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.

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]

Declining bee populations in Europe have caused alarm in recent years and the decline has been attributed to a multitude of factors / causes, for example :- Climate change Pesticides - especially neonicotinoids Varroa mites Viruses Now Russia is reporting mass bee deaths and major declines in their bee populations in recent months.  Read more...