Monument protection as a limiting factor for large scale vertical greening system implementation to counteract indoor heat stress – a GIS-based analysis for Berlin, Germany

Vertical greening systems (VGS), as proven strategy for adaptation to heat stress, are prohibited for monument-protected buildings and façades. To what extent monument protection effectively limits the implementation of VGS in heat stress affected city centers has not been quantified yet. In this study, the ratio of buildings under monument protection was quantified for Berlin, Germany, using a GIS analysis. It was then compared to a map of potential heat stress. This was done for the entire city and the inner-city area on block-scale. Therefore, maps of the Berlin buildings, monuments and blocks were processed. The ratio of buildings under monument protection was calculated for each block of Berlin. To not deform the outcome, urban green areas were cut out of the maps. Inside the 80 km2 city center of Berlin, which is severely affected by heat stress, the range of monument protection prohibiting VGS in the individual blocks ranges from 0 % to 100 ﻿%. However, 25.42 ﻿% of the building façades in the city center and 16.20 ﻿% for whole of Berlin are protected on average and therefore cannot be greened. Compared to other restricting factors, monument protection does not generally hinder large scale implementation of VGS in Berlin. Nonetheless, 102 potentially heat stress exposed blocks inhabited by 48,122 people cannot be greened due to monument protection. This demonstrates that VGS should be discussed as exception from monument protection that can be justified by a predominant public interest. It also points to the need of minimal invasive and mobile greenery technologies, which would enable heritage-protection conform greening.


Introduction
The number of 'summer days' (max. θ air ≥ 25 °C) and of 'hot days' (max. θ air ≥ 30 °C) in Berlin, Germany rose significantly from 1960 as well as the area of distribution of 'tropical nights' (min. θ air ≥ 20 °C), which can be attributed to climate change and the urban heat island effect (SenUVK 2016(SenUVK , 2018. High summer temperatures can cause working productivity loss (Lundgren et al. 2013) and raise morbidity plan on urban nature' (Küspert et al. 2019) as well as scientific research (e.g. Gill et al. 2007) describing it as crucial for the adaptation to climate change.
VGS, as a measure without concurring land use, provide a variety of positive effects for city dwellers, which have been assessed in numerous studies. Examples are air quality improvements (Pugh et al. 2012), indoor noise reduction (Pérez et al. 2016) and even a contribution towards more environmental justice (Felgentreff et al. 2022).  quantified cooling potential of VGS and "demonstrated cooling effects of façade greening through transpiration and shading". This is confirmed by Koch et al. (2020) and modelled by Hoffmann et al. (2021), who quantified VGS-induced indoor wall surface temperature reductions for non-retrofitted Wilhelmine period buildings (dominant building type in Berlin) of more than 1 °C. This is a highly relevant finding and a major argument for large scale VGS implementation, since especially indoor heat stress poses a high risk to city dwellers (Buchin et al. 2016).
VGS implementation is limited, among other reasons, like water availability (Pearlmutter et al. 2021) or design aspects, by two hard concerns that are checked when asking for a building permit: fire safety and monument protection. Fire safety rules must be followed by the VGS design approach while monument protected buildings must not be greened -without an official exceptional permit.
Before going into further detail on the legal situation, general definitional questions regarding VGS need to be clarified, since existing research on the subject lacks a uniform nomenclature. Several authors differentiate into 'living walls' and 'green façades' (Pérez et al. 2011;Radić et al. 2019). Green façades consist of climbing plants growing in the ground or in containers, which climb directly on the façade or on simple climbing aids. Living walls offer a much freer design, which results in substantially more expensive installation and maintenance. They grow in containers or on panels, that are attached to the façade or placed in front of it, which enables greening entire façades without the use of climbing plants.
This becomes especially interesting, when considering the greening of protected monuments. As Ottelé (2011) states, plants that grow and root directly at a façade can pose a risk, if walls are already damaged before, or when plants are being removed from façades. On the other hand, it is described as a misconception, that VGS will do any harm to façades they are attached to. As described before, there are several forms of VGS, which do not necessarily have to be attached to the façade they are covering. Thus, the appropriate choice of VGS prevents from façade damages.

