The Effect of Combined Drought and Temperature Stress on the Physiological Status of Calcareous Grassland Species as Potential Candidates for Urban Green Infrastructure.

Nature-based solutions are promising for climate adaptation and environmental management in urban areas, but urban conditions are stressful for vegetation. In particular, the interaction of drought and high temperatures may be detrimental. Guiding plant selection for urban greening with native species requires a far better knowledge of plant adaptations and stress acclimation. We tested the physiological responses of four candidate calcareous grassland species for green roofs and walls to the combined effects of drought and high temperatures under controlled conditions. The tested species proved relatively resistant to stress despite different strategies to protect the photosynthetic apparatus, maintain water balance, and repair damages. Based on the physiological responses, we rank the species in descending order of resistance to the stress factors tested: Trifolium medium > Festuca ovina > Carex flacca > Potentilla reptans, but all four can serve as potential candidates for green walls and roofs. Physiological stress screening of plant species for use on green roofs and walls supplements the habitat template approach to provide a stronger and wider base for prioritizations.

The impact of soil hydrological regimes and vegetation systems on plant performance and root depth distribution in bioswale microcosms.

Plant rooting patterns in bioswales, raingardens and other vegetated infiltration systems are essential, as they contribute biopores which maintain the infiltration function over time. However, fluctuating hydrological conditions, ranging from flooded to drained, can have a heavy impact on plant rooting, as well as consequences for plant contributions to other ecosystem services and ecological functions. This study tested the biomass allocation to roots and the vertical root profile of four plant species, alone or in competition with a grass, and their responses to the experimental manipulation of soil hydrology in soil column microcosms. The hydrological regimes were combinations of flooded and drained conditions, respectively, including Wet cycles (72 and 96 h), Dry cycles (24 and 144 h), Wet-dry cycles (72 and 264 h), and Control group (watered twice per week). When the species were exposed to repeated wet-dry cycling hydrological regimes, we found a clear shift in vertical root distribution and shallower rooting in wetter regimes. It was also found that alongside this shallower rooting, there were no changes to total biomass and only moderate adjustments to biomass investment in roots. Overall, differences in rooting patterns between hydrological regimes and species were moderate when the dicot species were grown alone. The addition of the grass Festuca rubra contributed to a strong increase in total root mass density that evened out the differences in rooting patterns but also gave a deeper rooting. Accordingly, mixed species systems may be a robust approach to vegetated infiltration systems.

The contribution of roadsides to connect grassland habitat patches for butterflies in landscapes of contrasting permeability.

Roadsides, in particular those being species-rich and of conservation value, are considered to improve landscape permeability by providing corridors among habitat patches and by facilitating species' dispersal. However, little is known about the potential connectivity offered by such high-value roadsides. Using circuit theory, we modelled connectivity provided by high-value roadsides in landscapes with low or high permeability in south-central Sweden, with 'permeability' being measured by the area of semi-natural grasslands. We modelled structural connectivity and, for habitat generalists and specialists, potential functional connectivity focusing on butterflies. We further assessed in which landscapes grassland connectivity is best enhanced through measures for expanding the area of high-value roadsides. Structural connectivity provided by high-value roadsides resulted in similar patterns to those of a functional approach, in which we modelled habitat generalists. In landscapes with low permeability, all target species showed higher movements within compared to between grasslands using high-value roadsides. In landscapes with high permeability, grassland generalists and specialists showed the same patterns, whereas for habitat generalists, connectivity provided by high-value roadsides and grasslands was similar. Increasing the ratio of high-value roadsides can thus enhance structural and functional connectivity in landscapes with low permeability. In contrast, in landscapes with high permeability, roadsides only supported movement of specialised species. Continuous segments of high-value roadsides are most efficient to increase connectivity for specialists, whereas generalists can utilize also short segments of high-value roadsides acting as stepping-stones. Thus, land management should focus on the preservation and restoration of existing semi-natural grasslands. Management for enhancing grassland connectivity through high-value roadsides should aim at maintaining and creating high-value roadside vegetation, preferably in long continuous segments, especially in landscapes with low permeability.

Temperatures and precipitation affect vegetation dynamics on Scandinavian extensive green roofs.

Standard succulent vegetation mixes developed mostly in temperate climates are being increasingly used on green roofs in different climate zones with uncertain outcome regarding vegetation survival and cover. We investigated vegetation on green roofs at nine temperate, cold, and/or wet locations in Norway and Sweden covering wide ranges of latitude, mean annual temperature, annual precipitation, frequencies of freeze-thaw cycles, and longest annual dry period. The vegetation on the roofs were surveyed in two consecutive years, and weather data were compiled from meteorological databases. At all sites we detected a significant decline in species compared to originally intended (planted/sown) species. Both the survival rate and cover of the intended vegetation were positively related to the mean annual temperature. Contrary to a hypothesis, we found that intended vegetation cover was negatively rather than positively related to mean annual precipitation. Conversely, the unintended (spontaneous) vegetation was favoured by high mean annual precipitation and low mean annual temperature, possibly by enabling it to colonize bare patches and outcompete the intended vegetation. When there is high mortality and variation in cover of the intended vegetation, predicting the strength of ecosystem services the vegetation provides on green roofs is difficult. The results highlight the needs for further investigation on species traits and the local factors driving extinction and colonizations in order to improve survivability and ensure a dense vegetation throughout the successional stages of a green roof.

