BRIT Green Roof Research Program BRIT Urban Ecology Program

Green Roof Research: Cover and diversity

This page describes ongoing research on vegetative aspects of BRIT's green roof.

More about the project

Byerley Best, B. and R.K. Swadek. 2019. Plant cover & diversity on a young prairie-style green roof relative to slope position & planting assemblage. Poster presentation at Botany 2019, annual conference of the Botanical Society of America, 27-31 Jul 2019, Tucson, Arizona, USA. [link to poster]

image of poster presentation from Botany 2019 conference for the Botanical Society of America

CONFERENCE PROGRAM ABSTRACT

Green roofs are an important tool in urban areas 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 don’t typically accommodate. A 20,000-square-foot test roof in North Central Texas was modeled after a local natural prairie system, Goodland Limestone Prairie Barrens/Glades, and incorporated harvested native prairie soil into the planting medium. As part of a larger effort to determine the efficacy of a biomimicry approach to this roof’s design, we examined plant cover and species richness relative to two roof slope positions (upslope and downslope) and three pre-determined planting assemblages at eight six-week intervals beginning about 20 months after the roof was first planted. We predicted that downslope areas would have higher coverage but fewer species due to faster establishment of a few dominant species where the soil moisture might be greater. Conversely, upslope areas should have lower vegetative cover due to drier conditions, but greater species richness from the more open community. The three plant assemblages each began with the same number of known species and an unknown but likely homogenous seed bank from the harvested soil; we thus predicted no difference in richness or cover among the groups when averaged over slope position. We found that species richness did not vary relative to slope position, but cover was consistently greater downslope than upslope. Planting assemblage did not appear to impact either richness or cover differentially when averaged over time. We saw an overall trend in all samples where cover generally kept increasing over time while richness peaked in the spring, troughed in the summer, and began to rebound in the fall, mirroring the bimodal precipitation for the region. This pattern is indicative of annual-dominated systems such as the one upon which the roof was modeled.

WHAT DID WE FIND?

  • Two years after planting this green roof, species richness does not differ relative to slope position or vegetation type.
  • Total vegetative cover is higher downslope than upslope.
  • Species composition differed more by slope than by vegetation type.

WHY DOES IT MATTER?

  1. Slopes are a way to create niches on green roofs and therefore encourage higher diversity. Here the number of species (richness) didn’t differ between slopes but the species composition did, so total roof diversity still increased.
  2. The roof design included three diverse planting assemblages but each used the same soil with a homogenous seed bank. Our results could suggest that design effort could be saved by letting the slope do the work.
  3. The biomimicry design was successful. The patterns of both richness and cover over time mimic what we see in the native prairie analog: a system dominated by annuals that flourish with the spring and fall rains and then subside.
  4. It might not matter, actually. The roof was still establishing during this study. A reexamination at a later date should yield different results, as the effect of perennial dominants should be stronger.

BACKGROUND

Excerpted from "Soil-based green roofs" (Byerley Best et al. 2015, in Green Roofs Ecosystems):

"When the designers of the headquarters building for the Botanical Research Institute of Texas (BRIT) (Fort Worth, Texas, USA) decided to create a green roof, there were few precedents in the region. The idea of mimicking a local prairie habitat was put forth by community shareholders with hopes of the roof becoming a form of “reconciliation ecology” (Rosenzweig 2003), allowing habitat expansion for native species within a built environment. The landscape architects agreed to a biomimicry approach, and collaborating scientists and students started investigating possible template ecosystems.

The biomimicry strategy, also called habitat template strategy (Lundholm 2006), depends upon comparisons between a self-organizing, natural ecosystem and a model designed to simulate desirable aspects of that system. The sequence was to (1) explore and describe the wild system, (2) create a model system, (3) compare performance of the model system with the wild prototype, and (4) progressively refine the model to optimize desired functions. The advantage of this process brought learning about a poorly understood ecosystem in tandem with developing a regionally appropriate design for living roofs.

The Model Ecosystems: Fort Worth Prairie Barrens and Glades
Exploration refined project participants’ perception of the plant communities associated with limestone, leading to a fundamental distinction between “barrens” and “glades” (Quarterman 1989; Homoya 1994; Baskin & Baskin 2003; Swadek & Burgess 2012). Barrens are habitats where shallow soil over bedrock restricts plant growth, vegetation usually occurring on soils 5–25 cm (2–10 inches) deep over weathered limestone. Glades by contrast have extensive areas of exposed bedrock, with distinctive plant communities restricted either to soils in deeper crevices or soils less than 5 cm (2 inches) deep. In general, both of these habitats are dominated by short perennial bunchgrasses, prickly pear cactus, and yucca, which are hardy, drought-tolerant species, and annual grasses and forbs, which can complete their life cycle with seasonal rains. Patches of bare soil with cryptogamic crusts are also common."

