LARGE-SCALE 35-MM AERIAL PHOTOGRAPHS FOR ASSESSMENT OF VEGETATION-MANAGEMENT RESEARCH PLOTS IN EASTERN CANADA

Two 35-mm cameras were mounted on a boom and suspended from a tethered helium-filled blimp to obtain nominally vertical aerial photographs ( 1:828 and 1:414 contact scale) of vegetation-management research plots . Photo and ground estimates of woody plant crown area (m2/ha) and roo tstock density (number/ha) were compared for several experimental vege tation-control treatments. Horizontal point-sampling estimates of tota l crown area made directly on 1:93 scale prints (enlarged from 1:414) correlated strongly with equivalent estimates made on the ground (n = 62, r2 = 0.97). An estimated 20 ground-truth plots were required to ad equately quantify photo bias and correct subsequent prediction of actu al total crown area on the plots studied. Much of the observed photo b ias could be attributed to the undersampling of small rootstocks. Excl usion of individual rootstocks less than 0.10 M2 in crown area (or, eq uivalently, <56 cm in height) resulted in a 1:1 relation between the t wo sampling methods for estimates of both total crown area (r2 = 0.98) and rootstock density (r2 = 0.97). If data for rootstocks in smaller size classes are not needed, uncorrected photo estimates may be approp riate for evaluation of treatment response. Ground-sampling costs aver aged $200 (Canadian) per plot, compared with photo costs of $104 per p lot (without ground truth) or $150 per plot (with 20 ground-truth plot s). Smaller scale photos (1:828 contact) cost 11% less than the larger scale tested, but resulted in significant undersampling of individual rootstocks less than 0.2 m2 in crown area (or, equivalently, <80 cm i n height).