Spectral indices to evaluate the physiological response of Mediterranean shrubs to browsing

Resumen

Herbivory by large ungulates may cause important physiological and spectral changes in plants, which may serve as ecological indicators of herbivore overabundance. This study examines the physiological and spectral response of plants to recent and prolonged browsing through remote sensing techniques. We selected 30 plants from two Mediterranean shrub species with contrasting palatability: Cistus ladanifer (non-palatable) and Phillyrea angustifolia (highly palatable). Plants were subjected to three treatments: 1) no browsing (deer exclosure), 2) simulated recent browsing (clipping 30% of branchlets), and 3) intense, long-life natural browsing by deer (Cervus elaphus). Leaf physiological variables (chlorophyll, flavonols, anthocyanins, Nitrogen Balance Index (NBI), chlorophyll fluorescence, and stomatal conductance) were measured, and 27 spectral indices were calculated from reflectance data. The less herbivore-adapted Cistus showed lower chlorophyll (-27.7%), nitrogen (-26.8%), and stomatal conductance (-19.24%) when browsed compared to non-browsed plants, while no differences were found in the more palatable Phillyrea. Reflectance indices R750/R800 and R740/R800 related with chlorophyll fluorescence, were significantly lower for the intense, long-life browsing treatment in both species. Other indices such as R690/R600 only showed significant differences between species. NDPI and SRPI indices, related to carotenoid/chlorophyll ratio, showed seasonally variable responses to browsing, with higher levels in spring for both herbivory treatments. We conclude that some spectral indices related to chlorophyll/carotenoids were able to detect physiological stress from deer herbivory, with key seasonal and species-specific variations for both, palatable and non-palatable shrubs. This demonstrates the potential and context-dependent use of spectral indices for the ecology and management of herbivore-dominated environments.