Comparison of invasive, native, and hybrid cattail tolerance to light and nutrients: Implications for understanding cattail invasions

Background/Question/Methods

Our study focused on understanding the ecology of invasions of North American wetlands by both the European cattail, Typha angustifolia, and the hybrid of it and the native North American species, T. latifolia. Very little is known about the mechanisms by which either T. angustifolia or the hybrid invades, if they also displace native T. latifolia, or which characteristics render particular wetlands invasible. The lack of data is at least in part due to the difficulty of distinguishing the hybrid from the parent species in the field.  However, we have developed a reliable set of field identification characteristics, confirmed by DNA markers, enabling us to collect and analyze rigorous ecological data.
Results/Conclusions

To compare ecological differences among all three taxa, we first studied their habitat requirements by surveying coastal and inland wetlands in Michigan. Although cattails are expected to grow in eutrophic habitats, we found invasive T. angustifolia mostly in oligotrophic, densely vegetated, sedge-rush, and mostly unshaded wetlands. In contrast, the native species formed mono-dominant stands at sites with high nutrients often shaded by trees. The hybrid occurred strictly at unshaded sites and formed extensive mono-dominant stands at sites with high nutrients and smaller, less dense stands at sites with lower nutrients. Going beyond correlative associations, we tested in controlled garden settings if these strong patterns were caused by response differences of the cattail taxa to the environmental variables.
We conducted an experiment with a factorial design to test the effect of light (full sun vs. 75% shading) and nutrients (sand vs. peat) on cattail performance. We found that nutrients have no effect on total biomass of T. angustifolia, but shading decreased production by 50%. In contrast,  shading had little effect on biomass production of T. latifolia, but low nutrients decreased biomass in 15-25%. The hybrid produced significantly more biomass than both parents, but only under full sun and high nutrients. This taxon was strongly affected by both shading and low nutrients, decreasing its production by almost 75%.

These observations suggest that, although cattail taxa are very similar, subtle physiological differences give rise to ecological mechanisms that can allow both invasive taxa to invade oligotrophic wetlands that are not primarily occupied by native cattail. With nutrients available, the hybrid can cause dramatic ecosystem modification of open coastal wetlands due to its massive biomass production. Our other studies showed that high nutrients can come from abundant hybrid litter accumulation without external nutrient input.