Using excess greenness and green chromatic coordinate colour indices from aerial images to assess lodgepole pine vigour, mortality and disease occurrence

• Colour indices related to ground measures of forest health, except stem rust.
• Canopy closure influenced the strength of these relationships.
• Colour indices can be calculated at spatial scales of trees or plots.
• Colour indices could help prioritize areas for ground assessments of stand health.

Here, we test the feasibility of using two colour indices to assess differences in stand-health metrics in 54 plots of lodgepole pine in British Columbia, Canada. Colour indices of excess greenness (EG) and green chromatic coordinate (GCC) were calculated over two spatial scales from colour images captured from a Cessna T210 flying at 600 m above ground level. EG and GCC were then compared to five ground-based metrics of stand health: vigour, mortality, foliar disease occurrence, western gall rust occurrence, and root disease symptoms. Colour indices, calculated at both tree and plot scales, significantly related to the ground-based metrics of stand health, except western gall rust occurrence. These relationships were influenced by canopy closure, but were unaffected by foliar nitrogen concentration. Using linear regression models, ground-based stand-health metrics accounted for 36.5–60.9% of the variability in colour indices. Within research projects, EG and GCC values could be used to set thresholds below which ground-checks of stand health would be warranted.