It is not a farfetched claim that trees are a dominant field of interest within horticulture and landscape professions, with multiple horticultural organizations, societies, and publishing journals focusing on trees (Götmark et al. 2016). Shrubs, which comprise a vast and interesting plant group, are much less well-represented in research and the literature. In Sweden, nursery production and sales of shrubs comprise a large proportion of the national total. The latest available statistics from the Swedish Board of Agriculture refer back to 2005 (Jordbruksverket 2006), reflecting a general lack of up-to-date information, but the total value of domestic nursery production in that year was 40.1 million euros, of which ornamental shrubs and hedge and landscape plants accounted for 8.27 million euros (20%). Similar data about shrubs only do not exist within Europe, but the total production of ornamental plants in the union (with the United Kingdom) had a turnover of 22,099 million euros in 2019 (European Commission 2020). Even at the user level (designers, landscape architects, landscape engineers, gardeners), shrubs comprise a very large proportion of the plant material used. As an example, a recent assessment of typical use of different categories of plant material in landscape design work in a residential area (Augustenborg) in Malmö, Sweden, showed that the costs of low shrubs comprise almost 48% of all plant costs (Slagstedt Johan, personal communication 2022). Therefore, shrubs constitute a very large proportion of the green infrastructure in urban environments and should merit a substantial research focus, especially toward site-specific guidance for selecting the correct shrubs for a particular site, that is currently lacking.
Some previous studies have focused on shrubs in their native context (Adelman and Schwartz, 2016; Kolbeck et al. 2003; Leuschner et al. 2017), thus providing guidance for the capacity of different species for survival and success in different climates and growing habitats. Based on findings in these publications, shrubs are important components in at least 9 of 11 global biomes (Archibold 1995; McKell 1989), shaping much of the vegetation in tropical savannah, arid regions, Mediterranean ecosystems, polar tundra, and high mountain tundra. They are also widespread in terrestrial wetlands, forest understory, and gaps in the forest canopy, where both shade-tolerant and pioneer (shade-intolerant) shrubs occur. The broad tolerance of shrubs for different climates and growing conditions in nature widens their use potential for urban environments compared with trees, indicating a need for site-related research of shrubs in urban environments.
Moreover, recent research of shrubs and the provision of ecosystem services (Blanusa et al. 2019) has demonstrated that shrubs and hedges are becoming particularly important in an urban context because of city densification, which may place pressure on space for parks and large-stature trees in the future (Haaland and van den Bosch 2015). Shrubs and hedges use less space than trees, at least in terms of width, and together with green walls and roofs may be critical to the future provision of effective green infrastructure in cities, particularly considering recent re-evaluations of the role of trees in the urban context (Abhijith and Kumar 2019; Blanusa et al. 2016; Pugh et al. 2012). However, those seeking to maximize the delivery of ecosystem services by shrubs need to use species that are capable of performing well in challenging urban sites, thus making site-specific selection of shrubs absolutely crucial.
There is currently a lack of quantitative assessments of stress tolerance in a large proportion of available species and cultivars of shrubs, which complicates any design process involving shrubs. The aim of this study was to evaluate drought tolerance of many common and less traditional shrubs intended for public planting to make a first contribution (dataset) to species selection. As water stress is a major constraint for landscape plants in urban environments and is likely to increase in many regions under future climate scenarios (Caretta et al. 2022), the quantitative drought tolerance of a species or a genotype must be a fundamental consideration in plant selection for urban environments. Previous research has demonstrated great intraspecific variation in drought tolerance within the same species for trees (Hannus et al. 2021; Hirons et al. 2021), another fundamental research aim is to determine whether this is also the case within species of shrubs.
In this study, we used water potential at the turgor loss point (ΨP0) as a key trait for evaluating drought tolerance of different species of shrubs. ΨP0 is a highly instructive trait because it represents a quantifiable measure of physiological drought tolerance. More negative ΨP0 values represent greater drought tolerance by allowing the leaf to maintain physiological function over a greater range of leaf water potentials (Lenz et al. 2006; Sack et al. 2003). ΨP0 has also been demonstrated to differentiate a wide range of species and cultivars with respect to drought tolerance and has helped to inform plant species selection guidance for green infrastructure (Hirons and Sjöman 2019). The current technique for determining ΨP0 uses vapor pressure osmometry to predict osmotic potential at full turgor (Ψπ100) and is sensitive enough to resolve differences in drought tolerance between closely related genotypes (Hannus et al. 2021).
