(Arboricultural-styled) 'Fact of the Day'

[Kveldssanger]

Quick question - are you all content with the new 'direction' of the posts, complete with pictures and longer write-ups?

[David Humphries]

Quick answer........Yes

 

Mix of images, graphs and text appeal to a wider audience

 

 

 

 

.

[Kveldssanger]

I was thinking that. Only when I started writing my blog did I figure how horrendously dire some of my long posts must have been, in terms of a wall-of-text appearance!

 

Curious to know whether most people access this thread from within arbtalk, or via internet searches. Stacking up a nice load of hits lately, which is great.

[Kveldssanger]

01/02/16. Fact #143.

 

When a species of tree suffers mortality as a result of a pest or disease, and such mortality occurs on a large scale, there is significant risk of co-extinction. Other species reliant upon the tree for habitat, be it exclusively (they spend their whole life using that tree species) or partially (a segment of a species’ lifecycle is spent on that tree species), stand a higher chance of also suffering large scale mortality, either immediately or soon after (a lag period). With around 10% of all tree species being threatened with extinction by a pest or pathogen, understanding the co-extinction effects of mass tree mortality is critical.

 

In this study, located on Gotland (an island in the Baltic Sea; off of the coast of Sweden), the authors look at the relationsip between Fraxinus excelsior mortality associated with ash dieback (Hymenoscyphus fraxineus), and the species’ epiphytic lichen community. In total, 20 wooded sites (including meadows, pasture, and of course woodlands of varying canopy closure levels) were selected that contained ash populations (at least 50 individuals of the species), and all 20 sites had been inventoried for lichens in 1989-1991. This allowed for the authors to compare current populations of lichen with populations prior to the onset of ash dieback.

 

The study was undertaken during July of 2009. By completing the study during this time, symptoms of ash dieback could be seen more readily, and the ash populations could be segmented into categories: dead (16%), dying (18%), infected (36%), and visually healthy (28%). In total, 1,066 ash were surveyed, and all 20 sites were found to be home to ash infected with ash dieback (though managed sites showed higher infection rates at 84%, compared to 63% in unmanaged sites and 66% in grazed sites – likey as managed trees, such as pollards, have an abundant amount of sprouts that may be more susceptible to the pathogen). The authors expect that the infected ash will all succumb to the pathogen in the coming 10 years. With the current figures from this study, the authors suggest that at least 34% of the ash (classed as dead and dying) will die, though as trees do not appear to routinely recover from infection, a further 36% may also die.

 

Ash pollards such as this are considered to be at higher risk of infection (and subsequent mortality). How will this impact upon local lichen populations, and the populations of other tree affiliates? Source: Explore South Lakeland.

 

As a result of this anticipated mortality, the authors suggest that the best case scenario will see 12-95% of the lichen species to suffer the same fate as the ash, though it is likley that at least 38% of the lichen community will disappear. Once 60-65% of the host ash are infected on a site, lichen communities will begin to suffer significantly as a result. Of course, lichen species that are not exclusively associated with ash will not suffer so greatly compared to those species reliant perhaps exclusively on ash, though their populations will still suffer losses. Concerningly however, most of the lichen species likely to suffer huge losses are red-listed species found only on ash. Additionally, extinction of lichen species is most likely on traditionally-managed sites, given their higher infection rates. The below tables outline such predicted extinction rates.

 

Graph ‘a’ shows how lichen species, either fully or partially reliant upn ash for habitat, will fare as a result of the infection and subsequent extinction of ash (both optimistic and expected co-extinctions are displayed). Graph ‘b’ shows how lichen species, again both exclusively found on ash and found across all tree species, will fare in different land use types.

 

Because extinctions only begin to significantly rise in occurrence at around 60-65% of ash being infected, the fact that few co-extinctions of lichens has been observed is perhaps not surprising – dead and dying individuals, at least in the area surveyed, were not high enough to cause such large-scale mortality of lichens. However, the authors note that lichen species may in fact go extinct before the ash do, at least on a local level, which highlights the need for further research into the relationship. Furthermore, sexually-reproducing lichen species are more likely to suffer extinction than vegetatively-reproducing species, as the latter is a much more rapid means of colonisation on a host tree. However, all species will ultimately be caught in a ‘bottle-neck’, where many lichen species are seeking to colonise few remaining ash hosts (particularly mature individuals), and this will see host-specific lichen species suffer most markedly.

