Gas and energy fluxes above a tropical forest in dong nai

 Nghiên cứu khoa học công nghệ 
Tạp chí Khoa học và Công nghệ nhiệt đới, Số 14, 11 - 2017 36
GAS AND ENERGY FLUXES ABOVE A TROPICAL FOREST 
IN DONG NAI 
KURBATOVA JU.A. (1, 2), KURICHEVA O.A. (1, 2), AVILOV V.C. (1, 2), 
BA DUY DINH (2), KUZNETSOV A.N. (2) 
1. INTRODUCTION 
Tropical forests play a gibbous role among all types of natural ecosystems in 
energy and material exchange between earth surface and the atmosphere, receiving 
and converting great amounts of radiative energy and precipitation, creating about 
one half of land surface evaporation and one third of land ecosystems primary 
productivity [18, 29]. The fluxes of energy, water vapour and carbon dioxide in 
primary seasonal tropical forests are less studied in comparison with rainforests, but 
may have significant differences due to prominent seasonal course of moisture 
regime [4, 6, 13, 35]. 
Southeast Asia (SEA) tropical forests make up 16.4% of all tropical seasonal 
forests [31]. Primary SEA tropical forests, covering less than 10% of all SEA forests 
[17], are among the most luxury earth ecosystems in terms of biodiversity, biomass 
and complexity of a stand structure [47, 32, 5]. At the same time, there is lack of 
investigations of the functioning of seasonally dry deciduous and semi-deciduous SEA 
tropical forests, while significant seasonal changes in the moisture and energy supply 
make it possible to reveal the peculiarities and interrelationships between fluxes of 
energy and mass in a given ecosystem under contrasting conditions. 
By now, there are few studies of energy, water and CO2 exchange on an 
ecosystem level in seasonal forests of SEA: in Thailand [25, 36, 45], Cambodia [33], 
Southwest China [14, 26, 43], and some publications about rainforests of Malaysia 
and Indonesia [8, 22, 23, 24, 41, 42]. Tanaka summarized the estimations of 
evapotranspiration (ET) in monsoon forests of Thailand and Kambodia [44]. He 
noted that in two evergreen forests ET increased in the dry season with higher 
radiation and VPD deficit, but, in contrast, in one evergreen forest and in two 
deciduous forests ET declined with increasing evaporative demand. Tanaka accented 
the need for studying the response of different kinds of SEA seasonal forests to 
inter- and intra-annual precipitation anomalies [44]. Saigusa reported tropical 
seasonal forests to be a net CO2 sink during the dry period and a net CO2 source 
during the wet period, whereas the flux of CO2 in rainforest was small throughout a 
year [39]. Long dry season significantly reduced GPP in a tropical seasonal forest. 
The objective of this study was to estimate the sums and dynamics of energy, 
water and CO2 fluxes in a seasonal tropical forest of Southern Vietnam in different 
meteorological conditions during more than 5-year field campaign via eddy 
covariance (EC) technique. We start the analysis of the data from the identification 
of the place of the given forest among other seasonal and perhumid tropical forests 
in terms of annual totals of net radiation, precipitation, evapotranspiration and net 
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Tạp chí Khoa học và Công nghệ nhiệt đới, Số 14, 11 - 2017 37
ecosystem exchange of CO2 (NEE). Second, we describe the way of utilization of 
the incoming solar radiation by the forest ecosystem for generation of turbulent 
latent (LE) and sensible (H) heat fluxes in different meteorological conditions. 
Third, we draw up the characteristics of the seasonal course of gross primary 
production (GPP), ecosystem respiration (Reco) and resultant NEE. 
2. SITE AND METHOD 
The meteorological and EC measurements were conducted in the newly 
formed Đồng Nai biosphere reserve (former Nam Cát Tiên National Park) in the 
Southern Vietnam. The site code in Asiaflux network list is NCT 
( The nature reserve on the territory was 
established in 1997 [9]. The measurement site (11°27'N, 107°24'E, 150 m a.s.l.) is 
located in the respectively homogeneous massive of lowland mixed tropical forest. 
The territory has numerous streams and lakes, most of which dry up in dry season, 
but in wet season wide areas in the forest are flooded. 
The climate is Am [30], with dry period from November to April. Mean 
annual rainfall and temperature were 2518 mm and 26.4oC, respectively, for the 
period 1980÷2010 at nearby Ðong Xoài weather station [12]. In some years, usually 
ensuant El Niño events, precipitation is less than during the 4 month-period. 
