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.

There are eleven thousand trees in Kew Gardens.  Each year, a few trees are lost due to natural causes, old age, disease etc.   In 2002, a drought resulted in the loss of  some 400 trees.  Such a prolonged dry spell is  likely to occur again and again as global temperatures rise, and climate change takes a hold. Modelling of future climate scenarios by Kew scientists suggests that towards the end of this century between a third and a half of Kew’s trees could be lost.  Trees like the English oak, beech, birch and holly could be vulnerable to warmer temperatures and extended dry spells.  There is a plan at Kew to replace gradually trees with species currently found in warmer areas, such the Mediterranean, Asia and Central America. Examples might include species such as the iberian alder, cherry hackberry and Montezuma’s pine.  Many of the plants in the gardens will survive, [including Kew’s ‘Old Lions’] as they were collected from in and around the Mediterranean; some of these date back to the victorian era or earlier. The ‘old lions’ of Kew are trees from the original grounds / garden that still survive. Examples include : Japanese pagoda tree (Styphnolobium japonica) Maidenhair tree (Ginkgo biloba) Oriental plane (Platanus orientalis) Caucasian elm (Zelkova carpinifolia) Black locust (Robinia pseudoacacia) The Caucasian elm dates from 1762, when an arboretum was planted.  It is thought that it might have been in a batch of plants from the Caucasus, planted in what is now the herbarium paddock.  In 1905, the height of the tree was recorded as 60 feet (18M), though they can grow to 100 feet.  A larger caucasian elm can be seen at  Tortworth. One species of oak that is common at Kew is the holly or holm oak (Quercus ilex).  This is a common, naturalised oak that was probably introduced into the country in the sixteenth century.  It is a hardy, slow growing tree and many new holm oaks were planted in 2008 to redefine the Syon Vista.  The wood of the tree is strong and, in the past, it was used in carts and farming equipment. Its acorns start off green in colour but turn a reddish brown; they are a tasty treat for pigs. The threat to Kew's trees is not unique, parks and urban spaces across the country need to plan for the future, to ensure that their trees can offer some resilience to changing weather patterns. Full details of Kew's planning here.  

Woodland covers some 1,000 hectares of the Ashdown Forest, that is roughly 40% of its area. Much of the woodland is relatively young. However, the forest’s capacity for regeneration / renewal is being damaged by overgrazing.  Local deer populations have grown and now represent a problem. Deer browse / graze on vegetation, shoots, flower buds and foliage are stripped off plants.  Young saplings are damaged and bark is eaten, especially when food is scarce.  Consequently, tree and shrub regeneration is limited.  Other species are affected by the feeding of the deer, either through loss of niches or food.  Among those at risk are small mammals and certain butterfly species. [caption id="attachment_34368" align="aligncenter" width="650"] deer damage[/caption] Damage is found in woodlands in many parts of the country, as deer populations have increased in recent times. In the 1970s, the deer population was estimated to be around 450,000 as compared to today’s estimates of over 2 million.  The National Forest Inventory highlighted that "40 percent of British forests have ‘unfavourable’ levels herbivore damage, which limits the survival of young trees and threatens biodiversity".  Apart from deer damage, there is damage by the grey squirrel populations. Deer browsing can : Prevent natural regeneration Affect biodiversity Affect woodland resilience Reduce food availability to the herd which can lead to starvation / loss of condition Deer are also hosts to ticks.  The ticks may be infected with Borrelia burgdorferi  bacteria and transmit them to humans, resulting in Lyme disease. Deer  also contribute to collisions with motor vehicles; more than 450 deer were hit by vehicles on Hampshire roads last year .  In Scotland,  government agency figures indicate that deer vehicle collisions  [DVCs] have almost doubled between 2008 and 2020.   Sadly, people are injured or killed in DVCs, and the repair cost to vehicles runs into millions. The solution to the ‘problem’ is not clear cut. Culling [the selective killing of animals] to control deer populations is one way in which numbers can be reduced, and the damage to woodland mitigated.  However, this approach has been met with opposition by many, including animal rights organisations.  There is the argument that whilst a reduction in deer numbers might fix some problems in the short term, the subsequent increase in plant growth and food availability might lead to increased breeding by the remaining deer and numbers would then increase again.  Also, unsuccessful or inaccurate shooting leads to animal suffering, mutilation and / or a lingering death.   Some might advocate rewilding and the introduction of apex predators (such as the wolf, lynx, wild cats*) as a means of reducing numbers but that might raise other problems! Deer have been 'part and parcel' of woodlands since mediaeval times, when the forests were used for hunting. In the Ashdown Forest, the number of red deer declined during the C17th,  and poaching was a factor in their decline. Fallow deer numbers also declined. [Fallow deer were introduced to England by the Normans around 1100 AD.]  The deer population roaming the forest has increased significantly in the recent decades, and now there are the relatively recently introduced species, muntjac and sika deer.   There are six species of deer in UK woodlands – the two native species, the red deer and roe deer and fallow, muntjac, sika and chinese water deer make up the four non-native species. The problem of over grazing is not unique to the Ashdown forest. For example, deer numbers in Scotland have doubled in recent years to almost a million since 1990.   Finding sustainable (and humane) solutions to the large numbers of deer is difficult. * Wildcats were once widespread in Britain, but by the end of the 18th century, they were to be found only in the northern regions. [caption id="attachment_34415" align="aligncenter" width="700"] Remnants of birch woodland near Loch Muick are subject to browsing by red deer (especially in the winter), so temporary fences have been put in place to allow for regeneration.[/caption]

