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Kveldssanger

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  1. Kveldssanger
    Man has always had a direct link to the landscape, though that link, whilst it is always there, may not be in the form that it once was. Keeping with the wood pasture theme, which I am really enjoying learning about through books and journal articles, I thought we’d look at how the manner in which we approach the ecosystem has changed over the centuries (quite briefly). Of course, what is written below doesn’t stop just at wood pasture – it has cross-over to other ecosystems, where the reasons for interaction with the landscape have altered through space and, more pertinently, time.
    Historically, wood pastures were managed for economic purposes. The grazing of animals on grasslands containing trees (and the feeding of the livestock with cuttings from pollarded trees, and a tree’s fruit crop), such as cattle and pigs, was for the direct benefit of communities, who relied upon the produce of the livestock (milk, meat, and so on) in order to make a living, and to generally therefore survive. Of course, the wood pastures needed to be conserved, so that they did not disappear, due to over-grazing. In this sense, they were actively conserved (by replanting dying and dead trees, and limiting grazing intensity), though largely because, without actively conserving them, the livelihood of many tens of thousands of people would be challenged. A by-product of this conservation of wood pastures, for the benefits created from grazing livestock, was that the sites were very rich in biodiversity – birds, fungi, insects, and plants, for example. The complex mosaic of niches within the wood pasture, ranging from open and disturbed soils through to the (perhaps sizeable) groves surrounded by the mantle and fringe vegetation, meant that a large number of organisms could viably frequent the site. However, for all of the biodiversity present as a result of the careful management and conservation of wood pastures throughout history, biodiversity was not the reason for management – until recently.
    The shift, in Europe, probably begun when wood pasture became disliked (for hope of a better word), during the 19th-20th century (varies depending upon the country). Foresters wanted to maximise output from the trees (coppice – sometimes with standards), and farmers wanted to maximise agricultural output. Therefore, the two practices, initially married, were divorced from one another (somtimes farmers were forced to stop grazing their livestock in wood pasture!). Wood pastures were thus either cleared of trees entirely, or alowed to regenerate into forest. With this came a decline in the richness of biodiversity and, eventually, this loss of biodiversity caused a rather evident of panic amongst conservationists. Ironically, therefore, the rationale behind creating and maintaining wood pasture became largely ecologically-driven, in place of economically-driven (though, particularly in Spain and Romania, wood pastures remain, are these are generally economically viable). Regardless of reason however, the status of wood pastures essentially went full-circle.

    A fantastic wood pasture in Estonia. Source: Ideas for Sustainability.
    Of course, this new found love for wood pastures does not necessarily mean that they can ever exist in the manner in which they did before. First and foremost, wood pastures are extensively grazed, and thus, for operations to be self-supporting financially, they must cover large expanses of land (unless the wood pasture is maintained for subsistence purposes, or grants are provided as a means of financial support). As farmers in Europe are generally in ‘the game’ for profit (they must make a living), managing livestock in wood pastures is probably not going to be all too popular, as it’d probably signal a marked drop in profits and / or a marked increase in labour input (at least, initially). Scope does exist to harvest edible mycorrhizal mushrooms from the wood pasture, such as truffles, though this is a specialised pursuit that is far from the current farming status quo of Europe.
    Furthermore, European culture has changed. Gone are the days of communities being self-sufficient, and instead many Europeans now work a job (that they may even hate) and buy their food from the supermarket (or even order it online). Therefore, is there even the desire to re-introduce wood pastures, for anything other than ecological reasons, or to supply the market with a niche animal product (such as Iberian ham from the black Iberian pig, in the holm oak dehesas of Spain). With this change in culture there has also been a change in learning priorities, and unfortunately many today seem to be fixated with knowing pointless facts about sports teams and celebrities. Functional and practical knowledge is largely gone. As a consequence, the management of wood pastures will be left to an expert few, where knowledge has either been gained academically, or via being passed-down through the generations (usually limited to rural areas where grazing still takes place). However, as more people now live in cities than in rural areas, and this trend will likely continue as rural areas are swallowed up by urban sprawl, or people move into cities for economic reasons, this tradition of passing practical knowledge on and keeping up the family tradition of extensive livestock grazing within wood pasture may very well become ever more the stuff of legend.

