USING HEMISPHERICAL PHOTOGRAPHY FOR ESTIMATING PHOTOSYNTHETIC PHOTON FLUX-DENSITY UNDER CANOPIES AND IN GAPS IN DOUGLAS-FIR FORESTS OF THE PACIFIC-NORTHWEST
Mj. Easter et Ta. Spies, USING HEMISPHERICAL PHOTOGRAPHY FOR ESTIMATING PHOTOSYNTHETIC PHOTON FLUX-DENSITY UNDER CANOPIES AND IN GAPS IN DOUGLAS-FIR FORESTS OF THE PACIFIC-NORTHWEST, Canadian journal of forest research, 24(10), 1994, pp. 2050-2058
Our primary objective in this study was to determine if hemispherical
(fish-eye) photographs could be used to estimate photosynthetic photon
flux density (PPFD) in mature and old-growth Pseudotsuga-Tsuga forest
s in the Oregon and Washington Cascade Range. LI-COR quantum sensors s
ampled PPFD at 10-s intervals at six points in each of two old-growth
stands from January to December of 1991. Direct measures of PPFD were
also made with photodiodes, which were calibrated against quantum sens
ors. They were used to sample PPFD for 6- to 14-day periods during the
summer at 15 points in each of four stands, two mature and two old gr
owth. Hemispherical photographs were taken at these sample points and
were digitized and analyzed using the program CANOPY. The software pre
dicts overall site openness and openness along the sun path. Regressio
n models were developed based upon those parameters. The models predic
ted the mean daily PPFD for each month of the year. Two major factors
were found that influenced regression models: stand age-structure (mat
ure vs. old growth) and sky conditions (dry vs. wet seasons). Canopies
in mature stands were more open than old growth stands, and their PPF
D models were more heavily influenced by overall site openness. Conver
sely, PPFD models for old-growth canopies were more heavily influenced
by openness along the sun path. Wet-season models were dominated by d
iffuse PPFD (overall site openness), and dry-season models were domina
ted by direct flux (openness along the sun path). Combining data sets
from stands of comparable age-structure and seasons considerably incre
ased regression R(2)-values.