(Arboricultural-styled) 'Fact of the Day'

[Kveldssanger]

Interesting, 'aint it! I'll look to do more of these, as I wrote that for my Lvl 4 though really want to look at other P&Ds as well. Got so many things I want to do it's just hard to find the time. Writing a lot on the benefits of trees right now, and have amassed 20,000 words for economic impacts and ecological impacts. In time, I'll share some of that. Plenty more things to look at there - hence why I'm hoarding books, in part.

[Kveldssanger]

11/03/16. Fact #171.

 

The cuticle is the primary barrier against uncontrolled foliar water loss, and is comprised of a continuous cutin membrane, waxes, and polysaccharides. The cuticle ultimately controls the transfer of water on both an intra-ceullular level and on an atmospheric level, thereby aiding with the reduction of water loss through the epidermis (Riederer & Schreiber, 2001; Watson, 2006). In conifers, water loss is controlled not only by very thick waxy cuticles, but by the leaf area being segmented into a massive abundance of smaller single leaf areas – this segmentation of the leaf ‘mass’ is known as a xerophytic adaption (Watson, 2006). Further, and as previously established, sun leaves will also be smaller in order to reduce surface area and, subsequently, transpirational loss (Givnish, 1988; Nobel, 1976).

 

The stomata, found below the cuticle layer, facilitate the rate of transpiration. They are usually less abundant on the upper epidermis than on the underside, as upper epidermis abundance would provide more risk of significant water loss by the leaf. Stomata begin to close when hydration levels of the soil begin to fall, thus ensuring that necessary water is retained within the plant. However, as stomata are required for gas exchange, their full closure halts any gaseous exchanges. To combat this however, the cuticle layer can provide for limited gas exchange so leaf operations can continue; albeit at a reduced level (Boyer et al., 1997).

 

Stomata control moisture loss through the two guard cells that surround and regulate each stomatal pore. To optimize the trade-off between carbon dioxide induction (which also occurs via the stomata) and transpirational water loss, stomata sense and respond to a range of environmental signals that include ambient carbon dioxide concentration and soil moisture levels. As moisture levels drop, stomata will reduce their size in response via the closure of the guard cells (Doheny-Adams et al., 2012). Abscisic acid will regulate such stomatal closure under drought conditions, and the flow of positively-charged potassium ions out of the guard cells will facilitate their closure by drawing out water alongside through osmotic processes (Karban, 2015).

 

In very drastic drought conditions, leaves may control water loss by abscising from the tree, thus reducing potential transpirational area and also reducing the overall water demand of the tree. Juglans ssp. for instance are known to shed leaves in times of severe drought, and such an act is defined as vulnerability segmentation (Tyree et al., 1993). Typically, leaves will be shed from the lower crown primarily, as their role is of lesser criticality than the upper crown’s leaves, and the increased competition for light in the lower canopy means retaining such leaves may be impractical (Achten et al., 2010). As a side note, buds will form within leaf axils as soon as leaves begin to develop. Therefore, if leaves are shed, new leaves can readily be grown again, and the same process will begin once more (Shigo, 1986). This adaption essentially means trees can continue to live following intentional or unintentional (herbivory, pruning, etc) defoliation – this is of direct benefit to the tactic of leaf shedding in drought conditions.

 

Additional means of controlling water loss include altering the angle at which the leaves face the sun (such as with Fraxinus excelsior), through the growth of small hairs that trap air and make it more difficult for water to escape in dry conditions or conversely aid with water release by keeping the leaf surface clear of moisture build-up during times of very high humidity, and by adopting one of two leaf macro-morphological adaptations: (1) grow rather thin leaves in times where conditions are adverse, shedding them once conditions improve, and / or (2) growing leaves that are small and covered with a leathery cuticle, thereby persisting through the adverse conditions and increasing photosynthetic rates once conditions improve again (Davis, 2015).

 

References

 

Achten, W., Maes, W., Reubens, B., Mathijs, E., Singh, V., Verchot, L., & Muys, B. (2010) Biomass production and allocation in Jatropha curcas L. seedlings under different levels of drought stress. Biomass and Bioenergy. 34 (5). p667-676.

 

Boyer, J., Wong, S., & Farquhar, G. (1997) CO2 and water vapor exchange across leaf cuticle (epidermis) at various water potentials. Plant Physiology. 114 (1). p185-191.

