Evaluation of several measures of canopy openness as predictors of photosynthetic photon flux density in deeply shaded conifer-dominated forest understory

摘要

A study was conducted at 2 sites to evaluate the effectiveness of 3 rapid methods of estimating the long-term integrated percentage of above-canopy photosynthetic photon flux density (PPFD) in a deeply shaded conifer-dominated forest understorey atthe University of Minnesota's Cloquet Forestry Centre. The first site was overstocked with a canopy of Pinus strobus and subcanopy of Acer rubrum and Abies balsamea with the understorey dominated by Corylus cornuta. The second site was understocked and the canopy dominated by Pinus resinosa and Pinus strobus with a patchy understorey. Sampling points were selected to to encompass a wide variation in light availability and data collected during August to November 1995 and June to August 1996. Methods included (i) hemispherical canopy photography, (ii) hemispherical sensors (LAI-2000), and (iii) instantaneous %PPFD. Transmitted PPFD was continuously measured using photodiodes at 60 measurement points ranging from 1 to 50% PPFD. Measurements from all methods were positively and linearly related to the mean daily %PPFD measured for 2 different periods of the year (foliage on and foliage off). However, the strength of the relationship and closeness to a 1:1 fit was weaker for the hemispherical photograph technique. During the foliage-on period, the hemispherical sensor (LAI-2000) explained 90% of the variation in mean daily %PPFD, while the instantaneous %PPFD and hemispherical photography explained 88 and 67%, respectively. Moreover, when examining low-light conditions only (<6%PPFD), hemispherical photographs failed to detect differences in %PPFD, while the other techniques were nearly as effective in low light as across the entire light gradient.