Nobuko Saigusa

Influence of spring and autumn phenological transitions on forest ecosystem productivity

We use eddy covariance measurements of net ecosystem productivity (NEP) from 21 FLUXNET sites (153 site-years of data) to investigate relationships between phenology and productivity (in terms of both NEP and gross ecosystem photosynthesis, GEP) in temperate and boreal forests. Results are used to evaluate the plausibility of four different conceptual models. Phenological indicators were derived from the eddy covariance time series, and from remote sensing and models. We examine spatial patterns (across sites) and temporal patterns (across years); an important conclusion is that it is likely that neither of these accurately represents how productivity will respond to future phenological shifts resulting from ongoing climate change. In spring and autumn, increased GEP resulting from an ‘extra’ day tends to be offset by concurrent, but smaller, increases in ecosystem respiration, and thus the effect on NEP is still positive. Spring productivity anomalies appear to have carry-over effects that translate to productivity anomalies in the following autumn, but it is not clear that these result directly from phenological anomalies. Finally, the productivity of evergreen needleleaf forests is less sensitive to phenology than is productivity of deciduous broadleaf forests. This has implications for how climate change may drive shifts in competition within mixed-species stands.

A parameterization of evaporation from bare soil surfaces

Abstract A simple model of evaporation from a bare soil surface is developed. This model combines two processes of water vapor transport: one is the vapor transport in air expressed by the bulk formula, and the other is molecular diffusion of vapor in the surface soil pore with the vapor being carried from the interior of the soil pore to the land surface. The resistance to the vapor diffusion in the soil pore is expressed using a new parameter, estimated by experimental means. General formulation of the so-called “surface moisture availability” is expressed with this model. The formulation shows that the “surface moisture availability” depends not only on volumetric soil moisture, but also on wind velocity, and on the ratio of the specific humidity of the air to that of the saturation value at the soil surface temperature. This dependence agrees with experiments performed with loam and sand under various conditions. In the evaporation parameterization used in current numerical simulations, the humidity o...

Gross primary production and net ecosystem exchange of a cool-temperate deciduous forest estimated by the eddy covariance method

Abstract An eddy covariance measurement system was installed to measure long-term turbulent fluxes of heat, water vapor, and CO2 over a cool-temperate deciduous forest in the central part of Japan. Previous to a long-term measurement, a comparison of flux measurements using open- and closed-path type infrared gas analyzers was conducted for CO2 and water vapor. The closure of the energy budget over the forest was also examined to test the validity of the flux measurements. Using a whole year data from 25 July 1998 to 24 July 1999, the relation between the night-time air temperature and the night-time CO2 flux over the forest (ecosystem respiration) was investigated. The results suggest that the night-time CO2 flux was significantly influenced by the vertical air temperature gradient over the canopy. The night-time CO2 flux increased exponentially with the temperature when the stability was nearly neutral, while it was insensitive to the temperature under stable conditions. Daily values of the net ecosystem exchange (NEE) and the gross primary production (GPP) of the forest were estimated, and they were parameterized as a function of the air temperature and the absorbed photosynthetic active radiation (APAR). The results show a clear seasonal change in GPP caused by changes in the light-use efficiency and the maximum rate of carbon fixation of the canopy with LAI. The estimated NEE based on the parameterization agrees well with the observed NEE during the observational period.

CO2 and water vapor exchange of a larch forest in northern Japan

In the northern part of East Asia, forests dominated by larch are extensively distributed and probably play an important role in the global carbon cycle. However, a knowledge of the CO2 balance of larch forests based on long-term flux measurements is very restricted in East Asia. Thus, a long-term flux measurement has been started in 2000 at a larch plantation on a flat terrain in Hokkaido, Japan to obtain more information on the CO2 and energy balances of larch forests. From September 2000 to August 2001 the net ecosystem CO2 exchange (NEE) changed seasonally in accordance with the annual cycles of phenology and climate. NEE was negative for six months of the growing season, May—September; the larch ecosystem was a carbon sink with a peak intensity of –0.38 mol m−2 d−1 for this period. In the leafless season from November to April the forest ecosystem was a carbon source with an intensity ranging between 0 and 0.05 mol m−2 d−1. Annual NEE from September 2000 to August 2001 was −24.4 to −32.4 mol m−2 yr−1 (=−293 to −389 gC m−2 yr−1); this value is compatible with those reported from other temperate forests. Annual evapotranspiration for the same period was 367 mm, which was only 29% of annual precipitation.

