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Data from: Norway and Sweden Green Roof (GF) plant data

Citation

Lönnqvist, Joel; Hanslin, Hans Martin; Johannessen, Birgitte Gisvold (2020), Data from: Norway and Sweden Green Roof (GF) plant data, Dryad, Dataset, https://doi.org/10.5061/dryad.2547d7wng

Abstract

Standard succulent vegetation mixes developed mostly in temperate climates are being increasingly used on green roofs in different climate zones with uncertain outcome regarding vegetation survival and cover. We investigated vegetation on green roofs at nine temperate, cold and/or wet locations in Norway and Sweden covering wide ranges of latitude, mean annual temperature, annual precipitation, frequencies of freeze-thaw cycles and longest annual dry period. The vegetation on the roofs were surveyed in two consecutive years, and weather data were compiled from meteorological databases. At all sites we detected a significant decline in species compared to originally intended (planted/sown) species. Both the survival rate and cover of the intended vegetation were positively related to the mean annual temperature. Contrary to a hypothesis, we found that intended vegetation cover was negatively rather than positively related to mean annual precipitation. Conversely, the unintended (spontaneous) vegetation was favoured by high mean annual precipitation, and low mean annual temperature, possibly by enabling it to colonise bare patches and outcompete the intended vegetation. When there is high mortality and variation in cover of the intended vegetation, predicting the strength of ecosystem services the vegetation provides on green roofs is difficult. The results highlight the needs for further investigation on species traits and the local factors driving extinction and colonisations in order to improve survivability and ensure a dense vegetation throughout the successional stages of a green roof.

Methods

Survey methods

We monitored vegetation cover and species presence/absence in permanent 1 x 1 m quadrats placed in transects with evenly spaced quadrats reflecting the surface area of the roof and avoiding edge zones and shaded areas. The total intended vegetation cover, total unintended vegetation cover, total moss cover and total bare vegetation cover as well as the percentage cover of each individual intended vascular plant species was recorded in each quadrat. Each quadrat was divided into smaller 0.1 x 0.1 m squares, corresponding to 1% cover, to facilitate estimation of plant cover. The exact locations of quadrats were recorded so that successional changes at the same spots could be monitored by surveys in consecutive years. Due to the difficulties of differentiating Phedimus hybridus from P. kamtschaticus when not in flower the two species were merged and are referred to as P. coll hereafter.

 

Weather variables

Weather time series for the years prior to the surveys were collected from the Norwegian Meteorological Institute (MET Norway) and Swedish Meteorological and Hydrological Institute (SMHI) as daily averages for precipitation and 6 h averages for temperature. To acquire complete full time series of data for all sites, weather data from several weather stations in the same town were merged when data were incomplete. All weather variables were compiled from July 15 until the same date of the following year when surveys were conducted. Since relevant weather indices are often highly correlated (Johannessen et al. 2017), we selected a subset to represent major gradients. Freeze-thaw cycles were defined as all changes between negative and positive temperatures recorded with six-hour resolution, irrespective of snow cover since such data were incomplete. The duration of the longest drought episode was defined as the longest consecutive sequence of days without precipitation (recorded in days), and mean annual temperature as the mean temperature recorded between July 15 and July 15 the following year. Total precipitation was the total precipitation during the time from July 15 in the preceding year until July 15 in the year of the survey. Samples from 2016 and 2017 were treated as separate replicates to account for the variability in weather and vegetation between the years.

Funding

Svenska Forskningsrådet Formas, Award: 2014-00854

Miljødirektoratet