Legal situation
In the monument protection law of Berlin in § 11 Abs. 1 (DSchG Bln. 1995) it is stated, that a monument can be altered in its appearance, only with a permit of the responsible monument protection authority. There is no differentiation between the different kinds of monuments (building monument, monument area, garden monument and archeological monument). The permit is to be chartered, if causes of the monument protection do not conflict or if a predominant public interest requests or justifies the measure 1 . To receive such a permit, a motion must be filed, that needs to be submitted to the lower monument protection authority (Untere Denkmalschutzbehörde), which is managing matters on district level. An agreement on the motion needs to be achieved in cooperation with the technical monument protection authority (Denkmalfachbehörde). If they do not come to an agreement, a so-called dissent-case, the motion needs to be passed through to the next level authority. This is the supreme monument protection authority (Oberste Denkmalschutzbehörde), which is located within the Berlin Senate for Culture and Europe and is responsible for the final decision. Until now, there is no official information on the share of protected façades or buildings available.

Aims of the study
This study aims on quantifying the ratio of buildings under monument protection in Berlin, for the entire city, the inner city and the individual blocks. This is done by a GIS-based method, that is presented in this study.
The generated map of the monument protection levels is compared to the map of Dugord et al. (2014), that shows potential heat stress risk levels. This allows for the identification of blocks that combine high potential heat stress risk, therefore needing climate change mitigation and adaptation measures, and high shares of protected monuments.
It is discussed whether the current rather complicated way of filing a motion that needs to be assessed by different authorities should be untightened, in terms of an exception or at least an easier processing for VGS implementation at protected monuments, under certain conditions.

Data sources and areas of interest
The Berlin Senate provides numerous maps of Berlin, among other things, on environmental data, in an online, free of charge web portal (FIS-Broker). For this work, maps of all buildings in Berlin 'BB' (SenSW 2021a), of the protected monuments 'PM' (LDA 2021), of the Berlin blocks 'BL' (AfS 2020), of its green spaces 'UG' (SenUVK 2021) and the population density 'PD' (SenSW 2021b) were processed.
The map of the inner-city area 'IC', which is limited by the urban circular railway, is given in Fig. 1. A map of the potential heat stress risk 'HS' was provided by the authors of Dugord et al. (2014).

