Tanveer – Year 12 Student
Editor’s Note: Year 12 student Tanveer writes insightfully here in response to a question set as part of the Newnham College Biological Sciences Essay Prize. All female students currently in Year 12 at a UK school may enter the annual essay competitions run by Newnham College, Cambridge. CPD
Many view the act of planting a tree as one of the most valuable things to do for the betterment of the environment due to the vast array of benefits it provides. One of the most commonly recognised benefits is the potential trees have to minimise the effects of global warming because of their ability to remove carbon dioxide from the atmosphere thus reducing the greenhouse effect. However, trees are also essential in maintaining biodiversity and can even prevent floods. This means that planting trees provides living organisms with food, habitats and defence, which are all instrumental to life itself. With global warming currently causing detrimental effects across all ecosystems, planting trees could significantly reduce the risk of the deterioration of the planet.  Therefore, action needs to be taken to plant these trees as an immediate response to shield all living organisms from the changes that the planet is currently experiencing.
Versatility of trees
Trees benefit us in more ways than we can imagine, making them exceptionally versatile. They can be of particular value in preventing natural disasters such as floods from occurring. One way they do this is by creating small channels in the soil called ‘macropores’, when they grow, which means that water can flow into these channels rather than over the surface and into a river. As well as this, the roots of a tree hold the soil together making it more cohesive so it is not washed away into the river. Consequently, less soil and sediments are found on riverbeds so there is more space for water and the risk of water overflowing is reduced. The Pontbren Project in Wales in 1994 provides evidence that suggests that trees do in fact prevent surface run-off of water thus preventing flooding. It was discovered that the soil in a woodland area of a particular farm absorbed water 60 times quicker than in the grasslands, which were just 10m away (Keenleyside et al. 2016).
As humans, we are not the only living organisms to rely on trees for survival – in fact it could be argued that other organisms depend on these trees much more; 80% of the world’s land based species live in forests where they have access to food and shelter both being fundamental to their survival. The increase in deforestation over the years has substantially decreased biodiversity due to the fact natural habitats have been demolished. This reduction in animal species may have more severe consequences than previously thought. Ander Dobson, a researcher at Princeton University said, ‘When biological diversity declines, and contact with humans increases, you have a perfect recipe for infectious disease.’ There has been some evidence to suggest that animals that thrive even in fragmented habitats are often the ones that carry pathogens whilst those that are the most negatively affected by the destruction of their habitat are immune to such pathogens. This means that the pathogens that the thriving organisms carry are more likely to spread to other animals or even humans. Planting more trees could therefore allow such situations to be avoided and reduce the risk of infectious disease. 
How do pathogens invade trees?
Despite the incredible ways trees protect us, they are constantly under the threat of pathogen invasion. Many living organisms could potentially develop diseases caused by pathogens, and trees are no exception. However, just like many of these organisms, they have ways to protect themselves against these harmful microorganisms. Plants have developed a complex detection system, which allows them to recognise pathogens as part of their innate immunity. Pathogens are generally detected by pathogen recognition receptors (PRR) found on the plasma membrane of the plant cells.  These receptors recognise the pathogen associated molecular patterns (PAMP) found on the surface of pathogens, which stimulates a PTI (PAMP’s triggered immunity) response. The pathogens also secretes harmful substances called effectors that they inject into the plant cells to hijack the plants immune system. These effectors activate specific sensory proteins (known as R-proteins) in the plant causing ion flux within the cell, which ultimately leads to the hypersensitive response. This prevents the spread of the infection to the whole plant by killing the cells in the infected region thus counteracting the pathogens ability to spread and destroy the whole organism. However, as the pathogens have evolved, their effectors have gained the ability to weaken the plants response to infection, making the plant basal defence less effective in detecting and responding to pathogens. As a result, the pathogen successfully invades and infects the tree. 
Pathogens can then spread between plants in a vast range of different methods. They can be carried by the rain and spread via droplets; however, this does not apply to viruses as they remain in the cells that are infected.  Many pathogens require the assistance of vectors such as aphids (which can aid the spread of Strawberry Mottle virus)  to help transport them to other plants that they can then infect. A number of pathogens can move between plants without any support such as nematodes, which can swim in wet soil to nearby plants. 
Relationship between planting more trees and the spread of plant pathogens
Just like all communicable diseases, the spread of a plant pathogen escalates by increasing the abundance of the potential hosts. This is because the more densely populated an area is with trees, the more likely the disease is to spread from one organism to another and wipe out a whole species in a particular area. An example of where this has occurred is the growth of Hevea, which is a species of tree essential for rubber production; it has led to an increase in the spread of leaf blight in South America. As well as this, an increase in the number of Tanoak trees grown led to a sudden surge of Sudden Oak Death.  Both cases of diseases are because of silviculture – the practice of growing trees in order to meet particular needs. Afforestation as a response to silviculture has increased patterns of diseases because it significantly increases the abundance of trees therefore the number of pathogen hosts. Any kind of variation in the environment (such as the introduction of a pathogen) can have a significant negative impact on trees because they are organisms that do not mutate often and are very long-lived. As a result, they cannot depend on processes such as natural selection in order to survive so it is much easier for pathogens to kill many trees of the same species. This shows that planting trees could potentially not be enough to improve the environment as plant pathogens may eradicate entire forests of the trees we plant rendering our efforts futile. However, the possibility of planting more trees increasing the likelihood of the spread of disease does not at all undermine the benefits of planting them and instead subsequent improvements should be made to prevent this from occurring.
