Research Article |
Corresponding author: Nan Liu ( liunan@tyut.edu.cn ) Academic editor: Yu-Pin Lin
© 2016 Nan Liu, Yujie Wang, Yunqi Wang, Zhanjun Zhao, Yangyi Zhao.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Liu N, Wang Y, Wang Y, Zhao Z, Zhao Y (2016) Tree species composition rather than biodiversity impacts forest soil organic carbon of Three Gorges, southwestern China. Nature Conservation 14: 7-24. https://doi.org/10.3897/natureconservation.14.6486
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Forest soil represents an important resource for mitigating the climate change. Besides, plant composition and diversity and their roles in ecosystem functioning are becoming a central issue in forest soil organic carbon (SOC) research. The primary objective of this research is to investigate the effects of tree species diversity and composition on potential of C sequestration of forest soil in Three Gorges area and provide basic information to future research on climate change. Two dominant forest ecosystems were selected: mixed conifer-broadleaf forest (Fm) and evergreen broadleaf forest (Fb). Then study transects were established and investigated. Soil samples were collected and determined for bulk density, SOC concentration and stock, nitrogen (N) concentration and C:N ratio. The results showed that the statistical differences of SOC concentrations and stocks between Fm and Fb were caused by tree species composition rather than the tree species diversity. And the most significant differences were found in the first two soil horizons (0–15 cm and 15–30 cm). The average C:N values of four different horizons in Fm were decreased with increasing soil depth as well as Fb. Not only SOC concentrations but also stocks of the two studied forests were decreased with increasing soil depth. However, Fm showed a larger capacity to store SOC with an average stock of 183.50 t/ha than that of Fb (100.44 t/ha) in study area. Thus, forest which is composed of conifer and evergreen broadleaf tree species may be the best choice for local afforestation and reforestation aimed at alleviating climate change in Three Gorges region.
Soil organic carbon, tree species composition, biodiversity, C:N ratio, Three Gorges
Scientists have long been concerned with soil carbon (C), because it is often the master variable determining soil fertility (
The potential C sequestration of forest ecosystems is widely accepted (
In natural ecosystem, nitrogen (N) is a primary nutrient that limits vital activities of plant and microbe (
Because of the alleviation effect on global warming, C sequestration ability of forest is expected for more and more focus (
Our study was carried out at Jinyun Mountain, Three Gorges area, southwestern China. The forest area is totally 1112.7 ha which accounts for 96.6% local land area, and typical subtropical forest species are abundant. The study area is bounded by the two major river systems of the region, i.e., the Yangtze River and the Jialingjiang River. Elevation ranges from 350 to 952 m. This region has a subtropical monsoon climate with long warm to hot humid summers and short cool to cold and cloudy winters with the lowest total number of sunshine days in China (about 1000 hours per year). The mean annual temperature is 13.6 °C and the average annual precipitation is 1611.8 mm. Soil type is Kandihumults of Ultisols (
In our study, we investigated two natural forest ecosystems in April, 2011: the mixed conifer-broadleaf forest (Fm) and the evergreen broadleaf forest (Fb). These two forests are close to each other (separated from each other by approximately 100 m) and have similar elevation and same aspect. The basic information, including vegetation, soil and topography characteristics, is showed in Table
Site | Dominant tree species | Shrub species | Herb species | Soil type | Range of slope (°) | Aspect | Mean elevation (m) | Range of canopy density | Total area (ha) |
---|---|---|---|---|---|---|---|---|---|
Fm |
Pinus massoniana Lamb. Cunninghamia lanceolata (Lamb.) Hook. Symplocos setchuensis Brand Lindera kwangtungensis (Liou) Allen |
Maesa japonica (Thunb.) Moritzi. Eurya nitida Korthals Sarcandra glabra (Thunb.) Nakai Smilax china Rosa multiflora Thunb. Eurya fangii Rehd. Elaeagnus bockii Diels Rubus assamensis Focke Rubus malifolius Focke Rubus corchorifolius L. f. Camellia cuspidata (Kochs) Wright ex Gard. |
Woodwardia japonica (L. f.) Sm. Lophatherum gracile Brongn. Oplismenus compositus (Linn.) Beauv. Hicriopteris glauca (Thunb.) Ching Commelina communis Linn. Stenoloma chusanum Ching Miscanthus sinensis Anderss. Hemerocallis fulva (L.) L. Conyza canadensis (L.) Cronq. Phyla nodiflora (L.) Greene |
