(Arboricultural-styled) 'Fact of the Day'

[daltontrees]

for trees that rely on wind dispersing their seed they will usually have much lighter and more aerodynamic seeds.

 

Less aerodynamic, surely?

[Kveldssanger]

Less aerodynamic, surely?

 

[Kveldssanger]

14/01/16. Fact #125.

 

Having a large number of a particular tree species in an area is not necessarily good by default - there must also be good genetic diversity of the population. Whilst there have perhaps not been too many total extinctions as a result of poor genetic diversity of a tree species, a lack of diversity does mean that a tree species is very susceptible to a pest or pathogen. We can look at the case of Castanea dentata to prove this.

 

The American chestnut, with a native range from New England to Alabama, would have accounted for up to 40% of the total forest overstorey cover, supported a wide range of species, and been classed as an important timber crop. However, its lack of genetic diversity (the species had a 14.5% genetic variation, from samples studied) resulted in its presence across its native range being drastically impacted by Cryphonectria parasitica (chestnut blight), simply because there was little inherent genetic variation that would have enabled some individuals to possess high levels of resistance. It's a bit like if I cloned myself 3,000,000 times - there would be a lot of me, and I may provide a lot of good services, but if a killer virus comes along that I am poor at fighting against, all 3,000,000 of me are at huge risk of mortality. Put the same killer virus in a group of 3,000,000 different individuals however, and it is far more likely that not everyone will be at risk.

 

This is why threatened species with declining or already very small populations can suffer as a result of a limited gene pool. This limited gene pool, even if the species can regain its foothold, means that it may be susceptible for many generations (particularly if the species cannot breed with an individual with different genes outside of that gene pool - look up 'genetic drift' if you are interested in this). And for species with small populations that have a very small gene pool, there is huge risk of one 'event' wiping out the population - locally, regionally, or even nationally. That is also why, when collecting seeds for seed banks, seeds are sourced from a wide range of locations that would harbour different genotypes. It is of little use collecting 100 seeds from the same tree, as they are genetically highly similar (if not identical).

 

It may also make one consider whether using clonal propagation, even in urban areas, is a good idea. London's recent i-Tree report suggests 21% of the 8,421,000 trees within Greater London are clonal - what does this mean in terms of urban forest resilience?

 

Source: Cooper, F. (2006) The Black Poplar - Ecology, History, and Conservation. UK: Windgather Press.

Edited January 14, 2016 by Kveldssanger

[Kveldssanger]

Please note with the fact above that I also added a lot of my own knowledge in, particularly in the latter half.

[Kveldssanger]

15/01/16. Fact #126.

 

Trees within the urban environment may more frequently suffer from water stress when compared to their rural counterparts. This is due to urban environments being heavily covered with impermeable surfaces, the limited rooting environment of many urban trees, and the fact that stormwater management systems swiftly remove run-off (within sealed pipe networks) during periods of heavy rainfall. Ironically, whilst surrounded by water, urban trees are unable to access what they may very well require. Therefore, in times of marked drought, urban trees may very well begin to suffer greatly.

 

The Great Drought of 1976, which was the driest summer for over 200 years, was one of the more distinct examples of how urban trees suffered as a result of drought conditions. Young and newly-planted trees displayed symptoms of stress, and ultimately died, within the very same growing season as the drought – particularly where they received no aftercare by local authorities. It was only because of the successful plea by such authorities to members of the public to use their less contaminated waste water (from bathing and washing up) that mortality of young trees was not incredibly damaging.

 

However, it was not just the young trees that suffered. Mature trees, whilst not necessarily showing symptoms during the year of the drought, at times did not re-flush, or flushed weakly, in the following growing season. For many, Johnston profoundly states, it would be their last.

 

Such a brief account of the drought of 1976 is a good example of how precariously-balanced even mature urban trees may be, with regards to life and death. Granted, the drought was significant and an outlier, but it should serve as a reminder to those within the industry (and associated industries, such as developers and engineers) that urban environments are not accommodating for trees and their water demands, even at the best of times. This is discounting the huge array of other biotic and abiotic stressors placed upon our urban trees, such as pollution of the air and soil, ground compaction, mechanical damage, and vandalism.

