7 minute read
Tree planting in urban environments for flooding mitigation
Taxodium distichum. © Henrik Sjöman
The Essential Tree Selection Guide, published in collaboration with the Royal Botanic Gardens Kew, highlights the latest research on the ability of trees to deliver important ecosystem services, deal with different growing environments and address flooding.
There is an increasingly positive attitude towards trees and tree planting in urban environments, not only among landscape professionals, but also from those who previously did not understand the importance of the urban canopy. A new language, which originates from the latest knowledge of the ecosystem services and functions that are linked to our trees, is now available to us. It enables us to communicate and argue for the value of the trees along our streets, in parks and in our private gardens. We can prove how important trees are in creating resilience towards the future challenges of increasing extreme weather.
As parts of the world increasingly suffer acute water shortages, making irrigation inappropriate, rain harvesting may prove an important approach to meeting future irrigation needs.
One of the more adverse effects of climate change on our urban landscapes is the increase in pluvial flooding. Any means of slowing down the water flow helps to mimic a natural hydrological cycle. Plants play an obvious role in this respect, and recently much attention has been paid to green roofs as part of a sustainable storm water management system, but also to trees and to non-vegetative elements such as permeable paving. Here we will discuss how trees contribute to a more sustainable approach to storm water management by means of their canopies and the soil in which they are planted.
The need to be plant and site specific
The primary way in which green space can help prevent the effects of pluvial flooding is by providing a natural route for rain and meltwater to infiltrate into the ground and contribute to groundwater recharge. In studies, where residential areas within the same river catchment have been investigated, the results show that the amount of space above ground was secondary to the type of soil below ground. One might think that a single home with a large garden would contribute less surface runoff to densely built-up areas. In fact, it can be equal if the single home area is built on heavy clay, but the densely built-up area is built on sandy soil through which the water could quickly percolate. This shows that any disturbance of the natural land cover could cause more detrimental effects to infiltration and groundwater recharge on clay soil. For example, building and removing tree cover on clay soil causes far more of an increase in surface runoff compared to similar activity on sandy soil. This is not to say that sealing surfaces on sandy soil will not cause runoff – any surface sealing whatever the location will have that effect. However, the underlying soil profiles and properties and existing vegetation determine how much. It is very important to start from the soil profile on site and adapt accordingly, as the same solution or capacity can differ greatly between sites and projects.
Rain gardens
In order to manage storm water, it is possible to introduce rain gardens, which tolerate temporary flooding. When designing these types of plantings, it is important to ensure that flooding does not endure for too long and result in anaerobic (oxygen-poor) ground conditions. This process is, of course, simpler on permeable soils and drainage will always be slower on clay soils, where it may be necessary to introduce artificial drainage. As well as ensuring the plant bed retains water for just a short period before drainage occurs, it is also important to select plants that are able to tolerate having their root systems submerged in water for a short period and recover relatively quickly. The capacity of trees to tolerate oxygen-poor ground conditions varies considerably.
As parts of the world increasingly suffer acute water shortages, making irrigation inappropriate, rain harvesting may prove an important approach to meeting future irrigation needs. Instead of leading water away, we can lead water from roofs and other built parts of the garden into containers for later use or directly into planting beds, allowing carefully selected vegetation to develop and deliver other important ecosystem services in the garden. However, it is imperative that the constructions which are to handle this excess water are designed to prevent anaerobic conditions.
Trees for flooding
In sites that suffer periodic flooding or waterlogging, the most important attribute for a tree species is quite simply the ability to ‘hold its breath’ during periods when the soil is oxygen deficient. One way that trees deal with this is by limiting the damage by transporting oxygen collected through lenticels on the trunk and branches down to the roots via the conduction pathways in the trunk. This has been demonstrated by studies of the white willow (Salix alba), crack willow (S. fragilis) and lodgepole pine (Pinus contorta). Another attribute that many wetlands trees share is the ability to grow an adventitious root system, in the form of new roots higher up the trunk. This is particularly valuable in environments that may receive a large supplement of alluvial sediment after flooding. It is especially evident among willow (Salix spp.), poplar (Populus spp.) and alder (Alnus ssp.). Growing adventitious roots at the water’s surface, where oxygen is more readily available than deeper down, is another successful strategy for coping with wet, oxygen-deficient soil conditions.
Wet conditions compromise the ability of roots to absorb nutrients, an oxygen-intensive process, which may leave the tree malnourished. Many trees that grow in river valleys subject to both the continuous erosion of minerals and organic material and brief flooding events during the growing season have developed strategies for dealing with poor soil conditions and obtaining nutrients from elsewhere. Among those with a natural ability to cope with poor soil conditions we find our native Scots pine (Pinus sylvestris), which is found in Europe in poor or occasionally waterlogged soils in environments such as marshes. Here, the pine represents an outand-out stress-tolerant strategist, growing very slowly in these wet and resource-poor sites and seldom to any great size. There are, however, species that employ a more successful strategy to cope with poor, periodically waterlogged soils and grow relatively quickly into large trees. These include those that cope with brief flooding in symbiosis with nitrogen-fixing bacteria that convert atmospheric nitrogen into nutrients that can be used by the tree.
Nitrogen-fixing tree species found in river valleys include the black alder (Alnus glutinosa), grey alder (A. incana), Caucasian alder (A. subcordata), Italian alder (A. cordata), honey locust (Gleditsia triacanthos) and black locust (Robinia pseudoacacia)
In sites that suffer periodic flooding or waterlogging, the most important attribute for a tree species is quite simply the ability to ‘hold its breath’ during periods when the soil is oxygen deficient.
Selecting right tree for the right site and function
The Essential Tree Selection Guide aims to create a better understanding of what trees can deliver and which trees are suitable for particular functions and specific environments. The Guide includes an in-depth description of over 550 different species and cultivars available today in European and North American nursery production – including common and traditional species as well as more unusual but promising ones.
Henrik Sjoman is senior researcher at the Swedish University of Agricultural Sciences & Scientific Curator at Gothenburg Botanical Garden where he teaches dendrology and plant use for urban environments.
Arit Anderson CMLI is an environmental advocator, TV presenter, writer and host of the podcast 'Growing Greener'.
