Influence Of The Exposure On The Estimation Of Seasonal Variations In Leaf Area Index Measured Using Digital Hemispherical Photograph

Exposure setting is one of the important error sources of leaf area index(LAI)measured using digital hemispherical photography(DHP).This study was conducted in a mixed broadleaved-Korean pine(Pinus koraiensis)forest,a secondary birch(Betula platyphylla)forest,a Korean pine plantation and a Dahurian larch(Larix gmelinii)plantation in the Xiaoxing'an Mountains.LAI was measured using DHP and LAI-2200 plant canopy analyzer in the middle of June to September,respectively.Firstly,according to the exposure of the automatic exposure under the forest canopy.the LAI is measured by multiplying the shutter speed to obtain the LAI measurement values under different shutter speed.and compared with the LAI-2200 measurement values to obtain the optimal setting.And then determine if the type of forest and the measurement period have an effect on the choice of the optimal exposure gear.On the other hand,we compared LAI values measured through these two methods.Then we tested whether the forest type and study period had a significant influence on the correlations between the measured values of these two methods.Finally,we constructed empirical models for correcting the errors caused by automatic exposure for LAI values measured through DHP at different study periods in different forest types.The results showed that LAI from DHP was underestimated by 20%-49%relative to the LAI from LAI-2200 in four study periods of the four forest types.Compared with the automatic exposure under the canopy,the exposure is multiplying changed,and the optimal setting is one stops of less exposure than the automatic exposure.The forest type and measurement period have no effect on the selection of the optimal shutter speed range.Compared to the LAI measurement of the automatic exposure,the LAI measurement of the optimal gear increased by an average of 10%(8%?12%).In addition.Forest type had no significant effect on the construction of empirical models between these two methods for measuring the LAI.whereas the study period showed significant effects on them.Two classified empirical models(A and B)were constructed,which are suitable for correcting the LAI from DHP in June and September,July and August in four forest types,respectively.After being corrected by the classified empirical models,LAI from DHP of the four forest types increased by 45%-79%.and the measurement accuracy can be improved to 90%(83%-94%).Classified empirical models between LAI from DHP and LAI-2200 could effectively correct the influence of automatic exposure on DHP and greatly improve its measurement accuracy.This study established a technical support for rapid and effective measurement of seasonal changes in LAI for different forest types.