Ecological Modelling

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1 Ecological Modelling 222 (2011) Contents lists available at ScienceDirect Ecological Modelling journal homepage: Potential solar radiation pattern in relation to the monthly distribution of giant pandas in Foping Nature Reserve, China Xuehua Liu a,, Xiangnan Cheng a, Andrew K. Skidmore b a Department of Environmental Science and Engineering, Tsinghua University, Beijing , China b Department of Natural Resources, International Institute for Geo-Information Science and Earth Observation (ITC), University of Twente, P.O. Box 6, 7500 AA Enschede, The Netherlands article info abstract Article history: Received 5 November 2009 Received in revised form 20 August 2010 Accepted 15 October 2010 Available online 17 November 2010 Keywords: Solar radiation Foping Nature Reserve Qinling Mountains Movement distribution Giant pandas Geographical information system (GIS) Solar radiation is an important parameter in ecological process modeling, hydrological modeling and biophysical modeling. However, models focusing on solar radiation in relation to giant panda habitat and seasonal distribution are limited. The research aims to form spatial models of 12 month solar radiation patterns and to investigate the relation between the solar radiation patterns and the monthly distribution patterns of giant pandas. The solar radiation model of Kumar et al. was adopted for this study in Foping Nature Reserve (NR), China. By comparing twelve monthly solar radiation patterns and calculating statistics such as maximum, minimum, mean and standard deviation of the solar radiation, diversified solar radiation patterns over different months were obtained. Maximum solar radiation occurred in June and July, while minimum solar radiation occurred in December and January. The annual sum of solar radiation was 6954 MJ/m 2 in Foping NR. The range in solar radiation was smaller in hot months and larger in cold months. Radio tracking data of giant pandas were collected for twelve months and the ensuing maps were overlaid with the twelve-month solar radiation map to analyze the relation between the giant panda s monthly distribution and solar radiation. Our results showed that giant pandas prefer areas with lower solar radiation in warm months and select areas with higher solar radiation in cold months, which illustrates that the distribution of giant pandas is indeed affected by solar radiation. To a certain degree, it also explains the behavior of seasonal movement by giant pandas in Foping NR Elsevier B.V. All rights reserved. 1. Introduction Solar radiation is the principle energy source for physical, chemical and biological processes on the earth s surface and drives processes as diverse as snow melting (Li et al., 2009), evaportranspiration (Bois et al., 2008), and crop growth (Garcia et al., 2008; Bansouleh et al., 2009). It also provides the energy for soil heat flux, soil temperature, surface and air temperature, water loss through evaporation and transpiration, and plant and animal activity. In other words, solar energy is a key explanatory variable in ecological process modeling, hydrological modeling and bio-physical modeling (He et al., 2003; Franklin, 2010). The amount of solar energy reaching the earth s surface is mainly affected by solar geometry, as well as geomorphologic and climatic factors. Solar geometry and geomorphologic factors control the radiation characteristics of terrain, particularly slope gradient and slope aspect (Kumar et al., 1997). Climatic factors influencing solar radiation include cloud and other atmospheric conditions such as Corresponding author. Tel.: ; fax: address: xuehua-hjx@mail.tsinghua.edu.cn (X. Liu). dust and aerosols, and these effects can be locally significant (Jia, 1997). Recently, scholars around the world have been attempting to study solar radiation in their target areas as an important variable in modeling. In forest research, Kumar and Skidmore (2000) explored radiation vegetation relationships in Australia for twelve species of eucalyptus forest and found that the mean radiation values differed significantly between species. In plant ecology, Shi et al. (2007) researched utilization and dissipation of solar radiation in two alpine plants in alpine mountains in China. In geography, Fan et al. (2003) analyzed solar radiation of northeast China using DEM, slope, aspect and weather station data. In agriculture, Yi and Yang (2001) and Li (1999) related solar radiation to crop physiology. In glacial research, Ding et al. (1998) made a theoretical calculation of solar radiation and used this as a basis to build models of the mass of glaciers. Research focusing on solar radiation in relation to animal ecology, and specifically giant panda habitat and seasonal distribution, is limited. Zeng et al. (2010) published research on golden takin (Budorcas taxicolor bedfordi) seasonal migration patterns in relation to solar radiation. Their results show that golden takin preferred intermediate elevation areas on exposed southern slopes correlated closely with higher solar radiation /$ see front matter 2010 Elsevier B.V. All rights reserved. doi: /j.ecolmodel