 

Such associated mortality in fact crosses over into other affiliate species associated with ash, however (such as insects, fungi, and other epiphytes), meaning that lichen will not be the only affiliate to suffer as ash dieback takes a hold. Perhaps, as highlighted in this research, areas with infection rates of above 60% should be targets for conservation management, in an attempt to reduce the loss of affiliate species. Such management may in fact encourage traditional management methods, such as pollarding, to cease upon ash, given managed ash are at a higher risk of infection.

 

Source: Jönsson, M. & Thor, G. (2012) Estimating coextinction risks from epidemic tree death: affiliate lichen communities among diseased host tree populations of Fraxinus excelsior. PLoS One. 7 (9). e45701.

[daltontrees]

Quick question - are you all content with the new 'direction' of the posts, complete with pictures and longer write-ups?

 

The addition of description under the pictures is good, when you just added pictures to break up text I found it irritating trying to figure out if the pitcures were relevant.

 

For me the articles are too long for practical use. I open up the thread, scroll down to today's 'fact' and if it's more than 1 1/2 screens or so (ecluding bibliography and pictures) I don't have time to digest it so I shut it down until I have time to read it all. Which I never do. But that's just me.

[Kveldssanger]

Always a work in progress, though am getting there. Lately I've been going through entire articles I find interesting. I'll have shorter ones, though doing a lot of writing about trees at the moment and am trawling so much literature that I have so much I want to write about.

[Kveldssanger]

02/02/16. Fact #144.

 

I recently looked into how neighbourhood affluence levels influenced tree populations (abundance and diversity) in a city in Brazil, and have since come across a similar research article that looked into such issues (amongst others) in southern California. In this instance however, it appears that the authors take a slightly different angle of approach, and assess whether residents’ preferences across different communities impacts upon constituent tree populations as well. Ultimately however, this study had three main aims, and they are: (1) to determine whether the socio-economic and environmental status of an area will influence tree cover and species diversity; (2) to ascertain whether patterns exist across spatial areas within an urban forest (such as between street trees and trees within residential properties, and across the three counties studied), and; (3) to assess the level of influence residents have in terms of determining tree species composition within the urban environment.

 

The three counties featured within this study were Los Angeles, Orange, and Riverside, and street tree inventories were completed during 2010-11. In each county, 12-13 residential neighbourhoods were identified (that differed in their characteristics) and subsequently surveyed. In these neighbourhoods surveyed, data relating to socio-economic and environmental variables (temperature, precipitation, population density, average income, etc), ecosystem services of the trees (growth rate, likelihood of root damage, flower showiness, and so on), and also tree type (native or exotic), were collected.

The macro-scale urban forest of southern California

 

Across all of the neighbourhoods surveyed, it was found that only 7% of all trees were native, whilst 46.5% were exotic species with the ability to regenerate naturally, and the other 46.5% unable to regenerate naturally. In terms of species diversity, all three areas were found to be somewhat similar, though Riverside had the lowest at 45 and Orange had the highest at 75. Los Angeles has 64 tree species identified in the survey. Only Quercus agrifolia, a native to the region, was found in significant numbers – all other abundant species were exotics, such as Cupressus sempervirens and Syagrus romanzoffiana, of which some were unable to reproduce naturally.

 

Los Angeles County (as well as the other Counties) was found to be home to a great number of Mexican fan palms (Washingtonia filifera), which can be seen lining this street within the County. Source: LA City Urban Forestry.

In terms of tree cover, Los Angeles and Orange County had a greater number of trees per neighbourhood than Riverside, though residential and parkland areas across all three counties had the highest diversity of tree species, as well as the highest canopy cover. Most trees in all counties were planted, in place of growing naturally.

