Using the CSR Theory when Selecting Woody Plants for Urban Forests: Evaluation of 342 Trees and Shrubs

Introduction

Trees and shrubs provide distinctive combinations of ecosystem services (ES) in urban settings. Recent projections suggest that an overreliance on a limited range of species may compromise the ES provided by urban forests (Pauleit et al. 2017). The suboptimal fitness of plants for their climate and the diminished performance of their ES are frequently compounded by the planting of species in locations that are not conducive to their growth and development. In the planning and design of urban green infrastructure, it is of the utmost importance to be able to anticipate the development of different species under a variety of growing conditions. This enables the planning and anticipation of their needs for establishment and longterm maintenance, as well as their long-term development in response to current and future climates. In a changing climate, characterised by increasingly hotter and periodically drier conditions, coupled with the densification of urban structures and the occurrence of severe outbreaks of diseases and pests, there are increasingly challenging conditions for the successful utilisation of plants (Koeser et al. 2014; Allen et al. 2015; Matusick et al. 2018; Yi et al. 2022). This is leading to a high proportion of trees and other plants dying or experiencing impaired development and establishment. (Kianmehr et al. 2024). In addition to the aforementioned factors, the natural mortality of trees in urban environments due to the aging of the urban tree population represents another significant cause of urban canopy loss (Roman et al. 2014). It is therefore crucial to gain a deeper understanding of the resilience of different tree and shrub species to challenging conditions and to anticipate their initial management needs, particularly in light of the potential for nontraditional plant material to be free from serious diseases and pests. When available plant literature directed to urban forestry is reviewed, the descriptions of trees and shrubs are often limited to their aesthetic qualities and size, with minimal attention given to climate and site-related guidance. The climate and site-related information that does exist is primarily based on the authors’ personal experiences and observations, rather than on rigorous scientific evaluations of their capabilities or limitations in challenging growing habitats (Sjöman and Nielsen 2010; Sjöman et al. 2018; Watkins et al. 2020), which means that there are many contradictory conclusions within the literature. This contributes to the pervasive reluctance to test unconventional plant material with the aim of diversifying the urban forest.

One type of information that is notably absent from the urban forestry community and seldom considered in the design of public green spaces is trait-oriented plant selection (Watkins et al. 2021). This approach considers the evolutionary properties that different species have developed in order to cope with the growing conditions and competition for resources in their natural environment.

One accessible method for classifying plant investments and thereby their tolerance to different plant environments, as well as for gaining insights into growth rate, is the CSR classification system (Hodgson et al. 1999; Pierce et al. 2017). This classification system is based on Grime’s theory of competitorstress tolerator-ruderal (CSR) plant strategies (Grime 1974, 1977, 2006; Grime and Pierce 2012). In the CSR system, the 3 principal strategies represent viable trait combinations that have evolved in response to competitive pressures, abiotic limitations to growth, or periodic biomass destruction. C-strategists (competitors) are plants that are adapted to stable and productive habitats and are successfully converting available resources into rapid growth and rapid attainment of large individual and organ size, thereby aiding resource pre-emption. S-strategists (stress tolerators) are adapted to resource-limited habitats, investing in the capacity to retain resources and repair cellular components in dense, persistent tissues. R-strategists (ruderals) are disturbance-favoured species that invest in traits not in the individual plant, but in propagules from which the population can regenerate in the face of repeated lethal biomass destruction events or disturbances. The CSR classification method employs a variety of functional traits, which are quantified in situ (or measured in material collected in situ), thereby providing plant users with a comprehensive understanding of the capacity of plant material to thrive in resource-rich or resource-limited habitats.

In the context of urban horticulture, it is essential to have a clear understanding of the evolutionary strategies of different tree and shrub species prior to the planning and design of planting schemes. This provides insight into their suitability for specific conditions, whereby their investments must align with the environments in which they are planted. Furthermore, knowledge of their growth rate is vital for anticipating their requirements for establishment management and for predicting their development (growth) on-site. The process of matching a plant to its designed location integrates multiple sources of information. Conceptual frameworks for this have been proposed for herbaceous plants and small shrubs by Kühn (2011) and for trees by Watkins et al. (2021) (Figure 1). Both of these frameworks connect locations in trait space, illustrating an ongoing trade-off between metabolic pace and stress tolerance in urban environments. The optimal location for a plant is determined by its evolutionary adaptations, which are designed to enhance its fitness.