According to [52], dominating at drained places are Lagerstroemia calyculata 
(Lythraceae), Haldina cordifolia (Rubiaceae), Tetrameles nudiflora (Datiscaceae), 
Afzelia xylocarpa (Caesalpiniaceae), Sterculia cf. cochinchinensis (Sterculiaceae). 
The forest may be characterized as primary, but probably disturbed in the second 
part of XX century by some human activity (selective cutting etc.). The forest has 
the complex structure with 4÷5 canopy sub-layers and rich biodiversity with about 
80 tree species. The mean canopy height is 36÷37 m with some emergents reaching 
46 m. The forest is semi-evergreen: about half of individual trees of upper sub-layers 
shed their foliage in a dry season. LAI is 5.1±0.9 m2 m-2 (n = 52) with ca. 0.3 m2 m-2 
reduce of the values in a dry season. Soils in the site area are Dystric Skeletic 
Rhodic Cambisol (Clayic) according to WRB with 2÷3% of organic carbon и 
0.45÷0.22% of nitrogen in the upper layer, and have a sufficient amount of 
phosphorus and potassium [21]. 
Eddy covariance [1, 2, 3, 7, 15] measurements were conducted at a 50-m 
height tower. NEE, LE and H were measured at 10-Hz frequency using open-path 
infrared gas analyser (LI-7500A, LI-COR Biosciences Inc., USA) and 3-dimentional 
sonic anemometer-thermometer (CSAT3, Campbell Scientific Inc., USA). 4-
component radiation, precipitation, temperature and relative humidity were 
registered at 50 m height. Soil heat flux was measured in 4 replications. CO2 and 
heat storage in the air inside the forest canopy were calculated using 8-level profile 
measurements of CO2 concentrations and air temperature, respectively. Soil heat 
storage was calculated using soil temperature measurements in 3 replications. 
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Tạp chí Khoa học và Công nghệ nhiệt đới, Số 14, 11 - 2017 38
The database under our analysis covered the period from November 
2011 to December 2016. The turbulent CO2 and heat fluxes for each 30-min period 
were calculated by means of EddyPro software (LI-COR Biosciences Inc., USA) 
with all standard EC corrections (frequency response, WPL etc.). Gaps in H, LE and 
NEE in 2012÷2016 made up 34%, 41% and 61%, respectively. For NEE, in night-
time/day-time 79% / 31% of values were missed, respectively. Gaps in fluxes were 
filled using updated look-up tables method [16, 37] in Online Eddy Covariance gap-
filling and flux-partitioning tool (Max-Plank Institute, Germany)1. Different gap-
filling methods of Flux Analysis Tool, Japan [46] were used for fluxes uncertainty 
evaluation. The estimation of flux totals uncertainty depending on different gap-
filling procedures for 2012 revealed the non-stability of totals and their strong 
dependance on night-time gaps and spikes, especially in the wettest months. As a 
most suitable method for gap-filling of these periods was recognized the method 
from [22]. The data on each step of processing (raw data, 30-minute fluxes, 
storages) were subjected to despiking and checking-up for physical plausibility. The 
ABD software by A. Deshcherevskij was used for the analysis [50, 51]. 
The energy balance unclosure, which is typical for EC measurements [19, 48], 
made up 17÷27%. Authors assumed that the turbulent fluxes had been 
underestimated and added the additional energy to daily, monthly and annual totals 
of H and LE using the Bowen ratio H/LE [15]. 
The prevailing winds during wet season had a predominantly south-west 
direction, in transitional periods a direction differed, and in dry season the winds 
were northern. 90% of the measured fluxes were collected from the circle with a 
radius of 600 m (380 m in day-time and 1360 m in night-time). 
For 2011÷2016, there were few 2-week gaps in the records of some parameters 
due to instrument malfunction, mistakes in data transfer or energy failure. 
Meteorological data gaps were filled using the data from other sensors or moving 
average diurnal variation of the adjacent data round the gap. Net radiation (Rn) in the 
second half of 2013 and in 2014 was recovered using the data on photosynthetically 
active radiation, average diurnal variation of albedo and downward long-wave 
radiation, air temperature in the canopy. Monthly and annual totals of precipitation in 
2016 were only roughly estimated due to 4-week gap in the wet season. 
3. RESULTS AND DISCUSSION 
3.1. Weather conditions during the experiment 
Seasonality in Southern Vietnam is determined by the volatile moisture regime 
related to the phase of large-scale El Niño-Southern oscillation in the Pacific ocean. 