Abhijith KV & Kumar P. 2019 Field investigations for evaluating green infrastructure effects on air quality in open-road conditions Atmos Environ. 201 132 147
Adelman C & Schwartz BL. 2017 Midwestern native shrubs and trees: Gardening alternatives to non-native species: An illustrated guide Ohio University Press Athens, OH, USA
Archibold OW. 2012 Ecology of world vegetation Springer Science and Business Media. Dordrecht https://doi.org/10.1007/978-94-011-0009-0
Bartlett MK, Klein T, Jansen S, Choat B & Sack L. 2016 The correlations and sequence of plant stomatal, hydraulic, and wilting responses to drought Proc Natl Acad Sci USA. 113 46 13098 13103 https://doi.org/10.1073/pnas.1604088113
Bartlett MK, Scoffoni C, Ardy R, Zhang Y, Sun S, Cao K & Sack L. 2012a Rapid determination of comparative drought tolerance traits: Using an osmometer to predict turgor loss point Methods Ecol Evol. 3 880 888 https://doi.org/10.1111/j.2041-210X.2012.00230.x
Bartlett MK, Scoffoni C & Sack L. 2012b The determinants of leaf turgor loss point and prediction of drought tolerance of species and biomes: A global meta-analysis Ecol Lett. 15 393 405 https://doi.org/10.1111/j.1461-0248.2012.01751.x
Blackman CJ, Brodribb TJ & Jordan GJ. 2010 Leaf hydraulic vulnerability is related to conduit dimensions and drought resistance across a diverse range of woody angiosperms New Phytol. 188 1113 1123
Blanusa T, Garratt M, Cathcart-James M, Hunt L & Cameron RW. 2019 Urban hedges: A review of plant species and cultivars for ecosystem service delivery in north-west Europe Urban For Urban Green. 44 126391
Blanusa T, Hadley J, Hunt L, Alexander P & Hobbs K. 2016 Provision of ecosystem services by hedges in urban domestic gardens: Focus on rainfall mitigation VI International Conference on Landscape and Urban Horticulture 1189 519 524
Caretta MA, Mukherji A, Arfanuzzaman M, Betts RA, Gelfan A, Hirabayashi Y, Lissner TK, Liu J, Lopez Gunn E, Morgan R, Mwanga S & Supratid S. 2022 Water Pörtner H-O, Roberts DC, Tignor M, Poloczanska ES, Mintenbeck K, Alegría A, Craig M, Langsdorf S, Löschke S, Möller V, Okem A & Rama B Climate change 2022: impacts, adaptation, and vulnerability. Contribution of Working Group II to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press Cambridge, UK [in press]
- Search Google Scholar
- Export Citation
Caretta MA Mukherji A Arfanuzzaman M Betts RA Gelfan A Hirabayashi Y Lissner TK Liu J Lopez Gunn E Morgan R Mwanga S Supratid S. 2022 Water Pörtner H-O Roberts DC Tignor M Poloczanska ES Mintenbeck K Alegría A Craig M Langsdorf S Löschke S Möller V Okem A Rama B Cambridge University Press Cambridge, UK [in press]
Filazzola A & Lortie CJ. 2014 A systematic review and conceptual framework for the mechanistic pathways of nurse plants Glob Ecol Biogeogr. 23 12 1335 1345
Götmark F, Götmark E & Jensen AM. 2016 Why be a shrub? A basic model and hypotheses for the adaptive values of a common growth form Front Plant Sci. 7 1095
Haaland C & van den Bosch CK. 2015 Challenges and strategies for urban green-space planning in cities undergoing densification: A review Urban For Urban Green. 14 4 760 771
Hannus S, Hirons A, Baxter T, McAllister HA, Wiström B & Sjöman H. 2021 Intraspecific drought tolerance of Betula pendula genotypes: An evaluation using leaf turgor loss in a botanical collection Trees (Berl). 35 2 569 581
Hirons AD, Watkins JHR, Baxter TJ, Miesbauer JW, Male-Muñoz A, Martin KW, Bassuk N & Sjöman H. 2021 Using botanic gardens and arboreta to help identify urban trees for the future. Plants, People Planet. 3 2 182 193
Hirons AD & Sjöman H. 2019 Tree species selection for green infrastructure: A guide for specifiers Trees and Design Action Group. https://www.tdag.org.uk/tree-species-selection-for-green-infrastructure.html
