by Rob Jackson, Estrella Luna-Diez, Graeme Kettles and Megan McDonald
Trees, like all other organisms, experience a lifecycle. Starting from a seed, the seedling emerges from the earth and grows in size, reliant on the water and nutrients in the ground and on sunlight for provision of energy (sugars) via photosynthesis. This process means that the trees are creating nutrients in themselves whilst also depositing other nutrients into the earth from root exudates, fruit and seeds, leaf litter and bark. Over the millennia, trees have therefore attracted a wide range of fauna to tap into their tasty offerings as well as offering a haven for shelter (animals) or as a ‘birthing station’ (e.g. insects bury their eggs in the bark or leaves).
Naturally, humans have also been attracted to trees for various resources such as timber for construction, fuel for fires, sap for rubber, and seeds and fruit for food. In many cases, we have artificially selected trees for traits that are beneficial to us, for example gaining food of a particular quality, and in quantity, and thereby growing trees as crops (apples, pears, cherries). We have also planted trees for ornamental purposes (horse chestnuts and planes) and for timber production (oak, cherry, pine, fir, spruce and larch).
Like human bodies, whose cellular content is at least half microbes, trees are not typically sterile entities. They will usually have microscopic organisms surrounding their roots, all over their surfaces and deep inside their tissues. In most cases, these are mutual interactions with partnerships between the microbes and the tree, exchanging nutrients between one another.. We are also starting to realise that microbes living inside trees can do remarkable things like utilise greenhouse gas (GHG) methane. This discovery emerges from the realisation that microbes in the soil can create methane gas, which dissolves in water just enough to pass through the tree root system and through the trunk to the atmosphere – methane utilising microbes can access this methane to help them grow. Trees are also responsible for sequestering huge amounts of carbon, both into their structure and into the soil. Thus, trees represent a vast carbon and greenhouse gas sink important for our climate.
However, we are now observing a nexus of problems for trees particularly with the widespread emergence of pests and diseases. In the UK, the most prominent disease in recent years was Dutch Elm Disease that absolutely devastated the UK elm population. This disease is caused by a beetle that carries with it a fungal pathogen. More recently, Ash Dieback Disease (ADD, caused by the fungus Hymenoscyphus fraxineus), Acute Oak Decline (AOD, caused by a bacterial consortia) and Horse Chestnut Bleeding Canker (HCBC, caused by the bacteria Pseudomonas syringae pv. aesculi) have also been shown to be widespread within the resident tree population. The increasing incidence of these diseases can be attributed to a range of factors, unfortunately all linked directly or indirectly to human activities. Trade and the exchange of live plants and wood products spreads pests and diseases throughout the world. The introduction of a new disease (by humans) can be exacerbated by our changing climate. For example, warmer winters allow insect populations to continue to grow and cause more serious disease outbreaks when spring arrives. Changes in seasonality can also allow insects/pathogens to expand their ranges into regions that previously did not need to worry about these pests. Finally, alterations in precipitation, and especially periods of drought, can make trees more vulnerable to both introduced and endemic pathogens.
The way in which we grow trees and use once-forested land for our own purposes (city expansion, agriculture) has reduced the absolute numbers and diversity of trees that are grown. We have also dramatically narrowed the gene pools of commercially grown trees by strongly favouring monoculture (growing trees that are essentially copies of each other). Classic examples would be extensive forest clearance in the Amazon for soybean and cattle farming and in South East Asia for oil palm plantations, spanning approximately 3-5 million hectares (larger than Belgium). The large scale of these clearances and subsequent loss of ecosystem services has a direct negative impact on planetary health, as these smaller forests are unable to support the fauna (biodiversity) that depend on trees and, of course, the loss of carbon and GHG storage. Moreover, the spread and emergence of pests and diseases further exacerbates these issues, especially in monoculture systems because diseases can sweep through the entire crop. Severe disease outbreaks in tree plantations or forests do not only result in tree death but also have knock-on effects to release more GHG, pollution, and increase the risk of flooding and fires. A UK “rush to carbon” that focused on extensive planting of fast-growing clones would be making a similar mistake: forsaking a healthy and biodiverse forest for a single-minded, but highly vulnerable, goal.
Future predictions on the impacts of pests and disease suggest a worrying outlook for humans and wildlife. Climate changes will no doubt be a major part of this problem, for example, imagine that the UK sits in a temperate zone, but current climate predictions indicate that the typical London climate will change from its current status to be more like Barcelona in 2050. This is just one example of how climate is predicted to change in one part of the world – globally, it is clear that big changes are on the way. What does this mean for trees in the context of pests and diseases? And how do we study this and try to make the necessary preparations and changes to counter the effects. In other words, how do we make our forests resilient?
What we know
Like the COVID-19 pandemic, trees around the globe are suffering from major disease and pest outbreaks –trees are stationary and widespread, they are unable to communicate their illness, and we do not know how to aid them beyond an initial response of introducing stricter biosecurity measures. Governments around the world reacted quickly to tackle the COVID-19 pandemic. In contrast, the parallel approach for trees and forests of the world is slower, but no less important. Unlike COVID-19, however, it is not a single disease that we face but many diseases caused by a very diverse set of pathogen species.
