On Friday I made a post on Piptoporus betulinus and talked briefly about its colonisation strategy. You may recall that one of the images, which showed the underside of the bracket, had quite an interesting texture. As luck would have it, I think that I have just learned the reason (or one of the reasons) for this.
Here we see the bracket’s underside – the black dots may mark the entry points for boring insects, or perhaps even be small insects feasting upon the fungus’ flesh.
In his book Mycelium Running, Paul Stamets states that the ‘scent’ of the birch polypore attracts beetles that will, upon arrival, burrow into the spore-rich underside of the bracket and feast upon the inner flesh. As the beetles burrow in and feast, they are covered in spores. Upon leaving the birch polypore, the spore-laden insects will travel to another birch where they will bore into the wood and lay their eggs – all whilst depositing birch polypore spores.
The spores will then germinate and begin to form a mycelial web, which acts as a food source for the growing larvae. Because the mycelium induces a brown rot, the wood properties also change. In fact, as the wood properties change, woodpeckers are drawn to the tree in the search of grub to feed on. As the woodpekcers search for the beetle larvae, all whilst ‘damaging’ the wood during such a pursuit, they too act as a vector for spores – as does the woodpecker create conditions for other insects and birds to begin using this birch tree as a food source. Amongst all of the comings and goings of insects and birds, fungal spores hitch a ride, and the birch becomes a “launching platform” from which the birch polypore can continue its existence.
I first came across the weeping willow-leaved pear some months ago by a residential car park. Funnily enough, my first thoughts were that it looked like a pear with willow-shaped leaves. I suppose I wasn’t wrong!
Since then, I have seen this tree dotted about all over the place, from private gardens to wide roadside verges.
It’s a very dainty tree that doesn’t reach tall heights (I have seen one that was about 5-6m high, but most have only been around 3-4m) with a very dense branching structure. In winter, there is very little attraction other than its ‘different’ form (a very unorderly and dense arrangement of pendulous branches), though it lights up during the growing season to produce not only a sublime spring blossom but a very wonderful summer appearance (the glossy green leaves atop contrast gloriously with their silvery undersides).
Because of its small mature height and compact form, I can see this being widely used in urban environments where there is a desire to improve amenity – perhaps in new developments, within smaller verges where larger trees wouldn’t be appropriate. A favourite of mine in the ‘small tree’ department, alongside Crataegus x lavallei ‘Carrierei’.
I feel this image really captures the tree’s character. Certainly one for an ornamental setting!
In urban areas, street trees will normally exist within a length of grass verge (of varying width) running parallel to the highway (path and / or road), or within planting pits inside the highway. As these trees mature, both the roots and the root collar can cause damage to the highway – particularly if rooting space is limited. When such damage occurs, there is generally the need for remedial works to take place. However, such remedial works can have an impact upon tree health, survival, and economic value (CAVAT, CTLA, and so on) – particularly as roots may be damaged or severed during the construction works, which has consequences for tree health.
An example of a street tree drastically uplifting the pathway and curb. Source: Seattle.gov.
In light of the above, research was undertaken during the late 1980s to early 1990s in Milwaukee, USA to establish exactly the above impacts to trees that have had highway repair works undertaken within their rooting environments. The reason for Milwaukee being the choice location was because the city had a third of its trees valued with the CTLA system in 1979, so there was relatively recent data to compare results to with regards to economic impacts.
Methodology
The authors looked at construction schedules for the 1981-1985 period within the areas where trees were assessed with the CTLA valuation system, and pinpointed locations where highway repair or widening had been undertaken (in order of descending ‘disturbance severity’, the four criteria established by the authors were: street widening and curb setback, curb and pathway replacement, curb replacement, and pathway replacement).
One hundred projects were then randomly selected over the 1981-1985 period, with 20 per year (allowing for tree condition to be assessed 4-8 years after highway repair works were undertaken). From each study, a single block where the repair works took place was identified. Then, using the criteria mentioned above, 50 blocks (10 per year) were selected based on the highest ‘disturbance severity’. These 50 blocks were then examined, and the top 25 in terms of species diversity were selected to feature within the study. The authors then chose the nearest block to those 25 blocks that did not have repair works undertaken and contained tree populations, and used them as the controls. The first 25 trees in both the construction and control blocks were then identified (from the 1979 survey) for surveying. This lead to 989 trees being sampled in total – 510 from construction blocks, and 490 from control blocks.
From each tree, the following data was collected: DBH at 1.4m, species, verge width (from pathway edge to curb), and the CTLA tree condition rating (100, 80, 60, 40, 20, and 0). The type of construction activity was also identified. Then, comparisons were drawn to the 1979 CTLA valuation survey, to determine whether the trees, assessed in 1989, had suffered as a result of repair works between 1981-1985.
Results
Of the 989 trees sampled, only 670 had actually survived from 1979-1989. 175 trees had been replaced since 1979, and another 144 were newly planted in different locations. The trees were of 15 different species, though Acer platanoides, Fraxinus pennsylvanica, and Gleditsia triacanthos were the only three to feature enough to have statistical analyses run on them.
With regards to tree condition (with the percentage being the average of the entire CTLA tree condition rating scores, I suspect – it’s not clarified), no significant difference was found between the construction (77.2%) and control (77.7%) blocks in the 1979 survey, though by 1989 there was a significant difference between construction (71.2%) and control (76.7%) blocks.
Table taken from the report.
