Prairie glades and barrens as ecological models for living roof systems: A case study

Background/Question/Methods

Green roofs are an important feature in urban cities for ecosystem services such as storm water management, urban wildscape integration, and reduction of urban heat island effect. Green roofs in the southwest US, however, require additional design specifications (e.g., drought resistance, extreme temperature tolerance) that northern US green roofs do not typically accommodate. Our proposed solution to this problem is the use of native southern plants from local growers that are naturally adapted to the required conditions. Specifically, we used North Central Texas prairie glades and barrens as a green roof model for a 20,000-square foot rooftop in Fort Worth, Texas.

Results from a pilot project helped determine the specific plant palette and soil/media mix for the roof system, including the use of native soil inoculated with beneficial bacteria and a healthy seed bank. Following planting and establishment, roof performance was assessed via monthly vegetation surveys and various climate parameters including temperature, humidity, rainfall, UV radiation, and evapotranspiration. Climate data from the green roof were compared to data gathered from a nearby in situ prairie and a standard reflective roof. 

Results/Conclusions

Data over a five month period show that, similar to the in situ prairie, the green roof is significantly cooler during the day than a standard reflective roof. At night, however, the green roof retains heat gathered during the day and exhibits warmer nighttime temperatures than both the standard roof and the prairie. This suggests that although this style of green roof may be useful for reducing building cooling costs in summer and perhaps even heating costs in the winter, it may not be as useful as a reflective roof for reducing heat island effects.

The plants in the roof design are native to North Central Texas and are tolerable to barren habitats with shallow soil over limestone and intense periods of heat and drought where soil moisture falls below permanent wilting point. Opuntia phaeacantha, Tetraneuris scaposa, Yucca pallida, Bouteloua curtipendula, and Schizachyrium scoparium are perennials currently performing well on the roof. Other species are expected to perform well, but have not undergone testing; these plants are not commercially available and require transplanting and seed harvesting from their native habitats. Bromus japonicus, present in the seed bank, appears to be the most problematic weed.