Mapping minor plantation species for New Zealand’s small-scale forests using Sentinel-2 satellite data

Main Article Content

Cong Xu
Bruce Manley
Ning Ye

Keywords

Minor species, small-scale forests, Sentinel-2 satellite, random forest, species classification

Abstract

Background: Relying solely on radiata pine (Pinus radiata D.Don) leaves New Zealand’s plantation forest industry vulnerable to fluctuations in market demand and at risk from a potentially devastating pest or disease outbreak. Therefore, the New Zealand government and forestry industry urge to diversify the forest resource and wood supply beyond the reliance on radiata pine. Unfortunately, the lack of accurate information on minor species’ area, composition, and location poses challenges to forecasting potential log supply and logistics planning.


Methods: The objective of this study is to classify minor species in New Zealand using imagery and phenological features extracted from data collected by the Copernicus Sentinel-2 satellite. The study collected reference data of minor species from large-scale forest owners and applied Random Forest classification using Sentinel-2 imagery to classify nine minor species classes in the Hawke’s Bay region of New Zealand.


Results: The study achieved an overall classification accuracy of 92.2% for minor species in New Zealand’s Hawke’s Bay region. Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) and Eucalyptus species had the highest accuracies, exceeding 90% for both producer’s and user’s accuracies. Acacia, larch, and other mixed species had lower accuracies, likely due to their lower occurrence. The most important input variable for classification was the Digital Elevation Model, indicating the significance of elevation in differentiating plantation species. The Greenness Index (GI) and Red edge bands also proved useful in the classification. The phenological measure Mean-EVI2 was found useful in classifying deciduous species such as larch and poplar.


Conclusions: To the best of our knowledge, this study is the first to map the spatial extent and distribution of minor plantation species in New Zealand at the regional level, providing promising results for potentially expanding the study to national-level species mapping.

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