*3.2. Phytomass in Relation to the Type of Media and the Media Depth*

**Test 4:** focused on the phytomass on various growing media depths. The question is whether a greater growing media depth supports a higher biomass with better performance, e.g., in regard to CO2 fixation.

#### 3.2.1. The 10-cm Plots

In 2017, phytomass was harvested from all representative parts from each of the different roof orientations and exposures, with three replicates. Table 7 shows the dry matter values for the fractional biomass analysis from the vascular higher plants, mosses, and lichens. The total carbon is the sum of the shoots and roots together. All areas were fully developed over more than 10 years. One visible difference was the spontaneous growth of mosses and lichens, which represent a very special type of extensive green roof. These roofs are located in a region in northeastern Germany with clean air, and the composition of the vegetation on these roofs resembles that of typical poor sandy ground conditions, as is typical in the surrounding Müritz National Park. These dry-adapted lichens, mostly from the group of Cladonia, are endangered plants and are seldom found in ground-level habitats. The green roof is a retreat area for these plants. They are susceptible to foot traffic on dry roof conditions in summer. All values in Table 7 were calculated for an area of 1 m2. In the calculation, the shoot and root phytomass of the vascular plants were calculated. The annual growth rate decreased after the establishment of full coverage with vegetation.

**Table 7.** Examples of the harvested dry phytomass in g/m<sup>2</sup> with SD from various media with 10-cm depth. Calculation for 1 m2; analysis based on the mean of 3 harvesting plots each.


<sup>1</sup> Zi, Blä, Opti = on building 2, est. 1999, Op, Ulo: = on building 2, est. 2001. <sup>3</sup> Total dry organic matter in plants. Data from further plots are available.

> Table 8 shows the correlation between vascular plant phytomass and its importance for CO2 fixation. Broadly speaking, the more phytomass, the greater the effects, whereas mosses and lichens are counter-productive in that they suppress the growth of endangered higher plant species in sites with low-nutrient media while only making a minimal contribution to CO2 fixation.

**Table 8.** Correlation and significance of harvested plants, n = 27 samples.


The dependence of CO2 fixation on the phytomass follows a regression line as presented in Figure 5. More plants cover means higher CO2-fixation in general.

**Figure 5.** Regression—CO2 fixation dependent on the phytomass of the higher plants from the 27 samples. All numbers are in grams.

The better growth rate on the 10-cm media is related to the different growing media used in these tests. Figure 6 shows the median values for CO2 fixed by the plants, which varies considerably, with the lowest values of about 2000 g/m<sup>2</sup> on the Ulo media compared to average values of about 6000 g/m2. In isolated cases, values that are twice as high are possible. If CO2 fixation is to be the primary aim of the roof, careful choice of the media plus fertilizer is one solution. Calculating based on the approximately 8.5 million m<sup>2</sup> new green roofs realized in Germany [13], this will result in carbon storage of between 17,000 and 51,000 t CO2/year after a few years.

**Figure 6.** Comparison of the CO2 fixation by the total phytomass (shoots and roots) of the vascular higher plants in g/m2 on the nine different growing media, 10-cm layer, 3 replicates each.
