Rikie Suzuki

Using digital camera images to detect canopy condition of deciduous broad-leaved trees

Background: Recent studies have described a technique that incorporates a digital camera to observe aspects of tree phenology, such as leaf expansion and leaf fall. This technique has shown that seasonal patterns of red, green and blue digital numbers (RGB_DN) extracted from digital images differ between species. Aims: To identify the different characteristics of phenology between species by examining RGB_DN, the relationship between the seasonal patterns of RGB_DN and ecological characteristics for various species were evaluated throughout the year. Methods: The relationship between the normalised RGB_DN values extracted from digital images and in situ leaf area index (LAI) and leaf chlorophyll content (indicated by soil and plant analyser development, SPAD) was examined for three dominant species for multiple years in a cool-temperate, deciduous, broad-leaved forest in Japan. Results: The RGB_DN values in spring were not useful in detecting the different characteristics of leaf-flush patterns between species. In contrast, RGB_DN values in autumn showed differences in leaf-colouring as well as in leaf-fall patterns and timings between species. Conclusion: Differences in autumn phenology between tree species can be detected by using the normalised RGB_DN technique, while the technique cannot be applied in spring.

Relationships between meridional profiles of satellite-derived vegetation index (NDVI) and climate over Siberia.

This study investigates the regional relationship between the satellite-derived vegetation index (Normalized Difference Vegetation Index, NDVI) and climatological parameters (temperature and precipitation) over Siberia on a 5-year (1986–1990) annual mean basis. The NDVI in Siberia shows a large value around the 60°N zone, and it gradually decreases toward the southern arid region and the low temperature polar region. This meridional profile (south–north regionality) of the NDVI was analysed in two meridional transects, along 75°E (arid–forest transect) and 110°E (forest–tundra transect). A modified warmth index (WI(0)) was utilized as a temperature index. In the 75°E transect, high positive (0.79) and negative (−0.58) correlations were found in the NDVI–precipitation and the NDVI–WI(0) meridional profiles, respectively. This fact implies that precipitation plays a role in providing water and dominates the vegetation distribution, while high temperature induces dryness and water shortages, i.e. the critical factor for the vegetation meridional profile is available water in the arid–forest transect. In the 110°E transect, a high positive (0.92) correlation was found, which suggests that the dominant factor for the NDVI profile is temperature in the forest–tundra transect, i.e. the critical factor for the vegetation meridional profile here may be temperature. Furthermore, these meridional profiles were scrutinized in terms of the station altitude and it was suggested that the NDVI tends to change depending on the WI(0) and precipitation at regional variations, which are basically the result of the altitude difference between the stations. The comprehensive relationship between the NDVI, WI(0) and precipitation regionalities is discussed in terms of six vegetation types. The result shows that high NDVI (>0.2) is observed when the WI(0) is over 40°C and precipitation is more than 2.5×WI(0)+50. Copyright

Assessing the use of camera-based indices for characterizing canopy phenology in relation to gross primary production in a deciduous broad-leaved and an evergreen coniferous forest in Japan

Abstract Recent studies have reported that seasonal variation in camera-based indices that are calculated from the digital numbers of the red, green, and blue bands (RGB_DN) recorded by digital cameras agrees well with the seasonal change in gross primary production (GPP) observed by tower flux measurements. These findings suggest that it may be possible to use camera-based indices to estimate the temporal and spatial distributions of photosynthetic productivity from the relationship between RGB_DN and GPP. To examine this possibility, we need to investigate the characteristics of seasonal variation in three camera-based indices (green excess index [GE], green chromatic coordinate [rG], and HUE) and the robustness of the relationship between these indices and tower flux-based GPP and how it differs among ecosystems. Here, at a daily time step over multiple years in a deciduous broad-leaved and an evergreen coniferous forest, we examined the relationships between canopy phenology assessed by using the three indices and GPP determined from tower CO2 flux observations, and we compared the camera-based indices with the corresponding spectra-based indices estimated by a spectroradiometer system. We found that (1) the three camera-based indices and GPP showed clear seasonal patterns in both forests; (2) the amplitude of the seasonal variation in the three camera-based indices was smaller in the evergreen coniferous forest than in the deciduous broad-leaved forest; (3) the seasonal variation in the three camera-based indices corresponded well to seasonal changes in potential photosynthetic activity (GPP on sunny days); (4) the relationship between the three camera-based indices and GPP appeared to have different characteristics at different phenological stages; and (5) the camera-based and spectra-based HUE indices showed a clear relationship under sunny conditions in both forests. Our results suggest that it might be feasible for ecologists to establish comprehensive networks for long-term monitoring of potential photosynthetic capacity from regional to global scales by linking satellite-based, in situ spectra-based, and in situ camera-based indices.

