Takafumi Miyama

The carbon budget of coarse woody debris in a temperate broad-leaved secondary forest in Japan

We evaluated the carbon budget of coarse woody debris (CWD) in a temperate broad-leaved secondary forest. On the basis of a field survey conducted in 2003, the mass of CWD was estimated at 9.30 tC ha-1, with snags amounting to 60% of the total mass. Mean annual CWD input mass was estimated to be 0.61 tC ha-1 yr-1 by monitoring tree mortality in the forest from 1999 to 2004. We evaluated the CWD decomposition rate as the CO2 evolution rate from CWD by measuring CO2 emissions from 91 CWD samples (RCWD) with a closed dynamic chamber and infrared gas analysis system. The relationships between RCWD and temperature in the chamber, water content of the CWD, and other CWD characteristics were determined. By scaling the measured RCWD to the ecosystem, we estimated that the annual RCWD in the forest in 2003 was 0.50 tC ha-1 yr-1 or 10%–16% of the total heterotrophic respiration. Therefore, 0.11 tC ha-1 yr-1 or 7% of the forest net ecosystem production was sequestered by CWD. In a young forest, in which CWD input and decomposition are not balanced, the CWD carbon budget needs to be quantified for accurate evaluation of the forest carbon cycle and NEP.

Development of an automatic chamber system for long-term measurements of CO2 flux from roots

To separate CO2 efflux from roots (Rr) and soil (Rs), we developed a system to measure Rr continuously. Using this system, seasonal variation in Rr was obtained in a temperate forest in Japan. We measured Rs, CO2 efflux from mineral soil (Rm) and environmental factors simultaneously, and the characteristic and seasonality of Rr were analysed in comparison with Rs. Rr and Rs showed different responses to soil water content: Rs decreased with decreasing soil water content, whereas Rr peaked at relatively low soil water content. Rr/Rs decreased from 64.8% to 27.3% as soil water content increased from 0.075 to 0.225 cm cm-3. The relationship between respiration and temperature appears to change seasonally in response to phenological and biological factors. Rr showed clear seasonal variation as a function of soil temperature. During the growing period, Rr exhibited a higher rate at the same soil temperature than during other periods, which may be due to phenological influences such as fine root dynamics. Rs decreased during the summer despite high soil temperatures. The seasonal peak for Rs occurred earlier than that for soil temperature. Rr/Rs ranged between 25% and 60% over the course of the year.

Characteristics of CO2 flux over a forest on complex topography

The CO2 flux over a mixed forest of evergreen and deciduous broad-leaved trees on complex topography was measured by using an eddy covariance method. To evaluate the CO2 flux over such a forest and to ascertain the effect of topography, the eddy covariance measurement was conducted at the top of each of two meteorological towers erected in a basin (1.6 ha). The CO2 flux measured by the eddy covariance method was also evaluated by comparing it with the CO2 flux from leaf photosynthesis and soil respiration as continuously measured by automatic chamber methods on the forest floor and in the foliage. The daily variations in the daytime CO2flux values measured at the two towers were each quite similar. However, for the night-time CO2 flux values, there was about 36% difference between the towers, even though the flux was measured in the same basin. The CO2 flux as measured by eddy covariance and including CO2 storage change (Fn) was 60% lower than that estimated by the chamber methods (Fchm). The CO2 flux as measured by using the relation between Fn and soil temperature at 2 cm depth obtained only under conditions of low stability (U*≥ 0.25 m s−1) was 32% lower than Fchm.

Development of the IRGA enclosed-chamber system for soil CO2 efflux measurement and its application to a spatial variation measurement

A new system was developed for measuring soil CO2 efflux. The chamber in this system contains a small infrared CO2 gas analyzer. This system does not need air tubes or pumps for circulating air, so it is expected to offer the advantages of mobility and durability. This system was verified by a comparison with measurements made by using a closed-dynamic-chamber (CDC) system. The spatial variation in the soil CO2 efflux in a broadleaved deciduous forest was measured using the new system. The soil CO2 efflux at sampling locations 50–70 cm apart varied within a range of 60%–150%. This variation was smaller than the variation due to differences in soil characteristic reflected in different moisture conditions, etc.

Automated foliage chamber method for long-term measurement of CO2 flux in the uppermost canopy

Direct monitoring of CO2 exchange in forest ecosystems is necessary for evaluation of the annual net CO2 flux. We developed an automated foliage chamber system for long-term in situ measurement of the CO2 flux in foliage. The chamber closes automatically for 5 min in every 30. We found a high correlation between the photosynthesis rates measured by this system and by a portable H2O/CO2 analyzer (LCA-3, ADC). We used the new system to continuously monitor the CO2 flux in foliage of Quercus serrata. There was a positive correlation between CO2 flux and short-wave radiation. The daily photosynthesis rate varied widely depending on the environmental conditions. It increased from May to July and decreased temporarily in August. The seasonal pattern of photosynthesis was almost the same as that measured by the eddy covariance method. These results suggest that the new system is effective for tracking long-term seasonal change of the CO2 flux in foliage.

