definearth

Not long after settlers arrived in Wisconsin from New England, New York, and Germany, logging of the dense pine forests there began. Once the Menominee Nation was forced to cede large chunks of resource-rich land in Central and Eastern Wisconsin in 1836, small-scale logging evolved into large-scale logging. Levees were constructed alongside harvesting sites to improve access by flooding, and canals aided in export. Not only did the exploding timber business provide critical support for the economy of Wisconsin, but also the cleared land that remained afterwards could be used for farming.

Railroads and the demand for pine-based paper led to another spike in the lumber business at the start of the 20^th century. Fire suppression began in an attempt to halt the fires that fed off the dry, dead heaps of tree branches and tops left on the forest floor by loggers. For many years fire suppression remained unsuccessful. In 1931, over 30,000 ha of the Pine Barrens of Wisconsin were destroyed by fire. Resources were exhausted at that point for a few reasons aside from the fire. Unsustainable practices like clear-cutting that were initially employed to meet the increasing demand for trees by local furniture, paper, and leather industries allowed people to harvest trees at a rate faster than the forests could rebuild. Drained soil nutrients, extreme droughts, and the Great Depression also added to the problem. In the end, the Timber Era of Wisconsin left the state with less than one percent of its original old growth forests.

Riverine forests (or forested, riverine wetlands) were preferred for tree harvesting because logs could be easily transported downstream. In the Crex Meadows of Northwestern Wisconsin, the lowland wetland plant Carex stricta was harvested from entire wetlands by the Crex Carpet Company for production of mats, rugs, and carpets. The Civilian Conservation Corps started replanting pine around the 1940’s, and in 1945 the Wisconsin Conservation Department took over the Crex Meadows area for wetland restoration.

Wetland restoration benefits and techniques

Early forest management techniques included building levees and ditching or draining, as flooding was believed to greatly advance tree growth. In fact, the opposite is true. Most species of trees cannot withstand two years of continuous flooding. Flooding can also be damaging depending on the time of year. According to a 1994 study on forest management practices in the South, yellow-poplar seedlings submerged for just three days during growing season will suffer more than those submerged during dormant season. By the time that seeds are submerged for two weeks, survival rate is down to 5%. Seedlings belonging to the majority of species will lose their leaves if submerged in the spring. Because of this, levees and floodwalls can prevent tree growth and worsen river flooding by allowing water to flow faster and higher than normal.

Long term consequences built up as a result of poor earlier management techniques. In the 1994 study (mentioned previously) published by The Society of Wetland Scientists about Southern forested wetlands, cypress trees which typically dominated vegetation composition failed to regenerate after logging operations due to their inability to germinate in still water. This occurred despite the mature plant’s ability to thrive in the same conditions, so long as water levels do not surpass 60 cm. Changing water level, nutrient inflows, and litterfall in natural cypress and tupelo forest systems are what contribute to their status among forests with the highest aboveground biomass and primary productivity. Periodic inundation provides nutrients and sediments that promote plant production, while stagnant water can sometimes be less productive. Poor drainage will build acidic peat soils and lead to lower productivity because of infrequent nutrient turnover from anoxia, N limitations, and low pH.

Bottomland hardwood forests found alongside streams in the Southeastern US (cypress is a softwood often found among hardwoods) are also unique in that they have higher basal area, biomass, and biomass production rates than adjacent uplands. Similar to cypress forests, temporary flooding in these settings is beneficial in increasing productivity by bringing ample water supply in the summer, dispensing new nutrients, and allowing for a more oxygenated root zone. One the other hand, wetland drainage causes damage similar to continuous flooding by reducing overstory canopy and restricting biomass production from trees, litterfall, and herbaceous plants. Both flooding and draining will transform cypress wetland landscapes by reshaping vegetation composition. Burning will also limit forested wetland regeneration. Severe or recurring fires will often convert cypress forests to prairie or willow stands.

Flow management has been shown to improve wetland CO₂ fluxes in the case of a restored coastal salt marsh in Shanghai, China, even leading to CO₂ fluxes that surpass that of a natural wetland. Fluxes of CH₄ at the restored wetland were also lower. This was because tidal flow management halted the continuous water flow which created anoxic conditions and caused methanogenesis. A culvert with a controlled value, along with tidal access that mimicked natural hydrology were the likely causes of higher CO₂ flux going into the restored wetland and lower outgoing CH₄ flux.

Higher plant biomass could have been the culprit of higher carbon uptake at the restored wetland. Higher CH₄ production at the natural wetland stemmed from anoxic conditions and a lack of influence from low tide, compared to the restored wetland. Water level at the restored wetland was maintained at 10 cm and flow rate was limited to 0.3 ms^-1 year-round. Flow was limited by turning off the flow pump and draining overlying water at the restored wetland for two weeks to mimic the natural wetland during low tide. Continuous flow stimulated oxidation reduction potential of the water-soil interface and thus, lowered CH₄ emissions in the restored wetland. In the past, other studies have linked wetland CH₄ emissions to salinity because sulfate-reducing bacteria will replace the methanogens and decrease CH₄ production. Also, CH₄ oxidation by sulfate reducers limits CH₄ emission. But this study opposes that idea because the natural wetland produced more CH₄ and had a higher salinity level than the restored wetland, meaning salinity is not the controlling factor of CH₄ emission in all wetlands with moderate salinity. Coastal ecosystems may flip from net C sources to sinks with adequate water exchange, meaning that tidal management is important in reducing wetland CH₄ emission. Flow management is important in restoration of both coastal and inland wetlands.

Takeaways

• Over-harvesting brought the Timber Era of the Midwest to a halt
• Logging altered natural wetland hydrology and resulted in a significant loss of old growth forest in some areas
• Research on restoration gives insight on how we can better support wetland ecosystems and tree growth with flow management