    The black Iberian pig grazing amongst a landscape of holm oak, in a Spanish dehesa. Source: Andrew Petcher.
    Society is simply in a different place than it once was. For this reason, the conservation of wood pasture is to be far from mainstream. People are certainly aware of nature (of which wood pastures feature), though more and more awareness comes from watching on the television and less from direct experience, and with this comes a discord. There is less emotional and cultural attachment to nature, and as a result, less of an impetus to associate with nature. Why help with the recreation of wood pasture when you can watch about its conservation on television, utter some lamentations, and then switch the channel and soon relegate it to a mere memory? That’s even assuming people watch such programs, in large numbers, in the first place.
    This probably turned out far more dystopian than I ever intended for it to come out as, though hopefully this illustrates how social changes have led to landscape management changes, with specific focus upon wood pastures in Europe. This is obviously applicable to other landscape types as well, of course. The principle generally carries across.
    Source (of inspiration): Hartel, T. & Plieninger, T. (2014) The social and ecological dimensions of wood-pastures. In Hartel, T. & Plieninger, T. (eds.) European wood-pastures in transition: A social-ecological approach. UK: Earthscan.
  2. Kveldssanger
    During the Carboniferous Period some 345-280m years ago, the continent of Pangaea began to drift northwards from its southern hemisphere origins, and also began to pivot 30 degrees to the west. Giant insects developed, amphibians evolved further and reptiles became more land-based. Plants were reproducing in an alternating manner of asexual and sexual methods across different generations (spores and seed respectively). Equisetum plants (includes Horsetails) became huge, forming Calamites in damp, swampy areas, and forming Cordaites (that had pollen sacs and ovules at branch tips - later forming the first conifers, such as ginkgos, during the Permian Period 280-225m years ago) in drier areas. Clubmosses grew to 30-45m in height. All of this development was fueled by the equatorial climate induced by the drift northwards of Pangaea, particularly at the northern-most end of the super-continent.
     
    Coal measures were thus formed very readily from such large plants in what now constitutes Northern Europe (given this segment of Pangaea was first to travel over the equator), once the sea began to engulf the swamps of massive clubmosses, semi-composting them and later compacting them down with silt and clay to form lignite, and eventually forming coal under continued compression events. It took 20m of rotted 'forest' biomass to produce a 1m-thick coal measure, so given many European coal measures are hundreds of metres thick, the length of time required to create such coal measures would have been hugely significant. This also adds significance to our eagerness to burn such stored coal, releasing carbon that has been locked away for hundreds of millions of years in mere decades.
     
    The southern area of Pangaea (Africa, Australia) contained smaller plants and thus smaller coal measures, only beginning to lay down larger coal measures (that never amounted to the extent of the earlier coal measures, as the steamy swamps that produced the huge clubmosses and subsequent coal measures no longer existed due to climate change) much later once the southern segment of Pangaea did reach the equator (during the Cretaceous and Tertiary Periods) when plants had evolved to develop roots and reproduce sexually via seed. Evolutionary-speaking, the development of reproduction via seed was critical to the survival of plants long-term, as germination could be delayed until conditions were desirable.
     
    Building on the above a little, though nonetheless as a tangent, Europe has so few tree species as tree populations could not retreat southwards as the ice sheets encroached into their territory, given the east-west running mountain ranges that are the Alps and Pyrenees, and the area that is now the Mediterranean Sea. Because of these blockading landscapes, tree diversity is rather low within Europe, when compared to the Americas and Asia.
     
    The UK's plant diversity is even more impoverished as when the ice sheets last retreated 12,000 years ago, the subsequent rising of sea levels bridged the gap to mainland Europe. Thus, only species that had colonised during the 6,000 years after the ice caps began to retreat are found today - any others that may have potentially once again reached these shores (I suspect sweet chestnut, plane, holm oak, etc, though definitely Norway maple and larch) were barred from doing given the mass of water in the way.
     
    Source: Davis, M. (2015) A Dendrologist's Handbook. UK: The Dendrologist.
  3. Kveldssanger
    The traditional view of mycelial fungi is that the development cycle can be split into two stages: vegetative and reproductive. The former relates to the mycelial spread within the host and the latter the production of sexual or asexual structures that produce spores.
     
