SPECIAL FEATURE Plant responses to heterogeneous environments U ¨ lo Niinemets A review of light interception in plant stands from leaf to canopy in different plant functional types and in species with varying shade tolerance Received: 15 April 2009 / Accepted: 26 February 2010 / Published online: 31 March 2010 Ó The Ecological Society of Japan 2010 Abstract Changes in the efficiency of light interception and in the costs for light harvesting along the light gra- dients from the top of the plant canopy to the bottom are the major means by which efficient light harvesting is achieved in ecosystems. In the current review analysis, leaf, shoot and canopy level determinants of plant light harvesting, the light-driven plasticity in key traits altering light harvesting, and variations among different plant functional types and between species of different shade tolerance are analyzed. In addition, plant age- and size- dependent alterations in light harvesting efficiency are also examined. At the leaf level, the variations in light harvesting are driven by alterations in leaf chlorophyll content modifies the fraction of incident light harvested by given leaf area, and in leaf dry mass per unit area (M A ) that determines the amount of leaf area formed with certain fraction of plant biomass in the leaves. In needle- leaved species with complex foliage cross-section, the degree of foliage surface exposure also depends on the leaf total-to-projected surface area ratio. At the shoot scale, foliage inclination angle distribution and foliage spatial aggregation are the major determinants of light harvest- ing, while at the canopy scale, branching frequency, foliage distribution and biomass allocation to leaves (F L ) modify light harvesting significantly. F L decreases with increasing plant size from herbs to shrubs to trees due to progressively larger support costs in plant functional types with greater stature. Among trees, F L and stand leaf area index scale positively with foliage longevity. Plant traits altering light harvesting have a large potential to adjust to light availability. Chlorophyll per mass increases, while M A , foliage inclination from the horizontal and degree of spatial aggregation decrease with decreas- ing light availability. In addition, branching frequency decreases and canopies become flatter in lower light. All these plastic modifications greatly enhance light har- vesting in low light. Species with greater shade tolerance typically form a more extensive canopy by having lower M A in deciduous species and enhanced leaf longevity in evergreens. In addition, young plants of shade tolerators commonly have less strongly aggregated foliage and flatter canopies, while in adult plants partly exposed to high light, higher shade tolerance of foliage allows the shade tolerators to maintain more leaf layers, resulting in extended crowns. Within a given plant functional type, increases in plant age and size result in increases in M A , reductions in F L and increases in foliage aggregation, thereby reducing plant leaf area index and the efficiency of light harvesting. Such dynamic modifications in plant light harvesting play a key role in stand development and productivity. Overall, the current review analysis dem- onstrates that a suite of chemical and architectural traits at various scales and their plasticity drive plant light harvesting efficiency. Enhanced light harvesting can be achieved by various combinations of traits, and these suites of traits vary during plant ontogeny. Keywords Bifurcation ratio Æ Biomass allocation Æ Foliage distribution Æ Leaf structure Æ Light interception Æ Review analysis Æ Shade tolerance Æ Shoot architecture Æ Stand age Introduction In deeply shaded understories, plant carbon gain can be improved most by capturing more light (Pearcy and Sims 1994). Thus, efficient light harvesting is of paramount importance for plants growing in competition in dense stands under natural conditions (Pearcy et al. 2004; Valladares and Niinemets 2008). At ecosystem scale, strong, close to linear, positive relationships between canopy productivity and the amount of intercepted light U ¨ lo Niinemets (&) Institute of Agricultural and Environmental Sciences, Estonian University of Life Sciences, Kreutzwaldi 1, 51014 Tartu, Estonia E-mail: ylo.niinemets@emu.ee Fax: +372-7313738 Ecol Res (2010) 25: 693–714 DOI 10.1007/s11284-010-0712-4