Abstract In recent years, digital repeat photography has received increasing attention as an inexpensive means by which seasonal changes in vegetation canopies can be monitored. Offering automation and an increased measurement frequency, colour indices derived from above-canopy digital repeat photography have proven a popular alternative to traditional observations of forest phenology. Nevertheless, previous work has demonstrated several features in time-series of colour indices that are unrelated to canopy structure, limiting their utility to track specific biophysical properties such as leaf area index (LAI). Whilst techniques such as digital cover photography and the use of radiometric sensors are better suited to this task, they are restricted by the need for careful calibration of above- and below-canopy reference sensors, ancillary information on canopy leaf angle distribution, and smaller measurement footprints. Using data collected at a deciduous broadleaf forest site in Southern England, we investigated a new method to derive continuous measurements of LAI, making use of automated digital hemispherical photography (DHP) from below the canopy. After applying simple data screening procedures, the LAI observations derived from our automated DHP system demonstrated very close agreement with those obtained from manually acquired DHP images, which were collected under optimal illumination conditions over the surrounding forest plot (r2 = 0.99, RMSE = 0.20, NRMSE = 13%). By combining our automated DHP system with an above-canopy time-lapse digital camera, we then investigated the relationship between the green chromatic coordinate (GCC) and LAI. Distinct hysteresis effects were observed, as were substantial differences between phenological transition dates derived from the GCC and LAI, particularly in the case of the onset of senescence. Our results indicate that phenological transition dates derived from colour indices cannot easily be linked to any one biophysical property. We recommend further investigation of the automated DHP approach, which provides time-series of LAI whose physical interpretation is straightforward, as an alternative to above-canopy digital repeat photography.

中文翻译：

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使用自动数字重复摄影跟踪森林生物物理特性：使用来自树冠下方的数字半球摄影的鱼眼透视

摘要 近年来，数字重复摄影作为一种廉价的监测植被冠层季节性变化的手段受到越来越多的关注。提供自动化和更高的测量频率，来自冠层上方数字重复摄影的颜色指数已被证明是传统森林物候观察的流行替代方案。然而，之前的工作已经证明了与冠层结构无关的颜色指数时间序列中的几个特征，限制了它们跟踪特定生物物理特性（如叶面积指数 (LAI)）的效用。虽然数字封面摄影和使用辐射传感器等技术更适合这项任务，但它们受到需要仔细校准冠层上方和下方参考传感器的限制，关于冠层叶角分布的辅助信息，以及更小的测量足迹。使用在英格兰南部落叶阔叶林场地收集的数据，我们研究了一种新方法，利用树冠下方的自动数字半球摄影 (DHP) 连续测量 LAI。在应用简单的数据筛选程序后，从我们的自动化 DHP 系统得出的 LAI 观察结果与从手动获取的 DHP 图像中获得的观察结果非常一致，这些图像是在周围森林图的最佳照明条件下收集的（r2 = 0.99，RMSE = 0.20， NRMSE = 13%）。通过将我们的自动化 DHP 系统与冠上延时数码相机相结合，我们研究了绿色色度坐标 (GCC) 和 LAI 之间的关系。观察到明显的滞后效应，以及源自 GCC 和 LAI 的物候过渡日期之间的实质性差异，特别是在衰老开始的情况下。我们的结果表明，从颜色指数得出的物候转变日期不能轻易与任何一种生物物理特性联系起来。我们建议进一步研究自动 DHP 方法，该方法提供 LAI 的时间序列，其物理解释很简单，作为冠上数字重复摄影的替代方案。我们的结果表明，从颜色指数得出的物候转变日期不能轻易与任何一种生物物理特性联系起来。我们建议进一步研究自动 DHP 方法，该方法提供 LAI 的时间序列，其物理解释很简单，作为冠上数字重复摄影的替代方案。我们的结果表明，从颜色指数得出的物候转变日期不能轻易与任何一种生物物理特性联系起来。我们建议进一步研究自动 DHP 方法，该方法提供 LAI 的时间序列，其物理解释很简单，作为冠上数字重复摄影的替代方案。