In terms of tree survival between these years, 81.4% of the trees on control blocks survived, whilst only 77.3% survived on the construction blocks – again, a significant difference.
Table taken from the report.
In relation to verge width, significant differences were again found. In both control and construction blocks, a lower width resulted in trees being poorer in condition, though where construction had occurred the decrease in condition was more distinct.
Table taken from the report.
Significant difference in tree condition between construction and control blocks was also observed between tree species (in this case, only Acer platanoides, Fraxinus pennsylvanica, and Gleditsia triacanthos could be analysed, as other species were not in enough abundance), using two-way ANOVA. However, using one-way ANOVA, there was no significant difference.
No significant difference between construction and control blocks was found with regards to tree diameter.
What does the data suggest?
The authors begin by asserting that highway repair works has a significant impact upon both tree condition and survival. For instance, a 22.7% mortality rate was observed in trees on the construction blocks, compared to 18.6% on control blocks. Similarly, whilst the condition of control trees did not significantly change during the survey period, it declined by 6.1% for trees affected by construction. Results also suggest that the width of the verge has a direct impact upon tree condition on both construction and control blocks, though trees on narrow verges that also were impacted by construction suffered more significantly.
How will this tree be impacted by the highway repair works direction surrounding it? Source: Bike Nopa.
Conclusions drawn from the data also suggest that tree species is not a significant determinant in tree condition and survival rate following construction. The authors state that this was to be expected, because all three species aforementioned are hardy species that are tolerant of urban conditions and disturbance. In terms of tree size (DBH), the authors note that they were surprised no significant difference was found between control and construction blocks, though because many of the trees were young (319 were under 10 years of age, and many more were planted in the 1960s and 1970s), this may have impacted upon the data. If all, or more, of the trees were mature, it may have been a different story entirely.
Turning attention towards economic implications of highway repair works to trees, assuming each tree was worth $1,100 (as was, I suspect, concluded in the 1979 survey), the 200,000 trees of Milwaukee would value in at $220,000,000. As around 3% of the tree population had a decline in condition resulting from highway repair works each year during the study period of 1981-1985, a total of 6,000 trees per annum (with a value of $6,600,000) would suffer, meaning an annual loss of $521,500 can be calculated. Additionally, as tree mortality associated with construction works was 4.1% higher than for control blocks, an additional hit of $270,600 would be taken. Therefore, the effect of highway repair works on the value of Milwaukee’s tree population was $792,100 per annum, between 1981-1985.
Source: Miller, R. & Hauer, R. (1995) Street Reconstruction and Tree Decline. In Watson, G. & Neely, E. (eds.) Trees & Building Sites. USA: International Society of Arboriculture.
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A couple of weeks ago I took a visit to Cambridge, not for the purpose of looking at trees, though nonetheless – as there were trees within the city centre – I couldn’t resist having a glance. Just as well I did, because I spotted a lovely Inonotus hispidus bracket on a weeping ash (Fraxinus excelsior ‘Pendula’).
I’m not quite sure what’s going on here, but there’s some absurd growth form on the main stem(s) – see the bottom part of the photo, and the area around the bracket. Perhaps an old branch tear out where the bracket is? No doubt that sapwood exposure has caused the fungus to colonise.The red circle shows the position of the bracket in this image. Unfortunately, I could not access the area to get closer shots.
The urban environment can be very challenging for a tree, particularly as conditions are commonly less than optimal. Many urban trees therefore very rarely have a long life expectancy – potentially only between 7-30 years, depending on the level of landscape adversity (Foster & Blaine, 1978; Moll, 1989; Nowak et al., 2004; Roman & Scatena, 2011). In order to improve conditions and ensure that an urban tree has the potential to fulfil a full-term and healthy life therefore, many factors must be considered (Kopinga & van den Burg, 1995).
1. Space
The size of the rooting environment is directly attributable to root growth; once tree roots have reached a point where they can no longer grow in a given direction, the progressively increasing demand for water and nutrients to aid in providing the energy for growth cannot be met – because root growth is confined to a closed soil environment with a limited ‘carrying capacity’ (Day & Bassuk, 1994; Grabosky et al., 2001; Kopinga & van den Burg, 1995). Very common in urban environments, particularly where a tree is bordered by built structures on two or more sides (such as roads or footpaths); at times, the available soil is less than the drip line of the tree, even soon after planting (Jim, 2001). Unless the roots can break free of the area that the root zone is confined to and can find a source of water and nutrients elsewhere, mass will be reduced and / or growth will halt (Day et al., 2010; Sanders et al., 2013). On average, trees require 2.46m of verge between a road and path to be able to grow without (1) causing problems and (2) experiencing (major) problems (Shigo, 1991), and where trees are confined to tree pits the design of the pit will be the limiting factor for tree growth (Roberts et al., 2006).
It is also necessary to note that trees need space above ground. Where aerial space is confined, trees may need to be pruned in order to ensure they are in-keeping with their surrounding environment. Pruning, most probably on a cyclical basis so that the tree retains an artificially maximum size, introduces wounds to the tree’s structure. Such wounds are undesirable – decay may be introduced, energy is required for compartmentalisation (particularly if decay becomes serious), and photosynthetic mass is reduced (at least, temporarily) (Shigo, 1986; Shigo, 1991). The introduction and progression of decay, in particular, is likely to reduce the life expectancy of an individual.
These raywood ash have a good amount of aerial space to grow within, whilst not impeding upon the highway nearby.