Wet season precipitation and temperature regime was relatively homogeneous, 
providing mean monthly air temperature at 50 m about 25÷27 ˚C and enough water 
1 https://www.bgc-jena.mpg.de/bgi/index.php/Services/REddyProcWeb 
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Tạp chí Khoa học và Công nghệ nhiệt đới, Số 14, 11 - 2017 39
for vegetating. In contrast, dry seasons dramatically varied in duration and 
droughtiness. Dry periods of 2012÷2013 and 2013÷2014 felt within neutral phase of 
El Niño, but were El Niño-liked, i.e. they lasted 2.5 weeks longer than average and 
had less rain and cloudiness and more contrast temperature conditions (fig. 1). 
Fig. 1. The moisture regime in Đồng Nai: precipitation (Pr), soil water content at 
5cm (SWC5cm), water vapour pressure deficit at 50 m (VPD50m) 
Dry period of 2015÷2016 felt with strong El Niño. Its duration was normal, 
but the wet season started only in mid-May, therefore April and May were as dry 
and hot as the few hottest months in the 30-year record of Dong Xoai station [12]. In 
April 30-min values of air temperature at 2 m (30.6ºС) and soil at a depth of 5 cm 
(28.8ºС) were the highest for the entire observation period, and absolute temperature 
maximum (30-min average was 40.8oC) for the whole time of observations was 
registered at an altitude of 30 cm. In March and April the soil water content at a 
depth of 5 cm was as low as 9÷11% vol., close to wilting humidity for loamy soil. 
By contrast, the dry season of 2016÷2017 was exceptionally wet against the 
backdrop of La Niña: the total precipitation for December-March 2017 was 269.2 
mm (for comparison, in 2015÷2016, no rain was observed from the end of 
December to the middle of April). Dry period of 2011÷-2012 also coincided with 
La-Niña event and was wetter and milder than long-term average (rainfall from 15 
of November to 15 of March was 143.4 mm). 
3.2. The formation of radiation balance 
In contrast with pulse seasonal course of precipitation, radiation dynamic is 
relatively smooth with slightly higher level in the first half of a year (fig. 2). 
Ecosystem short-wave albedo had little seasonal changes from ca. 12.0% in the peak 
of dry season to ca. 10.2% in wettest months. These small changes show that the 
forest remain almost evergreen in terms of radiation reflectance even in the peak of 
dry season after a 3-month drought. The rate of net radiation to incoming radiation had 
the strong seasonal course, making up ca. 0.6 in February and 0.8 during wet months. 
0
100
200
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400
500
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700
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0
5
10
15
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25
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-2
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6
Pr, 
mm/mon 
SWC, 
% vol. 
VPD, 
hPa 
Pr
SWC5cm
VPD50m
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Tạp chí Khoa học và Công nghệ nhiệt đới, Số 14, 11 - 2017 40
Fig. 2. The downward short-wave radiation (Rs) in Đồng Nai in different years 
The net radiation (Rn) totals (table 1) over the forest were found to be among 
the highest across 21 eddy covariance stations in tropical forests world-wide, 
including Brazil rainforests [10, 11, 18, 20, 22, 28, 38, 40]. Only some sites in 
monsoon climate in Costa-Rica [27] and wet climate in Malaysia [22] and Indonesia 
[18, 20] in some years receive the similar or greater amount of radiation. Since the 
main factor that determines the amount of solar radiation in tropics is cloudiness, 
authors link high radiation amount in NCT with noon-time minimum of rainfall and 
cloudiness in Đồng Nai [12], which suggests the influence of oceanic circulation to 
the climate of Southern Vietnam [34]. Rn was on average 6% higher in wet half of a 
year in comparison with a dry half of year, creating favored conditions for vegetating. 
Table 1. Annual totals of fluxes at the site in Đồng Nai: net radiation (Rn), 
precipitation (Pr), evapotranspiration (E), net ecosystem exchange (NEE) 
Flux Unit 
Year 
2012 2013 2014 2015 2016 
Rn MJ m-2 y-1 4851 4694 4940 5056 4780 
Pr mm y-1 2621 2634 2332 2351 2252 
E mm y-1 1555 1459 1589 1556 1396 
NEE gC m-2 y-1 -287 -452 -565 -350 -243 
3.3. Turbulent latent and sensible heat fluxes 
The latent heat annual totals (table 1) were on the same level as LE values in 
central Amazonian rainforests despite a 4-month dry season in southern Vietnam. In 
a dry part of a year almost half of Rn yet spent on evapotranspiration owing to 
persisting transpiration of evergreen undergrowth. In 4 driest months of a year LE 
totals reduced only by 30% (5-year average) in comparison with 4 wettest months. 
