Within-Canopy Variations in Functional Leaf Traits: Structural, Chemical and Ecological Controls and Diversity of Responses

Abstract

Plant canopies are characterized by extensive gradients in light availability that importantly alter the photosynthetic productivity of leaves in different canopy layers and result in acclimatory changes in leaf structural, chemical and physiological traits. These within-canopy variations are further importantly driven by species functional type and ecological characteristics such as shade tolerance (ecological controls). This chapter explores the within-canopy variations in key functional traits among different plant functional types and in species with different ecological potentials using a simple methodology to separate the importance of different leaf-level traits in foliage photosynthetic acclimation. As a major acclimatory change, foliage photosynthetic capacity per leaf area (A max A) increases with increasing long-term average integrated quantum flux density (Q int) in the canopy. Within-canopy variation in A max A results in a greater whole canopy carbon gain than having A max A constant through the canopy. The increase in A max A with Q int can potentially result from increases in leaf dry mass per unit area (M A), nitrogen content per unit dry mass (N M) and nitrogen allocation to rate-limiting photosynthetic proteins. This analysis indicates that the importance of these three key factors varies among plant functional types. In species with relatively low rates of canopy expansion and leaf turnover such as woody evergreens and woody deciduous species, within-canopy variation in A max A is primarily determined by M A, while in herbaceous species with high rates of canopy growth and leaf turnover, the variation is mainly driven by changes in N M and nitrogen allocation to rate-limiting proteins of photosynthetic machinery. Furthermore, there are large within-canopy modifications in structural traits such as leaf angles and spatial aggregation modulating light harvesting and light avoidance, and in chemical traits such as xanthophyll cycle carotenoid content and isoprene emission contributing to abiotic stress resistance. As the result of light-dependent alterations in these traits, lower canopy leaves have a greater light harvesting efficiency, while upper canopy leaves a greater capacity for excess radiation dissipation and resistance to abiotic stress. Plasticity for foliar modifications varies among woody species of different ecological potentials with shade-intolerant species tending to have a greater photosynthetic plasticity, while shade-tolerant species greater leaf areas and higher canopy light interception. This review emphasizes the overall large within-canopy variation in key foliage functional traits and underscores the important differences among plant functional types and in species with different ecological potentials in their acclimation to within-canopy environment.