Therefore, in order to ensure an individual is not adversely impacted by space, many questions must be asked prior to planting: (1) what is the ultimate size of the individual, and will the site be able to cater for its maximum size?; (2) can space for trees be designed into new areas set for construction?, and; (3) is the cost of on-going maintenance, where maximum size is not sustainable, in itself a sustainable measure, as a last resort?
Where the maximum size of an individual will exceed available space, the correct tree must thus be selected. Planting a large willow in a cramped car parking zone is likely to lead to its premature death (or removal) unless it is retained as a pollard, though planting a rowan may see the individual have a full and healthy life without any significant maintenance. Fastigiate varieties of trees (hornbeam, liquidambar, ginkgo, and oak, to name four) also exist thanks to cultivation techniques, and such individuals are particularly good for where aerial space is lacking (Patch, 1981; Ware, 1989). Such trees, incidentally, are also more likely to persist through severe wind storms with little damage (Cutler, 1991).
Unlike the raywood ash shown above, this horse chestnut lacks the aerial space needed for it to adopt a full crown (which has resulted in it being crown reduced). It also lacks the rooting space needed.
It is perhaps easier for Local Authorities to implement, particularly for the rooting environment, design requirements for new developments in the form of ample rooting space (tree pits, for example) – the Trees & Design Action Group (2014) details many measures and considerations in their recent publication Trees in Hard Landscapes. Similarly, planting schemes can be altered by Local Authorities during the planning process – such a time would be beneficial for ensuring that the right individuals are planted in the right places, with regards to their required space. Tree and Landscape Officers must therefore be heavily involved wherever vegetation is concerned. This does not however solve the issue of existing developments lacking the capacity to accomodate large trees.
2. Nutrients
The necessary abundance of nutrients within the soil is absolutely critical to the long-term survival of an urban tree (Kopinga & van den Burg, 1995; Pirone et al., 1988), though many urban soils are found to be lacking (de Kimpe & Morel, 2000). Where nutrients are therefore lacking, particularly nitrogen and potassium, the health of the individual will begin to decline – phosphorus deficiency may also be a significantly-limiting factor, where soil pH is too extreme as a result of pollution (Craul, 1994; Jim, 1998). At times, however, nutrients may not necessarily (but may indeed still) be lacking, though soil bulk density may simply be too high (Kopinga, 1991) – this can simulate the same effects as nutrient deficiencies can, given the tree simply cannot access the available nutrients.
In order to ensure trees do therefore receive the necessary amounts of nutrients required for long-term and healthy growth, many practical solutions exist. Firstly, soil can be amended via the application of either quick- or slow-release fertilisers that restore, though only on a temporary basis, soil nutrients (Davis, 2015; Pirone et al., 1988; Shigo, 1991) – in certain instances, it may be necessary to analyse the soil to ascertain the extent of any deficiency (Pirone et al., 1988), and this in itself may be somewhat costly if routinely undertaken. Where fertiliser application is perhaps not warranted, either due to cost or other limiting factor, mulch may be a preferable alternative (Davis, 2015; Sæbø & Ferrini, 2006). Scope also exists to artificially inoculate the soil with mycorrhizae so to improve nutrient uptake by trees (Harris et al., 2004) – a practice which is particularly effective when coupled with fertiliser application (Adesmoye et al., 2009; Appleton et al., 2003) – or look to improve soil conditions (improve aeration, for instance) to permit the natural succession of mycorrhizae into the soil (Saif, 1981). The amelioration of soil may also improve nutrient availability directly, particularly if the soil is compacted (Kopinga & van den Burg, 1995). As a more long-term measure however, careful species selection may be cost-effective. The planting of alder, which is a nitrogen-fixing species, can improve nitrogen availability, though utilising successional species in general will normally aid, to a degree, with soil amelioration, as well as mycorrhizal establishment, and thus nutrient availability (Põlme, 2014; Shigo, 1991; Temperton et al., 2003; Wiemken & Boller, 2002) – either for the successional species to themselves thrive via creating their own niche, or for the trees to act as nurses to other trees to be planted afterwards (or that may already exist) (Prévosto & Balandier, 2007).
Caution should however be exercised where any amendments are made to the soil. For example, whilst the use of fertiliser can certainly improve tree performance where nutrients are lacking, a dosage that is too high, or is applied too late in the growing season, is likely to be counter-productive to tree health (Benzian et al., 1974; Pirone et al., 1988; Shigo, 1991). This is a particular risk where an urban tree may already be stressed – increasing soil nitrogen causes the tree to absorb (via active transport) the nitrogen, which requires energy in itself, to then synthesise amino acids and other hormones, and to subsequently increase growth rate (Shigo, 1991). If low energy reserves exist within the tree prior to application, such application may worsen the capacity for the tree to operate in the long-term by further reducing energy levels, unless the energy ‘debt’ accumulated by the new growth is ‘paid back’ with a surplus.
3. Water
Trees require (though not in significant excess) water for a vast number of life processes, ranging from the uptake of nutrients from the soil (via active transport within root hairs) to transpirational cooling from the leaves. In fact, water demand (and subsequent stress) may be more significant in urban areas due to the urban heat island effect, which not only raises temperatures but also may reduce humidity (that in turns hastens transpiration by creating a larger ambient water vapour deficit around the leaves) (Cregg & Dix, 2001; Whitlow et al., 1992), the swathes of impervious surfaces that limit water percolation through the soil (Iverson & Cook, 2000, Roberts et al., 2006; Whitlow et al., 1992), the fact that large abundances of storm-water are transported within sealed pipe networks, and the lack of rooting space (Grabosky et al., 2001; Kjelgren et al., 2000).
