Testing Hopkins’ Bioclimatic Law with PhenoCam data

Abstract

Premise of the Study We investigated the spatial and temporal patterns of vegetation phenology with phenometrics derived from PhenoCam imagery. Specifically, we evaluated the Bioclimatic Law proposed by Hopkins, which relates phenological transitions to latitude, longitude, and elevation. Methods “Green‐up” and “green‐down” dates—representing the start and end of the annual cycles of vegetation activity—were estimated from measures of canopy greenness calculated from digital repeat photography. We used data from 65 deciduous broadleaf ( DB ) forest sites, 18 evergreen needleleaf ( EN ) forest sites, and 21 grassland ( GR ) sites. Results DB green‐up dates were well correlated with mean annual temperature and varied along spatial gradients consistent with the Bioclimatic Law. Interannual variation in DB phenology was most strongly associated with temperature anomalies during a relatively narrow window of time. EN phenology was not well correlated with either climatic factors or spatial gradients, but similar to DB phenology, interannual variation was most closely associated with temperature anomalies. For GR sites, mean annual precipitation explained most of the spatial variation in the duration of vegetation activity, whereas both temperature and precipitation anomalies explained interannual variation in phenology. Discussion PhenoCam data provide an objective and consistent means by which spatial and temporal patterns in vegetation phenology can be investigated.

Status of Phenological Research Using Sentinel-2 Data: A Review

“…In contrast to satellites, phenocams provide close-range remotely sensed imagery at much higher spatial and temporal resolutions, and do not require atmospheric corrections [114]. Though phenocams currently typically operate in only a few spectral channels (e.g., three visible and one NIR band), they have been successfully used across a wide range of ecosystems, and are reported to provide better agreement with GP observations, more robust estimates of productivity, and an ability to track changes at the canopy level [115][116][117][118][119]. While their field of view is considerably reduced compared to satellites, multiple phenocam devices have emerged in the last decade, and have been used effectively to track vegetation changes.…”

Sensitivity of Deciduous Forest Phenology to Environmental Drivers: Implications for Climate Change Impacts Across North America

“…Vegetation phenology is a sensitive indicator of climate change (Cleland et al, 2007;Körner & Basler, 2010) and strongly influences terrestrial nutrient and carbon cycles (Estiarte & Peñuelas, 2015). Plant phenology varies with long-term environmental variables such as climate and short-term environmental variables such as weather (Hopkins, 1920;Richardson et al, 2019), as well as the interaction of weather and climate (Zohner et al, 2016). Therefore, quantifying how green-up and senescence respond to environmental forcing is crucial for understanding biosphere-atmosphere interactions.…”

Evaluation of VEGETATION and PROBA-V Phenology Using PhenoCam and Eddy Covariance Data

“…Differences in the structure, ecophysiology and dynamics of the vegetation canopy at the start and end of the growing season [37] may partially explain this. The phenological dynamics for the timing of EoS tend to vary with species, age, dispersion and homogeneity, and can also differ across the same species, with differences of up to two weeks within the same ROI [26,43,71,76,77]. Further studies will use high resolution satellite data from Sentinel-2 to mitigate these issues and capture the spatial variability of EoS [78].…”