Comprehensive reassessment of Australia's land-surface phenology trends (1982–2022) using circular statistics and a harmonised NDVI dataset

Abstract

Land-surface phenology is critical to understanding Earth system responses to environmental change. However, there is a lack of studies that specifically examine Australian phenology trends over time periods long enough to robustly capture the effects of a changing climate. Here we utilise and demonstrate the methodological superiority of circular statistics for quantifying phenology in Australia. Next, we employ circular statistical methods across a long-term harmonised NDVI dataset (1982–2022) to analyse phenological trends across Australia's diverse landscapes. We find that forest ecosystems exhibit inertia to long-term shifts in rainfall regimes and increasing vapour pressure deficits, exhibiting stable growing season length, but increased maximum seasonal productivity (0.012 NDVI/decade). In contrast, shrublands and grasslands show significant phenological shifts, including earlier green-ups (−4.3 and − 2.0 days/decade, respectively), earlier senescence (−2.5 and − 1.7 days/decade), and earlier peaks (−2.5 and − 3.1 days/decade) linked to altered rainfall regimes and land use changes. Only modest increases in the length of season are observed because the start and end of seasons often advance simultaneously. Importantly, major cropping regions are experiencing shortened growing seasons (−3.5 days/decade), offset by increased maximum NDVI, stabilising productivity but raising concerns for future agricultural productivity. Increases in maximum NDVI are driving an amplification of Australia's vegetation cycles, with concomitant increases in rates of growth and senescence.