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Figure 1.

The hypothetical trait-based scheme for urban foresters, as presented by Watkins et al. (2021), builds upon the competitor-stress tolerator-ruderal (CSR) theory. It demonstrates the diversity of viable plant strategies, which are characterised by a trade-off between fast growth and high tolerance of stress. This allows for the identification of tree species that are well-suited to urban forestry sites. (A) Positions A and C represent the 2 extremes of the trade-off between competitive and stress-tolerant strategies, with position B representing a generalist strategy. In disturbed environments, a greater investment in reproduction and faster growth is required, resulting in more ruderal strategies (Position D). In more stressful situations, delayed sexual maturity allows for greater investment in dense structural and photosynthetic tissues (Position E). It should be noted that, in contrast to other similar graphs, the trait trade-off is fitted by a quadratic rather than a linear line of best fit. (B) Overlays environments found in urban forests upon this model, resulting in a method for identifying the most suitable tree species for urban forestry sites.

In a landmark study by Pierce et al. (2017), a plethora of species were positioned within the CSR model through the assessment of an array of traits pertaining to resource investment and environmental adaptation. The study demonstrated that the utilisation of a reduced set of 3 leaf traits, in lieu of the original 14, resulted in a relatively minor loss of information. Moreover, the multivariate plant functional space described by leaf traits alone was found to be representative of whole-plant functional variation. This is further verified in a comprehensive review by Reich (2014), which established a correlation between the investments made by leaves, stems, and roots of plants in order to cope with varying resource availability and climate situations. In the same study, Reich concluded that leaf investments provide a framework for assessing the capacity of different species to manage different habitats and for obtaining guidance on their investment priorities and thus their growth rate. The 3 leaf traits presented in Pierce et al. (2017) are leaf area (LA), leaf dry matter content (LDMC), and specific leaf area (SLA). Leaf area (LA) represents interspecific variation in plant size, while leaf dry matter content (LDMC) represents conservative resource use. Specific leaf area (SLA) represents acquisitive resource use. The combination of these traits provides a robust indication of the classification of the different species in the CSR model. This, in turn, allows for an overview of a large number of species and their plant economic investment, which is based on plant ecological strategies derived from functional traits. This, in turn, explains the trade-off between dry matter investment in leaf structure and the potential rate of resource return.

While these conceptual ecological frameworks offer a valuable means of describing or predicting plant fitness, the challenge of accessing the requisite data to ordain plants in CSR space persists. To date, the majority of published studies in this field have been conducted using either plants that have been cultivated specifically for the purposes of the study in question, or established plants in their planted location. This approach has the effect of constraining the potential for data to be shared between sites or projects. The aggregation of extensive trait databases from disparate studies offers promising avenues for macro-ecological inquiry (e.g., Ely et al. 2021) and the evaluation of hypotheses pertaining to ecological restoration (e.g., Crowther et al. 2022). Nevertheless, it remains uncertain to what extent these data can be employed in fine-grained inquiries, such as plant selection at either interspecific or intraspecific levels in complex stress environments, including urban forests.

The principal objective of this study was to evaluate the leaf economics spectrum for trees and shrubs that are currently used, or have the potential to be used, in urban forests in Northwestern Europe (in total 342 taxa—different species and genotypes) and categorise them using the CSR classification. In this study, the leaf economic spectrum is described through the investment of nutrients and dry mass in leaves. This reflects a mixture of direct and indirect causal relationships between traits independently of growth form, plant functional type, or biome (Wright et al. 2004). The second objective was to discuss how the CSR classification system applied plant ecological strategies to plant specification in urban environments.