Kikuta SB & Richter H. 1992 Leaf disks or press saps? A comparison of techniques for the determination of osmotic potentials in freeze thawed leaf materials J Expt Bot. 43 1039 1044
Kottek M, Grieser J, Beck C, Rudolf B & Rubel F. 2006 World map of the Köppen-Geiger climate classification updated Meteorol Z (Berl). 15 3 259 263 https://doi.org/10.1127/0941-2948/2006/0130
Krüssmann G. 1982 Choosing woody ornamentals–A concise manual for the correct use of woody landscape plants Timber Press Portland, OR, USA
Lenz T, Wright IJ & Westoby M. 2006 Interrelations among pressure-volume curve traits across species and water availability gradients Physiol Plant. 127 423 433
Leuschner C, Ellenberg H & Sutcliffe L. 2017 Vegetation ecology of central Europe Volume I, ecology of central European forests. Springer Cham. Switzerland
Mitchell PJ, Veneklaas EJ, Lambers H & Burgess SSO. 2008 Leaf water relations during summer water deficit: Differential responses in turgor maintenance and variations in leaf structure among different plant communities in southwestern Australia Plant Cell Environ. 31 1791 1802
- Search Google Scholar
- Export Citation
Mitchell PJ Veneklaas EJ Lambers H Burgess SSO. 2008 Leaf water relations during summer water deficit: Differential responses in turgor maintenance and variations in leaf structure among different plant communities in southwestern AustraliaPlant Cell Environ. 31 1791 1802
Pugh TAM, MacKenzie AR, Whyatt JD & Hewitt CN. 2012 Effectiveness of green infrastructure for improvement of air quality in urban street canyons Environ Sci Technol. 46 14 7692 7699
R Core Team 2020 R: A language and environment for statistical computing R Foundation for Statistical Computing, Vienna, Austria. https://www.R-project.org/
Rundel PW. 1991 Shrub life forms 345 370 Mooney HA, Winner W & Pell E Response of plants to multiple stresses. Academic Press San Diego, CA, USA
Sack L, Cowan PD, Jaikumar N & Holbrook NM. 2003 The ‘hydrology’ of leaves: Co-ordination of structure and function in temperate woody species Plant Cell Environ. 26 1343 1356
Sheffer M, Vergnon R, Cornelissen HC, Hantson S, Holmgren M, van Nes EH & Xu C. 2014 Why trees and shrubs but rarely trubs? Trends Ecol Evol. 29 433 434 https://doi.org/10.1016/j.tree.2014.06.001
Sjöman H, Hirons AD & Bassuk NL. 2018a Magnolias as urban trees—A preliminary evaluation of drought tolerance in seven magnolia species Arboric J. 40 6 1 10 https://doi.org/10.1080/03071375.2017.1415554
Sjöman H, Hirons AD & Bassuk NL. 2018b Improving confidence in tree species selection for challenging urban sites: A role for leaf turgor loss Urban Ecosyst. 21 6 1171 1188 https://doi.org/10.1007/s11252-018-0791-5
Sjöman H, Hirons A & Bassuk N. 2015 Urban forest resilience through tree selection – Variation in drought tolerance in Acer Urban For Urban Green. 14 4 858 865
Sjöman H & Östberg J. 2019 Vulnerability of ten major Nordic cities to potential tree losses caused by longhorned beetles Urban Ecosyst. 22 2 385 395
Tanentzap AJ, Mountford EP, Cooke AS & Coomes DA. 2012 The more stems the merrier: Advantages of multi-stemmed architecture for the demography of understorey trees in a temperate broadleaf woodland J Ecol. 100 1 171 183
Wilson BF. 1995 Shrub stems: form and function 91 102 Gartner BL Plant stems: Physiology and functional morphology. Academic Press San Diego, CA, USA
Yang J, Zhou J, Ke Y & Xiao J. 2012 Assessing the structure and stability of street trees in Lhasa, China Urban For Urban Green. 11 432 438
Total overview of the ΨP0 value of species in the study.
Total overview of the ΨP0 value of genera in the study.