Kandihumults of Ultisols | 15–24 | N | 820 | 0.85–0.97 | 17.3 |
Fb |
Lindera kwangtungensis (Liou) Allen Symplocos setchuensis Brand Castanopsis fargesii Franch. Adinandra bockiana Gordonia acuminata Chang |
Maesa japonica (Thunb.) Moritzi. Eurya nitida Korthals Sarcandra glabra (Thunb.) Nakai Smilax china Neolitsea aurata (Hay.) Koidz Eurya fangii Rehd. Symplocos lancifolia Sieb. & Zucc. Rubus malifolius Focke Camellia cuspidata (Kochs) Wright ex Gard. |
Woodwardia japonica (L. f.) Sm. Oplismenus compositus (Linn.) Beauv. Hicriopteris glauca (Thunb.) Ching Commelina communis Linn. Stenoloma chusanum Ching Miscanthus sinensis Anderss. Hemerocallis fulva (L.) L. Conyza canadensis (L.) Cronq. Phyla nodiflora (L.) Greene |
Kandihumults of Ultisols | 12–28 | N | 822 | 0.88–0.94 | 12 |
SOC concentrations and stocks and their vertical distributions were studied. The statistical differences of SOC in 0–100 cm between the two studied forests were analyzed by T-test. The statistical differences of SOC in each horizon (i.e.: 0–15cm, 15–30 cm, 30–50 cm and 50–100 cm) between the two studied forests, as well as those among horizons, were analyzed by one-way ANOVA respectively. And this method was performed to test the differences between tree species diversity of the two researched forests. The results were summarized to explain the effects of tree species composition and diversity on SOC accumulation. In order to study the effect of tree diversity on SOC sequestration, the correlations between SOC and tree species diversity indices of Fm, as well as Fb, were then estimated by regression analysis. As an important controller of SOC decomposition, soil C:N ratio was also analyzed. One-way ANOVA was performed to evaluate the differences between C:N ratios of the two studied forests so as to understand the condition of SOC decomposition. Data analysis was implemented by using Microsoft Office Excel 2003 (Microsoft Corporation, US) and SPSS-17 (IBM Corporation, US).
SOC stock was calculated according to following formula:
(1)
Where ST is SOC stock (t/ha), i is soil horizon code, n is the number of soil horizons, Ci is SOC concentration (g/kg), ρi is soil bulk density (g/cm3), hi is soil horizon thickness (cm), θi is volume proportion (%) of gravel with diameter (φ) >2 mm.
Tree species diversity was presented by following indices (
Simpson’s index (biodiversity index):
(2)
Shannon - Wiener index (biodiversity index):
(3)
Margalef index (richness index):
(4)
Where N is total number of trees in plot, i is tree species type, ni is number of individuals of tree species i, S is number of tree species.
SOC concentrations of the studied forests remarkably decreased with increasing depth of mineral soil. These correlations could be simulated as follows:
SOC concentrations of Fm and Fb may be calculated by above empirical models. But it indeed needs more samples for accuracy.
Significant differences (p<0.001) were found among the four soil horizons in both Fm and Fb (see Fig.
SOC concentrations (Fig.
Mean value (`x) ± standard deviation (σ) of SOC concentration, SOC stock and C:N ratio. The p values which are less than 0.05 indicate significant difference between Fm and Fb.
Fm | Fb | ||||||
---|---|---|---|---|---|---|---|
xˉ ± σ | xˉ ± σ | p | F | Critical values of F | t | Statistical values of t | |
SOC concentration | |||||||
0–15 cm | 52.38±22.49 | 31.02±8.88 | 0.0101 | 7.9880 | 4.3248 | —— | —— |
15–30 cm | 16.45±7.66 | 9.51±6.37 | 0.0338 | 5.1562 | 4.3248 | —— | —— |
30–50 cm | 10.62±9.77 | 5.19±6.31 | 0.2068 | 1.7229 | 8.3997 | —— | —— |
50–100 cm | 7.57±7.52 | 2.89±4.41 | 0.1539 | 2.2275 | 8.3997 | —— | —— |
0–100 cm | 85.62±30.17 | 46.18±18.44 | 0.0016 | —— | —— | 2.0796 | 3.6342 |
SOC stock | |||||||
0–15 cm | 75.23±29.09 | 56.96±14.58 | 0.0843 | 3.2841 | 4.3248 | —— | —— |
15–30 cm | 32.03±14.88 | 18.70±11.55 | 0.0294 | 5.4666 | 4.3248 | —— | —— |
30–50 cm | 29.00±24.38 | 15.09±18.60 | 0.2114 | 1.6865 | 4.4513 | —— | —— |
50–100 cm | 53.59±47.01 | 20.30±28.37 | 0.1092 | 2.8583 | 4.4513 | —— | —— |
0–100 cm | 183.50±71.59 | 100.44±50.38 | 0.0052 | —— | —— | 2.0796 | 3.1159 |
C:N ratio | |||||||
0–15 cm | 9.63±12.47 | 7.44±6.71 | 0.5853 | 0.3071 | 4.3248 | —— | —— |
15–30 cm | 7.30±8.41 | 4.27±4.45 | 0.3162 | 1.0544 | 4.3248 | —— | —— |
30–50 cm | 4.82±5.54 | 4.51±5.36 | 0.8910 | 0.0194 | 4.4513 | —— | —— |
50–100 cm | 4.89±8.53 | 3.83±4.72 | 0.7780 | 0.0821 | 4.4513 | —— | —— |
0–100 cm | 6.66±2.29 | 5.01±1.64 | 0.4464 | 0.8175 | 3.9574 | —— | —— |
SOC stocks in 0–100 cm of Fm and Fb (Fig.
