1 INTRODUCTION

Functional traits that impact tree fitness indirectly via their effects on growth, reproduction and survival are widely studied to understand plant ecological strategies (Díaz et al., 2016; Violle et al., 2007). Trait patterns have been described using various concepts, with the ‘economics spectrum’ currently being one of the most popular models to describe how traits related to resource economics (Chave et al., 2009; Reich, 2014; Wright et al., 2004). For example, the leaf economics spectrum describes leaf-level resource use trade-offs, opposing species with acquisitive or conservative leaf traits based on leaf nutrient concentrations, leaf mass per area (LMA) and leaf life span (Onoda et al., 2017; Osnas et al., 2013; Wright et al., 2004).

In analogy with the leaf economics spectrum, several studies have reported resource economics trade-offs in different tissues, e.g. wood (Chave et al., 2009; Fortunel et al., 2012) and root (Carmona et al., 2021; Prieto et al., 2015; Roumet et al., 2016) tissues, as well as at the whole-tree level (de la Riva et al., 2016; Díaz et al., 2016; Freschet et al., 2010). Some studies have suggested that the traits of leaves, stems and roots are co-ordinated and thus provide a useful framework for exploring the trade-off between acquisition and conservation of resources at the whole-tree level (Reich, 2014). However, other studies on tropical species have shown that trait variation may be decoupled among different organs, suggesting that a consistent, whole-tree economics spectrum is difficult to identify or would at least involve several functional axes (Baraloto et al., 2010; Fortunel et al., 2012). These studies indicate that the economics spectrum model is not ubiquitous at a whole-tree level and across climates. However, differences in the spatial scales considered may contribute to discrepancies among studies on the topic of trait relationships (Escudero & Valladares, 2016). Therefore, further understanding as to whether an economics spectrum exists at the whole-tree level is necessary, especially across a broad range of species and climates and by embracing broader functions related to nutrient and water use.

Non-structural carbohydrates (NSC) are the primary substrates and key energy sources for tree metabolic processes, and play a vital role in multiple functions such as growth, osmoregulation, defence and survival (Hartmann & Trumbore, 2016; Kozlowski, 1992; Sala et al., 2012). NSC produced by photosynthesis can be quickly used for immediate functions or stored for reserves and defence (Klein & Hoch, 2015; Weber et al., 2019), which suggests an allocation-based trade-off between carbon demand and supply. Thus, the content of NSC and allocation patterns in leaves, stems and roots are considered as indicators of balance between carbon sources and sinks (Martínez-Vilalta et al., 2016). For example, species with a lower LMA tend to have greater light capture potential and net photosynthetic rate, which may result in an increase in photosynthates that are then transported as soluble sugars (SS) for rapid plant growth (Li et al., 2016; Wright et al., 2002). Additionally, tree species with tough leaves and high LMA usually have more carbon investment in reserves and defence, allowing individuals to persist when carbon demand is greater than supply (e.g. under severe drought stress) (Anderegg & Anderegg, 2013; Poorter & Kitajima, 2007). However, few studies have directly examined the relationship between NSC and the economics spectrum in trees, and whether NSC in different organs co-ordinates with economic traits (e.g. LMA, nitrogen content and wood density) that define ecological strategies, is still poorly understood. Due to the diverse roles played by NSC in different tree organs (Hartmann & Trumbore, 2016), NSC in leaf and woody organs are likely to exhibit different dimensional characteristics in resource allocation strategies.

We explored the coordination of functional traits by measuring suites of traits characterizing leaves, stems and coarse roots across 60–90 angiosperm tree species from temperate, Mediterranean and tropical forests. Specifically, we sought to test the following hypotheses: (H1) As a product of photosynthesis, NSC may be co-ordinated with economics strategies, but due to the role played by NSC in different organs, we hypothesize that leaf NSC content would co-ordinate with leaf economic traits, while stem and root NSC content would be independent of acquisitive and conservative resource strategies in these organs. (H2) Functional differences between leaves and wood may lead to independent functional trade-offs at the leaf versus stem and coarse root levels. However, given the anatomical continuity of xylem tissues between stem and root systems in woody plants, we hypothesize that leaf, stem and coarse root traits would not be integrated along a whole-tree economics spectrum, but coarse root structural traits such as wood density and anatomical traits would be co-ordinated with stem structural traits.