| Forest,which accounts for 30%of area of the earth’s land surface and provides about 50%net primary productivity for the biosphere,plays a key role in global change researches.Forest canopy structure characteristics,which describe geometric characteristics of canopy at several levels,are very important for physiological processes of forest eco-system as well as understanding radiative transfer in the canopies.As one kind of forest canopy reflectance(CR)models,geometric-optical(GO)models are very suitable for structural parameter retrieval for discrete vegetation canopies because of their emphasis on vegetation structures and their interaction with radiative transfer processes within the canopy.Several scales of canopy structual parameters have been mathematically described by many previous GO modelers.While,there are still some deficiencies of existing researches about GO models of forest canopies.(1)Trees are usually assumed to be randomly distributed(such as the Poisson model)in natural forests based on a hypothesis of randomness of natural resources in many previous studies.Few studies have shown how trees are distributed in forest plantations and how these distribution patterns affect gap fraction(GF)and CR in these forests.(2)The previous researchers mainly focus on the influence of crown shapes(such as cylinder,cone,ellipsoid and cone+cylinder)on GF or clumping index(CI);while,crown size characteristics are also very important in addition to the shape characteristic for tree crowns.The characteristics of crown size have shown more obvious geographical spatial characteristics than crown shape characteristics at regional or global scale.Based on the above deficiencies of current researches,the main objectives of this paper are:(1)a tree distribution model for forest plantations will be developed;(2)a CR model for forest plantations will be developed based on the tree distribution model for forest plantations;(3)the influences of tree distributions and crown size characteristics on GF and CR are shown quantitatively,respectively.The main conclusions of this paper are as follows:1.A hypergeometric distribution model,which could quantitatively describe the degree of mutual exclusion among trees and completely replace the Poisson model,for forest plantations was developed.Based on GOST2,GOFP was developed using the hypergeometric distribution model for simulating CR of forest plantations.2.(1)The area ratios of four scene components(sunlit foliage,sunlit ground,shaded foliage,and shaded ground)of GOFP compare well with those simulated by a 3-D canopy visualization technique.A comparison is also made against DART(Discrete Anisotropic Radiative Transfer),showing that GOFP has the ability to simulate reflectance of canopies for forest plantations.(2)The simulations of GF in GOFP are verified by field measurements in four forest plantation stands.Another comparison is made against Landsat8 OLI and MODIS MOD09GA surface reflectance,respectively,showing that GOFP with the hypergeometric model outperforms GOST2 for simulating GF and CR of forest plantations.3.Tree distribution patterns have strong impacts on GF and CR.(1)The larger exclusion distance(RASD,Relative Allowable Shortest Distance between centers of any two adjacent crowns divided by the mean diameter of crowns)among trees,the lower GF is and larger CI is.GF of the hypergeometric distribution model is 41.5%lower than that of the Poisson distribution model at θ=0°.(2)In red band,CR simulated by GOST2 which is based on the Poisson distribution model is 57.4%greater than that simulated by GOFP;the latter is 26.3%greater than the former.(3)In most cases,the regular distribution model of trees could be replaced approximatively by the hypergeometric distribution model.(4)The influence of crown distribution patterns on CR increase with the increasing of the disparity between leaf reflectance(RT)and background reflectance(RG)as well as vegetation coverage.(5)If canopy structures are considered only,the saturation phenomenon of forest CR in optical bands is mainly due to saturable transmittance of canopy structures at different scales,not only due to high vegetation coverage or high leaf area index(LAI).4.Crown size characteristics have strong impacts on GF and CR.(1)Whether the radius or volume of tree crowns are fixed,the larger of projection of crown in the direction of view,the larger GF is,and the less CI is.When θ=60°,the canopy gap fraction with ellipsoid height(Hb)is 16 m is 92.8%lower than that with Hb=0.25 m,CI with Hb=16 m is 261.8%greater than that with Hb=0.25 m and expectation of CI in hemisphere space with Hb=16 m is about 250.8%greater than that with Hb=0.25 m when the crown radius is fixed.The influence of crown sizes on GF and CI is much more obvious than that of crown shapes on these two parameters.The influence of crown sizes on GF and CI is much more obvious than that of crown shapes on GF and CI.(2)PT and CR in NIR band with high crowns is larger than that with dwarf crowns at high θ,but less than the latter at low θ because of shadow effect among tree crowns.The innovation points of this paper are:(1)a hypergeometric distribution model,which could completely replace the Poisson model,for forest plantations was developed;GOFP was developed using the hypergeometric distribution model for simulating CR of forest plantations.(2)The saturation phenomenon of CR for forest is mainly due to the saturation of light transmittance at different levels within canopy,not merely due to high vegetation coverage or high LAI.(3)The influence of crown sizes on GF and CI is much more obvious than that of crown shapes on these two parameters. |
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