Fig. P1.

Spurious correlation. (A) The positive correlation between the canopy bidirectional reflectance factor (BRF) in the near-infrared region and the canopy foliar nitrogen concentration, %N. (B) Both the canopy foliar %N and the BRF are strongly and positively related to the broadleaf fraction of the leaf area, which characterizes variations in forest structure due to the proportions of structurally different broad- and needleleaf species. The canopy structure is the basis of this positive relationship between the BRF and %N. (C) When the BRF data are corrected for canopy structural effects, the residual variations are negatively related to %N. It was shown that whereas the BRF exhibits either positive or no relation with nitrogen in the NIR and shorter wavelengths, its structure-corrected counterpart is negatively correlated with %N in the 423- to 855-nm interval. The observed positive correlation is therefore an artifact resulting from variations in the canopy structure rather than %N.