SiK - Urban trees in climate change: climate impact monitoring and adaptation | Umweltbundesamt

Trees in the same room with all structural changes, e.g. the internal compression in Hamburg, compete with each other. Road and cycle route expansion, acceleration of the bus, upgrading the public transport system, new supply routes for water, gas, electricity or the Internet and the compact development of 10,000 new apartments per year are indispensable, but they are at the expense of inner-city open spaces, even on the roadside. Compensation in the sense of replacement sites for tree replanting due to lost cultivation can be done but even with compensated balances always a part of valuable tree substance – usually trees older than 50 years – are lost in exchange for new plants meaning a lack of quality in terms of ecological and urban climatic point of view for welfare. In particular, the old trees make a major contribution to the urban climate, due to their sufficient root space and their water sources and can survive dry phase without stress. Against this backdrop, urban development goals should consider the importance of the tree population with all other legitimate interests. This applies to both urban planning processes as well as short-term decisions in local redevelopment planning.

The importance of old trees

Facilitating resilience of the three population facing extended and summerly drought periods have to be further explored. Focus should be a selection and usage of appropriate regional and available soil substrates for tree planting in combination with choices of tree species and varieties which are considered promising for climate adaptive urban planting. This can be realized at favor sites (e.g. nurseries) and at real city locations.

Basic requirements for a tree’s resilience to climate changes are reasonably sized and well-stocked tree site. Only if the tree has sufficient space above and below ground, it can access water resources and survive dry periods better. Especially young trees are therefore quickly affected by drought stress, since they depend on the closer environment of the tree site. Old trees are therefore the best climate trees, as they have developed deeper sources of nutrients and water for years and survive dry periods well. The interim results from the SiK monitoring of selected tree locations in Hamburg show positive and negative examples of how much space city trees have, with which usage pressure they are confronted or which design features characterize the locations. 

Around half of the city trees are over 40 years old and have potential to cope with increasing problems such as diseases, pests, pollutants and the emerging consequences of climatic change. Protecting the existing and established tree population are central to a sustainable urban development.

Observing the effects of climatic changes on the pin oak (Quercus palustris) 

Examplarily the pin oak, as one of 23 city trees, the soil moisture change by sensors was recorded automatically and in a half-hourly rhythm for the summer of 2017. Visible is an almost complete soil coverage of a site due to stone pavement, irrigation bags can help the tree.

In total, 362 mm of rain was measured in the six months (509 mm in Fuhlsbüttel), the driest month was September (35 mm), the wettest in June (93 mm), rain fell on two out of three days. The soil moisture profiles were averaged for 3 flat sensors (30 - 40 cm installation depth), 4 sensors from the main root area (27 - 80 cm depth), 4 sensors from the edge area (40 - 70 cm depth) and 2 sensors from the subsoil (100 cm Depth). The following processes can be recognized:

• After the winter, the soil is moist at all depths, in the short term after rain also very humid, so favorable starting conditions for the tree.
• After blade drive, water consumption begins. From 15.5. decreases the water availability in the central root area, but the decrease is interrupted by short recoveries as a result of individual rains. Despite the June precipitation is the first time on 29.6. the state "dry" reached.
• On 30.6. After heavy rainfall (31 mm) water penetrates into the topsoil and the central root area, the water supply is optimal for 6 days. Within three weeks after the rain the state "dry" is reached again in the root area.
• Another rain event on July 25 (8.6 mm, also watering) brings very short relaxation. This time it takes 11 days for the soil to dry again.
• After that, rainfall occurs regularly, but only on 18.9. reach over 10 mm / d. The ground sensors do not show these (although some of the sensors had failed from the 28th of August).
• The sensors at 100 cm depth are not reached by roots. Here the soil reacts neither to the water supply by rain nor to the water consumption of the roots.

The measurements at high rise building Ericusspitze permit the following derivations: The requirements on the substrates would have to be changed by ensuring better water storage with a selection of more cohesive variants. In addition, drought stress could be prevented by including more frequent and consistent watering over a longer period of time as a rule element in the contractual growth management.

In the course of the project a concept was developed to adapt city trees to climate change. Within the concept measures and instruments were developed to sustain and developed the tree population to climate change.

Tree sites and the greening of buildings improve the quality of the air by binding greenhouse gases and air pollutants, reducing the urban heat island effect, thus contributing to a better urban climate. In addition, they serve as reserve areas for flood and heavy rain events. In order to adapt tree sites to climate change, tree species need to be identified that can cope well with urban stress, including heat and lack of water. A further measure is the optimization of tree locations. Screws for adapting the plant locations to the needs of urban trees are in the design of the planting pit and the tree pane and in the composition and stratification of plant substrates. At present some minimum standards are set by guidelines e.g. given with regard to the dimensioning of the plant pit and tree disc. The problem remains, however, that in urban areas and streets usually no space for appropriately sized plant pits and planted tree slices is available, meaning the symptomatic high risk of loss of vitality due to drought remains despite "DIN plant pits". A plant pit design with targeted supply of rainwater can help.

Planting often a space problem in inner city space