To many people, the dandelion is a pernicious weed.   First one makes its way into your garden, but then due to its reproductive capacity - there are dozens.  Each flower stalk may produce up to 200 seeds with their characteristic parachute for dispersal.  A plant may produce 10 or so flower stalks, so that it is a lot of seeds from a single plant.  Once the seeds  disperse and germinate, they produce a significant tap root which grows down into the soil seeking water and minerals.  The seedlings can survive in inhospitable places, like the cracks between street paving stones or on your drive. As the seeds are wind blown, they may travel considerable distances with the aid of their ‘parachutes'.  However, changes in agriculture, increasing use of herbicides etc have meant that countryside populations of dandelions have diminished.  This has affected the insects that feed upon the dandelions' nectar and pollen.  As the number of these insects falls, it affects the seed production of other plants that these insects visit. Interestingly, urban dandelions seem to benefit from the heat island effect in cities.  A city may be some 2oC warmer than its surrounding countryside, more so in the summer.   Dandelions seem to thrive in the heat, growing more rapidly than their 'rural cousins'.  Urban warmth also means that the plants can begin to flower soon after a milder and shorter winter, whereas other plants struggle to adapt to changing environmental signals.   The urban  dandelions provide a 'feast' for insects in early Spring when resources are limited.  Urban meadows can provide 90% of the nectar for pollinators, and 80% of the pollen. These provide sugars and proteins / amino acids for some 200 species of pollinators [ solitary bees, mining bees, bumble bees, hover flies and pollen beetles].  The gifts of the dandelions are helping many struggling insect populations. Interesting fact : Dandelions often reproduce by a sort of sub-sexual system (termed apomixis) that has resulted in some 200+ microspecies in the U.K.  These have been studied by Professor John Richards for some 40 years.  The species name Taraxacum officinale has a gg. (for aggregate) added to it - in recognition of all the variations of the plant that can be found.  Thanks to Angus for images. https://en.wikipedia.org/wiki/Taraxacum officinale  

The blog tends to write about trees in the context of woodlands and forests, but the trees to be found in hedgerows, parks, streets and gardens are important in many ways.  London is sometimes referred to as an urban forest, as it has some eight million trees within its boundaries.  Though their individual contributions of any given tree may be small, collectively the trees help with : Carbon storage  Carbon sequestration Air pollution removal Removal of pollutants (e.g nitrogen and sulphur oxides) Capture / removal  of particulates Reducing runoff, helping with flood mitigation  Noise réduction temperature regulation. The role of trees in temperature regulation has received some attention recently.   With climate change, many parts of the world have experienced periods of extreme hot weather / heatwaves.  Whilst it is true that extreme cold weather is associated with more deaths than hot, heat waves and the associated deaths are a significant problem.  Heatwaves, such as that experienced in 2015, have been associated with cardiorespiratory problems and premature deaths.   Cities, in particular, record higher temperatures than the surrounding countryside or the suburbs. the so-called urban heat island effect.  The lack of vegetation, the use of air conditioning systems, coupled with the dark asphalt of roads means that heat is retained / trapped; giving rise to the urban heat island effect.  On a summer’s day, some city centres may be some 10oC hotter than the surrounding countryside.  Europe saw its hottest summer last year, and its second warmest year.  Cities in southern and eastern Europe were particularly affected by recent heat waves. They tended to have significant urban heat island effects and low tree coverage.   In contrast, 27% of Gothenburg is covered by trees.  Studies / modelling by the Barcelona Institute for Global Health have indicated that increasing tree cover to 30% of the urban space could decrease the deaths associated with heatwaves / extremes of hot weather (perhaps by a third).  A number of cities, like Barcelona and Seattle have committed to increasing tree cover.  Indeed, Barcelona’s Trees for Life Master Plan is making progress towards its goal of covering 30% of the city with trees,  Barcelona is also encouraging green roofs.  Studies have also shown that ‘green spaces’ can have other benefits from reducing cardiovascular disease, poor mental health, and helping to improve cognitive functioning of children and the elderly.

Each day, our cities are exposed to pollution from cars, power stations, industry etc.  All of these release particulate matter (aka particulates) and various gases - such as carbon dioxide, carbon monoxide, sulphur dioxide, and nitrogen oxides.   The pollutants in many of our city streets frequently exceed the World Health Organisation (WHO) guidelines. Particulate matter is the sum of all solid and liquid particles suspended in air - it may include dust, pollen, soot, smoke and various liquid droplets.  These particulates are often divided into PM10 particles - that have a diameter of 10 micrometers or less, are inhalable and may pass into the lungs and then the bloodstream. And, PM2.5 particles have a diameter of less than 2.5 micrometers - these are very fine and often contribute to the haze in the air. Many particles form as a result of complex reactions between sulphur dioxide and nitrogen oxides. Read more...