The vascular plant adaptive radiation involves the evolution of various aspects of phenotype operating across a range of scales, from molecular, cellular, organ and whole plant life history characters. Adaptive or functional traits (characters that affect survival) and plant ecological strategies are typically measured and analysed at the level of organs (leaves, roots, stems, flowers and dispersules) and the whole plant (e.g. integrated suites of traits including canopy height, biomass production and relative growth rate, and the timing of reproductive events). At this scale, differential expression of traits reflects fundamental trade-offs in resource and biomass investment (Grime 1965), the importance of which has been confirmed world-wide (Díaz et al. 2016) and is known to limit plant adaptation to tenable trait combinations. The principal trade-off involves “attributes conferring an ability for high rates of resource acquisition in productive habitats and those responsible for retention of resource capital in unproductive conditions” (Grime et al. 1997), resulting in “a spectrum of plant functional types which in terms of resource processing range from ‘the acquisitive’ to ‘the retentive’ and correspond respectively to highly productive and chronically unproductive vegetation” Grime and Mackey (2002). This is now widely recognised as the ‘leaf economics spectrum’ or ‘plant resource economics spectrum’ (Wright et al. 2004; Reich 2014). The second main axis of functional trait variability involves plant and organ size traits, which together with the economics spectrum form the ‘global spectrum of plant form and function’ (Díaz et al. 2016). Beyond this ‘primary’ trade-off, ‘proximal’ traits are specific characters in response to selection pressures acting at particular moments of the life cycle: for instance, reproductive traits such as the pollination syndrome or seed dormancy breaking requirements (Grime and Pierce 2012). Integration of functional traits into ecological strategies is also a key concept because plant fitness may ultimately depend on the optimization of trait integration along the trade-off axes more than it does on the performance of any single trait (Guo et al. 2018).
Crucially, the extent to which plant and organ-level trade-offs are evident at smaller scales, such as tissues or cells, is little understood, although the partitioning of essential hydrocarbons and minerals between tissues with contrasting roles is likely to be fundamental to plant functioning (Grime and Pierce 2012). To date these fine-scale physiological and structural trade-offs have been investigated only in the context of specific leaf economics traits, usually leaf mass per area (LMA) or its inverse, specific leaf area (SLA) (Hassiotou et al. 2010; Villar et al. 2013; del la Riva et al. 2016; Xiong et al. 2016; Guo et al. 2017; John et al. 2017; Onoda et al. 2017; Belluau and Shipley 2018). For instance, greater LMA of deciduous trees is related to the ratio of mesophyll (chlorenchyma tissue) to intercellular airspace and epidermis (Villar et al. 2013). Reduced intercellular airspace is known to reduce mesophyll conductance to internal CO2 diffusion, limiting photosynthetic rates (see Ye et al. (2020) and references therein). It is also clear that the relative extent of tissue types comprising leaves varies with environmental factors, such as elevation (Liu et al. 2021). However internal anatomical variation has not been placed in the context of the wider ‘global spectrum’ trade-off between economics and size.
Notably, while resource economics variation is a property of the entire leaf, large leaves exhibit extensive lamina area but must be physically supported by prominent mechanical/vascular venation, in particular the main mid-vein, suggesting a division of ‘photosynthetic’ and ‘supportive’ roles between the lamina sensu stricto and venous portions. Thus, is it reasonable to predict a prominent division of tissue types (mesophyll chlorenchyma vs. mechanical/vascular) between lamina and venous leaf portions for species with larger leaves. Crucially, this generally agrees with Li et al.’s (2017) concept of modular function, whereby leaf tissue types reflect three main ‘modules’: the light capture module (essentially chlorenchyma), water-nutrient flow (vascular tissues), and gas-exchange (intercellular airspace and the CO2 diffusion pathway to stomata). Mechanical tissues could be said to represent a fourth ‘support’ module, but are often integrated with vascular bundles as xylary reinforcement.
