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| Titel |
Potential and limitations of using digital repeat photography to track structural and physiological phenology in Mediterranean tree-grass ecosystems |
| VerfasserIn |
Yunpeng Luo, Tarek EI-Madany, Gianluca Filippa, Arnaud Carrara, Edoardo Cremonese, Marta Galvagno, Tiana Hammer, Oscar Perez-Priego, Markus Reichstein, Pilar Martín Isabel, Rosario González Cascón, Mirco Migliavacca |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250144791
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| Publikation (Nr.) |
 EGU/EGU2017-8660.pdf |
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| Zusammenfassung |
| Tree-Grass ecosystems are global widely distributed (16-35% of the land surface). However,
its phenology (especially in water-limited areas) has not yet been well characterized and
modeled. By using commercial digital cameras, continuous and relatively vast phenology
data becomes available, which provides a good opportunity to monitor and develop a robust
method used to extract the important phenological events (phenophases). Here we aimed to
assess the usability of digital repeat photography for three Tree-Grass Mediterranean
ecosystems over two different growing seasons (Majadas del Tietar, Spain) to extract critical
phenophases for grass and evergreen broadleaved trees (autumn regreening of grass- Start of
growing season; resprouting of tree leaves; senescence of grass – End of growing season),
assess their uncertainty, and to correlate them with physiological phenology (i.e.
phenology of ecosystem scale fluxes such as Gross Primary Productivity, GPP).
We extracted green chromatic coordinates (GCC) and camera based normalized
difference vegetation index (Camera-NDVI) from an infrared enabled digital camera
using the “Phenopix” R package. Then we developed a novel method to retrieve
important phenophases from GCC and Camera-NDVI from various region of interests
(ROIs) of the imagery (tree areas, grass, and both - ecosystem) as well as from
GPP, which was derived from Eddy Covariance tower in the same experimental
site.
The results show that, at ecosystem level, phenophases derived from GCC and
Camera-NDVI are strongly correlated (R2 = 0.979). Remarkably, we observed that at the end
of growing season phenophases derived from GCC were systematically advanced (ca. 8 days)
than phenophase from Camera-NDVI. By using the radiative transfer model Soil Canopy
Observation Photochemistry and Energy (SCOPE) we demonstrated that this delay is related
to the different sensitivity of GCC and NDVI to the fraction of green/dry grass
in the canopy, resulting in a systematic higher NDVI during the dry-down of the
canopy.
Phenophases derived from GCC and Camera-NDVI are correlated with phenophase
extracted from GPP across sites and years (R2 =0.966 and 0.976 respectively). For the
start of growing season the determination coefficient was higher (R2 =0.89 and
0.98 for GCC vs GPP and Camera-NDVI vs GPP, respectively) than for the end of
growing season (R2 =0.75 and 0.70, for GCC and Camera-NDVI, respectively). The
statistics obtained using phenophases derived from grass or ecosystem ROI are
similar.
In contrast, GCC and Camera-NDVI derived from trees ROI are relatively constant and
not related to the seasonality of GPP. However, the GCC of tree shows a characteristic peak
that is synchronous to leaf flushing in spring assessed using regular Chlorophyll content
measurements and automatic dendrometers.
Concluding, we first developed a method to derive phenological events of Tree-Grass
ecosystems using digital repeat photography, second we demonstrated that the phenology of
GPP is strongly dominated by the phenology of grassland layer, third we discussed the
uncertainty related to the use of GCC and Camera-NDVI in senescence, and finally
we demonstrate the capability of GCC to track in evergreen broadleaved forest
crucial phenological events. Our findings confirm digital repeat photography is a vital
data source for characterizing phenology in Mediterranean Tree-Grass Ecosystem. |
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