Optimizing Green Roofs in a Mediterranean Climate: Idaho Fescue's Potential

Green roofs are increasingly valued for their role in urban sustainability, offering benefits such as stormwater management, temperature regulation, and biodiversity enhancement. These green spaces not only help mitigate the urban heat island effect but also provide aesthetic and recreational value, making cities more livable. However, selecting the right vegetation for these roofs is particularly challenging in climates like Portland, Oregon, which experiences dry summers and cool, wet winters. In such climates, plant selection must consider not only aesthetic and maintenance aspects but also the ability of plants to withstand varying weather conditions and optimize ecological benefits.

A key factor in determining suitable green roof vegetation is the plant's photosynthetic pathway—C3, C4, or CAM—each offering unique advantages and limitations under different environmental conditions. This study evaluates Idaho fescue (Festuca idahoensis), a C3 cool-season perennial bunchgrass, as a potential option for green roofs in Portland. Idaho fescue is known for its tolerance to cold and moderate drought resistance, making it a promising candidate for regions with similar climatic conditions. We aim to determine if Idaho fescue can improve green roof performance by providing effective cooling, stormwater retention, and resilience in Portland's diverse climate conditions, ultimately contributing to urban climate resilience.

To achieve this, we identified key physiological parameters of Idaho fescue, including its leaf area index, maximum carboxylation capacity, and the onset and complete points of stomatal closure. These parameters were then integrated into the Photo3 model, a comprehensive tool that simulates the three photosynthetic pathways. This enabled us to assess crucial plant functions, such as transpiration rates, carbon assimilation, and stomatal conductance, within the environmental conditions typical of Portland. By comparing Idaho fescue, a C3 plant, with other green roof species like blue grama grass (Bouteloua gracilis), a C4 plant, and white stonecrop (Sedum album), a CAM plant, we evaluated how different photosynthetic pathways impact plant performance and suitability for green roofs.

We ran simulations with the Photo3 model, incorporating environmental variables such as soil moisture, temperature, and solar radiation to predict carbon assimilation, stomatal conductance, and transpiration rates of Idaho fescue compared to C4 and CAM plants. These simulations provided a comprehensive analysis of how Idaho fescue's physiological traits, especially its photosynthetic pathway, affect its efficiency in urban green roof settings. Additionally, we examined the strengths and limitations of C3 photosynthesis in urban climates, where effective adaptation to varying water availability is crucial.

Our results indicate that Idaho fescue shows higher daytime transpiration rates, peaking at about 23 mm/day, and maintains effective carbon assimilation around 1.4 µmol/m²/s. In comparison, blue grama grass demonstrates lower transpiration rates, peaking around 15 mm/day, but achieves higher carbon assimilation, up to 3.8 µmol/m²/s, highlighting the water efficiency of C4 plants in hot, sunny environments. Meanwhile, white stonecrop exhibits minimal daytime carbon assimilation, close to 0 µmol/m²/s, and the lowest transpiration rates, below 5 mm/day, excelling in water conservation under arid conditions. These findings emphasize the importance of considering photosynthetic pathways when selecting vegetation for climate-responsive green roofs.

This study offers valuable insights into selecting climate-adapted vegetation for urban environments. By identifying the specific conditions under which Idaho fescue outperforms other plants, we provide practical guidance for those designing sustainable green infrastructure. Future research will include field trials to validate model predictions and should explore optimal soil substrates and watering regimes to further optimize green roof systems in diverse climate conditions.