Cladonia lichens on extensive green roofs: evapotranspiration, substrate temperature, and albedo

Introduction

Green roofs are constructed ecosystems, designed to provide ecosystem services such as the reduction of heat flux through the roof, the capture of storm water, and the provision of habitat for animals^1,2. Green roofs consist of a vegetation and growing medium layer (substrate) over engineered layers that provide a root barrier, drainage, and/or water retention layers¹. The majority of green roofs constructed in temperate climates are “extensive” green roofs, characterized by shallow growing media (<20cm), which minimizes the weight added to the building. Shallow growing media in such systems have led to a reliance on succulent plant species in the vegetation layer, to ensure survival during drought conditions. Other plant growth forms have also been used on extensive green roofs, with grasses, forbs, and mosses among the most frequent^3,4. Lichens are often found on conventional roof surfaces⁵ and form an important component of cryptogamic crusts in terrestrial ecosystems⁶. Cryptogamic crusts in arid environments perform a range of ecosystem services, such as stabilizing soil, fixing nitrogen, and enhancing soil water holding capacity⁷, but have not been intentionally planted in green roof ecosystems.

Lichens are symbiotic organisms, an association between a fungus (the mycobiont) and one or more algal and/or cyanobacterial photobionts^8,9. Typically the mycobiont forms 95% of the lichen body. They are the dominant plant life in harsh environments such as the Arctic, Antarctic, mountains, and dry land crusts⁸. Lichen species are widespread and can be found from the Arctic to deserts; they can survive frequent cycles of desiccation and rehydration, nutrient-poor soil, fluctuating temperatures, and UV light intensities^8,10. They can survive and grow on the bare surface of rocks and in poor soils such as heathlands, peat lands, sand dunes, and toxic spoil heaps¹⁰. Selection of vegetation types appropriate for extensive green roofs often involves identifying local habitats that have characteristics in common with green roofs (shallow soil, harsh abiotic conditions)¹¹; lichens are a common component of rock barrens, dunes, and heathland habitats that can be a source of plants for green roofing projects^12,13.

Lichens are lightweight and can be found growing naturally on bare tile or slate rooftops¹. This could make them a candidate for roofs on buildings with low weight-loading capabilities. Species of the genus Cladonia, large fruticose lichens that colonize bare soils, are common in cold temperate climates. These lichens produce bundles of hyphae that stabilize the soil and add organic matter. The light color of the lichen can reflect solar radiation, keeping the soil cool and moist¹⁴. These characteristics indicate the possible utility of Cladonia as a component of extensive green roofs. In this study, the performance of key green roof ecosystem functions in green roof modules planted with live Cladonia lichens was compared with substrate-only controls.

Methods

The study site was located on the east side of the roof of the five-story Atrium building at Saint Mary’s University in Halifax, Nova Scotia, Canada (44°39′N, 63°35′W ) (Figure 1). Data on soil temperature and water loss were collected between July and October 2012. Albedo was measured in September 2013. During the 2012 study period, the weather station on the adjacent green roof testing facility (~50m away from the study site) recorded the minimum monthly temperature as 6.7–20.7°C and the monthly maximum as 12–30°C. The monthly precipitation recorded from the green roof weather station averaged between 1.7 and 11.59mm. Albedo was collected on September 28, 2013 (average temperature: 14.12°C, total precipitation: 0mm).

Figure 1. Placement of lichen trial on the east side of the roof of the five-story Atrium building at Saint Mary’s University in Halifax, Nova Scotia, Canada (44°39′N, 63°35′W).

In order to quantify the influence of Cladonia on green roof substrate, a trial was set up to determine the effects of Cladonia on soil temperature, water loss, and albedo. Ten green roof modules were placed on the Atrium roof on top of the roof surface, which was made up of grey concrete pavers. Each module had a length and width of 36cm, with a freely draining base (Polyflat^®, Stuewe & Sons Inc., Oregon, USA). Modules contained a root barrier/water retention fleece (length and width 36cm) over the base (EnkaRetain and Drain 3111^®, Colbond Inc., North Carolina, USA) with 6cm depth of Sopraflor X substrate, purchased in 2011 (Soprema Inc., Drummondville, QC, Canada), over the root barrier/water retention layer. A soil test describing the composition of the Soprema X substrate can be seen in Supplementary Table 1. This experiment consisted of two substrate-only controls and eight modules covered 100% in Cladonia lichen approximately 6cm thick (Figure 2). The lichen was collected from a coastal barrens site (Chebucto Head (44°30′N, 63°31′W)) in May 2012 and placed on the surface of the substrate. A mix of two lichen species, C. terranova and C. boryi, was used; both species have similar colors and growth forms, and co-occur in lichen mats. Lichens were alive when transplanted and shoot tips were marked to determine incremental growth over the experiment.