Drivers behind differing urban forest characteristics

 

Socio-economic factors, in light of the data collected, were shown to have a significant influence upon urban forest structure and richness – the average income of the neighbourhood, the year in which the neighbourhood was built, and the level of education residents received (in this case a college degree or higher) were the most important influencing factors. There were greater numbers of trees (as well as a greater species richness) per neighbourhood in the wealthier (and better-educated) areas of all three countries, and Orange County and Riverside County had more trees in older neighbourhoods. Conversely, tree species diversity was low in cramped (high density) neighbourhoods.

 

Spatial differences in urban forest structure

 

In terms of differences between richness of street tree populations and residential tree populations, there was only a significant difference (in favour of residential trees) for Orange County (and between counties, street tree species composition was not significantly different). There was also a difference between the counties with regards to the amount of shade trees found, with Riverside County possessing more shade trees than Los Angeles County and Orange County – though there was always a greater proportion of shade trees found in streets, when compared to residential properties. Residential properties also had trees that, on average, were more water-demanding, and required more maintenance.

 

Tree species preference of residents

 

More fruit trees were found in older, more affluent neighbourhoods, compared to high-density neighbourhoods where trees were typically far less ‘showy’. Newer developments also had fewer trees that were demanding in terms of maintenance. Hotter areas were found to be home to the most shade trees, which correlated with the increase demand by residents in those areas for shade trees.

So what does this mean?

 

Because many of the trees in all of the neighbourhoods were planted, the authors suggest that socio-economic factors feature heavily in what is planted and where. As most of the tree species identified were not as common 20-30 years ago within the same counties, the demands of residents must have changed and the alteration in tree species composition would have shifted as a result. This is, according to the authors, perhaps because nurseries stock what is in demand. However, it is concerning that most trees are not native, though this may be because the counties’ generally arid conditions mean that the areas were largely void of trees prior to development.

 

The fact that species richness is higher in affluent neighbourhoods is also interesting, though perhaps not surprising given other studies showing much the same. The authors consider that because wealthier neighbourhoods have existed for longer periods of time than the newer low-income neighbourhoods, and because wealthier neighbourhoods can afford to heavily irrigate their trees, their tree species diversity is, on average, higher. Interestingly, the concept of ‘prestige trees’ is also raised, perhaps suggesting that wealthier households will seek to plant exotic and perhaps less common tree species as a means of ‘showing-off’.

 

The ‘wood streets’ in Riverside County are lined with trees, in this case what appear to be cypresses. Source: Grow-a-brain.

 

Residents’ levels of education, whilst shown to be an influencing factor in more diverse and plentiful tree populations, may not relate exclusively with affluence. Of course, well-educated individuals may live in poorer regions, for varying reasons. It is perhaps because they are likely more aware of the need for trees that areas of well-educated individuals were found to have ‘better’ urban forests.

 

The greatest species diversity, which was found in parks and residential areas, may be explained by (in the case of the latter) the desire by residents to have functional trees with good amenity value (traits are more important than species, and therefore there is a wide species diversity). The fact that street trees required little maintenance and had low water demands may be because municipal foresters are principally concerned with reducing the costs associated with their trees. Hence, we can observe a difference in the rationale behind the decisions made with regards to species selection, though publicly-owned parks do allow for more innovation on behalf of the municipality’s officers (given more space and a lack of a need to prune park trees, in general).

 

Focussing on resident preference for trees, whilst the data collected here was not robust enough to draw wholly accurate conclusions, the greater abundance of trees in affluent areas correlates with such residents’ desires to have trees within their neighbourhoods. Similarly, the desire by residents in hotter neighbourhoods to have shade trees was shown to also hold true in actuality (more shade trees were found in hotter areas). More research would need to be done to ascertain whether resident preferences do influence street tree populations and species compositions, however.

Source: Avolio, M., Pataki, D., Gillespie, T., Jenerette, G., McCarthy, H., Pincetl, S., & Clarke, L. (2015) Tree diversity in southern California’s urban forest: the interacting roles of social and environmental variables. Frontiers in Ecology and Evolution. 3 (73). p1-15.

[Kveldssanger]

Few days off, I think. Let you guys catch up.

[Kveldssanger]

05/02/15. Fact #145.

