A better understanding on the response of forest against current environmental factors is prerequisite for an accurate estimation of the potential global carbon cycle in association with future climate changes    . Therefore, much attention has been given to the subject of quantifying and identifying the carbon budget of forest ecosystems in the regional and global scales, whether the forest ecosystem is a sink or a source for atmospheric CO2   .
The net carbon acquisition of forest ecosystem, as well as the net primary pro- ductivity (NPP) depends principally on the photosynthetic capacity and respiration of the ecosystem in the growing season of a given forest   . The rate of instantaneous photosynthetic assimilation is one of the most important factors in determining the rate of CO2 exchange between the atmosphere and forest eco- systems   .
Many studies have been extensively focused on photosynthetic characteristics at the leaf level, including comparisons of different functional types, biochemistry and eco-physiology of plant species, and leaf morphologies   . However, the structural distribution of plant’s organs has rarely been studied even though it affects significantly the canopy photosynthesis rate at the stand level through the changes of the transmittance and interception patterns of the incident light within the canopy   .
The forest canopy photosynthesis depends upon its own eco-physiological and architectural properties  , and meteorological factors such as light incidence, air temperature and humidity, atmospheric CO2 concentration and wind speed  . Among all these parameters, the light condition that is measured by the photosynthetic photon flux density (PPFD) is a main driving parameter in determining photosynthetic characteristics, and other biological and environmental conditions within the canopy    . The light response of photosynthesis of an individual leaf and the attenuation of PPFD in the canopy largely depend on the distribution of leaf area within the canopy  so that the detailed distribution of PPFD at different levels of the forest canopy is a prerequisite for the accurate estimation of the photosynthesis rate   .
Eco-physiological measurements in the processes of the forest ecosystem can provide crucial parameters for the canopy photosynthesis modeling   . For example, the leaf area index (LAI), defined as the projected area of foliage per unit ground surface area (m2∙m−2), is a very useful quantitative measure for the quantity of forest foliages   and regulates the spatial and temporal distribution patterns of PPFD  . Thus, accurate measurements of seasonal variations of LAI and the leaf-level photosynthetic capacity that is affected by environment conditions (light, temperature, humidity and precipitation etc.) are required for estimating eco-physiological ecosystem processes (photosynthesis and respiration)   .
Cool-temperate deciduous forests are widely distributed in East Asia including southeastern Siberia, northern China, central and northern Japan and the Korean Peninsula   . These forests are regarded as significant sinks for atmospheric CO2  . In Korea, the forested region is located within the complex terrain of mountainous regions, which occupies about 65 % (6.3 × 106 hectare) of the whole territorial area. Deciduous and mixed forests of broad-leaved species are predominant with the area of 1.7 × 106 hectare and 1.9 × 106 hectare, respectively. They account for about 56 % of all Korean forests. In particular, Mongolian oak (Quercus mongolica) forest stand is one of the typical deciduous broad-leaved forest types in the Korean Peninsula  .
However, photosynthetic assimilation mechanisms, as well as biological pro- cesses and modeling at the forest stand level of Q. mongolica species in Korea have rarely been studied due to the lack of observed eco-physiological and phenological parameters for this forest, even though the CO2 sequestration behaviors in Q. mongolica forest stands may have a great effect on the balance of carbon budgets  .
The purpose of this study is to estimate the canopy net photosynthesis rate of Q. mongolica forest stand for the growing period at the Nam-San ecological- experimental site with optimally derived photosynthetic light response curves and LAIs at several layers within the canopy with the use of the measured photosynthesis rates and the modified PPFDs by total plant area index (TPAI).
2. Materials and Methods
2.1. Field Experimental Site
Field experiments were conducted in a secondary Mongolian oak (Quercus mongolica) forest at the Nam-San Ecological Experimental site located in central Seoul city surrounded by urban complexes having various CO2 sources in the west-central part of the Korean Peninsula (Figure 1). The experimental site is located at 126˚59'E and 37˚33'N with a northeastern aspect, a gentle slope and an elevation of 220 m in a cool-temperate zone under the influence of Asian monsoon climate with a mild in springs and autumns, hot and humid in summers, and cold and snowy in winters. The climate data from Seoul Weather Station (belonging to Korea Meteorological Administration) close to the experimental site indicate that the annual mean air temperature and annual total precipitation are 11.8˚C (minimum of −3.4˚C in January and maximum of 25.4˚C in August) and 1369.8 mm , respectively. A tower-based continuous measurement of CO2 flux between the forest and atmosphere, soil CO2 effluxes with automatic opening/closing chamber systems and various meteorological elements in the forest ecosystem have been made continuously since 2008  .
