A lot of research work now focuses on the resilience of woodlands and forests in the light of climate change, that is their ability to cope with conditions like drier, hotter summers and/or  warmer/wetter winters. It has generally been assumed that trees at the limit of their range in dry regions would be most affected by climate change (with rising temperatures and less water).  However, a major study of some six million tree annual ring samples, (involving 120+ species) coupled with analysis of historical climate data has shown that trees in drier regions show a certain resilience to drought.  Trees seemingly become less sensitive to drought as they approach the edge of their range.  Trees in wetter climates are less resilient when they experience drier conditions or drought.  It seems probable that many species in wetter woodland and forest ecosystems will face significant challenges if the climate does move to a drier and warmer state. Assisted migration may be needed.  One idea is to ‘exploit’ the genetic diversity found at the edge of a species range.  The slow natural migration of trees may not be able to keep pace with the speed of climate change. Full details of this study by the University of California can be found here : Drought sensitivity in mesic forests heightens their vulnerability to climate change The effects of climate change have become very clear in recent times.  This last year witnessed:- Record breaking wild fires in Canada, with the smoke extending across to the East coast of the States. [caption id="attachment_40597" align="aligncenter" width="675"] Canadian forest fire[/caption] Heat waves in parts of America , for example, Phoenix (Arizona) suffers the best part of a month with temperatures of 43oC. Parts of the North Atlantic Ocean saw unprecedented temperatures The global temperature in July was 1.5oC above the pre-industrial average, September saw temperatures 1.8oC above the pre-industrial average. Parts of Chile and Argentina saw a ‘heatwave’ in the middle of their winter. It is clear that ‘unchartered waters’ lie ahead.

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.

Much of England experienced a series of ‘tropical nights’ last summer, when night time temperatures were 20oC or above.  These tropical nights were associated with the heat wave that affected most of south east England.  Central London experienced its longest stretch of extreme daytime temperatures since the 1960’s -  temperatures of 30+oC were recorded on six consecutive days.  A number of experts have said that such heatwaves and associated tropical nights are likely to become more common as a consequence of climate change.   We were not alone in experiencing high temperatures by day and night, much of western Europe  sweltered in the heat this August. The problem was most marked in urban areas and large cities.  Some three-quarters of the population of Europe now live in urban areas. Extreme heat affects our health causing general discomfort, malaise, respiratory problems, headaches, heat stroke, heat cramps and heat-related mortality.  Read more...

Radiant energy from the sun falls on the Earth; some of this energy is absorbed by the planet and its atmosphere.  Some of the energy is radiated back into space.  The balance between the absorbed and radiated energy determines global temperature.  This balance is changed by a number of factors - the intensity of the solar energy, cloud reflectivity, the absorption of energy by various gases or surfaces. The reflectivity of the Earth’s surface  (the albedo) influences the amount of light energy that is reflected back into space. Snow has a high albedo, that is, it reflects much of the light back out into space.  Dark objects (like conifer plantations) reflect less light / radiation and absorb more thereby trapping heat that would otherwise be reflected back into space. The amount of energy that is ‘retained’ is also influenced by the presence of particular gases in the atmosphere - the so-called 'greenhouse gases', notably carbon dioxide and methane.  The amount of carbon dioxide in the atmosphere has increased dramatically from 280 ppm during the 10,000 years up to the mid-18th century to 415 ppm (as of 2019).  This increase has certainly contributed to the changes in climate that we have witnessed in recent years - extreme weather events such as heat waves and flooding.   Read more...

Dendrochronology seeks to gather information from the annual rings of trees, dated to their year of formation.   As trees age, they form new distinctive rings, outward from the centre, and each year a circle of new, dead wood is formed (by the cambium) around the trunk of most trees. The annual rings form because the water conducting xylem vessels or early wood formed in late spring and early summer (when growth is rapid) are wider and lighter than those formed in late summer and early autumn, which are narrower and darker (sometimes called the late wood).  A light and dark ring together constitute one year’s growth.   In winter, there is no growth. Trees and their annual rings are essentially long term recording devices that ‘house’ information about climatic variations, past climates and ecosystems; they can even offer insights into historical events. From an annual ring, it is possible to extract information about precipitation, temperature and other climate data about that year. Read more...

The level of carbon dioxide in the atmosphere has been carefully monitored since the 1950’s (for example at Mauna Loa).  For much of recorded history and prior to the industrial revolution, the level of carbon dioxide was about 280 ppm.  However, its current level of 400 ppm represents a significant increase and is probably the highest value for some 800,000 years.  It has risen because of the burning of fossil fuels (coal, oil, peat etc.) - which release carbon dioxide.  This has contributed to global warming and the phenomenon of climate change. However, the rise in global temperature has not been quite as great as some calculations / predictions have proposed.  The “greening of the planet” (that is an increase in the number of trees and plants growing on the planet) was held to be responsible for this ‘reduction’ in anticipated temperatures. Read more...