DataSheet_1_Daytime and seasonal reflectance of maize grown in varying compass directions.docx

High temporal and spatial resolution is required to meet the challenges of changing plant characteristics over time. Solar radiation and reflectance of vegetation canopies vary with the time of day and growing season. Little is known regarding the interactions between daily and seasonally varying irradiation and reflectance of row-planted crops that can be grown in any compass direction. The spectral reflectance of maize grown in four compass directions was recorded across the entire life cycle through highly frequent drone-based multispectral sensing to determine biomass changes over time and make early yield predictions. Comparison of information from spectral bands and indices indicated no differences among the four compass directions at the reproductive stage and only a few differences at the earlier vegetative growth stages. There was no systematic influence of row orientation on the relationships between spectral data, biomass, and grain yield, except at the early growth stages. Spectral relationships to biomass at the reproductive stage varied in row directions with R2-values close to 0.9, already observed at early growth stages for the indices NDVI, SR, GCI, and GNDVI. The spectral relationships to yield were closer in individual compass directions, with R2-values varying between 0.8–0.9 for the best indices GCI and GNDV after BBCH 61. A closer inspection of daytime changes indicated a diurnal trend with 15 and 20% decreased spectral values observed after midday at the growth stages BBCH 81 and 61, respectively, thus requiring standardization of flight timing during the day. Drone-assisted nadir-oriented spectral sensing could be a reference for terrestrial and satellite-based reflectance sensing to relate canopy reflectance to crop characteristics quantitatively.

为应对植物特性随时间动态变化带来的研究挑战，需采用高时空分辨率的观测方案。植被冠层的太阳辐射与光谱反射率会随每日时段及生长季发生显著变化。目前对于可沿任意方位种植的条播作物，其日变化与季变化的辐射及反射率之间的相互作用机制，仍缺乏系统研究。本数据集通过高频次无人机搭载多光谱传感系统，对沿四个罗盘方位种植的玉米全生命周期内的光谱反射率进行连续记录，旨在解析生物量随时间的动态变化规律并实现早期产量预测。对比不同光谱波段与植被指数的信息后发现，在生殖生长期，四个种植方位间无显著差异；仅在早期营养生长阶段存在少量差异。除早期生长阶段外，种植行方位并未对光谱数据、生物量与籽粒产量之间的关联关系产生系统性影响。生殖生长期内，光谱反射率与生物量的关联关系随种植行方位存在差异，其决定系数（R²）接近0.9；此类差异在早期生长阶段的归一化差分植被指数（NDVI, Normalized Difference Vegetation Index）、比值植被指数（SR, Simple Ratio）、绿色叶绿素指数（GCI, Green Chlorophyll Index）及绿色归一化差分植被指数（GNDVI, Green Normalized Difference Vegetation Index）中已被观测到。针对籽粒产量的光谱关联关系在单个种植方位中表现更强，在BBCH 61生长阶段之后，表现最优的GCI与GNDVI指数的决定系数R²介于0.8至0.9之间。对日间变化的进一步细致分析显示，存在明显的日变化趋势：在BBCH 81与BBCH 61生长阶段，午间过后的光谱反射率分别降低15%与20%，因此需对无人机日间飞行时间进行标准化处理。无人机搭载的天底指向光谱传感技术，可为地面及卫星平台的冠层反射率与作物特性定量关联研究提供参考范式。