Amy Heim

Leaf and Life History Traits Predict Plant Growth in a Green Roof Ecosystem

Green roof ecosystems are constructed to provide services such as stormwater retention and urban temperature reductions. Green roofs with shallow growing media represent stressful conditions for plant survival, thus plants that survive and grow are important for maximizing economic and ecological benefits. While field trials are essential for selecting appropriate green roof plants, we wanted to determine whether plant leaf traits could predict changes in abundance (growth) to provide a more general framework for plant selection. We quantified leaf traits and derived life-history traits (Grime’s C-S-R strategies) for 13 species used in a four-year green roof experiment involving five plant life forms. Changes in canopy density in monocultures and mixtures containing one to five life forms were determined and related to plant traits using multiple regression. We expected traits related to stress-tolerance would characterize the species that best grew in this relatively harsh setting. While all species survived to the end of the experiment, canopy species diversity in mixture treatments was usually much lower than originally planted. Most species grew slower in mixture compared to monoculture, suggesting that interspecific competition reduced canopy diversity. Species dominant in mixture treatments tended to be fast-growing ruderals and included both native and non-native species. Specific leaf area was a consistently strong predictor of final biomass and the change in abundance in both monoculture and mixture treatments. Some species in contrasting life-form groups showed compensatory dynamics, suggesting that life-form mixtures can maximize resilience of cover and biomass in the face of environmental fluctuations. This study confirms that plant traits can be used to predict growth performance in green roof ecosystems. While rapid canopy growth is desirable for green roofs, maintenance of species diversity may require engineering of conditions that favor less aggressive species.

The impact of mosses on the growth of neighbouring vascular plants, substrate temperature and evapotranspiration on an extensive green roof

Currently the majority of vegetation used on shallow extensive green roofs are species of Sedum, which are able to survive in the harsh green roof environment. While mosses frequently colonize green roofs in Europe, intentional planting of mosses on green roofs is less common, especially in North America. Mosses may contribute to the ecosystem services provided by green roofs, and may act as facilitators of vascular plants. This study examined the effect of three different moss species on soil temperature, water loss rates and the growth of neighbouring vascular plant species. Overall, the presence of mosses in this experiment impacted the neighbour species differently, suggesting that mosses are best used in particular species combinations. One species of grass showed a net benefit of moss neighbours, suggesting that facilitation may be operating. Mosses reduced soil temperature relative to bare substrates; net evapotranspiration of green roof modules planted with mosses varied depending on the identity of moss and neighbour species.

Spatial environmental heterogeneity affects plant growth and thermal performance on a green roof.

Green roofs provide ecosystem services, including stormwater retention and reductions in heat transfer through the roof. Microclimates, as well as designed features of green roofs, such as substrate and vegetation, affect the magnitude of these services. Many green roofs are partially shaded by surrounding buildings, but the effects of this within-roof spatial environmental heterogeneity on thermal performance and other ecosystem services have not been examined. We quantified the effects of spatial heterogeneity in solar radiation, substrate depth and other variables affected by these drivers on vegetation and ecosystem services in an extensive green roof. Spatial heterogeneity in substrate depth and insolation were correlated with differential growth, survival and flowering in two focal plant species. These effects were likely driven by the resulting spatial heterogeneity in substrate temperature and moisture content. Thermal performance (indicated by heat flux and substrate temperature) was influenced by spatial heterogeneity in vegetation cover and substrate depth. Areas with less insolation were cooler in summer and had greater substrate moisture, leading to more favorable conditions for plant growth and survival. Spatial variation in substrate moisture (7%-26% volumetric moisture content) and temperature (21°C-36°C) during hot sunny conditions in summer could cause large differences in stormwater retention and heat flux within a single green roof. Shaded areas promote smaller heat fluxes through the roof, leading to energy savings, but lower evapotranspiration in these areas should reduce stormwater retention capacity. Spatial heterogeneity can thus result in trade-offs between different ecosystem services. The effects of these spatial heterogeneities are likely widespread in green roofs. Structures that provide shelter from sun and wind may be productively utilized to design higher functioning green roofs and increase biodiversity by providing habitat heterogeneity.

Preserving plant diversity on extensive green roofs – theory to practice

The growing phenomenon of green roofs throughout the world mostly results in the establishment of extensive green roofs with low species diversity. However, research from the last few decades has shown that several advantages can accrue from diverse ecological systems, such as increased faunal diversity, storm water retention, thermal stability, pollution mitigation, and visual appeal. The maintenance of diversity on extensive green roofs has not been closely examined and few studies incorporate methods to ensure long-term coexistence into green roof design. Theoretical work has placed much focus on the niche and neutral theories that attempt to explain diversity as a result of the existence of different habitats or of demographic and dispersal patterns, respectively. These theories have resulted in several theoretical and practical recommendations for the maintenance of diversity in ecological systems and could suggest additional practices that would support biodiverse green roofs. We hereby review and d...

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

Green roofs are constructed ecosystems that provide ecosystem services in urban environments. Shallow substrate green roofs subject the vegetation layer to desiccation and other environmental extremes, so researchers have evaluated a variety of stress-tolerant vegetation types for green roof applications. Lichens can be found in most terrestrial habitats. They are able to survive extremely harsh conditions, including frequent cycles of desiccation and rehydration, nutrient-poor soil, fluctuating temperatures, and high UV intensities. Extensive green roofs (substrate depth <20cm) exhibit these harsh conditions, making lichens possible candidates for incorporation into the vegetation layer on extensive green roofs. In a modular green roof system, we tested the effect of Cladonia lichens on substrate temperature, water loss, and albedo compared to a substrate-only control. Overall, the Cladonia modules had significantly cooler substrate temperatures during the summer and significantly warmer temperatures during the fall. Additionally, the Cladonia modules lost significantly less water than the substrate-only control. This implies that they may be able to benefit neighboring vascular plant species by reducing water loss and maintaining favorable substrate temperatures.