Data processing
To process the data, the open source softwares QGIS 3.10, GRASS GIS, as well as RStudio (RStudio team, 2020) were used. First, maps of the Berlin buildings 'BB', of its protected monuments 'PM', of the Berlin blocks 'BL' and of the urban greenspaces 'UG' were imported into QGIS. All layers were reprojected into the chosen coordinate reference system EPSG:25833 and the fix geometries command was applied to allow for further processing.
Next, BB was filtered for several attributes that prevent buildings from the possibility of being greened: Building parts marked as either underneath the surface, destroyed or degenerated, or without walls (open warehouses) were removed. All remaining buildings were dissolved to lower the file size and processing time, resulting in a new layer 'BB-diss'.
The PM map, was also dissolved and unsuited objects for VGS, named as archeological and garden monuments, were deleted. Thereafter, the layer was dissolved again, to 'PM-diss'.
To not include urban greenspaces, which might be irritating for the visualized outcome, those were removed from the BB_diss, PM_diss and the BL layer. To do so, the difference command in QGIS was used, to delete overlapping parts of BB_diss and UG, PM_diss and UG and BL and UG, respectively.
Following, PM-diss, BB-diss and BL were imported to the GRASS GIS software. Larger data sets can be processed here with more computationally intensive commands (i.e., intersection), than in QGIS. PM-diss and BB-diss were then intersected (v.overlay command) which means, that overlapping parts of both layers are kept. The result was dissolved for easier further processing and exported as new layer 'BB-PM-inter-diss'. This layer shows all parts of buildings in Berlin, that are under monument protection.
To determine the share of protected buildings on blocklevel, BB-PM-inter-diss and BB-diss were each intersected with the BL layer in GRASS GIS and exported as BB-PM-BL-inter and BB-BL-inter.
Next BB-PM-inter-diss, BB-diss, BB-PM-BL-inter, BB-BL-inter and IC were imported back into QGIS and the fix geometries command was applied again for all of those layers.
To calculate the ratio of protected monuments on city level, first, a column was added to the attribute tables of BB-PM-inter-diss and BB-diss, where information about the area size was generated. Joining the area field of BB-diss to the BB-PM-inter-diss layer allowed for creating a new column, where the area of protected monuments is divided by the area of relevant buildings, which equals the ratio of protected buildings on city-scale.
Those last two steps were almost similarly repeated for calculating the ratio for the inner city, after clipping both, BB-PM-inter-diss and BB-diss by IC, newly generating BB-PM-IC and BB-IC. Columns containing information on the area size were added for both layers. Joining the area field of BB-IC to the attribute table of BB-PM-IC allowed for adding the ratio-column, in which the area of protected monuments in the inner city was divided by the area of relevant buildings in the inner city.
Lastly, for calculating the ratio for each of the over 15,000 blocks, BB-PM-BL-inter and BB-BL-inter were dissolved by the block numbers, resulting in one attribute per block in both attribute tables of the respective layers. Those were enlarged by information on the area size and the area field of BB-PM-BL-inter was joined to the attribute table of BB-BL-inter by the column 'block number'. Lastly, adding the ratio-column, as done before at both city scales, allows to visualize different levels of protected monument ratios on block level.
For an easier determination of particularly threatened areas, where both, shares of monument protection and the potential heat stress risk are high, the final block-level map was overlaid by the potential heat-stress risk map of Dugord et al. (2014).
To develop this map, Dugord et al. (2014) investigated the influence of land-use patterns on the land surface temperature and the location and concentration of vulnerable inhabitants (< 6 years old or > 65). Both factors were rated from 0 to 3 and then combined by multiplying them, leading to categorization of seven potential risk classes (Table 1).
To enable the extraction of different combinations of potential heat stress risk classes and monument protection ratios, heat stress risk classes were aggregated to three levels (0-2 = no -extremely low potential risk; 3-4 = low 97,120,874 m 2 built-up areas in Berlin, according to the analysis of this study. Highest shares occur in the districts of Friedrichshain-Kreuzberg, Charlottenburg-Wilmersdorf and Mitte. This is in line with the finding, that in the inner city, which is limited by the circular railway, 25.4 % of the buildings are listed monuments. Ratios of buildings that are under monument protection differ widely over all city blocks, starting at 0 % and rising up to 100 % (see Table 2; Fig. 1). There are 1,125 blocks where (almost) no greenery is possible due to protection ratios of 75 % and above (Table 2).
According to Dugord et al. (2014), there are 168 blocks with high to extremely high potential heat stress risk.
In 13 blocks inhabited by 5,026 people, high or extremely high potential heat stress meets a coverage 75 to 100 % of monument protected buildings. When looking at all levels above 50 % monument protected buildings and above a very low potential heat stress risk, 102 blocks with 48,122 inhabitants are determined. If all the blocks in Table 3 are added up, the number is smaller than the number of blocks in Table 2. This happens, if blocks are categorized in the monument protection ratio analysis of this paper, but not in Dugords' heat stress risk assessment. With the intersection geoprocessing command, blocks that exist in just one of the two layers disappear. Same applies for the intersection of the remaining blocks with the population density layer for blocks without inhabitants, leading to a smaller overall number of 13,928 blocks in further processed tables and maps.
The combined maps of monument protection ratios and potential heat stress levels can be seen in Figs. 2 and 3.
Most of the blocks that combine one of the two highest levels of potential heat stress and high monument protection ratios are located within the inner-city area that is limited by the circular railway. For those blocks, exceptions from monument protection, justified by a predominant public interest, could be demanded.
In the last step, the new layers were, again, intersected with a layer containing the population density. Therefore, a field with the area size was added to the attribute tables and multiplied with the number of inhabitants per area , that is part of the information in the population density layer. This had to be summed up to give the number of blocks and inhabitants for each possible combination of heat stress risk and monument protection ratios.