The vegetation at the site is classified as an approximately 49 - 55 years old deciduous broad-leaved forest, composed mainly of Q. mongolica. The predominant tree stand of Q. mongolica has an average diameter at breast height (DBH) of 23.2 cm and a total basal area of 22.1 m2∙ha−1. The continuous canopy height and density in Q. mongolica trees are about 15.1 m and 482 ha−1, respectively. The mid- and understory vegetation under the dominant tree canopy of this site is rarely composed of few trees with Sorbus alnifolia, Styrax japonica, Acer pseudo-sieboldianum species. All leaves of these deciduous species at the site usually begin to flush in late April, and to fall in November. The starting times of leaf unfolding and shedding of Q. mongolica trees were, respectively 27 April and 30 October in 2010. More detailed descriptions of the Nam-San eco-
Figure 1. The geographical location of (a) Seoul, Korea. The topography of (b) Mt. Nam in Seoul with the indication of the ecological-experimental site ( ) in the Q. mongolica forest stand during the whole growing season (May-October) in 2010.
maximum value is about 0.87 kg CO2 m−2 month−1 (1.02 kg CO2 m−2 month−1) in June. However, in spite of the mid growing period of August, the canopy net photosynthesis rate is as low as 0.43 kg CO2 m−2 month−1 due to the cloudy weather in August and the monthly total daylight hour was about 60% of that in June (Figure 10). During the whole growing period (May to October) of 2010 at the site, the estimated total canopy net photosynthesis rate with (PPFD)t (PPFD) is found to be about 3.3 kg CO2 m−2 (3.9 kg CO2 m−2) that is within the range of 3.3 - 4.3 kg CO2 m−2 yr−1 for various cool-temperate deciduous forests estimated by Falge et al.  and Muraoka et al.  .
In the whole growing season, the estimated total canopy net photosynthesis rate with (PPFD)t is approximately 14% (0.6 kg CO2 m−2) lower than that estimated with PPFD, suggesting the total canopy net photosynthesis rate being significantly affected by the seasonal variations of the effective TPAI and the relative (PPFD)t within different canopy layers. This also suggests that the present method has a great potential for more accurate estimation of the photosynthesis rate in the forest stand.
Field measurements have been conducted throughout the growing season of the Q. mongolica forest at the Nam-San ecological experimental site in Seoul, Korea from May to October in 2010. An empirical model for the estimation of canopy net photosynthesis rate of Q. mongolica forest stand has been developed using the field experimental data by constructing the photosynthetic light response curve, vertical distributions of the leaf area index (LAI) and the photosynthetic photon flux density (PPFD) measured at around noon within the canopy of five levels (0.5, 9, 12, 14 and 15 m heights).
It is found that LAIs within the canopy layers of the Q. mongolica forest stand show distinct seasonal variations with a maximum of 4.6 m2∙m−2 at the lowest canopy level (0.5 m above the ground) in July. The increasing rate of LAI is rather large during the leaf expansion period (late April to May) at all levels within the canopy compared to the decreasing rate during the leaf-litter fall period (late October to November). It is also found that the observed PPFD within the canopy can be properly regressed by the Beer-Lambert’s law with the observed total plant area index (TPAI) within the canopy. The estimated light extinction coefficients (β) are found to be a maximum value of 0.92 - 0.97 in the early growing season (May) and the late growing season (October) with a near constant value of 0.57 - 0.59 during the peak growing season (June to August), suggesting a significant amount of PPFD being extinguished by the branch and stem (wood silhouette area index) during the early and late growing seasons.
The photosynthetic light response curve for the leaves of Q. mongolica is found to show a great increase of the net photosynthesis rate with the increase of PPFD in the peak growing seasons of June and July in 2010. However, in the early growing period of May and the late growing period of September the PPFD exceeding 1000 μmol∙m−2∙s−1 is founded to cause the decrease of the net photosynthesis rate due to the thermal stress probably caused by higher temperature than that at LSP which varies with the growth of the leaf.
The total canopy net photosynthesis rate for the growing period of Q. mongolica forest (May to October) at this site is found to be about 3.3 kg CO2 m−2 that is about 1.3 times larger than the total soil respiration of 2.5 kg CO2 m−2 measured at the same site for the same period  . The presently estimated canopy net photosynthesis rate is comparable to those estimated by Falge et al.  and Muraoka et al.  , suggesting the potential usefulness of the present model for the estimation of the canopy net photosynthesis rate of Q. mongolica forest stand.
This study is mainly pertained to the estimation of the canopy net photosynthesis rate in the Q. mongolica forest stand with empirically derived the photosynthetic light response curve using limited measurement data. Some important parameters affecting the net photosynthesis rate are not evaluated independently. This is now on hand at the Nam-San ecological-experimental site based on direct measurements of photosynthesis rates, respirations and environmental conditions continuously with the automatic chamber systems.
This work was funded by the Korea Meteorological Administration Research and Development Program under Grant KMIPA 2015-6040.
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