In situ examination of the relationship between various vegetation indices and canopy phenology in an evergreen coniferous forest, Japan

We examined the relationship between four vegetation indices and tree canopy phenology in an evergreen coniferous forest in Japan based on observations made using a spectral radiometer and a digital camera at a daily time step during a 4 year period. The colour of the canopy surface of Japanese cedar (Cryptomeria japonica) changed from yellowish-green to whitish-green from late May to July and turned reddish-green in winter. The normalized difference vegetation index (NDVI), enhanced vegetation index (EVI) and plant area index (PAI) showed no seasonality. In contrast, the green–red ratio vegetation index (GRVI) increased from March to June and then decreased gradually from July to December, resulting in a bell-shaped curve. GRVI revealed seasonal changes in the colour of the canopy surface. GRVI correlated more positively with the evaluated maximum photosynthetic rate for the whole forest canopy, A max, than did NDVI or EVI. These results suggest the possibility that GRVI is more useful than NDVI and EVI for capturing seasonal changes in photosynthetic capacity, as the green and red reflectances are strongly influenced by changes in leaf pigments in this type of forest.

PALSAR 50 m Mosaic Data Based National Level Biomass Estimation in Cambodia for Implementation of REDD+ Mechanism

Tropical countries like Cambodia require information about forest biomass for successful implementation of climate change mitigation mechanism related to Reducing Emissions from Deforestation and forest Degradation (REDD+). This study investigated the potential of Phased Array-type L-band Synthetic Aperture Radar Fine Beam Dual (PALSAR FBD) 50 m mosaic data to estimate Above Ground Biomass (AGB) in Cambodia. AGB was estimated using a bottom-up approach based on field measured biomass and backscattering (σo) properties of PALSAR data. The relationship between the PALSAR σo HV and HH/HV with field measured biomass was strong with R2 = 0.67 and 0.56, respectively. PALSAR estimated AGB show good results in deciduous forests because of less saturation as compared to dense evergreen forests. The validation results showed a high coefficient of determination R2 = 0.61 with RMSE  = 21 Mg/ha using values up to 200 Mg/ha biomass. There were some uncertainties because of the uncertainty in the field based measurement and saturation of PALSAR data. AGB map of Cambodian forests could be useful for the implementation of forest management practices for REDD+ assessment and policies implementation at the national level.

Spatial Scale and Landscape Heterogeneity Effects on FAPAR in an Open-Canopy Black Spruce Forest in Interior Alaska

Black spruce forests dominate the land cover in interior Alaska. In this region, satellite remote sensing of ecosystem productivity is useful for evaluating black spruce forest status and recovery processes. The fraction of absorbed photosynthetically active radiation (FAPAR) by green leaves is a particularly important input parameter for ecosystem models. FAPAR_1d is computed as the ratio of absorbed photosynthetically active radiation (APAR_3d) to the incident photosynthetically active radiation at the horizontal plane above the canopy (PAR_1d, FAPAR_1d = APAR_3d/PAR_1d). The parameter FAPAR_1d is scale dependent and can be larger than 1 as a result of laterally incident PAR. We investigated the dependence of FAPAR_1d on spatial scale in an open-canopy black spruce forest in interior Alaska. We compared FAPAR_1d with FAPAR_3d( = APAR_3d/PAR_3d), the latter of which considers incident PAR as actinic flux (spheradiance) (PAR_3d). Our results showed the following: 1) landscape scale FAPAR_3d(30×30 m²) was always larger (0.39-0.43) than FAPAR_1d (0.19-0.27) due to the landscape heterogeneity and incident PAR regime, and 2) at the individual tree scale, FAPAR_1d was highly variable, with 34% (day of year [DOY] 180) to 52% (DOY 258) of , whereas FAPAR_3d varied across a much narrower range (0.2-0.5). The spatial-scale dependence of the ratio of PAR_3d to PAR_1d converged at the pixel size larger than 5 m. Thus, a 5-m or coarser resolution was necessary to ignore the lateral PAR effect in the open-canopy black spruce forest.

