Spatial autocorrelation in plant communities: vegetation texture versus species composition

Vegetation texture, describing plant communities by the range of characters present in them, can describe functional variation more effectively than using the identities of the species. We ask whether spatial trends, as seen in spatial autocorrelation (SAc), are stronger in vegetation texture. We also ask in what environment SAc is stronger, whether local or over longer distances, and with what measure of distance. Roadside vegetation was sampled across an orographic region of the South Island, New Zealand, from a high-rainfall zone up to just above treeline (the "Wet" side) and then down into a rain-shadow area (the "Dry" side). Species composition was recorded in quadrats placed in 30 clusters. Five functional plant characters were measured on all species encountered: characters that have been reported in the literature to correlate with light-capture, heat budget and nutrient strategy. SAc was calculated using a saturation-response type of relation in species composition, and also in texture using the five functional characters. There was significant species-composition SAc in almost all comparisons, but the maximum percentage of variation explained was 25%. The fit was almost always higher using local, short-distance, comparisons than using comparisons over a whole side (Wet or Dry). On average dissimilarity had reached 90% of its asymptote after ca 250 m. It is concluded that the SAc was mainly due to local factors. SAc relations almost always predicted the presence/absence of species more accurately than their abundance. Distances along the road generally gave a very slightly better fit to species composition than surface distances, suggesting that there might be dispersal limitation. On the Wet side, elevation in overall comparisons gave a still better fit, implying environmental control too. The strength of SAc in texture was strongest on the Dry side, and using overall comparisons, abundance weighting and surface distances, but in other cases texture SAc was weaker than SAc in species composition. It is concluded that the saturation-response formula used here has theoretical advantages over previous approaches. The SAc in this vegetation seems to be caused by a combination of dispersal limitation, broad environmental trends and especially local environmental effects such as disturbance. SAc in texture may have been weaker overall on the Wet side than on the Dry because the main environmental difference across the former is in frost, and characters correlated with frost resistance have not yet been included in texture analyses.