Youngkeun Song

Estimation of leaf area index and gap fraction in two broad-leaved forests by using small-footprint airborne LiDAR

In this study, we evaluated methods for reliably estimating leaf area index (LAI) and gap fraction in two different types of broad-leaved forests by the use of airborne light detection and ranging (LiDAR) data. We evaluated 13 estimation variables related to laser height, laser penetration rate, and laser point attributes that were derived from LiDAR analyses. The relationships between LiDAR-derived estimates and field-based measurements taken from the forests were evaluated with simple linear regressions. The data from the two forests were analyzed separately and as an integrated dataset. Among the laser height variables, the coefficient of variation (CV) of all laser point heights had the highest level of accuracy for estimating both LAI and gap fraction. However, we recommend that more evaluations be conducted prior to the use of CV in forests with complex structures. The simplest laser penetration variable, which represents the ratio of the number of ground points to the total number of all points (PALL), also had a high level of accuracy for estimating LAI and gap fraction at the study sites regardless of whether the data were analyzed separately or as an integrated data set. Furthermore, PALL values showed near 1:1 relationships with the field-based gap fraction values. Hence, the use of PALL may be the most practical for estimating LAI and gap fraction in broad-leaved forests, even when the canopies are heavily closed.

Drought impact assessment from monitoring the seasonality of vegetation condition using long-term time-series satellite images: a case study of Mt. Kenya region

Drought-induced anomalies in vegetation condition over wide areas can be observed by using time-series satellite remote sensing data. Previous methods to assess the anomalies may include limitations in considering (1) the seasonality in terms of each vegetation-cover type, (2) cumulative damage during the drought event, and (3) the application to various types of land cover. This study proposed an improved methodology to assess drought impact from the annual vegetation responses, and discussed the result in terms of diverse landscape mosaics in the Mt. Kenya region (0.4° N 35.8° E ~ 1.6° S 38.4° E). From the 30-year annual rainfall records at the six meteorological stations in the study area, we identified 2000 as the drought year and 2001, 2004, and 2007 as the normal precipitation years. The time-series profiles of vegetation condition in the drought and normal precipitation years were obtained from the values of Enhanced Vegetation Index (EVI; Huete et al. 2002), which were acquired from Terra MODIS remote sensing dataset (MOD13Q1) taken every 16 days at the scale of 250-m spatial resolution. The drought impact was determined by integrating the annual differences in EVI profiles between drought and normal conditions, per pixel based on nearly same day of year. As a result, we successfully described the distribution of landscape vulnerability to drought, considering the seasonality of each vegetation-cover type at every MODIS pixel. This result will contribute to the large-scale landscape management of Mt. Kenya region. Future study should improve this method by considering land-use change occurred during the long-term monitoring period.

Spectral Correction for the Effect of Crown Shape at the Single-Tree Level: An Approach Using a Lidar-Derived Digital Surface Model for Broad-Leaved Canopy

The spectral response of a single tree on remotely sensed images may be affected by the crown shape, yielding brighter sun-facing sides and darker sides facing away from the sun. To correct this crown-shape effect, we assumed that the roughness of the canopy surface within a single crown resembles the topography of the land surface, in the case of a closed upper-canopy layer (i.e., canopy surface) of broad-leaved deciduous trees. Previous topographic normalizations have proved to be useful in correcting the effect of rugged terrain at a large-landscape scale. However, are they also useful in correcting the rough canopy surface at the scale of a single crown? We applied conventional topographic corrections (cosine, Minnaert, and C) to a canopy surface model (CSM) of 66 trees, which was derived from airborne small-footprint Lidar data. The 12-band spectral data of each tree were acquired from the aircraft with 0.5-m spatial resolution and analyzed to validate the corrections. Among the tested methods, the C correction was the most successful in applying to the CSM, whereas the Minnaert and cosine corrections yielded errors in the reflectance of some steep canopy surfaces. We assessed the spectral feature of each tree in terms of tree vigor, by using the vegetation index employing the red-edge (700 nm) and green (541 nm) bands. Following the correction, the overall accuracy was improved from 75.5% to 84.5%. Thus, we suggest that the effect of crown shape be carefully considered and corrected in single-crown-level analyses of remotely sensed spectra.

Inter-annual growth of broad-leaved canopy estimated from the repeated LiDAR measurements

We estimated inter-annual growth of broad-leaved trees planted in the urban park, by using multi-temporal airborne LiDAR datasets acquired in 2004, 2008, and 2010. The annual changes in the LiDAR-estimated growth of average canopy heights were significantly correlated with one another over the study periods at the plot and individual-tree levels. A moderate and significant relation was shown with the increment of field-measured basal area. But the uncertainties remain in estimating short-term growth for small crown areas.

Indicators for the Quantitative Assessment of Tree Vigor Condition and Its Theoretical Implications : A Case Study of Japanese Flowering-cherry Trees in Urban Park

The vigor condition of trees is an important indicator for the management of urban forested area. But difficulties in how to assess the tree vigor condition still remain. Previous efforts were limited in the 1) measurement of single indicator rather than using multiple indices, 2) purpose-oriented measurement such as for air-pollution effect or specific pathological symptom, and 3) ordinal-scale evaluations by field crews 4) despite human errors based on his/her experiences or prior knowledge. Therefore, this study attempted to develop a quantitative and objective methodology for assessing tree vigor condition, by measuring multiple modules and building the profile inventory. Furthermore, the possibility and limitations were discussed in terms of schematic frames describing tree vigor condition. The vigor condition of 56 flowering cherry plants in urban park were assessed by in-situ measurements of following eight items; growth of crown(Gc), growth of shoots, individual tree