Materials and Methods

The method for ordinating species in CSR space follows the framework presented in Pierce et al. (2017) where LA, LDMC, and SLA are integrated in a single model. Data were assembled at the Swedish University of Agricultural Sciences Campus Arboretum in Alnarp, which hosts one of the largest collections of woody plants in Northern Europe. Alnarp is considered to have a fully humid, oceanic temperate climate with warm summers (Cfb in the Köppen-Geiger climate classification system)(Kottek et al. 2006). The trees and shrubs studied in the Arboretum at Alnarp were all established for over 10 years in unconstrained rooting space, were growing in full sun or only subjected to short periods of partial shade, and had no visual symptoms of stress. The species included in the study were limited to taxa with more than 6 individuals in order to achieve an adequate number of replicates—for a complete list of species, see Appendix.

The methodology used for collecting leaf trait data followed the standard presented in Pérez-Harguindeguy et al. (2013) which is a very well experienced methodology. Specific leaf area (SLA) is calculated as the ratio of one-sided area of a fresh leaf to its oven-dry mass and thus indicates how much leaf area a plant builds with a given amount of leaf biomass:

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Figure 2.

Ordination of trees and shrubs growing in Alnarp, showing the different evolutionary strategies of each life form classifying plants according to 3 principal strategies (Competitors [C], Stress tolerators [S], Ruderal [R]), which represent a spectrum of plant forms and functions arising under conditions of competition, abiotic restriction to growth, or periodic disturbance, respectively. In this assessment, the study taxa of trees and shrubs were distributed along the Competitor-Stress tolerator (CS) axis of the ternary plots. It indicates that long-lived organisms such as trees have a limited tolerance to recurrent disturbances with extensive biomass losses, while this tolerance is somewhat higher in shrubs concerning disturbance, showing that shrubs have a more general approach to disturbance as well as competition and stress (CSR) compared with trees.

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Figure 3.

Ordination of trees growing in Alnarp, with examples of species within the different sections of the model according to 3 principal strategies (Competitors [C], Stress tolerators [S], Ruderal [R]) which represent a spectrum of plant forms and functions arising under conditions of competition, abiotic restriction to growth, or periodic disturbance, respectively. This shows a clear concentration of CS strategists with a relatively high tolerance to stress (resource constrained conditions) with a relatively good growth rate. Among the species that are more pronounced C-strategists, there are several pioneer species from resource-rich habitats, while pronounced S-strategists originate from resource-limited habitats such as warm and dry environments to shade late successional phases in forest environments.

where A is the area of a given leaf and M_L is oven-dry mass of that leaf. Typical units are m² kg^–1 or mm² mg^–1.

Leaf area (LA) is the most commonly used metric for leaf size and is defined as the one-sided or projected area of an individual leaf, expressed in mm². Heat stress, cold stress, drought stress, nutrient stress, and high-radiation stress all tend to select for relatively small leaves, which makes LA a strong trait in the plant economic spectrum.

Leaf dry-matter content (LDMC) is the oven-dry mass (mg) of a leaf divided by its water-saturated fresh mass (g), expressed in mg g^–1. LDMC is widely used as an indicator of plant resource use and has been shown to correlate positively with leaf lifespan (Shipley and Vu 2002).

Results

In this assessment, the study taxa (170 trees and 172 shrubs) were distributed along the Competitor-Stress tolerator (CS) axis of the ternary plots. Analysis of data gathered in Alnarp Arboretum revealed that shrubs had a broader position in the model with a greater prevalence of stress tolerators and species with a more general approach to disturbance as well as competition and stress (CSR) compared with trees, where 6 shrub species are categorised as CSR compared to 1 tree species (Figure 2). Among the tree species evaluated, there was a large proportion that act as CS strategists: 69 out of 170 species (Figure 3). The S-strategists included tree species that are naturally found in very challenging environments where it is an advantage for a species to equip itself with expensive leaves/needles. Thus, they included many evergreen species that specialise in hot, dry environments, such as Pinus leucodermis, or shady environments, such as Tsuga heterophylla and Taxus cuspidata (Figure 3). In contrast, the tree species categorised as C-strategists were distinct pioneer species in resourcerich habitats, such as Pterocarya fraxinifolia, Liriodendron tulipifera, and Sorbus ullungensis. Among the species included in the study, there were fewer C-strategists among the shrubs, but significantly more species that are distinct S-strategists and more generalist (CSR-strategists). Even among shrubs, the S-strategist species were those that specialise in dealing with very challenging conditions by having evergreen needles/leaves, such as Taxus baccata, Buxus sempervirens, and Berberis × fricartii, which gives them flexibility in shady conditions (Figure 4). The S-strategists among the shrubs also included many species with pronounced high tolerance to exposed and periodically very dry environments, such as Potentilla fruticosa, Rosa rugosa, and Spiraea trilobata (Figure 4). For a full summary of the categorisation within the CSR-classification of the different species in the study, see Appendix.

