Trees & Avenues

For more than 200 years now, avenues of trees have been a key traditional element of many landscapes in Central Europe. They thus represent important natural habitat, including for species enjoying protection at national level, or even Europe-wide. In farming areas these may often represent the only place of shelter for species, especially birds, bats and insects. 

Trees shape the character of the local landscape

A further benefit arising out of the presence of trees in our surroundings concerns the chance we gain to better acquaint ourselves with nature and the processes that govern it.

And as we list the functions trees serve, there is no way we can omit their aesthetic role. Greenery is a key feature of the spatial layout to city and village alike. It often goes a long way to determining its prestige and attractiveness. Thanks to their size, trees emphasise the character of a local landscape, in the same way contributing to all that makes a given place unique. The presence of trees impacts upon value in monetary terms too, given the way that research shows how much more we are ready to pay for a house or flat whose windows look out on to trees. Through their species, habits and layouts, trees can also be the ”calling cards” for regions. For example, Poland’s Varmia and Masuria region is famous for lakes; but what would they be without trees, and the beautiful lines and avenues they often form?

Trees clean up the air

A leading function served by trees is their ”treatment” of the pollution introduced into the atmosphere by human activity. Leaves act as a specific kind of filter, the gaseous exchange processes they have evolved to run at the same time taking in or holding on to both particulate pollutants and dust, and pollutant gases and other chemical compounds. These plants are astoundingly effective in this role, for example reducing levels of particulates in cities by 75%.

Trees are quite well-known for their role in the abatement of greenhouse gases. The photosynthesis they engage in uses up carbon dioxide as it liberates oxygen. A single small tree generates around 118 kg of oxygen each year. It is worth remembering that, over that period of time, each adult person will require around 176 kg of oxygen.

The favourable influence of greenery on air quality also results from the transpiration process, by which water evaporates out of small pores called stomata that are present on leaves and stems. In 24 hours, a single tree may release 200, or even as much as 400, litres of water. In line with the laws of physics, evaporation takes heat, with the result that a tree can lower temperatures locally by as much as 11oC on a hot day. The cooling impact of a single tree in fact compares with that of five typically-sized air-conditioning units working for a whole day. No surprise, then, that the shade of a tree has long been a favoured place to rest during hot summers.

Naturally, the absorption of pollutants is not something a tree does without costs to its health. The mean longevity of trees growing in urban areas is only half that of their counterparts lucky enough to have germinated and grown amidst the fields and woods of the countryside. A further fact contributing to the early demise of city trees is shortage of space, especially when it comes to the rather small cubes of soil we leave at the disposal of street-tree roots.

Purifying water and the soil

A major problem our environment has to deal with is the pollution running off from the surfaces of cultivated fields. First and foremost, these are components of fertilisers that crop plants have not had time to take in, and that have also failed to transfer deeper into the soil. The compounds of phosphorus, potassium and nitrogen that are mostly involved are ideal food for algae, which bloom when the runoff reaches lakes and ponds. The more-severe blooms associated with sustained eutrophication can produce more or less permanent devastation in the lakes and ponds we are so keen to visit. And, while the point sources of pollution characterising factories or households are amenable enough to the installation of treatment facilities, the same cannot be said of diffuse pollution carried away from the very extensive areas in which crops are grown. Fortunately, treatment plants are not needed, since the simplest and most effective way of dealing with runoff and thus limiting the pollution caused by farming is to leave boundary strips of uncultivated (above all tree-planted) land around the edges of fields. Several belts of such vegetation are able to reduce concentrations of nitrates in runoff water by as much as 98%, as well as phosphates by around 25%.

However, the role of plants as barriers helping to protect against pollution is not confined to agricultural areas. In urbanised and industrialised zones likewise, the root systems of trees remove from soil and effectively deactivate heavy-metal compounds, achieving 40-70% levels of efficiency in this process. Furthermore, the bacteria that accompany plants ensure the breakdown of a wide variety of organic compounds, including the very harmful polycyclic aromatic hydrocarbons (PAHs).

The influence of trees on harvests

Notwithstanding “received wisdom” on the issue, the presence of areas of trees in the middles of fields increases crop yields, even after account has been taken of reduced harvests in their immediate vicinity. This reflects a favourable influence on microclimate, the retention of water, and the role in combating erosion by water and the wind. Tree roots help to stem rapid runoff during downpours, while crowns are well known as effective windbreaks. The process of snowmelt in spring is also slowed down by trees, with the result that risks of flooding are reduced.

Soil close to trees tends to be of undisturbed structure, also having more organic matter thanks to the decay of fragments of plant matter. The capacity of the soil to hold water is increased in this way.

Guardians of biodiversity

It is impossible to overestimate the natural functions that trees play, especially as habitat for plants and animals. Old, decaying trees are particularly important from this point of view, since many organisms live lives hidden away within them. Many useful animals occur in, on or around trees, not least bees, birds of prey, and insect-eating birds and bats. All of these are friends of the farmer, as allies in the war against pests. In turn, naturalists hardly pause for breath as they name several tens of rare and protected species that reside in single trees, and best of all avenues. Many birds, mammals and insects are to be met with, including such species of benefit to human beings and their activities as tawny owls, buzzards, woodpeckers, various insect-eating warblers, shrikes and so on. Particularly noteworthy insect inhabitants include the protected hermit beetle, which colonises holes in limes, willows, cherries and oaks. The diversity of organisms inhabiting lines of trees departs markedly (in a positive sense) from what are often species-poor stands of planted forest made up of single species.

Why avenues and rows of trees?

For more than 200 years now, avenues of trees have been a key traditional element of many landscapes in Central Europe. They thus represent important natural habitat, including for species enjoying protection at national level, or even Europe-wide. In farming areas these may often represent the only place of shelter for species, especially birds, bats and insects. The interiors of old trees in fact constitute a rich ecosystem, given that they are home to hundreds of species of insect, fungus and microorganism, among which the one cited most frequently, and enjoying the strictest protection, is the hermit beetle. Roadside trees make the microclimate less severe, slow down winds, favour precipitation, protect the soil against erosion, and also help climate globally by sequestering carbon dioxide.

Of course, they are also beautiful, an asset and beautifying element of the landscape, and a pleasant feature or talking point that makes travel a nicer experience than it otherwise would be.