(Arboricultural-styled) 'Fact of the Day'

[Kveldssanger]

Thanks for those of you that helped this thread take off. It's been a good few months of learning and discussing stuff. More of the same next. Have a good one, all.

[Kveldssanger]

01/01/16. Fact #113.

 

So I thought I'd start off 2016 with a detailed look at how trees can be impacted by increased atmospheric carbon dioxide. This post is taken from an assignment I did recently, so is referenced quite significantly (I have provided the reference list, complete with links, at the bottom of the post). Of course, what I have written is not a complete guide to how elevated carbon dioxide impacts trees - it's only a mere brick in an entire structure that makes a house, that is yet to be completed even in spite of all the research that has been undertaken over the years (will this 'house' ever even be built completely?). Us humans are - after all - looking at only a tiny segment of our world's existence, and as trees have existed for millions of years - at times in conditions where the atmosphere's composition has been completely different - we don't really have an 'absolute' reference point that will stand unequivocally in the face of scrutiny. There's also the risk of projections not actually manifesting into actuality, because of the infinite number of potentially 'causers' and 'effectors' across the world's ecosystems (within individual ecosystems, and across a cohort of ecosystems) Further, a lot of research is limited to what researchers can get funding for, which is actually a rather disconcerting aspect when assessing the works from any field of scientific research!

 

Anyway, on with this morning's post…

 

Increased atmospheric carbon dioxide is not necessarily bad for trees. In fact, as trees require carbon dioxide for photosynthesis, there are many benefits of higher levels of carbon dioxide within the air. For instance, increased photosynthetic rates are brought about by the increase in exposure to atmospheric carbon dioxide (Rogers et al., 1983, Tissue et al., 1997, Medlyn et al., 1999), which may therefore increase productivity of forest ecosystems for many years – particularly in summer, when temperatures are higher (Tissue et al., 1997), and most notably on leaves exposed to the sun (a 98% increase in photosynthetic capacity compared to 41% for shade leaves) (Herrick & Thomas, 1999). Assuming other factors, such as water availability and nutrient availability, are not lacking, increased carbon dioxide can in fact be beneficial for forests (Broadmeadow et al., 2005, Kimmins, 1997).

 

Such elevated carbon dioxide levels also aid with growth – up to 30% in young trees (Medlyn et al., 2001) – primarily down to the increase in photosynthesis, which has an added effect of increasing leaf area also due to better growth. Such a leaf area increase can then further aid with photosynthetic rates; observed long-term increases in net photosynthesis are however typically lower than the heightened short-term response (Liberloo et al., 2006; Hyvönen et al., 2007). In addition, increased leaf mass improves the ecology of the soil as nutrient cycling from the increased leaf litter brings about an increase in nutrient availability (Johnson et al., 2001); again however, how sustainable this is in the longer term is questionable (Hyvönen et al., 2007; Kirby & Watkins, 2015).

 

Continuing, the efficiency of water usage by plants with elevated atmospheric carbon dioxide conditions has the potential to increase, with studies showing that plants exposed to higher concentrations of carbon dioxide not wilting as they would under normal drought conditions (Rogers et al., 1983; Guehl et al., 1994). However, much like the increase in the ability to photosynthesise, it is unclear whether such a continued efficiency is sustainable long-term (Eamus, 1991). Further to this, the increase in leaf area as a result of increased carbon dioxide may mean that water usage is not more efficient when assessing water efficiency to overall leaf area as a ratio (Picon et al., 1996).

 

Increases in nitrogen uptake are also present within elevated atmospheric carbon dioxide conditions – via combinations of increased fine root production, increased rates of soil organic matter decomposition, and increased allocation of carbon to mycorrhizal fungi. Regardless of the specific mechanism, research indicates that the larger quantities of carbon entering the forest soil system under elevated conditions results in greater nitrogen uptake, even in nitrogen-limited ecosystems (Finzi et al., 2007). Such indications may have beneficial implications for forests of the future, whereby, even in nitrogen depleted areas, growth is stimulated – of course, as increased carbon dioxide causes greater uptakes of soil nitrogen, if nitrogen is significantly depleted or entirely lacking in a soil ecosystem, increased carbon dioxide may actually have a negligible or negative impact upon forest health (Norby & Iversen, 2006).