A sycamore within a heavily-paved area, which will limit water availability to its rooting system. Additionally, it is impossible for this sycamore to achieve a mature size – not only is it lacking rooting space, but the flats just out of view to the right restrict its aerial space as well. This entire estate is filled with sycamore, unfortunately. Completely the wrong species choice.
Given most trees found within urban areas are transplanted, it is first critical to reduce the water stress that manifests itself in the years following planting (Pirone et al., 1988; Harris et al., 2004; Mincey & Vogt, 2014). On average, staggered watering over one season for a young tree will need to amount to between 240-640L (Buhler et al., 2006) at 15-30L intervals (Davis, 2015). Even at such an early stage in the tree’s life however, may watering be an issue – particularly where the tree is maintained by the Local Authority. Budget constraints have lead to a stalling in allocation for planting and aftercare, and compiled with the budgetary shift towards maintaining existing trees (Johnston, 2010), watering may not be feasible even at such an early stage. Potential means around such an issue may come in the form of encouraging resident involvement, which can and does work where implemented properly (Mincey & Vogt, 2014), or selecting species that are more tolerant of dry conditions (May et al., 2013; Roloff et al., 2009) – Castanea sativa, Gleditsia triacanthos, and Koelreuteria paniculata are just three examples, though particular cultivars of certain species may also display heightened tolerance to dry conditions (Percival et al., 2006). Where trees are to be planted into newly-developed areas however, there is scope for Local Authorities to require for tree pits to be designed at the planning stage that integrate storm-water management systems for irrigation purposes (Bartens et al., 2009; Coutts et al., 2012) – implementation of such a system will aid with long-term health of the trees within the area benefiting from the irrigation, particularly where storm-water can be retained for longer periods of time.
Where storm-water management systems do not exist, and their implementation is not cost-effective or practicable, management options for larger trees are largely preventative in place of reactive. Reactive measures in the form of artificially watering mature trees during drought periods are likely to be incredibly costly and probably wasteful, unless many controls are put in place to reduce wastage (May et al., 2013), though irrigation can be very successful at reducing both stress and higher than average rates of mortality (Hickman, 1993). Wherever possible therefore, the surface surrounding the trees should remain free of impervious materials, so to improve rainwater percolation, though selective removal (thinning) of denser stands within urban areas may also potentially decrease competition for soil moisture; assuming the gain in leaf area of the remaining trees does not exceed (potential) total leaf area of the pre-thinned stand. Interestingly however, mature trees are usually less likely to suffer from drought stress than young trees – tolerance increases with age, by-and-large (Niinemets & Valladares, 2006). This does not of course mean that larger, older trees can be allowed to suffer as a result of neglect.
4. Light
As light is required for photosynthesis, at least a certain amount of light is required by an individual for the synthesis of sugars – of course, shade tolerance influences exactly how much light is ‘optimal’ (Niinemets & Valladares, 2006; Valladares & Niinemets, 2008). Ensuring a tree does not suffer as a result of heavy shading is, theoretically, rather straightforward – particularly where new plantings schemes are being designed. Planting schemes, typical with new developments, can ensure that trees are planted where light conditions will be suitable. For instance, there is little justification in planting an oak within a courtyard that, even during summer, will receive very little natural light. Growth will likely be stunted (Dover, 2015; Jutras et al., 2010; Kjelgren & Clark, 1992), and such poor vigour may have adverse impacts upon the individual’s long-term health – notably when wounding occurs, and the individual cannot synthesise anti-microbial compounds in the necessary abundance (Lewinsohn et al., 1993). The planting of a beech, on the other hand, may be more justified, given its greater shade tolerance (Valladares et al., 2002; Welander & Ottosson, 1998). Similarly, where an existing group of established trees already exist within the urban environment, the planting of a young individual within the shade of the mature trees will likely be detrimental and may have longer-term implications with regards to its health. The same can also be said for narrow streets lines with tall buildings. Trees that are not shade tolerant should not be planted along such streets, as they will suffer as a result of reduced photosynthetic ability (Dover, 2015).
A small Japanese cherry resides between a large Italian alder and European lime. What chance does this tree have of achieving good form and maintaining good vigour into maturity? Probably little. It simply cannot compete.
It is also important to recognise that trees may also suffer from high light-stress. In urban areas in particular, the array of artificial surfaces are usually more reflective of light than their natural counterparts (Dover, 2015). Therefore, where an individual is situated within an area with a high percentage of solar irradiation being reflected, there is the potential risk of both photodamage and photoinhibition – this is particularly a risk where the individual may operate optimally at lower light intensities, or where nutrient deficiencies exist (Cakmak & Marschner, 1992; Critchley, 1981; Marschner & Cakmak, 1989) – which is in itself more common within urban environments (Kopinga & van den Burg, 1995). Research into the impact of very high light intensities in urban trees is however lacking, and most research has focussed on much smaller higher-tier plants. Regardless, where trees are planted, care must be taken to ensure that light levels are neither too low or too high.
