Measurement And Application Of Dynamics In Leaf Area Index Of Typical Shrubs And Grasses In A Semiarid Region

Leaf area index（LAI）quantifies the leaf area in an ecosystem and is a key vegetation parameter affecting processes such as photosynthesis,respiration,and precipitation interception.As a fundamental attribute of global vegetation,LAI has been classified as an important climate variable by the global climate change research community.In the past decades,scholars in China and abroad have extensively explored the measurement methods and application scope of LAI,but there is still a lack of in-depth understanding regarding the dynamics of LAI in shrubs and grasses in arid and semiarid regions,the assessment of LAI on climate change,and the application of LAI as a parameter in the modeling of key processes of water cycle.Thus,further research on accurate LAI dynamics measurements of shrubs and grasses,the application of LAI in assessing vegetation response to climate change,and as a parameter in rainfall interception models are important for predicting future dynamics of arid ecosystems,improving watershed management and supporting ecological services.In this sthdy,a typical semiarid watershed on the Loess Plateau,the Liudaogou watershed in northern Shaanxi Province,was used as the study area.The typical vegetation of Caragana korshinskii,Salix psammophila,alfalfa（Medicago sativa）and Stipa bungeana were selected as the study objects.The experiments related to leaf area index measurements and rainfall partitioning were conducted from 2016-2022.The main findings are as follows:（1）The seasonal dynamic of leaf area index and the woody-to-total area ratio of two deciduous shrub canopies in a semiarid loessial region were clarified.The results showed that the LAI-2200 data were consistent with the plant area index（PAI）estimated using upward-pointing webcams（UPWs）but not downward-pointing webcams（DPWs）.Shrub PAI derived from the DPWs was much smaller than PAI derived from the UPWs over the season.LAI derived from the direct methods was significantly linearly correlated with the PAI minus WAI obtained from upward digital hemispherical photography（P<0.05）.This result indicates that the upward digital hemispherical photography for deriving PAI and WAI performs good,which can be utilized to calculate LAI accurately.The woody-to-total area ratio（α）of deciduous shrub canopies was a dynamic parameter that varied throughout the season,decreasing as the canopy leaves developed and increasing when they fell.α=1 for the leaf-off season and<0.1 when the foliage peaked.αdiffered little between the two shrub canopies,and it could be used to convert PAI to LAI for C.korshinskii and S.psammophila.The UPWs demonstrated excellent potential for continuous LAI measurement for the two shrub canopies,but the DPWs should be further tested before use.The fisheye-webcam is inexpensive and can easily monitor LAI at field scale and can be used to evaluate LAI products derived from remote sensing.（2）The dynamic of LAI and its response to climate change in typical grasslands in semiarid regions were revealed.The results showed a tendency to become colder in spring and drier in summer and autumn during 2016-2021.In response,LAI tended to decrease in alfalfa and S.bungeana plots with interannual variation rates of 0.21 year^-1and 0.12 year^-1,respectively.Additionally,linear regression methods showed that precipitation had the greatest effect on LAI of two grasslands during the growing season（P<0.01）,followed by relative humidity and temperature（P<0.01）,a negative effect of wind speed（P<0.01）,and no significant effect of solar radiation on LAI were found.Conversely,the path analysis indicated that five climatic factors explained 65%and 87%of the monthly LAI variation in alfalfa and S.bungeana plots.Solar radiation and precipitation were the largest decision coefficients affecting the monthly LAI of alfalfa and S.bungeana,respectively,due to the different time-to-peak LAI and root growth depth of these two grass species.（3）The changes of rainfall partitioning and canopy interception modeling after progressive thinning in two shrub plantations on the Loess Plateau were elucidated.The results showed that under（moderate thinning）MT and（heavy thinning）HT,the throughfall（TF）rate increased by about 12%and 20%,respectively,compared to（no thinning）NT.The stemflow（SF）and observed interception（I）rates decreased by about26%and 33%,respectively,under MT,and the corresponding values for HT were about50%and 52%,respectively.The observed I rate decreased proportional to the percentage of biomass removed from the C.korshinskii and S.psammophila plots.The results also revealed a significant linear correlation between the plant area index（PAI）value and the canopy water balance of the two shrub plantations.The performance of the revised Gash model（i.e.,relative error[RE]<20%）was satisfactory according to the Nash-Sutcliffe model efficiency（NSE）coefficient（0.34-0.71）.Based on the RE values,the performance of the revised Gash model was better when applied to the plots subjected to MT and HT than NT for S.psammophila.Changes in the canopy storage capacity and canopy evaporation rate strongly affected changes in simulated interception loss.The model can facilitate water management in semiarid shrub plantations by accurately simulating the effect of thinning on interception loss.（4）The stemflow temporal dynamics between standing live and dead branches of two xerophytic shrub species were quantified.The results demonstrated that the stemflow（SF）variables and temporal dynamic processes differed between standing live and dead branches of the two shrubs.For C.korshinskii,the mean SF volume（SFV）and funneling ratio（FR）were 1.9 and 1.8 times higher for live branches than dead branches,respectively,whereas for S.psammophila,the mean SFV and FR were 1.3and 1.6 times higher for dead branches than live branches.These contradictory results can be explained by differences in the canopy structures of the species,such as the surface area that contributes to the generation of SF in the upper canopy,and the degree of branch bark decomposition and decay.In addition,the SF variables were significantly temporally dependent on the rainfall characteristics.SFV,SF intensity（SFI）and SF duration（SFD）were positively and linearly correlated with the rainfall amount,intensity and duration at the inter-event scale（P<0.001）.The influence of rainfall intensity on SF time lag to rain were not significant at the intra-event scale（P>0.05）.Considering the enrichment ratio due to canopy dry deposition and bark decomposition,the periodic flattening of dead S.psammophila branches to restore their growth is less significant than allowing them to die and renew naturally.In general,this study developed a new method for automatically and accurately measuring LAI dynamics,confirming the key contribution of LAI in assessing the impact of climate change on vegetation and as a canopy parameter applied to canopy interception models,and providing new ideas for vegetation restoration and management in semiarid areas.