Carlo Ricotta, Giancarlo Avena and Fernando Ferril INTRODUCTION

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1 This file was created by scanning the printed publication. Errors identified by the software have been corrected; however, some errors may remain. The Influence of Vegetation Cover Density and Topographic Parameters on the Thermal Emission of the Beech Forests of Simbruini Mountains (Central Italy) Carlo Ricotta, Giancarlo Avena and Fernando Ferril Abstract. - Numerous studies have noted a strong negative correlation between radiometric surface temperature and spectral ratio-based vegetation indices. Since vegetation, through the process of transpiration, considerably enhances evaporative fluxes to the atmosphere, greater vegetation biomass should be associated with increased reduction in surface temperature. In irregular terrains with variable sun-sensor-surface geometry one of the major problems concerning the evaluation of surface temperatures of vegetation from remotely sensed data is that the thermal response of the earth surface is strongly influenced by topographic factors, such as altitude and insolation angle. In this paper, which uses remotely sensed inputs of surface temperature and vegetation fractional cover, the relations between the surface temperatures of the beech forests of Simbruini Mountains (Central Italy) and the main vegetation cover density and topographic parameters are discussed. According to the results obtained, 79.0% of the thermal response variance of the beech forests is due to variations of the topographic parameters. INTRODUCTION Thermal infrared measures of the Earth's surface from terrestrial and airborne sensors have been used since the 1960s to investigate energy budget conditions of vegetation. With the launch of Landsat 4 and 5 plant ecologists now have access to a wide-area synoptic and multitemporal thermal infrared band TM6 with a emote Sensing Laboratory, Department of Plant Biology, University of Rome "La Sapienza", Rome, Italy. 623

2 relatively high resolution (Lathrop e Lillesand 1987). A number of authors (Asrar et al. 1988, Smith and Choudhury 1990, Smith and Choudhury 1991, Hope and McDowell 1992) had focused on the widely observed negative correlation between radiometric surface temperature and remotely sensed measures of actively transpiring vegetation such as vegetation indices. One hypothesis to explain this observation is that variability in transpiring leaf area produces spatial variability in surface temperature by modulating evaporative cooling at the land-surface-atmosphere interface (Fried1 and Davis 1994). In mountainous regions, however, this relationship may be complicated by the effect of topography since a variation in altitude and insolation angle should also result in variated surface heating. The combined effect of topography and evapotranspiration in the energy exchange process at the surface produces a complex problem in the interpretation of observed thermal emissions (Conese et al. 1989). The present work aims at estimating by Landsat 5 TM data the variability of the beech forest surface temperatures located on the Simbruini Mountains (Central Italy) depending on the major remotely sensed vegetation cover density and topographic parameters, such as the Normalized Difference Vegetation Index (NDVI), altitude and insolation angle. STUDY AREA The beech forests of the Municipalities of Trevi nel Lazio and Filettino (Central Italy) were selected for analysis. There are dense 5600 ha forests located in the watershed of the Aniene River with elevational extent from about 900 to 1700 m. Due to environmental importance, the area was designated in 1983 as part of the Natural Park of Simbruini Mountains. The study area belongs to the Mountainous/Sub-alpine bio-climatic belt (Blasi 1994). The climate is strongly influenced by the position of the Simbruini Massive, which forms a transversal barrier to the western sea-winds. Meteorological data came from the station of the Ministero dei Lavori Pubblici located in Filettino (1062 metres above sea level.). The average annual temperature is 10.7 "C. The average annual rainfall is rnm with a principal maximum in winter and a secondary maximum during fall. The average rainfall in the quarter June-August is mm with a consequent lack of summer aridity. A short period of summer sub aridity is still possible during particularly dry years. The range of relief is approximately 1,600 metres. The maximum elevation above sea level is 2,156 metres (MSL). The topography of the area is complex with a variety of site exposures. The beech forests represent a nearly continuous formation with high cover density values due to the absence of intense grazing and cutting activities. The dominant tree species is Fagus sylvatica. More rare arboreal species are Acer pseudoplatanus, Taxus baccata, Tilia platiphyllos and Ulmus glabra.