 

Source: Johnston, M. (2015) Trees in Towns and Cities – A History of British Urban Arboriculture. UK: Windgather Press.

[Kveldssanger]

16/01/16. Fact #127.

 

When one is asked of the natural home of trees, the response may very likely be either woodlands or forests. Whilst these words do accurately describe the home of trees, they do not tell the full story. Wood pastures, or even landscapes void of any woodland characteristics, are also the natural homes of trees. Rackham describes five tree-land covers, and these are: (1) forest – trees grow characteristically close to one-another, and the ground flora is either non-existent or comprised of highly shade tolerant species; (2) savanna – grasslands and heathlands complete with trees, with ground flora being not of shade tolerant species; (3) coppice – sites are cut at intervals frequently enough to allow for light-demanding herbaceous species to flower; (4) farmland trees – those within hedgerow, or standing in isolation or in small groups within a worked field, and; (5) maquis – trees are reduced, as a result of browsing and burning, to the height of shrubs.

 

In defining these five distinct trees-lands, Rackham then proceeds to explore such landscapes across the world, and delves into the historical attributes of current landscapes, in addition to other forms of historical evidence, that indicate the tree-lands of the past.

 

Coppice history

 

Rather profoundly, Rackham begins by making the assertion that woodland history resides not only within historical accounts, but also in the woodland features we can see today – such as giant coppice stools. Stools that support multiple stems of previously-coppiced broadleaved trees and exceed 2m in diameter act as living proof of centuries of coppice. Such stools are not limited to the UK either – France, Greece, Hungary, Italy, Norway, and many other European countries (and beyond – Japan also has a rich coppice history, as its native tree genera are similar to that of Europe) sport such massive stools. Curiously, for coppice to exist in two such areas of the globe, centuries before commercial travel and worldwide communication, means that human civilisation must have evolved along similar lines in both regions.

 

Turning attention towards Australia, Rackham notes that it is an entirely different ‘planet’ – mainly because species of Eucalyptus grace the landscape. Additionally, for what fungi do in Europe in terms of degrading coarse woody debris, fire does in Australia (though termites also play a role, albeit a smaller one). Eventually, fire was harnesses by man to artificially alter fire frequency in order to manage the landscape. Interestingly, this different means of recycling of the tree-lands complements Eucalypts, as they possess a permanent woody lignotuber that can sprout if the tree is killed by fire. Over the course of many fire events, such continues resprouting may in fact have a similar visual impact as coppicing does – a stool forms. Again, an archaeological indicator of past land-use by man, perhaps.

 

 

Across the Pacific in North America, coppicing is also evident. Whilst the tree species are different to those within Europe, they could still be used by man as a perpetual (in theory) wood source. However, coppicing in North America takes on three different forms because active management of tree-lands has only been going on for 150-200 years. These forms are: (1) modest-sized coppice stools of the archetypal European coppice; (2) big stools where trees have resprouted from their lignotuber (common in Texas, where Native Americans worked the lands with fire for many centuries), and; (3) self-coppice stools, created by species such as Tilia americana, which send up basal sprouts during old age – eventually the main trunk dies and collapses, and the sprouts form new stems. In fact, this characteristic of the genus Tilia is not isolated to North America – Tilia japonica and Tilia maximowiczii have similar qualities in Japan, and Tilia cordata within Europe most likely can achieve the same result as well.

 

However, such stools may not always persist – as may other features, such as veteran standards and ancient woodland indicators. Usually, the reason for such a loss in archaeological indicators is down to man’s conversion of ancient woodland to plantation (PAWS), or grazing with sheep.

 

Regardless, Rackahm poses the question of how exactly coppicing originated, and how did trees use this regenerating ability prior to the invention of the axe? The answer may (at least, in part) reside within the chapter entitled ‘Coppice Silviculture: From the Mesolithic to the 21st Century‘, within the publication ‘Europe’s Changing Woods and Forests: From Wildwood to Managed Landscapes‘, of which I wrote about briefly in Fact #16..

 

Buildings

 

Early structures built by man, from the humble abode to the mighty cathedral, would have been constructed from wood. The size of the beams used, and the species that provided the wood, are two examples of how a landscape archaeologist may determine how the tree-lands of the local area may have looked. In the UK for example, many young oaks were harvested for single timber beams – annually, hundreds of thousands of oaks may have been harvested. This would have meant oak coppice would have been in demand, or even the harvesting entire succeeding oak stands within a woodland.

