Predicting Pinus radiata female strobilus production for seed orchard site selection in New Zealand

Abstract

A ground-based estimate was made of the number of emergent female strobili carried by 74 open-grown stands (10–12 years old) of Pinus radiata growing across a range of latitudes and altitudes of the South Island and North Island of New Zealand. The intent of this study was to develop and evaluate an empirical model to describe the relationship between environmental variables and emergent female strobilus numbers. The model suggested that variability in female strobilus number was necessarily associated with climate and site factors. Temperature and to a lesser extent rainfall variables were associated with the number of seed cone-buds that achieved anthesis in the spring. The step-wise multiple regression analysis selected mean minimum February temperature, integrated soil water stress (February–March), accumulated growing degree days (weighted against spring rainfall prior to seed cone-bud initiation), in that order. The first variable was found to explain around 54% of the variance in strobilus numbers across sites, while taken together all of the variables explained 64% of among-site variability. Model evaluation was carried out across both similar eastern South Island sites and different North Island sites at which it was developed. It was found to predict strobilus counts accurately across the South Island sites, but less accurately across the warmer North Island sites. Mean minimum February temperature was still the most important predictor of strobilus production across sites, but this relationship was non-linear. Thus, a non-linear modelling approach was adopted to better fit the model across regions New Zealand-wide. Long term nation-wide data for mean minimum February temperature were then used in a climate modelling system to generate a temperature map for New Zealand, to assist with seed orchard site selection. This map illustrated that oceanic (as opposed to semi-continental) environments around the New Zealand coast are likely to be the most productive sites for seed orchard production. Contrary to a priori perceptions, the northern regions of the North Island were depicted to offer a greater number of candidate sites for seed orchard siting than the more south eastern semi-arid regions of New Zealand.