tri-PRI: A three band reflectance index tracking dynamic photoprotective mechanisms in a mature eucalypt forest

•The new tri-PRI tracks leaf-level xanthophyll epoxidation state dynamics in a mature eucalypt forest.•Indices using two-bands do not perform well.•The new formulation is robust to constitutive pigment pool size effects across the canopy profile.•tri-PRI achieved results comparable to PLSR and continuum removal using full spectrum data.