2 646 X. Liu et al. / Ecological Modelling 222 (2011) The giant panda (Ailuropoda melanoleuca) is an endangered species with a natural distribution limited to southwestern China. The southern slopes of the Qinling Mountains with their large tracts of suitable habitat form a major distribution area for giant pandas. According to the Third National Survey on Giant Panda Population and Habitat implemented during 2000 and 2002, the number of giant pandas reached 91 individuals in Foping County, the study area used in this research (SFA, 2006). In Foping County, the average annual number of solar hours is more than 1727 (Liu and Zhang, 2003). The giant panda population in Foping NR moves in a regular seasonal pattern (Liu et al., 2002). The area between the elevation of 1400 and 2700 m harbors the highest density of giant pandas. The animals ascend in May from their autumn winter spring habitat at low elevation to their summer habitat at high elevation, and descend again in September and October. One reason for this regular, seasonal migration could be that the giant pandas migrate to find new bamboo shoots, rich in nutrients, palatable and easy to digest (Schaller et al., 1985; Pan et al., 1988; Yong et al., 1994; Wei et al., 1996). A second reason could be that giant pandas prefer habitat with cool temperatures and a wet climate (Reid et al., 1991; Taylor and Qin, 1989). This study aims to (1) analyze the spatial patterns in solar radiation over 12 months and search for a spatial relationship with the monthly distribution of giant pandas in Foping NR; (2) compare the differences in monthly solar radiation between Foping NR and other regions with giant pandas; and (3) compare the differences in solar radiation at giant panda locations between warm months and cold months. Findings may provide an insight in the habitat selection and migration behavior of the giant panda in the Qinling Mountains. 2. Methods 2.1. Study area Foping NR is situated in the northwest corner of Foping County, in the southern part of Shaanxi Province (Fig. 1). It covers part of the middle section of the southern slopes in the Qinling Mountains. Its geo-location is N, E and it covers an area of 293 km 2. The Qinling Mountains with their east west orientation play a very important role as natural geographical barrier. The Qinling Mountains stop cold air moving from north to south while receiving warm southerly air, thus creating refuges. These areas represent the most northerly refuges for many species, including the giant panda. Foping NR was established in 1978 with the aim to protect the giant panda and its forest habitat. The terrain of Foping NR descends from 2904 m in the northwest region to 980 m in the south-east region. The area below 1500 m consists of steep slopes and narrow valleys of the middle mountains, and has an active human population. Between 1500 and 2000 m, the terrain changes to gently sloping, wide valleys and flat mountain ridges; and the area above 2000 m comprises steep slopes and broad mountaintops (Ren et al., 1998). Foping NR is located on the southern slopes of the Qinling Mountains and has a subtropical monsoon, humid climate, with a growing period for plants of days (Pan et al., 2001). Foping NR is covered by well developed vegetation (Ren et al., 1998), with two major bamboo species growing in the understory, namely Bashania fargesii and Fargesia qinlingensis, which are both preferred by the giant panda (Pan et al., 1988; Tian, 1989, 1990; Yong et al., 1994) Data preparation The data in this research included (i) a DEM (1:50 000) of Foping NR plus a 2 km wide region surrounding the NR, since some radiotracking locations were situated outside the reserve boundary; (ii) a boundary map of the research area; and (iii) radio-tracking data of two giant pandas collared in Foping NR during These valuable tracking data were provided by Foping NR. As an endangered animal species, the giant panda is generally not allowed to be collared. Permission to conduct radio collared, behavioral research into giant pandas was only granted for six pandas in Foping NR between 1991 and 1995, six pandas in Wolong NR between 1981 and 1983, and three pandas in Tangjiahe NR between 1984 and Since then, no panda has been collar tracked until Based on the radio collar data for Foping NR, Liu (2001) and Liu et al. (2002) published their findings on migration patterns and habitat selection of giant pandas. From the six pandas collared in Foping NR, only two adult pandas with four years or more of tracking data were selected for this research. The other four pandas yielded less than two years of tracking data or were cubs. In total, 465 tracking records were obtained for Panda127, covering 34 months and collected over 5 years (Liu et al., 2002). From Panda45, 400 tracking Adopted from Liu, Fig. 1. The geo-location of Foping Nature Reserve, Shaanxi, China.

