Remote sensing is an invaluable tool for tracking decadal-scale changes in vegetation greenness in response to climate and land use changes. While the Landsat archive has been widely used to explore these trends and their spatial and temporal complexity, its inconsistent sampling frequency over time and space raises concerns about its ability to provide reliable estimates of annual vegetation indices such as the annual maximum normalised difference vegetation index (NDVI), commonly used as a proxy of plant productivity. Here we demonstrate for seasonally snow-covered ecosystems, that greening trends derived from annual maximum NDVI can be significantly overestimated because the number of available Landsat observations increases over time, and mostly that the magnitude of the overestimation varies along environmental gradients. Typically, areas with a short growing season and few available observations experience the largest bias in greening trend estimation. We show these conditions are met in late snowmelting habitats in the European Alps, which are known to be particularly sensitive to temperature increases and present conservation challenges. In this critical context, almost 50% of the magnitude of estimated greening can be explained by this bias. Our study calls for greater caution when comparing greening trends magnitudes between habitats with different snow conditions and observations. At a minimum we recommend reporting information on the temporal sampling of the observations, including the number of observations per year, when long-term studies with Landsat observations are undertaken.

中文翻译：

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高山生态系统中基于陆地卫星的绿化趋势因夏季观测的多年代际增加而被夸大


遥感是追踪响应气候和土地利用变化的植被绿度年代际变化的宝贵工具。虽然 Landsat 档案已被广泛用于探索这些趋势及其空间和时间复杂性，但其随时间和空间变化的不一致采样频率引发了人们对其提供年度植被指数的可靠估计能力的担忧，例如通常用作植物生产力代理的年度最大归一化差异植被指数 （NDVI）。在这里，我们证明了对于季节性积雪覆盖的生态系统，由年度最大 NDVI 得出的绿化趋势可能会被显著高估，因为可用的 Landsat 观测数据的数量会随着时间的推移而增加，而且大多数情况下，高估的幅度会随着环境梯度的变化而变化。通常，生长季节短且可用观测值较少的地区在绿化趋势估计中经历最大的偏差。我们表明，欧洲阿尔卑斯山的晚融雪栖息地满足了这些条件，众所周知，这些栖息地对温度升高特别敏感，并带来了保护挑战。在这种关键背景下，估计绿化幅度的近 50% 可以用这种偏差来解释。我们的研究呼吁在比较具有不同雪况和观测结果的栖息地之间的绿化趋势幅度时更加谨慎。在使用 Landsat 观测进行长期研究时，我们建议至少报告有关观测值时间采样的信息，包括每年的观测值数量。