How can we relate anatomical trade-offs to macroscopic functional trait trade-offs and plant adaptive/ecological strategies?
Currently, the only theory of plant strategies that can provide a general explanation for the evolution of the joint economics and size trade-off (i.e. that considers the context of the natural selection pressures that delimit plant functioning) is Grime’s (1974) CSR (Competitor, Stress-tolerator, Ruderal) theory (discussed by Pierce and Cerabolini 2018; also Grime and Pierce 2012, Pierce and Fridley 2021). Competitor species dominate stable, resource rich habitats by pre-empting resources using traits permitting rapid growth to large size (i.e. large ‘size of whole plants and their parts’, sensu Díaz et al. 2016). Stress-tolerators dominate in habitats where limiting and variable abiotic factors constrain metabolic performance, and are robust and slow-growing (conservative resource economics), some eventually becoming large. Ruderals dominate in habitats where biomass is periodically destroyed (disturbance) and are characterised by rapid growth using extremely ephemeral leaves (acquisitive economics), with populations persisting as propagules rather than mature individuals. Aside from this theoretical context, the CSR scheme also provides a practical quantitative framework for the comparison of individuals and species. In practice, quantification of CSR strategies is performed using leaf functional traits that represent resource economics and size trade-offs (Cerabolini et al. 2010; Pierce et al. 2012, 2013). Specifically, the resource economics spectrum (S to R-selection) is determined from leaf fresh and dry mass and area measurements that allow calculation of structural/photosynthetic tissue density traits. A leaf size axis arising perpendicular to this terminates in the extreme of C-selection (Pierce et al. 2013). The CSR score represents not economics or size per se, but the trade-off between these multiple functions.
Calculation of CSR strategies is relevant to ecology across a range of scales encompassing the centimetre-scale of species coexistence within communities (Pierce et al. 2014) to biomes at the global scale (Pierce et al. 2017). Indeed, CSR analysis has been used to predict vegetation responses such as local scale shifts in plant community composition along succession gradients (Zanzottera et al. 2020), regional scale functional shifts in response to climate and soil (Dalle Fratte et al. 2019a; Zhang and Wang 2021) and can help explain species global and native range sizes (Liao et al. 2021). Specifically, the CSR analysis method of Pierce et al. (2017), calibrated using the global-scale economics/size trade-off, is now a well-established method applied worldwide to explain intra- and interspecific functional variability and environmental responses for wild plants in natural circumstances (e.g. Dayrell et al. 2018, Vasseur et al. 2018; Dalle Fratte et al. 2019b; Dudova et al. 2019; Baltieri et al. 2020; Behroozian et al. 2020; Giupponi 2020; Ferré et al. 2020; Escobedo et al. 2021; Fernandes et al. 2021; Han et al. 2021; Hooftman et al. 2021; Lazzaro et al. 2021; Mugnai et al. 2021; Tameirão et al. 2021; Watkins et al. 2021). It provides both a dependable method and theoretical context for comparing plant functional variability.
To what extent are different tissue types with different functions associated with CSR strategy variation? To a limited extent this has been investigated within the Poaceae family (Pierce et al. 2007), for which greater intracellular airspace (decreased resistance to CO2 diffusion for photosynthesis) was positively associated with R-selection (and negatively with S-selection). However, beyond this single family, we hypothesise that anatomical/ecological strategy associations may be generally evident across a broader taxonomic range of Angiosperms. Here we quantify the relative extent of leaf tissue types for species with known, measured CSR strategies from a range of flowering plant families, to investigate the specific hypotheses that: 1). R- to S-variation is associated with an increasing extent of mechanical and fibro-vascular tissues and decreasing relative extent of mesophyll chlorenchyma, and 2). C-selected species in particular exhibit lamina and mid-vein anatomies optimised for contrasting functions, with the lamina exhibiting a greater relative extent of chlorenchyma and the midvein specialised in terms of mechanical and vascular tissues (the division of functions being less evident for S- and R-selected species).