    However, a more contemporary approach to discerning the development cycle of fungi reveals four distinct stages: arrival, establishment, exploitation, and exit. Such development stages are considered to be triggered by the changing conditions of the substrate (such as drying, cell degradation, etc), as in meeting the changing conditions the fungus must adapt its own behaviour (dubbed 'functional modes') to successfully persist within the host. Curiously, a fungus can compartmentalise itself so that, at different stages of decay within the same host, it adopts a different functional mode.
     
    Each mode is briefly described below:
     
    (1) Arrival: two known mechanisms exist: arrival as propagules (airborne and seemingly only favourable when localised conditions are optimised at the arrival surface - good supply of nutrients, a good microclimate, and a lack of competitors), or arrival as migratory mycelium (contact of an infected host to a non-infected host, such as with H. annosum and A. mellea).
     
    (2) Establishment & exploitation: three principal concepts of this mode are understood: the fungus must successfully gain access to the host and begin to command available resources; the fungus must begin to successfully convert potential energy resources into actual energy sources, and; the fungus must successfully 'wall-off' an area to resist against competitors or the host tree itself, or in turn have a rapid exit strategy (panic fruiting on the wound surface or within the inside of the hollow(ing) host, for example).
     
    (3) Exit: in the rawest sense, exit can be achieved either through the formation of reproductive structures or by the outgrowth of mycelium (relating back to the arrival strategies). The efficacy and extent of the exit mode is dictated by two factors: the extent to which resources are re-allocated from the mycelial biomass within the wood structure to the biomass of external structures, and; the effect of the environment on the form these exit structures adopt.
     
    At a slight tangent, the r-K continuum further dictates exit strategy, with r-strategists (Deuteromycotina, Zygomycotina) taking a more rapid, economical stance to reproduction (not developing massive or durable exit structures), whilst K-strategists (Ascomycotina, Basidiomycotina) limit commitment of non-reproductive biomass, though also possess greater ability to develop exit structures that persist and are thus durable and do so at more 'fixed' stages. Within the continuum, certain species do however hold the ability to possess more than one mode of exit (such as with F. hepatica and L. sulphureus having the ability to develop either as a basidiospore on primary fruiting structures and as a conidiospore on auxiliary structures).
     
    Source: Rayner, A. & Boddy, L. (1988) Chapter 5: Development Cycles. In Fungal Decomposition of Wood: Its Biology and Ecology. UK: John Wiley & Sons.
     
  4. Kveldssanger
    Severing roots out of purpose is hardly ever something that an arborist would find desirable, though it nonetheless occurs rather commonly where construction takes place and also where abatement of nuisance is practiced for terrestrial encroachment of a tree.
     
    Current research indicates that the severance of roots is, by-and-large, highly variable. In one instance, root severance may have very little adverse impact on tree stability, though in other cases may weaken a tree by over 20%.
     
    Roots that 'guy' a tree (exist uphill of the trunk) or reside on the outer (tension) side of a lean are ultimately far more crucial to the tree than the majority of the remaining root crown. Severing roots may not therefore simply be a case of "no more than 25% of the root crown can be lost", as context is key. If a guying / tension root is severed, the impact upon stability will be far more significant than if a compression root (or even multiple roots) is lost.
     
    Statistically (from a survey done on willow oak), when assessing strength loss due to buttress root severance at the base of the trunk, a loss of 50% of the buttress roots will reduce the mechanical required force to move the tree one degree by a third (33%). However, due to the oscillating nature of winds, such a loss in root mass will result in a much higher decline in strength, particularly for larger trees with more wind sail (or where root decay is evident). Interestingly, such a loss can at times be achieved simply by severing a single guying / tension root, which suggests that trenching may be of particularly significant adverse impact to trees in more exposed sites.
     
    Additionally, research indicates that severing roots closer than at a radius three-times the trunk diameter is not recommended, as tree stability declines significantly once this threshold is surpassed.
     
    Source: Smiley, E. (2009) Root pruning and stability of willow oak. In Watson, G., Costello, L., Scharenbroch, B., & Gilman, E. (eds.) The Landscape Below Ground III. USA: International Society of Arboriculture.
  5. Kveldssanger
    The Midland Reafforesting Association was created in 1903 with the intention of undertaking afforestation projects (amounting to 14,000 acres) across the Black Country, England. Whilst in principle such projects were met with support by the government and other organisations, less than 1% of the target was planted so by 1925 the project was terminated and the Midland Reafforesting Association dissolved.
     