 

Despite the benefits, elevated carbon dioxide levels also come with many adverse impacts. Whilst water-use efficiency is a common consequence of increased carbon dioxide within the atmosphere, this does mean there is decreased evapotranspiration. Such reduction in moisture being lost from trees may then have the implication of altered rainfall patterns and increased temperature – more infrequent rainfall and decreased cloud cover, respectively (Field et al., 1995) – given the reduction in evapotranspiration, which can then impact upon the trees themselves, inducing water stress down the line. Not only this, but increased carbon dioxide (and thus increased temperature) brings about, in itself, differing rainfall patterns. Whilst the UK might not initially suffer readily, Eastern Europe and the Mediterranean will experience severe drought and increased occurrence of wild fires (Kirby & Watkins, 2015), both of which may spell disaster for the constituent tree populations.

 

As the climate warms (in part, due to increased atmospheric carbon dioxide), the emergence of new continental pests and diseases within the UK will also increase. Already, we have seen a significant increase in the abundance and diversity of exotic pests and diseases within the UK – most within the last 5-10 years (Percival & O'Callaghan, 2015) – though as the climate warms and trading patterns change in response to different demands from different regions of the world due to the change in climate, transport may act as an increasing vector for new pest emergence (Brasier, 2008; Kirby & Watkins, 2015). Endemic native species might be particularly vulnerable to such exotic pests and pathogens with which they have not co-evolved (Anderson et al., 2004). Research even suggests that pests and diseases "can be considered biotic forcing agents capable of causing consequences of similar magnitude to climate forcing factors", which could be disastrous when compiled with the effects of a changing climate (Flower & Gonzalez-Meller, 2015). Further, as the effects of climate change on tree–pathogen interactions cannot be accurately predicted given their complexity (Pautasso et al., 2012; Sturrock et al., 2011), it is difficult to predict, with accuracy, the impact future pests will have on forest health – though it is likely not to be promising.

 

Warming in general may also have different impacts on different communities within a forest ecosystem. Higher canopy species are likely to be impacted before the under-canopy species, given their micro-climate will not change at the same rate as the more exposed upper canopy micro-climate would. Such impacts of warming and changing weather patterns in response to the warmer, more volatile weather system, may therefore have lag periods for particular forest communities (Kirby & Watkins, 2015). Coppice woodlands may in fact be particularly vulnerable to warming, as their tendency to not have, when so heavily managed, reproduced via seed (as the trees were harvested before seed-bearing age), means they may be considered 'stuck in the past' from a genetic perspective; any regeneration would have largely been via layering or suckering, in place of sexual reproduction through seed germination (Buckley & Mills, 2015).

 

Increased atmospheric carbon dioxide has also been found to alter, though varying between species, phenology – bud break, frost hardiness, flowering, fruiting, and seed production (Asshoff et al., 2006; Medlyn et al., 2001). For instance, the bud break during spring of Pinus sylvestris (Scots pine) is significantly hastened under elevated carbon dioxide levels, as is bud setting during autumn (Jach & Ceulemans, 1999), whilst the frost hardiness of some species is improved under elevated carbon dioxide conditions and in others reduced. For those that suffer reduced frost hardiness, survivability over winter may suffer in response (Bigras & Bertrand, 2006; Lutze et al., 1998; Repo et al., 1996).

 

Curiously, UK woodlands under private ownership may be at most risk of the threats of a warming climate and the evolving relationship between trees and both pests and diseases. A recent survey of private woodland owners highlighted that they are "not generally convinced of a need to adapt", as they "feel the future is uncertain, more usually in relation to tree disease than to climate change itself" (Lawrence & Marzano, 2014). Therefore, perhaps the impending and growing threat is not simply due to climatic changes, but the lack of willingness to respond pro-actively.

 

Sources:

 

Anderson, P., Cunningham, A., Patel, N., Morales, F., Epstein, P., & Daszak, P. (2004) Emerging infectious diseases of plants: pathogen pollution, climate change and agrotechnology drivers. Trends in Ecology & Evolution. 19 (10). p535-544.

 

Asshoff, R., Zotz, G., & Koerner, C. (2006) Growth and phenology of mature temperate forest trees in elevated CO2. Global Change Biology. 12 (5). p848-861.

 

Bigras, F. & Bertrand, A. (2006) Responses of Picea mariana to elevated CO2 concentration during growth, cold hardening and dehardening: phenology, cold tolerance, photosynthesis and growth. Tree Physiology. 26 (7). p875-888.

