Evergreen, Semi-evergreen, and Deciduous Woody Plants in a Semi-arid Savanna: Photosystem Energy Dynamics and Near-canopy Remote Sensing with CCI/PRI

Abstract

Woody encroachers have transformed the Southern Great Plains across its semi-arid regions. Little is known how these encroachers—particularly the drought tolerant Juniperus—may be vulnerable to hotter temperatures and water limitations in future climate projections and how that will affect their light utilization for photosynthesis and dissipation of excess energy. Additionally, photosynthetic changes in drylands are not adequately captured with remote sensing partly due to use of greenness indices in the presence of evergreens. Responses among three co-occurring encroachers to seasonal water-limitations were compared using photosynthetic rates, energy partitioning of absorbed light, and sustained energy dissipation, while the recently created Chlorophyll Carotenoid Index (CCI) was tested to track the photosynthetic phenology of trees varying in leaf persistence. Declining stem water potential (-5 MPa) during the summer dry period elicited strong photochemical stress in J. ashei. The actual quantum yield for PSII decreased to 4%, regulatory light-induced energy dissipation increased to 84%, while the maximal quantum yield of PSII in a dark-adapted condition (Fv/Fm) declined to 75% from sustained dissipation. Photochemical stress was less severe in the drought avoiders with neither decreasing in Fv/Fm, while regulatory light-induced dissipation increased to 43% in Prosopis glandulosa and Quercus fusiformis remained near ~50%. Consequently, water stress elicited stronger photochemical stress in J. ashei relative to the other two drought avoiders. Canopy CCI was the only index to track the slow-adjusting chlorophyll/carotenoid and Fv/Fm across the deciduous (Chl:Car R2 = 0.88, p < 0.02, Fv/Fm Rrmc2 = 0.54, p < 0.001) to evergreen species (Chl:Car R2 = 0.44, p < 0.07, Fv/Fm R2 = 0.74, p < 0.0006) plus foliage density (R2 = 0.81, p < 0.006) of the deciduous P. glandulosa. Consequently, CCI may improve estimates of productivity in drylands with its relationships across species, but future studies at greater spatiotemporal scales are needed. The photochemical reflectance index had stronger relationships with Fv/Fm and Chl:Car than CCI, which may be due to the incomplete knowledge on variation sources in CCI. Dynamic parameters at midday had species-specific moderate relationships or non-significant relationships underscoring the difficulty relating instantaneous measurements to slow-adjusting reflectance signals in drylands.