| Popis: |
Grasslands in mountainous areas often show distinct responses in the timing of their growing season in relation to topographical variation. However, it is unclear which factors of topography (elevation, aspect, and slope) affect which phases of the timing in seasonal growth (start, peak, end, length of growing season). Here we investigated these relationships between topography and growing season timing in the three key grassland types in mountainous areas in South Island, New Zealand. From a near-daily NDVI (Normalized Difference Vegetation Index) dataset over a 16 year period (2001–2016), we extracted five annual land surface phenology indices: start, end, length, peak of the growing season and peak NDVI. Averages in these phenology indices were correlated with three topographical factors. The start of growing season occurred later by 7.1, 5.2 and 3.5 days per 100 m elevation in the three grassland types (Alpine, Tall Tussock and Low Producing grasslands). The end of the season occurred earlier by 1.8, 1.6 days and later by 0.4 days per 10-degree more south-facing (colder) aspects in the three grasslands. A longer growing season was observed at lower elevation and on north-facing (sunny) slopes in alpine grasslands. A later season peak occurred at higher elevation and on north-facing slopes in alpine grasslands and at higher elevations and on steeper slopes in non-alpine grasslands. Higher peak NDVI was detected at the lower elevation. Our results show that different facets of a landscape's topography affect different stages of a grassland's growing season, and these responses also differ between grassland types. This highlights the importance of considering all topographical features when relationships between the physical environment and biological responses are investigated. |