Results
Over the entire city of Berlin, a total of 16.2 % of the buildings are under monument protection. This corresponds to a protected built-up area of 15,737,721 m 2 out of the overall Table 1 Potential risk classes according to Dugord et al. (2014) Potential heat stress risk class Factors Extremely high potential risk 9 High potential risk 6 Medium potential risk 4 Low potential risk 3 Extremely low potential risk 2 Negligible potential risk 1 Residential uses without potential risk 0  According to the lower monument protection authority, monuments can have historical, artistic, scientific or urbanistic relevance (BA Neukölln 2019). It is debatable, whether those factors outweigh the missed positive effects of VGS. As stated before, a predominant public interest is one of the reasons to grant permission for altering protected façades, in this case for greening respective façades.
Ultimately, the outcome of the combined maps on potential heat stress risk (Dugord et al. 2014) and monument protection ratios offers the possibility of identifying priority areas for climate adaptation and mitigation measures. In all 168 blocks with high and extremely high potential heat stress, greening building façades should be part of heat mitigation strategies. This also applies when monument protection would prohibit such measures, which it does for 16 of those blocks (50-100 % monument protected buildings). In those cases, it should be possible to refer to the monument protection law of Berlin, i.e., to claim a predominant public interest, which is valued over monument protection matters. One of the building types frequently under monument protection is the 'Dense block development, rear courtyard (1870s-1918), 5-6-storey' emerged during the Wilhelminian period . According to Hoffmann et al. (2021) these buildings could benefit from façade greenings as well,

Discussion
Results regarding the ratios of monument protected buildings have to be contextualized and discussed on different scales.
The identified ratio of 16.2 % monument protected buildings on city-scale cannot be seen as a major limiting factor to greening façades on a large scale in Berlin. Likewise, the ratio on inner-city-scale of 25.4 % is not to be seen as a primary factor when searching for reasons that prevent from large scale VGS implementation. This assessment is supported by several other difficulties that affect VGS installation, like, e.g., water availability. Increasing water demand in cities leads to a call "towards a new water reuse paradigm, by integrating circular economy […] principles into urban water management" (Pearlmutter et al. 2021, p. 3). Therefore, rainwater and greywater should be used for VGS irrigation instead of drinking water. Pearlmutter et al. (2021) estimated the ratio of buildings that could be greened and maintained, when using only rainwater (= 13 %) or an optimized rainwater-greywater system (= 64%). Accordingly, even with the optimized system, there would still be 11 % of the buildings that cannot be greened due to water availability, before reaching monument protection as limiting factor.
On block-scale the situation differs widely. There are 1,125 blocks in Berlin, where almost no VGS is possible due to 75-100 % monument protected buildings. In those blocks, none of the positive effects of VGS, like air quality improvements (Pugh et al. 2012), indoor noise reduction (Pérez et al. 2016) or especially the mitigation of heat stress Fig. 3 Combined map of potential heat stress risk and protected monument ratio on block-level in Berlin, Germany, zoomed to relevant extent there even are positive synergy-effects when implementing VGS at protected buildings. Since VGS protect façades from solar radiation, rain and wind, thus extending their lifespans (Pfoser 2016), VGS installation could also be seen as a positive aspect for protected monuments. Naturally, the chance of free-climbing plants to damage monument façades needs to be assessed from case-to-case to prevent possible destructions. If no concerns on this matter are found, one could argue that VGS might contribute to monument preservation and draw extra attention on the monument protected buildings. Regular and professional maintenance would most certainly be necessary to prevent plants from covering entire façades and completely blocking the view on the monuments. A dialogue between preservationists and ecologists needs to be held, to discuss whether VGS can also be seen as an enriching measure from a monument protection point of view.
When talking about VGS and its public benefits, it needs to be taken into account, that so-called green gentrification can be a major problem. Perini and Rosasco (2013) found potential increase in property values when VGS are installed, which might force tenants to leave their homes due to rising rents. This underlines the need for political regulation that needs to go hand in hand with such greening measures, but most certainly should not hinder VGS implementation.
Finally, regarding the methodology of this paper, it needs to be said that the elimination of urban greenspaces might not be sufficient in providing a final map that only contains relevant blocks. Areas like Teufelsberg or Tegel Airport which stand out from Fig. 1 due to their immense size could have been eliminated by hand. Here it was decided not to do so to ensure the repeatability of the procedure, which leads to slightly misleading first impressions when looking at Fig. 1. Also, the approach on analyzing the ratio of protected buildings and protected façades, respectively, must not be seen as airtight or fully reflecting the actual façade ratio that is under monument protection. A two-dimensional observation, that is looking at building areas cannot implicate height of buildings nor their orientation, and so cannot deliver fully accurate results. It must be kept in mind that this research provides numbers for areas, covered by protected buildings, which suggest ratios of protected façades. Nevertheless, the calculations can give an idea about the share and the extent to which monument protection limits implementation of VGS and the identification of blocks that need to be greened anyway.
