Variations in leaf βC along a vertical profile of irradiance in a temperate Japanese forest.

The vertical profile of stable carbon isotope ratios (δC) of leaves was analyzed for 13 tree species in a cool-temperate deciduous forest in Japan. The vertical distribution of long-term averaged δC in atmospheric CO (δ) was estimated from δC of dry matter from NADP-malic enzyme type C plant (Zea mays L. var. saccharata Sturt.) grown at a tower in the forest for 32␣days, assuming constant Δ value (3.3‰) in Z. mays against height. The δ value obtained from δC in Z.␣mays was lowest at the forest floor (-9.30 ± 0.03‰), increased with height, and was almost constant above 10␣m (-7.14 ± 0.14‰). Then leaf Δ values for the tree species were calculated from tree leaf δ C andδ. Mean leaf Δ values for the three tall deciduous species (Fraxinus mandshurica, Ulmus davidiana, and Alnus hirsuta) were significantly different among three height levels in the forest: 23.1 ± 0.7‰ at the forest floor (understory), 21.4 ± 0.5‰ in lower canopy, and 20.5 ± 0.3‰ in upper canopy. The true difference in tree leaf Δ among the forest height levels might be even greater, because Δ in Z. mays probably increased with shading by up to ∼‰. The difference in tree leaf Δ among the forest height levels would be mainly due to decreasing intercellular CO (C ) with the increase in irradiance. Potential assimilation rate for the three tree species probably increased with height, since leaf nitrogen content on an area basis for these species also increased with height. However, the increase in stomatal conductance for these tree species would fail to meet the increase in potential assimilation rate, which might lead to increasing the degree of stomatal limitation in photosynthesis with height.