9 Wrap-up Section and Research Needs
Peatlands are shallow unconfined aquifers. Groundwater in peatlands embody the principles of an ecohydrological system including their development and function and, in turn, their groundwater relations. In a given hydrogeomorphic setting, subject to climate, groundwater exchanges are controlled by the character of adjacent and underlying mineral aquifers and aquitards, such that incipient peatlands form where persistent saturation hinders decay of plant material. This then forms the matrix, peat, that hosts the groundwater, in an ever-evolving ecohydrological cycle.
While the principles of groundwater flow and storage are not different from those in mineral matrixes, the physical and chemical properties of the peat matrix impart distinct characteristics and challenges to measuring and understanding groundwater relations. The distinct characteristics arise from the relative instability of the medium, which undergoes physical breakdown and consolidation on time scales orders of magnitude shorter than for mineral materials. The surface chemistry of peat particles facilitates oxidation-reduction reactions, imparts enormous cation-exchange capacity, and changes the wetting behavior that controls capillary relations. The interaction of peatlands with adjacent groundwater systems dictates their class, form and function (e.g., bog versus fen versus swamp) because groundwater inflows offset the tendency for acidification caused by organic acids released when peat and plant matter decay. The outcome of this chemical balance and wetness condition dictates the plant community composition (species) and form (e.g., mosses, sedges, or woody plants), which feeds back to the hydrology of the system by influencing the structure of the peat matrix, and water exchanges by evapotranspiration and runoff.
The challenges facing researchers and practitioners charged with evaluating peatland function span the range of scales from pore spaces to entire ecosystems. This book on groundwater in peat and peatlands has highlighted many of the important groundwater processes. However, there remain many uncertainties that present research challenges. Some of these uncertainties and associated challenges are discussed below.
The effect of global warming on peatland biogeochemistry and ecohydrology is much more complex and uncertain than described herein; changes in atmospheric temperature are predicted more confidently than changes in precipitation. The well-known Clausius-Clapeyron Equation indicates that vapor pressure rises non-linearly with air temperature and, as such, a warmer climate implies a potentially wetter atmosphere. However, it is not clear if the potential increase in precipitation is sufficient to offset potentially higher evapotranspiration losses, or whether systems will be required to adjust to a persistently drier state.
Nor are the hydrological impacts of potential changes to the seasonality of precipitation and temperature certain across the range of climates in which peatlands are found. For instance, higher latitude systems in subarctic Canada will likely have a shorter period of snowpack accumulation, thus less snow to melt and longer growing seasons. Vegetation shifts will be likely; drier systems will have more trees, thus more interception of precipitation and greater transpiration, contributing to a positive feedback loop that increases peat decomposition, alters hydraulic properties, and so on. Such shifts may produce compounding disturbances with increased frequency and intensity of peatland wildfires or other natural and human disturbances that can further alter the processes that sustain peatland functions.
The class of peatland could change, for example, if the weak groundwater inflow to poor fens ceases; then they will evolve to bogs. Increased decomposition rates could slow or even reverse the development of domed bogs, reconnecting such areas to groundwater systems. Protracted periods of summer drying that enhance woody vegetation may result in fens transitioning to swamps, or for peat swamps to lose organic soil and become mineral swamps. In short, our ability to predict these changes fundamentally relies on the mechanistic understanding of peat and peatland ecohydrology and its feedbacks with biogeochemical processes. Yet, the linkages between ecohydrology and biogeochemical processes remains a critical research question.
Peatland-scale processes that control flow direction, rates, and persistence produce feedback that results in distinct, patterned peatland forms. The cause of patterning in peatlands (ridge/flark, hummock/hollow) remains somewhat speculative—a feedback mechanism among biotic productivity, decay, water table elevation, and water flow. In the case of ridge-flark microtopography in fens, the orientation of ridges perpendicular to flow decreases the rate of drainage, increases surface (i.e., depressional) water storage, and enables a threshold (i.e., “fill and spill”) runoff response once the water storage capacity of each flark is exceeded. However, ridge/flark systems such as ladder fens are a conduit for water loss from large domed bogs (as shown in Figure 7), capable of high water transmission rates following snowmelt or heavy rainfall.
While the ecohydrological and biogeochemical feedbacks that produce microtopography are uncertain, development of microtopography on measurable time scales is possible. Long-term monitoring of water levels over decades, combined with analysis of archived air-photograph or satellite images over the same period, indicate that microtopographic change accompanies persistent changes in peatland water storage. For example, Sphagnum lawns have been found to develop hummock-hollow topography in cases where peatlands lose water through sustained high rates of drainage. Such changes enable colonization of peatlands by trees on the relatively dry hummock surfaces, which presents the potential for feedback resulting in drying of the peatland through increased evapotranspiration. More research is required to evaluate whether such a process driven by increased drainage might also result from increased evapotranspiration driven by climate warming, and what those changes imply for downgradient ecosystems.
