Response Of Leaf Spectral Reflectance To Water Stress In Quercus Aliena Var. Acuteserrata, Baotianman

In recent years,the temporal and spatial distribution of regional precipitation as well as the rainfall frequency and intensity has been changed,as a result of the intensifying climate change.Thechange in precipitation pattern may lead to more frequent and severe regional drought which is an important factor of tree mortality and growth decline.To understand the influence of decreased precipitation to natural deciduous broad leaved tree species Quercus aliena var.Acuteserrata water status in warm temperate zone and its spectral reflectance response,in this study we used two methods including rapid dehydration test and forest in situ throughfall elimination to explore the spectral response mechanism to leaf water status change.The results are as following.(1)Relationships of leaf water status indicators and their sensitive spectral indicesAmong the different moisture indices,lower variability were found in RWC and EWT in different growth stages and canopy positions compared with that of SWC and LMP.RWC and EWT were more stable in representing whole water status.Moreover,RWC and EWT showed better spectral sensitivity than other water status indices,therefore they were suitable for large-scale remote sensing detection.Spectral reflectance difference analysis and spectral reflectance sensitivity analysis showed that the leaf spectral sensitivity is strongly influenced by growth stage;in short wave infrared region(SWIR),spectral reflectance of mature leaves in the initial stage of dehydration stress changed slightly,but new expended leaves showed rapid change during the dehydration process.Through the correlation analysis between 15 different spectral indices and moisture indices,we found the relationships of water index(WI)– RWC and double difference index(DDn(1530,525))– EWT were strong,but the WI – RWC relationship may likely influenced by leaf growth stage and spatial position,while the correlation of DDn(1530,525)– EWT was more stable.(2)The influence of throughfall elimination(TFE)to Quercus leaf water and spectral responsesQuercus leaf EWT showed a seasonal pattern of increased at first and then decreased in the growing season both in TFE and control plots.The intensity of thoughfall elimination has not changed the seasonal change pattern of leaf EWT.Within the growing season of Quercus,all spectral indices including water content reflectance index(WCRI),three band ratio index(Ratio1200),DDn(1530,525)and WI showed significant correlations with EWT,the correlations of EWT with WCRI,Ratio1200 and DDn(1530,525)were stronger than that of WI,and WCRI showed highest correlation coefficient.Leaf EWT showed significant differences(P<0.05)between the TFE and control plots in September 2014.The correlations between EWT and WCRI,Ratio1200 and DDn(1530,525)were significant(P<0.05),while the most significant correlation was between EWT and DDn(1530,525)(P<0.01).DDn(1530,525)is the most sensitive spectral index to water status change induced by thoughfall elimination in our study.(3)The influence of throughfall elimination to Quercus leaf pigment content and its spectral responseQuercus leaves chlorophyll a(Chl a),chlorophyll b(Chl b),total chlorophyll(Chl)and carotenoid(Car)content showed seasonal change patterns of rapid increasing during May-June,keeping high content level from June to September,decreasing rapidly from September to October;Car/Chl showed inverse seasonal change pattern in contrast to the pigments content;Chl a/b presented small fluctuation from May to September,and sharp increase during September-October.Leaf pigments content and pigments ratios showedslight differences between TFE and control plots in corresponding periodthe significatnt difference appeard in Chl b in July,September and October(P<0.05),this indicated that Chl b has the highest sensitivity to throughfall elimination;the difference of Car content is not significant,this indicated that Car was not sensitive toTFE treatment.The spectral reflectance around 550 nm was the most sensitive waveband to pigments concent,so we built sample ratio index(SR(750,550))on the basis of spectral reflectance at 550 nm.Strong positive relationships between SR(750,550)and Chl a、Chl b、Chl and Car content were found with high significant level(P<0.001).Similarly,significant negative relationships were also been found between photochemical reflectance index(PRI)and Car/Chl(P<0.001).Moreover,the spectral reflectance near 550 nm showed significant change of pigments induced by TFE treatment in our study.The SR(750,550)was also sensitive to leaf Chl a、Chl b and Chl content change induced by TFE treatment(P<0.01),while was not sensitive to changes of Chl a/b.PRI was sensitive to changes of leaves Car/Chl induced by throughfall elimination.