One of the most impactful issues for trees in the world right now is the Mountain Pine Beetle (Dendroctonus ponderosae), which is killing huge swathes of pine trees in North American forests. In Canada, for example, an area of 18 million ha (larger than the area of England, Wales and Northern Ireland combined) has already been affected. Moreover, recent research shows that bark beetle epidemics influence wildfire severity, thus causing knock-on effects to the broader forest system. In the UK, we have outbreaks of invasive insect pests such as leaf miner (Cameraria ohridella) in horse chestnut and oak processionary moth (Thaumetopoea processionea) in oak; fungal pathogens such as ADD and red band needle, blight of pine caused by Dothistroma septosporum; oomycete pathogens including Phytophthora ramorum on larch and Phytophthora cinnamomi in Sweet Chestnut; and bacterial pathogens, for example,. AOD and HCBC. Scanning the horizon, we know that there are major pests and diseases beyond our island but close by, which if they gain entry, would threaten several native species (for example the Xylella bacterium towards several tree species) or severely threaten the survival of trees already under attack (the Emerald Ash Borer, for example, is attacking ash trees suffering from ADD). Identifying the pests and pathogens causing disease is only the first step in designing effective control strategies to protect our native forests. Below we highlight some of the newest research that reveals the tools used bypathogens to attack trees and some novel strategies which we can employ to protect them from these attacks.
The latest research
The importance of trees has always been appreciated, but unfortunately, support for research into trees has diminished over the past few decades. With valiant efforts from colleagues in Forest Research, the Animal and Plant Health Agency and a determined group of academic colleagues, this is now changing with new funding streams from government and specific recognition of trees in the new UK Plant Science research strategy. However, trees are complex, slow growing and challenging organisms to work on, which means the scale of research, and the urgency for doing it, are major challenges. There is much to do.
A key pathway to fighting pests and diseases is to understand firstly, how pests and pathogens attack trees and secondly, how trees resist pests and diseases. This means we require detailed knowledge of how pests and diseases live, survive and spread, and especially how they attack and damage trees. In parallel, we need to examine tree genetics and physiology to identify the important characteristics distinguishing healthy, resistant trees from those susceptible to diseases and pests. Moreover, we need to know how pests, pathogens and trees perform in the ‘gladiatorial arena’ when the climate they live in changes. This knowledge will help us guide practitioners on appropriate tree planting schemes and promote management practices that help trees to fight back.
The Birmingham Institute of Forest Research (BIFoR) is well placed to address these challenges, with research programmes studying disease in ash, oak and cherry as well as plant immunity, using state-of-the-art facilities at the University of Birmingham. The influence of increasing levels of CO2 is also being studied to evaluate how this mediates physiological changes in trees, insect colonisation and disease manifestation. This is being done using trees grown in elevated CO2 levels, using the largest temperate forest Free-Air Carbon-dioxide Enrichment (FACE) facility in the world (Fig. 1), just 60km NW of Birmingham, and our new Wolfson Advanced Glasshouses, based on the University of Birmingham campus.. A centrepiece of our research efforts focuses on the iconic oak tree, which suffers from oak powdery mildew (PM) attacking its leaves and AOD, the bacterial disease which attacks the stem.
Figure 1. The Free-Air CO2 Enrichment (FACE) facility of the Birmingham Institute of Forest Research: BIFoR FACE is one of the three largest climate-change experiments in the world. Pictured is one of six arrays surrounding mature trees and undercanopy; three arrays spray ambient air over the enclosed trees whilst another three arrays spray CO2 (150 ppm above an ambient level which is currently ~410 ppm) over the trees. Photo courtesy of Prof. Jo Bradwell.
Powdery mildew, priming of oak defence and the impact of elevated CO2
Oak trees (Quercus robur and Q. petrea) are a keystone tree species of the UK landscape, with ~121 million trees supporting 2300 species (not including microbes) of which 326 are obligate associates (i.e. if oak dies off those 326 species also die off). In addition to supporting these vast ecosystems, oaks are an important part of British culture and have huge economic, ecological and social relevance. For example, they are versatile and tolerant to different environmental challenges and provide a source of high-quality timber. All these characteristics make oaks excellent candidates for use in UK woodland regeneration. However, British oak woodlands suffer widespread tree loss associated with biological threats. In addition, the oak regeneration capacity is highly compromised by the extreme vulnerability of oak seedlings and saplings to the fungus Erysiphe alphitoides, which causes the deadly powdery mildew (PM) disease. (Fig. 2) Consequently, the PM fungus hinders the establishment of new oak forests. Current methods used by tree nurseries to control the PM fungus depend on the use of chemical fungicides, which are extremely limited due to their toxicity to human health and the environment1. Thus, in BIFoR, we investigate the immune strategies that allow oaks to fight against the PM fungus.
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