone to erosion[xli].
Littering
Sadly, there is no escaping the damaging effects of mankind, even on the Champion Tree Trail. There was litter, plenty of it, with discarded doggy poo bags and plastic bottles. Research[xlii] has shown there to be a positive relationship between littering and personality disorders - no surprise there - while litter breeds more litter[xliii].
Littering, the act of improperly disposing waste materials in public places, has long been recognised as a significant environmental issue. While traditionally seen as a problem of aesthetics and public health, recent studies have highlighted its broader ecological impacts, including its contribution to climate change.
Littering contributes directly to greenhouse gas emissions through the decomposition of organic waste and the breakdown of synthetic materials. Organic waste, such as food scraps, when left to decompose in the open air, produces methane (CH4), a potent greenhouse gas. Methane is over 25 times more effective than carbon dioxide (CO2) at trapping heat in the atmosphere over a 100-year period, and significantly contributes to global warming[xliv].
Furthermore, plastic waste, which constitutes a significant portion of litter, has a direct and indirect role in climate change. When exposed to sunlight, plastics release methane and ethylene gases because of photodegradation[xlv]. The production of plastics, predominantly from fossil fuels, is energy-intensive, releasing substantial CO2 emissions during the manufacturing process. The lifecycle of plastic, from production to degradation, plays a critical role in greenhouse gas emissions.
Littering also affects soil health and plant life, thereby impacting carbon storage in soils and vegetation. Soil contaminated with plastics can inhibit the growth of plants by altering the soil structure and reducing water infiltration. The presence of litter in forests and other vegetated areas can hinder plant growth, reducing the amount of CO2 absorbed by plants for photosynthesis[xlvi]. The bottom line for all of this is not to fell trees, whatever the reason, and be sure to keep hold of all litter until you reach home. That includes plastic bottles.
With climate change all around, the Champion Tree Trail had set me thinking.
***
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References
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