Effects Of Warming On Water Use,Morphological And Stoichiometric Characteristics Of Young Cunninghamia Lanceolata Trees

Plant water use efficiency（WUE）reflects the tradeoff between plant C capture and water consumption,which affects the capacity of plants adapting to environmental changes and then,ecosystem production.Globally,the ongoing warming caused decrease in soil water availability has been expected to aggravate water limitation in a series of terrestrial ecosystems,particularly in the subtropics in China where there has a seasonal-drought.The Chinese fir（Cunninghamia lanceolata）plantation in south of China has the largest area globally,and plays an important role in wood production and carbon sequestration in China.However,there remains unclear that to which extent and how global warming affects the WUE in Chinese fir plantation,which is of significance for the scientific prediction of the production and carbon sequestration capacity as well as for the adaptation management of Chinese fir plantation in the context of global warming.In the current study,therefore,we performed a factorial atmospheric and soil warming manipulated experiment with randomized block design in a Chinese fir plantation since2016 at the Chenda observation site of the Sanming Forest Ecosystem National Field Science Observation and Research Station in Fujian Province.This includes four treatments:control without any warming（CT）,soil warming by 4℃（SW）,atmospheric warming by open-top chambers（AW）,and soil plus atmospheric warming（SW+AW）,each with 4replicate plots.We aimed to investigate changes in water use(e.g.,stem sap flux density（SFD）and leafδ¹³C)and morphological and chemical traits of vegetative organs（i.e.,<2mm fine roots,branches,and leaves）of Chinese fir,and,to explore the regulation of adjustment of morphological and chemical（C,N,P concentration and stoichiometries）traits of vegetative organs on WUE.The main results are as follows:（1）There is a significant interaction between atmospheric warming and soil warming on annual SFD.Relative to the control,soil warming and atmospheric warming significantly decreased the annual SFD,whereas soil plus atmospheric warming significantly increased the annual SFD,especially in summer and autumn.The increased SFD might be attributed to an increased demand to cool down by increasing transpiration under high-temperature stress.Leafδ¹³C,indicating WUE,was just significantly reduced by atmospheric warming（AW and SW+AW treatments）,which was possibly due to the inhibited photosynthetic capacity by atmospheric warming.Regardless of warming method（SW or AW）,both the annual SFD and the leafδ¹³C were positively and significantly related to the photosynthetic active radiation（PAR）and the saturated water vapor pressure deficit（VPD）.Both atmospheric warming and soil warming inhibited the growth of DBH,indicating a negative effect of warming on the growth of Chinese fir regardless of warming method.（2）Soil warming significantly reduced the P concentration of fine roots and leaves,significantly decreased the N concentration of fine roots,whereas had no significant effect on the N concentration of branches and leaves.Atmospheric warming significantly reduced the P concentration of fine roots,branches,and leaves,especially the P concentration of the first-order fine roots,and the one-year old branches and leaves.However,it was not the case of N concentration.Soil plus atmospheric warming just significantly decreased the P concentration of leaves,but not the case of fine roots and branches.Additionally,soil plus atmospheric warming had no effect on the N concentration of fine roots,branches,and leaves.Relative to the control,both soil-and atmospheric warming increased the C:P and N:P ratios of fine roots and leaves,indicating an alleviated N limitation but an intensified P limitation.Possibly,it could be explained by that warming enhances nitrogen mineralization and subsequent plant N uptake but simultaneously decreases plant P uptake.（3）Relative to the control,all warming treatments significantly increased the specific root length（SRL）and reduced the root diameter（RD）of fine roots（especially the higher-order roots）.In addition,soil warming treatments（including AW and SW+AW）also increased specific root surface area（SRA）and decreased root tissue density（RTD）.These results suggest an increase in root nutrient and water uptake efficiency under warming.Atmospheric warming treatment（including AW and SW+AW）increased specific branch length（SBL）,suggesting an enhanced light capture capacity.All warming treatments reduced leaf thickness（LT）,which indicates a reduced construction cost of leaves.However,only the atmospheric warming treatment significantly reduced leaf area（LA）,indicating that Chinese fir in the AW treatment responded to the insufficient water supply by reducing the transpiration area.There were some correlations among morphological traits within or among organs,indicating that Chinese fir improves the capacity to absorb,store,and transport water and nutrients by coordinating and trade-offing different morphological traits within or among different organs under warming,and thereby,improves the adaptability to warming.（4）There was a marginal significant positive relationship between annual SFD and leafδ¹³C,indicating a faster adjustment rate of photosynthesis relative to transpiration of Chinese fir after warming.There was no significant correlation between the annual SFD and the morphological traits of all vegetative organs,and the leafδ¹³C was positively correlated with leaf area only,indicating that the adjustment of morphological traits of individual modules of vegetative organs might have a limited impact on water use of Chinese fir.However,the annual SFD,and especially the leafδ¹³C,correlated significantly to the P concentration of fine roots,branches,and leaves,indicating that P and its stoichiometries played an important role in regulating the water use of Chinese fir.Overall,relative to the control,soil and atmospheric warming decreased the annual SFD.While soil plus atmospheric warming significantly increased the annual SFD,it could just be a passive response of Chinese fir to high-temperature stress.Simultaneously,atmospheric warming decreased WUE,which,together,may be an important mechanism responsible for the decreased growth of Chinese fir after warming.Not the adjustment of morphological traits of vegetative organs,but the changes in P concentration and its stoichiometries of various vegetative organs,are of pivotal significance in affecting the WUE of Chinese fir after warming.Thereby,this study provides theoretical support for revealing the adaptation mechanism of Chinese fir plantation to global warming,and also forms a basis for in-depth understanding of the relationship between plant stoichiometry and hydraulic processes in the context of global warming.