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  

Loss of nitrogen fixing species. Some plants can ‘fix’ atmospheric nitrogen.  That is they can take nitrogen from the air and use it to make complex nitrogen-containing organic compounds (such as amino acids / proteins).  This fixation of nitrogen is due to the presence of symbiotic bacteria in root nodules.  Gardeners often make use of ‘nitrogen fixers’, such clover, peas and beans to augment soil fertility. A recent study has investigated the changes in the makeup of the flora in European forests (over several decades) from 1940 to 2019.  What they found was that the proportion of nitrogen fixing plants has declined.  The changes did not seem correspond to any changes in temperature  or aridity / rainfall during the time period, but to nitrogen accumulation in the environment.  When nitrogen levels are low, nitrogen fixing plants have an advantage, but when nitrogen levels increase their advantage over other plants is lost. Nitrogen compounds in the soil can result from the intensive use of fertilisers on nearby agricultural land or atmospheric deposition of various pollutants.  Nitrogen levels have increased tenfold since the start date of the surveys.  This loss of nitrogen fixing plants might, in the long term, result in a loss of ecosystem resilience. For further info - visit https://www.science.org/doi/full/10.1126/sciadv.adp7953 The great green wall project. There are a number of large scale tree planting projects, many associated with offsetting global warming.  The great green wall aims to grow a belt of trees some 8000 km in length, and 15 km wide in the Sahara.  The planned route supported trees in the past.  The aim is to ‘stabilise’ the desert, limiting further expansion into the Sahel, as the tree roots help to stabilise the soil, limiting erosion.  Desertification is associated with drought and overgrazing.  The idea of such a barrier was taken up and approved by countries south of the Sahara in 2002, during a special summit.  The trees selected are drought resistant species, that also serve to fertilise the soil and contribute fruits, fodder and fuel wood for local communities. Though millions of trees have been planted, the project needs more funding if it is to succeed. Further details about the great green wall can be found here and here. Dealing with drought ? [caption id="attachment_35526" align="alignleft" width="300"] drought[/caption] Drought is a problem not only for woodlands but also for crops, resulting in substantial food loss across the globe.  The damage to crops is likely to increase as fresh water availability declines.  During drought, the availability of water in the topsoil decreases, leaving water only accessible in the  deeper subsoil.  Plants seek water through their roots and whilst roots generally grow downwards, they also tend to spread outwards to form a network. So, if the roots are mainly located in the upper layer of the soil, they may not be able to absorb water as the soil dries.   Now, research at the University of Nottingham has found that the plant growth regulator abscisic acid plays a critical role in a plant’s response to drought.  The abscisic acid  promotes the production of another growth regulator - auxin.  The two enhance the plant’s geotropic response* - so that the roots permeate deeper into the soil in search of water. Full details in the research paper here : https://www.sciencedirect.com/science/article/abs/pii/S0960982224016439?dgcid=coauthor * Geotropism is a plant’s response to gravity.