 

Davis, M. (2015) A Dendrologist’s Handbook. UK: The Dendrologist.

 

Doheny-Adams, T., Hunt, L., Franks, P., Beerling, D., & Gray, J. (2012) Genetic manipulation of stomatal density influences stomatal size, plant growth and tolerance to restricted water supply across a growth carbon dioxide gradient. Philosophical Transactions of the Royal Society of London B: Biological Sciences. 367 (1588). p547-555.

 

Givnish, T. (1988) Adaptation to sun and shade: a whole-plant perspective. Functional Plant Biology. 15 (2). p63-92.

 

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

 

Nobel, P. (1976) Photosynthetic Rates of Sun versus Shade Leaves of Hyptis emoryi Torr. Plant Physiology. 58 (2). p218-223.

 

Riederer, M. & Schreiber, L. (2001) Protecting against water loss: analysis of the barrier properties of plant cuticles. Journal of Experimental Botany. 52 (363). p2023-2032.

 

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

 

Tyree, M., Cochard, H., Cruiziat, P., Sinclair, B., & Ameglio, T. (1993) Drought‐induced leaf shedding in walnut: evidence for vulnerability segmentation. Plant, Cell & Environment. 16 (7). p879-882.

 

Watson, B. (2006) Trees – Their Use, Management, Cultivation, and Biology. India: The Crowood Press.

[Kveldssanger]

12/03/16. Fact #172.

 

Where there is human activity, there is usually noise. A particular type of human activity, which is that of vehicular travel, is a principle means of noise pollution, and wherever the highway supporting such vehicular travel may reside (in an urban, rural, or largely isolated area), there are adverse consequences to the noise pollution (for human health and ecosystem health). Therefore, it is important that we can understand what tree species are best-placed to buffer the most amount of noise, as particularly for urban locations it may influence how buffer planting schemes may be designed. Beyond the urban setting, recognising how the impacts of noise from a road will be dampened by constituent tree species is also important, as it may potentially influence exactly how such a site is managed.

 

In the study that is the focus of this post, the authors investigated how deciduous and coniferous tree species influence noise levels at staggered distanced away from roads in the Sonbolrood forest, Iran. The forest is home to the coniferous tree species Cupressus sempervirens var. horiztalis, Juniperus spp., Taxus spp., Thuja orientalis, and the deciduous tree species Alnus spp., Fagus orientalis, and Platanus spp. For the purposes of this study, 25 plots containing predominantly coniferous species, and another 25 plots containing predominantly deciduous species, were identified, with each one measuring 20m x 50m. All plots were located adjacent to a road, and also had their total tree populations counted.

 

At each plot location, in the adjacent roadway, a trumpet was sounded four times (with an approximate sound level of 100 decibels), and the decibel level was measured (at an elevation of 1.8m) at distances of 20m, 100m, and 300m (the latter two were to understand how noise is dampened over much greater distances) for each of the four sounds. Therefore, a total of 600 measurements were taken).

 

In terms of what the authors found, they first recognised that the more trees present within a plot the greater the noise reduction. However, there were observed differences between the effectiveness of deciduous and coniferous stands in dampening such sound, with deciduous trees reducing sound more significantly at densities of up to 40 trees when compared to coniferous trees. However, once coniferous stands reached beyond 40 trees in the sample areas, they dampened sound more effectively than their deciduous counterparts (as shown by the two graphs below). This, the authors allege, is because the crowns of deciduous trees are generally broader and less regularly shaped, thereby meaning they dampen sound waves more readily (as they have more matter with which to buffer against the waves). Furthermore, though also applicable for some constituent coniferous species such as Thuja orientalis, the form of deciduous trees sees them adopt, in general, a lower H : D ratio (height:diameter), and such larger trunks have more mass with which to reflect or refract sound waves. Of course, because deciduous trees abscise their foliage during winter, they may potentially dampen noise less effectively than coniferous species when their foliage is absent. In this sense, a mixed stand may well be most optimal for noise buffering.

 

Average sound reduction by coniferous trees (left) and deciduous trees (right), in relation to plot density.

