Photochemical Reflectance Index (PRI) captures the ecohydrological sensitivity of a semi-arid mixed conifer forest

The Photochemical Reflectance Index (PRI) corresponds to the
de-epoxidation state of the xanthophyll cycle and is one of the few
pigment-based vegetation indices sensitive to rapid plant physiological
responses.  As such, new remotely-sensed PRI products
present opportunities to study diurnal and seasonal processes in evergreen
conifer forests, where complex vegetation dynamics are not well reflected
by the small annual changes in chlorophyll content or leaf
structure.  Because PRI is tied explicitly to short and
long term changes in xanthophyll pigments which
are responsible for regulatig stress, this study
characterized PRI in a semi-arid, sub-alpine mixed conifer forest, in
order to assess its potential as a proxy for water stress by
extension of its association with photoprotection.  To determine
the sensitivity of PRI to seasonal changes in
ecohydrological variability and gross primary
productivity, canopy spectral measurements were combined with eddy
covariance flux and sap flow methods.  Seasonally, there
was a significant relationship between PRI and sap flow
velocity (R2=0.56), and multiple linear regression analysis
demonstrated the PRI response to dynamic water and energy limitations in
this system.  Although PRI was an effective indicator of
stomatal response to ecohydrological constraints on a seasonal time scale,
top-of-canopy leaf-level gas exchange, chlorophyll fluorescence, and
hyperspectral reflectance measurements suggest that diurnal PRI
saturates under conditions of severe
light stress.  This research indicates that
remotely-sensed PRI has potential to fill spatial and temporal gaps in the
ability to distinguish how water availability influences
carbon dynamics of forested ecosystems.