Increased above-ground resource allocation is a likely precursor for independent evolutionary origins of annuality in the Pooideae grass subfamily.

Semelparous annual plants flower a single time during their 1-yr life cycle, investing much of their energy into rapid reproduction. By contrast, iteroparous perennial plants flower multiple times over several years, and partition their resources between reproduction and persistence. To which extent evolutionary transitions between life-cycle strategies are internally constrained at the developmental, genetic and phylogenetic level is unknown. Here we study the evolution of life-cycle strategies in the grass subfamily Pooideae and test if transitions between them are facilitated by evolutionary precursors. We integrate ecological, life-cycle strategy and growth data in a phylogenetic framework. We investigate if growth traits are candidates for a precursor. Species in certain Pooideae clades are predisposed to evolve annuality from perenniality, potentially due to the shared inheritance of specific evolutionary precursors. Seasonal dry climates, which have been linked to annuality, were only able to select for transitions to annuality when the precursor was present. Allocation of more resources to above-ground rather than below-ground growth is a candidate for the precursor. Our findings support the hypothesis that only certain lineages can respond quickly to changing external conditions by switching their life-cycle strategy, likely due to the presence of evolutionary precursors.

Adaptive differentiation among populations of the Mediterranean dry grassland species Brachypodium retusum: The role of soil conditions, grazing, and humidity.

PREMISE OF THE STUDY: Genetic differentiation in plant species may result from adaptation to environmental conditions, but also from stochastic processes. The drivers selecting for local adaptation and the contribution of adaptation to genetic differentiation are often unknown. Restoration and succession studies have revealed different colonization patterns for Brachypodium retusum, a common Mediterranean grass. In order to understand these patterns, we tested population differentiation and adaptation to different environmental factors. METHODS: Structured sampling of 12 populations from six sites and two soil types within site was used to analyze the spatial and environmental structure of population differentiation. Sampling sites differ in grazing intensity and climate. We tested germination and growth in a common garden. In subsets, we analyzed the differential response to stone cover, grazing and soil moisture. KEY RESULTS: We found significant differences among populations. The site explained population differentiation better than soil, suggesting a dominant influence of climate and/or genetic drift. Stone cover had a positive influence on seedling establishment, and populations showed a differential response. However, this response was not related to environmental differences between collection sites. Regrowth after clipping was higher in populations from the more intensively grazed Red Mediterranean soils suggesting an adaptation to grazing. Final germination was generally high even under drought, but germination response to differences in soil moisture was similar across populations. CONCLUSIONS: Adaptive population differentiation in germination and early growth may have contributed to different colonization patterns. Thus, the provenance of B. retusum needs to be carefully considered in ecological restoration.

Stress acclimation and particulate matter accumulation in Pinus sylvestris saplings affected by moderate combinations of urban stressors.

To predict how the function of urban vegetation and the provision of ecosystem services respond to combinations of natural and anthropogenic drivers, a better understanding of multiple stress interactions is required. This study tested combined effects of moderate levels of drought, soil salinity and exposure to diesel exhaust on parameters of physiology, metabolism, morphology and growth of Pinus sylvestris L. saplings. We found that plant responses were primarily dominated by single stressors and a few two-way interactions. Stressor combinations did not have considerable additional negative effects on plant performance compared to single stressors. Hence, synergistic and antagonistic interactions were rare and additive effects frequent. Drought cycles caused most negative effects, from chlorophyll a fluorescence and epicuticular wax content to growth responses, while soil salinity caused fewer negative effects but contributed to reduction in fine root growth and fluorescence parameters at low air contamination. Interestingly, the air contamination alone had only marginal effects on plant morphology and growth, but contributed an antagonistic effect, dampening the negative effect of drought and salinity on the maximum quantum efficiency of PSII photochemistry (Fv/Fm) and fine root biomass. Although, these effects were moderate, it appears that exhaust exposure had a cross-acclimation effect on plant responses to drought and salinity. We also found that salinity had a negative effect on the accumulation of particulate matter on shoots, illustrating that the plant stress situation can affect the provisioning of certain ecosystem services like pollution attenuation. These findings have implications for the understanding of the complex natural and anthropogenic stress situation of urban, and how to maintain the ecological functions and delivery of ecosystem services.