Productivity was a poor indicator of five ecosystem services in experimental wetlands

Background/Question/Methods

Current ecological theory states that productivity and diversity are correlated, along with many other ecosystem functions and services. Wetland restoration projects often target productive and diverse vegetation as functions of value to humans (i.e., ecosystem services), but it is not clear which services are positively correlated and co-restorable. We measured six ecosystem services in a site with three near-replicate created wetlands; wetlands were similar in size, shape, inflows (quantity and quality), soils, and plantings, but had different drainage rates. We asked how the wetlands would differ in provision of services, how services would correlate with one another, and if any individual service (e.g., net primary productivity; NPP) could indicate all others. We measured: flow attenuation and stormwater retention by monitoring flows through the inlet and outlet weirs of each wetland, NPP and diversity support by repeatedly sampling and harvesting vegetation, erosion resistance by stressing soil until particles detached, and water quality improvement by analyzing removal of four common contaminants.

Results/Conclusions

Natural variation in clayey subsoils led to differences in wetland drainage rates; on average, the fastest-draining wetland infiltrated inflows three times faster than the slowest-draining wetland, which remained inundated for extended periods. That difference in drainage translated into differences in the level of services wetlands provided, which varied up to 1.6x for flow attenuation, 18.0x for stormwater retention, 2.2x for NPP, 2.2x for diversity support, 1.8x for erosion resistance, and 17.7x for water quality improvement. The fastest-draining wetland ranked highest in five of the six services but lowest in NPP; the slowest-draining wetland ranked highest in NPP but lowest in the other five services. The wetland with intermediate drainage provided intermediate levels of all services. We conclude that hydrologic regime is an overarching determinant of wetland services, and that some services could be co-restorable in well-drained sites. On the other hand, not all services were positively correlated. Thus, we caution against assessments focused of individual indicators, especially NPP, which was negatively correlated with hydrologic and water-quality services. Direct measurements and interdisciplinary research are key to understanding linkages among ecosystem services and how best to restore multiple wetland services.