At present, our forests and many across much of Europe have a medley of different species, and this has been the case for many hundreds of years.  They have survived minor fluctuations in climate and weather.  However, now climate and weather are changing in significant ways.  There are more extreme weather events, ranging from unprecedented rainfall to drought and periods of very high temperatures.  Winters seem to be be warmer and wetter, summers hotter and drier. Consequently, there is concern that many tree species being planted today will not be able to survive in the conditions that they are likely to experience in 50 or a 100 years time.  Species like the European Beech (Fagus sylvatica) are likely to struggle (like many did in the heat wave of 1976).  The root system of the beech is shallow, and though it has large roots spreading out in many directions, it cannot access water that may be present at deeper levels in the soil.   Though it is not known how native trees might adapt or be able to respond to a changing climate, it is possible that the number of tree species per km2 able to survive through to the next century may well fall by a third to a half in a warmer climate (depending on how quickly the warming occurs). Examination of some 60 plus European trees species at University of Vienna by Johannes Wessely et al suggested that the English or Pedunculate Oak (Quercus robur) may be a species that could cope with changing climatic conditions. It seems that native UK Oaks are genetically diverse, and this gives rise to variation and the potential to adapt to changing conditions.  Oak is wind pollinated and its light pollen can be dispersed over long distances, which promotes outbreeding and genetic diversity. Whilst the oak has always been valuable as a species for :- Timber production : it is used in furniture making and in the past thousands of oaks were used in the building of ships such as the Mary Rose. Carbon sequestration / storage - it is long lived and has a large above ground biomass Biodiversity : it provides a ‘home’ for many species of animals, plants and fungi. It offers food and shelter for many invertebrate species, numerous insects and spiders); its leaves often show the ‘scars’ of their feeding activities. Its bark is an ideal substrate for many lichen and bryophyte species (epiphytes). The roots of the trees establish mycorrhizal associations with various fungi. Now, the Oak may prove to be valuable in a warmer world as a species for timber production and reforestation projects.  The Oak’s ability to support other plant, animal and fungal species would also be important in terms of biodiversity and resilience..   Forests with a smaller number of tree species are thought to be less resilient to climate change and less biodiverse.   [caption id="attachment_41217" align="aligncenter" width="675"] A solitary oak[/caption]

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.

Rapid onset climate change, and the spread of new pests and diseases are creating unprecedented challenges to the long-term survivability of UK woodlands.  This looming threat is becoming ever more tangible, and the need for strategies of resilience building is urgent. Promoting diversification within and amongst woodlands has been identified as one such strategy with the potential for significant, positive impact. DiversiTree is a UKRI-funded project led by the James Hutton Institute which is measuring the impact a more diverse mixture of tree species has on building resilient woodland ecosystems, as well as how woodland managers and others understand woodland diversity, and what they are CURRENTLY doing to promote resilient woodlands. The project also hopes to generate practical advice and results which managers can use to make better informed decisions regarding the species mix of their woodlands, especially with regard to conifers. A key question which often accompanies discussions of woodland diversity is the planting of non-native species within British woodlands. The DiversiTree project is taking an evidence-focussed approach to its assessment and are investigating how ecological resilience interacts with woodlands with different priorities or objectives and what this might mean for the longer-term ecological sustainability of the forests of the UK. In actuality, many native woodlands are rather species poor, and could potentially benefit from a period of managed diversification with native species, non-natives, or a mixture depending on local objectives and context. What is critical here, is understanding the ecological role ANY tree can serve in a complex landscape, and planting in a manner which enhances and strengthens a woodland’s biodiversity.   If you’d like to learn more about our work and keep updated with our progress, please follow us on Twitter @DiversiTree_UK (https://twitter.com/DiversiTree_UK?s=20) or email [email protected] with any questions.  Seumas Bates (Environmental Anthropologist, Bangor University)  

