Review: advances in in situ and satellite phenological observations in Japan.

To accurately evaluate the responses of spatial and temporal variation of ecosystem functioning (evapotranspiration and photosynthesis) and services (regulating and cultural services) to the rapid changes caused by global warming, we depend on long-term, continuous, near-surface, and satellite remote sensing of phenology over wide areas. Here, we review such phenological studies in Japan and discuss our current knowledge, problems, and future developments. In contrast with North America and Europe, Japan has been able to evaluate plant phenology along vertical and horizontal gradients within a narrow area because of the country's high topographic relief. Phenological observation networks that support scientific studies and outreach activities have used near-surface tools such as digital cameras and spectral radiometers. Differences in phenology among ecosystems and tree species have been detected by analyzing the seasonal variation of red, green, and blue digital numbers (RGB values) extracted from phenological images, as well as spectral reflectance and vegetation indices. The relationships between seasonal variations in RGB-derived indices or spectral characteristics and the ecological and CO2 flux measurement data have been well validated. In contrast, insufficient satellite remote-sensing observations have been conducted because of the coarse spatial resolution of previous datasets, which could not detect the heterogeneous plant phenology that results from Japan's complex topography and vegetation. To improve Japanese phenological observations, multidisciplinary analysis and evaluation will be needed to link traditional phenological observations with "index trees," near-surface and satellite remote-sensing observations, "citizen science" (observations by citizens), and results published on the Internet.

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.

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.

Natural forest biomass estimation based on plantation information using PALSAR data.

Forests play a vital role in terrestrial carbon cycling; therefore, monitoring forest biomass at local to global scales has become a challenging issue in the context of climate change. In this study, we investigated the backscattering properties of Advanced Land Observing Satellite (ALOS) Phased Array L-band Synthetic Aperture Radar (PALSAR) data in cashew and rubber plantation areas of Cambodia. The PALSAR backscattering coefficient (σ0) had different responses in the two plantation types because of differences in biophysical parameters. The PALSAR σ0 showed a higher correlation with field-based measurements and lower saturation in cashew plants compared with rubber plants. Multiple linear regression (MLR) models based on field-based biomass of cashew (C-MLR) and rubber (R-MLR) plants with PALSAR σ0 were created. These MLR models were used to estimate natural forest biomass in Cambodia. The cashew plant-based MLR model (C-MLR) produced better results than the rubber plant-based MLR model (R-MLR). The C-MLR-estimated natural forest biomass was validated using forest inventory data for natural forests in Cambodia. The validation results showed a strong correlation (R2 = 0.64) between C-MLR-estimated natural forest biomass and field-based biomass, with RMSE  = 23.2 Mg/ha in deciduous forests. In high-biomass regions, such as dense evergreen forests, this model had a weaker correlation because of the high biomass and the multiple-story tree structure of evergreen forests, which caused saturation of the PALSAR signal.

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 (σ(o)) properties of PALSAR data. The relationship between the PALSAR σ(o) HV and HH/HV with field measured biomass was strong with R(2) = 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 R(2) = 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.

West-east contrast of phenology and climate in northern Asia revealed using a remotely sensed vegetation index.

The phenology of the vegetation covering north Asia (mainly Siberia) and its spatial characterstics were investigated using remotely sensed normalized difference vegetation index (NDVI) data. The analysis used the weekly averaged NDVI over 5 years (1987-1991) using the second-generation weekly global vegetation index dataset (0.144 degrees x 0.144 degrees spatial resolution). In the seasonal NDVI cycle, three phenological events were defined for each pixel: green-up week (NDVI exceeds 0.2), maximum week, and senescence week (NDVI drops below 0.2). Generally there was a west-early/east-late gradient in the three events in north Asia. In the zonal transect between 45 degrees and 50 degrees N, the timing of green-up, maximum, and senescence near 60 degrees E (Kazakh) was about 3.4, 8.7, and 13.4 weeks earlier than near 110 degrees E (Mongolia) respectively. It has been suggested that vegetation near Kazakh only flourishes during a short period when water from snow melt is available from late spring to early summer. In Mongolia, abundant water is available for the vegetation, even in midsummer, because of precipitation. In the 50-60 degrees N zonal transect, the green-up and maximum near 40 degrees E were about 3.8 and 3.9 weeks earlier than near 115 degrees E, respectively. As for the week of senescence, there was no clear west-east trend. This west-to-east phenological gradient was related to the weekly cumulative temperature (over 0 degrees C). Weeks in which the cumalative temperature exceeded 40 degrees C and 140 degrees C had a similar west-east distribution to green-up and maximum NDVI.