Spatial dependence and the relationship of soil organic carbon and soil moisture in the Luquillo Experimental Forest, Puerto Rico Hongqing Wang*, Charles A.S. Hall, Joseph D. Cornell and Myrna H.P. Hall Systems Ecology Laboratory, State University of New York, College of Environmental Science and Forestry, Syracuse, NY 13210, USA; *Author for correspondence (e-mail: hqwang@mailbox.syr.edu) Received 31 October 2000; accepted in revised form 27 August 2001 Key words: Elevation, Gradient, Luquillo Experimental Forest (LEF), Spatial correlation, Spatial variability, To- pography Abstract We used geo-spatial statistical techniques to examine the spatial variation and relationship of soil organic carbon (SOC) and soil moisture (SM) in the Luquillo Experimental Forest (LEF), Puerto Rico, in order to test the hypo- thesis that mountainous terrain introduces spatial autocorrelation and crosscorrelation in ecosystem and soil properties. Soil samples (n = 100) were collected from the LEF in the summer of 1998 and analyzed for SOC, SM, and bulk density (BD). A global positioning system was used to georeference the location of each sampling site. At each site, elevation, slope and aspect were recorded. We calculated the isotropic and anisotropic semi- variograms of soil and topographic properties, as well as the cross-variograms between SOC and SM, and be- tween SOC and elevation. Then we used four models (random, linear, spherical and wave/hole) to test the semi- variances of SOC, SM, BD, elevation, slope and aspect for spatial dependence. Our results indicate that all the studied properties except slope angle exhibit spatial dependence within the scale of sampling (200 – 1000 m sampling interval). The spatially structured variance (the variance due to the location of sampling sites) accounted for a large proportion of the sample variance for elevation (99%), BD (90%), SOC (68%), aspect (56%) and SM (44%). The ranges of spatial dependence (the distances within which parameters are spatially dependent) for aspect, SOC, elevation, SM, and BD were 9810 m, 3070 m, 1120 m, 930 m and 430 m, respectively. Cross correlograms indicate that SOC varies closely with elevation and SM depending on the distances between samples. The correlation can shift from positive to negative as the separation distance increases. Larger ranges of spatial dependence of SOC, aspect and elevation indicate that the distribution of SOC in the LEF is determined by a combination of biotic (e.g., litterfall) and abiotic factors (e.g., microclimate and topographic features) re- lated to elevation and aspect. This demonstrates the importance of both elevation and topographic gradients in controlling climate, vegetation distribution and soil properties as well as the associated biogeochemical processes in the LEF. Introduction Soil organic carbon (SOC) is an important source of nutrients and energy for above- and belowground bi- otic activities in terrestrial ecosystems (Detwiler 1986; Hall et al. 1992; Lugo and Brown 1993). There- fore, the spatial variation in SOC contributes greatly to the spatially heterogeneous distribution of flora and fauna (Silver et al. 1994; Scatena et al. 1996). Con- versely, the spatial distribution of SOC pools them- selves, as well as biological processes in ecosystems, depend upon spatial variation in abiotic factors such as solar radiation, soil temperature and soil moisture (SM) (Parton et al. 1987, 1988). Other studies have shown that the variability in topography, geomorphol- ogy, drainage characteristics, parent material and the nature of disturbances (e.g., land use changes, hurri- canes, landslides) and recovery across the landscape of an ecosystem also determine the spatial variation in SOC in the ecosystem (Silver et al. 1994; Scatena 671 Landscape Ecology 17: 671–684, 2002. © 2003 Kluwer Academic Publishers. Printed in the Netherlands.