GISdevelopment.net ---> AARS ---> ACRS 1992 ---> Poster Session Q A primary analysis of influence factors on the ground based spectral reflectance of grassland vegetation Wang yanrong, Yun Shipeng Inner Mongolia University Abstract It describes the ground based spectral reflectance of herbaceous vegetation and its soil-background on grassland in XiLin Gol, Nei Mon Gol, it analyses the influence of variations in the structure factors of community (such as life forms, water ecotype, yield etc), the soil surface humidity, the accumulation of dead biomass and the sun-view angle on the spectral reflectance of vegetation. The results indicates that : (1) the reflectance of different life forms and water ecotypes are significantly different,, especially in near-infrared bands, the reflectance between meadow and Com. Stipa grandis, Com. A neurolepodium Chinese and significantly different when the yelds of them are similar. (2) the sensitivitys of the reflectance in red bands are higher than in near infrared bands when the variations of the total yields are lower. (3) the reflectance in red bands in near-infrared bands will decline when the soil surface humidity increases and the influence of dead biomass on the spectral reflectance vegetation and soil-background will be significant when the dead biomass is more than 100 g m-2. (4) the most infavourable time for the spectral reflectance measurement is before 9 O’ clock a.m. and after 4 O’clock p.m and from 12 to 2 O’ clock in the afternoon. Spectral reflectance Influence factors. The near-ground spectral reflectance of vegetation is complex function of many factors. It is not only limited by the special way than plants, making up the community, reflect, absorb and transmit the radiation energy, but also is affected by the community structure, soil –back-ground and other non-vegetation factors, it is certainly necessary to understand, the influence factors in order to increase the accuracy of estimating the yield by using near-ground spectral reflectance of grassland vegetation. The near-ground spectral reflectance of vegetation and the influence factors were measured by two hand-held radiometers, one, named L83 – 10 types, made by Peking Teachers University, had ten bands and the spectrum is from 450nm to 900nm. The other, named RS – II type, is made by Peking University spectrally configured to simillate bands 1, 2, 3, and 4 of Landsat – 4 Thematic Mapper (TM)The chosen experiment site was at the natural pasture land in Xi Lin Gol Steppe Natural Protective Region. The field measurement period was from July to August 1986’ and 1987 which is the most vigorous stage for plants growing. The near-ground spectral reflectance of grassland vegetation There area receptively typical near-ground spectral reflectance for different natural grassland vegetation during the same period of time (fig. 1). There is a near-infrared reflectance peak and a lower valley in red band for the spectral reflectance of the grass ad mesic forbs meadows in the valley, The spectral reflectance character are almost similar for different normal typical steppe communities and the spectral reflectance increases obviously in the visible light bands for Artemisia frigida community, a degenerated typical steppe community. The spectral reflectance in visible light bands and in near-infrared bands is higher that typical steppe communities for desert-steppe vegetation. A primary analysis of influential factors on the ground based spectral reflectance for grassland vegetation. 1 The influence of life types and water ecotypes. The reflectance characters of different life forms and different water ecotypes are significantly different, especially in near-infrared bands. The order of reflectance in near-infrared is: Shrub>small half shrub > perennial Xerix plants>xeric – mesic plants mesad plants The different spectral reflectance for different communities is generated by the difference of the composition of the life forms and ecotypes, for example, the reflectance characters are about similar for Com. Stipa grandis and Com. Aneurolepidium Chinese, However compared with them the meadow’s reflectance is significantly different when the total yield of them are similar. The reflectance in near-infrared bands for meadow is 25 – 30% higher, compared with normal typical steppe vegetation. 2 the influence of yield. The sensitivities of reflectance in visible light bands are higher than in visible light bands are higher than in near-infrared bands for the variation of yield when the total yield is lower. For example, in meadow, the average variation is 9.06% in visible light bands and 4.21% in near-infrared bands only when the yield varies from 200gm-2 to 430gm-2. When the yield is higher than 4350 or 500gm-2, the reflectance tends to be saturate in visible light bands, and the average variation of reflectance is only 2.16%, but the sensitivities in near-infrared bands increase and the average variation is 8.30%. The sensitivity of reflectance in the band that the spectrum length is 550nm is the lowest, the variation of reflectance is about 3% when the yields vary on any graduation. 3 The influence of soil back-ground The measured spectral reflectance represents a complex mixture involving contributions from vegetation and soil-back ground. The soil-background is one of the important factor that affect the spectral reflectance of grassland vegetation. 