    Predominant drivers behind the failure of the project included the residents' acceptance of the industrialised and bleak landscapes as if they were the norm and status quo, the lack of necessary funding from bodies that verbally supported the efforts of the Midland Reafforesting Association (particularly as the subsoiling / ripping of poorer-quality sites being very expensive), 'technical difficulties' (species selection, poor site quality, etc), and the fact that one of the core motives for the afforestation project, that of such forest creation improving land value, was at the time not supported by crucial evidence in favour of such a claim.
     
    Unfortunately, and somewhat ironically, only one-sixth of the derelict 14,000 acres remained by 1953. Most had been built on due to demand for infrastructural services and homes for the rising population of the UK. The remaining derelict land, which would amount to around 2,400 acres, did funnily enough regenerate naturally, gradually 'greening' the residual areas left behind after continued construction.
     
    To top the whole thing off, The Black Country Urban Forestry Unit (BCUFU) that was formed in 1985 to continue the efforts of the project from 1903, which evolved into the National Urban Forestry Unit (NUFU) in 1995, also disbanded (prematurely) due to a lack of funding from central government. The project managed to plant 837 acres of woodland over six years with a £8.5m budget, before calling it a day.
     
    Not all is lost in failure however, as the lessons drawn from the demise of such an ambitious project paved the way for research into species selection for plantations, management of plantations, and planting techniques.
     
    Source: Webber, J. (2008) Greening the Black Country: The Work of the Midland Reafforesting Association in the Early Twentieth Century. Arboricultural Journal. 31 (1). p45-62.
  6. Kveldssanger
    During the 18th and 19th centuries, coppice woodlands underwent an 'improvement' period, which involved either (1) selective removal of more undesirable species with artificial planting / propagation of more desirable species, and (2) conversion to high forest (perhaps even the former followed later by the latter).
     
    In reference to point (1), whilst many coppice woodlands only saw such improvement come in the form of gap-filling with Fraxinus excelsior and other desirable species, composition of other woods dramatically changed. For example, in the South-east Lowlands, certain coppice woodlands saw the introduction of Castanea sativa, Fraxinus excelsior, Corylus avellana, and Alnus glutinosa, whilst simultaneously seeing the removal of Acer campestre and, in somewhat of a paradox, Fraxinus excelsior, where the overriding objective of coppice was to harvest Corylus avellana poles. In the Western Uplands, Quercus petraea, and to a lesser extent Quercus robur, were selectively planted with the intention of subsequent harvesting for the leather tanning process and for charcoal to be sold into the metal industry, whilst Corylus avellana was selectively removed.
     
    In reference to point (2), the conversion of coppice to high forest was driven by local demands (or even general neglect). Where action was deliberate, Quercus spp. were principally planted, though Fagus sylvatica was also planted in abundance (notably in the Chilterns and the Cotswolds) as, after a period of undesirability (due to its poor coppicing ability), it could now flourish within the high forest, continuous cover-esque style management regimes. Ultimately however, Quercus spp. planting was more evenly-spread than Fagus sylvatica planting.
     
