ABSTRACT

We investigate the temporal dynamics of shifts in phenological responses of a range of key stages of the growing season in New Zealand’s three indigenous grassland types over the last 16 years (2001–2016). A near-daily Normalized Difference Vegetation Index (NDVI) time series from MODerate Resolution Imaging Spectroradiometer (MODIS) was used to extract five annual growth phenology indices, namely the Start, End, Length, Peak and Peak NDVI of a growing season. The start of the growing season advanced (i.e. happened earlier) by a median of 7.2, 6.0 and 8.8 days per decade in Alpine, Tall Tussock and Low Producing grassland, whereas the end of the season advanced by a median of 4.5, 0.4 and 0.4 days in the three types respectively. The length of growing season was extended by 3.2, 5.2 and 7.1 days per decade in these three grassland types. Over 86% of the investigated grassland areas showed an advancing (earlier) start of the growing season, and 74% of Alpine grassland showed a trend toward an earlier end of season. Over 63% of all grassland types showed an increase in growing season length. A trend toward earlier growing season peak and overall increasing NDVI in the three grassland types indicate a tendency for increasing vegetation vitality in grassland ecosystems in recent years. The start of growing season was correlated with atmospheric pressure (negatively) and precipitation (positively) changes in winter–spring months, while the timing of the season end is positively correlated with air temperature and solar radiation in summer–autumn months. Our study shows that different grassland types differ in magnitude – but not in direction – of their recent shifts in timing of key growing season stages with high-alpine grasslands showing the strongest response. This study highlights the usefulness of remote sensing for monitoring ecosystem-level phenological shifts over large areas and long time periods.

摘要

我们调查了过去 16 年（2001-2016 年）新西兰三种本土草原类型生长季节一系列关键阶段物候响应变化的时间动态。来自中分辨率成像光谱仪 (MODIS) 的近每日归一化差异植被指数 (NDVI) 时间序列用于提取五个年度生长物候指数，即生长季节的开始、结束、长度、峰值和峰值 NDVI。在高山、高草丛和低产草地，生长季节的开始提前（即发生得更早）每十年中位数为 7.2、6.0 和 8.8 天，而生长季节末提前（即发生更早）的中位数为 4.5、0.4 和 0.4天数分别在三种类型中。这三种草原类型的生长季节长度每十年分别延长3.2、5.2和7.1天。超过 86% 的调查草原区域显示出生长季提前（提前）开始，74% 的高山草原显示出生长季提前结束的趋势。超过 63% 的草地类型显示生长季长度增加。三种草地类型的生长季高峰期提前和NDVI整体增加的趋势表明近年来草地生态系统的植被活力有增加的趋势。生长季节的开始与冬春季月的大气压力（负）和降水（正）变化相关，而季节结束的时间与夏秋季月的气温和太阳辐射正相关。我们的研究表明，不同草原类型在关键生长季节阶段的时间变化的幅度上有所不同，但方向不同，高山草原表现出最强烈的反应。这项研究强调了遥感在监测大面积和长期生态系统层面物候变化方面的有用性。