 

As trees transpire, they cool their surrounds. In urban areas, where the heat island effect may at times be quite significant, such transpirational activity is necessary for reducing both day-time and night-time temperatures. However, transpiration is not constant, and trees therefore may provide different ‘levels’ of cooling at different parts of the day. Efficacy may in fact be weather-related, or even impacted by the permeability of the ground surrounding the tree’s rooting system – this is because such factors influence how much water is available to the tree. In turn, transpiration rates are affected, which has a knock-on effect of impacting upon ambient temperatures. In this study, the authors investigate seven of the most common urban tree species (Acer platanoides, Aesculus hippocastanum, Betula pendula, Fagus sylvatica, Prunus serrulata, Quercus robur, and Tilia europaea) in Gothernburg, and ascertain their benefits with regards to cooling via transpiration.

 

From (most of) the study trees (which were ‘avenues’ or groups of a single species), the authors measured stomatal conductance, transpiration rates, and photosynthetic rates both during the day- (solar noon) and night-time (1-4 hours after sunset) on warm summer days; one day where no clouds were present, and one where clouds were moderately present. Certain tree species, including Acer platanoides, only had their measurements taken on one day. The reason for some tree species only having one reading was because of their locations (the map below highlights all locations across the city), and because the equipment used to take the readings was malfunctioning. Data relating to leaf area, solar radiation, and energy loss (heat) beneath the tree canopies was also collected, amongst other things (the table below the map shows some of the collected data relating to the trees surveyed).

 

A map showing the locations of all the tree species. Tilia europaea (A-C), Quercus robur (D), Betula pendula (E), Acer platanoides (F), Aesculus hippocastanum (G), Fagus sylvatica (H), and Prunus serulata (I).

 

Some of the information that was gathered by the authors relating to tree properties.

 

In addition to the tree data captured, meteorological data was also taken from the sites. Air temperatures, relative humidity levels, atmospheric pressure, recent rainfall, and so on, were captured, and the article itself contains a very large table outlining the weather conditions at each site.

 

Results

 

The authors found that day-time stomatal conductance of leaves was two-times greater in sun leaves than in shade leaves. The greatest differences between stomatal conductances during day and night were observed in Quercus robur and Prunus serrulata along streets and in Tilia europaea within parkland, whilst the lowest difference was observed in Aesculus hippocastanum and Tilia europaea along streets.

 

Transpiration rates were also found to be higher in sun leaves; on average, three-times higher. However, there was marked variation between species, with street-based Quercus robur and Betula pendula sun leaves having the greatest transpiration rates of the entire study on the sunny, warm, dry day. In addition, park-based Tilia europaea sun leaves were found to transpire at almost twice the rate of its street tree counterparts. Not only this, but individuals growing on streets with wide lawns transpired at higher rates than ones growing on narrow lawns. Transpiration may therefore be location-specific, even when assessing the same species. Such transpirational differences were not found within shade leaves.

 

Despite the above, and induced by the falling solar radiation levels and air temperatures, the transpiration rates of all individuals begun to drop 2-3 hours before sunset. However, transpiration did persist following sunset (at a rate of up to 20% of daytime rates) – for at least four hours (transpiration may have persisted after, though the data collection stopped 4 hours after sunset). Such night-time rates were also different between species, with Prunus serrulata transpiring at a much greater rate than Betula pendula. The other species all sat within the range these two species established, though day-time transpiration rates were somewhat correlated to night-time transpiration rates – as in, trees that transpired more during the day also did so at night.

 

Both stomatal conductance and transpiration rates were also shown to be markedly impacted by recent rainfall episodes – particularly over the 5-30 days prior to the data collection. Where there had been less rainfall, stomatal conductance and transpiration was reduced. However, impermeable surface to permeable surface ratios in the immediate surrounds to the study trees was also determined to be a strongly-influencing factor, in terms of how much of the rainfall actually was made available to the trees. Trees that sat in parks or along wide street verges had higher stomatal conductance rates than those on narrow verges – particularly in sun leaves. Across all species, Quercus robur and Prunus serrulata had higher transpiration rates compared to the other tree species, at similar levels of ground permeability.