Measured heat storage in soil and air accounted for 40% of morning-afternoon 
sensible heat flux in the hot months. 
0
200
400
600
800
1 2 3 4 5 6 7 8 9 10 11 12
Rs, MJ m¯² 
mon¯¹ 
2012
2013
2014
2015
2016
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Fig. 3. The radation balance (Rn), latent (LE) and sensible (H) heat fluxes 
In contrast, in hot April 2016 turbulent heat twice exceeded the latent heat. 
Nevertheless, the Bowen ratio rapidly resumed to the usual wet season rate after the 
first rains. 
3.4. CO2 fluxes 
Peaks of CO2 concentration in the seasonal course above the forest and inside 
the canopy fell at April-May and, in some years, there was a secondary peak in 
November-December (fig. 4). Usually, the forest was a moderate to significant sink 
of CO2 from the atmosphere (except for a hottest month of a year), which resulted in 
an annual CO2 sink in an amount of -250-550±100 gC m-2 y-1 (table 1), which is 
higher than mean in other tropical forests. On 4-year average, Reco and GPP in the 4 
driest months made up 60% and 65% of these in 4 wettest months, respectively. 
The structure of the ecosystem CO2 fluxes has changed dramatically during 
the drough ... 
as contrasted with other years, when the forest was a steady net CO2 sink in all 
months except the hottest one. 
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 Nghiên cứu khoa học công nghệ 
Tạp chí Khoa học và Công nghệ nhiệt đới, Số 14, 11 - 2017 48
TÓM TẮT 
DÒNG NĂNG LƯỢNG VÀ KHÍ TRAO ĐỔI TRONG RỪNG NHIỆT ĐỚI 
CỦA ĐỒNG NAI 
Bài báo trình bày kết quả nghiên cứu, đo đạc liên tục hơn 5 năm về các dòng 
năng lượng, hơi nước và khí CO2 trên hệ sinh thái rừng nhiệt đới bán thường xanh 
thuộc Vườn Quốc gia Cát Tiên thông qua trạm quan trắc dòng sử dụng kỹ thuật 
Eddy-Covariance đầu tiên tại Việt Nam. Những thay đổi của khí hậu trong khu vực 
đã ghi nhận ở thời kỳ mùa khô 2011÷2012, 2016÷2017, thời kỳ hạn 2011÷2012, 
2013÷2014 và đặc biệt là thời kỳ mùa khô 2015÷2016 khí hậu “ẩm ướt” hơn thường 
lệ. Khảo sát trong số 20 trạm quan trắc dòng trong rừng nhiệt đới của khu vực, bao 
gồm bức xạ thuần và tổng lượng mưa đều ghi nhận được một trong số những giá trị 
cao nhất ở các thời kỳ này. 
Mặc dù thời kỳ mùa khô chỉ kéo dài khoảng 3,5 tháng, song các kết quả 
nghiên cứu đã chỉ ra tổng lượng bốc thoát hơi E và tổng sản lượng sơ cấp GPP hàng 
năm trong thời kỳ mùa khô tại đây gần với các dữ liệu của rừng nhiệt đới ẩm. Đối 
với các dòng sinh ra chủ yếu do sinh vật sống trong hệ sinh thái, sự suy giảm mùa 
khô có ý nghĩa quan trọng nhất đối với hô hấp hệ sinh thái hơn là đối với GPP và 
E, Đại lượng E ít nhạy cảm nhất với hạn hán. Rừng Nam Cát Tiên là một bể chứa 
CO2 quan trọng từ khí quyển giai đoạn 2012÷2015, ngoại trừ những tháng nóng nhất 
của năm. Điều kiện nóng và khô bất thường trong tháng 4 và tháng 5 năm 2016 đã 
gây ra sự gia tăng dòng năng lượng hiển nhiệt và hô hấp của hệ sinh thái. Sau cơn 
mưa đầu tiên, trong khi các dòng năng lượng nhanh chóng quay trở lại trạng thái 
thông thường của thời kỳ mùa mưa thì rừng vẫn đóng vai trò là nguồn phát CO2 vào 
khí quyển trong khoảng thời gian thêm 1,5 tháng. 
Keywords: Eddy covariance, seasonally dry tropical forest, Đồng Nai 
biosphere reserve, net ecosystem excange, evapotranspiration. 
Nhận bài ngày 24 tháng 9 năm 2017 
Hoàn thiện ngày 02 tháng 11 năm 2017 
(1) A.N.Severtsov Institute of Ecology and Evolution, RAS 
 (2) Vietnam - Russian Tropical Center