Effects Of Warming On Photosynthetic And Water Physiology Of Subtropical Cunninghamia Lanceolata

There is no doubt that global precipitation and temperature patterns will change as a result of climate warming,which will affect forest ecosystems.Trees have the ability to regulate their physiology in response to climate change,in which photosynthetic and hydraulic physiology are the most important physiological processes.The subtropical zone is the transition zone between the tropics and the temperate zone,and trees of this area may be more sensitive to climate warming.Over the past 20 years,simulated warming experiments have been conducted mostly at middle and high latitudes,and there are still fewer global warming simulation experiments at low latitudes.Therefore,it is necessary to conduct large-scale field warming experiments at low latitudes to predict the effects of climate warming on tree growth and adaptation.Cunninghamia lanceolata is an important fast-growing silvicultural and timber tree species in China.Meanwhile,different seasons in the subtropics have different hydrothermal conditions,which may exacerbate the effects of climate warming.Therefore,the study of photosynthetic and hydraulic physiology of fir in different seasons under warming scenario needs to be addressed urgently.This study was conducted in the context of global warming,relying on the Fujian Sanming Forest Ecosystem National Field Scientific Observation and Research Station,and using soil cable warming to simulate climate warming.The effects of warming on photosynthetic physiology and hydraulic physiology of Cunninghamia lanceolata in different seasons were studied after six years of warming.The results are as follows:1)The maximum net photosynthetic rate（Pn）and stomatal conductance（Gs）of the leaves in spring were significantly higher in the warming treatment than in the control.However,leaf nutrients（C,N,and P）,chlorophyll content（Chl a,Chl b,and TChl）,and Rubisco enzyme content were not significantly different from the control.Correlation analysis showed that Pn was highly significantly and positively correlated with Gs,so the increase in photosynthetic rate in the warming treatment might be mainly due to stomatal factors.There was no significant difference between Pn and Gs of the warming treatment and the control in autumn.2)Between spring and autumn,the warming treatments differed from the control in response pattern.The warming treatment had higher specific leaf area（SLA）,thinner leaves,higher leaf area per unit mass,and higher photosynthetic nitrogen use efficiency（PNUE）as well as phosphorus use efficiency（PPUE）in spring;Whereas,in autumn leaf SLA decreased,and investments in photosynthesis reduced with the decrease of PPNE and PPUE.The control,on the other hand,had higher PNUE,PPUE and SLA in autumn than in spring,and leaf Pn increased significantly.3)The specific xylem hydraulic conductivity（K_S）and specific leaf area hydraulic conductivity（K_L）of the branches in warming fir trees were not significantly different from that of the control,nor were they significantly different in seasons;in autumn,the specific xylem leaf area(A_xylem/A_leaf)of the warming treatment increased significantly compared with the control,probably due to the lower soil water content in autumn in the warming treatment.In order to avoid hydraulic failure caused by leaf transpiration,Cunninghamia lanceolata reduced their investment in leaf area and invested more in xylem to ensure water supply and improve hydraulic security.4)There was no significant difference in tracheid average diameter（D）,tracheid density（TD）,total tracheid area(A_tracheid)and total tracheid/xylem area(A_tracheid/A_xylem)between warming treatment and control.However,the distribution of tracheids in different diameter classes is different.The number of tracheids with diameter above 19um and below9um in warming treatment is higher than that in the control,which may be due to the development of larger diameter tracheids in the seasons with the most appropriate environmental conditions to promote hydraulic efficiency,and the increase of small diameter tracheids in the season with unfavorable environmental conditions to ensure hydraulic safety.5)The stomatal size（S_s）of Cunninghamia lanceolata leaves was significantly reduced by the warming treatment,but stomatal density（S_d）was significantly increased in spring.Between spring and autumn,S_s of the leaves increased significantly in the warming treatment.6)The malondialdehyde（MDA）content of control was significantly higher than that of the warming treatment in spring,and the total antioxidant capacity（T-AOC）was also slightly higher than that of the warming treatment,indicating that the leaf damage of control trees was greater than that of warming trees in the spring,probably because of the poorer soil aeration in the control due to more rainfall in spring,which had some effect on metabolism.There were no significant differences between MDA and T-AOC in the warming treatment and control in autumn.Seasonally,both T-AOC and MDA were higher in autumn than in spring for warming treatment,indicating that the warming trees were was under unfavorable conditions in autumn.The activities of several antioxidant enzymes differed between treatments and seasons,suggesting different regulatory mechanisms.