Leaf tissue density (LTD), specific leaf area (SLA) and leaf nitrogen concentration (LNC) are recognized as key economic traits and have been widely paid attention in the libraries (Craine et al. 2001; Wright et al. 2004). These traits have been observed to have wide variations across co-existing species (Niinemets 1999; Poorter et al. 2009), reflecting the adaptation of plants to different growth strategies (Grime, 2001; Westoby et al. 2002). Leaf tissue density represents carbon (C) investment in tissue construction (Ryser 1996), indicating a conservative trait of slow-growth plants (Reich 2014). Generally, leaves with dense tissue will invest more C into tissue construction, rendering longer life span and greater resistance to biotic and abiotic stresses such as herbivory and drought (Niinemets 1999).
By contrast, high SLA, which associates with high photosynthetic rates (Poorter and Bongers 2006), equates to a larger light-capturing leaf area deployed for a given investment in leaf mass, which enables plants to rapidly grow under ample supply of resource-light, water and nutrients (Westoby 1998). Meanwhile, leaf photosynthesis requires large investments in leaf nitrogen (N) in form of proteins and enzymes (Evans 1983; LeBauer and Treseder 2008), and therefore leaves with high SLA and LNC often have high capability to capture light and assimilate C (Reich et al. 1998; Rotundo and Cipriotti 2017). SLA and LNC are often considered as acquisitive traits of fast-growth plants (Wright et al. 2004). Moreover, these key economic traits are shown close correlations with each other (Witkowski and Lamont 1991; Wright et al. 2004). Generally, LTD is negatively correlated with SLA and LNC (Niinemets 1999). Meanwhile, SLA is positively correlated with LNC (Reich et al. 1997; Poorter and Evans 1998; Tjoelker et al. 2005). These key economic trait correlations largely underlie the ‘leaf economic spectrum’ (LES) (Wright et al. 2004; Reich 2014).
The LES favors coordination among a set of key economic traits along a functional trait spectrum, reflecting changes in plant growth strategy from fast, resource-acquisitive syndromes to resource-conservative syndromes (Wright et al. 2004; Reich 2014). The functional trait spectrum explains most parts of interspecific variation in key economic traits in terms of C assimilation and nutrient use (Wright et al. 2004), probably due to trait convergence driven by strong selection (Reich 2014). Specifically, species at the one end of the spectrum with high SLA and LNC will have high C assimilation capability, namely, resource acquisitive strategy (Wright et al. 2004). By contrast, species at the other end of the spectrum with high LTD will invest more C into tissue construction, leading to high tissue construction cost, namely, resource conservative strategy (Wright et al. 2004). Together, the LES highlights leaf photosynthesis and tissue construction, which are directly associated with leaf anatomical structure. There are some reports on leaf economic traits and anatomical structure, such as literature surveys (Niinemets 1999; Sack et al. 2013), models (Blonder et al. 2011), glasshouse experiments (John et al. 2017). However, how leaf anatomical structure is involved in these key economic traits remains unresolved, but is important to comprehensively understand mechanisms underpinning the LES.
Leaf functions are tightly coupled with their anatomical characteristics (Terashima and Hikosaka 1995). For example, mesophyll tissues are responsible for light capture and C assimilation (Ren et al. 2019). Specifically, palisade tissues with more chloroplasts can maximize light interception and C assimilation, while spongy tissues with larger intercellular air space can maximize gas transportation. By contrast, veins are specialized for the transportation of water, nutrients and carbohydrate (Sack and Holbrook 2006). Generally, increased investment of vein mass/volume provides greater hydraulic capability, but potentially reduces mesophyll light capture and C assimilation (Sack and Scoffoni 2013). Meanwhile, the veins entail substantial construction cost (Sack and Scoffoni 2013). Across species, the major veins can contribute to substantial amount to the leaf mass per unit area (Niinemets et al. 2007). The tissue density of minor veins has been demonstrated to be 5-fold higher than that of mesophyll tissues (Poorter et al. 2009). This may be because that mesophyll tissues are comprised of highly-watery parenchyma cells (Canny and Huang 2006), while veils are primarily comprised of lignified cells (Sack and Scoffoni 2013). Therefore, plant leaves with high vein fraction per leaf volume/high vein length per leaf area are expected to have high LTD.
Nitrogen is the most abundant nutritional element in leaf tissues and up to 80% of N is allocated to chloroplasts in leaves (LeBauer and Treseder 2008). This is because N is a key component of chlorophyll and photosynthesis requires massive proteins and enzymes (Pibeam, 2011). For example, c. 50% of photosynthetic N is comprised of Ribulose-1,5-bisphosphate carboxylase/oxygenase (Evans 1998). Meanwhile, a larger number of chloroplasts are contained in the palisade tissues and the chlorophyll formation closely associates with the amount of assimilated N (Terishima and Inoue 1985). Therefore, plant leaves with high fraction of palisade tissue are expected to have high LNC.
In this study, we attempted to elucidate how leaf anatomical structure affects three key leaf economic traits, such as LTD, LNC and SLA across co-existing species. Leaf anatomical structure and three key economic traits were examined using 102 species in Chinese temperature forests, including woody plants, grasses and forbs. We predicted that species with high vein fraction per leaf volume/ high vein length per leaf area would have high LTD, and that species with high palisade tissues per leaf area would have high LNC. For monocotyledonous species, cells are not differentiated in mesophyll tissues and photosynthesis occurs in the whole mesophyll tissues (Esau, 1977). Therefore, we measured the thickness of mesophyll tissues instead of palisade tissue thickness.