5. Genetics
Because of the poor conditions of many urban environments, the genetic provenance of individuals sourced for urban tree planting schemes is incredibly important. Without optimal genetic properties that enable the individual to cope with the harsh urban setting, the individual is unlikely to achieve a significant age. Used routinely within forestry to ensure stock quality is high (Lines, 1987), there is no question that careful genetic selection of characteristics exhibited by trees that are desirable for urban areas should take place – for instance, high salt tolerance is a necessary trait for any urban tree alongside a main route to be successful.
Therefore, nurseries should be locating seed sources where the parent exhibits preferable traits (personal communication with Barcham indicates such a practice is already occurring) – as should customers be demanding such a practice. Building on the above point to provide for an example, where salt tolerance is required then sourcing seed from successful specimens in coastal areas is a distinct possibility; as is sourcing seed for sites contaminated with heavy metals from similar and existing sites (Wilkins, 1997). Despite this, evidence points towards sourcing from local trees wherever possible (Mortlock, 2000; Wilkins, 1997; Wilkinson, 2001) as, in doing so, the sourced seed is more genetically optimised for the environment in which it will grow. Long-term, this could mean the difference between premature death and survival.
6. Safeguarding
Even in spite of ensuring all aforementioned factors are in order, an individual will likely fail to achieve a full life if it suffers from significant damage in the form of vandalism or otherwise – damage via such means can introduce decay and dysfunction (Foster & Blaine, 1978; Gilbertson & Bradshaw, 1985; Kopinga & van den Burg, 1995). Vandalism is particularly a problem on young trees, and at times premature death can reach rates of 30% (Gilbertson & Bradshaw, 1985; Gilbertson & Bradshaw, 1990; Pauleit et al., 2002), though vandalism may occur on older trees as well and impacts can be very severe. Risk of damage as a result of mower damage is a further possibility, and whilst many injuries are minor, numerous injuries over time may facilitate greater dysfunction.
In order to prevent such damage therefore, it is necessary for trees to be protected from being damaged by such means. Of course, it is not practical for every single tree to be physically protected by a cage – this should only be reserved for young trees, or incredibly important trees (Harris et al., 2004). Additionally, one can never entirely remove vandalism as a risk to trees – education can however seek to reduce damage, by informing people of the benefits of trees and how to properly care for them.
However, where damage is as a result of other avoidable damage, such as mower damage, an element of control exists to safeguard trees from harm. Shigo (1986) boldly suggests that, where mower operatives are found to have damaged a tree, they should lose a day’s pay. Whilst this is perhaps not sustainable as a matter of routine, employers (such as Local Authorities and large landscaping companies) can mandate the need for acute awareness when operatives are working within the vicinity of trees. The monitoring of an operative’s work can be a further manner in which to minimise risk, as can a disciplinary if an operative continues to cause damages to trees.
Whilst Fistulina hepatica can be found upon its host wherever there is heartwood to be degraded, it’s normally found on the main trunk (particularly closer to the butt , because of the greater abundance of heartwood available). However, during the summer I spotted a small fruiting body up in the crown of a mature oak tree, on a branch with a diameter of around 15-20cm. So keep your eyes out for fungi where you may not expect them!
Here we can see the fruiting body emanating from a branch attached to a major limb.The red circle shows its location in this image, and demonstrates how high up in the crown it is.Having taken the branch attached off (there were visible signs of decay, stemming from this hollow that was likely an old branch attachment), we can see that the fungus was fruiting out of said hollow. Pulling off the bark revealed not only the brown rot associated with Fistulina hepatica, but also a stringy white rot of another fungal species.
Because different artificial surfaces will have varying effects upon the amount of oxygen within the soil beneath, it is important to select the surface that will be of lowest impact where trees are situated nearby. After all, this will ensure constituent trees live longer and healthier lives.
This post looks into a study undertaken in the world-famous Vondelpark, Amsterdam. With ten million visitors a year, there is a marked degree of foot traffic on the site, and thus surfaces to direct traffic (pathways, mainly) are absolutely necessary. However, because not only due to the high water table in the park, but also the fact that root inspection of the trees has revealed blue-coloured roots (suggesting poor soil oxygen), the average life expectancy of any tree does not place above 50 years – by this age, the tree will usually become unstable and be left prone to windthrow. Of course, in a public park, the element of risk is likely unsustainable, and therefore the tree is removed.
Taken in September 2008, this photo demonstrates how busy the park may be. Source: Wikimedia Commons.
Whilst the water table is not something that can actively be lowered, the poor soil oxygen status of the rooting environment can be improved. The author suggests that, based on past anecdotal evidence from park managers, the cause of the poor oxygen levels is due to the choice of surface-hardening material used to construct the pathways – following installation of pathways, tree health was seen to visibly decline. The surfaces used in the park are – in the pursuit of a more natural-looking park – not the archetypal paved or asphalt pathways, but instead comprised of a mix of sand, loam, gravel, and sometimes a cement-like material. Manufacturers of such mixes claim that the surfaces are permeable to both water and oxygen (and thus do not impact upon tree health) – this conflicts with the views of the park managers who witnessed tree health decline following the installation of pathways made with such mixes. Based on these concerns of the park managers, a study was undertaken and identified that soil oxygen levels were at (on average) 5% – at below 12% (though it varies between species), conditions become highly unfavourable for root growth. Therefore, an in situ study was commissioned to test different mixes, in the hope that the results would provide the park managers with a better direction on what to construct new pathways out of (as the pathways all were in need of renewal, it was the perfect time for a study).