 

Brasier, C. (2008) The biosecurity threat to the UK and global environment from international trade in plants. Plant Pathology. 57 (5). p792-808.

 

Broadmeadow, M., Ray, D., & Samuel, C. (2005) Climate change and the future for broadleaved tree species in Britain. Forestry. 78 (2). p145-161.

 

Buckley, P. & Mills, J. (2015) Coppice Silviculture: From the Mesolithic to the 21st Century. In Kirby, K. & Watkins, C. (eds.) Europe's Changing Woods and Forests: From Wildwood to Managed Landscapes. UK: CABI.

 

Eamus, D. (1991) The interaction of rising CO2 and temperatures with water use efficiency. Plant, Cell & Environment. 14 (8). p843-852.

 

Field, C., Jackson, R., & Mooney, H. (1995) Stomatal responses to increased CO2: implications from the plant to the global scale. Plant, Cell & Environment. 18 (10). p1214-1225.

 

Finzi, A., Norby, R., Calfapietra, C., Gallet-Budynek, A., Gielen, B., Holmes, W., et al. (2007) Increases in nitrogen uptake rather than nitrogen-use efficiency support higher rates of temperate forest productivity under elevated CO2. Proceedings of the National Academy of Sciences. 104 (35). p14014-14019.

 

Flower, C. & Gonzalez-Meler, M. (2015) Responses of Temperate Forest Productivity to Insect and Pathogen Disturbances. Annual Review of Plant Biology. 66 (1). p547-569.

 

Guehl, J., Picon, C., Aussenac, G., & Gross, P. (1994) Interactive effects of elevated CO2 and soil drought on growth and transpiration efficiency and its determinants in two European forest tree species. Tree Physiology. 14 (7-8-9). p707-724.

 

Hyvönen, R., Ågren, G., Linder, S., Persson, T., Cotrufo, M., Ekblad, A., et al. (2007) The likely impact of elevated [CO2], nitrogen deposition, increased temperature and management on carbon sequestration in temperate and boreal forest ecosystems: a literature review. New Phytologist. 173 (3). p463-480.

 

Herrick, J. & Thomas, R. (1999) Effects of CO2 enrichment on the photosynthetic light response of sun and shade leaves of canopy sweetgum trees (Liquidambar styraciflua) in a forest ecosystem. Tree Physiology. 19 (12). 779-786.

 

Jach, M. & Ceulemans, R. (1999) Effects of elevated atmospheric CO2 on phenology, growth and crown structure of Scots pine (Pinus sylvestris) seedlings after two years of exposure in the field. Tree Physiology. 19 (4-5). p289-300.

 

Kimmins, H. (1997) Balancing Act: Environmental Issues in Forestry. Canada: UBC Press.

 

Kirby, K. & Watkins, C. (2015) Evolution of Modern Landscapes. In Kirby, K. & Watkins, C. (eds.) Europe's Changing Woods and Forests: From Wildwood to Managed Landscapes. UK: CABI.

 

Lawrence, A. & Marzano, M. (2014) Is the private forest sector adapting to climate change? A study of forest managers in north Wales. Annals of Forest Science. 71 (2). p291-300.

 

Liberloo, M., Calfapietra, C., Lukac, M., Godbold, D., Luo, Z., Polles, A., et al. (2006) Woody biomass production during the second rotation of a bio‐energy Populus plantation increases in a future high CO2 world. Global Change Biology. 12 (6). p1094-1106.

 

Lutze, J., Roden, J., Holly, C., Wolfe, J., Egerton, J., & Ball, M. (1998) Elevated atmospheric [CO2] promotes frost damage in evergreen tree seedlings. Plant, Cell & Environment. 21 (6). p631-635.

 

Medlyn, B., Badeck, F., de Pury, D., Barton, C, Broadmeadow, M., Ceulemans, R., et al. (1999) Effects of elevated [CO2] on photosynthesis in European forest species: a meta‐analysis of model parameters. Plant, Cell & Environment. 22 (12). p1475-1495.

 

Medlyn, B., Ray., A., Barton, C., & Forstreuter, M. (2001) Above-ground Growth Responses of Forest Trees to Elevated Atmospheric CO2 Concentrations. In Karnosky, D., Ceulemans, R., Scarascia-Mugnozza, G., & Innes, J. (eds.) The Impact of Carbon Dioxide and Other Greenhouse Gases on Forest Ecosystems. UK: CABI.

