Response Of Photosynthetic Characteristics Of Cinnamomum Camphora To Extreme Heat And Drought In Urban Environments

With the rapid development of urbanization,the urban environment has become the "harbingers" of global change.Extreme weather is increasing in frequency and intensity in the context of global warming.A series of extreme events have threatened the normal production and life of humans and social development,and caused serious impacts and damage to the ecological environment and vegetation growth.The response of vegetation growth in urban environments to natural extreme heat and drought events is still lacking,which severely limits the understanding and prediction of future impacts of climate change(especially extreme climate events)on global vegetation growth.In this study,Li-cor 6800 was used to observe the response of photosynthetic properties(including net photosynthetic rate,P";stomatal conductance,Gs;evaporation rate,E;light use efficiency,LUE;water use efficiency,WUE)of Cinnamomum camphora(L.)to natural extreme heat and drought in urban paved and unpaved environments,and explored the response mechanism;several classical models of stomatal conductance were fitted and evaluated according to the measured data,and two new models of stomatal conductance were proposed;the suitability of slope parameters of stomatal conductance in terrestrial ecosystem models were evaluated by using new model parameters.The main results of this study are as follows:(1)There were significant differences in environmental factors(photosynthetic photon flux density,PPFD;air temperature,Tair;vapor pressure deficit,VPD;relative humidity,RH)between the pavement and non-pavement during early and late extreme heat and drought stages(P<0.05).Compared with the non-pavement,the pavement had higher PPFD and lower RH in the early stage of extreme heat and drought.On the whole,the Tair and VPD of the non-pavement were higher than that of the pavement.Compared with the early stage of extreme heat and drought,the Tair and VPD of the late stage of extreme heat and drought were increased,and the RH was decreased.(2)In the early stage of extreme heat and drought,only Gs had a significant difference in photosynthetic characteristics between the pavement and non-pavement(P<0.05).In the late stage of extreme heat and drought,Pn,Gs,E and WUE of the pavement and non-pavement were significantly different(P<0.05).Pn,Gs and E of the pavement were higher than those of the non-pavement,WUE of non-pavement was higher than that of pavement.The photosynthetic characteristics(Pn,Gs,E,LUE,WUE)in the late stage of extreme heat and drought decreased compared with that in the early stage of extreme heat and drought,except that WUE in the non-pavement increased.The partial correlation analysis of environmental factors and photosynthetic characteristics showed that Pn,E and Gs were significantly positively correlated with PPFD and Tair(P<0.001),and significantly negatively correlated with Tieaf(P<0.001).The relationship between Pn,E,Gs and VPD and RH on pavement changed from positive correlation to negative correlation.LUE,WUE were significantly correlated with PPFD,Tair,VPD,RH and Tleaf in most cases(P<0.05).(3)In this study,three classical stomatal conductances models(BWB model,BBL model and USO model)and two improved models(BBL_D model and USO_D model)were evaluated for their ability to simulate stomatal conductances under extreme heat and drought in urban environments.Four main findings were made:a.In this study,the model fitting effect(determination coefficient,R2;root mean square error,RMSE)was similar to that of other non-urban plants,but there were significant differences in fitting parameters,indicating that these model structures had universality.In this study,the best fitting effect is BWB model,USO model and USO_D model.b.Aiming at the defects of BBL model and USO model,this study improved corresponding models to form BBL_D and USO_D.The VPD effect of BBL model was expressed as "VPD/Do ",and it was found that the variation of fitting parameter D0 was too large.After the VPD effect of BBL_D model wa changed to "VPDD0",D0 was more stable than that of BBL model,but the fitting effect of BBL_D model was not improved compared with BBL model.The VPD response of the USO model was " VPDgk",and the parameter gk was fixed at 0.5.Whether this treatment can accurately reflect the regulation of VPD on stomatal conductibility is worth exploring.In this study,gk was changed to a variable parameter based on USO model.The results showed that gk was greater than 0.5 in both pavement and non-pavement and in the early and late extreme heat and drought stages,indicating that USO model reduced the regulation effect of VPD on stomata.In addition,the fitting effect of USO_D model was improved compared with that of USO model.c.All stomatal conductance models have an important parameter,the stomatal conductance slope parameter g1,which is used to reflect the sensitivity of stomatal conductance to the environment.The results showed that g1 in the early stage of extreme heat and drought was generally higher than that in the late stage of extreme heat and drought,indicating that the sensitivity of vegetation stomatal conductance to environmental changes decreased due to heat and drought.The g1 of the non-pavement was generally higher than that of the pavement,indicating that the vegetation on the non-pavement had a stronger ability to adapt to heat and drought.d.Compared with the g1 values in typical terrestrial ecosystem models(such as SiB2,CLM4.5 and CABLE),it was found that g1 of TEMs was generally higher than that of this study,especially that of the SiB2 model or CLM4.5 model based on the BWB model,which was about 97 times higher than that of the B WB model in this study.Secondly,g1 of CABLE model based on BBL model was about 3 times higher than the average g1 of BBL model in this study.g1 of CABLE model based on USO model was about once higher than the average g1 of USO model in this study.If g1 is too large,the fitted Gs is too high,which cannot correctly simulate the regulation of stomata by changes in environmental conditions,and thus causes large errors in the estimation of vegetation transpiration,CO2 flux and energy flux by TEMs.Cities are the forerunners of future global changes and the "natural laboratories"for studying them.By observing the photosynthetic characteristics of Cinnamomum camphora(L.)in urban environment under natural extreme heat and drought conditions,this study analyzed the differences of microenvironment and photosynthetic characteristics of Cinnamomum camphora(L.)between pavement and non-pavement,clarified the response mechanism of plant photosynthetic characteristics to environmental factors,and explored the suitability of stomatal conductance model and terrestrial ecosystem model in urban environment and extreme heat and drought conditions.This study provides an empirical basis for understanding the effects of global climate change on vegetation growth in the future and provides data support and theoretical basis for the optimization of stomatal conductance model and terrestrial ecosystem model.