Deriving a consistent δ¹³C signature from tree canopy leaf material for palaeoclimatic reconstruction

Contemporary studies of stable carbon isotope values (δ¹³C) from leaves in the canopies of mature forest indicate that reduced irradiance and respired CO₂ might bias palaeoenvironmental reconstructions developed using the ¹³C content of unsorted plant material formed within comparable ancient environments. Here, we investigated whether any simple morphological and/or chemical characteristics could identify the position of the leaf within a canopy of the evergreen mountain beech (Nothofagus solandri var. cliffortiodes). Leaf mass per unit area, carbon content and δ¹³C values of both bulk and lignin components of the leaves increased exponentially from the ground up through the canopy. Nitrogen, remobilized before death, was unsuitable as an indicator of canopy position. Leaf mass per unit area on the forest floor indicated that leaves from the sunlit part comprised approximately 30% of the fallen leaves; leaves originating from the upper canopy made up the remaining litter. Application of leaf mass per unit area to a 22 000-yr-old sequence dominated by mountain beech leaves from Mount George (New Zealand), demonstrated that this signal was preserved in the fossil record. Sunlit leaf material can be successfully differentiated from that originating deeper within the palaeo-canopy, thereby allowing robust δ5¹³C analyses for palaeoenvironmental reconstruction.