Water stress interacts with early arrival to influence inter and intra-specific priority competition: A test using a greenhouse study

Background/Question/Methods

Communities are dynamic and often functions of chance; thus, understanding how arrival order affects interactions may increase predictability of community assembly. We conducted a greenhouse experiment that examined the effects of priority for two study species (Panicum capillare L. and Polygonum persicaria L.) to determine if this confers competitive advantages under varying water stress. We predicted that growth measures of older individuals would be greater compared to younger individuals regardless of water stress. Furthermore, we hypothesized the effects of intraspecific priority competition would be greater than interspecific priority competition [priority (cohort) competition effect], regardless of stress treatment.  Each species was planted as a younger or older cohort within the same species (intraspecific) or with a different species (interspecific), so there were two planting combinations for each species, resulting in two individuals pot ^-1. For each species, three factors were examined: cohort, water stress, and priority competition. Thus, our experiment had 72 pots (36 species ^-1) (cohort (2) x water stress (3) x priority competition (2) x 6 replicates = 72). We used GLM  MANOVA and ANOVA’s on the dependent variables: root and shoot biomass, total biomass, root/shoot ratio, height, number of leaves, and length of longest leaf, using Tukey’s adjustment form multiple comparisons for each dependent variable.

Results/Conclusions

MANOVA results showed three significant interactions on biomass. Panicum and Polygonum root weight were affected by an interaction of stress x cohort; however, stress effects were only seen in older individuals where root biomass was higher under drained conditions. Shoot weight for Panicum was significantly affected by cohort, while Polygonum was significantly affected by a stress x cohort interaction. Polygonum total biomass was significantly affected by two interactions: stress x cohort and competition x cohort. Priority competition x cohort interacted to affect increase in Panicum height and the interaction of stress x cohort affected Polygonum. The number of new leaves produced by Polygonum was affected by a competition x cohort interaction and regardless of neighbor older Polygonum grew more leaves than their younger counterparts. Our results indicate that order of arrival does significantly alter competitive hierarchies in plant communities. Older individuals maintained a competitive advantage over younger individuals regardless of stress. Stress, however, reduced most growth parameters for older individuals more than younger individuals, suggesting that stress effects were masked in young plants. This study provides evidence for the importance of early arrival and establishment on the competitive and physiological abilities of plants.