Like the flow of water in peat and peatlands, solute transport is complicated by ecohydrological feedbacks and linkages. However, studies of solute transport in peatlands have only recently begun to consider complex peat structures and peat surface chemistry. Solute transport in peat is not only subject to dual/multi-porosity processes; as an organic substrate, it is highly reactive with a wide range of chemicals, compounds, and elements. This makes understanding reactive transport in peat difficult but critical if we are to understand the feedbacks between ecohydrology and biogeochemistry that govern many key peatland processes.
At the peatland scale, it is thought that the linkages between hydrophysical peat properties, microtopography, and the movement of nutrients and carbon gives rise to regions of elevated nutrient/carbon cycling, resulting in both positive and negative feedbacks. However, the relative strength and importance of such processes and feedbacks have yet to be resolved. Understanding the movement of nutrients, carbon, and other elements/compounds from the pore to landscape scale of peatlands underpins much of our collective understanding of peatlands, but current limitations to our knowledge of processes in peat limits our understanding of peatlands.
The high compressibility of peat has been described. However, apart from the suggestion that decomposition decreases mean pore diameters and overburden increases pore compression—both of which reduce K—there is still much more to learn and know. For example, specific yield is almost exclusively used to relate water exchanges to water table position, but, given the compressibility of certain peats, coupled with extreme water table drawdown in some settings, the inclusion of specific storage may be essential to evaluating water storage changes. However, this approach has not been extensively adopted by the peatland hydrology community.
Peat’s high compressibility results in mire breathing that causes the water table to be closer to the surface than it otherwise would be in a more rigid media, resulting in hydraulic properties that vary over short (hours to days) time scales. Characterization of hydraulic properties such as bulk density and porosity are mostly based on a fixed, sampled field volume, but peat cores shrink as soil water pressure decreases, so expressing porosity of a sample under tension, based on field volume of the sample, understates the proportion of saturation compared to what would be calculated if the reduced sample volume were used. Explicitly accounting for volume change in estimation and expression of hydraulic parameters needs more attention.
Advancements in hydrological modeling of peatlands are ongoing in several key areas. At the plot and point scale, modeling has focused on simulating the flux and storage of water, solutes, and energy, typically in one dimension. Modeling can incorporate mobile versus immobile porosity mechanistically, to demonstrate and evaluate the partition of water accordingly, in a peat matrix. However, some simulations, so presumably some peats, do not exhibit this behavior. A better understanding is needed about which peats (e.g., Sphagnum peat, woody peat) and their state of decomposition require complex porosity to be considered.
Studies undertaking 2- and 3-D simulations in peat and peatlands remain rare, yet the addition of extra dimensionality to modeling of water, solute, and energy fluxes will likely reveal new understandings of peatland hydrology, just as 1-D models substantially advanced understanding over the last few decades. At regional scales, groundwater modeling has focused on representing wetlands in land surface schemes to improve coupling of hydrological with atmospheric models. Incorporating feedbacks without unmanageable complexity is essential to realistically incorporating peatland functions into global climate models.
Modeling and quantification of water and solute fluxes in peat and peatlands requires a suite of parameters; ideally, these would be measured but commonly are estimated or taken from literature describing other sites. Unlike mineral soils with measurable components (percent clay, percent slit, percent sand, organic matter content, and bulk density) that facilitate the development and widespread adoption of pedotransfer functions, peat is predominantly organic in nature with very little mineral component. Several properties of peat lend themselves to the development of pedotransfer functions, however. Since the degree of decomposition increases with depth, there a systematic change in physical, hydraulic, and thermal properties of peat. As such, the degree of decomposition and bulk density as well as other key properties including the botanical origin of peat and its moisture content can be used to infer other properties that control the flux and storage of mass and energy. However, linking measurable chemical indices of the organic peat—i.e., carbon isotope ratios or C/N ratios—to hydrophysical properties has not been done. Thus, our lack of knowledge on the range of physical, chemical, and hydraulic properties across all peat types limits our ability to develop a universal pedotransfer function.
Our understanding of peatland hydrology is dominated by what we have learned from northern peatlands. This reflects not only the large area occupied by peatlands in the northern hemisphere but also the capacity of northern researchers to generate funding and regional interest in peat and peatlands from a scientific, industrial, and social establishment. Now, interest in the large peatlands of the subtropical and tropical regions is growing. Global interest has increased with recognition of their role in the global carbon budget because of air-quality impairment from extensive peat-fire smoke plumes, such as from Sumatra in 2015, and the role of land-clearance and agriculture in their demise.
Many processes governing the occurrence, growth, and degradation of tropical peatlands; mechanisms controlling water and solute flows; and even some basic approaches to restoration are shared with better-studied northern peatlands. However, the distinct climate, botanical origins of peat thus hydraulic structure, and scales of exploitation for resource development of tropical/subtropical peatlands are very different and require exclusive focus and extensive research. While an excellent cadre of scientists have reported on tropical peatland form, hydrology, and carbon biogeochemistry, the global importance of these peatlands warrants increased research effort.