Validity of leaf areas and angles estimated in a beech forest from analysis of gap frequencies, using hemispherical photographs and a plant canopy analyzer

摘要

Using both a Li-Cor Plant Canopy Analayzer (PCA) and the hemispherical photographs technique, we measured the gap fraction in two young beech pole stands of known leaf tip angle distribution. The average contact number at various zenith angles (K (theta) function ) was determined and leaf area index was calculated using the method proposed previously. The following cases were examined: 1) data from PCA using five, four or three rings, 2) data from hemispherical photographs, arranged in rings, and divided into azimuth sectors (90, 45 and 22.5 deg) or averaged over azimuth (360 deg). These results were compared with a semi-direct estimation of the leaf area index derived from allometric relationships established at tree level. We also compared the G(theta) functions calculated using direct measurements of the leaf tip angle distribution with those deduced from transmittance data. The two indirect techniques gave the same estimation of the gap fraction at all zenith angles. When data were processed using the random model (averaged over azimuth), the PCA and photographs provided the same values of leaf area index, these values being considerably lower than those from allometric relationships (-25percent). When data from hemispherical photographs were divided into narrow azimuth sectors (22.5 deg), assuming a quasi-random model, the estimate of leaf area index was improved, but remained about 10percent below the allometric estimates. Leaf area index estimated using the random model was found to be 75 percent of that estimated using allometric relationships. It is shown that the underestimation of the leaf area index observed considering all five rings on the PCA isdue to an inappropriate use of the random model. It is also shown that the increase in leaf area index that was observed when neglecting one or two rings (PCA) was caused by an important error in the estimation of the slope of the function K (theta). We quantified this bias which depends on the leaf angle distribution within the canopy. Errors made on K function by the PCA are often compensated by an arbitrary omission of one or two rings. The consequences of neglecting these rings are discussed, together with the respective interest of both techniques.