马占相思林冠层水分利用效率的气候调节

Climatic Control of Water Use Efficiency of Acacia mangium Forest Canopy

由于受直射光、散射光或受遮阴叶层分布以及一天中光、温和水汽压亏缺变化的剧烈影响,植物冠层的水分利用效率(WUE)较为复杂.通过测定叶片的碳同位素比率(δ13Cp),估测叶片新固定碳同位素的比率(δ13Cplant)和冠层水平的碳同位素甄别率(⊿canopy),计算华南地区鹤山丘陵地马占相思林冠层WUE.结果显示:从冬季至夏季,最高气温(Tamax)和水汽压亏缺(Dmax)明显增高;冬季早上和傍晚的δ13Cplant最高,春夏季傍晚有较低的δ13Cplant,秋季的δ13Cplant日变动较小,冬季冠层有较其他季节高的δ13Cplant和较低的⊿canopy.由春季至冬季,δ13Cplant趋向于增高,而⊿canopy则趋向于下降.夏季马占相思林冠层平均(±SD)WUE[(2.01±0.38)mmol mol-1]明显低于冬季和春季值[分别为(6.90±0.26)、(5.65±0.14)mmol mol-1],年均WUE为(4.36±2.32)mmol mol-1.环境变化引起叶片胞间和空气CO2浓度的变化,从而改变⊿canopy和δ13Cplant,造成不同季节冠层WUE的差别.冠层WUE与水汽压亏缺(D1,kPa)、气温(Ta,℃)和叶片水势(Ψ,MPa)成显著负相关.利用δ13Cplant计算的WUE与根据Wang和Leuning的Ball-Woodrow-Berry模式求算的WUE契合较好.结果表明,以新固定碳的稳定同位素比率方法是研究环境胁迫对冠层WUE限制的有效方法.

Plant canopies are dramatically affected by leaves as sunlit fraction receiving direct light and shaded fraction only receiving diffuse light or being shaded and by variation of light,temperature and water vapor deficit in a day.When scaling up from leaves to canopy there are additional complications that affect the measurement of water use efficiency（WUE）.The carbon isotope ratio of leaves（δ13Cp） was seasonally measured,and the integrated canopy-level carbon isotope discrimination（⊿canopy） and the ratio of isotope of newly fixed carbon（δ13Cplant） were estimated for calculating the canopy WUE in an A.mangium plantation in the hilly land of Heshan in South China.A general increase was found in the maximum air temperature（Tamax）,atmosphere saturation deficit（Dmax） and leaf water potential（Ψ,MPa） from winter to summer on the site.The results showed the highest δ13Cplant values occurred in the morning and at the dusk in winter,while the lowest generally appeared at the dusk in spring and summer.No obvious variation of δ13Cplant occurred in autumn.A higher δ13Cplant and a lower ⊿canopy occurred in winter compared with those in other seasons.An increasing δ13Cplant trend and a decreasing ⊿canopy one from spring to autumn were observed.The average（±SD） WUE [（2.01±0.38） mmol mol-1] of the canopy of A.mangium occurred in summer,and it was much lower than that observed in winter and spring [（6.90±0.26） and（5.65±0.14） mmol mol-1]）.The difference in canopy WUE in different seasons at this site was primarily due to environment conditions,which would cause the change in integrated ratio of intercellular to ambient CO2 concentration and be consistent with the changes in ⊿canopy and δ13Cplant.There was a strong negative correlation of WUE with air temperature and water vapour pressure defi cit（D,kPa）,as well as with leaf water potential（Ψ,MPa）.An fair agreement between WUE measured using δ13Cplant and that using Ball Woodrow-Berry model following Wang and Leuning（1998） was observed.This result indicated that the stable isotope composition of newly-fi xed carbon was a useful canopy-scale tool that would help to study the constrain of environmental stress on canopy WUE .