3 MATERIALS AND METHODS The study area was extracted from a Landsat TM 190/3 1 I1 Q scene from 29 July Collateral information consisted of the vegetation map of the study area at 1:25,000 scale obtained through photointerpretation of infrared aerial photographs. Minimal and maximal air temperatures were respectively 12.8 "C and 26.8 OC at Filettino on 29 July In the month prior to satellite overpass there was 79.4 mm of rain. 17 July was the last rainy day prior to satellite overpass with 70.0 mm of rain. On 29 July 1993 the atmosphere was clear and any atmospheric attenuation was assessed to be systematic across the study area. The TM data were rectified to a Universal Transverse Mercator grid using a nearest neighbour resampling method. A digital terrain model (DTM) was created by acquiring on a digitizer the contour lines every 25 m of the national cartography mapping sheets at 1:25,000 scale of the study area. Relative sun incidence was calculated from the DTM. Relative sun incidence displays direct illumination which is equal to the cosine of the angle between the surface normal and the incident beam for each pixel. The solar azimuth and altitude angles at the time of satellite overpass (10:09 local time) necessary to calculate this illumination model were read from the header file of the computer compatible tape (CCT). In order to test the hypothesis that thermal emissions are related to the topographic parameters, such as altitude and insolation angle and the amount of vegetative cover, a systematic sampling (Congalton 1988) of 216 pixels was performed on the vegetated areas of the image. To avoid autocorrelation effects particularly on band TM 6 the starting point was located at random and each successive unit was taken at an interval of 10 lines and 10 columns thereafter. For each of those pixels the values of altitude (MSL), relative sun incidence (COS Z), NDVI (TM4-TM3/TM4+TM3) and surface temperatures were extracted from the data set. THERMAL DATA CALIBRATION The satellite-derived normalized difference vegetation index (NDVI) is used as the index for the vegetation density. The physical quantity that is measured remotely is the spectral radiance reaching the sensor [m~/cm2 sr pm] quantized in digital numbers (DN) over a 8-bit brightness range. Digital counts of the bands TM3, TM4 and TM6 were converted to spectral radiances according to the method proposed by Markham and Barker (1986). The improved darkest object subtraction technique described by Chavez (1988) was used to correct digital numbers from the red and near-infrared bands for differences in atmospheric scattering in these two bands. This simple correction was applied to minimize compression in the NDVI values which results from

4 unequal atmospheric effects on the red and near-infrared radiances (Hope and McDowell 1992). Spectral radiance L in the thermal-infrared band and surface temperature Ts are related in the form (Singh 1988): Note that the accuracy of Eq. 1 decreases with the increase of the range of temperature chosen. For this reason the parameters a and b were determined for temperature range of 40 OK only and are reported by Singh (1988). This technique does not account for atmospheric attenuation of the thermal radiance. However, although the 8-14 pm atmospheric window is far from being transparent, Landsat TM thermal IR data collected under clear atmospheric conditions can be correctly calibrated in surface temperatures with acceptable accuracy even when corrections for atmospheric effect are not applied. This is possible because under clear atmospheric conditions the amount of atmospheric path radiance is nearly equal to the amount of attenuated target radiance (Bartolucci et al. 1988). Moreover, since the objective of this study was to investigate the correlation coefficients of the relationships between Ts and the examined independent variables, a systematic offset in the values would not affect the interpretation of the results. The relationship between the surface temperature of the beech forests (Ts) and the three independent variables MSL, COS Z and NDVI were investigated. Linear correlation coefficients were calculated to quantify the magnitude of the hypotesized relationships, and scattergrams were constructed to indicate the nature of the relationships. RESULTS AND DISCUSSION Scattergrams depicting the relationship between surface temperatures of the beech forests and the independent variables are presented in figure 1 and 2, and the associated correlation coefficients are given in table 1. Table 1. - Correlation coefficients (R) and coefficients of determination (R~) for surface temperatures of vegetation versus values from MSL, COS Z and NDVI. *) Not significant at the level. MSL COS Z NDVI

5 These linear correlation coefficients show that at time of satellite overpass COS Z has the strongest association with surface beech forests temperatures. This fact can be explained with the obvious warming of vegetation surfaces caused after sunrise on illuminated slopes. It is very likely that at the time of satellite overpass (10:09 local time) the temperature difference between illuminated and shaded surfaces is near its daily maximum (Conese et al. 1989). On 29 July 1993, the relationship between MSL and the surface temperature of the beech forests was weaker with a negative correlation coefficient. Moreover, including MSL along with COS Z in the calculation of a bivariate correlation coefficient did explain 79% of the variance of the surface temperature of the beech forests (table 2). On the contrary, according to Smith and Choudhury (1991), the simple linear regression of the surface temperature of the beech forests with the corresponding remotely sensed vegetation index amount had no significant correlation coefficient. In fact, no strong inverse relationships between surface temperatures and vegetation cover density indexes are evident for homogeneous forests with complete ground cover. This relationships exist only for crop and pasture lands where fractional vegetation cover varies from bare soil trough to crops with complete ground cover giving a wide range of surface temperature and NDVI values. Table 2. - Bivariate and multivariate coefficients of determination (R~) for surface temperatures of vegetation versus values from MSL, COS Z and NDVI. All coefficients are significant at the level. MSL MSL MSL NDVI COS Z COS Z NDVI COS Z NDVI A week relationship between NDVI and surface temperature of the beech forests is still evident in combination with the topographic variables. The multiple correlation coefficient of the surface temperature of the beech forests with the three independent variables MSL, COS Z and NDVI increase their explained variance by less than 1% with respect to the bivariate correlation coefficient of the topographic variables alone with surface temperature. CONCLUSIONS This study has defined some of the problems encountered when attempting to use remote sensed data to estimate surface temperatures of vegetation in complex

