Cohort-specific tuning of foliage physiology to interacting stresses in evergreens.

Leaf economics spectrum is the basic concept linking leaf carbon gain strategies, fast vs slow turnover, to foliage functional traits (Wright et al. 2004). At the heart of the economics spectrum is the reverse correlation among traits responsible for leaf longevity and physiological activity; thus, high leaf longevity in evergreens is associated with traits improving leaf robustness such as high leaf dry mass per unit area and density reflecting enhanced biomass investment in support biomass (Wright et al. 2004). Increased support investments inevitably result in low photosynthetic capacity per dry mass placing evergreens towards the slow return end of the leaf economics spectrum (Wright et al. 2004, Reich 2014). The economics spectrum describes the basic co-variation patterns among leaf functional traits across plant functional types, but it does not implicitly consider several potentially confounding sources of variation such as leaf age (e.g., Niinemets 2014). Evergreens characteristically support multiple leaf age classes with potentially contrasting structural, chemical and physiological characteristics, but due to practical reasons, often only the leaves from the newest fully mature leaf flush are included in analyses involving evergreens. However, older leaves contribute a major proportion of total canopy leaf area in many evergreens (Whitehead et al. 1994, Niinemets et al. 2005), implying that age-specific differences in foliage performance play a major role in whole canopy carbon gain. While the basic age-dependent modifications in foliage functional traits have been studied in evergreens (Kitajima et al. 1997a, Radoglou and Teskey 1997, Ellsworth 2000, Niinemets and Lukjanova 2003, McGarvey et al. 2004, Niinemets et al. 2005), there is limited information on responses of different-aged leaves to environmental stresses with a few exceptions (e.g., Mulkey et al. 1992, Ogaya and Penuelas 2006). Many evergreen species also have a complex phenology of foliage development characterized by multiple foliage flushes during the same growing season. In this issue of Tree Physiology, the study of Morales et al. (2014) conducted in a Chilean temperate rain forest with the evergreen relatively shade-intolerant species Eucryphia cordifolia Cav. demonstrates that seasonality in environmental conditions can strongly affect foliage growth and development as well as physiological potentials of fully mature leaves. They observed major differences in leaf structural, chemical and physiological traits among leaf flushes produced during a given growing season (Figure 1, Morales et al. 2014). Such flush-specific differences in foliage characteristics have been observed in several warm temperate and subtropical conifers (Whitehead et al. 1994, Teskey 1997, Niinemets et al. 2002, McGarvey et al. 2004) and in evergreen angiosperms in climates with seasonal distribution of precipitation (Mulkey et al. 1992, Kitajima et al. 1997b), and collectively emphasize the importance of the timing of leaf flush in foliage functioning. The study of Morales et al. (2014) further found that differences among leaf cohorts in E. cordifolia were amplified in the low-light environment and under water stress (Figure 1). Although stress led to immediate physiological adjustments such as changes in stomatal conductance, the scaling among foliage physiological potentials (maximum carboxylase activity of Rubisco) and leaf nitrogen content was also altered (Figure 1). Such modifications in foliage functioning to stress during foliage growth suggest that the timing of leaf flush is an important driver altering the leaf economics spectrum in Commentary Tree Physiology Advance Access published November 25, 2014