How accurate are methods for predicting phenology in New Zealand

Abstract

Thermal accumulation (degree day) methods are routinely used to predict plant and insect phenology Depending on the data available prediction may involve three separate steps with associated errors estimating daily heat units from maximum and minimum temperatures; interpolating daily maxima and minima from monthly averages; and predicting future monthly averages from past climate potentially including climate change This research investigated the potential error in thermal accumulation totals arising from each of these factors at nine New Zealand sites Ten simple heat unit calculations were tested including two littleknown and three new methods Those utilising the true daily mean temperature performed best followed by 4step triangle and sine approximations Interpolating between monthly mean temperature extremes introduced much error into daily estimates and the use of predicted rather than observed monthly normals further increased error Specific recommendations are made for thermal accumulation methods depending on what temperature data are available