Applied Vegetation Science 17 (2014) 53–62 Differences in spatial autocorrelation (SAc), plant species richness and diversity, and plant community composition in grazed and ungrazed grasslands along a moisture gradient, North Dakota, USA Lindsey M. Meyers, Edward S. DeKeyser & Jack E. Norland Keywords Dissimogram; Patchiness; Spatial structure; Disturbance; Invasive species; Prairie/ grassland Management Nomenclature http://plants.usda.gov/java/ Received 1 October 2012 Accepted 18 March 2013 Co-ordinating Editor: Peter Adler Meyers, L.M. (corresponding author, Lindsey.Meyers@ndsu.edu), DeKeyser, E.S. (Edward.DeKeyser@ndsu.edu) & Norland, J.E. (Jack.Norland@ndsu.edu): School of Natural Resource Sciences, North Dakota State Univeristy, Department 7680, PO Box 6050, Fargo, ND 58108, USA Abstract Questions: How are plant species richness and diversity related to spatial auto- correlation under grazed and ungrazed conditions in grassland communities? Is spatial autocorrelation dissimilar within different plant communities? Location: The grasslands of southeast North Dakota, USA. Methods: The study was conducted during the summer of 2009 in six grass- lands under different management (grazed vs ungrazed with fire) along a mois- ture gradient. Spatial autocorrelation at each site was measured along 50-m transects among three different landscape positions representing different mois- ture levels. Dissimogram multivariate analysis was used to measure the nugget, dissimilarity and patch diameter. Additionally, richness and Shannon diversity indices and nonmetric multidimensional scaling analysis were compared to the dissimogram results. Results: Mean patch diameter was significantly higher in grazed mid-slope sites than in ungrazed sites. Spatial autocorrelation was not different between the landscape positions (higher, dry areas were not different from lower, mesic areas). However, plant species richness, diversity and community composition were found to be different between landscape positions and management, with key differences in the presence of certain invasive species. Conclusions: Disturbance and moisture level were reflected differently in the spatial autocorrelation analysis compared to the richness, diversity and ordina- tion analyses. This indicates that disturbance and management impacted spatial autocorrelation separate from plant community composition and species diver- sity. This also highlights changes in the plant community caused by different management actions and the spread of invasive species. Introduction A pattern universally seen in plant ecology is that spatial autocorrelation (SAc) will decrease with increasing dis- tance (Nekola & White 1999; Mistral et al. 2000). This rela- tionship results in dissimilarity increasing within or between plant communities as the distance between sam- ples increases, and can be smoothly continuous (as a gradi- ent) or characterized by severe discontinuities (as distinct patches; Legendre 1993; Fortin 1999). This pattern is a result of both biotic and abiotic factors (Dray et al. 2012). Abiotic factors create spatial arrangements at differing scales due to soil type and moisture level, nutrient avail- ability and removal of dominant species through grazing (Collins 1992; Glenn et al. 1992; Fisk et al. 1998). Biotic factors affecting vegetative spatial structure may include competition, establishment, mortality, plant morphology and dispersal of plant species, along with stochasticity (Legendre 1993; Brownstein et al. 2012). Other factors may include pluvial and drought periods (Teague et al. 2004), fire, climate variability and herbivory (Olff & Ritchie 1998; Del eglise et al. 2011). Therefore, the spatial structure of vegetation varies spatially and temporally with spatial autocorrelation as a component of vegetative spatial structure (Cadenasso et al. 2006). Investigations into the factors that affect vegetative SAc, such as grazing, are still Applied Vegetation Science Doi: 10.1111/avsc.12040 © 2013 International Association for Vegetation Science 53