Base-line selection of black spruce by large-scale aerial photography
Following selection of black spruce (Piceamariana (Mill.) B.S.P.) for growing space efficiency using a base line of basal area increment vs. crown length × crown radius, the corresponding crown data were determined by large-scale (1:1250) aerial photography for 174 ground-measured trees to determine whether aerial photography would provide a more accurate and cost-effective alternative for plus-tree selection. Values obtained for ground- and photo-measured tree heights and crown lengths corresponded closely, but values of crown radii measured from the ground by a right-angle prism did not correspond in the same manner to values derived from crown horizontal projection areas on the photos. Nonetheless, the different base lines of basal area increments vs. various ground- and photo-measured crown parameters generally identified the same trees as having the greatest growing space efficiency. The inclusion of crown length together with crown radius did not improve the goodness of fit for regressions of basal area increments vs. crown parameters. Apparently, the two horizontal dimensions are adequate to describe growing space for black spruce, perhaps because its crown form is relatively constant. Our results indicate that large-scale aerial photography is well suited to plus-tree selections of black spruce if ground mesurements are not also required to establish base lines; the extra cost of the photos is offset by the greater speed in measuring tree heights or crown horizontal projection areas. Thus, while the technique is sufficiently accurate, it is not cost effective for establishing growing space efficiency base lines where the basal area increment is determined directly from increment cores.