Fine-scale substrate heterogeneity in green roof plant communities: The constraint of size.

Heterogeneity-diversity relationship (HDR) is commonly shown to be positive in accordance with classic niche processes. However, recent soil-based studies have often found neutral and even negative HDRs. Some of the suggested reasons for this discrepancy include the lack of resemblance between manipulated substrate and natural settings, the treated areas not being large enough to contain species' root span, and finally limited-sized plots may not sustain focal species' populations over time. Vegetated green roofs are a growing phenomenon in many cities that could be an ideal testing ground for this problem. Recent studies have focused on the ability of these roofs to sustain stable and diverse plant communities and substrate heterogeneity that would increase niches on the roof has been proposed as a method to attain this goal. We constructed an experimental design using green roof experimental modules (4 m2) where we manipulated mineral and organic substrate component heterogeneity in different subplots (0.25 m2) within the experimental module while maintaining the total sum of mineral and organic components. A local annual plant community was seeded in the modules and monitored over three growing seasons. We found that plant diversity and biomass were not affected by experimentally created substrate heterogeneity. In addition, we found that different treatments, as well as specific subplot substrates, had an effect on plant community assemblages during the first year but not during the second and third years. Substrate heterogeneity levels were mostly unchanged over time. The inability to retain plant community composition over the years despite the maintenance of substrate differences supports the hypothesis that maintenance of diversity is constrained at these spatial scales by unfavorable dispersal and increased stochastic events as opposed to predictions of classic niche processes.

Fine-scale substrate heterogeneity does not affect arthropod communities on green roofs.

Green roofs, which are roofs with growing substrate and vegetation, can provide habitat for arthropods in cities. Maintaining a diversity of arthropods in an urban environment can enhance the functions they fill, such as pest control and soil development. Theory suggests that the creation of a heterogeneous environment on green roofs would enhance arthropod diversity. Several studies have examined how arthropod diversity can be enhanced on green roofs, and particularly whether substrate properties affect the arthropod community, but a gap remains in identifying the effect of substrate heterogeneity within a green roof on the arthropod community. In this paper, it is hypothesized that creating heterogeneity in the substrate would directly affect the diversity and abundance of some arthropod taxa, and indirectly increase arthropod diversity through increased plant diversity. These hypotheses were tested using green roof plots in four treatments of substrate heterogeneity: (1) homogeneous dispersion; (2) mineral heterogeneity-with increased tuff concentration in subplots; (3) organic heterogeneity-with decreased compost concentrations in subplots; (4) both mineral and organic heterogeneity. Each of the four treatments was replicated twice on each of three roofs (six replicates per treatment) in a Mediterranean region. There was no effect of substrate heterogeneity on arthropod diversity, abundance, or community composition, but there were differences in arthropod communities among roofs. This suggests that the location of a green roof, which can differ in local climatic conditions, can have a strong effect on the composition of the arthropod community. Thus, arthropod diversity may be promoted by building green roofs in a variety of locations throughout a city, even if the roof construction is similar on all roofs.

Green roof and photovoltaic panel integration: Effects on plant and arthropod diversity and electricity production.

The combination of green roofs with photovoltaic (PV) panels has been proposed to provide synergistic benefits as the panel is cooled by the presence of the vegetation, and thus produces more electricity, while the solar panel enhances growing conditions for vegetation, and increases abiotic heterogeneity, resulting in higher plant diversity. We tested these hypotheses in a non-irrigated green roof in a Mediterranean climate with replicated plots including green roofs only, green roofs with a PV panel, and a conventional roof surface with a PV panel. We found that presence of a panel resulted in higher heterogeneity in substrate moisture, but there was no effect on plant diversity. Plant species showed enhanced growth in plots with PV, including greater growth of Sedum sediforme and longer flowering time of annual species. On the other hand, arthropod diversity was lower during part of the year, and abundance of some arthropod taxa was lower in green roof plots with PV. The presence of the green roof also did not improve electricity production by the panels. We conclude that in a Mediterranean climate, it would be appropriate to examine the use of irrigation in green roofs with PV panels, including effects on the plant community and on electricity production.