Components and seasonal variation of night‐time total ecosystem respiration in a Japanese broad‐leaved secondary forest

The Yamashiro Experimental Forest is a broad-leaved secondary forest in Kyoto, Japan. On its complex terrain, low wind speed, high air stability, and local advection are common at night. To reduce the uncertainty in measuring woodytissue respiration at night, we used automated stem chambers to measure stem respiration continuously (for 5 min at 30 min intervals) on stems of Quercus serrata Murr. (deciduous) and Ilex pedunculosa Miq. (evergreen) throughout 2003. Using these data, we estimated night-time respiration for the total ecosystem and its various components, and we report foliar and soil respiration rates for 2003. Annual average night-time respiration of soil, evergreen leaf, deciduous leaf, evergreen woody tissue and deciduous woody tissue were estimated as 0.0794 (63.2%), 0.0101 (8.0%), 0.0160 (12.7%), 0.0064 (5.1%) and 0.0137 (10.9%) mg CO2 m-2 s-1, respectively. The contribution of soil respiration to the total ecosystem respiration rate reached its minimum (49.1%) on 12 June (DOY 163) and its maximum (82.4%) on 29 November (DOY 333). Seasonal change of growth respiration was marked, indicating that the seasonal variation of growth respiration must be evaluated carefully to estimate total ecosystem respiration. Therefore, long-term continuous measurement using automated chambers and averaging provides an effective means of evaluating the annual night-time ecosystem respiration.

Seasonal ozone uptake by a warm-temperate mixed deciduous and evergreen broadleaf forest in western Japan estimated by the Penman-Monteith approach combined with a photosynthesis-dependent stomatal model.

Canopy-level stomatal conductance over a warm-temperate mixed deciduous and evergreen broadleaf forest in Japan was estimated by the Penman-Monteith approach, as compensated by a semi-empirical photosynthesis-dependent stomatal model, where photosynthesis, relative humidity, and CO2 concentration were assumed to regulate stomatal conductance. This approach, using eddy covariance data and routine meteorological observations at a flux tower site, permits the continuous estimation of canopy-level O3 uptake, even when the Penman-Monteith approach is unavailable (i.e. in case of direct evaporation from soil or wet leaves). Distortion was observed between the AOT40 exposure index and O3 uptake through stomata, as AOT40 peaked in April, but with O3 uptake occurring in July. Thus, leaf pre-maturation in the predominant deciduous broadleaf tree species (Quercus serrata) might suppress O3 uptake in springtime, even when the highest O3 concentrations were observed.

Using Capacitance Sensors for the Continuous Measurement of the Water Content in the Litter Layer of Forest Soil

Little is known about the wetting and drying processes of the litter layer ( layer), likely because of technical difficulties inherent in nondestructive water content (WC) monitoring. We developed a method for continuously measuring the WC of leaf litter (the “LWC method”) in situ using capacitance sensors. To test variants of this approach, five (for the LWC_5) or ten (for the LWC_10 method) Quercus serrata leaves were attached around capacitance sensors. The output voltage used for each LWC method was linearly correlated with the gravimetric WC (LWC_5: ; LWC_10: ), producing different slopes for each calibration line. For in situ continuous measurements of WC in the layer, two sensors were used, one placed on top of the layer and the other at the boundary between the and mineral layers. The average continuous WC of the layer was then calculated from the output voltage of the two sensors and the calibration function, and this value was linearly correlated with the gravimetric WC . However, because the layer characteristics (e.g., thickness, water-holding capacity, and species composition) may differ among study sites, appropriate approaches for measuring this layer’s moisture properties may be needed.

Increased phytotoxic O3 dose accelerates autumn senescence in an O3-sensitive beech forest even under the present-level O3.

Ground-level ozone (O3) concentrations are expected to increase over the 21st century, especially in East Asia. However, the impact of O3 has not been directly assessed at the forest level in this region. We performed O3 flux-based risk assessments of carbon sequestration capacity in an old cool temperate deciduous forest, consisting of O3-sensitive Japanese beech (Fagus crenata), and in a warm temperate deciduous and evergreen forest dominated by O3-tolerant Konara oak (Quercus serrata) based on long-term CO2 flux observations. On the basis of a practical approach for a continuous estimation of canopy-level stomatal conductance (Gs), higher phytotoxic ozone dose above a threshold of 0 uptake (POD0) with higher Gs was observed in the beech forest than that in the oak forest. Light-saturated gross primary production, as a measure of carbon sequestration capacity of forest ecosystem, declined earlier in the late growth season with increasing POD0, suggesting an earlier autumn senescence, especially in the O3-sensitive beech forest, but not in the O3-tolerant oak forest.

In Situ CO2 Efflux from Leaf Litter Layer Showed Large Temporal Variation Induced by Rapid Wetting and Drying Cycle

We performed continuous and manual in situ measurements of CO2 efflux from the leaf litter layer (R LL) and water content of the leaf litter layer (LWC) in conjunction with measurements of soil respiration (R S) and soil water content (SWC) in a temperate forest; our objectives were to evaluate the response of R LL to rainfall events and to assess temporal variation in its contribution to R S. We measured R LL in a treatment area from which all potential sources of CO2 except for the leaf litter layer were removed. Capacitance sensors were used to measure LWC. R LL increased immediately after wetting of the leaf litter layer; peak R LL values were observed during or one day after rainfall events and were up to 8.6-fold larger than R LL prior to rainfall. R LL declined to pre-wetting levels within 2–4 day after rainfall events and corresponded to decreasing LWC, indicating that annual R LL is strongly influenced by precipitation. Temporal variation in the observed contribution of R LL to R S varied from nearly zero to 51%. Continuous in situ measurements of LWC and CO2 efflux from leaf litter only, combined with measurements of R S, can provide robust data to clarify the response of R LL to rainfall events and its contribution to total R S.