Environmental controls on stable isotope ratios in New Zealand Podocarpaceae: Implications for palaeoclimate reconstruction

• Oxygen and carbon isotopes from New Zealand Podocarpaceae were analysed.
• Vapour Pressure Deficit exerts primary control on stable isotope signature of most genera studied.
• Mean Annual Sunshine is the primary control on stable isotopes in leaves of Phyllocladus.
• Topographic climate features affect the stable isotopes of leaves more than latitude.
• Results have implications for interpreting stable isotopes in palaeoclimate proxies.

Stable isotope ratios of various proxies are widely used for palaeoclimate reconstruction, and it is often assumed that isotope ratios reflect vegetation abundance or type. However, very little research exists on the isotopic equilibration of extant biomes under variable environmental conditions. In this study, carbon and oxygen isotope ratios from leaves of various Podocarpaceae genera, endemic to New Zealand, are linked to environmental parameters from the Land Environments New Zealand model. The dominant influence on stable isotope ratios within the majority of Podocarpaceae studied here is vapour pressure deficit (VPD). A simple latitudinal trend does not exist, and neither temperature nor rainfall (decoupled from VPD) controls the stable isotope ratios. The results suggest that modern spatial heterogeneity in VPD affects the stable isotope values of vegetation, and that historic VPD variability would change the stable isotope ratios of Podocarpaceae without necessitating a change in vegetation type, density, or productivity. This represents an alternative model for temporal isotope change within geochemical proxies and reinforces the need for increased stable isotopic research in modern plant ecosystems to better understand modern, and eventually palaeoclimatic processes affecting the terrestrial biosphere.