Figure 2. A Cladonia module used in the lichen trial.

Each module had a length and width of 36cm with a substrate depth of 6cm.

Results

The albedo of the Cladonia modules was significantly higher than the control modules (Table 1, Data Set 1). Substrate temperatures for July and August were significantly lower in the Cladonia modules compared to the controls (Table 1). In September, there was no significant difference in soil temperature between the Cladonia modules and the controls. In October, the Cladonia modules had significantly higher substrate temperatures than the controls (Table 1, Data Set 2). Regarding water loss, there was no significant difference between the Cladonia modules and the control for August, but in September and October, Cladonia modules lost significantly less water than the controls (Table 1, Data Set 3).

Discussion

Compared to the substrate-only controls, the Cladonia modules were cooler during hot conditions and retained more moisture in the substrate. These cooler temperatures may have been a result of shading and higher albedo. Even though albedo was only measured in 2013, the Cladonia lichen always had a lighter coloring than the bare substrate. Lighter colors are associated with higher albedo. Likewise, the lichen cover resulted in less net evapotranspiration compared with the bare substrate treatment. Interestingly, the soil temperature during October (average air temperature on date of measurement: 17.9°C) was significantly warmer in the Cladonia modules compared to the controls. This implies that a Cladonia mat may help reduce heat loss during the winter and help cool the roof during the summer.

Plants surrounded by Cladonia may benefit from such temperature regulation (cooler temperatures in the summer, warmer temperatures in the winter) and greater water availability. Other studies have shown that plants with mat-forming growth can benefit less drought-tolerant species¹⁶. In addition to this, Cladonia lichens do not appear detrimental to plant growth. Vascular plant species growing out of these lichen mats is a natural occurrence on the coastal barrens of Nova Scotia and, during the trial, seedlings of trees and grasses (carried in by the wind or by birds) were observed growing out of these modules, although control modules also had such seedling growth. Some lichen species also play a key role in soil development in conditions of high abiotic stress^8,17. If lichen inclusion on a green roof could improve the substrate properties for neighboring species, the entire system would benefit.

In this study, while the lichens remained in the roof modules for the duration of the experiment, it is not clear that the lichens remained alive. Fruticose lichens grow an average of 1.5–10mm per year¹⁴. However, the relative growth rate of transplanted Cladonia mats compared to established Cladonia mats can besignificantly lower¹⁸. The growth of Cladonia in these modules was undetectable. Additionally, during the 2012 growing season the Cladonia lichen did not suffer from wind damage and appeared equivalent in size as to when initially established. However, during the 2013 growing season the Cladonia lichen mats fragmented, decreasing in size (personal observation). This fragmentation led to wind damage that only occurred during the 2013 growing season. Since green roofs can be exposed to high winds, unattached lichen could decrease any value the lichen may have to offer. This fragmentation could have been due to a number of factors such as incompatible soil chemistry, wind damage, or winter damage. While the fragmentation is undesirable aesthetically and functionally, it is possible that such fragments could colonize other parts of the green roof, should they be deposited in suitable areas.

More research is necessary to determine whether this method of transplanting lichens onto green roof substrates results in viable lichen populations. Further, our transplanting method involved sourcing lichens from the wild. While these species are very common in the region, this kind of collection would clearly not be feasible on an industrial scale. Additionally, the propagation of this genus is slow. Expansion of existing colonies can occur through the fragmentation of the thallus and subsequent growth of branches that fall to the ground, vertical growth from the base, and apical growth leading to larger clumps¹⁹.

Alternatives to Cladonia lichens providing high albedo and lack of competition with plant roots could include light-coloured gravel, stones, or woody debris, however more research is needed to determine the effect these materials would have on the green roof system. Finally, while green roofs are becoming more popular in all regions, lichens, in varying degrees, are sensitive to air pollution and many species are likely to perform poorly where air quality is low¹⁴. Our study establishes that Cladonia lichens could provide roof cooling and water retention services in a green roof environment, but more work is necessary to explore the long-term viability of fruticose lichens in these systems.