A Simple Method for Retrieving Understory NDVI in Sparse Needleleaf Forests in Alaska Using MODIS BRDF Data

Global products of leaf area index (LAI) usually show large uncertainties in sparsely vegetated areas because the understory contribution is not negligible in reflectance modeling for the case of low to intermediate canopy cover. Therefore, many efforts have been made to include understory properties in LAI estimation algorithms. Compared with the conventional data bank method, estimation of forest understory properties from satellite data is superior in studies at a global or continental scale over long periods. However, implementation of the current remote sensing method based on multi-angular observations is complicated. As an alternative, a simple method to retrieve understory NDVI (NDVIu) for sparse boreal forests was proposed in this study. The method is based on the fact that the bidirectional variation in NDVIu is smaller than that in canopy-level NDVI. To retrieve NDVIu for a certain pixel, linear extrapolation was applied using pixels within a 5 × 5 target-pixel-centered window. The NDVI values were reconstructed from the MODIS BRDF data corresponding to eight different solar-view angles. NDVIu was estimated as the average of the NDVI values corresponding to the position in which the stand NDVI had the smallest angular variation. Validation by a noise-free simulation data set yielded high agreement between estimated and true NDVIu, with R2 and RMSE of 0.99 and 0.03, respectively. Using the MODIS BRDF data, we achieved an estimate of NDVIu close to the in situ measured value (0.61 vs. 0.66 for estimate and measurement, respectively) and reasonable seasonal patterns of NDVIu in 2010 to 2013. The results imply a potential application of the retrieved NDVIu to improve the estimation of overstory LAI for sparse boreal forests and ultimately to benefit studies on carbon cycle modeling over high-latitude areas.

Utility of information in photographs taken upwards from the floor of closed-canopy deciduous broadleaved and closed-canopy evergreen coniferous forests for continuous observation of canopy phenology

Abstract Hemispherical photographs taken on forest floors are used to monitor seasonal changes in canopy openness or leaf area index in ecological studies. Those analyses usually use black and white images converted from the original colour images. Photographs taken by downwards-facing cameras installed on towers are used to provide detailed information on leaf expansion, maturation and senescence of various tree species through the analysis of red, green and blue ‘digital numbers’ (DNRGB) extracted from those images. To examine the usefulness of colour information encoded in upwards hemispherical photographs in monitoring canopy phenological characteristics, we examined the consistency of DNRGB values between downwards and upwards images in deciduous broadleaved and evergreen coniferous forests in Japan. In the deciduous broadleaved forest, the DNRGB values in the upwards images were able to detect canopy phenology almost as well as those in the downwards images. However, we found the effects on DNRGB of (1) the spatial heterogeneity among observed points, (2) low-vegetation (before the beginning of leaf-expansion and after the end of leaf-fall period) and (3) white balance settings. In the evergreen coniferous forest, in contrast, the DNRGB values in the upwards images did not capture canopy phenology. These different results may be attributable to the light attenuation characteristics in the canopies due to the geometries of leaves and branches. Thus, the DNRGB values obtained from upwards images are almost as good as those of downwards images for monitoring detailed canopy phenology in deciduous broadleaved forests with a closed canopy.

Unmanned Aerial Survey of Fallen Trees in a Deciduous Broadleaved Forest in Eastern Japan

Since fallen trees are a key factor in biodiversity and biogeochemical cycling, information about their spatial distribution is of use in determining species distribution and nutrient and carbon cycling in forest ecosystems. Ground-based surveys are both time consuming and labour intensive. Remote-sensing technology can reduce these costs. Here, we used high-spatial-resolution aerial photographs (0.5–1.0 cm per pixel) taken from an unmanned aerial vehicle (UAV) to survey fallen trees in a deciduous broadleaved forest in eastern Japan. In nine sub-plots we found a total of 44 fallen trees by ground survey. From the aerial photographs, we identified 80% to 90% of fallen trees that were >30 cm in diameter or >10 m in length, but missed many that were narrower or shorter. This failure may be due to the similarity of fallen trees to trunks and branches of standing trees or masking by standing trees. Views of the same point from different angles may improve the detection rate because they would provide more opportunity to detect fallen trees hidden by standing trees. Our results suggest that UAV surveys will make it possible to monitor the spatial and temporal variations in forest structure and function at lower cost.

Spatio-temporal distribution of the timing of start and end of growing season along vertical and horizontal gradients in Japan

We detected the spatio-temporal variability in the timing of start (SGS) and end of growing season (EGS) in Japan from 2003 to 2012 by analyzing satellite-observed daily green-red vegetation index with a 500-m spatial resolution. We also examined the characteristics of SGS and EGS timing in deciduous broadleaf and needleleaf forests along vertical and horizontal gradients and then evaluated the relationship between their timing and daily mean air temperature. We found that for the timing of SGS and EGS, changes along the vertical gradient in deciduous broadleaf forest tended to be larger than those in deciduous needleleaf forest. For both forest types, changes along the vertical and horizontal gradients in the timing of EGS tended to be smaller than those of SGS. Finally, in both forest types, the sensitivity of the timing of EGS to air temperature was much less than that of SGS. These results suggest that the spatio-temporal variability in the timing of SGS and EGS detected by satellite data, which may be correlated with leaf traits, photosynthetic capacity, and environment conditions, provide useful ground-truthing information along vertical and horizontal gradients.