    Source: Peterken, G. (2015) Woodland History in the British Isles – An Interaction of Environmental and Cultural Forces. In Kirby, K. & Watkins, C. (eds.) Europe's Changing Woods and Forests: From Wildwood to Managed Landscapes. UK: CABI.
  7. Kveldssanger
    I’m currently engrossed in Vera’s book Grazing Ecology and Forest History, which I cannot rate high enough for its readability, lucidity, and coherency. Whilst on my holiday last week, I read through the third chapter on the study of palynology and how this relates to interpreting how treed landscapes may once have looked, and I have to be honest when I say that the entire field was somewhat (though not wholly) new to me, and what was suggested within the book certainly made me think. For this reason, I’m going to write a little bit about pollen studies and how it can effectively be used, or even ineffectively used, to determine what our landscapes once looked like, with regards to what trees existed, and in what abundance / distribution, according to Vera.
    Vera begins by ‘setting the scene’, by describing how the Swedish geologist, Von Post, in 1916, produced what is considered the first pollen diagram. Prior to the utilisation of pollen, typically accumulated in peat bogs and lakes (which are regional pollen sinks), larger parts of plants were used in an attempt to understand what the landscape once looked like (up until the last Ice Age some 12,000 years ago) – leaves, fruits, tree stumps, and possibly even larger seeds were three means of how the landscape’s vegetation history was being deciphered, and again these were usually found within peat bogs. In this sense, prior to 1916, the vegetation composition of a landscape was being understood through assessing how plant macro-fossils were distributed (vertically) in peat bogs. For example, if a stump of a pine tree was found below the leaves of a willow tree, one could suggest that pine trees existed prior to willow trees in the geographical area. After 1916, pollen, which readily remains desposited in such aforementioned naturally-occurring sinks, could instead be used. Granted, pollen generally only persists for wind-pollinated species, with the exception of poplars, so one cannot, in theory, decipher the exact presence of tree (and plant – grasses, etc) species – one can instead only interpret, based on the facts gathered.
    By-and-large, following Von Post’s landmark pollen diagram in 1916, studies into pollen presence had suggested that the landscape was once almost wholly covered in trees (where tree cover was possible, due to biotic and abiotic factors). This is because, when pollen studies have been undertaken, the large majority of pollen found has been from tree species (usually, non-arboreal pollen amounts for no greater than 5-20% of total pollen in the sinks). Historically, and prior to 1934, when Firbas published a paper on how one can also identify and use the pollen of grasses and shrubs to determine landscape composition, there was also a choice to ‘ignore’ the pollen of non-tree species. Because of these factors, scientific opinion was generally that grasslands and wood pastures are an advent of agriculture and man’s influence upon the landscape, in place of wild ungulates (auroch, bison, boar, deer, Przewalski’s horse, and so on) influencing upon the vegetation composition. Therefore, prior to modern man, the European and American landscape was largely void of expansive steppes and pastures, where the land was potentially habitable by trees. The wild ungulates were thus not seen as responsble for carving the landscape, and thus only existed in low numbers within treed landscapes. Instead, these wild herbivores followed the regression and regeneration of trees in the landscape.

    An artistic depiction of the auroch (Bos primigenius). Interestingly, the breeding of cattle in an attempt to re-create the auroch is being
    undertaken, by breeding characteristically-similar (to the auroch) domesticated cattle and allowing them to exist in the wild.
    Image source: Open Up!
    However, where it gets interesting is when one looks at what tree species were present in the pollen records. Before we look further at this however, we must recognise that the dense forest will generally be host only to shade tolerant tree species (beech, lime), assuming it has reached its ‘climax’ (prior to this climax, more light-demanding species will be present, initially with birch, hazel, pine, willow, and so on, and then with species such as oak). For this reason, if we assume that the historic landscapes were covered with high forest, we can assume that much of this high forest will be of climax species, as man was not historically around to carve apart such landscapes with cattle and for arable activities. Despite this, this is not what the pollen records show. In fact, hazel (Corylus avellana) and oak (Quercus robur) contribute quite significantly to pollen records, and as neither species will regenerate in high forest (because they are not shade tolerant), how is it possible that large tracts of the landscape were high forest? Unless the species were able to regenerate significantly enough in high forests to feature so readily in pollen records, which goes against the species’ understood biology and ecology, there must have existed landscapes where significant light was able to reach the floor. This is where Vera suggests that the landscape could very well have been shaped by wild grazing animals, who kept large areas adjacent to groups of trees or forests open (where there was the ‘mantle and fringe’ vegetation), and the thorny scrub that grew within such a grazed landscape enabled for hazel to grow in thickets and oak to succeed within such thickets (of hazel, and particular thorny scrub, on which ungulates would not generally graze). Oak, in particular, can in modern day be observed not to regenerate in high forest, but in grazed areas amongst thorny scrub (I myself saw this the other day at Dunwich Forest and nearby heathlands, where oak was regenerating not amongst high forest, but within the gorse and bramble scrub).