 

Similar means of landscape archaeology can be applied in other countries and regions of the world – Colombia, Japan, and parts of Australia (Tasmania and Queensland). Typically, the wood carpenters would head into the woods and use the smallest tree necessary to produce the exact beam needed for construction (as it was pointless harvesting large trees for timber, because the time spent dividing up the timber would have been better spent felling smaller specimens). Tree-lands were thus managed in a way that would provide for small trees year-in year-out, and larger beams and boards may well have been imported from elsewhere. In medieval England, for example, large oak boards (Baltic oak) were imported from specialist suppliers in Eastern Europe, where oaks grew slowly and reached huge sizes.

 

Pictures

 

Following the invention of photography, early pictures produced with the technology are a great way of analysing past tree-lands – often, the trees would only be in the background, with the focus of the image being something entirely different. Aerial imagery from the early twentieth century also offers a great insight into the tree-lands of the past.

 

Similarly, old paintings also provide for an historical account of landscapes gone by. As far back as the Bronze Age, paintings may have shown what the tree-lands may have looked like. However, until the dawn of Romanticism in the 1800s, paintings would have likely been too lacking in detail to distinguish tree species from one another.

 

 

There is also the risk of artists cherry-picking tree-lands, with a desire to paint only single-stemmed trees being very much evident – coppice woodlands may therefore have been neglected in this medium, as may have old wood pastures. Perhaps the fact they are easier to draw lead to this bias towards single-stemmed specimens, or perhaps nobody thought coppice woodlands were worth drawing for other reasons. Ancient trees may have been a very notable focus of many artists, as a matter of fact – this is likely due to them being very emotionally evocative, and important within their own right.

 

Rackham notes that Japanese artists may have been the best at drawing and painting trees. From the 14th century onwards, artists on Japan could capture recognizable tree species and genera – pines were very frequently drawn and were abundantly present in pictures of woodlands, whereas their spread in Japan today is nowhere near as distinct. This indicates how the arboereal landscape of Japan, in terms of constituent species, has altered. Without such artistic reference, would this have been so readily identifiable?

 

Savanna and wood pasture

 

Scattered trees within grazed landscapes were common sights in the centuries gone by, from Europe to Africa, and from North America to South America (though the holm oak dehesas of Spain are a gorgeous example of wood pastures still in existence). These grazed landscapes may have been man-made, or even natural (as Vera suggests in his book ‘Grazing Ecology and Forest History‘). Many exist today as ex-wood pastures, and younger trees have grown up around the old worked trees (typically pollards).

 

 

Savannas, on the other hand, signify the transition between forest and grassland (an ecotone, of sorts). The reason for savannas existing would have normally been because there was some driver behind the lack of ability for trees to colonise the land, be it because of human management or natural cause (altitude, for instance). In today’s age, evidence of old savannas in West Africa can be found now amongst woodland – those who worked the savanna were murdered or taken away for the slave trade, and the lack of management following their disappearance lead to woodland forming (perhaps from the seed bank within the soil, or from seed dispersed from the trees that remained).

 

In Australia, savannas actually cover over half the continent. This landscape originated with the Aborigines, and may show signs of old pollarding practices (either because of man taking an axe to the tree, or forest fires – the marri and jarrah eucalypts of Western Australia, when scorched by fire, sprout up in the main stem, creating a pollard-like structure over time).

 

Pollards and ancient trees

 

Whilst the giant coppice stools signified past coppicing operations, pollards signify wood pasture (as elucidated to above). Perhaps commonly undertaken to feed livestock, it was a more physically-demanding practice than coppicing, but enabled for trees to be retained as their growth was above the grazing line – this was crucial for farmers, who could essentially rear cattle and grow (some of) the fodder within the same parcel of land.

 

Many pollards in wood pasture are now lapsed, given the dying art of grazing (down to various reasons, which won’t be explored here). Old oak and beech pollards stand proud in the UK, as will oaks in Hungary, whilst elms and ash pollards can be found in Norway. The holm oak dehesas of Spain (and also Greece) are another fine example of (perhaps still active) pollarding operations.

