Phylogenetic diversity and plant trait composition predict multiple ecosystem functions in green roofs.

Plant selection and diversity can influence the provision of key ecosystem services in extensive green roofs. While species richness does predict ecosystem services, functional and phylogenetic community structure may provide a stronger mechanistic link to such services than species richness alone. In this study, we assessed the relationship between community-weighted trait values from four key leaf and canopy functional traits (plant height, leaf area, specific leaf area, dry leaf matter content), functional diversity, and phylogenetic diversity to ten different green roof functions, including ecosystem multifunctionality, in experimental polycultures. Functional traits of dominant plant species were a major driver for indicators of multiple green roof functions, such as substrate nitrate-N, substrate phosphorus, aboveground biomass and ecosystem multifunctionality. In contrast, functional diversity alone increased substrate organic matter. Moreover, both functional/phylogenetic diversity and identity predicted canopy density, substrate cooling. This study highlights the first line of evidence that distinct aspects of phylogenetic and functional diversity play a major role in predicting multiple green roof services. Therefore, we provide further evidence that to maximize green roof functioning, a very careful selection of plant traits and polycultures are needed.

The effect of environmental heterogeneity on species richness depends on community position along the environmental gradient.

Environmental heterogeneity is among the most important factors governing community structure. Besides the widespread evidence supporting positive relationships between richness and environmental heterogeneity, negative and unimodal relationships have also been reported. However, few studies have attempted to test the role of the heterogeneity on species richness after removing the confounding effect of resource availability or environmental severity. Here we constructed an individual-based spatially explicit model incorporating a long-recognized tradeoff between competitive ability and stress-tolerance ability of species. We explored the impact of the level of resource availability (i.e. the position of the community along a gradient of environmental severity) on the heterogeneity-diversity relationship (HDR). The results indicate that the shape of HDR depends on the community position along the environmental gradient: at either end of the gradient of environmental severity, a positive HDR occurred, whereas at the intermediate levels of the gradient, a unimodal HDR emerged. Our exploration demonstrates that resource availability/environmental severity should be considered as a potential factor influencing the shape of the HDR. Our theoretical predictions represent hypotheses in need of further empirical study.

Performance of dryland and wetland plant species on extensive green roofs.

BACKGROUND AND AIMS: Green roofs are constructed ecosystems where plants perform valuable services, ameliorating the urban environment through roof temperature reductions and stormwater interception. Plant species differ in functional characteristics that alter ecosystem properties. Plant performance research on extensive green roofs has so far indicated that species adapted to dry conditions perform optimally. However, in moist, humid climates, species typical of wetter soils might have advantages over dryland species. In this study, survival, growth and the performance of thermal and stormwater capture functions of three pairs of dryland and wetland plant species were quantified using an extensive modular green roof system. METHODS: Seedlings of all six species were germinated in a greenhouse and planted into green roof modules with 6 cm of growing medium. There were 34 treatments consisting of each species in monoculture and all combinations of wet- and dryland species in a randomized block design. Performance measures were survival, vegetation cover and roof surface temperature recorded for each module over two growing seasons, water loss (an estimate of evapotranspiration) in 2007, and albedo and water capture in 2008. KEY RESULTS: Over two seasons, dryland plants performed better than wetland plants, and increasing the number of dryland species in mixtures tended to improve functioning, although there was no clear effect of species or habitat group diversity. All species had survival rates >75 % after the first winter; however, dryland species had much greater cover, an important indicator of green roof performance. Sibbaldiopsis tridentata was the top performing species in monoculture, and was included in the best treatments. CONCLUSIONS: Although dryland species outperformed wetland species, planting extensive green roofs with both groups decreased performance only slightly, while increasing diversity and possibly habitat value. This study provides further evidence that plant composition and diversity can influence green roof functions.

Natural analogues of degraded ecosystems enhance conservation and reconstruction in extreme environments.

Ecosystem rehabilitation strategies are grounded in the concept that coexisting species fit their environments as an outcome of natural selection operating over ecological and evolutionary timescales. From this perspective, re-creation of historical environmental filters on community assembly is a necessary first step to recovering biodiversity within degraded ecosystems; however, this approach is often not feasible in severely damaged environments where extensive physiochemical changes cannot be reversed. Under such circumstances management goals may shift from restoring historical conditions to reconstructing entirely new ecosystems or replicating natural ecosystems that may be locally novel but of regional conservation importance. This latter goal may be achieved by introducing to damaged sites species already adapted to filters maintaining the degraded state, through targeting assemblages from natural ecosystems biophysically analogous to the degraded state, here termed "degraded-state analogue" (DSA) ecosystems. This hypothesis predicts that, in high-stress sites where recruitment of previous inhabitants is strongly microsite-limited, DSA species will be primarily propagule-limited; furthermore, communities invaded by DSA species should shift in structure to reflect properties associated with high-value DSA target ecosystems. We tested these predictions by experimentally sowing long-abandoned limestone quarry floors with 18 perennial grass and forb species characteristic of rare natural limestone pavements called "alvars." Alvar species established successfully under a range of microsite conditions manipulated to alter suspected constraints on colonization, including nitrogen deficiency, excessive CaCO3, and competition with weeds. Alvar species performed equivalently to seeded weed species known to thrive on quarry floors. Resident communities doubled in species richness following alvar species addition, supporting 17-20 species/0.18 m2 (95% confidence interval) and providing refuge to regionally restricted or threatened species including Iris lacustris, Solidago ptarmicoides, and Liatris cylindracea. In contrast, maximum-diversity reference plots on a pristine alvar supported 20-23 species/0.18 m2. Strong propagule limitation but weak microsite constraints on quarry colonization by alvar species combined with establishment of species-rich communities comparable to natural alvar biodiversity hot spots confirms that targeting DSA assemblages in ecosystem reconstruction can promote both efficient site colonization and ex situ biodiversity conservation within difficult-to-restore anthropogenic wastelands.

Experimental separation of resource quantity from temporal variability: seedling responses to water pulses.

We tested the hypothesis that higher temporal variability in water supply will promote higher species richness of germinating and surviving seedlings using assemblages of 70 species of herbaceous plants from limestone pavement habitats. In a two-factor greenhouse experiment, doubling the total volume of water added led to greater germination (measured as number of germinated seeds and species) and establishment (survival and biomass) but the effects of temporal variability depended on the response variable considered. Low pulse frequencies of water addition with total volume added held constant resulted in greater temporal variability in soil moisture concentration that in turn promoted higher density and richness of germinated seedlings. Low pulse frequencies caused an eight-fold greater mortality in the low total volume treatment and biomass production to decline by one-third in the high total volume treatment. The effects of increasing temporal variability in water supply during recruitment stages can thus be opposite on different components of plant fitness and may also depend on total resource quantity. While greater species richness in more temporally variable soil moisture conditions was attributable to sampling effects rather than species-specific responses to the water treatments, species relative abundances did vary significantly with temporal variability. Changes in the amplitude or frequency of resource fluctuations may alter recruitment patterns, and could have severe and relatively rapid effects on community structure in unproductive ecosystems.