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Figure 4.

Ordination of shrubs, growing in Alnarp, classifying them according to 3 principal strategies (Competitors [C], Stress tolerators [S], Ruderal [R]), which represent a spectrum of plant forms and functions arising under conditions of competition, abiotic restriction to growth, or periodic disturbance, respectively. This shows a broader spectrum of strategies with the exception of pronounced C-strategies compared to trees. Due to the limited ability of shrubs to compete with trees for sunlight, many species have developed characteristics to cope with more extreme conditions such as hot and dry sites or as undergrowth in mature forest environments, resulting in a large proportion of specialists and thus a large proportion of S-strategists.

Figure 5 presents a 2D diagram in which the data collected in the study and their corresponding Competitive Strategy Position (CSR) have been plotted. The diagram includes the variables of stress tolerance and competitiveness (growth rate), which have been used to construct the S-C axis, as illustrated in Figure 2. Position A in Figure 5 encompasses distinct C-strategists that flourish in environments characterised by high resource availability and low disturbance. Such conditions are exemplified by swale bottoms and open park environments, which afford ample space above and below ground. Position B is designated for generalist plants that demonstrate a relatively extensive range of adaptive capacity and are classified as CS strategists. Such environments may be likened to those found in parks and gardens, which offer a somewhat limited supply of resources. These include shaded environments and short periods of drought. Position C refers to plants that are well adapted to high-stress environments, such as paved areas or drought-prone sites where resource constraints can be very pronounced. These species are therefore classified as pronounced stress strategists, which can cope with these resource-limited conditions through expensive and resilient investments, but at the expense of rapid growth. Position D refers to plants with a higher degree of adaptation to disturbed sites, making them more suitable for pioneering roles in new plant communities, such as young micro-forest plantations.

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Figure 5.

The S-C axis in variation, overlaid with hypothesised fitness in urban environments, where position A relates to situations with high resource availability and low disturbance (e.g., swale bottoms or parks); position B relates to plants with generalist that have a relatively wide range of adaptive capacity; position C relates to plants that are well fitted to high stress environments such as paved areas or sites prone to drought; and position D relates to plants with a higher degree of adaptation to disturbed sites, making them more suitable for pioneer roles within new plant assemblages (S = logC² + logC – 76.13 [1]; S = logC² + logC – 104.65 [2]).