The study was therefore set up, and sought to test soil oxygen status (oxygen containers were placed underground and connected to 2mm tubes so that measurements could be taken) 18 times under five different mixes between June 2006 and March 2007. These were: Mix A (crushed natural stones and transparent bitumen fixed with latex material); Mix B (a loamy to gravely mix with a grain size distribution from 0-8mm, with a more loamy composition than Mix D); Mix C (crushed slag from the steel industry); Mix D (another loamy to gravely mix with a grain size distribution from 0-8mm, with a more gravely composition than Mix B), and; Mix E (crushed dolomite with a grain sized distribution from 0-10mm). Exact mix ratios were not available, as all mixes were sourced from manufacturers who could not provide such information. Budget constraints were also noted, which lead to the survey lacking the means of measuring oxygen diffusion rates.
The first batch of results.The second batch of results, taken after there was impetus for extending the survey.
Results from the study (above) suggested that oxygen levels in the soil were highest beneath Mix A and C, and lowest under Mix B and E – though, under all mixes, soil oxygen levels dropped after periods of heavy or prolonged rainfall. However, soil beneath Mix B and E not only suffered the most from such rainfall, but took the longest to re-oxygenate to ‘normal’ levels. Interestingly, the author notes that Mix A, which was said to be wholly impermeable by the manufacturer, had the lowest impact upon oxygen levels in the soil. Because of these findings, the research continued from between the dates of August to December 2008 (where another 10 readings were taken), to assess whether longer-term impacts were any different. Curiously, results had changed – Mix B was still very much the worst, though Mix C also had lead to lower oxygen levels in the soil than it had shown during the first study period. Now, Mix A and D were considered to be best.
From this research, it was concluded that there was a marked difference in soil oxygen levels beneath the different mixes, though because the study was only done over a short period the long-term impacts of such mixes could not be ascertained – the author notes that such mixes will all deteriorate progressively after the first year, reducing permeability of water and oxygen into the soil (this may have been what occurred with Mix C). The author also notes both that research into more permeable mixes should continue, as they are likely to provide better soil conditions beneath, and that rainfall significantly impacts upon oxygen levels.
Unfortunately, beyond this, there is not much of a conclusion in relation to the data captured (and no indication of what mix the paths were repaired with), perhaps because the author states that the reasons for the differences identified in the study were not understood. Instead, the author remarks: “park managers need to consider oxygen permeability of surface-hardening materials of footpaths as well as aesthetic and mechanical properties”, and “in future work, the measurements should be repeated with more replicates, a good control, and over extended periods”. Here’s hoping for more research, then!
Source: Couenberg, E. (2009) A preliminary study evaluating oxygen status beneath different surface-hardening materials for park use. In Watson, G., Costello, L., Scharenbroch, B., & GIlman, E. (eds.) The Landscape Below Ground III. USA: International Society of Arboriculture.
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Without question, going out and exploring the world to find interesting trees is enjoyable. Similarly, looking at incredible photos of trees (perhaps from across the world, by simply peering at a computer screen) can be rewarding. However, when trees feature as part of a drawing or painting, or even are the feature of the artwork, there is no denying that – particularly if the artist is skilled – the same sense of awe is evoked. The New Sylva, for example, is full to the brim with incredible illustrations.
One artist for me that I consider to produce some incredible work is Albert Bierstadt. Bierstadt painted landscapes of the American West, of which many included trees. Below are some of my favourites (click on the source for a larger image).
In an age where we can potentially acquire a massive array of tree species simply by going down to the local nursery, or by purchasing seeds, saplings, or standards online, we very much forget how plant collections used to be amassed. Before the age of the internet, and before the age of mass travel, botanic collections were much harder to add to, and the individuals who went all over the world to acquire new (sometimes unidentified specimens) were both incredibly gutsy and knowledgeable (in terms of identifying species, collecting seed, cuttings, or otherwise, and then transporting them back, via ship, to the homeland).
This post will revolve around the Dutch East India Company and Japan’s island of Dejima (a man-made island off of the Nagasaki coast).
A map of how Dejima Island would have looked, from above. Note its only connection is a bridge from Nagasaki. Nobody except the Dutch and Chinese could reside upon this island. Source: Wikimedia Commons.
First we must understand that, from 1641, only the Dutch and Chinese were allowed to trade with Japan, and must do so via this island (as the Nagasaki harbour was the only port foreign vessels could dock at). Once per year (though eventually becoming once every four years), as a ‘diplomatic mission’, foreign individuals from China and The Netherlands (who must be native to those countries) could travel from Nagasaki to Tokyo – at these opportunities, plant collectors could work their magic.
So this brings me to the individual who we’ll be focussing on this morning: Philipp Franz Balthasar von Siebold (1796-1866). Philipp, born into a family with a tradition of practicing medicine, adopted this familial trend and worked within Würzburg, Germany. However, he grew bored of working in Würzburg, thus sought work elsewhere, and subsequently was hired by the Dutch East India Company as a surgeon. By 1823, he was sent to Dejima Island (having originally been sent to work on Java), though immediately ran into a problem. Because only Dutch nationals could live on Dejima Island, and given Philipp’s very strong Bavarian accent, he was soon threatened with expulsion from the island – until his colleagues persuaded the Japanese that his heavy Bavarian accent was in fact just because he lived in a remote area of The Netherlands up in the hills…!
