Forest age class within 1.5km radius around den

[popovs]

Add a target to the targets pipeline that includes forest age class by % area within a 1.5 km radius around each den for each year.

Currently, this function is written as st_proportion_age_class, but a bug needs to be fixed where the function fails to take into account the age of a forest stand prior to being cut. As that data isn't necessarily in the VRI, instead use a weighted mean stand age for all other trees within the 1.5km radius, where the weighting is by % area of stand.

[popovs]

Mean % area by group across all dens, all years:

[popovs]

Mean % area by young (< age class 3) and old (>= age class 3) forest across all dens and years:

[popovs]

Mean % area by young (< 3), medium (3-7), and old (8, 9) forest across all dens and years: