(Arboricultural-styled) 'Fact of the Day'

[Kveldssanger]

06/01/16. Fact #118.

 

Frequently residing in front gardens, the Magnolia is appreciated by-and-large for its amenity value. Whilst there is no question that the genus produces some stunning specimens, its rich history is also something to be admired.

 

 

The origin of the Magnolia

 

Fossil records suggest that the genus has existed from the Cretaceous period (145-66 million years ago), making the Magnolia the first flowering plant. Before this time, only conifers and cycads graced the earth - which themselves came after both ferns and horsetails. Its historic distribution, prior to the last age, would have been across most of mainland Europe and the rest of the northern hemisphere, though since the last ice age its native range has predominantly been Asia and eastern America - the mountain ranges across Europe, which span east to west, restricted the retreat of plant species (as seed could not feasibly retreat over mountains), thus trapping them and sentencing them to death.

 

Its method of pollination was, and still is largely, through beetle-type insects. These beetles, which would travel between specimens, were attracted by the fragrance of the flowers, and the edible tissues and pollen contained within. In fact, to improve its means of successful pollination, the inner tepals of the flower could remain tightly shut for days on end, which allowed visiting beetles to feed safely and - at the same time - get covered in pollen. The Magnolia is however monoecious, meaning both male and female organs are found on the same specimen. To counteract the risk of self-fertilisation by visiting beetles, individuals will not mature their male and female organs at the same time. This enables pollen from one specimen's male flower to reach the female organ of another.

 

 

The Magnolia hunters

 

The first Magnolia came to the UK in 1688 from the USA, courtesy of John Bannister. Bannister, a missionary, on his travels to Virginia, returned with Magnolia virginiana. Since then, a cascade of introductions occurred from the USA, and by 1800 most American species had been introduced and started being cultivated.

 

Records are less clear over when the Magnolia was first introduced from Asia, though it is considered that Magnolia denudata and Magnolia coco were both introduced around a century later than Magnolia virginiana - the former was brought over, in 1780, by Sir Joseph Banks.

 

The author of this book remarks that it was not these early Asiatic introductions that were so significant, but introductions from two plant hunters in the 20th century.

 

The first, George Forrest, was sent by the Royal Botanic Gardens in Edinburgh to collect plant specimens in Asia, though during his travels in China he remarked how dangerous it was - to the point that, in 1905, after the British begun invading China's Lama (Tibet) region, any foreigner was seen as a threat. As a result, he and his team of 17 plant collectors had to flee, with hostile natives on their tail. After a period of nine days, and having stumbled into a village inhabited by friendly Lissoos (a sub-tribe of the Tibetans) at death's door, the village leader managed to smuggle him and the only other survivor of his team out of China. On his travels, George Forrest managed to collect 31,000 plant species, of which eight were of the genus Magnolia and three new to cultivation.

 

The second, Ernest Henry Wilson, whilst having far less of a tale, introduced over 1,000 plant species from Asia into cultivation, including eight new species of Magnolia, making him the most significant 'Magnolia hunter' the world has ever seen. His introductions were: M. wilsonii, M. dawsoniana, M. delavayi, M. sprengeri, M. officinalis, M. sinensis, M. sargentiana, and M. s. robusta.

 

Source: Rankin, G. (1999) Magnolia. China: Hamlin.

[Kveldssanger]

I realise these latest ones have been a bit longer - I'll start breaking them up with the photos I use on my blog, I think! Better than a wall of text.

 

That Magnolia book can be ordered for 1p on Amazon.

[Kveldssanger]

06/01/2016. Fact #119.

 

Anecdotally, I am certain that many of you will attest to tree presence being highly favoured in urabn environments - not only for the environmental and ecological benefits they provide, but for the benefits they offer humans on an economic and social level.

 

The focus of this post is a study by Frances Kuo, published in 2003, entitled The Role of Arboriculture in a Healthy Social Ecology. Kuo sought to analyse exactly how, in the US city of Chicago, urban trees influence human social interactions, and the results are - whilst not unexpected - very interesting.

In terms of methodology, Kuo remained brief in description (instead referring readers to the respective journal articles for each study that was only summarised in this article) selected different housing areas of Chicago that met four different criteria: (1) a variation in green cover immediately surrounding the area (from areas laden with trees to aread void of them); (2) a constant with regards to other environmental features, for control purposes; (3) housing areas contained residents that were randomly assigned abodes (publix housing), so to negate the bias encountered where studying social populations where people have chosen to live in the area, and; (4) residents have no influence over how the vegetation in the area is managed. In light of these criteria, two housing developments were identified (shown below - the first Robert Taylor Homes and the second Ida B. Wells), and the residents' social undertakings were assessed and subsequently separated into different categories. Results are listed below.

 

 

 

Enticing residents to venture 'outdoors'

When shown different pictures of trees within an urban landscape, residents were found to strongly prefer more trees in a landscape than less (54 per hectare, in this study), and stated that if their courtyards had more trees contained within them then they would feel more encouraged to utilise the grounds. This suggests that trees can be strategically planted to entice residents to actually use the outdoor space surrounding their property, which in itself brings social interaction in - largely - a positive manner.

 

Encouraging adults to use outdoor spaces

Whilst the above study was hypothetical, Kuo found that, when transferred into reality, results were very similar. Not only are adults more likely to use outdoor space if the space contains plenty of trees, but venturing adults will be disproportionately concentrated in areas where there are many trees compared to where there are few or no trees. Additionally, the closer the trees were to properties, the more likely it was to have adults use the outdoor space nearby - to the point that, where there were no trees at all, the space was not used.

 

Encouraging children to use outdoor spaces

Much like with adults, children also are disproportionately drawn to areas with plenty of trees. In these heavily-treed areas, it is also more likely that the children are engaged in play (in place of other activities) - particularly creative play. Kuo suggests here that not only can it be said that trees draw both adults and children out in greater concentrations, but this greater density of people encourages social interaction among communities.

 

Promoting social interactions between residents

Building on the comments above, results suggest that the frequency of activities such as talking, playing card games, and reparing cars, is positively linked to tree cover. 73% more individuals were shown to interact socially where tree cover was significant, with the results showing adults were most noticably impacted.

 

Facilitating child-adult interactions

Mixed-age groups consisting of children and adults are more likely to be observed in areas of high tree cover. This is important for children in particular, as it enables them to develop the necessary skills for adulthood, and also stops children from engaging in ani-social behaviour (due to adult supervision).

 

Improving neighbourhood social 'ties'

Trees encourage neighbours to interact informally with one another, improving social ties amongst communities and making communities stronger as a unit - to the point that residents will share resources with one another out, particularly where there is poverty. Individuals living in areas with higher tree cover also reported significantly greater amounts of social interactions with neighbours, compared to barren areas. This ties in with the greater use of outdoor space by residents in heavily-treed areas.

 

Heightening the sense of safety

Kuo concludes that, as a result of studies into how safe residents feel in areas with plenty of trees and areas lacking, those in locations with many trees reported feeling much safer. Additionally, residents with nearby access to tree cover reported feeling more adjusted to their home, compared to those where trees were not available nearby.

 

Reducing graffiti and other minor disorders

Vandalism, graffiti, and incidences of littering were found to be at lower rates in areas with higher green cover, as were anti-social activities such as strangers loitering, individuals making a lot of unecessary noise, and such forth. Kuo suggests this may be because green space improves peoples' level of care and awareness, making anti-social behaviour both less attractive harder. However, the increased cohesiveness of areas with many trees may also make anti-social behaviour less enticing.

 

Impacting crime levels

Put simply: the greener the surrounds of the building, the lower the observed crime rate - this applies to crimes such as property crime and violent crime.

 

Source: Kuo, F. (2003) The role of arboriculture in a healthy social ecology. Journal of Arboriculture. 29 (3). p148-155.

[Kveldssanger]

07/01/2016. Fact #120.

 

The manner in which woody plants grow vegetatively is impacted by many different environmental qualities – light availability, water supply, frequency of flooding, ambient temperature, soil fertility, soil salinity, soil bulk density, atmospheric and soil pollution, wind speeds, frequency of fire, and pests and diseases. The focus of this post will be exclusively on how wind affects vegetative growth.

 

First and foremost, we must recognise that wind can be both good and bad for vegetative growth. For example, whilst the wind can aid with seed dispersal to ensure the species can continue to exist, the wind may also be the cause of major limb failure, entire windthrow, or soil erosion – additionally, the wind can be the harbinger of arboreal pestilence (as we have seen, to some degree, with Hymenoscyphus fraxineus). Furthermore, not all tree species are equal in their response to wind – conifers are more likely to suffer from the wind than are broadleaved trees. To complicate matters here however, it is also important to understand that fast-growing pioneer species (such as birch, pine, poplar, and willow) will be more adversely impacted than later-successional species (such as beech and oak), given they are far more exposed as a result of both their quick growth rate and unestablished environmental surrounds (these species create woodland; they don’t enter into stabilised woodlands).

 

 

In addition, strong winds following periods of heavy rainfall, or a marked lack of rainfall (drought), are likely to have significant adverse impacts – windthrow at the root plate and stem failure are the most notable impacts, respectively. Not only this, but disease and dysfunction manifesting within the roots and stems of trees will leave them more susceptible to windthrow when compared to the susceptibility of a healthy tree. This may be particularly apparent after a dense stand has been thinned, exposing once-sheltered trees to wind gusts they have not adapted to (in both their rooting and aerial structures).

 

 

Building on the concept of exposure, forests that reside along the coastline have their ‘edge trees’ damaged by the wind. For example, exposed sycamores suffered damage to 46% of their leaves as a result of strong coastal winds. Damage manifested as a result of foliar tearing, the collapse of epidermal and mesophyll cells, and dehydration via the disruption of protective foliar waxes. Even mild winds of 6m per second will oft dehydrate leaves (such as of aspen).

 

 

Mild winds will also bring about a decrease in primary elongation whilst initiating increased secondary thickening, thus altering the form of the main stem. Trees in exposed settings and therefore shorter, thicker, and more tapered than sheltered counterparts, in order to resist swaying violently and to retain uniform stress throughout. In conifers, the adaptive secondary thickening (xylem increment) actually is more discernible on the leeward (compression) side – this may be exacerbated when the very same conifer leans with the wind, given conifers lay down reaction wood on the compression side in an attempt to self-right themselves (becoming a ‘sabre tree’, as penned by Mattheck).

 

In essence, all the above changes can be attributed to three different types of change brought about by wind – a change in water relations, food relations, and hormone relations.

 

Looking firstly at water relations, wind has been shown to actually have varying effects depending upon species. To demonstrate, Norway spruce and Swiss stone pine are observed to transpire less during wind, whilst larch and alder transpire more. It is considered that this is due to varying stomatal responses, with stomatal size being a particular determinant – smaller stomata dehydrate quicker and thus closer faster. However, as a general rule of thumb, woody plants exposed to wind will initially have their transpiration rates increase rapidly, with rates then tailing-off gradually (dependent upon the species) as the stomata close because of dehydration.

 

Turning towards food relations, exposure to strong winds will usually lead to reduced photosynthetic rates. This is because the strong winds cause foliar injury and / or foliar shedding. However, the effect wind has upon leaf temperature also drives the change in photosynthesis. As winds will cool leaves, the conductance of leaves will alter – this will impact upon photosynthesis. Strong winds may also increase respiration rates of leaves, with the consequence of reduced carbohydrate supplies.

 

Lastly, hormone production and translocation is markedly impacted by wind, with auxin and ethylene being the main hormones affected. This drives the physiological changes mentioned earlier – namely a decrease in primary elongation and an increase in secondary thickening. For example, the reaction growth laid down on the compression side of conifers is attributed to a high auxin gradient, which prompts carbohydrates to be mobilised and used in the leeward region for growth. Conversely, tension wood laid down by broadleaved trees in identical conditions is linked to a deficiency in auxin.

 

Source: Kozlowski, T. & Pallardy, S. (1997) Growth Control in Woody Plants. UK: Academic Press.

Edited January 8, 2016 by Kveldssanger

[Kveldssanger]

I seem to have scared everyone off bar Jules, lately! I hope people aren't commenting because they are too busy reading or researching, in place of feeling like their potential comments would be out of place.

 

Comments on the style of stuff I'm putting out are also welcome, and what I write is to help everyone learn - not just myself.

 

[TIMON]

I have to admit I'm unable to keep up!

I like the fact that I'm building a reference library courtesy of someone else's hard work and research. So your labours are appreciated even though I don't have the time to read as quickly as you post.

Thanks

Timon

[Kveldssanger]

08/01/2016. Fact #121.

 

The importance of a diverse, plentiful, and healthy urban tree population of a town or city is well understood. For local authorities, who typically will own the largest amount of a town or city's tree population, there is normally a 'strategy' employed that involves retaining and replacing trees for the (direct and indirect) benefits of the residents. However, many trees also exist within private property (such as front and rear gardens) and, unless a tree has statutory protection (a Tree Preservation Order, for UK trees), there is no means of safeguarding its presence. This study, which looks at homeowner attitudes to private tree maintenance, therefore offers a good insight into how residents seek to manage their own trees. Perhaps, the results of this study can be used as an indicator for homeowner attitudes in other urban areas, in the USA and beyond.

 

Before we go on, I think it is worth noting that the two questions the authors of the study sought to answer were: (1) What are urban homeowners’ past and future tree planting and care behaviour patterns?, and (2) Are there variations in perceptions, attitudes, and behaviour patterns by geographic district?

 

In order to answer these two questions, the authors set up a web-based survey and encouraged a randomly-selected sample of homeowners (not those who rent) from across all of Seattle's 53 districts to respond - personalised and hand-signed letters were sent out to inform the randomly-selected homeowners of the survey, and a week later personalised postcards were sent to those who had yet to respond. In addition to the personalised means of contacting the homeowners, each respondent was informed that if they completed the survey they would be able to claim a free meal at a local restaurant that was, at the time, being featured heavily in local newspapers due to their advertising regime.

 

In total, 2,485 homeowners were contacted, and 751 (31%) of the homeowners responded to the survey. Response rates varied between districts, with Central Seattle (including areas such as Madison Park and Capitol Hill) having the highest response rate at 41%, and South East Seattle having the lowest (15%).Demographically, the average respondent age was 51, and the average time the respondent had lived at their property was 14.6 years. Furthermore, 38% of respondents had a college degree, and 44% having a graduate or professional degree - this reflected in total earnings per year, with over 50% earning between $75,000-$150,000. In terms of ethnicity, 88% were white, 5% were Asian, 3% mixed race, and the remaining 4% of other ethnicity - this reflected the overall ethnic distribution of Seattle rather accurately. It is important to stress that the authors did not analyse how* demographics in the same - and across different neighbourhoods - impacted upon private tree management.

 

 

Moving onto the results of the study, the authors segmented the results for city-wide analysis into two sections - planting behaviour and pruning behaviour.

 

Planting behaviour

The respondents of the survey showed a distinct preference towards planting small ornamental trees, in addition to alack of desire to plant large coniferous and deciduous trees. However, there was a very marked intent by respondents to have their next planting be a fruit tree.

 

 

Respondents were also asked as to what fuelled their most recent tree planting, of which nearly half stated it was either as part of a larger landscaping project within their property's grounds, or to replace a tree that had previously existed within their property.

 

 

Of these recent plantings, 48% of respondents said they planted the saplings in spring, whilst 36% planted in autumn, 13% in summer, and 3% in winter.

 

Additionally, the survey asked whether the respondents would consider the possible future installation of solar panels as a determining factor in deciding what tree to plant, if at all, of which 7.5% said yes. Also, the survey results showed that the respondents have less of a desire to plant trees in the future, with an average of 3.4 trees planted oer household in the past dropping to 2.1 per household in the future.

 

Pruning behaviour

88% of respondents stated that the tree(s) within their property had been pruned. Of this 88%, 60% did their own pruning work, 28% hired in certified arborists, and 10% hired uncertified persons. The motivation behind the pruning was, from the results, down to an attempt to improve the tree's shape (64.2%), to remove dead or damaged wood (59%), to provide clearance for utility lines (23.4%), to increase sunlight levels (22.1%), to increase fruit production (19%), and to improve the view (13%). Multiple reasons could be ticked, per respondent.

