Competitive advantage of invasive clonal plants in changing wetlands

Background/Question/Methods:

Downstream wetlands bear the brunt of impacts from both intensifying land use and increasingly variable weather by accumulating runoff, sediment and nutrients.  How does long-term tracking of vegetation in downstream wetlands help us identify effects of both landscape change and climate change?

Results/Conclusions:  

Along the Southern California coast, during 40 years of salt marsh research, the native halophytic vegetation was diverse during benign weather, but encroached upon by native cattails (Typha domingensis) when extreme river flooding deposited sediments and lowered soil salinity. Tall, clonal cattails had a competitive advantage under brackish conditions but yielded dominance to halophytes in dry years, when soils were hypersaline. Then, a monotype of the native clonal Sarcocornia pacifica (Salicornia virginica) developed when extreme sedimentation elevated topography and eliminated tidal flushing, and a severe drought led to extremely saline soil. Landscape change (altered watersheds releasing copious sediment) and climate change (associated with the Pacific Decadal Oscillation) interacted synergistically.  The “sequence effect” (first drier soil, then prolonged hypersaline conditions) reduced plant diversity.

In Wisconsin, aerial photos revealed that clones of native cattails (T. latifolia) expanded and retreated in Green Bay coastal wetlands as water levels rose and fell prior to 1992. But when invasive cattails (T. angustifolia + T. x. glauca) appeared in the downstream wetlands of urbanizing watersheds, the widely-tolerant cattail clones expanded both up- and downslope and persisted as near-monotypes, likely aided by eutrophication. In inland wetlands, landscape changes and longer growing seasons facilitate dominance by another invasive clonal graminoid, reed canary grass (Phalaris arundinacea), which now dominates >200,000 ha of Wisconsin wetlands.

Clonal wetland plants tend to develop monotypes where land use intensifies and water levels reach extremes.  The strong competitive advantage of clonality allows monotypes to develop in downstream wetlands that receive excess runoff, sediments and nutrients.  Knowing that extreme conditions are likely to intensify, land stewards can develop strategies for landscape-change adaptation as well as climate-change adaptation, including guidelines for realistic restoration goals.  Biodiversity might best be restored in upstream wetlands, while ecosystem services that are enhanced by inflows (such as denitrification) could be restored downstream.

[Symposium sponsored by ESA’s National Vegetation Classification Panel.]