    A young Quercus robur growing amongst gorse and bramble, and protected from the impacts of grazing as a result.
    Vera also raised concerns over interpreting the high amounts of tree (arboreal) pollen in pollen records as meaning the landscape was largely comprised of trees. This is because the pollen sinks, as already stated, are generally regional (peat bogs and lakes, of which large lakes are more often used). Because tree pollen is released early in the season, and is usually released in high abundance at an elevated level in the canopy, there is a much greater chance of tree pollen travelling greater distances, where it will reach these regional pollen sinks. Conversely, grasses release pollen during the summer, and at levels just above the ground, where winds are less strong and there is a greater chance of the pollen not travelling too far (because of the lower wind speeds, and the trees and shrubs in leaf ‘trap’ the pollen in situ). As a result, a regional sink, such as a peat bog, even if large areas of land, even almost adjacent to the bog, were grassland or pasture that were bordered by trees, there is still a very high probability of non-arboreal pollen not accounting for more than 10-25% of the total pollen distribution in a sample. Not only this, but even if we assume that the landscape was wood pasture where animals grazed, the suppression of the grass by grazing herbivores and the fact that open-grown trees have much larger, fuller crowns, means that pollen ratios between arboreal and non-arboreal sources will likely register as if the area was instead a forest (for example, the total crown area of an area of wood pasture and of high forest may not be all that different). Trees in wood pasture will also have more clearance for pollen to travel great distances, and thus end up in these regional sinks at high levels. Even modern-day records suggest exactly this, and in this sense a wood pasture can be interpreted as, if assessed on pollen records alone, high, dense forest. Of course, this suggests that pollen records only tell part of the story, and it is easy to mis-interpret findings based on pollen studies.

    What could be considered mantle and fringe vegetation (regenerating birch amongst gorse), with Dunwich Forest’s pines in the background.
    Bear in mind deer (at least) are found on the site, so there is some grazing pressure.
    If you have found this post interesting, then please do consider buying the book. There is no way that I can give the whole picture here, and instead I have only given a fragment. Hopefully, it makes sense, and hopefully it gives an indication of why suggesting that the landscape was once comprised of massive expanses of high forest is perhaps not entirely accurate. In the modern day, there is no doubt that grazing by cattle has suppressed the regeneration of forest, and man’s conservation efforts with heathlands and grasslands has also stopped forest regeneration; as has man’s carving-up of the landscape for building and development. However, historically, when wild herbivores were still actually in existence, as man hadn’t pit-falled the last auroch to its death, the landscape may have not been covered exclusively by high forest where conditions allowed. Considering that fire is not seen as a massive driver behind the regression of forests to grassland and then back to a form of woodland at a later date, and the beaver is not considered to have been the only mammalian influence behind the loss of forest patches (again, according to Vera and the sources he immersed himself in), perhaps wild ungulates had more of a role in shaping the landscape than is generally considered. Food for thought, no doubt. Graze on that literary resource, and head out for pannage in your local library.
    Source: Vera, F. (2000) Grazing Ecology and Forest History. UK: CABI Publishing.
  8. Kveldssanger
    The presence of trees in the urban environment is certainly beneficial for the health and well-being of local residents, who will, generally-speaking, benefit more than they suffer as a result of the trees’ presence. Somewhat anecdotally, it has also been shown that it is the presence of healthy trees that has such beneficial impacts. In this sense, if tree populations suffer at the hands of a biotic or abiotic stressor, visibly decline in health, and potentially die in time (even on a massive scale), then the impact will also be ‘felt’ by the local residents.
    As a means of adding weighting to this statement, we can look at a US-based study that assessed whether the presence of the tree pest emerald ash borer (Agrilus planipennis), as it ravaged urban ash (Fraxinus spp.) populations, brought about adverse health responses in humans as well. The beetle has spread quite rapidly in the US since its arrival in 2002, and now occupies an area of land quite massive in scale (as shown by the map below). Specifically, the study looked at whether the presence of emerald ash borer had associations with the rate of mortality caused by cardiovascular and lower-respiratory-tract issues. The authors of this study selected these causes of death, because they are the first and third highest causes of death, respectively. Furthermore, there is reason to suggest that these two causes of death can be influenced by the presence (or lack) of trees.

