Improving modeling of the 'dark part' of canopy carbon gain.

Canopy photosynthesis is the integrated contribution of carbon gains of all leaves forming the canopy. As the simplest form of integration, canopy photosynthesis is given as the product of the number of leaves and the average leaf net assimilation rate. Although numerically correct, such calculation is wholly impractical as typical moderately dense broad-leaved plant stands contain thousands and conifer stands contain tens to hundreds of thousands of leaves per m2 ground area. Furthermore, there are major environmental gradients, in particular, gradients in light, temperature and humidity, from canopy top to bottom (for reviews, see Valladares 2003, Niinemets and Anten 2009), such that individual leaves are exposed to vastly different environmental conditions. As the dependencies of foliar physiological characteristics on key environmental drivers, light and temperature, are strongly non-linear, the correct daily average leaf photosynthesis rate cannot be obtained using daily average values of environmental drivers (de Pury and Farquhar 1997, Cescatti and Niinemets 2004, Niinemets and Anten 2009). Thus, first of all, canopy models need to provide a reliable description of canopy environmental conditions as the input for photosynthesis algorithms. There has been much progress since the pioneering study of Monsi and Saeki (1953) introducing the Lambert–Beer model of light absorption to describe exponential reduction of light availability with cumulative leaf area index and the Duncan et al. (1967) model including sunlit and shaded leaf area fractions. Currently available modeling schemes can describe the 3D heterogeneity in foliage distribution and particularly detailed schemes can even characterize the exact position of single foliage elements (Cescatti 1997, Chelle and Andrieu 1998, Disney et al. 2000, Bittner et al. 2012). Coupled to soil–vegetation–atmosphere transfer models, it is further possible to predict within-canopy gradients in temperature, humidity and CO2 concentration (e.g., Baldocchi et al. 2002, Akkermans et al. 2012). A further complication in simulating canopy photosynthesis is that foliar photosynthetic potentials and respiration rates vary through the canopy as a result of acclimation to variable within-canopy environmental conditions (Hirose and Werger 1987, Gutschick and Wiegel 1988, Baldocchi and Harley 1995, Harley and Baldocchi 1995, Lloyd et al. 2010, Niinemets 2012, Niinemets and Keenan 2012, Peltoniemi et al. 2012). This variation has been often ignored when simulating vegetation carbon gain at scales ranging from canopies to the globe, but canopy-level studies have conclusively demonstrated that such variations importantly alter whole-canopy carbon gain (e.g., the classical studies of Hirose and Werger 1987, Gutschick and Wiegel 1988, Baldocchi and Harley 1995). Furthermore, recent modeling studies underscore that even world-scale productivity estimates and spatial mapping can be significantly improved by considering within-canopy variations in foliar photosynthetic potentials (Friend 2010, Bonan et al. 2011, 2012). While within-canopy trait variations have been studied for a number of temperate ecosystems (see Niinemets 2007 for a review) and incorporated in large-scale models (Bonan et al. 2011, 2012), information on within-canopy gradients in foliar photosynthetic traits is particularly limited for the tropics (for a few studies reporting trait gradients for tropical forests, see Carswell et al. 2000, Kenzo et al. 2006, Cavalieri et al. 2008, Commentary