Influence of solar radiation and groundwater table on carbon balance of phreatophytic desert shrub Tamarix

doi:10.3773/j.issn.1005-264x.2010.04.003

Abstract

Abstract:

Aims The groundwater table has changed and air pollution has been reducing solar radiation on the southern periphery of China’s Gurbantonggut Desert. Our objective was to investigate the response and adaptation of Tamarix ramosissima, a native dominant desert shrub in Central Asia, towards variation in groundwater and photosynthetically active radiation (PAR), in terms of ecophysiological activities, morphological characteristics and community carbon/water balances.

Methods During the growing season from 2005 to 2007, we carried out experiments in the original habitat of T. ramosissima, where the groundwater table fluctuated from 2.9 to 4.5 m. Photosynthesis, transpiration, leaf water potential, water-use efficiency and root distribution were examined to reveal the water-use strategy of the species, and CO₂ and H₂O fluxes above an undisturbed T. ramosissima ecosystem were measured by eddy covariance method to evaluate net carbon assimilation, water loss and leaf area index (LAI).

Important findings Physiological activity and community carbon uptake of T. ramosissima did not respond to sustained drought in upper soil or rainfall pulses, and its photosynthetic consistency is achieved by its water-use pattern. Special stomatal behavior and root distribution are two main mechanisms. Tamarix ramosissima tends to maximize its carbon gain at the cost of higher water consumption, attributable to its phreatophytic root system that ensures sufficient groundwater supply and avoids the effects of water deficiency in upper soil. Tamarix ramosissima can adapt to moderate fluctuation of groundwater table, but severe decline will threaten its survival, and hence the overexploitation of groundwater will cause severe degradation of Tamarix-dominated perennial vegetation and disturb the original ecohydrological processes in this arid region. PAR is another important environmental factor positively correlating with community carbon uptake. The LAI indicates that the seasonal pattern in community carbon assimilation represents the combined effects of groundwater table and PAR on the phenological photosynthesis capacity. It shows that the integrated study on different scales is an effective approach to further the understanding of desert shrub adaptive strategies and ecosystem processes under variable environmental conditions.

Key words: community carbon/water flux, desert ecology, ecophysiological response, leaf area index, phreatophyte, plant water-use strategy

XU Hao, LI Yan, XIE Jing-Xia, CHENG Lei, ZHAO Yan, LIU Ran. Influence of solar radiation and groundwater table on carbon balance of phreatophytic desert shrub Tamarix[J]. Chin J Plant Ecol, 2010, 34(4): 375-386.

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URL: https://www.plant-ecology.com/EN/10.3773/j.issn.1005-264x.2010.04.003

https://www.plant-ecology.com/EN/Y2010/V34/I4/375

Figures/Tables 11

Fig. 1 Energy closure in Tamarix ecosystem. G, soil heat flux; H, heat flux; LE, latent heat flux; Rn, net radiation.

Fig. 2 Vertical distribution of surface area of absorbing roots of Tamarix ramosissima (A, B) and soil water content (C, D) in its habitat with natural and no precipitation. Bars represent the standard error of the average.

Fig. 3 Seasonal variations in leaf water potential (ψ) and maximal transpiration rate (Trmax) of Tamarix ramosissima.

Fig. 4 Seasonal changes in apparent quantum efficiency of photosynthesis (Φ), photosynthetic photon flux density at light compensation point (Ic), net photosynthetic rate at light saturation point (Ps), and photosynthetic water use efficiency of Tamarix ramosissima.

Fig. 5 Effects of potential evaporation on physiological activities of Tamarix ramosissima. A, The correlation between net photosynthetic rate at light saturation point / potential evaporation and the maximum transpiration rate (Trmax). B, The correlation between potential evaporation and photosynthetic water use efficiency (WUE).

Fig. 6 Diurnal pattern of H2O flux in Tamarix ecosystem and its seasonal variation.

Table 1 Seasonal variation of meteorological factors, water table, carbon budget and leaf area index (LAI) in Tamarix ecosystem

月份 Month	5月 May	6月 June	7月 July	8月 August	9月 September
每日净辐射 Daily net radiation (W·m^-2)	4 471	4 392	5 170	4 635	2 638
每日光合有效辐射 Daily photosynthetically active radiation (mmol·m^-2·d^-1)	47 745	47 462	50 848	39 629	34 533
平均温度 Average temperature (℃)	19.1	27.0	24.3	22.1	17.5
潜在蒸发力 Potential evaporation (mm)	202.7	205.3	208.5	160.0	138.0
降水 Precipitation (mm)	6.8	7.2	11.1	44.2	1.1
地下水位 Water table (m)	3.12	4.04	3.69	4.43	3.87
群落碳收支 Community carbon budget (g C·m^-2·d^-1)	-0.40^a	-0.42^a	-0.51^a	-0.33^a	-0.24^a
群落蒸腾耗水 Community water loss (mm·d^-1)	0.95	0.84	0.91	0.21	0.46
叶面积指数LAI	0.19	0.17	0.25	0.15	0.18
碳平衡 Ecosystem carbon balance (g C·m^-2·d^-1)	-0.07^b	-0.10^b	-0.11^b	-0.19^b	-0.26^a
水分蒸散 Ecosystem evapotranspiration (mm·d^-1)	1.17	1.07	1.28	1.64	0.50

Fig. 7 Diurnal patterns of net ecosystem carbon exchange, soil carbon flux and net community carbon budget of Tamarix and their seasonal variations.

Fig. 8 Relationship between carbon budget of Tamarix community and photosynthetically active radiation (PAR). Each plot represents the average of continous 20 d during the same period in the growth season of 2005-2007.

Fig. 9 Relationship between leaf area index (LAI) of Tamarix community and water table. Notes see Fig. 8.

Fig. 10 Relationships between net carbon uptake of Tamarix community and photosynthetically active radiation (PAR) or water table.

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