    A map of where the emerald ash borer has been found in the USA, on a county level.
    In all of the US states that had at least one case of the emerald ash borer, the authors analysed information, from 1990-2007, outlining the cause of death for citizens. With this data, the authors looked at whether the presence of the beetle, and how long it had existed in a state, had any influence upon the mortality rates associated with the two causes of death mentioned. These comparisons were then related to the estimated abundance of ash trees in the states, and demographic data obtained via census records.
    Following on from data analysis, the authors identified that the presence of the beetle had a more significant impact upon respiratory-related mortality rates in wealthier counties (where there was a greater access to ash trees, compared to poorer counties) and, on average, there were 6.8 more deaths per 100,000 individuals in states host to the pest than prior to the state’s date of infestation. However, the impact of its presence increased over time, therefore meaning that areas that have been infested for longer will experience a higher average mortality rate than those areas more recently infected (as can be seen in the below table). At the time of the study, the authors therefore estimated that 6,113 respiratory-related human deaths had been caused by the presence of the emerald ash borer between 2002-2007, because of its impact upon its host ash trees (which almost always will die, or are cut down upon detection of them being infected). Therefore, we can perhaps observe that dying trees (and an increasing lack of trees) means dying humans. However, the fact that there is an anticipated 2-5 year lag associated with beetle presence and human mortality rates, the real effects of the ash borer upon human health may not yet be fully appreciated. Curiously, the authors also suggest that the media coverage of the ash borer’s presence may induce stress in some individuals, and such stress may potentially exacerbate (or create) health issues. Perhaps this highlights the emotional relationship people have with trees, and at times there may even be a sort of cross-kingdom empathy (and associated grief).

    The impact upon respiratory-related human mortality rates caused by the emerald ash borer.
    The observed impacts upon cardiovascular-related mortality rates per 100,000 individuals was even higher. In counties host to the emerald ash borer, an additional 16.7 deaths per 100,000 can be attributed to the pest’s presence. Therefore, 15,080 deaths can be directly linked to the effects of the borer, meaning that a total of 21,193 individuals have suffered mortality, between 2002-2007. Much like with respiratory-related deaths, the duration of time for which the beetle has been present has an impact upon the rate of mortality (see the below table). It was also found that individuals in counties with moderate levels of average income were most markedly affected.

    The impact upon cardiovascular-related human mortality rates caused by the emerald ash borer.
    Not provided in the journal, I decided to plot both sets of figures relating to the mortality rate increase observed with the presence of the borer, and assess the two lines (as shown below). What we can crudely see is that, as the years progress, the dispiraity increases between the two data sets, in favour of cardiovascular-related deaths. However, respiratory-related deaths consistently remain, across the six year period, 39-41% lower than cardiovascular-related deaths. Therefore, enriching cardiovascular health may likely be a more significant focal point with regards to any mitigation measures that may take place, though we must obviously be aware that there is likely going to be a levelling-off threshold, by where no more ash trees exist and therefore the mortality rate cannot suffer any further (accompanied by lag times). During this time, re-planting may of course occur, and offset any adverse impacts associated with ash mortality. Of course, these new trees will take time to mature, and therefore it may be many decades before health impacts begin to markedly reverse.

    Comparing the two data sets (blue line: cardiovascular mortality rates; orange line: respiratory mortality rates).
    There is no question that such data is indeed very interesting, and the results don’t necessarily remain limited to the emerald ash borer. Across the world, we can observe trees dying or being removed because of pest or disease outbreaks, so one can suspect that similar impacts may be associated with, for example, ash removal because of ash dieback, elm removal because of Dutch elm disease, or tree removal caused by Xylella fastidiosa. Granted, the actual impacts may differ (either be more severe, or less severe), though there will certainly be impacts. The findings that more affluent individuals were more adversely impacted by the emerald ash borer is very interesting, though perhaps not surprising, as environmental inequality certainly exists within urban districts (with more affluent areas having a more abundant tree presence). In this sense, less affluent neighbourhoods may already be suffering as a result of other stressors, and the lack of ash trees in the near locality means that they simply aren’t impacted by their death, because they are not directly experiencing such significant tree mortality. Other socio-demographic issues may also be implicated in this equation, such as the highest level of education an individual has. Similarly, the reduction is ecosystem sevices (pollution removal, encouragement to exercise outdoors, and so on) associated with the death of ash trees may have an impact upon respiratory and cardiovascular health, and this impact will be most pronounced where the most trees have been lost, which is (in this case) in the more affluent areas.

    ATTENTION: Have you seen this pest? It is wanted for the death of over some 100,000,000 ash trees, in the USA. Source: Aetree.
    Source: Donovan, G., Butry, D., Michael, Y., Prestemon, J., Liebhold, A., Gatziolis, D., & Mao, M. (2013) The relationship between trees and human health: evidence from the spread of the emerald ash borer. American Journal of Preventive Medicine. 44 (2). p139-145.

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