Assembly rules were sought, as relative constancy in the proportion of species from different guilds that can coexist at a point. The community used was a regularly-mown lawn, sampled at the end of the mowing/regrowth cycle. There was a significant tendency towards constant proportions of representatives from the Graminoid and Forb guilds. E.g.. at 2-species points there was one graminoid species plus one forb species more often than expected. This effect was seen whether or not the two bryophyte species were included with the forbs. There was little evidence that the rule was based on grass/legume interactions. Optimisation of the Graminoid vs Forb+bryophyte classification showed that some forbs were better assigned to the 'graminoid' guild, apparently because of the role of their laminae in the upper canopy. This optimised guild classification gave an even stronger tendency towards guild proportionality. Guilds comprising species that tended to be in the Upper vs Basal strata of the lawn also exhibited an assembly rule, again with significant tendency towards constancy of guild proportions. This effect was seen only using stratum assignments based on species' positions at the end of the mowing/regrowth cycle. A search for intrinsic guilds starting from random initial configurations, resulted in classifications quite similar to the optimised 'Graminoid' vs 'Forb+bryophyte' guilds. With further optimisation using the whole dataset, the three optimised classifications converged to become identical. The resulting guilds are termed 'intrinsic' guilds, being determined by the observed tendency of species not to co-occur. Similar trends, but generally non-significant, were seen in plots that had been disturbed, by complete removal of the vegetation, nine months earlier. It is suggested that explanations for the assembly rules lie in structural differentiation within the canopy towards the end of the mowing/regrowth cycle, based on the interaction of lamina shape and position. The results represent the most convincing demonstration yet of assembly rules for a plant community. The study includes the first search for intrinsic guild structure, based on observed species coexistence patterns.