 

 

Rackham notes that whilst ancient trees can be found dotted over the landscape, they are disproportionately-clustered in areas of old wood pasture. They are not typical of ancient woodland (and thus should not be classed as indicators of the wildwood), where giant coppice stools are more frequent. Where ancient trees can be found in woodland, it is very likely that the land use changed to facilitate woodland regeneration, in place of the ancient trees growing up with existing woodland. In fact, the decline in wood pasture and savanna landscapes can be attributed to a decline in the practice of grazing over the last 200 years – the slave trade in Africa, and the push for forestry in Europe and The Americas, has lead to this.

 

Source: Rackham, O. (2012) The Ghosts at the Ends of the Earth: Tree-Land in Four Hemispheres. In Rotherham, I., Jones, M., & Handley, C. (eds.) Working & Walking in the Footsteps of Ghosts – Volume 1: The Wooded Landscape. UK: Wildtrack Publishing.

[Kveldssanger]

That was a very enjoyable one! The missus is now famous as well, given she's gracefully modelling that wonderful oak tree!

Edited January 16, 2016 by Kveldssanger

[Kveldssanger]

17/01/16. Fact #128.

 

Trees and other plants are continually buffeted by the elements, and have thus adapted their physiology over many thousands of years to suit the environment in which they naturally will grow. Below, we look at some of the adaptations plants may go through in order to safeguard themselves against the harsh winters that afflict many parts of the world.

 

Bud break / growth cessation

 

The phenological timing of spring bud burst and autumn growth cessation will tie in closely with when the period of colder weather ends and begins, respectively – the length of the night is the principal means of determining phenology (Hurme et al., 1997; Leinonen & Hanninen, 2002). Such processes for temperate and boreal tree species ensure that they are in an improved position to cope with the colder weather. For instance, the timing of bud burst in spring is crucial for the avoidance of spring frost damage, which may reduce growth, or in extreme cases kill the specimen (Arora et al., 2003). However, since spring frost damage can not be totally prevented by the adaptation that is late bud burst, it is likely that the adaptation is a compromise between the likelihood of the occurrence of damaging frosts and a sufficiently long growing season, in addition to a species’ ability to cope with cold weather periods before and following bud break (Calmé et al., 1994). Further, deciduous trees will shed their leaves so to reduce the rate of water loss during the winter, and to safeguard their foliage from potentially severe frost damage.

 

 

Bark thickness

 

A thick rhytidome (outer bark) can help to protect trees from frost damage during the winter, by acting as a ‘buffer’ zone that shields the phloem, cambium and xylem from significant cold damage. This is particularly necessary with conifers, as frost-related injuries correlate highly with bark thickness. The thicker bark can retain heat in the cambium zone, thus keeping bark temperature above freezing even during winter in many periods (Gurskaya & Shiyatov, 2006). Species such as Sequoia giganteum and Pseudotsuga menziesii have very thick rhytidomes – up to 60cm for the sequoia (Dujesiefken & Liese, 2015).

 

Cellular changes

 

As cell temperature approaches freezing point, accumulations of sugars, organic compounds and amino acids within cells will artificially lower the cell’s freezing point, and allows mild cold tolerance up to around -1°C to -2°C (Morin et al., 2007; Thomas, 2000). As temperature drops further, cells can essentially ‘supercool’ themselves so their temperature can drop to around -40°C but the water inside will not freeze. In this state, the cells act as if they are a solid when in fact they are still liquid. This is partly achieved by proteins within the cell changing in structure, acting as an anti-freeze protein (AFP) of sorts. The plasma membrane will also become ‘gel-like’, changing its state from a liquid during the warmer temperatures (George et al., 1974; Karban, 2015; Thomas, 2000).

 

Assuming both of the above adaptations don’t work as it’s that inhospitably cold, cells will shift water from within their structure and store it extracellularly, allowing the water to then freeze outside of the cells, and tolerating the temporary cellular desiccation during these absurdly cold conditions. Such a tactic can give trees tolerance up to around -196°C. As the water outside the cells freeze, the small amount of heat released also helps to keep the cells themselves slightly warmer (Ashworth & Abeles, 1984; Thomas, 2000).

