Vegetation cover and plant diversity on cold climate green roofs

During the vegetation survey, the following variables were recorded. When referring to their use as modelling parameters (and for convenience here) these variables are italicized: Species richness (number of vascular plant species detected on a roof section) Cover of individual vascular plant species in quadrats (%) Total vascular plant cover (%) Total moss cover (%) Substrate depth (mm) Age (years) Roof area (m2) Roof width (m) Roof length (m) Aspect (in cardinal direction: North, East, South or West) Stratified random sampling was used to estimate the vegetation cover on the selected roof sections. For this, sampling quadrats (each consisting of a 1×1 m wooden frame) were evenly spread along transects on each roof section, avoiding edge zones such as ventilation shafts. The number of quadrats used on each section (3-54) was limited by the size of roof sections. The quadrats covered a minimum of 6% of the roof section surfaces and vegetation cover was estimated visually with grids in each quadrat. The buildings’ owners or suppliers of the green roofs provided the age of the roof and the substrate depth was measured in each sampling quadrat as an average of three insertions using a 3 mm diameter metal rod. Total vascular plant and total moss cover were recorded separately as growing in separate layers, so their combined cover in a sampling quadrat could potentially exceed 100%. Vascular plant species were determined to species level when possible, using floras (Mossberg and Stenberg, 2010; Krok et al., 2013), but in exceptional cases only to genus (e.g. Taraxacum). The percent cover of each detected vascular plant species was recorded in each sampling quadrat and mean values were calculated for each roof section. Species represented by small single specimens were assigned a minimum of 0.1% cover. All vascular plant species found outside the sampling quadrats were also recorded, during a further limited time spent searching for rare species on each roof section after scrutinizing the quadrats. The species found outside of quadrats were not assigned any percent cover values, but were included in the list of species, and thus contributed to the recorded species richness, in each section.