Applications of structural equation modeling(SEM)in ecological studies:an updated review

Aims:This review was developed to introduce the essential components and variants of structural equation modeling(SEM),synthesize the common issues in SEM applications,and share our views on SEM’s future in ecological research.Methods:We searched the Web of Science on SEM applications in ecological studies from 1999 through 2016 and summarized the potential of SEMs,with a special focus on unexplored uses in ecology.We also analyzed and discussed the common issues with SEM applications in previous publications and presented our view for its future applications.Results:We searched and found 146 relevant publications on SEM applications in ecological studies.We found that five SEM variants had not commenly been applied in ecology,including the latent growth curve model,Bayesian SEM,partial least square SEM,hierarchical SEM,and variable/model selection.We identified ten common issues in SEM applications including strength of causal assumption,specification of feedback loops,selection of models and variables,identification of models,methods of estimation,explanation of latent variables,selection of fit indices,report of results,estimation of sample size,and the fit of model.Conclusions:In previous ecological studies,measurements of latent variables,explanations of model parameters,and reports of key statistics were commonly overlooked,while several advanced uses of SEM had been ignored overall.With the increasing availability of data,the use of SEM holds immense potential for ecologists in the future.

Review of drought impacts on carbon cycling in grassland ecosystems

Grasslands play a key role in both carbon and water cycles.In semi-arid and arid grassland areas,the frequency and intensity of droughts are increasing.However,the influence of a drought on grassland carbon cycling is still unclear.In this paper,the relationship between drought and grassland carbon cycling is described from the perspective of drought intensity,frequency,duration,and timing.Based on a large amount of literature,we determined that drought is one of the most prominent threats to grassland carbon cycling,although the impacts of different drought conditions are uncertain.The effects of a drought on grassland carbon cycling are more or less altered by drought-induced disturbances,whether individually or in combination.Additionally,a new conceptual model is proposed to better explain the mechanism of droughts on grassland carbon cycling.At present,evaluations of the effects of droughts on grassland carbon cycling are mainly qualitative.A data fusion model is indispensable for evaluating the fate of carbon cycling in a sustainable grassland system facing global change.In the future,multi-source data and models,based on the development of single and multiple disturbance experiments at the ecosystem level,can be utilized to systematically evaluate drought impacts on grassland carbon cycling at different timescales.Furthermore,more advanced models should be developed to address extreme drought events and their consequences on energy,water,and carbon cycling.

Spatial variation of net radiation and its contribution to energy balance closures in grassland ecosystems

Introduction:Low energy balance closure(EBC)at a particular eddy-covariance flux site has increased the uncertainties of carbon,water,and energy measurements and has thus hampered the urgent research of scaling up and modeling analyses through site combinations in regional or global flux networks.Methods:A series of manipulative experiments were conducted in this study to explore the role of net radiation(Rn)in the EBC in relation to spatial variability of vegetation characteristics,source area,and sensor type in three sites of the Inner Mongolian grassland of northern China.Results:At all three sites,the residual fluxes of EBC peaked consistently at 110 W m^(-2).The spatial variability in net radiation was 19 W m^(-2)(5%of R_(n))during the day and 7 W m^(-2)(16%)at night,with an average of 13 W m^(-2)(11%)from eight plot measurements across the three sites.Large area measurements of Rn significantly increased by 9 W m^(-2)during the day and decreased by 4 W m^(-2)at night in the unclipped treatments.Net radiation decreased by 25 W m^(-2)(6%of Rn)at midday and 81 MJ m^(-2)(6%)during a growing season with heavier regular clipping than that in unclipped treatments.The Rn was lower by 11–21 W m^(-2)(~20–40%of Rn)measured by CNR1 than by Q7.1 at night,while there was only 6 W m^(-2)(~1–2%of Rn)difference during the daytime between these two types of commonly used net radiometers.Conclusions:Overall,the inclusion of the uncertainty in available energy accounted for 65%of the~110 W m^(-2)shortfalls in the lack of closure.Clearly,the unclosed energy balance at these three grassland sites remains significant,with unexplored mechanisms for future research.

Arbuscular mycorrhizal fungi protect a subtropical tree species exposed to simulated acid rain by accelerating photosynthetic ability,antioxidant enzymes and osmolyte accumulation

Acid rain(AR),which occurs frequently in southern China,negatively affects the growth of subtropical tree species.Arbuscular mycorrhizal fungi(AMF)mitigate the detrimental effects induced by AR.However,the mechanisms by which AMF protect Zelkova serrata,an economically important tree species in southern China,from AR stress remain unclear.We conducted a greenhouse experiment in which Z.serrata plants were inoculated with AMF species Rhizophagus intraradices and Diversispora versiformis,either alone or as a mixed culture,or with a sterilized inoculum(negative control).The plants were subjected to three levels of simulated sulfuric AR and nitric AR(pH 2.5,4.0 and 5.6)to examine any interactive effects on growth,photosynthetic capabilities,antioxidant enzymes,osmotic adjustment and soil enzymes.AR significantly decreased dry weight,chlorophyll content,net photosynthetic rate and soluble protein(SP)of non-mycorrhizal plants.Mycorrhizal inoculation,especially a combination of R.intraradices and D.versiformis,notably improved dry weight,photosynthetic capabilities,catalase,peroxidase,superoxide dismutase,SP and root acid phosphatase activity of Z.serrata under harsh AR stress.Moreover,the benefits from AMF symbionts depended on the identity of AM fungal species and the gradient of AR stress.Our results indicate that AM fungi protect z.serrata against AR stress by synchronously activating photosynthetic ability,antioxidant enzymes and osmolyte accumulation.These findings suggest that a combination of R.intraradices and D.versiformis may be a preferable choice for culturing Z.serratain southern China.

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.

Effects of changes in precipitation on energy and water balance in a Eurasian meadow steppe

Introduction:Water such as precipitation is the most critical environment driver of ecosystem processes and functions in semi-arid regions.Frequency and intensity of drought and transient waterlogging are expected to increase in the meadow steppe in northeastern China.Using a 4-year dataset of eddy covariance flux measurements,ground measurements of biomass,phenology,and meteorological conditions,we investigated the changes in energy fluxes at multiple temporal scales and under different precipitation regimes.Results:The meadow steppe was latent heat(LE)dominated when soil water content was>0.3 m3 m−3,but switched to sensible heat(H)dominated status when soil water content fell below 0.3 m3 m−3.LE dominated the energy exchange of the meadow grasslands on a yearly basis.Intensive precipitation had a profound impact on water-energy balance that could reduce the damages of drought by elevating deep soil moisture.The influence of LE on waterlogging depended on timing,with increased LE at the beginning of growing season and decreased LE after waterlogging.Spring and summer droughts resulted in different energy partitioning between latent and sensible heat energies,with spring drought dramatically decreased the LE fraction due to the change in water.In contrast,summer drought had little impact on LE due to the sufficient water input from large precipitation events at the beginning of the growing season.Conclusions:There existed great seasonal and interannual variabilities in energy balance and partitioning in the meadow steppe over the 4-year study period,which were strongly influenced by changes in precipitation.The water loss through latent heat was more sensitive to spring drought than to summer drought,while summer drought had negligible impact on LE.Waterlogging contributed to LE by enhancing its values during and after the waterlogged periods at the beginning of the growing season in a dry year,but lowering its value after the waterlogged periods in growing season.