Impact of genetic relatedness and functional diversity on green roof community performance
In recent years, phylogenetic information has increasingly been used to shed light on the drivers of community assemblage such as competition and facilitation. This concept, commonly studied in natural terrestrial plant communities, is largely unexplored in green or vegetated roofs. Green roofs differ from natural systems in that their species assemblages are predetermined and they are designed for a specific function. However, genetic and phenotypic diversity can impact species interactions in this novel context. My study examined green roofs as ecological communities by comparing genetic relatedness and levels of functional diversity among nine Sedum species, a plant commonly used to construct green roofs. These nine species were divided into three functional groups based on phenotypic traits, and planted in a simulated green roof. Replicate trays were manipulated for number of functional groups, ranging from monoculture, two, or three groups per tray. Four chloroplast intergenic regions were used to estimate a phylogeny and measure phylogenetic relatedness among species. I hypothesized that greater functional diversity, and lower genetic relatedness would positively impact survival and biomass and thus improve the ability of the roof to perform its designed functions such as stormwater retention.
By measuring phylogenetic relatedness as well as survival, biomass, and water retention capabilities, I can infer the impact that diversity and relatedness will have on green roofs as an engineered plant community. Initial results from chloroplast intergenic region sequences in the nine species show that there are genetic differences among species at multiple loci. Thus, metrics of genetic relatedness such as the Net Relatedness Index (NRI), calculated using mean phylogenetic distance can serve as a useful measure of diversity even at the within-genus level. Additionally, preliminary measurements of green roof performance (survival, biomass, etc.), when seasonally applicable, suggest that trays with higher functional diversity exhibit a greater degree of survival and greater biomass than monoculture trays. Further analysis is needed to determine whether genetic relatedness correlates with functional diversity, as well as how these variables impact roof community performance. Explicit quantification of genetic relatedness as well as traditional focus on functional diversity in engineered green roof communities can serve to better inform the design and intentional species assemblages of the roofs, and thus improve the ecological and environmental health of urban areas.