Tuyển tập các báo cáo nghiên cứu về lâm nghiệp được đăng trên tạp chí lâm nghiệp quốc tế đề tài: Validity of leaf areas and angles estimated in a beech forest from analysis of gap frequencies, using hemispherical photographs and a plant canopy analyzer. | Ann. For. Sci. 56 1999 1-10 Inra Elsevier Paris 1 Original article Validity of leaf areas and angles estimated in a beech forest from analysis of gap frequencies using hemispherical photographs and a plant canopy analyzer Isabelle Planchaisa b Jean-Yves Pontaillera Laboratoire d ecophysiologie végétale CNRS bâtiment 362 université Paris-Sud 91405 Orsay cedex France bDépartement des recherches techniques Office national des forets boulevard de Constance 77300 Fontainebleau cedex France Received 11 December 1997 accepted 1 July 1998 Abstract - Using both a Li-Cor Plant Canopy Analyzer 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 Q 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 or averaged over azimuth 360 . 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 0 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 -25 . When data from hemispherical photographs were divided into narrow azimuth sectors assuming a quasi-random model the estimate of leaf area index was improved but remained about 10 below the allometric estimates. Leaf area index estimated using the random model was found to .