 

Norby, R. & Iversen, C. (2006) Nitrogen uptake, distribution, turnover, and efficiency of use in a CO2-enriched sweetgum forest. Ecology. 87 (1). p5-14.

 

Pautasso, M., Döring, T., Garbelotto, M., Pellis, L., & Jeger, M. (2012) Impacts of climate change on plant diseases—opinions and trends. European Journal of Plant Pathology. 133 (1). p295-313.

 

Percival, G. & O'Callaghan, D. (2015) Pest and Disease Control [seminar]. Barcham. 1st July.

 

Picon, C., Guehl, J., & Aussenac, G. (1996) Growth dynamics, transpiration and water-use efficiency in Quercus robur plants submitted to elevated CO2 and drought. Annales des Sciences Forestieres. 53 (2-3). p431-446.

 

Repo, T., Hänninen, H., & Kellomäki, S. (1996) The effects of long‐term elevation of air temperature and CO on the frost hardiness of Scots pine. Plant, Cell & Environment. 19 (2). p209-216.

 

Rogers, H., Thomas, J., & Bingham, G. (1983) Response of agronomic and forest species to elevated atmospheric carbon dioxide. Science. 220 (4595). p428-429.

 

Sturrock, R., Frankel, S., Brown, A., Hennon, P., Kliejunas, J., Lewis, K., Worrall, J., & Woods, A. (2011). Climate change and forest diseases. Plant Pathology. 60 (1). p133-149.

 

Tissue, D., Thomas, R., & Strain, B. (1997) Atmospheric CO 2 enrichment increases growth and photosynthesis of Pinus taeda: a 4 year experiment in the field. Plant Cell and Environment. 20 (9). p1123-1134.

[Kveldssanger]

02/02/16. Fact #114.

 

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.

 

Whilst the water table is not something that can actively be lowered, the poor soil oxygen status of the rooting environment can be. 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 the pathways, tree health was seem 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 - 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.

 

Results from the study 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 long-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".

 

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.

[tree-fancier123]

The Vondelpark looks a nice space for the city dwellers to get away from the concrete jungle.

This book came out before your reference, saw it a while back and thought they could do with this stuff around all the city street trees, quite expensive research needed still, as you've alluded to, but the cost of sorting out dead and dying trees is also a burden on the taxpayer.

[ame=http://www.amazon.co.uk/Pavements-Integrative-Studies-Management-Development/dp/0849326702/ref=sr_1_2?ie=UTF8&qid=1451757261&sr=8-2&keywords=porous+pavements]Porous Pavements (Integrative Studies in Water Management and Land Development): Amazon.co.uk: Bruce K. Ferguson: 9780849326707: Books@@AMEPARAM@@http://ecx.images-amazon.com/images/I/51VtKL1dufL.@@AMEPARAM@@51VtKL1dufL[/ame]

[Kveldssanger]

Probably nearly as much as that book's price!

 

But no, I completely agree with you. Plus, it's the right thing to do - else we're just avoiding the cause of the problem in favour of tackling the symptoms with money.

[Kveldssanger]

03/01/2016. Fact 115.

 

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.

 

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.

 

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.

 

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, 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.

 

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.

 

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.

 

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.

 

The authors then begin their conclusions by asserting that highway repair works has a significant impact upon both tree condition and survival. For instance, a 22.7% mortality rtate 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 imapcted by construction suffered more significantly.

 

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 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.

[Kveldssanger]

04/01/2016. Fact #116.

 

For some, the idea of "urban agriculture" may be somewhat of an oxymoron, though when we understand what agriculture is - basically either cultivating land to produce crops or to raise livestock - perhaps the term isn't so audacious. However, for this post, I am not talking about small-scale urban agriculture, but large-scale, city-wide agriculture.

 

For this post, we must head off to Spain - specifically, the city of Seville. Here, some 14,000 citrus trees grace the city's streets, and unleash a beautiful aroma during the spring blossom, and then transform the streets in summer by providing shade.

 

Unlike many street trees in the UK, where plenty of fruit trees in urban areas have their crop left to rot on the ground (I'm looking at you, Prunus cerasifera - for those of you that haven't tried its fruit, please do!), these citrus fruits are used once they ripen. In fact, they are sold! And one of the importers of this citrus fruit crop is the UK, where cooks will make Seville orange marmalade during January and February.