3.1 The spectral reflectance of soil-background for different grassland vegetation. The spectral reflectance of soil-background is significantly different in the natural conditions for different vegetations. (Fig. 4). The spectral reflectance of soil-background is similar for the different normal communities of typical steppe and its degenerated community’s soil-background reflectance is obviously different from them. There is linear reflectance character for the soil background of desert-steppe communities and psammphytic vegetation. The reflectance in TM3 and TM4 bands of soil-background is arranged as the following : Meadow < typical steppe < desert-steppe or psammophytic vegetation The t-test indicates that the reflectance character of soil background is not significantly different between different normal typical steppe communities. The reflectance is significantly different in 0.01 level between typical steppe and desert-steppe or psammophytic vegetation communities, and 0.05 level, it is significantly different between desert-steppe and psammophytic vegetation. So it is necessary to build estimation models for different vegetation types in order to avoid more errors. For example, when the PVI greenness models of typical steppe communities is used to estimate the yield of meadow, the estimated value will be less than the real yield and when using it to estimate the yield of desert-steppe the estimated value will be more than the real yield. The opposite situation will take place when using the ratio greenness models of typical steppe to estimate the other vegetation types. 3.2 The influence of soil background surface humidity. The observations and correlation analysis for normal typical steppe communities indicate that (fig. 5) there is a significantly non-linear negative correlative relation between the reflectance in each bands of soil background and soil surface humidity. The reflectance variation ranges are all lower than 5% when the humidity fluctuates from 1.00% to 4.00%, when the humidities vary from 4.00% to 6.00% and from 6.00% to 10.00% the reflectance variations are from 5 to 10% and about 10% respectively. There is a correlative relation between ratio greenness index (G) and humidity. When using the ratio greenness models to estimate the yield, the increase in soil surface humidity will make the estimated value more than real value. The perpendicular greenness index is influenced by the humidity, too. When soil surface humidity increases, the angle between the soil-back –ground regression line and the axie made by reflectance in TM 3 band will become bigger and PVI will be lower. The estimated value will be lower than real yield. 3.3 The influence of the accumulation of dead biomass The observation for Com. Aneurolepidium Chinese and Com. Stipa grandis indicates that (fig. 6) there is a significant influence on soil-back ground reflectance when dead biomass is more than 100gm-2 the variation of reflectance in each band is more than 15%, when the dead biomass is lower than this, the influence will decline obviously. For example, the reflectance variation in each band of soil – background is only from 5% to 10% and the tendencies of reflectance variation are similar when the dead biomass varies from 70 to 100gm-2, and from 30 to 50gm-2 respectively. 2.4 The influence of sun – view angle. The observation of the spectral diurnal variation for Com. Aneurolepidium Chinese and Com. Stipa grandis indicates that (fig. 7) the diurnal fluctuation is minor and similar different near – infrared bands, the reflectance are higher in the morning and in the afternoon but lower at noon. The extreme different value of reflectance is 12% and 18% respectively. The reflectance in red bands is the most unstable, which appears a lower valley in the morning and in the afternoon respectively and a higher peak at noon. The extreme different value of reflectance in same band may be more than 40%, relating to the photosynthesis intercliel for communities. Conclusion and discussion It is necessary to build the respective estimation models of yield for different vegetation types because there are significantly different reflectance for vegetation if the specific composition and soil – background are not similar when the yields are about equal. The sensitivity of reflectance in red bands is higher for the variation is yield when the total yield is at a lower level. When the yield reaches a higher level, the reflectance tends to saturate and the reflectance in near infrared bands becomes a sensitivity Indies for the variation of yield. There is a significant non-linear negative correlative relation between the reflectance in each band and soil – background surface humidity. And its influence can’t be ignored when the humidity is more that 5%. The influence of dead biomass on vegetation and soil – back ground will be smaller when its accumulation is less than 100gm-2. The most infavourable for the spectral reflectance measurement is before 9 O’clock a.m. and after 4 O’ clock p.m. and from 12 to 2 O’clock in the afternoon for Com. A-neurolepidium Chinese and Com. Stipa grandis. References Wang Yanrong and Yun Shopeng, Plant Ecology and geobotany Journal, 1990; 14 (3); 258 ~ 266. Wang Yan rong and Yun Shipeng, Acta Scientiarum Naturalism Universitatis Intramongolicae, 1991 ; 22 (4); 545 ~ 550.