3 X. Liu et al. / Ecological Modelling 222 (2011) records were used, covering 29 months, collected over 4 years (Liu et al., 2002). More information about the radio tracking data and their analysis can be found in Liu (2001) and Liu et al. (2002). All spatial data were geo-referenced with the same Universal Transverse Mercator (UTM) coordinator system Deriving potential solar radiation surfaces in Foping NR Spatial models based on 12 months of solar radiation data were created for Foping NR. Using ARCINFO software (ESRI, 1991), the SHORTWAVE2.AML program provided by Kumar et al. (1997) was executed to calculate solar radiation, and spatial patterns were shown. We then analyzed the statistical characteristics of the solar radiation. The solar radiation model provided by Kumar et al. (1997) computes potential solar radiation (the amount of shortwave radiation received under clear-sky conditions) over large areas using only digital elevation and latitude data. The model comprises three main components viz.: (1) the direction of solar radiation on a tilted surface (I p ), (2) the diffuse solar radiation (I d ) and (3) the reflected radiation (I r ). The potential solar radiation for any site is the sum of I p, I d and I r. I p = I s cos i I d = I o d cos 2 ˇ 2sin I r = ri o r cos 2ˇ 2sin where I s is the shortwave solar radiation striking a surface normal to the sun s rays, i is the angle between the normal to the surface and the direction to the sun, I o is the extraterrestrial intensity of beam radiation, d is the radiation diffusion coefficient, is the solar altitude angle, ˇ is the tilt angle of the surface (slope), r is the ground reflectance coefficient and r is the reflectance transmissivity. In this study, the parameters used for calculating solar radiation were (1) the longitude and latitude of the centre of Foping NR, i.e N, E, (2) the first and last days of each month according to the Julian Calendar, and (3) a time interval of 30 min, which is the interval between two consecutive calculations and may be set to a larger value in a region with flat terrain and smaller value in a mountainous region with changing shadow. Theoretically, the smaller the interval is, the more accurate the calculated result. However, smaller intervals results an increase in computation. The solar radiation model for each of the twelve months was expressed in the same color scale (i.e. 0 MJ/m 2 month to 900 MJ/m 2 month). Consequently, differences in solar radiation pattern were shown and compared, and the maximum, minimum, mean and standard deviations for each month were also calculated Analyzing the relationship between giant panda distribution and solar radiation patterns The radio-tracking location data for the giant pandas were mapped for a twelve month period and overlaid with the solar radiation maps for each month to ascertain possible spatial relationships. Thus, the distribution of giant panda locations could be displayed in relation to solar radiation, indicating the giant panda s preferences. To clearly show the pattern relating the giant panda s monthly location to solar radiation, the areas containing most radio-tracking locations over the twelve months were enlarged for visual analysis. The maximum, minimum, mean and standard deviation of the solar radiation was calculated for each radio-tracked location, for each month. Further, the proportion of giant panda locations to solar radiation for two coldest months (January and December) and two warmest months (June and July) were calculated to show the animals preference for certain solar radiation. 3. Results 3.1. Different patterns in monthly solar radiation in Foping NR Fig. 2 shows the different spatial patterns of solar radiation over twelve months in Foping NR, as well as the giant panda locations in those months. Fig. 3A shows the statistical curves of the mean, maximum, minimum and standard deviation of the solar radiation values over twelve months for all areas in Foping NR. Solar radiation reaches a maximum in July and a minimum in December. The solar radiation patterns for Foping NR for two coldest months of January and December show that Foping NR receives low average monthly solar radiation from 270 to 320 MJ/m 2 approximately, while the received average monthly solar radiation for two warmest months of June and July is between 820 and 840 MJ/m 2. The differences in