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  

Looking around the garden this morning one might be forgiven for thinking that autumn is already here.  Several trees seem to think the short days of autumn have arrived and are preparing to shed their leaves. Certainly the walnut and hazel are of this opinion.  However, other signs indicate that this is not the case, my twenty year old cedar has died, as have several Cryptomerias and other conifers - this despite assiduous trips with the water can.  In fact, the plants are responding to this summer’s weather, a long period without rain (or very little) plus the very hot weather that we have experienced; particularly in the South and South East.  The plants are not reacting to any changes in day length but to drought. Plants detect changes in the photoperiod they experience through their phytochrome system; under normal circumstances their biological clock will tell them when to prepare for winter.   Whilst many well established trees and plants with extensive root systems can withstand a period of drought, younger plants or those in poor or freely draining soil are likely to die.  Such plants simply cannot take up enough water to replace that lost through transpiration; that is, water evaporation through the stomates (pores) in the leaves.    The intensity of the heat this year coupled with the reduced rainfall has affected many plants and trees and it may be some time before we see the full effect of this summer’s weather.  Beech trees suffer more during periods of droughts because they have shallower roots, while oaks reach moisture deeper in the soil and continue to grow.  In the drought of 1976, many beech trees died and survivors often exhibited reduced growth some 40 years on*. Some plants respond to drought stress by bringing forward their reproductive process, so that fruits and seeds are produced early.  This is an attempt to ensure the survival of the species for when conditions are more favourable to growth.  The Woodland Trust, noted for its phenology records, recorded ripe blackberries in late July (lots of reports on its page on Facebook).  Hawthorn, Rowan, Holly and Elder have all been recorded as fruiting earlier this year.  This was true of the filbert in my garden this summer.  This earlier production of fruits and seeds may have ‘knock on’ effects on the wildlife that feed upon them.   Whilst the effects of the extreme weather are obvious in our parks, gardens etc, the effects on rivers and streams are dramatic.   Chalk streams, in particular, have been affected; they are unique habitats.  The River Ver is a chalk stream in Hertfordshire.  The summer has seen large sections of its upper reaches reduced by several kilometres due to the hot weather (and water abstraction).  The effects in and around such streams may be felt for many years as the water levels fall so the habitats for fish, amphibians and invertebrates (such as dragonflies / mayflies) are reduced or lost.  Recolonisation of such drought impacted systems can be slow and prolonged.   An accessible and detailed paper on Beech trees and drought is available here : research gate.net [caption id="attachment_22186" align="alignright" width="300"] Mature Beech on Box Hill, Surrey.[/caption] [caption id="attachment_10575" align="alignleft" width="300"] Mature beech[/caption]

Whilst it is not possible to attribute a particular weather event to climate change alone, what is clear is that climate change / global warming intensifies certain meteorological events. High temperatures and reduced rainfall have lead to the extreme fires seen recently in Australia and on the pacific coast of America.  Floods and periods of drought are now more common and the last two decades have seen some of the warmest years on record - in the U.K. .  Whilst too much water can result in the death of trees and plants as the soil becomes water-logged - so oxygen cannot reach the root -  drought also stresses trees and other plants. Drought-induced death of trees is associated with the failure of the water transporting system (xylem), but the process is poorly understood. Tree, indeed plant survival, is dependent on a continuous supply of water to the leaves. Read more...

Climate change means that we are likely to experience more extreme weather events from intense and prolonged rainfall to severe drought and heat.  The plants and trees of woodlands may experience periods of flooding, during which their roots may be deprived of oxygen (as the soil is waterlogged) and consequently - die.  Similarly trees and other plants may be subject to drought - resulting in the stunting of growth or even death.  It had been thought that most trees had roots that penetrated deep into the soil (in search of water and minerals) but the aftermath of the Great Storm of 1987 revealed that this was not the case for many types of tree.  The root systems of many trees are, in fact, relatively shallow - the root plate may only reach down some five to six feet. Read more...

Climate change is now a fact of life and one aspect of this is the occurrence of more extreme weather events.  These can take the form of high winds / hurricanes, extended periods of heavy rainfall or conversely periods of drought.  Clearly extreme weather can affect all ecosystems; woodlands and forests are no exceptions.   Consequently in recent years, a number of organisations have been looking at different tree species in order to understand more about drought resistance (or the ability to withstand prolonged flooding - when the roots are deprived of oxygen). INRA (French National Institute for Agricultural Research) has been looking at a small tree of the cypress Family (Cupressaceae) - Callitris tuberculata.   This grows as a small evergreen tree or shrub in Western Australia.  It can survive extreme drought and has been described as “the most drought resistant tree in the world”. Read more...

In the early 1970’s, Professor Barry Commoner (Washington University) said that ‘the first law of ecology’ could be stated as “Everything is connected to everything else”. That is, there is one ecosystem for all living organisms and what affects one – affects all. He was much concerned with the interactions of organisms, and the concept of sustainability. Traditionally, connectedness in ecology is concerned with how one organism affects another, or how an organism affects the environment or how a change in an environmental factor affects the organisms in the ecosystem. Read more...

Why do hedgerows exist?  Many were originally planted to contain livestock (sheep, cattle, pigs, chickens), others existed to define boundaries – who owned which parcels of land.  Hedges often delineate fields; field is derived from the old English feld which means an area of felled trees.  However, hedge construction and management was particularly associated with the process of enclosure, which transformed the landscape of England.  This was often connected with the change of land use - from arable to pasture for sheep.  Read more...