Spectral reflectance and photosynthetic properties of Sphagnum mosses exposed to progressive drought
Ecohydrology, 1, (1), . (doi:10.1002/eco.5).
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This article explores the utility of spectral reflectance signals to assess changes in the photosynthetic efficiency (?PSII) of
Sphagnum mosses exposed to reductions in water availability. Reflectance was measured in parallel to moisture content and
chlorophyll fluorescence in five species of Sphagnum exposed to progressive drought. Decreases in moisture availability
caused a significant reduction in ?PSII for all samples tested. An objective was to ascertain whether Sphagnum ?PSII was
better correlated with (i) spectral indices directly related to photosynthetic processes through association with xanthophyll
cycle pigmentation (the photochemical reflectance index; PRI), or (ii) indices indirectly correlated with photosynthetic activity
but able to detect changes in canopy morphology (the normalized difference vegetation index; NDVI), chlorophyll stability
(the structure insensitive pigment index; SIPI), or canopy moisture content (the floating water band index; fWBI). Strongest
correlations were found between ?PSII and indices which were indirectly related to ?PSII. Both the SIPI and the NDVI exhibited
linear correlations although these relationships were sometimes sample-specific. The fWBI was the index least affected by
sample-specific relationships and showed a strong curvilinear correlation with ?PSII. Photosynthetic efficiency was correlated
with the PRI but relationships were much weaker than for the other indices used, and in some cases negative. The NDVI and
the fWBI show the most potential for monitoring Sphagnum photosynthetic activity at the ecosystem scale. The results from
this study will help to monitor and understand the responses of these key species to hydrological disturbances.
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