 

Neighbourhood differences

On a neighbourhood level, past research has suggested that planting of 6.4 trees per acre is needed to reach Seattle's goal of achieving 33% canopy cover (not including trees planted to replace old ones). However, no districts in Seattle, according to the results of this survey, will have enough trees planted to meet this target. Of course, the amount of future tree plantings varied between districts (from 2.4-5.4 trees per acre). Respondents were also asked whether they felt they had the knowledge needed to be able to select the right tree to plant, of which it was found between 8-32% of respondents were confident in their ability to select the correct tree.

 

 

Analysing the results

In light of the aforementioned results, the authors were concerned over the falling desire by respondents to plant more trees. They were unable to assert whether this is a city-wide phenomenon, or is a trend across the USA. However, it was noted that the desire for tree planting by respondents may rise (or even fall) in the future. Regardless, the Seattle urban forest managers are concerned over the lack of intent to plant trees as the needed rate.

 

Turning attention towards the time of planting, concerns were raised following results showing 84% of trees were planted during periods of the year that were unfavourable. A reason for this, the authors allege, is that nurseries advertise during the spring, and as many homeowners reported they didn't have the knowledge needed to be able to select the right tree, it is very likely their knowledge also was lacking on awareness of when to plant.

 

Furthermore, there is a discernible trend in "downsizing" mature tree height, with a very small desire for big trees to be planted. This may have marked implications for stormwater management, pollution reduction, and so on. As there are fewer spaces for large trees to be planted, mainly due to in-filling within cities, the future for large urban trees looks bleak in Seattle. The authors therefore suggest that there may be a need for the Seattle urban forest managers to work with homeowners to encourage, and even subsidise, large tree plantings.

 

Looking at the clear desire for homeowners to begin planting fruit trees (42% want to plant tem in the future), the authors remark that this desire could be harnessed to get homeowners more interested in urban forestry on the whole. As there is a clear desire for homeowners to grow their own fruit (at least, in part), there may be scope to encourage homeowners to consider the needs of the urban forest on the whole, which may help the city achieve its 33% tree cover target. However, as fruit trees are generally small, there is a risk that an over-use of fruit trees may be detrimental to the goal.

 

Respondents' desire to not use certified arborists for tree work was also a concern for the authors. As tree health can be greatly impacted by improper pruning works, the fact that only 28% of the 88% of respondents who had pruned their trees had used certified arborists, means many trees may not be receiving the appropriate level of care. Again, there may be scope here to enocurage the use of trained arborists.

 

Survey limitations

The authors recognise that non-response bias (due to a lack of interest, or alanguage barrier) will have likely skewed the results obtained, particularly in districts where there is greated cultural diversity and / or the average income per household is lower. In fact, higher rates of response were received from the more affluent districts, which means data from poorer regions of Seattle may be both lacking and not as reflective of homeowner attitudes.

 

Source: Dilley, J. & Wolf, K. (2013) Homeowner Interactions with Residential Trees in Urban Areas. Arboriculture & Urban Forestry. 39 (6). p267-277.

[Kveldssanger]

I have to admit I'm unable to keep up!

I like the fact that I'm building a reference library courtesy of someone else's hard work and research. So your labours are appreciated even though I don't have the time to read as quickly as you post.

Thanks

Timon

 

I am getting through stuff at a rate of knots at the moment, though am finding learning is an exponential curve! Putting the jigsaw together gets more enjoyable and a little easier, in time.

 

I am glad you'll nonetheless go through everything, and hope there's plenty of stuff you can use for your own research or learning needs.

[Kveldssanger]

10/01/2016. Fact #122.

 

Bulk density is an indicator of soil compaction. It is calculated as the dry weight of soil divided by its volume. This volume includes the volume of soil particles and the volume of pores among soil particles. Bulk density is typically expressed in g/cm³.

 

Soil bulk density is, to some degree and in combination with other soil characteristics, reflective of the mechanical resistance roots meet in the soil. At higher bulk densities, root growth can be restricted as the forces exerted by the roots as they push through the soil cannot 'overcome' the resistant forces of the compacted soil. Further to this, increased bulk density reduces soil porosity and therefore offers less viable space for roots to grow within (given roots grow most often within aerated pockets of the soil structure). By-and-large, root growth is optimal at 1.2g/cm³ and limited seriously when bulk densities exceed 1.6 g/cm³ (Bassett et al., 2005). Road foundations in Denmark are typically compacted to bulk densities exceeding 2 g/cm³ however (Buhler et al., 2007), which indicates the extent of the problem for urban trees in particular.

