Wetland Compensation and Landscape Change in a Rapidly Urbanizing Context.

While there are regulatory requirements that regulators should assess the impact of landscape-scale changes on the success of US Clean Water Act wetland compensatory mitigation sites, these requirements are poorly specified and very little work has been done to characterize how landscape change impacts CWA compensation sites. We created a rapid assessment method with both site-based and landscape-scale components, and used it to assess a population of wetland compensation sites in suburban St. Paul, Minnesota in 1997. We resampled the sites in 2010. The watersheds of these 22 compensation sites are characterized by rapid urbanization, the increase in impervious surfaces, and the loss of agriculture. This has resulted in extreme hydrographs at compensation sites and a fragmenting landscape context of more and smaller undeveloped patches. The ecosystem services provided by these compensation sites in 2010 are not significantly different than in 1997, indicating resilience in the face of landscape change, but not showing a trajectory of improvement. Reference sites were established for each ecosystem service, but two reference sites declined dramatically; results point to the importance of understanding ongoing landscape change even at benchmark sites. Compensation sites are typically located in rapidly changing and fragmenting landscapes, and understanding the relationship between landscape and compensation site will be important to ensuring appropriate compensation for impacts regulated by the Clean Water Act.

Effect of light on seed germination of eight wetland Carex species.

BACKGROUND AND AIMS: In wetland plant communities, species-specific responses to pulses of white light and to red : far-red light ratios can vary widely and influence plant emergence from the seed bank. Carex species are the characteristic plants of sedge meadows of natural prairie wetlands in mid-continental USA but are not returning to restored wetlands. Little is known about how light affects seed germination in these species-information which is necessary to predict seed bank emergence and to develop optimal revegetation practices. The effects of light on germination in eight Carex species from prairie wetlands were investigated. METHODS: Non-dormant seeds of eight Carex species were used to determine the influence of light on germination by examining: (a) the ability of Carex seeds to germinate in the dark; (b) the effect of different lengths of exposures to white light on germination; (c) whether the effect of white light can be replaced by red light; and (d) whether the germination response of Carex seeds to white or red light is photoreversible by far-red light. KEY RESULTS: Seeds of C. brevior and C. stipata germinated >25 % in continuous darkness. Germination responses after exposure to different lengths of white light varied widely across the eight species. Carex brevior required <15 min of white light for > or =50 % germination, while C. hystericina, C. comosa, C. granularis and C. vulpinoidea required > or =8 h. The effect of white light was replaced by red light in all species. The induction of germination after exposure to white or red light was reversed by far-red light in all species, except C. stipata. CONCLUSIONS: The species-specific responses to simulated field light conditions suggest that (a) the light requirements for germination contribute to the formation of persistent seed banks in these species and (b) in revegetation efforts, timing seed sowing to plant community development and avoiding cover crops will improve Carex seed germination.

Founder control and coexistence in a simple model of asymmetric competition for light.

Size asymmetry in plant light acquisition complicates predictions of competitive outcomes in light-limited communities. We present a mathematically tractable model of asymmetric competition for light and discuss its implications for predicting outcomes of competition during establishment in two-, three-, and many-species communities. In contrast to the resource-reduction model of symmetric competition for a single resource, the model we present predicts that outcomes of asymmetric competition for light will sometimes depend on the timing of establishment and the consequent hierarchy among species in canopy position. Competitive outcomes in the model depend on the minimum light requirements (L(c)) and self-shading of species lower in the canopy compared to the light available (L(out)(*)) beneath species higher in the canopy. Succession progresses towards species with decreasing values for L(c), but arrested successions occur when initial dominants have relatively high values for L(c) but low values for L(out)(*), leading to founder control. A theoretically limitless number of species may coexist in competition for light when dominance is founder controlled. These model predictions have implications for an array of applied ecological questions, including methods to control invasive species in light-limited restored ecosystems.