Multi-scale Monitoring Of Drought Stress In Forests Based On SIF And Multi-Source Remote Sensing Indices

Forests are the mainstay of terrestrial ecosystems,playing an important role in maintaining the ecological environment,the global carbon cycle,and providing a range of ecosystem services.Drought stress is one of the most common climate disasters in China,characterized by frequent occurrence,wide ripple effects,and strong destructive power.The increasing frequency and extent of drought under the global warming trend has negatively impacted forest growth by affecting photosynthesis and respiration,causing harm to forest ecosystems.Effectively monitoring drought stress in forests can help improve fined forest management and promote sustainable forestry development.Traditionally widely used drought monitoring methods have limitations.They lack a direct relationship with the physiological state of plants.Solar-induced chlorophyll fluorescence(SIF)is a long-wave signal released by vegetation photosynthesis after light absorption,which can directly reflect the dynamic changes of actual photosynthesis in plants.In this paper,we construct a multi-scale drought monitoring method system based on SIF and other remote sensing indicators,and evaluate the drought stress monitoring capability of SIF and other indicators from leaf,single plant canopy and regional scales,mainly including research based on ground trial actual measurement data and satellite remote sensing data.The study at leaf and canopy scales was achieved by designing ground experiments.The responsiveness of SIF,normalized vegetation index,enhanced vegetation index,and leaf area index under different degrees of drought stress was analyzed by natural water depletion in Ginkgo and Poplar.First,at field water holding levels of 70-75%,50%-55%,25%-30%,and 10%-15%(corresponding to no drought,mild drought,moderate drought and severe drought conditions),SIF,vegetation reflectance spectral information,and LAI were collected by both leaf scale and canopy scale,respectively.Next,the SIF data were normalized using the acquired photosynthetically active radiation,and the vegetation index is calculated from the spectral information.Then,we quantified the decrease of each indicator under different stress levels and compared the ability of different indicators to analyze the response to drought.Finally,the SCOPE model was used to simulate the chlorophyll fluorescence in the canopy,which was used to verify the fluorescence results obtained from the actual measurements.The study at the regional scale was based on forest type areas in typical provinces in northern and southern China as the study area.SIF,canopy fluorescence yield(SIFyield)data,NDVI,EVI,LAI,and fraction of absorbed photosynthetically active radiation were analyzed for their responsiveness under drought stress.The temporal and spatial ranges of drought stress indicated by the Standardized Precipitation-Evapotranspiration Index(SPEI)values were used as a reference(SPEI <=-0.5 indicates the occurrence of drought).Firstly,the standardized anomalous values of SIF,SIFyield,NDVI,EVI,LAI,and f PAR were calculated.The temporal and spatial response abilities of each variable to drought stress were analyzed.Secondly,the correlation between each variable and the drought indicator SPEI was quantified.Finally,validation of the sensitivity of each indicator in regional spatial and temporal drought monitoring based on random forest classification model importance analysis.The research indicates:(1)Under the drought stress set up in the field experiment,both ginkgo and poplar SIF responses were timelier,with greater decreases than NDVI,EVI and LAI.Ginkgo,being a more drought tolerant species,did not experience significant degradation in its chlorophyll content during this drought treatment.Poplar showed significant degradation of chlorophyll,resulting in elevated spectral reflectance in the blue-violet and red-light absorption bands.The spectral reflectance of the water absorption bands of Ginkgo and Poplar leaves was elevated.The comparison of normalized SIF values revealed that SIF response timely under drought stress both ginkgo and poplar: the leaf of ginkgo SIF decreased by 8.05% under mild water stress and 36.10%under severe water stress;an individual of ginkgo SIF decreased by 62.7% under mild water stress and 71.79% under severe water stress;the leaf of poplar SIF decreased by 5.12% under mild water stress and 15.71% under severe water stress;an individual of poplar SIF decreased by16.70% under mild water stress and 53.55% under severe water stress.The vegetation index and leaf area index of ginkgo and poplar showed no significant decreasing trend in this drought treatment.They change more slowly and insignificantly in drought stress and are less responsive to drought stress than chlorophyll fluorescence.The simulation results of the SCOPE model showed that the SIF decreased gradually with the deepening of drought stress,which verified that the SIF decreased significantly with the deepening of drought stress in the actual measurement.(2)The analysis of satellite remote sensing data showed that SIFyield is more sensitive to drought in multiple forest types.At the regional scale,the results show that on a temporal scale,SIFyield showed an earlier response to drought stress than other variables and the abnormal change of SIFyield was higher than other variables by 10% or more.Spatially,the range of drought areas indicated by SIFyield and SPEI had more coincident areas than other variables.The overall correlation between SIFyield and SPEI was also higher during the drought period.In the cold temperate coniferous broad-leaved forest area of Heilongjiang,the maximum is 0.73.Additionally,the variable importance assessment using a random forest model also indicated that SIFyield is more sensitive to forest droughts.