6 vegetated mountainous regions. In irregular terrains the influence of topography can strongly affect the thermal levels of vegetation. The comprehension of the sources of this influence is thus determinant for a correct interpretation of remotely sensed thermal data. Results of the correlation analysis on 29 July 1993 in mid-summer sub-aridity conditions reveal that at time of satellite overpass COS Z alone accounted for 41.0% of the variance in surface temperature of the beech forests. This results indicate that insolation angle was, under the conditions of this experiment, a more important factor controlling surfaces temperatures of the beech forests than altitude or vegetation cover density. Moreover, the combined effect of differential heating and altitude explains 79.0% of the variance of surface temperature of the beech forests of Simbruini Mountains. On the contrary, the hypothesized negative relationship between NDVI and surface temperature is not significant due to the limited range of surface temperatures and fractional vegetation cover density values of the beech forests of the study area. However, remotely sensed data provide only an instantaneous assessment of thermal conditions and the use of remote sensed thermal data is just a first step to describe surface temperatures of vegetation. Future works towards accurately define thermal trends of vegetation, should necessarily include field measurements to convert instantaneous satellite-derived thermal data to daily predictions. Figure 1. - Scattergram of surface temperatures versus MSL 628

7 COS Z Figure 2. - Scattergram of surface temperatures versus COS Z REFERENCES Asrar, G., Harris, T.R., Lapitan, R.L. and Cooper, D.I Radiative surface temperature of the burned and unburned areas in a tallgrass prairie, Remote Sensing of Environment, 24/ Bartolucci, L.A., Chang, M., Anuta, P.E. and Graves, M.R Atmospheric effects on Landsat TM thermal IR data, IEEE Transactions on Geoscience and Remote Sensing, 26/ Blasi, C Fitoclimatologia del Lazio. Regione Lazio, Ufficio Parchi e Riserve Naturali, Roma. Chavez, P.S An improved dark-object subtraction technique for atmospheric scattering correction of multispectral data, Remote Sensing of Environment, Conese, C., Maselli, F. and Maracchi, G Influence of orography on the thermal response of coniferous forests measured by TM data. User contribution to satellite remote sensing programmes, Proceedings of the IX EARSEL Symposium held in Espoo, Finland, on 27 June 1989, pp Congalton, R.G A comparison of sampling schemes used in generating error matrices for assessing the accuracy of maps generated from remotely sensed data, Photogrammetric Engineering and Remote Sensing,

8 Friedl, M.A. and Davis, F.W Sources of variation in radiometric surface temperature over a tallgrass prairie, Remote sensing of Environment, Hope, AS. and McDowell, T.P The relationship between surface temperature and a spectral vegetation index of a tallgrass prairie: effects of burning and other landscape controls, International Journal of Remote Sensing, Lathrop, R.G. and Lillesand, T.M Calibration of Thematic Mapper thermal data for water surface temperature mapping: case study on the Great Lakes, Remote Sensing of Environment, Markaham, B.L. and Barker, J.L Landsat MSS and TM post-calibration dynamic ranges, exoatmospheric reflectances and at-satellite temperatures, Landsat Technical Notes, Singh, S.M Brightness temperature algorithms for Landsat Thematic Mapper data, Remote Sensing of Environment, Smith, R.C.G. and Choudhury, B.J On the correlation of indices of vegetation and surface temperature over south-eastern Australia, International Journal of Remote Sensing, Smith, R.C.G. and Choudhury, B.J Analysis of normalized difference and surface temperature observations over southeastern Australia, International Journal of Remote Sensing, BIOGRAPHICAL SKETCH Carlo Ricotta is a Ph.D. candidate in Botanical Sciences at the Department of Plant Biology of the University of Rome "La Sapienza", Italy, where he graduated in Geology in His research interests include the extraction of spatial information from remotely sensed imagery to study vegetation dynamic and landscape change. Giancarlo Avena is Full Professor of Phytogeography and Director of the Department of Plant Biology at the University of Rome "La Sapienza". He graduated in Geology from the University of Rome "La Sapienza" in 1962 and specialized in Physical Geography and Geobotany. His current research interests include the application of remote sensing and associate techniques for the study of vegetation. Fernando Ferri is technical co-ordinator of the Remote Sensing Laboratory of the Department of Plant Biology at the University of Rome "La Sapienza", where he graduated in Geology in Mr. Ferri provides technical consultation on GIs and remote sensing projects that affect vegetation mapping.