Flood disturbance in arid region rivers can be extreme, changing the abiotic and biotic conditions within riparian ecosystems. Intense flood pulsing reworks the floodplain and channel sediments resulting in colonization site renewal. Resource pulsing occurs concomitantly with flood pulsing causing a temporary increase in water and nutrient availability to riparian plant communities. Prosopis forests, located on the outer perimeter of the floodplain, experience infrequent flood and resource pulsing resulting in relatively stable community dynamics in between flood events. We asked what level of flood disturbance intensity was necessary to change understory community dynamics in Prosopis forests? Our objectives were to determine the level of flood scour necessary to change richness, diversity (Shannon Index), and the competitive hierarchy in Prosopis understories.
Results/Conclusions
Using a randomized block design, three levels of flood scour intensity (vegetation removal, 2 cm soil removal, and 5 cm soil removal) were applied at five sites located in Prosopis understories along the Hassayampa River near Wickenburg, Arizona, USA. The vegetation and soil removal treatments simulated erosional effects from floods of varying intensity. Sites were monitored monthly for 13 months, and then a final time 24 months after treatment application. Results of uneven univariate repeated measures analysis revealed that vegetation cover and richness increased over time for all treatments. However, two-years after treatment application only diversity had increased significantly. The cover of grasses, specifically annual grasses, was significantly lower in 5 cm soil removal plots two-years after treatment application. The competitive hierarchy also changed in the 5 cm soil removal plots with annual forb species outcompeting annual grasses. These results reveal that the scour that accompanies an approximate 25-year flood (5 cm soil removal) is necessary to impact competitive dynamics by reducing competitive exclusion by dominant species. Competitive release occurred when transient seed banks produced by community dominants were removed allowing the less successful competitors to establish from persistent seed banks and via immigration from surrounding communities.
