Comparison of phenology dates in deciduous forests from near-surface and remote sensing

Deciduous forest phenology regulates physical processes affecting the lower atmosphere, including the timing and amount of carbon uptake and the partitioning of surface energy fluxes into sensible and latent heat. In order to understand the relation between forest phenology, climate, and associated feedbacks, observations from the scale of individual trees and forest stands must be related to broader scale global data from satellite borne radiometers. This paper will discuss a comparison of spring onset and fall dormancy dates determined from the MODIS EVI and NDVI products to corresponding metrics obtained with digital repeat photography at 17 deciduous forest sites, over 78 site-years. An identical methodology for estimating phenology dates will be applied to satellite data and greenness time series extracted from digital repeat photography of forest canopies. Preliminary results indicate that while determination of phenology dates using these two data sources generally compare well, there is a mean bias across all site years of approximately 11 days toward an earlier spring onset when satellite data is used. Possible causes for this bias will be explored on a site by site basis. Human observations, including determination of phenology dates from series of digital repeat photography and direct observation of trees, will be used to evaluate the comparison of satellite and near-surface phenology detection.