Mayuko Jomura

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.

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.

CO2 efflux from leaf litter focused on spatial and temporal heterogeneity of moisture

Leaf litter respiration (RLL) was directly measured in situ to evaluate relationships with the water content in leaf litter (WC), which is distributed heterogeneously under natural conditions. To do so, we developed a small, closed static chamber system using an infrared gas analyzer, which can measure instantaneous RLL. This study focuses on the measurement of CO2 effluxes from leaf litter using the chamber system in the field and examines the relationship between RLL and WC among seven broadleaf species in a temperate forest. The measurements focused on the position of leaves within the litter layer, finding that both RLL and WC were significantly higher in the lower layer. The value of RLL increased with increasing WC, and the response of RLL to WC was similar among all seven species. Moreover, the temporal variation in WC differed among three species and was associated with leaf litter thickness. The observed heterogeneity in WC induced by the physical environment (e.g., position and thickness of leaf litter) affects the variation in WC and, therefore, both RLL and the decomposition rates of organic matter in the litter layer.

Differences between coarse woody debris and leaf litter in the response of heterotrophic respiration to rainfall events

Heterotrophic respiration strongly influences carbon cycles at the ecosystem and global scales. We used an automated chamber system to measure the heterotrophic respiration of coarse woody debris (CWD) and leaf litter in a secondary broadleaved forest in southern Kyoto Prefecture. This system, which targeted only organic matter, could detect heterotrophic respiration responses to changes in environmental factors, especially rainfall events. The temporal trends and responses of respiration to environmental factors differed dramatically between CWD and leaf litter. CWD respiration showed a clear diurnal change corresponding to changes in CWD temperature and a clear decrease during rainfall events. Leaf litter respiration did not change clearly but increased at the beginning of rain events and returned to pre-rain rates when soil water content declined. The temporal patterns of the residuals between the observed respiration and baseline respiration, developed from the temperature–response curves under pre-rain conditions, differed between CWD and leaf litter respiration. The typical trend in CWD respiration response to rainfall events was a clear decrease and then gradual increase in the residuals after the event. The response of leaf litter respiration to wetting was an increase in the residuals during rainfall events and then a gradual decrease during drying. The difference in the responses of these respirations to wetting and drying processes are likely caused by differences in the physical characteristics of the CWD and the leaf litter layer. Measurements targeting only organic matter using an automated chamber system could detect the responses of heterotrophic respiration to environmental factors.

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.

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.

Biotic and Abiotic Factors Controlling Respiration Rates of Above- and Belowground Woody Debris of Fagus crenata and Quercus crispula in Japan

As a large, long-term pool and source of carbon and nutrients, woody litter is an important component of forest ecosystems. The objective of this study was to estimate the effect of the factors that regulate the rate of decomposition of coarse and fine woody debris (CFWD) of dominant tree species in a cool-temperate forest in Japan. Respiration rates of dead stems, branches, and coarse and fine roots of Fagus crenata and Quercus crispula felled 4 years prior obtained in situ ranged from 20.9 to 500.1 mg CO2 [kg dry wood]–1 h–1 in a one-time measurement in summer. Respiration rate had a significant negative relationship with diameter; in particular, that of a sample of Q. crispula with a diameter of >15 cm and substantial heartwood was low. It also had a significant positive relationship with moisture content. The explanatory variables diameter, [N], wood density, and moisture content were interrelated. The most parsimonious path model showed 14 significant correlations among 8 factors and respiration. Diameter and [C] had large negative direct effects on CFWD respiration rate, and moisture content and species had medium positive direct effects. [N] and temperature did not have direct or indirect effects, and position and wood density had indirect effects. The model revealed some interrelationships between controlling factors. We discussed the influence of the direct effects of explanatory variables and the influence especially of species and position. We speculate that the small R 2 value of the most parsimonious model was probably due to the omission of microbial biomass and activity. These direct and indirect effects and interrelationships between explanatory variables could be used to develop a process-based CFWD decomposition model.

Testing a method for reconstructing structural development of even-aged Abies sachalinensis stands

Accuracy of a stand reconstruction technique was examined by comparing the estimated values of the aboveground biomass, total stem volume, stem volume growth, and stand density of Abies sachalinensis stands to those observed between 1980 and 1998 in Hokkaido, northern Japan. Census data from two stands established in 1973, one fertilized and the other unfertilized, were used for the examination. The stand statistics in the past were estimated from the DBH and height of individual trees measured in 1998, data on the aboveground biomass and stem volume with bark for nine living trees of various sizes harvested in each plot in 1998 or in 1999, and data from the stem analysis of the same harvested trees. We showed that the reconstructed patterns of the frequency distribution in aboveground biomass and in stem volume were generally the same as those observed in both plots and in any year in the past (except for 1982 and/or 1980), and that the reproduced patterns of stand development over time were similar to those observed directly in the past. Accuracy in predicting stand statistics was generally in the order of ±10% relative error. We consider that the present method of stand reconstruction could be used to estimate aboveground biomass, total stem volume, and stem volume growth of a stand in the past. Interpretation of results for the early years (1982 and 1980) and for the stand density requires caution.