[Continuation of the interview with the forestry expert Dr. Somidh Saha from the Karlsruhe Institute of Technology (KIT) on the topic of urban trees. Click here for part one of the interview.]

Dr. Saha, what recommendations do you have to better protect urban trees?

Dr. Saha: Urban trees are often felled down as an excuse for “development and infrastructure” projects. As a scientist, this is very painful to me and it happens all over the world. Urban planners often claim that they do compensatory tree planting for each felled tree “somewhere”. But, they should ask themselves about the environmental cost for cutting multiple 70 years old sycamore or linden tree in comparison to planting some small saplings that require at least 7 decades to compensate.

Moreover, when urban authorities plant new trees in the “renovated quartiers or streets” then they don’t provide enough rooting space to newly planted trees. It means the citizen will never get a big tree in that location anymore, where there was once, a big tree standing and providing ecosystem services.

This unscientific system in the name of development must be stopped. Because this is simply not sustainable. Moreover, this happens also in country like Germany which is globally renowned for environmental protection (for example, recent felling of big Platanus trees in Freiburg i. Br. in the area of Siegesdenkmal which is newly named as Europaplatz). My suggestion is to reform or create new laws at state and national level to completely ban the felling of big trees in German cities in the name of development.

How can tree management be improved?

Dr. Saha: There is an urgent need to reform the management plan for urban trees and forests under increasing threat from climate change. We need to reform the activities related to site preparation, species selection, planting, tree care, irrigation etc. In many cities, trees are still managed in a very old-fashioned way. For example, it is still common to plant a drought intolerant species in one urban area where you have a problem of drought and they are often planted as street-monoculture. We need to change this system. It also requires research and collaboration between municipalities, research institutes and universities. In my newly established junior research group Sylvanus, this is currently a topic of research.

So an important starting point, cause without authorities and allies little changes...

Dr. Saha: That is right. The local authorities as well as other stakeholders from society and politics should be involved in improving public awareness on the importance and management of urban trees. In my opinion citizen can play a huge role in preservation and maintenance of urban green spaces. They can also contribute in creating knowledge through citizen science. For example, in our research, we are trying to engage citizens and municipal authorities in conservation and planning of urban trees and forests to increase resilience under climate change.

In your opinion, are there model cities in which the management of stressed trees is already exemplary and from which other municipalities can learn?

Dr. Saha: No. I don’t know any model city or I don’t know any comparative studies between the cities of Germany. This is a good point and we should initiate a study like this.

You found in your research that urban trees often lack sufficient root space. Is it still possible to adjust anything for the existing urban trees? What should we focus on more when planting trees in the future?

Dr. Saha: Yes, root space is serious issue in our city of Karlsruhe and many other cities. Root space decision should be made on the target desirable size of the trees. For example, if an urban forester want to have a 15 m tall tree then at least 80 to 150 m³ of rooting space should be provided, which varies between species. However, such guidelines are often not followed. In future, we need to make sure that we provide required root space based on the species, neighborhood structure and desired crown size and height.

How can the irrigation of urban trees be optimized?

Dr. Saha: This is a very good question. Currently urban irrigation is done in ad hoc basis and it is not precise. Urban irrigation should be more precise and the amount of irrigated water per tree per week should be based on factors like tree species, period of drought, tree size, local irradiation, and water holding capacity of soil or substrate. We also lack research knowhow on species-specific water balance model. In our research group, we are working on this topic and expect to get first results by 2021.

How can the irrigation of urban trees be optimized?

Dr. Saha: This is a very good question. Currently urban irrigation is done in ad hoc basis and it is not precise. Urban irrigation should be more precise and the amount of irrigated water per tree per week should be based on factors like tree species, period of drought, tree size, local irradiation, and water holding capacity of soil or substrate. We also lack research knowhow on species-specific water balance model. In our research group, we are working on this topic and expect to get first results by 2021.

It is often discussed that in times of increasing drought, citizens should also water the trees in front of their houses. Does this really help if a city tree evaporates more than 100 litres per day?

Dr. Saha: No, but you are raising several interesting points here. First, if a tree is still transpiring during a day of drought then it is not in a win-win situation in terms of ecophysiology. Because, xylem cavitation and can happen in that tree if water is not enough in soil. Xylem is type of pipe-like (“tracheids”) tissue in vascular plants (e.g. trees) that transport water and nutrients from soil to leaves. Cavitation can occur if tension with the water exceeds the atmospheric pressure by bubbles which are filled with water vapor. This can become severe if plants continue to loss water through transpiration during drought. Plants can repair the cavitation in most circumstances. However, during intense and prolonged drought, embolism can occur by complete collapse of water column in xylem. Frequent events like this can lead to dieback of trees which often starts from the upper of the tree crown. The xylem cavitation is a species-specific characteristic as well.

In this situation, we need a tree species, which closes the stomata during drought to control transpiration (e.g. oak). However, it means less cooling of the urban environment, which is nonetheless also a very desirable ecosystem service. This trade-off between the vulnerability of a tree to xylem cavitation and cooling capacity should be considered while selecting the species. If you have right species at right place, planted with right shoot and root growing space then you can increase the resistance of the tree to drought, hence, reduce the dependence on irrigation. Secondly, a citizen should be only encouraged to water the trees, which are visibly under stress. However, we need to know about the precise amount of minimum water for the irrigation in order to avoid the waste of water. I have read in recent news that in future, Germany may face problem of water scarcity if recharge of groundwater is not enough. In this scenario, we should design our urban tree and forestry management in a way so that we reduce the consumption of water.

In your research publications you also talk about the need to improve density management in cities. What does this mean?