Perhaps in light of this, or maybe for other reasons, Philipp did not accept payment for his work as a surgeon on Dejima Island. He instead accepted only gifts. This, in addition to his very good ability to perform cataract surgery, lead to him becoming highly respected by the Japanese. In light of this, Philipp had a request granted to have a skilled draughtsman work alongside him, so he could have assistance in recording plant and animals specimens from Japan whilst travelling with the ‘diplomatic mission’ from Nagasaki to Tokyo (which, by this time, took place only once every four years). Philipp also married a Japanese woman who he likely had met on the island, and when their daughter was born the Japanese allowed him to live on the Nagasaki hillside with his family
In 1826, Philipp travelled with his draughtsman on the ‘diplomatic mission’ to Tokyo. It was a huge success – many species of plant were identified. However when in Tokyo, he did accept, as a gift, a map of Japan – a serious offence at the time. This lead to his imprisonment for over a year in 1828, when it was eventually discovered by the Japanese. Upon release in 1830, he was pardoned, though also banished from Japan. Philipp therefore returned, with his wife and daughter, to The Netherlands, though at the same time managed to smuggle out some exotic plants he acquired upon his travels (which are listed below). He would not return to Japan again for almost 30 years.
Returning with Philipp were the following plant species: Acer palmatum, Catalpa ovata, Chamaecyparis pisifera, Fraxinus sieboldiana, Hamamelis japonica ‘Arborea’, Ilex latifolia, Ligustrum japonicum, Malus sieboldii, Pinus densiflora, Thujopsis dolabrata, Trachycarpus fortunei, and Tsuga sieboldii. Perhaps, the Acer palmatum is the most interesting one of this list.
I have previously spoken about branch shedding (at times also known as cladoptosis) over on my thread on Arbtalk – see here, here, and here. The purpose of this post is to try and draw together everything I have researched on branch shedding, so to provide for a more centralised reference point for those seeking to learn more about the process. I highly recommend that anyone looking to learn more about branch shedding checks out Alex Shigo’s A New Tree Biology and T. T. Kozlowski’s Shedding of Plant Parts. Whilst both were written over 30 years ago, the material is very detailed and, in the case of the latter book, heavily referenced. Things have advanced since then somewhat, but these publications offer crucial context and ‘set the scene’, so to speak.
Natural branch shedding – what is it?
The shedding of lateral twigs and branches is a frequently-observed phenomenon of woody plants, with many species of angiosperm and gymnosperm having the capacity to shed such laterals (among gymnosperms however, only the species of Coniferales (conifers) and Gnetales are able to ‘practice’ cladoptosis – to expand, only 2 of the 9 genera of Pinaceae (pines) possess such an ability). In these species, Millington & Chaney (1973) outline two distinct mechanisms by which a branch may be shed: (1) physiological processes (cladoptosis), and (2) an interaction of biotic and mechanical agents (‘self-cleaning’ or ‘natural pruning’).
The physiological mechanism is much akin to how leaf abscission (shedding – during autumn for deciduous trees) operates. Separation of the branch from the adjoining structure occurs along well-defined ‘cleavage zones’ and is preceded by both the weakening of tissues local to the region and formation of a periderm (cork-like tissues). This process is known as cladoptosis, and typically only operates successfully for small branches and twigs – large branches may also be shed from the bole, however (of course, more infrequently).
Interactions of biotic and mechanical agents, on the other hand, can be dubbed instead as ‘self-cleaning’ or ‘natural pruning’. Particularly in dense stands (woodlands), branches lower on the bole often will die as a consequence of significant shading brought about by significant competition. These dead branches are colonised by fungal saprophytes (fungi that colonise upon deadwood) and insects, which in time will decay, weaken, and eventually facilitate failure of the branch in loading conditions (rain, wind, snow, animal activity, or otherwise).
Turning attention towards cladoptosis, the process, as elucidated to above, involves the senescence (physical deterioration) and subsequent death of a branch through the re-allocation of energy to other parts of the tree (Kozlowski et al., 1991). This is achieved by the tree ‘identifying’ a branch which it needs to lose (as it is not sustainable to retain it – usually due to poor light exposure) and thus begins the process of abscission, which sees the tree grow a layer of specialised tissue where the parent branch (or stem) meets the branch, which cuts off the vascular supply to the branch (Thomas, 2000). This tissue is corky, full of antimicrobial substances, and sits above the ‘wound’ – the branch is shed beyond this point, allowing for this ‘protection’ zone to remain (Bhat et al., 1986). This ‘wound’ is then occluded during the following growing seasons by a callus that adopts a circular shape (Kozlowski et al., 1991; Shigo, 1986; Watson, 2006), and once fully occluded there may be a distinct gall-like shape (a raised ‘bump’) that remains (Rust & Roloff, 2002). Ethylene is one of the plant hormones responsible for this process, as it encourages the re-allocation of resources away from shaded areas of the crown (Karban, 2015).
In certain species, such as oak, the process may be staggered (Shigo, 1986). The tree may first create a protection zone out at a distance from the branch junction, which leads to failure at that point. A long stub is then left, which can then be shed further down the line, by the same processes. Shigo (1986) also details that conifers behave slightly differently to broadleaved trees. Whilst a conifer branch is alive, resin is impregnated into the core wood of the branch junction (that can at times propagate out into the branch itself). As the branch begins to die and the shedding process begins, the tree seals off the small area surrounding the branch base to resist progress of potential pathogens. Once the branch has died, it will break where the resin core ends.