 

Research has however found that plants will ‘learn’ how to adapt to the cold, so trees exposed to very cold conditions annually are more readily able to undertake such cellular changes (Karban, 2015). For example, one study saw naive rye that was killed at -5°C, whilst ‘hardened’ rye that had been subject to yearly cold periods could withstand -30°C temperatures (Thomashow, 1999).

 

 

Xylem embolism protection

 

Physiological studies of the responses of temperate woody plants to winter xylem embolism, which involves the freezing of xylem water, suggest that such plants minimize the impact of overwinter embolism by replacing previously-embolised vessels with new and functional vessels every year, and / or by re-filling embolised vessels by generating positive xylem pressures – at least, positive xylem pressure is found in some species, such as Acer spp., Juglans spp., and Betula spp. – to re-assimilate gas bubbles that appeared during freezing (Sakr et al., 2003; Tyree, 1983; Zhu et al., 2000).

 

Internal regulation of temperature

 

Certain plants are even able to adapt – via their mitochondria – cellular respiration rates, sometimes increasing respiration rate to the equivalent of that of a hummingbird in flight (Nagy et al., 1972). This increase in respiration raises internal temperature, thereby reducing risk of cells freezing (Karban, 2015) when temperatures fall during colder periods (Knutson, 1974; Nagy et al., 1972).

 

References:

 

Arora, R., Rowland, L., & Tanino, K. (2003) Induction and release of bud dormancy in woody perennials: a science comes of age. HortScience. 38 (5). p911-921.

 

Ashworth, E. & Abeles, F. (1984) Freezing behavior of water in small pores and the possible role in the freezing of plant tissues. Plant Physiology. 76 (1). p201-204.

 

Calmé, S., Bigras, F., Margolis, H., & Hébert, C. (1994) Frost tolerance and bud dormancy of container-grown yellow birch, red oak and sugar maple seedlings. Tree Physiology. 14 (12). p1313-1325.

 

Dujesiefken, D. & Liese, W. (2015) The CODIT Principle: Implications for Best Practices. USA: International Society of Arboriculture.

 

George, M., Burke, M., & Weiser, C. (1974) Supercooling in overwintering azalea flower buds. Plant Physiology. 54 (1). p29-35.

 

Gurskaya, M. & Shiyatov, S. (2006) Distribution of frost injuries in the wood of conifers. Russian Journal of Ecology. 37 (1). p7-12.

 

Hurme, P., Repo, T., Savolainen, O., & Pääkkönen, T. (1997) Climatic adaptation of bud set and frost hardiness in Scots pine (Pinus sylvestris). Canadian Journal of Forest Research. 27 (5). p716-723.

 

Karban, R. (2015) Plant Sensing & Communication. USA: University of Chicago Press.

 

Knutson, R. (1974) Heat production and temperature regulation in eastern skunk cabbage. Science. 186 (4165). p746-747.

 

Leinonen, I. & Hanninen, H. (2002) Adaptation of the timing of bud burst of Norway spruce to temperate and boreal climates. Silva Fennica. 36 (3). p695-701.

 

Morin, X., Améglio, T., Ahas, R., Kurz-Besson, C., Lanta, V., Lebourgeois, F., Miglietta, F., & Chuine, I. (2007) Variation in cold hardiness and carbohydrate concentration from dormancy induction to bud burst among provenances of three European oak species. Tree Physiology. 27 (6). p817-825.

 

Nagy, K., Odell, D., & Seymour, R. (1972) Temperature regulation by the inflorescence of Philodendron. Science. 178 (4066). p1195-1197.

 

Sakr, S., Alves, G., Morillon, R., Maurel, K., Decourteix, M., Guilliot, A., Fleurat-Lessard, P., Julien, J., & Chrispeels, M. (2003) Plasma membrane aquaporins are involved in winter embolism recovery in walnut tree. Plant Physiology. 133 (2). p630-641.

 

Thomas, P. (2000) Trees: Their Natural History. UK: Cambridge University Press.

 

Thomashow, M. (1999) Plant cold acclimation: freezing tolerance genes and regulatory mechanisms. Annual Review of Plant Biology. 50 (1). p571-599.