Seasonal variations of the exchange of CO2 and H2O between a grassland and the atmosphere: An experimental study

Abstract Seasonal variations in the CO 2 and H 2 O fluxes were investigated experimentally over a grassland in central Japan during the growing periods of 1993 and 1994. The CO 2 flux over the grassland was measured by an aerodynamic method. The surface heat budget was routinely measured and the data were used to analyze the CO 2 and H 2 O exchange between the grassland and the atmosphere. Conditions were relatively cool and wet due to the prolonged rainy season from July to August 1993, being hot during the 1994 summer. The annual maximum value of sensible heat flux density was observed prior to the growing period on March 1993 and on April 1994, accounting for 40–50% of the net radiation. The annual maximum of latent heat flux density was observed in August, accounting for about 85% of the net radiation in both years. The latent heat flux observed in August 1994 was quite high, being 135 W m −2 (monthly mean, daily basis), while that in 1993 was 96 W m −2 . The grassland absorbed CO 2 during the period from the beginning of May to October in 1994, and maximum of daily net ecosystem CO 2 exchange during the growing period amounted to values more than 40 g CO 2 m −2 day −1 , in both 1993 and 1994. The dependence of daily net ecosystem CO 2 exchange on leaf area index (LAI) and incoming photosynthetic photon flux density Q p was investigated. When the Q p was higher than 45 mol m −2 day −1 , the daily CO 2 exchange strongly increased with an increase in LAI. The daily canopy surface conductance was calculated based on the Penman-Monteith equation, and it showed a linear increase with LAI when the LAI had small values (typically less than one). With an increase in LAI from one to four, the maximum level of surface conductance became dominated by water vapor pressure deficits.

A model and experimental study of evaporation from bare-soil surfaces

Abstract A model is constructed for estimating evaporation from bare-soil surfaces. In the model, the evaporation is parameterized with the soil-water content for the upper 2 cm of the soil (Kondo et al.), and the heat and water transport within the soil layer below 2 cm is explicitly described by the heat conduction and moisture diffusion equations. Experiments on evaporation from loam packed in pans are also carried out. The present model well simulates the observed evaporation and vertical profiles of soil temperature and water content. Long time simulations of evaporation by the present model an compared with the force-restore method and the bucket model for a drying period of over several months. The decrease in evaporation rate for the bucket model is comparatively small. However, the evaporation by the present model and the force-restore method decreases rapidly several days after the beginning of the drying period. Differences between the evaporation by the present model and that by the force-rest...

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.

Temporal variations of atmospheric CO2 concentration in a temperate deciduous forest in central Japan

In order to examine the temporal variation of the atmospheric CO2 concentration in a temperate deciduous forest, and its relationship with meteorological conditions, continuous measurements of CO2 and meteorological parameters have been made since 1993 on a tower at Takayama in the central part of Japan. In addition to an average secular increase in atmospheric CO2 of 1.8 ppm yr−1, diurnal variation with a maximum during the night-time to early morning and a minimum in the afternoon is observed from late spring to early fall; the diurnal cycle is not so clearly observed in the remaining seasons of the year. A concentration difference between above and below the canopy, and its diurnal variation, can also be seen clearly in summer. Daily mean concentration data show a prominent seasonal cycle. The maximum and the minimum of the seasonal cycle occur in April and from mid August to mid September, respectively. Day-to-day changes in the diurnal cycle of CO2 are highly dependent on the day-to-day variations in meteorological conditions. However, CO2 variations on longer time scales (>10 d) appear to be linearly related to changes in respiration. At Takayama, variations in the 10-d standard deviation of daily mean CO2 data and 10-d averaged respiration show distinct relationships with soil temperature during spring and fall seasons. In spring, respiration has a stronger exponential dependence on soil temperature than in fall. Interestingly, in summer when soil temperature becomes greater than about 15 °C, biological respiration becomes more variable and independent of the soil temperature. Thus, at the Takayama site, the Q10 relationship is seasonally dependent, and does not represent well the biological respiration process when the soil temperature rises above 15 °C.

What makes the satellite-based EVI–GPP relationship unclear in a deciduous broad-leaved forest?

Recent studies have suggested that gross primary production (GPP) of terrestrial vegetation can be estimated directly with the satellite-based Enhanced Vegetation Index (EVI). However, the reported EVI–GPP relationships showed wide variability, with the regression functions showing widely scattered data. In the present study, we examined the possible reasons for this variability in the EVI–GPP relationship using daily EVI values from satellite and field measurements and daily flux-based GPP in a cool-temperate deciduous broad-leaved forest in Japan. The variability appears to be caused by noise due to cloud contamination in the satellite data as well as the different seasonality of EVI and GPP, especially during the leaf-expansion period. Our findings indicate that improvement of cloud screening and consideration of the leaf-expansion period are critical when applying the EVI–GPP relationship.

Comparison of gross primary productivity derived from GIMMS NDVI3g, GIMMS, and MODIS in Southeast Asia

Gross primary production (GPP) plays an important role in the net ecosystem exchange of CO2 between the atmosphere and terrestrial ecosystems. It is particularly important to monitor GPP in Southeast Asia because of increasing rates of tropical forest