In summary, there are two broad types of methods for estimation of LAI, either employing the ��direct�� measures involving destructive sampling, litter fall collection, or point quadrat sampling ��indirect�� methods involving optical instruments and radiative transfer models. The dynamic, rapid and large spatial coverage advantages of remote sensing techniques, which overcome the labor-intensive and time-consuming defect of direct ground-based filed measurement, allow remotely sensed imagery to successfully estimate biophysical and structural information of forest ecosystems.A range of LAI definitions exist in the research literature, which complicates the comparison between works, and thus, the first focus of this paper is a compilation of LAI definitions.
The second focus of the paper is the explanation of the gap fraction method theory. Thirdly, LAI estimation methods and sensors are discussed. Finally, remotely sensed LAI estimation and scaling issues associated with it are discussed.2.?TheoryIn the early period of LAI research, due to the complicated distribution of foliage elements within the canopy, a modified Beer��s law light extinction model was developed. The model estimates LAI by mathematically analyzing light intercepting effect of leaves with different angular distribution based on a very common simplified assumption that all of foliage element and live parts within canopy are randomly distributed. The point quadrat method [45,46] was an early method used to mathematically analyze the relationship between projection area and foliage elements with all possible angular and azimuthal distributions.
In this model, the extinction coefficient served as an important parameter to characterize the effect of leaves�� angular and spatial distributions on radiation interception. An algorithm was developed  to calculate extinction coefficients based on the assumption that the angular distribution of leaf area in a canopy is similar to the distribution of area on the surface of prolate and oblate spheroids. Because of the assumption of randomly located foliage elements within canopy, the LAI obtained from gap fraction  theory was not the true LAI, thus, a term called effective LAI was created to more accurately describe the result. However, gap fraction theory only applies to the percentage or proportion of gaps accounting for the whole hemispherical bottom-up view of a canopy.
Gap size (dimensional information) is another very useful information to characterize clumping and overlapping effect, therefore, the gap size theory is a another stage for LAI ground-based filed indirect measurement development.Recently LAI research focus has shifted from an empirical and statistical stage GSK-3 to process-based modeling stage due to the involvement of remotely sensed datasets and numerical ecological model implementation.