 

The author notes, at this point, that whilst urban agriculture is "gaining ground", it may always be limited by legal issues associated with cultivating fruit and nut trees on city streets. For example, whilst there is a growing trend of public apple orchards in Seattle, USA, there is a street-wide ban on cherries and pears. The reason? Public safety. Cherries and pears are a threat to national security, it seems! I jest, but even the author is a little bedazzled as to why such a heavy-handed approach has been adopted.

 

Quite hilariously, and again in the USA (but this time in the city of San Francisco), there has been a recent rise in 'Guerilla Grafters'. People are, in essence, going around the streets of San Francisco and grafting, onto ornamental fruit trees, fruit-bearing wood. Such is the desire for street trees to produce fruit, it appears - and that's not a bad thing, as the benefits are many.

 

Source: Dover, J. (2015) Green Infrastructure - Incorporating plants and enhancing biodiversity in buildings and urban environments. UK: Earthscan.

[Kveldssanger]

05/01/2016. Fact #117.

 

The effect of acid rain on trees manifests in two ways – upon foliage, and roots (Kimmins, 1997). The symptoms include direct damage to plant tissue (particularly the foliage), reduced density of foliage within the crown, distinct areas of dieback, and whole tree death (Singh & Agrawal, 2007).

 

Studies into the impact of acid rain upon the foliage of forest trees conclude that necrotic (dead) patches are particularly common, with injuries being primarily to the epidermis (outer-most layer of a leaf). Acid rain therefore may have potentially severe impacts upon leaf structure (Fan & Wang, 2006). The foliage of coniferous species is more prone to the foliar effects from acid rain, likely given conifers do not shed foliage annually (Percy, 1986). Persistent (long-term) damage via acid rain may actually bring about a potential shift away from coniferous forests and towards deciduous forests. Exactly how this will impact upon the boreal (northern) forests however is not fully known, though the observed transition between northern broadleaved forests and boreal coniferous forests is seemingly rapid (Johnson, 1983).

 

Forest ecosystems either at high altitude or prone to misty conditions can also be damaged by the acidic nature (both sulphur and nitrogen are present) of the water droplets that constitute the clouds (Kimmins, 1997). A study into how such acidic mist impacted upon forest trees identified that growth was rapidly reduced and calcium was directly leached from the foliage, in turn leading to foliar injury (DeHayes et al., 1999).

 

Turning attention towards the impact upon the root system of a tree, as the forest soil increases in acidity – due to acid rain – seedling germination lowers (Percy, 1986). Therefore, if soils do continue to acidify, the future of forests may be under threat as a result of a possible lack of regeneration. The lowering of soil pH via the accumulation of sulphur and nitrogen ions within the soil (Kimmins, 1997) means important nutrients are leached from the soil, and increases in the abundance of phytotoxic (toxic to plants) heavy metals, such as aluminium, occur. Such changes in the soil chemical characteristics reduce soil fertility, which ultimately has a negative impact on growth and productivity of forest trees both above and below the ground (DeHayes et al., 1999; Singh & Agrawal, 2006; Menon et al., 2007), with the example of a reduction in fine root mass being evident.

 

The accumulation of toxic heavy metal ions, which may be brought about (at least in part) by acid rain, is also known to have negative impacts upon the ability for decomposers (fungi and insects) of a forest ecosystem to function properly. This leads to imbalances in nutrient cycling, litter decomposition, and productivity of the ecosystem (Pennanen et al., 1998), which in turn can impact upon vegetation life, and may lead to stresses that can increase in severity over the years.

 

Such soil acidification ultimately can change the entire vegetation composition of a forest – much like how acid rain damage to leaves can alter composition – with one study highlighting how a pine and spruce forest transitioned towards and then into a mixed and birch forest (Koptsik et al., 2001). Further studies have concluded similarly, by identifying that certain tree species will begin to die from ill-health following the change in soil properties (Johnson, 1983; Swaine, 1996; van Breemen et al., 1997). As a result, acid rain can initiate a transition away from forests dominated by particular species, and towards forests dominated by different species. This has impacts for the species that rely on the trees for habitat and food, as well – birds, insects, fungi, lichens, mammals, and bacteria are but just a few examples of the types of organisms that will be impacted.