Discussion

Given that a significant proportion of the available guidance for selecting trees and shrubs for urban environments is based on the observations and personal experiences of various authors, rather than on controlled scientific studies, it can be challenging to translate these experiences to other climates or to contexts characterised by more complex urban situations. Such information often originate from arboretums, botanical gardens, or authors’ own gardens, which may be the source of many plant literatures towards use of trees and shrubs in a more horticultural approach (Sjöman and Nielsen 2010; Sjöman et al. 2018; Watkins et al. 2020). In order to supplement these recommendations based on observation regarding the selection of an appropriate plant for a specific location and function, it is possible to utilise studies of functional traits that can elucidate why certain species are more suited to, for instance, resource-limited habitats or whether they will undergo a slower establishment and development. Although studies on functional traits focus on how different species have developed traits to cope with different types of climate and growing environments and compete for resources in their natural growing environments, these studies are of great importance when used in urban forestry despite the different competitive situations (Watkins et al. 2021). The inherited attitude to growth and investment of traits to cope with different resource constraints remains strong even when a tree or shrub is planted in a different competitive situation (Laughlin 2023). This makes the plant economic perspective a valuable addition to the more observation-based guidance, as it can explain why different species are slower in their growth and how they cope with, for example, more stressful environments. There is clear evidence demonstrating that plant economics is a driving factor for the ability of different species to compete within different plant environments (Diaz et al. 2016). Functional traits have thus been used as the main foundation for constructing classifications to identify ecological strategies that indicate how species cope with environmental factors and are assembled in a given community (Grime 1977; Westoby 1998; Westoby et al. 2002; Reich 2014; Pierce et al. 2017). Understanding the investment strategy employed by different species can provide good guidance on their ability to handle different plant environments even in cultivation and predict their development. In this study, we evaluated 342 species and cultivars of shrubs and trees based on 3 leaf traits (SLA, LDMC, LA) with the aim of classifying the species from a leaf economic perspective. Inspired by Pierce et al. (2013), we opted to communicate this through the CSR model based on Grime’s theoretical triangular scheme of Competitor, Stress tolerator, and Ruderal plant strategies (Grime 1974, 1977, 2006; Grime and Pierce 2012). These 3 principal strategies represent viable trait combinations arising under conditions of competition, abiotic limitation to growth, and periodic biomass destruction, respectively, and predictions require data on only 3 morphological functional traits representing the extremes of leaf economics and the leaf size spectra (Rosado and de Mattos 2017). Due to its applicability across distinct habitats, CSR theory, despite important shortcomings (Wilson and Lee 2000), has been proven to be powerful because of its efficiency (Caccianiga et al. 2006; Cerabolini et al. 2010; Negreiros et al. 2014; de Paula et al. 2015). The CSR analysis tool proposed by Pierce et al. (2017) can therefore be considered a valuable way to clarify some of the mechanisms explaining plant community assembly and environmental filtering across scales (Rosado and de Mattos 2017). Our analysis of 342 species revealed that the C-strategists comprised many species that have their natural origin as pioneer species in resource-rich habitats, i.e., environments with a very good supply of water, nutrients, and sunlight. They included e.g., 2 species of wing nuts, Pterocarya fraxinifolia and P. insignis, which originate from river valleys or other moist terrain with a good supply of nutrients from a constant supply of sediment and more or less unlimited supply of sunlight (Maharramova et al. 2018; Song et al. 2020). Other species identified as successful in resource-rich habitats were Magnolia obovata, Liriodendron tulipifera, and Sorbus ullungensis. This confirms the reliability of the CSR model in positioning these species as distinct C-strategists, as both in nature and in cultivation they show very strong growth in resource-rich habitats (Dirr 2011). It is important to clarify that the role of the competitive strategist is to leverage the resources available through sunlight, water, and nutrition to facilitate vigorous growth and thus create a competitive advantage. This vigorous growth is also evident in the cultural context, with the aforementioned species described as easily established and fast-growing even when free-growing without any competition from other adjacent trees or shrubs (Sjöman and Anderson 2024).

