The role of plant interactions (e.g. interference, facilitation) in structuring communities remains uncertain, especially in arid and semi-arid plant communities where debate over the importance of interference persists. While interference has been historically viewed as a distinguishing feature of water-limited communities, more recent work suggests that facilitation may be more common than expected, and potentially more important in governing plant patterns and ecological processes. Under the assumptions that (a) spatial structure directly influences plant-plant interactions and (b) the results of pattern analysis can elucidate these interactions, researchers often interpret regular plant patterns as evidence of interference and aggregated patterns as evidence of facilitation. These studies often require point pattern data that can be spatially imprecise and difficult to collect in the field. We used a relatively novel approach to rapidly obtain a large dataset of highly precise spatial data in a Great Basin, shrub-dominated plant community in the Western USA. Our objectives were to test three a priori hypotheses related to aggregated plant spatial patterns (species-independent aggregation, intraspecific aggregation, and interspecific aggregation) using traditional (bivariate O-statistics) and novel (the K2 statistic) spatial analyses using the software packages R and Programita. Monte Carlo simulation envelopes were used for significance testing.  

Results/Conclusions

The community was composed of five shrub species, three Asteraceae and two Chenopodiaceae. Shrub spatial patterns were largely aggregated within the study plot, suggesting facilitation as a stronger force than interference in community organization. Aggregation was detected at the plot level when the overall pattern was analyzed, independent of species. Intraspecific aggregation was also detected in four of the five shrub species present in the plot. Interspecific aggregation was only detected between two of twenty possible species pairs; interestingly, in both cases one member of the pair was an Asteraceae and the other was a Chenopodiaceae, supporting the idea that more distantly related species are less likely to compete strongly. The scale of aggregation was ≤1.0m in all cases, suggesting that shrubs have formed small clusters with their immediate neighbors. Regular patterns were weak, rarely detected, and not a distinguishing feature of the community under study. Our results strongly suggest that this semi-arid shrubland community is significantly structured by positive plant interactions. We discuss the significance of the results, as well as the utility of integrating new data collection techniques and novel spatial analysis, relative to the ongoing debate on the role of facilitation and competition in semi-arid plant communities.