 

Soil acidification within forests may therefore be very destructive, in time. However, as germination is only significantly stunted at a pH of 2.0-3.5 (Percy, 1986), such an issue may only be a distant concern for now. Despite this, one study that looked at modelling future soil profiles on a heavily acidified site concluded that pH is unlikely to revert back towards what it once was, even if drivers of acidification (such as acid rain) slow. This is because soil profiles suffer from past inputs into their system (Małek et al., 2005), thereby meaning that the future extents of acid rainfall upon forest soils could be very damaging.

 

Sources:

 

DeHayes, D., Schaberg, P., Hawley, G., & Strimbeck, G. (1999) Acid rain impacts on calcium nutrition and forest health alteration of membrane-associated calcium leads to membrane destabilization and foliar injury in red spruce. BioScience. 49 (10). p789-800.

 

Fan, H. & Wang, Y. (2000) Effects of simulated acid rain on germination, foliar damage, chlorophyll contents and seedling growth of five hardwood species growing in China. Forest Ecology and Management. 126 (3). p321-329.

 

Johnson, A. (1983) Red spruce decline in the northeastern US: hypotheses regarding the role of acid rain. Journal of the Air Pollution Control Association. 33 (11). p1049-1054.

 

Kimmins, H. (1997) Balancing Act: Environmental Issues in Forestry. Canada: UBC Press.

 

Koptsik, G., Koptsik, S., & Aamlid, D. (2001) Pine needle chemistry near a large point SO2 source in northern Fennoscandia. Water, Air, and Soil Pollution. 130 (1-4). p929-934.

 

Małek, S., Martinson, L., & Sverdrup, H. (2005) Modelling future soil chemistry at a highly polluted forest site at Istebna in Southern Poland using the “SAFE” model. Environmental Pollution. 137 (3). p568-573.

 

Menon, M., Hermle, S., Günthardt-Goerg, M., & Schulin, R. (2007) Effects of heavy metal soil pollution and acid rain on growth and water use efficiency of a young model forest ecosystem. Plant and Soil. 297 (1-2). p171-183.

 

Pennanen, T., Perkiömäki, J., Kiikkilä, O., Vanhala, P., Neuvonen, S., & Fritze, H. (1998) Prolonged, simulated acid rain and heavy metal deposition: separated and combined effects on forest soil microbial community structure. FEMS Microbiology Ecology. 27 (3). p291-300.

 

Percy, K. (1986) The effects of simulated acid rain on germinative capacity, growth and morphology of forest tree seedlings. New Phytologist. 104 (3). p473-484.

 

Singh, A. & Agrawal, M. (2007) Acid rain and its ecological consequences. Journal of Environmental Biology. 29 (1). p15-24.

 

Swaine, M. (1996) Rainfall and soil fertility as factors limiting forest species distributions in Ghana. Journal of Ecology. 84 (3). p419-428.

 

van Breemen, N., Finzi, A., & Canham, C. (1997) Canopy tree-soil interactions within temperate forests: effects of soil elemental composition and texture on species distributions. Canadian Journal of Forest Research. 27 (7). p1110-1116.

[daltontrees]

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.

 

A good study but there is a weakness in the argument. It can be approached from two different directions.

 

The first is that it is a longstanding deficiency of CTLA that it does not take into account the life costs and expected duration nof a tree. For this reason, any tree of a species or in a position or of a size to do damage to a highway will probably occasion repairs which, if these will shorten its life, means the tree should be down-valued even before it is damaged and possibly even before it causes damage.

 

The second is that those trees that were valued at $1,100 or whatever costed the city possibly millions of dollars of highway repairs. Was the inevitability of this reflected in the initial valuations? I am going to say almost cetainly not because CTLA doesn't allow for this to be done.

 

As an epilogue, the failure of the trees that had caused so much damage that they were badly damaged during remedial highway works may have saved the city millions more in further highway repair works as the trees might have continued to outgrow their situation. Again CTLA does not allow for this to be reflected in valuation.

[Kveldssanger]

Some very good points you raise there, Jules. I find, from going through the ISA conference proceedings publications (The Landscape Below Ground series and Trees & Building Sites) are a little lacking in terms of background information and setting context. I realise they're overviews of what was presented in spoken word, though I find myself still wanting having read many of the articles.