The tree species analysed in the study included a large proportion of species shown previously to be CS strategists (Figure 3). This is explained by the fact that many species of trees invest in deep root systems to create an advantage during drier periods, which makes them a combination of the C-strategists’ investments to create an advantage and the S-strategists’ investments to manage or avoid a stressful situation which is demonstrated by the hickory species (Carya spp.) in the study as having distinct CS strategies, as noted in Grime (2006), and where the ability of the genus to invest in deep taproots that can reach resources at depth and thus escape resource limitation during hot and dry summer months is recognized even in cultivation (Sjöman and Anderson 2024). Species with pronounced Stress tolerance strategies among the trees were dominated by evergreen tree species such as firs (Abies spp.), pines (Pinus spp.), yews (Taxus spp.) and hemlocks (Tsuga spp.), all of which are species from different types of resource-limited habitats such as warm, dry environments or cold, shady habitats. The costly investment required for the production of evergreen needles is substantial; however, this strategy has proven to be highly effective in addressing more challenging environmental conditions. The inherent flexibility of evergreen species throughout the year is attributable to their ability to photosynthesise when conditions are most conducive, such as when overgrown and emerging species lack leaves, or in cold climates with a limited growing season, or in warm, periodically dry habitats (Keddy 2007). Even among shrubs, many evergreen species in the study were found to be distinct S-strategists, such as Lonicera nitida and Berberis × fricartii. It is perhaps not surprising that the majority of the shrubs are S-strategists and thus more disturbance-tolerant, since shrubs are present in most vegetation systems globally and extend much farther than trees, occupying tree-free habitats such as high mountain areas, deserts, steppes, and habitats with recurrent disturbances (Rundel 1991; Archibold 1995). A reason for the greater prevalence of shrub species among S-strategists (Figure 4) is their limited ability to compete for sunlight with tall trees and face limited light interception conditions under a compact canopy of overgrown trees. This means that shrubs have developed unique properties and investments in order to handle these sometimes very stressful conditions. Apart from investment in leaf structure, being a low and multi-stemmed shrub also creates resilience to disturbance such as browsing by deer (Tanentzap et al. 2012). Moreover, a single-stemmed tree faces a lethal risk if the stem breaks and dies due to e.g., harsh weather conditions, falling trees/branches, drought through risk of cavitation, disease, or browsing and trampling by animals, whereas a shrub can afford to lose some of its stems and still survive (Wilson 1995; Ryan and Yoder 1997; Scheffer et al. 2014; Götmark et al. 2016).

Many studies have reported extensive intraspecific variation in leaf economic traits (e.g., Poorter et al. 2012; Laforest-Lapointe et al. 2014; Morrow et al. 2022), which can vary between different ecotypes within the same species. These findings must be more carefully explored in the future, to evaluate the capacity of different ecotypes to manage a changing climate. We may increasingly have to depart from treating species as a unified concept and instead move towards considering the unique investment strategy of different ecotypes to manage competition and/or stress (Sjöman et al. 2024). The results in this study show the importance of evaluating unique genetic plant material of different plant collections regarding their functional traits in increasing understanding of how different species and ecotypes prioritise different investments in order to create tolerance and success in their development. This is of critical importance in efforts to match the right plant to the right place and function in urban environments. This intraspecific variation was also obvious in the dataset of this study with variation in leaf traits between different genotypes of the same species in e.g., Prunus lauracerasus and Salix lanata (see Appendix). It is crucial to recognise that the plant material examined in this study represents a narrow subset of the genetic diversity present across the species’ entire geographical range. This is a significant consideration when interpreting the data, as other material may exhibit differing investment priorities, reflecting the specific plant environments and climates from which the unique genetic material originates. A more extensive international screening of a broader genetic pool within species is of significant importance for the enhancement of plant material currently in cultivation, enabling it to be upgraded to better withstand future climate scenarios. The present study has focused on species originating from temperate climates. However, further complementary studies of species and ecotypes in other climates are essential to gain a more comprehensive understanding of the diverse genetic types of a single species across different climatic zones. This is a crucial aspect when attempting to match plants for future climate scenarios, particularly in light of the observed climate shift towards a warmer climate in Northern Europe, which urban plants must adapt to (Lyon et al. 2022).

However, a shortcoming of the CSR framework today is that it does not provide a mechanistic framework for categorising the specific environments in which the different species are likely to be best fitted, especially for stress strategists. This is because their investments can be based on specific stress conditions, such as drought, flood, shade, etc., while competition strategists have a more significant and clearer use of their repertoire of growing environments to achieve successful development, which is mainly in resource-rich habitats. However, in recent studies on drought tolerance of trees and shrubs (e.g., Sjöman et al. 2018; Sjöman et al. 2023), there is a clear correlation between the most drought-tolerant species when assessing estimated water potential at leaf turgor loss with the stress strategists in this study, such as Acer tataricum, Berberis julianae, Chaenomeles japonica, Cotinus coggygria, and Spiraea betulifolia, indicating that the leaf economical and ecophysiological perspectives are compatible when it comes to drought tolerance between species from different datasets. Further research is however needed to provide a more detailed description of the unique stresses to which the different stress strategies are adapted to. Nevertheless, in this study, we want to test and explore leaf economy perspectives linked to the CSR model and see how this can become a tool for selecting the right plant for place and function in urban environments. In Figure 5, we further explore the axis of variation of the Competitive-Stress tolerator by using the C and S ordinations in a 2D perspective. This arrangement clearly identifies the adaptive limits to growth according to Watkins et al. (2021), as well as the areas of trait space that result in inefficient resource use relative to environmental tolerance. As a result of this step, we can not only derive quadratic equations describing the distribution of shrubs and trees along axes of evolutionary variation, but also refine the hypothesis of Watkins et al. (2021) to quantitatively describe design uses for tree and shrub species in urban environments. Through this model (Figure 5) it is possible to further link the CSR perspective towards use potential of trees and shrubs for urban environments and for specific functions such as for stormwater management.

