The biosphere removes nearly a quarter of anthropogenic greenhouse gas emissions each year through biogeochemical processes. These fluxes, along with biophysical exchanges of energy and water, play an important role in local to global climate dynamics and human well-being. With an urgent need to decrease the amount of greenhouse gases in the atmosphere to avoid runaway climate change, much recent work has focused on identifying and quantifying the role that terrestrial ecosystems could play in mitigating climate change. Restoration of coastal and deltaic wetlands, with their often carbon-rich organic soils and high productivity, presents an attractive but largely untested land-based climate mitigation strategy.
The benefits associated with wetland restoration stem from two key areas. First, drained agricultural peat soils can be large greenhouse gas sources. Second, the slow decomposition rates of inundated wetland soil organic matter along with high productivity leads to soil carbon accumulation and protection. While these tenets are generally widely appreciated, they have rarely been tested and measured at the ecosystem scale, over multiple years.
In this dissertation work, I studied the coupled biogeochemical and biophysical impacts of a long-term wetland restoration ecological experiment in the Sacramento-San Joaquin River Delta in California, USA. By continuously measuring greenhouse gas and energy fluxes at the ecosystem scale over a variety of land cover types, including four restored wetlands of various ages and structures, I was able to characterize the carbon, greenhouse gas, and biophysical impacts of degraded peat soil restoration to freshwater deltaic wetlands.
I find that these restored freshwater deltaic wetlands are highly productive, sequestering carbon in the soil as productivity outpaces ecosystem respiration. This productivity comes at the cost of substantial methane emissions, however, making these wetlands greenhouse gas neutral to sources over a century. Despite this fact, transitions from high-emission degraded peat soil agricultural land uses to restored deltaic wetlands often reduce greenhouse gas emissions overall. Furthermore, I analyze how the biophysical impacts of this restoration activity – the changes to the way the ecosystems exchange heat and water – affect the surface temperature and boundary layer. I find that along with potential biogeochemical benefits, restored deltaic wetlands have an evaporative cooling effect due to rougher, wetter canopies.
These findings shed light on the viability of freshwater wetland restoration as a land-based climate mitigation solution. Restored wetlands should be part of a climate solution, but aren’t a ‘quick fix’. Ecosystem restoration is a dynamic process, with interannual variability, succession, and disturbance influencing the long-term performance of these ecosystems. Despite the incurred methane emissions in our restored wetlands, the flooded conditions effectively inhibit heterotrophic soil respiration and thus sequester carbon and create soil, refilling the deeply subsided ‘islands’ that have formed over the past century and a half. Other positive co-benefits, like local to regional biophysical cooling, along with habitat enhancement, must be considered to understand the full potential of restored wetlands as a part of the climate solution.
