Spatiotemporal variations of CO_(2) fluxes in a Cynodon-dominated riparian zone of the Three Gorges Reservoir(TGR),China

Aims Riparian ecosystems play an important role in overall ecosystem function,including the global carbon cycle of terrestrial ecosystems,at both landscape and global scales.Yet few studies have reported in situ measurements of CO_(2) in riparian areas where flooding is a unique disturbance to carbon cycling.Methods At multiple locations across riparian zones(RZ)with different water submergences in Xiangxi Bay(XXB),we studied seasonal variations of CO_(2) exchange between this Cynodon-dominated community and the atmosphere for 2 years by using static chambers.Important Findings We found that the seasonal changes in CO_(2) fluxes were apparent and dependent on the biophysical environment.In the beginning of spring,low gross primary productivity(GPP)in lightly flooded zones(LFZ)resulted in a positive net ecosystem exchange(NEE),indicating a net CO_(2) source.With the increase in temperature,more species and vegetation abundance appeared,and the increased GPP turned the LFZ from a net CO_(2) source into a sink.This transi-tion seemed predominantly controlled by the physiological growth of vegetation.The mean NEEs,REs and the light-use efficiency(α)of the vegetation at HFZ and MFZ were significantly higher than those at LFZ and UFZ.Yet the coefficients of variation(CV)of NEE and RE at MFZ and HFZ were lower than those at LFZ and UFZ.Submergence promoted the emission and uptake of CO_(2) to the atmosphere.Elongated submergence reduced the number of spe-cies and lowered the spatial variability of the RZ,further lowering the variation of the CO_(2) exchange.

The effects of nutrients on stream invertebrates:a regional estimation by generalized propensity score

Introduction:The effects of nutrients on stream conditions within individual streams or small areas have been studied extensively,but the same effects over a large region have rarely been examined due to the difficulty of applying large-scale manipulative experiments.In this study,we estimated the causal effects of nutrients within the Western United States on invertebrate richness,an important biological indicator of stream conditions,by using observational data.Methods:We used the generalized propensity score method to avoid the common problem of statistical inference using observational data,i.e.,correlation established based on observational data does not imply a causal relationship because the effects of confounding factors are not properly separated.Results:Our analysis showed a subsidy-stress relationship between nutrients and invertebrate taxon richness in the whole Western United States and in its sub-ecoregions.The magnitude of the relationship varies among these subecoregions,suggesting a varying nitrogen effect on macroinvertebrates due,in large part,to the varying natural and anthropogenic conditions from ecoregion to ecoregion.Furthermore,our analysis confirmed that causal estimation results using regression can be sensitive to the imbalance of confounding factors.Conclusions:Stratifying data into ecoregions with relatively homogeneous environmental conditions or adjusting data by generalized propensity score can improve the balance of confounding factors,thereby allowing more reliable causal inference of nutrient effects.Invertebrates respond to the same nutrient levels differently across different site conditions.

Urbanization,economic development,and environmental changes in transitional economies in the global south:a case of Yangon

Background:Transitional economies in Southeast Asia—a distinct group of developing countries—have experienced rapid urbanization in the past several decades due to the economic transition that fundamentally changed the function of their economies,societies and the environment.Myanmar,one of the least developed transitional economies in Southeast Asia,increased urbanization substantially from 25%in 1990 to 31%in 2019.However,major knowledge gaps exist in understanding the changes in urban land use and land cover and environment and their drivers in its cities.Methods:We studied Yangon,the largest city in Myanmar,for the urbanization,environmental changes,and the underlying driving forces in a radically transitioned economy in the developing world.Based on satellite imagery and historic land use maps,we quantified the expansion of urban built-up land and constructed the land conversion matrix from 1990 through 2020.We also used three air pollutants to illustrate the changes in environmental conditions.We analyzed the coupled dynamics among urbanization,economic development,and environmental changes.Through conducting a workshop with 20 local experts,we further analyzed the influence of human systems and natural systems on Yangon’s urbanization and sustainability.Results:The city of Yangon expanded urban built-up land rapidly from 1990 to 2000,slowed down from 2000 to 2010,but gained momentum again from 2010 to 2020,with most newly added urban built-up land appearing to be converted from farmland and green land in both 1990–2000 and 2010–2020.Furthermore,the air pollutant concen-tration of CO decreased,but that of NO_(2)and PM_(2.5)increased in recent years.A positive correlation exists between population and economic development and the concentration of PM_(2.5)is highly associated with population,the economy,and the number of vehicles.Finally,the expert panel also identified other potential drivers for urbanization,including the extreme climate event of Cyclone Nargis,capital relocation,and globalization.Conclusions:Our research highlights the dramatic expansion of urban land and degradation of urban environment measured by air pollutants and interdependent changes between urbanization,economic development,and environmental changes.