June..A blog by Andy Hirons. Outdoor events and soil compaction

Music festivals, open-air cinema, outdoor theatre, and other community events are increasingly popular, and with good reason. Who wouldn’t enjoy a picnic listening to a great local (or international) artist, appreciate a natural backdrop to a play or enjoy art installations hidden within the nooks and crannies of their local park. Of course, it is right that our parks and gardens are used to enrich culture in this way. In fact, at least some managers of these areas are under significant pressure to make the most of public outreach and commercial opportunities afforded by parks and gardens.

Most of us recognise the societal value associated with using parks and gardens for outdoor events. However, almost without exception, these events risk serious damage to the trees found on these sites. Perhaps the most significant concern relates to soil compaction caused by foot and vehicular traffic associated with these events (Figure 1). Indeed, in some cases we seem to, quite literately, love our parks to death. In light of this, I thought it would be helpful to briefly explore some of the ways soil compaction is damaging to trees and some approaches that can be taken to ameliorate the damage.

Figure 1: Soil compaction associated with foot traffic. ©Andrew Hirons

Damaging levels of soil compaction are very rarely a feature of natural forest ecosystems. However, soil compaction is widespread in amenity landscapes and should be a major concern for those managing trees.

We usually measure the level of soil compaction by measuring the bulk density of the soil i.e. the mass of soil in a known volume of soil. We usually measure this in grams per cubic cm (g cm-3) even if scientific convention suggests kilograms per cubic metre (kg m-3) is more appropriate. [It gives the same number so it is probably not worth the argument in most cases.] In healthy forest soils, the bulk density will be less than 1.2 g cm-3. In these soils, roots are not physically restricted from developing and the soil is well-aerated. However, both foot and vehicular traffic can substantially increase the bulk density so that the soil becomes seriously restrictive for root development. This is best described as an increase in soil strength (usually measured in MPa) but it is important to acknowledge that it is not the same as bulk density.

Soil compaction becomes really problematic at bulk densities around 1.4 g cm-3. I say ‘around’, as both soil texture and moisture level can affect this threshold: sandy soils becoming limiting at higher bulk densities than silty or clay soils and; more moist soils restrict root development less than drier soils assuming the same bulk density. Of course, there are also different sensitivities to soil compaction across species.

High (>1.4 g cm-3) soil bulk densities physically restrict root development, they reduce aeration, negatively effect water movement around the roots and are detrimental to a wide range of soil biota. As a result, fine root development is seriously perturbed and this, in turn, effects the acquisition of water and nutrients. Ultimately, this leads to a gradual decline in the condition of the crown.

Therefore, for those managing trees, a useful aim is to maintain soil bulk densities at less than 1.4.g cm-3 within the critical rooting area for the tree. This is often taken as the area under the crown of a tree, but physiologically active roots often extend beyond the so-called ‘dripline’. By far the best way to achieve this is to prevent soil compaction occurring in the first place. Use of fences is sometimes appropriate, but softer approaches including mulching, not mowing and using ground-cover to ‘nudge’ people away from root zones can also be effective, particularly for large trees.

Subtly managing peoples behaviour using these approaches can often be a cost-effective option: few people want to picnic on a pile of mulch or in long grass. Where serious levels of soil compaction are already evident a few options exist to ameliorate this. The most effective option is the use of air cultivation (Figure 2). This involves the use of compressed air and a pneumatic tool (such as the Arb-Ex or Air Spade) to break up the compaction. This can be highly effective, particularly when the air tool is also used to integrate mulch into the decompacted soil. One of the major limitations is the time expenditure taken and the relative incompatibility with turf.

 Figure 2: Air cultivation using pneumatic tools (in this case the Arb-Ex) can be very effective for ameliorating soil compaction. ©Andrew Hirons

The use of air injection, using systems such as the VOGT Geoinjector, can also be useful in dealing with compaction. These systems can greatly aid soil aeration and they are much more sympathetic to turf. A larger area can be treated during the day but the method does not fundamentally alter soil bulk density across a large volume of soil. Nevertheless, it is likely to play a positive role in accelerating natural processes involved with reducing soil bulk density though: it is, therefore, well worth considering.

 Figure 3: Air injection systems, such as the VOGT Geoinjector shown here, can be very useful to restore soil aeration and accelerate natural soil decompaction activity. Granular material can also be injected using this system. ©Andrew Hirons

I believe that we need to have a range of tools in the arborist’s toolkit to deal with soil compaction: prevention and amelioration have their place.

So how does this all relate to the outdoor events? There is no doubt that these events are, in many cases, hugely detrimental to the tree’s rooting environment. Given the fact that the success of these events is at least in part a result of the landscape in which they are enjoyed, it seems reasonable for those tasked with managing the landscape to build the cost of root-zone amelioration into the cost of the event. I have to admit, as a biologist, this is where my knowledge gets hazy. Does such a mechanism to recoup costs associated with repairing damage caused outdoor events exist? If not, why not? Maybe somebody can help me out here…

The trend towards outdoor events is perfectly legitimate, but only if our landscapes can be maintained and preserved. If we do not make a concerted effort to rectify the damage do to soil and root systems, the risks from trees associated with these events may well increase as they begin to deteriorate in condition. Proactively resolving this tension between outdoor events and the rooting environment is, therefore, strategically important for our amenity landscapes.

If you would like any more information on soil compaction and management options to ameliorate it, please do feel free to contact Dr Andy Hirons ahirons@myerscough.ac.uk