11 minute read
Technical
Amenity grass
and climate change
Judged by area alone, domestic lawns are the most common feature of urban green space worldwide. Research suggests in the UK they may cover up to 248,000ha, an area more than seven times the area of local nature reserves in the UK, whereas in America turf grass and lawn coverage are estimated at 16,400,000ha, making it the single largest irrigated crop.
Whilst the lawn, as part of a designed landscape, can be traced to aristocratic palaces in the 1600s, lawns as part of a residential property increased as developments expanded from urban centres into surrounding rural land, allowing more space between homes. Grass around houses as part of a development helped create the illusion of a continuous green field and fostered a sense of community; maintenance of grass in a front garden allowed the homeowner to contribute to keeping the neighbourhood neat and tidy.
As well as an important component for amenity or recreation, research has shown turf in towns and cities has an important role in reducing urban heat island effect. Grass reflects around 25% of incoming radiation from the sun (compared to 15% by trees and 10% by the built environment) but also removes energy as latent heat by the process of evapotranspiration (water drawn up through the roots and stems evaporates from the leaves, which cools the surface of the leaves and the surrounding air). Studies have shown greenspace surfaces are significantly cooler than tarmac and concrete (up to 25oC) even in temperate countries such as the UK.
Until, that is, a shortage of rain renders the sward brown and lifeless.
Although still alive and useable for amenity and sports, when summer droughts inhibit the water supply to shallow-rooted plants such as turf, they close their stomata to limit the water lost through transpiration. Transpirational cooling is therefore reduced and areas of grass in this state will not provide any cooling to counter the urban heat island.
When in this dormant phase grass behaves like bare soil: it heats up in the day even more than the built environment due to lack of thermal mass.
It is during hot, dry periods of the year when humans seek green spaces for cooling, and droughts often occur at the same time as Temporary Use (hose pipe) Bans when irrigation of green spaces competes with other demands for water such as health, hygiene, and commercial uses.
Research suggests the threat of drought is likely to worsen with climate change; in the UK summer temperatures are anticipated to rise whilst precipitation falls, increasing evapotranspiration and limiting water supplies. Whilst such droughts therefore have the potential to reduce the cooling effectiveness of turf, without additional irrigation they also have the potential to render large areas brown for longer periods of the year. Whilst the turf is unlikely to die, it’s amenity use is reduced.
Whilst brown turf has represented little more than an inconvenience for most in the UK this summer, for Association members who cultivate and sell turf to the trade and public, the drought has proved a significant challenge and prompted trials into alternatives.
Turf growers must consider not only the suitability of the finished product for consumers, but also ensure turf can be successfully cultivated year-round throughout the country, in farms without the need for unsustainable reliance on irrigation.
Accredited Supplier Harrowden Turf grow and sell turf throughout the country and, whilst the drought did not interrupt sales this year, the business is mindful of the challenges they are likely to face in the future from climate change and invested in technology and new products to combat this.
The most obvious solution to water shortages is cultivating and selling grass species which are drought tolerant. Members who have visited countries with hotter climates or more acute shortages of water will have noticed broader-leaved species in turf mixes prevalent in domestic gardens and public areas. Examples of this are Buffalo turf or Buffalo grass, which are native to North America but frequently found in warmer climates due to their drought resistance and low maintenance.
Festuca Arundinacea mix (left)
This species, commonly referred to as tall fescue, is deeper rooting and has remarkable drought and heat tolerance which makes it ideal for the recent hot, dry weather the UK has just experienced. It has a dark green colour which is comparable to conventional turf mixes in the UK.
These species tend to feature leaves which are much broader and provide a sward which tends to be less formal than the turf traditionally cultivated in the UK. It also does not tolerate close mowing to the same extent as traditional turf with narrow leaves. These grasses are also vulnerable to cooler weather; whilst they remain green during prolonged periods of hot, dry weather, they tend to turn brown if the temperature falls below 10oC.
Turf growers in countries with a temperate climate are therefore faced with a challenge - reduce reliance on traditional mixtures which tolerate lower temperatures and offer some resistance to drought, or trial mixtures more suited to warmer climates at the expense of hardiness in cold weather. There is no perfect solution, particularly given the range of benefits associated with traditional seed mixes: low maintenance, close mowing capability, high shoot density and resistance to disease.
The solution being trialled by Harrowden Turf is a middle-ground between traditional narrow-leaved turf species traditionally cultivated in temperate climates and species found in much warmer climates. Species such as Poa supina and Poa pratensis have been chosen, which maintain their green colour during drought conditions, whilst withstanding close mowing, hardiness in the winter and low maintenance. During this year’s long, dry summer, the turf grown at the farm in Thetford remained significantly greener than traditional mixes.
