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 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.
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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