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Remote sensing phenology at European northern latitudes: From ground spectral towers to satellites
Lund University.
Responsible organisation
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Plant phenology exerts major influences on carbon, water, and energy exchanges between atmosphere and ecosystems, provides feedbacks to climate, and affects ecosystem functioning and services. Great efforts have been spent in studying plant phenology over the past decades, but there are still large uncertainties and disputations in phenology estimation, trends, and its climate sensitivities. This thesis aims to reduce these uncertainties through analyzing ground spectral sampling, developing methods for in situ light sensor calibration, and exploring a new spectral index for reliable retrieval of remote sensing phenology and climate sensitivity estimation at European northern latitudes.

The ground spectral towers use light sensors of either nadir or off-nadir viewing to measure reflected radiation, yet how plants in the sensor view contribute differently to the measured signals, and necessary in situ calibrations are often overlooked, leading to great uncertainties in ground spectral sampling of vegetation. It was found that the ground sampling points in the sensor view follow a Cauchy distribution, which is further modulated by the sensor directional response function. We proposed in situ light sensor calibration methods and showed that the user in situ calibration is more reliable than manufacturer's lab calibration when our proposed calibration procedures are followed.

By taking the full advantages of more reliable and standardized reflectance, we proposed a plant phenology vegetation index (PPI), which is derived from a radiative transfer equation and uses red and near infrared reflectance. PPI shows good linearity with canopy green leaf area index, and is correlated with gross primary productivity, better than other vegetation indices in our test. With suppressed snow influences, PPI shows great potentials for retrieving phenology over coniferous-dominated boreal forests.

PPI was used to retrieve plant phenology from MODIS nadir BRDF-adjusted reflectance at European northern latitudes for the period 2000-2014. We estimated the trend of start of growing season (SOS), end of growing season (EOS), length of growing season (LOS), and the PPI integral for the time span, and found significant changes in most part of the region, with an average rate of -0.39 days·year-1 in SOS, 0.48 days·year-1 in EOS, 0.87 days·year-1 in LOS, and 0.79%·year-1 in the PPI integral over the past 15 years. We found that the plant phenology was significantly affected by climate in most part of the region, with an average sensitivity to temperature: SOS at -3.43 days·°C-1, EOS at 1.27 days·°C-1, LOS at 3.16 days·°C-1, and PPI integral at 2.29 %·°C-1, and to precipitation: SOS at 0.28 days cm-1, EOS at 0.05 days cm-1, LOS at 0.04 days cm-1, and PPI integral at -0.07% cm-1. These phenology variations were significantly related to decadal variations of atmospheric circulations, including the North Atlantic Oscillation and the Arctic Oscillation.

The methods developed in this thesis can help to improve the reliability of long-term field spectral measurements and to reduce uncertainties in remote sensing phenology retrieval and climate sensitivity estimation.

Place, publisher, year, edition, pages
Department of Physical Geography and Ecosystem Science, Lund University , 2015. , 174 p.
Keyword [en]
Remote sensing, climate sensitivity., calibration, ground spectral tower, Plant phenology index (PPI), northern latitude
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:polar:diva-3505ISBN: 978-91-85793-49-5 (print)OAI: oai:DiVA.org:polar-3505DiVA: diva2:1083439
Public defence
2015-06-11, 10:00
Supervisors
Available from: 2017-03-21 Created: 2017-03-21 Last updated: 2017-03-21Bibliographically approved

Open Access in DiVA

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http://lup.lub.lu.se/record/5366415
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Citation style
  • apa
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  • de-DE
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  • nn-NO
  • nn-NB
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  • Other locale
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