Woodlands TV has just released another film, which discusses the pros and cons of tree planting in an area, as opposed to allowing natural regeneration.  Many years ago, Kenneth Mellanby [Director of a Research Station near Huntingdon] thought that it would be interesting to watch what happened to a barley field if it was left to its own devices.  The 4 acre field was ploughed for a final time in 1961 and then ‘abandoned’. The field sat next to an area of ancient woodland - Monks Wood. The changes in the field were monitored in succeeding years and eventually becoming known as the Monks Wood Wilderness Project.   In the first 10 to 15 years, a thorny scrubland developed with hawthorn and bramble.  Birds were attracted by their fruits and berries.  The birds dispersed the seeds of the fruits across the plot, and also brought in seeds from other areas (such as the adjacent woodland).  The bramble and hawthorn scrub afforded protection from the wind and herbivores [rabbits, deer etc] for young, delicate tree seedlings.  Wind blown seeds arrived in the area, such as ash and field maple.  Other tree seeds were deposited by birds, Oak began to grow.  Acorns were probably carried in by Jays, who bury them as a winter food store.  An acorn is a rich food source - containing fat, carbohydrates, proteins and micronutrients.  A single jay can scatter many hundreds of acorns each year.  They store more than they need, so some will germinate and grow on to saplings.  Whilst many different birds spread tree and shrub seeds, jays have been known to carry acorns significant distances and probably contributed to the northward spread of the Oak as the glaciers of the last Ice Age retreated. A mature oak tree can provide food and / or a home for many species of animals, plants and fungi. The former barley field has been monitored over the decades, and by 2014 it had become an oak woodland.  This despite the usual depredations of Rabbits, Hares, Muntjac Deer and Roe Deer which have been present at various times over the years.  So given time, former agricultural land can revert (rewind) to mixed deciduous woodland with a diverse collection of plants and animals.  For further details of Monks Wood - visit https://www.naturewalkswithcarol.com/monks-wood-nature-reserve.html The woodlands TV film features ecologist Connor McKinnie who highlights key factors to consider when planning woodland creation, such as tree protection methods, planting pattern, and species. This is the second film with Connor, exploring aspects of rewilding.  The first film is presented below : [embed]https://youtu.be/HZNX9hTzQaU?si=-Ivv4aziPzi4a3cA[/embed] The latest film may be viewed on YouTube by following this link : https://youtu.be/Qx7uiOlGjFY

The Lambir Hills is a national park on the island of Borneo.  It is a small park composed largely of mixed dipterocarp forest, with some small areas of 'kerangas' (heath forest). The forest is monitored and biologists previously recorded  237 species of birds,  64 species of mammals,  46 species of reptiles and  20 species of frogs 1175 different species of tree in the national park.  But now, large mammals (e.g. gibbons and sun bear) are absent or very rare due in part to the small size of the forest but also the impact of illegal hunting.  The most recent surveys failed to find many of the park's resident birds and mammals. As a result of the loss of these species, the make up of the forest has changed markedly in the matter of a few decades. Whilst there are now more trees, with lots of saplings, the variety of trees has declined, particularly of those tree species that rely on animals to disperse their fruits / seeds. The seeds are ingested and pass through the gut of birds and mammals, and finally expelled with a small amount of ‘fertiliser’.  The loss of seed dispersers is having a significant effect on the composition / make up of the forest. There is concern that this sad tale is being played out across the forests and woodlands of Europe.  Seed dispersal is essential in maintaining the biodiversity of woodlands and forests.  Many European species of plants and trees are dependent on animals / birds for the dispersal of their seeds, and it is known that the numbers of certain animals and bird are in decline.  The dispersal of seeds is increasingly important as the landscape becomes increasingly fragmented (due to agriculture / roads, urban expansion species may need to extend their range as a result of climate change (fires, drought etc). But which animal / bird disperses which plant seed / fruit?  There are thousands of plant species and hundred of different animals that might act as dispersers (even ants).  Sara Mendes at the University of Coimbra has started on this mammoth task.   She has identified nearly 600 species that have adaptations for animal dispersal (such as flesh fruit), and nearly 400 animals that ‘transport’ such seeds.  Some of the dispersers eat more than one type of fruit, so the number of interactions is legion.  However, sadly many of the seed dispersing animals / birds are dwindling in numbers or rated as threatened by the IUCN.  For example, the garden warbler (a migratory species) is known to spread the seeds of some sixty different plants but its numbers are in decline.  The redwing is another migratory bird, but which is amber listed. Another worrying feature is that many plant have five or fewer animals / birds to disperse their seeds.  Unfortunately, in many cases, we do not know which disperser species are important to which plant species, or whether another bird or animal could take on the role. Dipterocarps :  a family (Dipterocarpaceae) of tall, hardwood, tropical trees mainly found in southeastern Asia . They have a 2-winged fruit and are a valuable source of  timber, aromatic oils, and resins.