On this oak, we can see two dead laterals that are in the process of being shed. Notice the ‘snub’ at the base of either dead branch.
It is also important to note that shedding may occur when epicormic sprouts are no longer required by the tree. Usually a response to very heavy pruning (particularly at the wrong time of year – when leaves are not present or are not fully formed) due to the depletion of energy reserves that must be regained (sprouting initiates in areas where energy levels are low), the spouts may over time become unnecessary due to shading and due to the regaining of energy reserves. Usually this shedding occurs during the third year following sprouting (Shigo, 1991).
Branch shedding is ultimately a deliberate act by trees (Thomas, 2000). Certain species will be more prone to cladoptosis than others, even when of the same genus – dwarf or infertile (or both) cultivars are less likely to shed branches, due to their lower energy demand, when compared to forest species (Shigo, 1991); as are amenity trees less likely to shed lower branches due to the lack of competition for light (Shigo, 1986). However, the intricate array of drivers behind the branch-shedding process are not fully understood, though as trees evolved in groups within forests it is of little wonder why cladoptosis does occur – one simply has to look at a forest to see that branches lower on the trunk do not exist, as the intense shading makes their retention less than worthwhile (Shigo, 1986).
Another example of a dead branch being shed on oak. Once shed, there will be the typical ‘bump’ on the stem, typically indicative of a branch once being attached at that point.
Natural branch shedding – why does it occur?
Millington & Chaney (1973) state that branch and twig abscission will occur as a result of an array of physiological and environmental factors: low vigour, water supply, age, and unique site factors. However, the relationship between these four main drivers is poorly understood.
Typically, branches that abscise are weak and lack vigour. For illustrative purposes, insect or fungal infections may trigger a decline in vigour, in turn initiating cladoptosis. In hybrid black poplar, for example, twigs that arise from small buds and make poor growth are usually shed come autumn (fall). Further, if a branch produces a serious abundance of flowers year-on-year, inter-nodal distances (the distance along the shoot between buds) progressively reduce, photosynthesis of the branch is thus impacted, and the decline in the ‘carbohydrate budget’ of the branch eventually leads it to become compromised – the branch is then abscised. In support of such a claim, Quercus alba (white oak) have been observed to shed twigs with less distance between nodes and retain only the twigs with greater inter-nodal spacing.
Cladoptosis will also vary significantly with age. When young, Quercus alba will very rarely – if at all – shed any twigs. However, come maturity, twigs abscise frequently. The retention of leaves throughout winter on young specimens is thought to be a driver behind the lack of abscission. Cupressaceae (cypress) species will also shed commonly in maturity, though not so before. This trend is bucked however by Castilla elastica (Panaman Rubber Tree), which sheds twigs frequently when young, though by maturity has developed branches that need not be shed.
Relating to summer branch drop (a phenomenon by where branches may drop, without warning, during very dry periods), water deficits will also initiate branch shedding. In very dry summers in Ohio for example, many angiosperms were observed to drop branches before 15th July – branches of the trees continued to abscise until – and even partially into – autumn (Schaffner, 1902). Ephedra spp. (ephedras) will, for example, shed branches as a defence mechanism against water stress – as will Araucaria araucana (monkey puzzle), when on thin, dry, and sandy soils. Exactly whether summer branch drop is ‘intentional’ remains open to debate, however.
This image, which was taken on 17th July 1901, shows a Populus deltoides that has shed numerous branches during summer. The author (Schaffner, 1902) notes that this phenomenon, dubbed as ‘self-pruning’ (where foliage and branches were shed prematurely), occurred throughout the summer – all the way up until leaf abscission during autumn (when the remaining leaves were shed).
Natural branch shedding – what are the benefits?
Trees will naturally shed their branches so that their crown is not clogged by a profuse amount of branches. This process typically sees lower branches shed as higher branches occlude the light from the lower ones – this trait is particularly common with Pinus spp. (pines) and other excurrent species (Shigo, 1986; Shigo, 1991; Thomas, 2000). Essentially, if a branch is not producing enough carbohydrates (through photosynthesis) to maintain its own ‘mass’, then it will likely be shed – retaining the branch is counter-productive to the energy system of the tree, which seeks to retain efficiency at all times (Karban, 2015; Rust & Roloff, 2002; Thomas, 2000). This can be seen in trees of any age.
Cladoptosis also ensures that the tree does not have an unnecessary wind sail area. By retaining only the branches necessary for efficient energy production, unnecessary branches do not increase the wind sail of the tree; such an increase in wind sail would increase risk of windthrow, and may in fact require additional root growth to accommodate for increased wind sail (Thomas, 2000), which in itself requires more energy.
Cladoptosis also ensures that organisms located within the tree’s dead and dying branching structure do not pose any more of a threat to the tree than is ‘necessary’ in the long term. A tree can always re-grow branches (and roots), so cladoptosis is also beneficial in a defensive aspect (Shigo, 1986). Further, where species such as oak have been pruned and epicormic growth (twiggy growth along the branches) has ensued, the shedding of the very small twigs through ‘cladoptosis’ is very frequent in the years following – the twigs are ‘pinched off’ by the branch as it lays down its annual growth ring.
For species such as willow and poplar, the shedding of branches can even be a way of propagation. As willows and poplars are commonly found along water courses, one of their propagation techniques is to shed branches via cladoptosis, then having these shed branches travel downstream and then potentially take root when washed-up (Thomas, 2000).
Arboriculture 