Another perspective that links plant economics, the CSR model, and plant use is the relative priorities of different species with regard to growth. Species originating from resource-rich habitats with a good supply of water, nutrients, and sunlight have developed the ability to exhibit rapid growth in order to create a competitive advantage. In contrast, species that have developed strategies for more resource-limited habitats exhibit defensive growth in response to resource limitations. This trait of more limited growth based on evolutionary selection where tolerance of habitat is prioritized over vigorous growth in resource limited habitats is an approach that is also maintained in cultivation. In a landmark study by Grime and Hunt (1975) it is shown that species originating from resource-limited habitats (S-strategists) maintained lower growth even in cultivation under ‘optimal’ conditions, which has since been confirmed by numerous studies (Laughlin 2023). From an urban horticultural perspective, this is of great importance in the establishment and growth of shrubs and trees as extended establishment management with irrigation is necessary for the slow S-strategists, while establishment and growth is significantly faster for the C-strategists (Figure 6). When using vegetation to deliver important ecosystem services, the size of the trees and the shrub vegetation is often important, such as shading over playgrounds, which means that using fast-growing C-strategists is tempting, but in order to reach this rapid growth, it is crucial that resource-rich conditions be offered. This perspective on growth rate based on this CSR summary in the study makes it possible to tailor establishment management and predict how and when different functions can occur where, for example, using shrubs as ground cover in urban plantations, it is important to know which ones are fast and slow in order to predict management and when final function of the plantation will be reached.

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Figure 6.

Species originating naturally from resource-rich habitats have developed unique traits with a rapid turnover in growth to create a competitive advantage (C-strategy), traits that they retain in cultivation as they establish and grow rapidly in resource-rich environments such as parks and gardens. Species originating from resource-limited habitats have a slower turnover in growth due to limited resources (S-strategy), a characteristic they also have in cultivation, even on resource-rich habitats, resulting in significantly slower establishment and growth. Figure developed with inspiration from Chapin (1980) and Laughlin (2023).

The intention of this study was to establish a link between a well-known and widely used model in biology and the utilisation of urban plants. It has been observed that a significant number of trees and shrubs are planted in conditions that do not allow them to flourish, resulting in a considerable proportion of these plants either dying or exhibiting impaired growth and establishment (Kianmehr et al. 2024), and thus underscores the necessity to enhance the precision of our approach and to embrace diverse ecological and biological perspectives. This entails a departure from the prevailing tradition in urban forestry, which has been largely guided by the experiences and observations of a limited number of individuals to a more trait orientated plant selection (Watkins et al. 2021). The development of a more nuanced language surrounding the selection and utilisation of plants hinges upon the availability of species-specific data, as exemplified by the findings of this study. Currently, such data is scarce, impeding the ability to engage in informed discourse on plant selection with other professional groups, including politicians and construction companies. The integration of a plant economic perspective offers a compelling biological rationale for the selection of specific species in given circumstances. Moreover, it enables a more constructive dialogue during the planning phase, addressing potential challenges in the establishment process. Further research is required to ascertain the specific stress conditions to which the different species are adapted, based on their stress strategies. However, if this can be achieved, the CSR approach can serve as a foundation for communicating the selection of an appropriate plant for a given site and function. This entails identifying the characteristics of the site in the planning phase, including its opportunities and constraints, as well as the investment priorities of different species and their alignment with the site-specific conditions.

Conflicts of Interest

The authors reported no conflicts of interest.