DOI: 10.5061/dryad.bvq83bk4w
title: Vegetation cover and plant diversity on cold climate green roofs

description: Both vegetation abundances and community compositions play important roles
 for the functions of green roofs (e.g. stormwater retention, habitat
 provision, aesthetic appearance). However, green roof vegetation can
 change significantly over time, which may consequently affect the
 functions related to them. This study investigated vascular plant covers
 and species compositions on 41 roof sections located in Sweden’s subarctic
 and continental climate zones. For the roof sections with a known
 originally intended vascular plant composition (n=32), on average 24±9% of
 the intended species were present in surveys while unintended species made
 up 69±3% of the the species found. The Intended species dominated plant
 cover (93±3%) and Sedum acre (58±36% cover) was the most commonly found
 species. As revealed in previous studies, substrate depth had a positive
 relationship with plant cover and species richness. The vascular plant
 cover of the roofs in this study was not related to species richness as
 hypothesized but instead had a significant negative correlation with moss
 cover. The results in this study emphasize the importance of substrate
 depth for both plant abundance and species diversity, and that even in a
 cold climate, colonising unintended species can have a great contribution
 to the species richness of green roofs. However, since most colonising
 species formed sparse cover on the roofs, their potential benefit to green
 roof functions that benefit from a dense vegetation cover (e.g. stormwater
 management and thermal performance) could be limited while the intended
 vegetation performs these functions more effectively.
description: During the vegetation survey, the following variables were
 recorded. When referring to their use as modelling parameters (and for
 convenience here) these variables are
 <i>italicized</i>: 
 <i>Species richness</i> (number of vascular plant species
 detected on a roof section) Cover of individual
 vascular plant species in quadrats (%) Total
 <i>vascular plant cover</i> (%) Total
 <i>moss cover</i> (%) <i>Substrate
 depth</i> (mm) <i>Age</i>
 (years) <i>Roof area
 </i>(m<sup>2</sup>) <i>Roof
 width </i>(m) <i>Roof length
 </i>(m) <i>Aspect </i>(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 <i>age</i> of the roof and the <i>substrate
 depth</i> was measured in each sampling quadrat as an average of
 three insertions using a 3 mm diameter metal rod. Total <i>vascular
 plant</i> and total <i>moss cover</i> 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 <i>et al.</i>, 2013), but in exceptional
 cases only to genus (e.g. <i>Taraxacum</i>). 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
 <i>species richness</i>, in each section.