Harms, N. E. and J. T. Cronin. 2021. Phenology and competitive interactions and implications for management of the invasive wetland plant, Alternanthera philoxeroides. ERDC Technical Notes, ERDC/EL TN-21-1. Vicksburg, MS: US Army Corps of Engineer Research and Development Center.


Phenological differences between invading plants and members of recipient communities may be implicated in the success of invaders due to priority effects. Application of management when the invader has the phenological advantage (e.g. early in the year), may benefit other species by increasing resource availability. We used a combination of field observations and a mesocosm experiment to explore whether phenological differences exist between the invasive wetland plant, Alternanthera philoxeroides (alligatorweed), and native species, and whether interspecific differences in phenology contribute to alligatorweed success. We documented early-season growth of alligatorweed and other species at x sites in Louisiana, USA. We then conducted a mesocosm competition experiment between alligatorweed and a common native species, smartweed (Persicaria pensylvanicum), over a full year to detect differences in timing of growth and competitive interactions under two resource (fertilizer) levels. In the field, alligatorweed was consistently the most abundant species with peak abundance occurring early in southern and central sites and later in northern sites. In the mesocosm experiment, peak abundance for alligatorweed occurred in the spring (470 stems/m2) and for smartweed, it occurred in summer (697 stems/m2), x weeks later. Despite our prediction, this difference in phenology did not translate into a competitive advantage for alligatorweed. Alligatorweed experienced the largest negative effect of competition in the high resource treatment (Hedge’s g = 2.26), whereas the lowest competitive effect occurred for smartweed under low nutrients (g = 0.44). Biomass allocation varied between species, with alligatorweed allocating the vast majority of biomass in roots (x%), regardless of competition treatment, which may be an advantage in the long term. This study suggests that early emergence is not the primary determinant of successful alligatorweed invasion. However, the ability to quickly colonize disturbed habitat likely contributes to alligatorweed invasion. Additionally, under field conditions, damage by introduced biological control agents on early-establishing populations may be responsible for controlling alligatorweed and shifting community composition toward native species.

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