| The leaf area index (LAI), defined as half the total green leaf area per unit ground surface area, is a commonly used parameter for quantifying forest canopy structures, which controls many forest ecology processes. Especially, accurately measuring the seasonal changes of LAI is essential for modeling the response of plants to climate change and predicting the growth of trees. Recently, optical methods (indirect methods) are usually used to measure LAI for a forest stand because of their convenience and effectiveness. However, the optical LAI should usually be corrected against direct LAI due to inherent limitations. Although the direct method can measure LAI accurately, it is time-consuming and labor intensive, and most of direct methods are destructive. Therefore, the method for improving the accuracy of optical LAI and for directly measuring the dynamic changes of LAI in different forests within non-destructive is gaining increasing attentions.In the present study, we measured the annual maximum LAI in a mixed conifer and broadleaf forest (e.g., broadleaved-Korean pine forest) by combining the litterfall and the needle life span of evergreen needleleaf species; meanwhile we measured the annual maximum LAI in broadleaf forests using the litterfall method. Using these values as a reference, we evaluated the accuracy of LAI derived from DHP and LAI-2000. In the mixed broadleaved-Korean pine forest, DHP Le (effective LAI) and LAI-2000 Le underestimated direct LAI by 61% and 32%, respectively. The difference between direct LAI and corrected DHP LAI or LAI-2000 LAI was all less than 6%. In broadleaf forests, the mean difference between direct LAI and corrected DHP LAI was 21%. These results suggest that the optical method is able to measure the annul maximum LAI in different forest stands as long as considering the influence of woody materials and clumping effects, especially for mixed conifer and broadleaf forests, the accuracy of corrected DHP LAI and LAI-2000 LAI can be over 94%.Second, we explored a direct method to measure the seasonal changes of LAI in a mixed conifer and broadleaf forest within non-destructive by combining the leaf seasonality observations and litterfall, and we also recommended two methods to improve the accuracy of optical methods in measuring LAI. First, for mixed conifer and broadleaf forests, the accuracy of DHP LAI was determined by woody materials, clumping index, needle-to-shoot area ratio and exposure setting, the accuracy of DHP LAI after considering the above factors was over 85%; in contrast, the former three factors determined the accuracy of LAI-2000 in measuring LAI, and its accuracy was over 91% after correcting the error caused by these factors. For broadleaf forests, DHP Le underestimated direct LAI by 14-55% during from May 21 to October 1, but it overestimated direct LAI by 78% and 226% on May 12 and October 11, respectively. The accuracy of corrected DHP LAI with scheme D was over 83%. Second, based on optical methods, we can quickly and correctly measure the seasonal changes of LAI in different forests by using the empirical models between direct and optical LAI. The results showed that the accuracy of DHP and LAI-2000 in measuring the seasonal changes of LAI in different mixed conifer and broadleaf forests based on classified models were over 87% and 92%, respectively. For broadleaf forests, the DHP LAI derived from classified models did not differ with direct LAI significantly in each study period (P< 0.01).From 2009 to 2013, we measured the interannual changes of leaf area ratio, total annual leaf litter and LAI for major species in this study site. The results showed that the interannual changes in leaf area ratio for Pinus Koraiensis were larger than both Abies nephrolepis and Picea spp.; the leaf area ratio for Tilia amurensis, Acer mono, Betula costata and Ulmus laciniata in 2009 differed significantly with that other years, and the leaf out date for Fraxinus mandshurica delayed than other species. The interannual changes in annual leaf litter varied with species, and the interannual changes in LAI varied with different forest stands.Finally, we constructed the correlations between LAI and basal area for major species in mixed broadleaved-Korean pine forest. Based on these correlations, we measured the annual maximum LAI for major species and the forest stand in both 2005 and 2010, and we analyzed the spatial pattern distribution of the LAI using the semivariogram model. The results showed that the spatial heterogeneity of LAI for mixed broadleaved-Korean pine forest and major species were mainly derived from spatial autocorrelation, and the spatial pattern distribution for most LAI did not change clearly as time-varying. |
PDF Full Text Request