 

Interestingly, but probably unsurprisingly, there was also a near uniform decrease in the decibel level measured at all three distances away from the sound's origin, and this applies for both predominantly broadleaved and coniferous plots. Of course, for the two distances beyond the plots themselves, stand composition and density was not measured, though such a feat would have been wonderfully impressive at the 300m distance, in particular. Again, the two graphs below demonstrate this sound level reduction associated with tree density.

 

How deciduous plots (top) and coniferous plots (below) dampened sound levels, at all three distances, away from the sound of the trumpet.

 

In relation to the graph displaying sound level reductions in coniferous plots, it is actually interesting to note that the decibel measurements at 20m, 100m, and 300m were all relatively similar, and only when these plot stands had tree densities of over 26-29 individuals was there sufficient dampening of the trumpet's sound. With regards to broadleaved stands, the authors suggest that shade tolerant species including Fagus orientalis may, because they can persist in the lower canopy and form dense branching structures, effectively dampen sounds and thereby supplement, particularly in summer, the effectiveness of deciduous stands (or, if they exist underneath stands of conifers, those also).

 

Drawing upon the results provided, we can begin to understand, albeit in a rather basic sense, exactly how trees can aid with noise level reductions. Admittedly, this study was undertaken in a forest, and it is almost impossible that an urban highway would be bordered by 100m+ of tree belt, but if focussing on the results from 20m it may perhaps be best to select deciduous species principally, though consider planting conifers within, at high densities. Beyond the scope of this study, but something worth considering too, is whether deciduous species that actively sprout from the base, such as Tilia spp., will aid with the dampening of sound yet further.

 

Source: Nasiri, M., Fallah, A., & Nasiri, B. (2015) The effects of tree species on reduction of the rate of noise pollution at the edge of Hyrcanian forest roads. Environmental Engineering and Management Journal. 14 (5). p1021-1026.

[sandspider]

Interesting, thank you.

 

Just considering buying a house by a fairly busy road and was wondering which trees to plant to reduce the sound...

[Macca]

Interesting, thank you.

 

Just considering buying a house by a fairly busy road and was wondering which trees to plant to reduce the sound...

 

 

 

The Leyland cypress, Cupressus × leylandii is a really good noise barrier grows really fast!

[sandspider]

That's already planted, plus some laurels, which aren't big enough yet to do much. Still pretty noisy though!

[Kveldssanger]

Success! Perhaps get some hornbeam hedging in as well, or some Ilex?

 

As for trees, if you have a decent-sized front garden then Ilex may serve well, as may some smaller maples (including our native field maple)?

[Kveldssanger]

15/03/16. Fact #173.

 

In the UK, the introduced tree species horse chestnut (Aesculus hippocastanum) has been, over the centuries, a highly amenable tree. It has graced many important landscapes, features heavily as a mature population in many cities and their parks, and is a favourite amongst people of all ages for the conkers it drops and its stellar flowering display in spring. However, in recent times its amenity value in particular has been challenged, and namely by the various pests and pathogens obligated to the species: Cameraria ohridella (horse chestnut leaf miner), Guignardia aesculi (leaf blotch), and Pseudomonas syringae pv. aesculi (bleeding canker). Of course, such pests also impact upon the health of the tree. The repeated defoliations over successive years by the two leaf pests and pathogens, in addition to the bark lesions induced by Pseudomonas syringae pv. aesculi, will be progressively more taxing to the infected tree. Energy reserves may, therefore, eventually be entirely depleted, and the tree then ‘starves’ itself to death (if bark lesions haven’t girdled, and thus killed, the tree, by this point). At the same time, we must also remember that urban areas outside of parks may not be entirely suitable for the species (abiotic stressors), and therefore the trees may suffer as a result of human activity as well.

 

An example of horse chestnut foliage being attacked by the leaf miner. Source: The Wild Diary.

 

Because of this war of attrition the horse chestnut is suffering from, it is very much critical that we understand the drivers behind this evident multiple-pronged attack. In this case, we look at a study by Glynn Percival and Jonathan Banks, who investigated whether there is a relationship between Cameraria ohridella and Pseudomonas syringae pv. aesculi (Pae). Specifically, they sought to understand if the presence of the former heightened the severity of the latter, upon four year old horse chestnuts. Furthermore, they investigated whether secondary plant metabolites synthesised in response to Pae were impacted by a double-pronged attack courtesy of leaf miner. In order to ensure leaf blotch did not hamper the study, they treated all trees with a fungicide to prevent its manifestation.

