Forecasts of increased droughts and recent woody mortality events highlight the need for improved monitoring capabilities and understanding of how current woody encroachers in semi-arid savannas respond to water limitations and excess radiation. However, current abilities to track physiological responses to water in savannas by remote sensing technologies are severely hampered by the heterogenous structure of savannas (woody-grass matrix), evergreens (little variation in greenness), and tradeoffs in temporal, spatial, and spectral resolutions of remote sensing instruments. Further, strategies among woody encroachers relating to photoinhibition, energy dissipation, and impacts on photosystems remains unclear.
By moving beyond greenness indices and utilizing near-canopy remote sensing techniques for individual trees, we may be able to better capture (and decouple from surroundings) physiological changes of savanna trees ranging from evergreen to deciduous phenologies. We explore the utility of the chlorophyll carotenoid index (CCI) and photochemical reflectance index (PRI) versus common greenness indices with novel tree canopy remote sensing techniques in tracking seasonal leaf-level photosynthetic changes in semi-arid savanna trees. The susceptibility of woody plants with varying phenologies and strategies for water limitations (drought avoidance vs tolerance) to excess energy and photoinhibition during dry periods is explored with leaf-level fluorescence, pigments, and stem water potential measurements.
Results/Conclusions
Pre-dawn water potential for all three species decreased from July to September with Juniperus experiencing up to 2 MPa lower than the others. The maximum quantum yield of PSII (Fv/Fm) in Juniperus ashei (evergreen and drought tolerant) decreased from non-stressed values of 0.82 in July to slightly stressed values of 0.76 in September, while experiencing higher non-photochemical quenching (sensible heat release) than the semi-evergreen or deciduous species. Quercus (semi-evergreen) and Prosopis (deciduous) did not have stressed Fv/Fm values until just prior to leaf senescence. The greater sensible heat loss and lack of seasonal recovery of Fv/Fm from late summer water deficits indicates that the drought tolerant evergreen underwent earlier photoinhibition during periods of high moisture deficits relative to two trees with greater water availability.
CCI appeared to track the late-season down regulation of PSII (ΦPSII) across all three woody species. The normalized difference vegetation index (NDVI) and PRI appeared to have weaker trends in tracking both ANet and ΦPSII in the evergreen and semi-evergreen trees. The combination of physiological and structural bands in CCI appears to hold better promise in tracking woody savanna photosynthetic physiology (ΦPSII) than traditional greenness indices.