Monitoring and detecting plant pathogens to reduce the spread of disease
To avoid major outbreaks of plant pathogens monitoring and predicting the spread of the diseases they cause is essential. This would allow adequate measures to be put in place to reduce the risk of the disease spreading further. A particular pathogen called P. ramorum caused the disease sudden oak death in the state of California and killed millions of oak and tanoak trees across the heterogeneous landscapes. This disease is an example of a time when monitoring the progress of a pathogen would provide as a huge advantage in helping prevent further spread. Figure 1 (below) shows a map that was used to locate areas with trees that tested positive with Sudden Oak Death.  In order to monitor the spread of the pathogen the California Department of Food and Agriculture used a realistic geographical model. The map itself provides information about host index (the number of trees) and locates areas with trees that tested positive for the pathogen. The model also helps to outline the risk of the disease spreading to ecoregions, which have a high density of hosts thus a higher susceptibility to invasions by pathogens. As well as this, the magnified image A shows a link between increasing host index and the number of effected trees. It is clear to see that places with a higher host index tend to have more affected trees. This shows that planting more trees could encourage plant pathogens to spread at a greater rate. Nevertheless, by monitoring and detecting the diseases caused by plant pathogens, the spread of them can be controlled effectively to minimise the number of trees damaged. This is because infected trees can be destroyed on a smaller scale to stop the disease spreading any further.
How can planting trees cause plant pathogens to spread?
Despite the fact that the spread of plant pathogens cannot be avoided in certain circumstances, planting trees in a reckless fashion can hand pathogens the power to wipe out an exuberant number of trees. An example of such careless planting is the deliberate reduction of genetic diversity to produce multiple clones of trees with favourable characteristics. Reducing the gene pool and therefore the diversity of a species of trees in this way means that infection rates soar. This is down to the fact that every member of a certain species will produce the same R proteins in response to the pathogen so any ineffectiveness in immune response is prominent in all trees. As well as this, cloning many trees makes it significantly easier for pathogens to evolve mechanisms that will hinder the trees immune response. The reason for this is that the pathogen will mutate in such a way that allows it to avoid detection by one plant as all the plants are identical they are all effected by this pathogen. This means that all the plants are incapable of protecting themselves leading to the possibility of the eradication of an entire species.  If trees were planted in a way that maintains genetic diversity, it would restrict the pathogens ability to infect all trees reducing the spread of the disease it causes.
A surge of a disease can be caused by a non-native tree species being introduced to a certain area. As species of plants differ in their immune responses, a non-native species may be immune to a particular pathogen, which a native species is not immune to. A healthy non-native plant may carry this pathogen (whilst remaining unaffected) and pass it on to a native plant. An example of this occurring is the spread of the fungal disease Ash Dieback (Hymenoscyphus fraxineus), which originated in Asia. Although it does not affect native species of tree hosts like the Chinese Ash Tree, its introduction to Europe had devastating consequences on the European Ash. Unlike the native trees, the European Ash had not evolved to tackle the fungus that causes Ash Dieback (as part of its basal defence system) so it had no means of defence against it.  The yellow line on figure 2 below shows recently planted trees are generally at a higher risk of contracting the fungal disease which provides further evidence that planting more trees may increase the spread of pathogens. The disease is reported to cost the UK a tremendous amount of money, which has been approximated to be £15 billion.  This emphasises the risk plant pathogens pose on the economy as well as the trees we all rely on heavily for survival. By imposing further restrictions regarding the import of trees, such catastrophes could be avoided, preventing the global spread of plant pathogens. This is an example of why it is of the utmost vitality to assess the risk of disease before planting trees in any location.
Why planting trees is enough despite the risk of spreading plant pathogens
A single acre of trees that are planted have the ability to absorb as much carbon dioxide as a car releases after travelling 26000 miles and produces enough oxygen to keep 18 people breathing for a year.  This alone shows how fundamental trees are to the environment as they can effectively combat the current climate crisis by reducing the level of CO2 in the atmosphere as well as sustaining life itself. Being irreplaceable, we have no option but to continue to plant trees in order to keep all ecosystems and the organisms within them functioning. This makes the risks of the spread of plant pathogens pale in comparison to the benefits of planting trees (especially if the they are planted responsibly) as they can restore the planet by making it a healthier and safer place for us and for future generations.
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