 

In terms of what they found, it was identified that a combination of leaf miner and bleeding canker increased lesion sizes of Pae by 42%. Because the size of a lesion is one of the current recognised means of ascertaining pathogenicity of Pae, this is significant, as it suggests that if a horse chestnut suffers as a result of more than one biotic stressor, its health is quite likely going to suffer far more. By a similar token, where the two biotic agents were attacking the same host tree, leaf chlorophyll content and chlorophyll fluorescence were impacted slightly more significantly than when only leaf miner was present. For chlorophyll content, hosts only host to leaf miner had a loss of 86.1% of their content, and this increased marginally to 86.3% when bleeding canker was present too. Chlorophyll fluorescence was similarly impacted, with an adverse change of 72.4% and 75.2%, respectively. Both suggest the host trees are not photosynthesising in an efficient manner, and therefore cannot produce the carbohydrates required to defend themselves against such attacks – they will likely, in time, need to draw upon their energy reserves. Conversely, when only bleeding canker was present and leaf miner presence was controlled via insecticide application, chlorophyll content decreased and chlorophyll fluorescence was adversely impacted by only 12.3% (not significant – unsurprising as Pae is not principally a leaf pathogen) and 31.7% (significant), respectively. Leaf miner is, in this case, clearly has a massive impact.

 

Horse chestnut bleeding canker upon the main stem of an individual. Source: Beterebomen.

 

Looking now towards the synthesis of defensive enzymes against infection, it was found that in trees where only bleeding canker was present that such enzymes were at a level significantly higher than control trees in the areas surrounding lesions. Specifically, β-1,3-glucanase, which breaks down specific parts of the cell wall within fungal cells, was observed to increase by 57.7%, whilst peroxidase, a metabolite that increases lignin production in the host tree, increased in presence by 51.6%. Conversely, when the host tree was also being attacked by leaf miner, these metabolites were found in the locality of lesion sites at far reduced levels of 15.4% and 17.7%, respectively. Therefore, it can easily be recognised that these secondary metabolites, crucial to the effectiveness of the tree’s defensive response to attack, are markedly suppressed by leaf miner, and thus the tree will be in a far worse position to defend itself. Combined with reduced photosynthetic potential, one can really begin to recognise how, in the natural world, 1+1 may not necessarily equal two but instead five (basically, the compound impacts are synergistic, instead of additive). Furthermore, as photosynthesis is important in enabling a tree to create such secondary metabolites, horse chestnuts can, if they are impacted by leaf miner and bleeding canker (and bear in mind this is excluding leaf blotch, which is also common in the UK), suffer from a negative feedback cycle by where there is a continuous decline in tree health up until a point of human intervention (a spiral of decline, if you will).

 

Without question, it is also worth noting that this study was done on young trees grown under controlled conditions. Because young trees are usually more vigorous, if this study was to be done on old trees that were located out in the ‘real’ landscape, we could probably expect the effects to be jut as bad, if not worse.

 

Thus, next time you see a horse chestnut that is being battered by leaf miner and bleeding canker, don’t let anybody tell you it’s largely an amenity problem – it’s not! It’s a health problem, that may very well eventually kill the tree outright, or let another pest or pathogen come in and finish the job. Combined with leaf blotch and abiotic stressors common in urban locations (pruning, ground compaction, pollution, drought, and so on), urban horse chestnuts in particular may very well be suffering very significantly. If we don’t do something, we can very well expect for horse chestnuts to gradually decline in terms of mature populations, and I very much expect this isn’t something that anybody wants.

 

Source: Percival, G. & Banks, J. (2014) Studies of the interaction between horse chestnut leaf miner (Cameraria ohridella) and bacterial bleeding canker (Pseudomonas syringae pv. aesculi). Urban Forestry & Urban Greening. 13 (2). p403-409.

[David Humphries]

No mention by the authors of potential other pathogens to be mindful of that may/will also affect Aesculus (amongst many other tree species) such as the bacterium Xylella fastidiosa ?

 

 

 

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[Kveldssanger]

This was in 2014 (probably written a good few months before, maybe even in late 2013), so perhaps the threat was not as evident as it is now. The scope of this was just assessing these two, as even leaf blotch was cut out. No doubt the horse chestnut is being brutalised, and many other tree species as well. Rather depressing, to be honest.