 

Studies with tree seedlings in containers have shown that soil compaction reduces vertical root penetration, thereby meaning areas of high bulk density might increase the likelihood of shallow rooting. In larger, mature trees, higher bulk density can decrease the fine root density profile quite significantly – by up to 60% (Watson & Kelsey, 2006). This can impact upon water and nutrient uptake. Therefore, if a tree develops and matures in a soil with high bulk density, not only may it have shallow roots but also have a reduction of fine root mass. Research has however shown that trees have a remarkable ability to reach down to points of lower soil bulk density, where surface compaction is an issue (Nambiar & Sands, 1992) – assuming deeper root penetration is actually possible. Interestingly, a certain amount of compaction, in turn increasing bulk density, can aid with root growth – given the increase in root-soil contact (Alameda & Villar, 2009). This is particularly the case for roots within very loose, sandy soils (such as sand dunes).

 

Additionally, with the loss of macro-pore space as soil density increases, water infiltration and gas diffusion is reduced, soil oxygen concentration is decreased, and carbon dioxide concentration can increase – possibly to toxic levels (Watson & Kelsey, 2006). This deterioration in quality of the soil environment thereby renders the soil less favourable for root growth and nutrient uptake (respiration for active transport is less feasible, for instance) (Batey, 2009; Day et al., 2010; Kozlowski, 1999), as well as mycorrhizal fungi establishment (Shigo, 1986).

 

The overall impact of soil bulk density upon root morphology and function does ultimately vary between species however (Bassett et al., 2005). Species will preferentially reside at different places upon a larger continuum of bulk densities, with tolerance ranges also differing – certain species may tolerate a wider range of soil densities than other species.

Sources:

 

Alameda, D. & Villar, R. (2009) Moderate soil compaction: implications on growth and architecture in seedlings of 17 woody plant species. Soil and Tillage Research. 103 (2). p325-331.

 

Bassett, I., Simcock, R., & Mitchell, N. (2005) Consequences of soil compaction for seedling establishment: Implications for natural regeneration and restoration. Austral Ecology. 30 (8). p827-833.

 

Batey, T. (2009) Soil compaction and soil management–a review. Soil Use and Management. 25 (4). p335-345.

 

Buhler, O., Kristoffersen, P., & Larsen, S. (2007) Growth of street trees in Copenhagen with emphasis on the effect of different establishment concepts. Arboriculture & Urban Forestry. 33 (5). p330-337.

 

Day, S., Wiseman, P., Dickinson, S., & Harris, J. (2010) Tree root ecology in the urban environment and implications for a sustainable rhizosphere. Journal of Arboriculture. 36 (5). p193-205.

 

Kozlowski, T. (1999) Soil compaction and growth of woody plants. Scandinavian Journal of Forest Research. 14 (6). p596-619.

 

Nambiar, E. & Sands, R. (1992) Effects of compaction and simulated root channels in the subsoil on root development, water uptake and growth of radiata pine. Tree Physiology. 10 (3). p297-306.

 

Shigo, A. (1986) A New Tree Biology. USA: Shigo and Trees Associates.

 

Watson, G. & Kelsey, P. (2006) The impact of soil compaction on soil aeration and fine root density of Quercus palustris. Urban Forestry & Urban Greening. 4 (2). p69-74.

[daltontrees]

06/01/16. Fact #118.

 

Fossil records suggest that the genus has existed from the Cretaceous period (145-66 million years ago), making the Magnolia the first flowering plant.

 

 

Sorry but I'm not clear on this. Are you saying that the genus contained the first flowering plant? Or that it contains the oldest surviving flowering plant species

 

It's hard to pin all this down, plus since that book was written in 1999 there have been strong claims from water lilies, amborella, foxtails and star anise. Nad perhaps there is a distinction to be made about the first flowering plants and the first terrestrial flowering plants. From what I can tell from a sift of conflicting literature the proptotype angiosperms relied on water in the reproduction process (and were aquatic rather than terrestrial) but it's not even clear if these can properly be considered angiosperms.