 

Such foliar activity led to between 9-64% of incoming short-wave solar radiation being ‘reflected’ back as latent heat energy (associated with transpirational cooling). The lower end of the spectrum was occupied by Tilia europaea residing within narrow-verged and heavy-traffic streets, whilst Quercus robur occupied the upper echelons. Such latent heat ‘reflection’ lead to the reduction in heat energy beneath the tree canopies, and the data collected at each site is shown below. However, it was found that day-time cooling effects were somewhat – or fully – negated by the ‘mixing’ of air in the vicinity of the trees (the air during the day was more volatile, perhaps induced by higher temperatures), meaning that transpirational cooling was actually more effective later during the daylight hours and soon after sunset.

 

A comparison between species and the day and night.

 

What does this data mean?

 

In essence, it can be asserted that both the species of a tree, and its location, will influence upon the cooling effects associated with its presence. For cooling effects to be most significant, not only should species selection be carefully undertaken, but urban trees should be provided with the necessary levels of moisture required to sustain ‘healthy’ operations (irrigation may be necessary, in periods of drought – even for larger trees), and surrounding surfaces should be permeable in nature (and if not, irrigation should be strongly considered). For the best cooling effects, trees should be situated within parks. However, as the benefits of tree cooling are most desired in streets where there is a greater accumulation of heat energy, it may have to be accepted that constituent trees will not operate to maximum beneficial capacity – though not utilising trees should not even be considered, as all trees provide some benefit. It may indeed be a case of selecting species that are more effective at cooling the street localities, though also tolerate urban conditions and will not cause damage or other adverse impact in their environment (for small streets with narrow verges, Prunus serrulata may be a good choice of tree – assuming its roots do not cause damage to the pavements).

 

Because this study suggested that transpirational cooling during the day might not be overly significant (or even non-existent), and as such a conclusion is in-line with other studies, there must be a degree of acceptance that trees are not miracle-workers. Of course, the shading (shadowing) trees provide will lower ambient temperatures (at which point, larger and denser-canopy trees will be preferable), though it is during the later parts of the day (and the early night hours – at a point, transpiration no longer impacts upon cooling during the night) when trees will cool the air, through transpiration, most significantly. This is still nonetheless highly important, as the removal of heat during the evening (and even during the night) can be of marked benefit to residents – not only is it difficult to sleep when it is warm (and costly to cool a property), but increased temperatures have also been attributed to increased restlessness and crime.

 

An urban scene within Gothernburg, filled with large trees. During the middle of the day, it may perhaps only be their shading that reduces ambient temperatures, though as the evening draws in their foliar transpiration will have a positive effect on air temperatures. Imagine, for a minute, if these trees weren’t here – how uncomfortably hot (and bright!) might it be? Source: Skyscrapercity.

 

Source: Konarska, J., Uddling, J., Holmer, B., Lutz, M., Lindberg, F., Pleijel, H., & Thorsson, S. (2016) Transpiration of urban trees and its cooling effect in a high latitude city. International Journal of Biometeorology. 60 (1). p159-172.

[Kveldssanger]

07/02/16. Fact #146.

 

Trees provide a huge number of ecosystem services, ranging from the amenity value they provide to the ecology they support through their mere existence. In a setting where there is no target zone, a tree can be allowed to simply persist in any sort of state without there being the risk of injury or damage to property, though where the tree resides within a busy environment this is not the case. Therefore, a particular conflict arises between retaining viable and niche decaying (or hollow) habitat in an old tree and reducing the risk associated with the retention of such a tree. In this study, the authors assess how balancing such conflicting demands impacts upon saproxylic beetles including the endangered Osmoderma eremita.

 

The study area for this project was a large area (159ha) of parkland in the city of Rome, Italy. The parkland is characterised by areas of woodlands and scattered mature trees, with a species composition that features exotic cedars, pines, and cypress, though is dominated by old Quercus ilex (holm oak). These holm oaks were the focus of this research, and 1,247 individuals were surveyed in the areas of the park that were designated for Visual Tree Assessment surveys by the park’s managers (presumably because these oaks were seen to be of greatest potential risk to the public).