Dr. Saha: I mainly mean here the management of urban woodlands. Sometimes, urban woodlands are overstocked as thinning is often not common in urban woodlands, which are usually community forests. Trees growing in dense urban forests face more intra- and interspecific competition for water, nutrients, space, and light. An acute tree competition can act as a predisposing factor of drought-induced mortality. Therefore, in my opinion, urban forests should not be kept as too dense and thinning should be done to increase overall forest health, if necessary. However, an in-depth assessment of forest health and density should be done prior to such action.

Many cities in Germany heavily continue building and plan a lot of infrastructure projects. Increasingly, new buildings are also connected with the construction of underground car parks. This closes off the root area and the planting of large trees is no longer possible. Could this development be changed? Are there alternatives to city trees?

Dr. Saha: Again, you raised very interesting and challenging questions. I have two answers. First, we must keep a minimum rooting volume based on desirable tree size. The civil and architecture engineers do not prefer adequate rooting space not only because space shortage but also because the pressure from roots can destroy nearby utility networks (e.g. water and sewage lines). We need to plant trees with finer root texture like for example birch or willow rather than deep root species such as oak, beech or lindens in these locations. Moreover, we should regularly observe the below ground development and do the root pruning, if necessary. This will increase the cost of management which will create new challenges. However, use of technology such as earth penetrating laser cameras can reduce labour costs.

Building and architecture is not my field. But, I feel not very happy with many infrastructure projects where there are very less scopes of tree planting. In my opinion, if construction projects become greener (i.e. less steel and concrete) then, there is a better chance also to create space for tree planting. There are no proven alternative to city trees!

You say that the public needs to be made more aware of the value of urban trees, that sensitization is so important. What would you like to see?

Dr. Saha: Our target is to aware citizen about the threats and necessary actions to save trees and forests from climate change impacts. Public should know about the way we are managing trees and forests and approaches to increase resilience of trees and forests. Increase in awareness among citizens will help to improve the management of trees, forests, and green spaces under climate change privately owned by them. People should be able to get involved in the management of trees and forests in urban areas in a participatory way. That will also improve social cohesion between different segments of society for the betterment of urban trees and forests. Public participation in urban green activities will also help to increase the overall public wellbeing. Currently we are running a very nice experiment on “Close to Nature Garden Management” in the town of Rheinstetten involving local residents, which is receiving very good feedback from public.

At the Karlsruhe Institute of Technology (KIT), a junior research group called Sylvanus was founded to examine the effects of climate change on trees and forests. Can you describe the work of this new junior research group?  How can you help cities and municipalities to take the right decisions for their forests and urban trees?

Dr. Saha: We have set ourselves ambitious research goals in the KIT junior research group Sylvanus. We are dealing with important research questions that have not yet been sufficiently addressed scientifically. Let me give a little bit of explanation.  Impacts of climate change on natural and built ecosystems are visible all over the globe. Scientists and policymakers are recommending to increase the resilience of ecosystems for minimizing the impacts of climate change. Resilience could be a stochastic process with higher challenges of prediction related to uncertainties. Resilience is often defined as a capacity of a system (e.g. natural forests or urban forest) to recover after a disturbance (e.g. drought). However, this could be stochastic which means that there is no guarantee that the structure and composition of the forests at pre-disturbance level can be repeated at the post disturbance. It means there is no guarantee of ecosystem services from the forests if disturbances get severe and recurrent.   

Nonetheless, the expectation from human society to get services from ecosystems is growing with a global increase of population. The supply of ecosystem services must come from sustainable management of resources where sustainability can be seen as a normative process. 

I postulate that neither resilience nor sustainability can be achieved to an infinite level and trade-offs between resilience and sustainability may widen significantly after a certain threshold. Quantification of such limit and transferring it to practice is one of the most significant challenges we are facing under climate change. Solutions to the resilience vs. sustainability dilemma would also vary over space for example between countries and ecosystems and time for instance from a yearly to centurial scale. In the next five years, group members and I would like to investigate this dilemma in social-ecological systems such as forests. We have selected forests as a model system because our research can focus on natural ecosystems (e.g., multifunctional forests, national parks) as well as for built ecosystems (e.g., trees, parks, woodlands in urban areas or “urban forests”). The research group will combine basic research and applied research in the field of ecology, forestry, sustainability science, bioclimatology, and philosophy under inter- and transdisciplinary framework. The scope of the research group is global, but currently, we can envisage field works and case studies in Germany, India, China, Chile, and the USA.

In our Sylvanus research group, we are doing trans- and interdisciplinary research. It means, in addition to crosslinking scientific fields, we are also working in collaboration with the municipalities. Our aim is to support them. We regularly meet city officials and stakeholders in workshops and seminars. We provide evidence based suggestion to municipal administrations to improve urban tree and forest management. For example, in collaboration with City Karlsruhe and City Rheinstetten, we are working on the development of an adaptive forest and tree management plan for urban areas under climate change.

Mr. Dr. Saha, many thanks for this interview.

The interview was conducted by Jana Kandarr and Oliver Jorzik (ESKP).

Part one of the interview with Dr. Somidh Saha from the Karlsruhe Institute of Technology (KIT).



  Tost, H., Reichert, M., Braun, U., Reinhard, I., Peters, R., Lautenbach, S., ... Meyer-Lindenberg, A. (2019). Neural correlates of individual differences in affective benefit of real-life urban green space exposure. Nature Neuroscience, 22, 1389-1393. doi:10.1038/s41593-019-0451-y



Published: 21.11.2019, Vol. 6

Cite as: Saha, S. (2019, 21. November). Better care for urban trees. Earth System Knowledge Platform [www.eskp.de], 6. doi:10.2312/eskp.020

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