Carbon Dioxide Fluxes and Their Environmental Controls in a Riparian Forest within the Hyper-Arid Region of Northwest China

Impact of meteorological variability on diurnal and seasonal net ecosystem productivity in a desert riparian forest ecosystem

The desert riparian forests are susceptible to meteorological changes and contribute significantly to the net ecosystem productivity (NEP) variations of arid ecosystems. However, the responsive patterns of their NEP variations to the meteorological variabilities remain inadequately comprehended. To address this gap, we utilized seven years of eddy covariance flux measurements in a representative desert riparian forest to investigate the NEP variations and its response to changing meteorological factors across diverse temporal scales. The results revealed significant periodic variations in half-hourly NEP, with dominant cycles spanning from five hours to one year, with a principal oscillation period of one day. Key meteorological factors including global solar radiation (Rg), relative humidity (RH), air temperature (Ta), soil temperature (Ts), and vapor pressure deficit (VPD) exhibited synchronization with NEP on daily scales. This synchronization, coupled with the observed one-day periodic NEP variations, provides robust evidence supporting the existence of a circadian rhythm in the ecosystem carbon exchange of desert riparian forest regulated by meteorological conditions. Seasonal patterns were significant in the impact of Rg phase, Ta diurnal amplitude, and VPD diurnal amplitude on NEP diurnal amplitude and phase. The NEP diurnal amplitude significantly, directly, and positively affected daily NEP in both the dormant and growing seasons, whereas its phase yielded significant negative effects (P< 0.05). The averages, amplitudes, and phases of diurnal meteorological conditions controlled the daily NEP by regulating NEP diurnal amplitude and phase. These findings provide evidence that the variability in circadian rhythms, caused by the increase in diurnal Ta and VPD, significantly impact the daily NEP at an ecosystem scale. This study enriches our comprehension of the meteorological mechanisms governing diurnal and seasonal carbon uptake dynamics within desert riparian forests, providing fresh insights into the direct and indirect roles of climate change in shaping patterns of ecosystem carbon exchange.

Flooding constrains tree water use of a riparian forest in the lower Heihe River Basin, Northwest China

Riparian forests in floodplains are occasionally or regularly submerged by flooding. However, controversy exists regarding the effects of flooding on water use in riparian forests, and this controversy severely restricts our ability to better utilize limited water resources to restore damaged riparian forests in arid regions.The evapotranspiration (E_t) and transpiration (T) of riparian P. euphratica forests in the arid regions of northwestern China were determined using eddy covariance and sap flow technology across a 3-year period. Fortunately, the flooding introduced by ecological water diversion was occurred in 2014 and 2016 but not in 2015. Our results showed that the magnitude and seasonal pattern of E_t across 3 years was comparable (approximately 900 mm), but the T was higher in 2015 (431 mm) than in the other two years (288 mm in 2014 and 290 mm in 2016). The interannual patterns in the transpiration were consistent with the net ecosystem productivity at the site. Given the similar meteorological conditions (e.g. net radiation, temperature, relative humidity, and vapor pressure deficit) among the 3 years, two aspects may contributed to the suppressed tree water use and productivity under flooding: 1) the increased soil salinity reduce the roots water uptake from soil by increasing root water potential via osmotic adjustment; and 2) the depressed tree growth (e.g. the leaf area) via suspended water upward transport along soil-plant-atmosphere continuum. Although flooding is widely known beneficial for the regeneration, we suggest that it is not appropriate for the rejuvenation of phreatophyte (e.g., Populus spp.) in arid regions. Our results were drawn from only three years of measurement and therefore longer time series are needed to confirm or refine those conclusions.

Uncertainty in gap filling and estimating the annual sum of carbon dioxide exchange for the desert Tugai forest, Ebinur Lake Basin, Northwest China

In most eddy covariance (EC) studies, carbon flux measurements have a high defect rate for a variety of reasons. Obtaining the annual sum of carbon dioxide exchange requires imputation of data gaps with high precision and accuracy. This study used five methods to fill the gaps in carbon flux data and estimate the total annual carbon dioxide exchange of the Tugai forest in the arid desert ecosystem of Ebinur Lake Basin, Northwest China. The Monte Carlo method was used to estimate the random error and bias caused by gap filling. The results revealed that (1) there was a seasonal difference in the friction velocity threshold of nighttime flux, with values in the growing season and non-growing season of 0.12 and 0.10 m/s, respectively; (2) the five gap-filling methods explained 77–84% of the data variability in the fluxes, and the random errors estimated by these methods were characterized by non-normality and leptokurtic heavy tail features, following the Laplacian (or double-exponential) distribution; (3) estimates of the annual sum of carbon dioxide exchange using the five methods at the study site in 2015 ranged from −178.25 to −155.21 g C m⁻² year⁻¹, indicating that the Tugai forest in the Ebinur Lake Basin is a net carbon sink. The standard deviation of the total annual carbon dioxide exchange sums estimated by the five different methods ranged from 3.15 to 19.08 g C m⁻² year⁻¹, with bias errors ranging from −13.69 to 14.05 g C m⁻² year⁻¹. This study provides a theoretical basis for the carbon dioxide exchange and carbon source/sink assessment of the Tugai forest in an arid desert ecosystem. In order to explore the functioning of the Tugai forest at this site, a greater understanding of the underlying ecological mechanisms is necessary.