Variation in foliar δC values within the crowns of Pinus radiata trees

Although herbaceous species generally show little within plant variation in δ13C, trees show large spatial and temporal differences. We found that the aspect of exposure and branch length accounted for up to 6‰ δ13C difference within the foliage of individual trees of Pinus radiata D. Don. The foliage on branches 0.5 m in length was as much as 4‰ more depleted in 13C than foliage on 10-m long branches, and an additional 2‰ more depleted on the shaded side than on the exposed side. We confirmed that on clear days, relative branch hydraulic conductivity, defined as the ratio of transpiration to the water potential gradient, was much higher in short branches than in long branches. Stomatal conductance remained high in foliage on short branches during the day, whereas it declined progressively in long-branch foliage under similar conditions. These differences were sufficient to explain the observed variation in δ13C in foliage on long and short branches.