Poa Supina mix (right)
This species is native to areas such as Morocco and Mongolia. As an Alpine grass it has great shade tolerance whilst also coping with heat fairly well. It is also very hardwearing so can cope with heavy traffic. The only potential downside to this species is its pale green colour, which is distinct from lawn mixes traditionally sown in the UK. However, it recovers very quickly from stress so will green up much quicker than conventional turf mixes.
As the climate changes, concessions may have to be made by growers and specifiers which mean some degree of flexibility on traditional features.
Owen Baker - Technical Officer (Policy & Research) Turf sward commonly found in temperate climates. Featuring narrow leaved grass species which form a fine, smooth surface.
Turf sward commonly found in warmer climates. Featuring broader leaves and a coarser surface.
Effects of climate change on tree health
Shortly after the UK temperature record was broken in July this year, the Met Office confirmed the period between January and June was the driest in England since 1976. Research suggests warmer, drier summers are likely to become more common in future years, so what will the long-term effect be of this hotter, drier weather on plant health in the UK?
For landscape professionals the summer weather has always trodden a fine line between delight and frustration, depending largely on whether a business earns money from growing plants or maintaining them.
Research into the effects of climate change on trees has previously raised concerns regarding increase in temperature as well as a lack of precipitation. While a gradual increase in temperature may favour some species, it is also likely to increase the effect of evapotranspiration which can result in soil water deficits. This, combined with a lack of rainfall for successive years, has the potential to cause a significant change in the landscape.
Severe drought events have been rare in the UK. Whilst the profession has become accustomed to grass turning brown at the height of summer, most mature, native plants are adapted to tolerate the historic extremes of UK summers and rarely show obvious external symptoms or long-term damage.
By the end of July this year, mature trees and shrubs started to show signs of drought stress.
Many mature deciduous trees and some conifers undergo a period of abscission - the process of deciduous trees dropping leaves or coniferous trees dropping needles - when under drought stress; a practice that has been observed in the UK during the summer. This is part of a trees defence mechanism - shedding the part of the plant that is responsible for water loss via transpiration. However, this response also means the tree is losing surface area of the component responsible for photosynthesis, making this process a delicate balance.
Most of the dropped leaves originate from the lower, interior of the canopy rather than the top of the tree or tips of branches. The process sacrifices the least productive leaves, which are shaded by the leaves above and on the outside of the canopy. As in autumn, the leaves are likely to turn yellow before being shed, which is caused by the plant harvesting the nutrients from the excess leaves before dropping them.
An increased frequency of hot, dry summers in the UK may have a more serious impact on tree health than a single event. Research has suggested many European tree species will be negatively affected by increasing intensity and frequency of hot, dry conditions in a future climate.
Stressed trees have a higher susceptibility to diseases, parasites and insects, whose spread and reproductive success is heightened by rising temperatures. The intensity and
number of stress factors occurring at the same time influence the degree of loss of vitality and can vary greatly within a genus or a species as well as between species.
Research has outlined the benefit of healthy trees in urban areas, where they have the potential to mitigate the effects of climate change, but these benefits rely on the trees having a high level of resistance and adaptability to climate and soil conditions to survive.
Research investigating the effects of climate change on European street trees has concluded Quercus and Aesculus face severe challenges from hotter and drier climate conditions and predicts these species may be lost from the urban tree canopy of European cities that continue to experience intense heat effects. In contrast, studies have shown Tilia and Platanus species are well adapted to dryer and waterstressed urban climates.
Research has also considered the effect of maturity on a trees ability to resist the effects of climate change. Studies which looked at Pinus specimens discovered trees less than 40 years of age were more resistant to extreme droughts compared to trees aged between 40 and 120 years. It is believed older trees which are larger in size face a greater hydraulic challenge in terms of lifting water from roots to crown against the effects of gravity. In addition, mature larger trees are often associated with larger crowns which are directly exposed to increased solar radiation and higher evaporative demand during drought events.
Challenging climatic conditions emphasise the need for greater focus on the aspects to which specifiers and contractors continue to have control; correct planting, suitable soil and adequate management can help prevent the failure of trees. Existing species susceptible to heat must be monitored and maintained appropriately to increase life expectancy and prevent premature felling.
Where specimens are replaced or planted on new sites, greater consideration should be given to drought tolerance and heat tolerance, in addition to the more commonly considered factors of frost tolerance, air pollution, ability to resist compacted soil and resistance to pests and diseases.
Cases of trees shedding all leaves due to lack of water is currently uncommon in all but neglected young trees but may become more common in the future if the climate continues to change in the ways predicted. The ability of a tree to recover from more serious shortages of water varies between trees; some will be unable to produce sufficient photosynthates to properly harden off and survive winter later in the year.
Owen Baker - Technical Officer (Policy & Research)
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