but are not the most susceptible for the formation of summertime indoor nightly heat stress under current climatic conditions. From the authors' point of view, the mitigation of (morbidity-and mortality-raising) heat stress (Oudin Åström et al. 2011), for potentially particularly threatened city-blocks, is a clear example of a predominant public interest.
The selection of the potential heat stress risk map to create the combined map was made for this work to include both, the underlying heat hazard derived from land use patterns and the vulnerability of the population. This allows for identifying blocks in need of particularly timely change.
On the other hand, the focus could also have been on the map of the impact of land use on the heat hazard, which is more constant over time than the distribution of vulnerable people, when categorized just by the inhabitants' age and the population density as it was done by Dugord et al. (2014). Other aspects contributing to heat vulnerability, like socio-economic factors, should be considered in future research on this topic.
The monument protection law also states that advertising structures covering protected façades for a maximum of six months and whose content primarily reflects public interest, are not to be seen as in rivalry to the respective monument (DSchG Bln. 1995) 2 . Deciduous climbing plants, like woodbine, have a growing season of around 6 months, where their leaves cover the façade. Accordingly, it is questionable whether advertising structures should be favored by law, due to no further public benefits opposite to VGS.
Also, monument protection law allows to rebuild monuments in a way that is similar to their historical appearance, meaning that historically greened façades can be 'regreened', if proof is found of the historical VGS.
Additionally to the heat stress risk analysis in terms of morbidity and mortality by Dugord et al. (2014), a next research field can be working productivity loss due to heat stress (Lundgren et al. 2013). Identifying monetary loss due to reduced productivity can help arguing for heat stress mitigation measures, like VGS, especially, if the lost value is bigger than costs of VGS implementation. Identification of heat stress vulnerable blocks in terms of productivity loss at the workplace, as well as threshold temperature values, from which productivity declines, should be the first steps in research on this topic.
Generally, it is questionable whether VGS implementation and monument protection have to be conflicting, or if not applicable.
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Conclusion
This study provides an overview on shares of monument protected building areas in Berlin, that are, due to their protected status, not suitable for VGS, without permission by the respective authority. Additionally, monument protection ratios have been calculated on block-level and were overlaid with the potential heat stress risk assessment by Dugord et al. (2014). Thereby, blocks have been identified, that combine high potential heat stress risk and high shares of monument protected buildings. Generally, monument protection does not hinder VGS from being installed on a larger scale, due to the overall small percentage of buildings under monument protection in the entire city (16.20 %) and the inner-city (25.42 %). Nevertheless, VGS implementation, as measure to mitigate indoor heat stress, should be granted by the monument protection authorities for the identified blocks that combine high potential heat stress risk and high shares of protected buildings. It can be argued with a predominant public interest of not being exposed to heat stress, that overrules monument protection matters.

Authors' contributions
All authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by Emil Johann Roesch. The first draft of the manuscript was written by Emil Johann Roesch and all authors commented on and edited previous versions of the manuscript. Thomas Nehls revised the manuscript. All authors read and approved the final manuscript.
Funding This research was conducted as part of the project Urban Vertical Green 2.0: Vertical greening for liveable cities -co-create innovation for the breakthrough of an old concept, which has been financially supported by the German Federal Ministry of Education and Research BMBF (FKZ: 01LF1803A). The project is part of the Joint Programming Initiative Urban Europe, within the call The Sustainable Urbanisation Global Initiative (SUGI) Food-Water-Energy Nexus; shortened ERA-NET Cofund SUGI. This research was also funded as part of the project "climate protection through educational vertical green (FABIKLI) which has been financially supported by the German Federal Ministry for the Environment, Nature Conservation, Nuclear Safety and Consumer Protection BMUV (FKZ: 03KF0149B). Open Access funding enabled and organized by Projekt DEAL.

Data availability Available upon request.
Code availability Available upon request.

Conflicts of interest/Competing interests
The authors have no relevant financial or non-financial interests to disclose.
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