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Xie J.,Beijing Forestry University | Jia X.,Beijing Forestry University | He G.,Beijing Forestry Carbon Sequestration Administration | Zhou C.,Beijing Forestry Carbon Sequestration Administration | And 5 more authors.
Landscape and Urban Planning | Year: 2015

Knowledge of surface-atmosphere CO2 exchanges in urban forests is imperative. We measured net ecosystem CO2 exchange of an urban forest in Beijing over a two-year period (2012-2013). The aim was to examine seasonal controls of environmental variables on ecosystem carbon (C) cycle assessed with eddy covariance technique. Net ecosystem production (NEP) was 30% less in 2012 (200±27g Cm-2) than in 2013 (287±35g Cm-2). Both years were warmer than the long-term average. Seasonal and annual ratios of ecosystem respiration (ER) to gross primary productivity (GEP) were higher in 2012 than in 2013. Dry conditions decreased GEP more than ER, while warm conditions increased ER more than GEP. Heat stress exerted controls over seasonal changes in NEP. Daytime NEP ceased to increase beyond 11μmol CO2m-2s-1 and 13μmol CO2m-2s-1 in 2012 and 2013, respectively, where mean photosynthetically active radiation was >700μmolm-2s-1 and air temperatures was greatest (>27°C) in mid-summer days. The extremely strong precipitation day (176mm) was recorded as having the greatest C discharge to the atmosphere (5.6g Cm-2). Intermediate-size precipitation events (>2mm and ≤15mm) acted to increase C-sink strength. The results highlight the roles of environmental stresses and their alleviation in regulating C fixation in the face of warmer climate and increasing intensity of extreme precipitation events. © 2015 Elsevier B.V.

Xie J.,Beijing Forestry University | Zha T.,Beijing Forestry University | Zhou C.,Beijing Forestry Carbon Sequestration Administration | Jia X.,Beijing Forestry University | And 12 more authors.
Agricultural and Forest Meteorology | Year: 2016

The impact of extreme weather events on water-carbon coupling and ecosystem water use efficiency (WUE) in arid to semi-arid conditions is poorly understood. Evapotranspiration (ET) and gross ecosystem production (GEP) were based on continuously eddy-covariance measurements taken over an urban-forest reserve in Beijing, in a 3-year period (2012-2014) to calculate WUE (GEP:ET). Our objective was to investigate the seasonal response of WUE to changing environmental and drought conditions at different timescales. Annually, the forest produced new plant biomass at 2.6 ± 0.2 g C per kg of water loss. Within each season, interactions of surface conductance (gc) and normalized difference vegetation index (NDVI; i.e., gc × NDVI) in spring, net radiation (Rn) and air temperature (Ta; i.e., Rn × Ta) in summer, and Rn and vapor pressure deficit (D; i.e., Rn × D) in autumn were found as the significant variables explaining seasonal variation in WUE. Daily WUE correlated positively with Ta and NDVI during the growing season, but a negative relationship during excessively dry periods (i.e., 2014). Daily WUE decreased during warm and dry days or remained nearly constant at low levels due to proportional decreases in GEP and ET. An extreme drought during the leaf expansion led to a greater decline in GEP than in ET, causing WUE to be lower in 2012 and 2014 than that in 2013. In contrast, an extreme drought during the leaf coloration led to a greater decline in ET than in GEP, causing higher WUE in 2013 and 2014 than that in 2012. We concluded that: (i) high soil water content (SWC) during leaf expansion was more important than high SWC in mid-summer or autumn for maintaining a high seasonal WUE; and that (ii) seasonal water availability combined with variable drought severity and duration during periods of changing Ta, caused seasonal ET and GEP to respond differently, introducing significant variation in seasonal WUE. © 2016 Elsevier B.V.

Xie J.,Beijing Forestry University | Chen J.,Michigan State University | Sun G.,U.S. Department of Agriculture | Zha T.,Beijing Forestry University | And 10 more authors.
Agricultural and Forest Meteorology | Year: 2016

The impacts of extreme weather events on water-carbon (C) coupling and ecosystem-scale water use efficiency (WUE) over a long term are poorly understood. We analyzed the changes in ecosystem water use efficiency (WUE) from 10 years of eddy-covariance measurements (2004-2013) over an oak-dominated temperate forest in Ohio, USA. The aim was to investigate the long-term response of ecosystem WUE to measured changes in site-biophysical conditions and ecosystem attributes. The oak forest produced new plant biomass of 2.5±0.2 g C kg-1 of water loss annually. Monthly evapotranspiration (ET) and gross ecosystem production (GEP) were tightly coupled over the 10-year study period (R2=0.94). Daily WUE had a linear relationship with air temperature (Ta) in low-temperature months and a unimodal relationship with Ta in high-temperature months during the growing season. On average, daily WUE ceased to increase when Ta exceeded 22°C in warm months for both wet and dry years. Monthly WUE had a strong positive linear relationship with leaf area index (LAI), net radiation (Rn), and Ta and weak logarithmic relationship with water vapor pressure deficit (VPD) and precipitation (P) on a growing-season basis. When exploring the regulatory mechanisms on WUE within each season, spring LAI and P, summer Rn and Ta, and autumnal VPD and Rn were found to be the main explanatory variables for seasonal variation in WUE. The model developed in this study was able to capture 78% of growing-season variation in WUE on a monthly basis. The negative correlation between WUE and P in spring was mainly due to the high precipitation amounts in spring, decreasing GEP and WUE when LAI was still small, adding ET being observed to increase with high levels of evaporation as a result of high SWC in spring. Summer WUE had a significant decreasing trend across the 10 years mainly due to the combined effect of seasonal drought and increasing potential and available energy increasing ET, but decreasing GEP in summer. We concluded that seasonal dynamics of the interchange between precipitation and drought status of the system was an important variable in controlling seasonal WUE in wet years. In contrast, despite the negative impacts of unfavorable warming, available groundwater and an early start of the growing season were important contributing variables in high seasonal GEP, and thus, high seasonal WUE in dry years. © 2015 Elsevier B.V.

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