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- Materials and methods
As sessile organisms, plants should possess a tremendous capacity to adjust to the environment in which they have been dispersed. In tropical rainforests, light is one of the most limiting resources for plant growth and survival (Whitmore 1996). Irradiance is a very heterogeneous resource, and can be as high as 47 mol m−2 day−1 above the canopy and as low as 0·15 mol m−2 day−1 at the forest floor (Chazdon 1988). Moreover, the light environment changes continuously over time, from minutes in the case of sun flecks, to years in the process of canopy-gap closure (Chazdon 1988). Plants adjust continuously to the changing light environment: photosynthetic induction occurs within minutes (Rijkers et al. 2000); photosynthetic adjustments in days (Cai, Rijkers & Bongers 2005); morphological changes take weeks or months (Ackerly 1997); whereas architectural changes may have a time lag of years (Sterck et al. 1999). Modelling approaches show that such acclimation responses enhance the growth (Sims, Gebauer & Pearcy 1994), survival (Sterck et al. 2005) and, ultimately, fitness of the plant (Bradshaw 1965).
Perhaps the most efficient way to acclimate and forage for light is by adjusting the leaf characteristics. Sun and shade leaves differ predictably in a number of functional traits (Bongers & Popma 1988). Sun leaves grow in the exposed conditions of the canopy, and because the amount of light is not limiting they can maximize their photosynthetic capacity by producing thick leaves to increase nitrogen content on an area basis and the volume of photosynthetic machinery per unit leaf area (Björkman 1981; Gulmon & Chu 1981). Overheating of leaves due to excessive light capture needs to be prevented, which can be done through convective cooling of the leaves or by heat loss through transpiration. Sun leaves increase convective heat loss by reducing the boundary-layer resistance (Givnish 1984), which may be realized with small or slender leaves (Parkhurst & Loucks 1972; Bongers & Popma 1988). Heat loss through high transpiration rates is supported by a large water supply to the leaves, possibly facilitated by relatively thick internodes in proportion to leaf area (cf. Westoby & Wright 2003).
Shade leaves grow in the shaded understorey, where light is a limiting resource. Shade leaves increase the efficiency of light capture through a high specific leaf area (SLA) (Evans & Poorter 2001) which, in combination with a high chlorophyll content on a mass basis, leads to a similar chlorophyll content on an area basis compared with sun leaves (Chazdon et al. 1996; Poorter et al. 2000). Shade leaves therefore realize a similar light capture to sun leaves at lower biomass investment. Shade leaves reduce their respiration costs through a lower N content on an area basis (Björkman 1981; Sims & Pearcy 1989). To enhance light capture, they invest more N in chlorophyll at the expense of investment in Rubisco, which is reflected in a high chlorophyll : nitrogen ratio (Hikosaka & Terashima 1995; Poorter & Evans 1998). Shade leaves are thin (Björkman 1981) and less tough, as wind and desiccation stress are lower in the understorey.
Plasticity is defined as the differential response of a genotype to different environments (Bradshaw 1965). The requirements for optimal leaf functioning differ between low and high irradiance. Generally it is assumed that leaf traits showing high plasticity in response to irradiance are more important to plant functioning in different light environments than traits that show little or no plasticity (Bongers & Popma 1988; but cf. Rice & Bazzaz 1989). The magnitude of plasticity differs among leaf traits and species: physiological traits were found to have higher plasticity than morphological traits (Valladares et al. 2000).
Plasticity is thought to be greatest for early successional pioneer species, as they occur in variable, heterogeneous environments (caused by rapid gap formation and closure) with a high resource availability (Bazzaz 1979; Bazzaz & Wayne 1994; Valladares et al. 2000). The high resource availability provides early successional species with sufficient carbon and nutrients to be able to invest rapidly in acclimation to changing light conditions (Grime, Crick & Rincon 1986). Acclimation is defined here as the morphological and physiological adjustments made by individual plants to (changes in) the environment. Pioneer species have short-lived leaves (Reich, Walters & Ellsworth 1992), therefore they can rapidly track changes in the light environment by replacing old leaves at a high rate (Ackerly 1997; Valladares et al. 2000). This may explain their high short-term acclimation potential at the whole-tree level, but not the magnitude of the plastic response at the leaf level. A clear consensus on the link between plasticity and successional stage has not been reached, as greater (Strauss-Debenedetti & Bazzaz 1991, 1996); similar (Sims & Pearcy 1989; Kitajima 1994); and lower leaf plasticity (Popma, Bongers & Werger 1992) has been found for pioneers compared with shade-tolerant species.
It has been suggested that plasticity in leaf traits depends not only on the regeneration niche of a tree species, but also on the changes in irradiance it experiences during its life cycle (Popma et al. 1992). Tall, late-successional species that start in the shaded understorey and grow to the exposed conditions of the forest canopy experience large ontogenetic changes in light availability, and should have a large capacity for plastic responses. Such species were indeed found to have higher plasticity compared with species that always remained in the understorey, or pioneer species that always remained in the high light conditions of gaps (Popma et al. 1992). Tall species make larger ontogenetic shifts in crown exposure than small species (Poorter et al. 2005), therefore a close association has been found between plasticity in leaf traits and the maximum height of the species (Thomas & Bazzaz 1999; Cai et al. 2005).
In general, differences between sun and shade leaves and plasticity have been investigated using a rather low number of species or leaf traits (but cf. Bongers & Popma 1988; Popma et al. 1992), making sound generalizations difficult. Also, species have often been classified subjectively into different functional groups (early vs late-successional, shade tolerant vs pioneer), without a proper quantification of the light demand of the species and the ontogenetic changes therein.
Here we evaluate the differences between sun and shade leaves for 38 tropical moist forest tree species, and link them to a quantitative measure of the juvenile and adult crown exposure (CE) of the species. We focus on 16 morphological and physiological leaf traits that are important for the C, water and heat balance of the leaves. The following questions were addressed: (1) How do sun and shade leaves differ in their leaf traits? (2) Does plasticity differ among leaf traits? (3) Is plasticity related to the juvenile CE, adult CE, ontogenetic change in CE, or maximum adult stature of the species?