Abstract
Changes in plant phenology affect the carbon flux of terrestrial forest ecosystems due to the link between the growing season length and vegetation productivity. Digital camera imagery, which can be acquired frequently, has been used to monitor seasonal and annual changes in forest canopy phenology and track critical phenological events. However, quantitative assessment of the structural and biochemical controls of the phenological patterns in camera images has rarely been done. In this study, we used an NDVI (Normalized Difference Vegetation Index) camera to monitor daily variations of vegetation reflectance at visible and near-infrared (NIR) bands with high spatial and temporal resolutions, and found that the infrared camera based NDVI (camera-NDVI) agreed well with the leaf expansion process that was measured by independent manual observations at Harvard Forest, Massachusetts, USA. We also measured the seasonality of canopy structural (leaf area index, LAI) and biochemical properties (leaf chlorophyll and nitrogen content). We found significant linear relationships between camera-NDVI and leaf chlorophyll concentration, and between camera-NDVI and leaf nitrogen content, though weaker relationships between camera-NDVI and LAI. Therefore, we recommend ground-based camera-NDVI as a powerful tool for long-term, near surface observations to monitor canopy development and to estimate leaf chlorophyll, nitrogen status, and LAI.
Highlights
Plant phenology is the timing of key, annually occurring life history events, such as spring leaf development and autumn senescence, and the corresponding concurrent shifts in physiology
Yang et al.[12], Keenan et al.[13], and Liu et al.[14] found that the RGB camera-based phenological metrics were limited in their ability to characterize physiology: a critical decoupling existed between the peak of the camera-based greenness index and that of leaf chlorophyll concentration
Using TIMESAT-identified seasonal transition points (Fig. 2), we identified the date of leaf-out when camera-Normalized Difference Vegetation Index (NDVI) abruptly increased after bud break, indicating the onset of rapid leaf expansion
Summary
Plant phenology is the timing of key, annually occurring life history events, such as spring leaf development and autumn senescence, and the corresponding concurrent shifts in physiology. Long-term, high-resolution monitoring of plant phenology and seasonal variation in carbon fluxes across different scales is necessary for accurate estimation of the impact of environmental change on terrestrial carbon cycles[2], especially in deciduous forest systems where vegetation is governed by seasonal patterns. Digital camera-based phenological monitoring (e.g. the PhenoCam network) has been adopted to capture seasonal canopy transitions and detect key phenological events at high spatial and temporal resolutions[1, 3,4,5,6,7,8,9,10]. Visible-band digital cameras, which only use red, green and blue bands, may not accurately reflect certain plant physiological characteristics. To fully characterize the seasonal physiological variation of forest tree species and their impact on forest productivity, it is necessary to monitor and integrate plant processes at higher spatial and temporal resolutions. A simplified version of the NDVI camera using the light emitting diode (LED)-sensor technology has been reported to measure ground-based NDVI without reporting spatial information[27, 28]
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