SOC stocks of Fm and Fb were decreased from 0-15 cm to 30-50 cm firstly, then they were increased (Fig.
As shown in Fig.
Concentrations of SOC and soil N of Fm (r=0.6656, n=50, p<0.001) were linearly and remarkably correlated as well as those of Fb (r=0.5566, n=34, p<0.001). The results showed that soil N may have important effects on SOC. However, as a metric of SOC quality, the soil C:N ratios of the studied forests were not statistically different (Fig.
Average values of tree species diversity indices were shown in Table
The main differences of SOC concentrations of Fm and Fb were presented in 0–15 cm and 15–30 cm soil with p value of 0.0101 and 0.0338 respectively. The reason may be that roots are mainly distributed in 0–50 cm soil horizon (
Our total C stock estimates of 0-100 cm mineral soil under the two forest ecosystems (100.44–183.50 t/ha, Fig.
Published values of SOC in comparable mixed conifer-broadleaf forest and evergreen broadleaf forest in subtropical region.
Locatoin | Vegetation type | SOC stocks (t/ha) | Soil type | Source |
---|---|---|---|---|
Earth | Virgin and secondary forests | 180.00–240.00 | Ultisols |
|
Earth | —— | 83.00 | Ultisols |
|
China | Mixed conifer-broadleaf forest | 130.00–150.00 | —— |
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China | Evergreen broadleaf forest | 124.00–142.00 | —— |
|
Fujian, China | Mixed conifer-broadleaf forest | 30.90–107.10 | Ultisols |
|
Fujian, China | Evergreen broadleaf forest | 95.00–127.90 | Ultisols |
|
China | Mixed conifer-broadleaf forest | 225.70 | —— |
|
China | Evergreen broadleaf forest | 129.20 | —— |
|
Chongqing, China | —— | 144.8 | Ultisols |
|
Three Gorges region, China | Mixed conifer-broadleaf forest | 92.33–127.13 | Ultisols and Alfisols |
|
Three Gorges region, China | Evergreen broadleaf forest | 151.63–290.82 | Ultisols and Alfisols |
|
Central Spain | Mixed conifer-broadleaf forest | 80.00–100.00 | Inceptisols and Alfisols |
|
Central Spain | Evergreen broadleaf forest | 40.00–70.00 | Inceptisols and Alfisols |
|
Three Gorges region, China (Chongqing section) | Mixed conifer-broadleaf forest | 183.50 | Ultisols | This study |
Three Gorges region, China (Chongqing section) | Evergreen broadleaf forest | 100.44 | Ultisols | This study |
Both the average soil C:N ratios of Fm and Fb were decreasing with increasing soil depth. And the average C:N ratios of Fb were less than those of Fm in each soil horizons. The C:N ratio provides some indication about the relative quality and biochemical stability of soil organic materials (
The relationship between tree species diversity and SOC under studied forest ecosystems was not linear in our study. However,
Tree species composition significantly and statistically influenced SOC concentrations and stocks of Fm and Fb. In first two soil horizons (0-15 cm and 15-30 cm), these differences were even more significant. However, SOC of Fm and Fb were not influenced by tree species diversity due to the very low linear coefficients. And the statistical difference between biodiversity indices of Fm and Fb was not significant. Thus, in contrast to plant species composition, biodiversity may not make difference in forest soil C sequestrations. The average C:N values of Fm in four different horizons were decreased with increasing soil depth as well as Fb. And the values were larger in Fm. But the difference between C:N ratios of Fm and Fb was not remarkable. C:N ratio contributed little to the difference between SOC of the two studied forests. Not only SOC concentrations of Fm and Fb were decreased with increasing soil depth but also SOC stocks reduced from surface soil to bottom. Fm showed a large capacity to store SOC rather than Fb in the area. Thus, mixed conifer-broadleaf forest may be the best choice for local afforestation and reforestation aimed at alleviating climate change in Three Gorges region. However, conflict issues can still be found in the relation between SOC and tree species diversity in studies all over the world. It needs more detail researches in different scale to explain.
The work was funded by projects of National Natural Science Foundation of China (31400618, 41201265, 41503074), Qualified Personnel Foundation of Taiyuan University of Technology (Grant No: tyut-rc201277a), and the project supported by Youth Foundation of Taiyuan University of Technology (No. 1205-04020102, No. 2013Z070). We are grateful to the Ecologist Kristen Manies from U.S. Geological Survey and Ecologist Charlotte Reemts from Texas Field Office of Nature Conservancy for their constructive comments that improved this manuscript.