In recent times, we have heard much about various initiatives to plant more trees, such as The Queen’s Green Canopy tree planting project to mark the platinum jubilee. Tree planting is part of the government’s plan to mitigate certain aspects of climate change as the trees will absorb carbon dioxide from the atmosphere, which is one of the principal greenhouse gases.  Once absorbed and used in photosynthesis, much of this carbon is then locked away for many years in the form of complex compounds, such as cellulose and lignin. The peak of carbon dioxide uptake by UK woodlands and forests was estimated to be just under 20 million tonnes in 2009.  However, since that time the amount has actually declined.  Many of the conifer plantations that were planted back in the 1970’s and 1980’s have now been felled / harvested, so they no longer contribute to the uptake of carbon dioxide.  It is important that these clear felled areas are replanted and tree cover restored; indeed, in places increased.  Planting rates have risen in Scotland but the performance by the rest of the UK is somewhat limited.  Even with new planting, it takes time for such new forests / woodlands to reach the CO2 absorption levels seen in the 2000’s. Continuous cover forestry (CCF) is a different approach to woodland forest management; it seeks to avoid clear felling and  promotes a mosaic of trees by age and species. There are a number of factors that influence new planting, such as the availability of land.  Using high grade agricultural land for tree planting would affect targets for increased agricultural productivity and domestic food supply.  Also, with climate change and the increasing number of extreme weather events (storms, flooding, drought etc) greater thought needs to be given to risks such as forest fires.  Recent months have demonstrated the ferocity of forest fires in France, California and Portugal.  We cannot assume that the UK will be exempt from such events.  Similarly, we have witnessed powerful storms (such as Storm Arwen) which felled many thousands of trees (and impacted on public services such as electricity and train travel).  The warming climate is also associated with ‘new’ diseases and parasites in our woodlands and forests. New planting needs resilience ‘built in’ to the plan. The Environment, Food and Rural Affairs Committee in its recent report (2021/2) has made a number of important comments in relation to tree planting in the UK, notably : It noted that the Forestry Commission had said that nurseries in the UK will struggle to expand production to deliver the number of young trees required for the Government to achieve its planting ambitions. There is a lack of a sufficiently skilled and large workforce to achieve England’s tree planting ambitions. To meet the Government’s tree planting goals, the UK will need to import seeds and young trees (until domestic capacity increases); and this carries the risk of the introduction of pests and diseases.  

If you have planted new trees and there's been a spell of extremely dry weather, as happened in the summer of 2022, you might be very worried for their survival, but there are several things you can do to increase their chances.  This isn't just a problem for the first season because saplings can be at risk for the first three years after planting; it's also not a binary, live or die thing for your planting - usually some trees will survive and some will perish but the challenge is to maximise the percentage that make it through a drought.  Let's first consider how dry conditions affect trees.  The saplings' first response is often to increase root development to take up more water.  Your young plants will also seek to reduce water loss so the leaves will wilt or can even be shed even though it's only July or August, and this can lead to the saplings not having the ability to photosynthesise adequately.  So you need to do what you can to avoid water loss and, if practical, to apply water. So, to increase the chance of your saplings surviving you can, if practical, actually spread water around them but that may have to be done fairly regularly during the period of drought.  Ideally irrigation should happen overnight when evaporation is lower and that will also limit pest problems and "leaf burn" that may be associated with irrigation in full sun. An additional strategy is to weed the young trees thoroughly to reduce the competition for water.  This can be combined with firming up the soil around the base of your saplings to avoid the soil drying out through fissures.  To reduce evaporation and stop the weeds returning you can use a mulch around the base of your young trees - ideally a mulch mat or by spreading some woodchips or bark. A few weeks after the drought has passed but while the leaves are still on the saplings you can assess your losses and order new trees to replace those that have succumbed to the dry weather.  This process is described by foresters as "beating up" but when it comes to the actual planting this needs to be done carefully - only replace trees that you are sure have died rather than those which just lost their leaves early. For urban trees you can be prepared for a period of dry weather by installing watering tubes or bags which reduce surface run-off and make sure that the water reaches the tree roots.   It is unclear how many millions of trees have been lost to the 2022 drought but it certainly includes many mature trees as well as many newly-planted saplings and hedgerow shrubs.  It seems that droughts like this are part of a process of climate change and more such events are probable - so our woodlands will be under increasing stress over the next few years.  For those who are establishing new woodlands a sensible approach would be to plant only a proportion of the area each year and to select a wide variety of tree species.  It also suggests that for new woodlands you should, where possible, include the adopting of areas of natural regeneration - where trees have come up of their own accord - these self-set trees are likely to be very resistant / resilient to periods of extremely dry weather. [caption id="attachment_38870" align="aligncenter" width="600"] Drought - green shoots of recovery?[/caption]