Gap fraction and foliage clumping index play key roles in plant light interception;therefore

they have strong influences on plant growth and canopy radiative transfer processes.Leaves always aggregate in crowns

which are important objects innumerous geometric-optical models of forests.Researchers have mainly focused on the influences of crown shape（e.g.

cylinder

cone

ellipsoid

and cone+cylinder）on gap fraction and foliage clumping index.However

size is also an important characteristic of tree crowns.Crown sizes result from the interactions between plants and environments during the long-term evolution process of the plants.In fact

crown size characteristics have more obvious geographical spatial features than crown shape.Therefore

crown size should be given considerable attention when studying plant light interception and canopy radiative transfer processes in the fields of phytogeography

remote sensing and global change.The main objective of this study is to exhibit how crown size characteristics influence the gap fraction and foliage clumping index of forest canopies.First

a simple and general distance factor

which is defined as the relative allowable shortest distance between centers of any two crowns divided by the mean diameter of tree crowns

is proposed to quantitatively describe the degree of repulsion effect among trees in forest canopies.Second

the Poisson distribution model of trees is completely replaced by the hypergeometric model

which is more suitable for quantitatively describing the repulsion effect and spatial relationship among trees in forest stands.Finally

seven sizes of ellipsoids（from prolate ellipsoids to oblate ellipsoids）are selected for the tree crowns

and the influences of crown size on gap fraction and foliage clumping index are exhibited by means of fixing the radius and the volume of the tree crowns

respectively.Results show that the following:（1） the influences of crown size characteristics are significantly greater than the effects of crown shape on gap fraction and foliage clumping index

regardless if the radius or volume of tree crowns are fixed.（2） From prolate ellipsoids to oblate ellipsoids

crown size characteristics show significant and regular effects on both gap fraction and foliage clumping index.In extreme cases

whenθ=60°

the canopy gap fraction when ellipsoid height（H

b

） is 16 m is 92.8%lower than that when H

b

=0.25 m

and the foliage clumping index when H

b

=16 m is 261.8%greater than that when H

b

=0.25 m when the crown radius is fixed.Atθ=0°

the canopy gap fraction when H

b

=16 m is 414.1%greater than that when H

b

=0.25 m

and the foliage clumping index when H

b

=0.25 m is 11397.2%greater than that when H

b

=16 m when the volume of crown is fixed.The larger the projected area of the crown in the observed direction

the lower value of the gap fraction and the higher value of the foliage clumping index

indicating that leaves tend to distribute randomly in forest canopies.