Seasonality in the early Holocene: Extending fossil-based estimates with a forest ecosystem process model

Abstract

Past seasonality changes are often poorly represented by Quaternary proxies because one season, or one factor, dominates the reconstructed signal. During the early Holocene in New Zealand, mean annual temperatures were at least 1.5°C warmer than present. However, treelines were lower, suggesting summers were cooler. Here we use a forest ecosystem process model, LINKNZ, to explore past precipitation and temperature seasonality changes in an intermontane basin of the Southern Alps, New Zealand. Pollen and macrofossils from the basin show that during an early-Holocene warm event (11 500 to 9500 cal. yr BP) podocarp and broadleaved species dominated. In exploratory modelling runs, mean annual temperatures were increased by up to 2°C, precipitation was reduced by 20—30%, and temperature seasonality reduced. When mean annual temperature was increased by 1.0—2.0°C, LINKNZ reconstructed wet forest associations, very different to those in the early-Holocene fossil assemblages. Acceptable matches were made with the early-Holocene vegetation using elevated temperature scenarios with up to 30% lower annual precipitation and decreased temperature seasonality. Longer growing seasons apparently compensate for cooler summer temperatures. We suggest that during the early Holocene in this area, westerly wind flow over the Southern Alps to the west of the basin was less, reducing spill-over rainfall and vapour pressure deficits. Warm oceans generated milder, cloudier climates reducing seasonal contrasts. Inverse modelling is recommended as a tool for exploring past climate scenarios when proxies fail to reconstruct important climate variables.