solar radiation between each monthly maximum and minimum are larger in the cold months than in the warm months. The sum of all solar radiation for the twelve months was 6954 MJ/m 2 for Foping NR Relationship between solar radiation and giant panda distribution The giant panda radio-tracking location data covering twelve months were overlaid on solar radiation maps of Foping NR in Fig. 2 to show their spatial relationship. The spatial patterns of the radiotracked locations reveal that the migration of the giant pandas from the valleys to the mountain tops correlated with a change in the solar radiation from low to high, and that they move in the reverse direction when the solar radiation shifts from high to low. Areas with a high density of giant panda locations were enlarged for the twelve monthly maps (Fig. 4), in order to highlight the relationship between giant panda locations and solar radiation values. As can be seen, during June and July most radio-tracking locations of giant pandas are situated in areas receiving the least solar radiation. In all remaining months, the giant pandas select sites with more solar radiation. The solar radiation pattern over twelve months with minimum, maximum, mean, and standard deviation values for the tracking locations of giant pandas is depicted in Fig. 3B. It shows that the monthly pattern of solar radiation over the course of a year for the tracking locations is similar to that for Foping NR as a whole. Fig. 5 depicts the difference in solar radiation between the tracking locations and Foping NR as a whole. The maximum solar radiation of the tracking locations is very similar to, but slightly lower than that of Foping NR, while the minimum solar radiation of the tracking locations is much higher than that of Foping NR. This indicates that the giant pandas prefer areas with higher minimum solar radiation. Consequently, the mean solar radiation of tracking locations is higher than that of Foping NR for most months of the year except for the summer months of June and July (Fig. 5), implying that giant pandas prefer to use higher-radiation areas during most of a year, and only use lower radiation areas in June and July. In cold months the difference between the solar radiation mean for giant panda locations and for Foping NR is about 80 MJ/m 2. Fig. 6 depicts the percentage of giant panda locations for two winter and two summer months, indicating panda preference for habitat with certain levels of solar radiation. In the two winter months (January and December) all tracking locations of the

4 648 X. Liu et al. / Ecological Modelling 222 (2011) Fig. 2. Maps depicting the spatial pattern of solar radiation over twelve months in Foping Nature Reserve, Shaanxi, China. Each map has been overlaid with giant panda locations. giant pandas were located in areas with a solar radiation range between 57 and 536 MJ/m 2 month, and about 68% of the locations had a higher solar radiation than 291 MJ/m 2 month (the average radiation for this two cold months). In the two summer months (June and July) all tracking locations were found in areas with a radiation range between 752 and 893 MJ/m 2 month, and about 68% of the locations had a radiation lower than 832 MJ/m 2 month (the average radiation for the this warm months). This indicates that giant pandas prefer high radiation areas in the cold months, and that they select low and median radiation areas in the warm months. 4. Discussion 4.1. New application of a solar radiation model in a giant panda nature reserve This research demonstrates a new application for solar radiation models. Kumar et al. (1997) stated that the integration of solar radiation over long periods of time for large areas can make the solar radiation model useful in a variety of areas such as plant growth, species location, water balance studies, biodiversity and identification of possible flora and fauna sites. This research has aimed to

5 X. Liu et al. / Ecological Modelling 222 (2011) Solar radiation (MJ/m 2.month) Solar radiation (MJ/m 2.month) Fig. 3. Statistical features of 12 months of solar radiation for all areas (A) and for all tracking locations of giant pandas (B) in Foping Nature Reserve, Shaanxi, China. Fig. 4. Enlarged maps with solar radiation patterns overlaid with giant panda tracking locations for twelve separate months in Foping Nature Reserve, Shaanxi, China.