Photograph of model habitat. Woodland (background), Walnut barrens with Indian paintbrush ( Castilleja indivisa), and a Walnut glade (foreground) in the spring. In the late summer this habitat is typically very dry due to minimal, shallow soils. ( Photo Rebecca Swadek)
Woodland (background), Walnut barrens with Indian paintbrush ( Castilleja indivisa), and a Walnut glade (foreground) in the spring. In the late summer this habitat is typically very dry due to minimal, shallow soils. ( Photo Rebecca Swadek)

DESIGN & SAMPLING METHODS

  • A total of 34 native species were chosen for the roof. Planting assemblages were designed to simulate native prairie plant communities, centering each assemblage of species around a single dominant species: Opuntia phaeacantha, Yucca pallida, and Bouteloua curtipendula (or Buchloe dactyloides).
  • Our sampling plots were 0.25 m^2 in size. This size was chosen based on species area curves for our native prairie analog. Plot locations were randomized by superimposing a grid over the assemblage sections across the roof, denoting a boundary for upslope vs. downslope, and using a random number generator to assign final sampling areas. Grid squares were 1.5 m^2, creating an allowance for smaller plots to move slightly and account for local trampling effects and impeding roof hardware or structural obstacles.
  • Sampling occurred every 6 weeks from March through November 2012. Six plots per assemblage (9 per slope position) were assessed at each sampling interval.
  • For each plot, we listed every plant species, their relative percent cover, and whether alive of standing dead. We also calculated the total live vegetative cover as well as cover of bare ground and litter.

 

drawing of green roof from aerial perspective, showing the arrangement of grass, yucca, and cactus planting assemblages in sections across the roof. Location of sampling plots are denoted with plot ID numbers, showing plots in each of three sections at both upslope and downslope positions.
Roof map with planting assemblages and location of plots

 

    Compilation of photographs of the sampling plots in all three assemblages (grass, yucca, cactus) and two slope positions (high and low). The plots are marked with string and marker flags stick up in the middle of each plot. You can see green cactus and yucca plants, straw-colored grasses, and exposed gravel mulch in the different plots.
    Example of 2012 roof plots of all three assemblages and two slope positions

    RESULTS TO DATE (in progress)

    THOUGHTS FOR FUTURE

    • Volunteers. Plant species found on the roof that were not part of the three planting assemblages have three potential sources: seed bank from the native soil incorporated into the planting medium, potting soil of nursery-grown transplants used in the assemblages, and colonization. Colonization events were likely both pre-installation (while roof trays were tended at ground-level for three months in a temporary nursery) and post-installation (bird- and wind dispersal). Tracking provenance of volunteers will help us refine establishment and management recommendations.
    • What is the trajectory of the plant communities on the roof compared to the native prairie it was designed to mimic? Will the cover of spring annuals decrease over time towards a climax community? We don’t expect so. The Fort Worth Prairie (which includes Goodland/Walnut barrens modeled here), much like the rest of the southwest, experiences variation in annual species richness and cover based on winter and spring rainfall. We hope to reassess cover and diversity in 2022, ten years after the first assessment.
    • Click here to view a live video feed of the roof
    photograph of the study roof in May 2015. The vegetation is lush and very green. The yuccas are blooming with tall stalks of white flowers. The cactus in the foreground is low and very abundant. Buildings in downtown Fort Worth are visible in the background. The sky is mostly white with clouds.
    BRIT green roof, May 2015
    Photograph of the study roof taken in November 2018. The vegetation is thick, a mix of greens and browns. Individual cactus and yucca plants are visible. Old flowering stalks of yuccas are visible rising above all other vegetation. Buildings of Fort Worth downtown are visible in the background. The sky is blue with white clouds.
    BRIT green roof, Nov 2018
    Closeup photograph showing two smiling female botanists side by side, posing on the roof in their safety harnesses. Rebecca Swadek is on the left in a yellow shirt and white helmet, and Brooke Byerley Best on the right in a gray shirt, a blue helmet, and sunglasses.
    Rebecca Swadek and Brooke Byerley Best on the roof, Aug 2013

     

    This page describes ongoing research on vegetative aspects of BRIT's green roof.