 

Tyree, M. (1983) Maple sap uptake, exudation, and pressure changes correlated with freezing exotherms and thawing endotherms. Plant Physiology. 73 (2). p277-285.

 

Zhu, X., Cox, R., & Arp, P. (2000) Effects of xylem cavitation and freezing injury on dieback of yellow birch (Betula alleghaniensis) in relation to a simulated winter thaw. Tree Physiology. 20 (8). p541-547.

[Kveldssanger]

18/01/16. Fact #129.

 

This study, undertaken across six neighbourhoods of differing socio-economic values in Cincinnati, Ohio, was set up with the hypothesis of trees beneficially impacting upon property values. The results aligned with the hypothesis, though the findings from the study will be discussed below.

 

The six neighbourhoods featured in this study, all of which contained principally small and single-family homes that were between 60-80 years old, were Bond Hill, Carthage, Clifton, Hyde Park, Kennedy Heights, and North Avondale. From these six areas, the authors identified 100 residential property sales in each neighbourhood that were up for sale at the going market value, and subsequently visited each property and recorded the trees present within the property’s grounds (collecting data such as dominant genus, whether the trees were deciduous or coniferous, canopy cover percentage, the diameter of the trees, and the level of maintenance the trees have received – including tree health). Visits were done in winter of 2005-2006 and again in Summer of 2006, with the second visit serving to determine whether deciduous trees had a differing impact upon property value depending upon the time of year.

 

 

The results from the winter survey suggest that tree cover had a progressively increasing benefit to house prices in relation to an increase in canopy cover in all six neighbourhoods, with an average increase of $561 per 1% increase in canopy cover. Because the mean property value across the six neighbourhoods was $166,357, and the canopy cover average was 24.8%, the average value of the trees within the property is $13,913 (or 8.4% of the house price).

 

Summer results were very much similar, with the study data showing that every 1% increase in canopy cover lead to a $580.92 increase in property value. With the average property price being $166,357 and the mean tree cover being 27.1% (likely an increase due to the presence of foliage crowns), tree canopies account for $15,743 or 9.5% of the value of a property.

 

Looking at the two surveys combined therefore, the authors found that tree presence accounted for an increase of $780 per 1% increase in canopy cover during summer, and an increase of $669 in winter – these differences are not statistically significant. The reason for the difference in value increase, the authors posit, is because buyers cannot conceptualise what canopy cover will look like in the summer with regards to deciduous trees. With the mean sale price of the 600 properties being $188,730 and a mean canopy cover of 25.8%, the average value of the tree canopy was $20,226 (or 10.7%).

 

So did tree attributes (such as genus, and whether they were coniferous or deciduous) have an impact upon house sale prices? With regards to whether a tree loses its leaves in winter or not, there was no significant difference. In relation to tree genus (of which 40 genera were identified in the study), again there was no significant difference. Ultimately, the buyers were simply content in paying more for properties with trees, regardless of species. Similarly, the number of trees within a property was not significant, nor was their stem diameter.

 

However, the six neighbourhoods did show variation with regards to how beneficial tree presence was to property value. In the more affluent neighbourhoods (Hyde Park and North Avondale) tree presence had a significantly beneficial impact upon property retail value, whereas in less affluent areas (Carthage) the presence of trees may have little to no impact – or even drive down house prices slightly. This may be down to the fact that trees cost money to maintain, and the less affluent neighbourhoods had less disposable income to spend on tree maintenance.

 

Source: Dimke, K., Sydnor, D., & Gardner, D. (2013) The effect of landscape trees on residential property value of six communities in Cincinnati, Ohio. Journal of Arboriculture. 39 (2). p49-55.

[daltontrees]

18/01/16. Fact #129.

 

This study, undertaken across six neighbourhoods ...

 

... neighbourhoods had less disposable income to spend on tree maintenance.

 

Source: Dimke, K., Sydnor, D., & Gardner, D. (2013) The effect of landscape trees on residential property value of six communities in Cincinnati, Ohio. Journal of Arboriculture. 39 (2). p49-55.

 

I got as far as multicolinearity and gave up!

 

If I manage to magic up time from somewhere I may refer to thi article again in the Tree valuation thread.