 

The gardens of Villa Borghese. Source: Wikimedia Commons.

 

Alongside the VTA, (initially) all holm oaks were surveyed for saproxylic beetles (every fourth day) between 14:00 and 19:00 from 19th July to 28th August. This involved exploring all hollows up to 5m above ground level. After a period of time, only those oaks with more significant decay were surveyed. Further, from 1st September to 30th September, between 09:00 and 18:00, the holm oaks were surveyed three times for beetle larvae and frass.

 

In all, ten species of saproxylic beetle were identified from 66 of the 1,247 holm oaks. However, 41% of the beetle population was found within the holm oaks considered to be at highest risk of falling (category D3 – see table below), with some species being found exclusively in such trees. For Osmoderma eremita, four D3 trees were host to the species, and the remaining seven host trees were deemed less likely to fail.

 

The number of beetles found, of different species, in the holm oaks surveyed. D3 relates to the holm oaks at highest risk of falling, and the data shows how much of the population of beetles exists within these trees.

 

In relation to the properties of the trees that contained the beetles, it was found that host holm oaks had an average DBH of 71.5cm, height of 16m, and spacing (relative to other trees) of 13m (these are big trees!). 63% of the hollows within such oaks, which were home to saproxylic beetles, had a fair amount of mould, and many of the cavities were moderate in size (an average of 18cm in width – only a few cavities of 50cm width were found to have beetles), and the decaying wood within was situated some way from the opening of the hollow. Of these hollows, many had a north-eastern or south-eastern orientation, were no higher than 2m from ground level, and were free from bird nests. Holm oaks with damage to the root collar were also found to be most likely to be host to more species of beetle (higher species richness), most probably because of the greater amount of internal hollowing and decay close to the base of the tree (where the largest hollows can form).

 

The authors suggest that, in order to preserve viable habitat for Osmoderma eremita, there is a need to retain living and large holm oaks that are close together (no more than 100m apart, but ideally far less), that have a lot of internal wood mould, and have nest-free cavities situated on NE and SE sides of the tree – this ensures there is high humidity but also a relatively cool internal temperature, and a lack of predators in the form of nesting birds. Therefore, the removal of such hazardous (category D3) oaks found to be host to the species is damaging in the sense that the beetle’s habitat is fragmented and also largely removed, meaning remaining beetle populations may become isolated (even when found only a few hundred metres from other suitable holm oaks) and the carrying capacity of a landscape is reduced.

 

An overhead shot of some of the park. We can see how dense the tree cover is. Source: Personal Drones.

 

Concerningly, because 41% of holm oaks host to saproxylic beetles were considered to be of high risk to members of the public (because of their poor condition), there is an evident conflict of interest associated with management of many trees that are ecologically critical for rare and threatened insects. In fact, large trees home to beetles including Osmoderma eremita were already on a felling schedule, which is catastrophic for the saproxylic species’ longevity on the site. In place of felling, it is suggested that old trees are retained via the installation of supporting structures that reduce the risk of large limbs falling, or such hazardous limbs have some of the more unnecessary side-branches removed to reduce mechanical loading. However, if limbs must be removed, they should be left on the ground beneath the tree, and the cuts made should mimic a natural fracture created under over-loading conditions.

 

This begs the question – are park managers, who are responsible for many mature and veteran trees, aware of their ecological benefits, and are they prepared to extra spend money on retaining such trees for their habitat whilst also satisfying the need to reduce the risk to visitors (and do they even have the expertise to request this)? Because many of the holm oaks found to be host to saproxylic beetles were already on a felling list, it suggests that park managers (at least in this instance) were more than prepared to accept the loss of habitat in the pursuit of public safety (did they even know about the beetles being there?). How willing would park managers be to change their stance, and strike more of a balance between the two conflicting aspects of tree management?

 

Source: Carpaneto, G.M., Mazziotta, A., Coletti, G., Luiselli, L. and Audisio, P., 2010. Conflict between insect conservation and public safety: the case study of a saproxylic beetle (Osmoderma eremita) in urban parks. Journal of Insect Conservation. 14 (5). p555-565.