Acclimation of photosynthetic capacity to irradiance in tree canopies in relation to leaf nitrogen concentration and leaf mass per unit area

The observation of acclimation in leaf photosynthetic capacity to differences in growth irradiance has been widely used as support for a hypothesis that enables a simplification of some soil-vegetation-atmosphere transfer (SVAT) photosynthesis models. The acclimation hypothesis requires that relative leaf nitrogen concentration declines with relative irradiance from the top of a canopy to the bottom, in 1 : 1 proportion. In combination with a light transmission model it enables a simple estimate of the vertical profile in leaf nitrogen concentration (which is assumed to determine maximum carboxylation capacity), and in combination with estimates of the fraction of absorbed radiation it also leads to simple ‘big-leaf’ analytical solutions for canopy photosynthesis. We tested how forests deviate from this condition in five tree canopies, including four broadleaf stands, and one needle-leaf stand: a mixed-species tropical rain forest, oak ( Quercus petraea (Matt.) Liebl), birch ( Betula pendula Roth), beech ( Fagus sylvatica L.) and Sitka spruce ( Picea sitchensis (Bong.) Carr). Each canopy was studied when fully developed (mid-to-late summer for temperate stands). Irradiance ( Q , µ mol m − 2 s − 1 ) was measured for 20 d using quantum sensors placed throughout the vertical canopy profile. Measurements were made to obtain parameters from leaves adjacent to the radiation sensors: maximum carboxylation and electron transfer capacity ( V a , J a , µ mol m − 2 s − 1 ), day respiration ( R da , µ mol m − 2 s − 1 ), leaf nitrogen concentration ( N m , mg g − 1 ) and leaf mass per unit area ( L a , g m − 2 ). Relative to upper-canopy values, V a declined linearly in 1 : 1 proportion with N a . Relative V a also declined linearly with relative Q , but with a significant intercept at zero irradiance ( P < 0·01). This intercept was strongly related to L a of the lowest leaves in each canopy ( P < 0·01, r 2 = 0·98, n = 5). For each canopy, daily ln Q was also linearly related with ln V a (P < 0·05), and the intercept was correlated with the value for photosynthetic capacity per unit nitrogen (PUN: V a / N a , µ µ mol g − 1 s − 1 ) of the lowest leaves in each canopy ( P < 0·05). V a was linearly related with L a and N a (P < 0·01), but the slope of the V a : N a relationship varied widely among sites. Hence, whilst there was a unique V a : N a ratio in each stand, acclimation in V a to Q varied predictably with L a of the lowest leaves in each canopy. The specific leaf area, L m ( cm 2 g − 1 ), of the canopy-bottom foliage was also found to predict carboxylation capacity (expressed on a mass basis; V m , µ mol g − 1 s − 1 ) at all sites ( P < 0·01). These results invalidate the hypothesis of full acclimation to irradiance, but suggest that L a and L m of the most light-limited leaves in a canopy are widely applicable indicators of the distribution of photosynthetic capacity with height in forests.