Estimating past leaf-to-air vapour pressure deficit from terrestrial plant δ¹³C

δ¹³C was determined in lignin extracted from present-day cladodes of Phyllocladus alpinus (a small coniferous tree) from seven well-lit sites across New Zealand. The δ¹³C values ranged from -30.9‰ to -23.6‰ and were compared with monthly means of temperature, precipitation, relative humidity and vapour pressure deficit from the nearest recording stations. Of these parameters, the leaf-to-air vapour deficit of the first month of cladode growth and expansion proved to be the most significantly correlated with lignin δ¹³C, over a range of 0.3 to 0.8 kPa, corforming the importance of atmospheric moisture content on stomatal conductance. The carbon isotopic signature of lignin from fossilised cladodes preserved under the Kawakawa Tephra (22.6 k ¹⁴C yr BP) on the North Island is identical to that of the whole tissue, suggesting that for this species at least, fossil material can be used to approximate the lignin δ¹³C. The δ¹³C of species- and organ-specific fossil terrestrial plant material therefore provides an excellent method to quantify past changes in leaf-to-air vapour pressure deficit.