Environmental factors interact with spatial processes to determine herbaceous species richness in woody field margins

The species richness of hedges in an agricultural landscape may be determined by the environment and by the spatial processes which occur in that landscape. Here, we divided the environmental predictors into three groups: site conditions, hedge stand and landscape structure. We determined their independent and joint effects on the richness of four guilds of herbaceous species in 92 hedge stands in a north-Mediterranean intensive agricultural landscape. The fine—(at <250 m) and broad—(>550 m) spatial patterns of the key environmental predictors and the pure spatial effects on species richness were measured using a computation of the principal coordinates of a matrix of geographical neighbours integrated into a variation partitioning. The total explained variation of species richness among hedgerows was highest for wetland herbs (62 %), with increasing rates for rare plants (33 %), forest herbs (43 %) and arable weeds (47 %). 43–11 % of that variation was spatially structured and mostly explained by some of the key environmental predictors, such as proportion of a given landuse, presence of woody species and dead trees. This indicates that complex relationships between herbaceous species distribution and spatial processes exist in woody field margins and much of that is related to key factors which are spatially structured, both at fine or broad-scales, with implications for management and landuse planning.