6 650 X. Liu et al. / Ecological Modelling 222 (2011) Solar radiation (MJ/m 2.month) Solar radiation (MJ/m 2.month) Solar radiation (MJ/m 2.month) Fig. 5. Comparison between monthly solar radiation maximums, minimums and means for all radio-tracking locations and those for Foping Nature Reserve, Shaanxi, China. study spatial patterns of solar radiation in an area frequented by giant pandas and the relationship between solar radiation and the animal s monthly distribution patterns. Research results indicate that solar radiation may indeed be one of the environmental factors influencing the giant pandas migration as well as their spatialtemporal distribution in an area. Past applications of the solar radiation model were on dry eucalypt (silvertop ash-stringybark) forests by Bridges and Dobbyns (1991) and Kumar and Skidmore (2000), but very little research exits on solar radiation in relation to animal species. Zeng et al. (2010) recently published research on solar radiation and golden takin, and this study represents the first research on giant pandas and their habitat in relation to solar radiation. The modeling results showed solar radiation patterns for twelve different months, with the warm months of the year receiving the highest quantities of solar radiation within a relatively small range, and the cold months receiving little solar radiation, but within a larger range Linking solar radiation patterns to giant panda monthly distribution patterns Overlaying giant panda radio-tracking locations on solar radiation maps for twelve different months shows that giant pandas migrate from the valleys to the mountain tops when the solar radiation changes from low to high and move back when the radiation shifts from high to low. According to Liu et al. (2002), the giant pandas in Foping NR had an obvious seasonal pattern of migration, rapidly ascending in May from low elevation habitats with bamboo Bashaina fargesii to high elevation habitats with bamboo F. qinlingensis (original Latin name: Fargesia spathacea), and then descending slowly from high to low elevation areas in September and October. Three reasons have been hypothesised for this behavior (Pan et al., 1988): (i) temperature, (ii) bamboo-shoots, and (iii) habit. Temperature is a measurement of the heat reflectance from ground to air. The giant panda is often described as an animal species living in a cool and moist environment. As the temperature changes during the year due to the level of solar radiation, the giant pandas in Foping NR migrate between the cool and the warmer habitats. Two bamboo species occur at different elevations and sprout at different times. The giant pandas can avoid the rising temperatures, and at the same time feed for longer periods on the palatable bamboo shoots, which contain more nutrients and water, if they migrate to high elevation areas. Gradually, the offspring of giant pandas will learn to maintain this habit. This study supports the hypothesis that the giant pandas migration behavior in Foping NR is related to solar radiation and temperature Impact of minimum solar radiation on habitat selection by giant pandas An interesting result obtained in this study is that the minimum level of solar radiation for the tracking locations is much higher than the minimum level for Foping NR, which indicates that the minimum level of solar radiation is important to giant pandas when selecting a habitat. It is well known that for many species a minimum temperature forms the limiting factor for their distribution, especially for plant species (Sakai and Weiser, 1973; Woodward, 1987; Fang and Yoda, 1991). It is the first time that the importance of temperature-related solar radiation has been identified in giant panda research Accuracy of solar radiation calculations In this study, the solar radiation calculation model was based on a digital elevation model (DEM), and the model accuracy greatly

7 X. Liu et al. / Ecological Modelling 222 (2011) radiation value is approximately 300 MJ/m 2 month with a high variance. Further analysis of the tracking locations of giant pandas revealed the relationship between the giant panda distribution and the solar radiation distribution. A statistical analysis of the tracking locations and the relevant solar radiation values concluded that the giant pandas preferred habitat with high radiation most of the year, especially in the cold months, and selected areas with lower radiation during the summer months (June and July). Overall, giant pandas preferred areas that have a higher minimum solar radiation than the average minimum for Foping NR. These results add to the research field of the giant panda, its habitat and conservation. Acknowledgements Average radiation: 832 MJ/m 2.month Sincere thanks to Dr. Lalit Kumar for providing the program code to run the solar radiation calculations. Thanks are also due to Mr. Zhang Shuang for his GIS support. This work was partially supported by the SFA-CWCA s International Cooperation Project for Giant Panda Conservation (No. WH0633). References Fig. 6. Percentage of tracking locations of giant pandas versus range in solar radiation in two winter (A) and two summer (B) months for Foping Nature Reserve, Shaanxi, China. depends on the accuracy of the DEM used (Skidmore, 1989a,b; Schmidt et al., 2004). Errors in the DEM could lead to error creation when computing aspect and slope and therefore have a direct effect on the calculation of solar radiation (Skidmore, 1990; Kumar et al., 1997). In addition, incoming radiation is reduced with a decrease in atmospheric transparency, and the decrease in incoming solar radiation becomes more pronounced the higher the latitude (Kondratyev, 1969; Kumar et al., 1997; Satterlund and Means, 1978). For better modeling, it is necessary to obtain temperature data from ground measurements to check the solar radiation calculations. However, such data were lacking in this research. Nevertheless, it was valuable to look at the spatial-temporal patterns. According to Liu (2001), climate data analysis showed that the highest temperatures in Foping NR occurred in July and August, which is different to the solar radiation pattern showing the highest values to occur in June and July. Further research will be needed on the relationship between solar radiation and air temperature in Foping NR. 5. Conclusions This study applied the solar radiation model by Kumar et al. (1997) to Foping NR and created solar radiation distribution maps over twelve months based on GIS support and a DEM of Foping NR. Through analysis of relevant statistical parameters, the temporal and spatial variations in solar radiation were accurately displayed for Foping NR. 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