To celebrate World Wetlands Day 2017, Helen Moor of Stockholm University has given a background to her recent essay review published in Journal of Ecology; Towards a trait-based ecology of wetland vegetation.
Wetland ecosystems, from marshes to bogs, can provide numerous benefits to society. They can act as water reservoirs and attenuate floods, filter pollutants and improve water quality, and they store vast amounts of carbon in the form of peat. The maintenance of such ecosystem services has become an important target of natural resource management. To achieve this, we must understand exactly how services are being generated.
Different wetlands excel at different services. What they can do for us depends on the wetland type (its hydrology and vegetation) as well as on the location in the landscape (in relation to human influences and needs). An isolated peat bog high up in the hills may do little with regard to water quality in our rivers, but through the build-up of organic matter and peat storage, it has the potential to sequester carbon for centuries (Yu 2011). A marsh in a floodplain can reduce flood damage by soaking up and temporarily storing overflowing storm waters (Bullock & Acreman 2003) and even small wetlands like ponds and ditches scattered in agricultural landscapes can contribute their share to better water quality in rivers, lakes, and, ultimately, the sea (Blackwell & Pilgrim 2011).
Wetland types differ in their service potential because of the specific way they work. Different conditions entail different plant communities that in turn foster different processes, either directly or indirectly via their effects on the physical environment or the soil microbial community. The balance of many such processes is what ultimately constitutes a service (or disservice) to society. For example, whether a wetland is good for climate regulation depends on the balance between the uptake and release of greenhouse gases. If the wetland sequesters a lot of carbon dioxide but at the same time emits methane, its net effect on greenhouse gas regulation may be nil.
Because such process rates are difficult and costly to measure, it would be useful to estimate them from simpler and more easily accessible data. The ultimate goal is a model that, based on basic information about abiotic conditions in a particular wetland, can tell what plants occur and what processes can be expected to be dominant. With such a model in hand, one could even predict what might happen if conditions change, whether due to climate change or due to management interventions.
This is the holy grail of plant functional ecology.
Trait-based ecology: a dry affair?
The study of the functional characteristics (traits) of plants has exploded in the last two decades. Researchers have been disentangling which traits have adaptive value in particular environments in order to predict which plants may be present and to forecast changes in plant abundances under climate change. In hot climates, for example, plants with small and thick leaves that minimise water loss fare better and should be more abundant (Wright et al. 2004). That is the response side.
Others have been relating traits of existing plant communities to rates of ecosystem processes, also with regard to ecosystem service potential. In mountain grasslands, for example, plants with higher concentrations of nitrogen in their leaves tend to exhibit higher rates of primary productivity and represent higher quality fodder for cattle (Grigulis et al. 2013). That is the effect side.
A third cornerstone of the theory is how traits relate to each other, within and across species. Do tall species always have deeper root systems? Are nitrogen-rich leaves always thinner and more palatable to herbivores? The strategies plants adopt to cope with different environments are subject to trade-offs, which cause recurring patterns of trait co-variation across species. The best-known example is the plant economics spectrum (Wright et al. 2004, Reich et al. 2014), an axis of specialisation with regard to resource use. The spectrum ranges from highly acquisitive plants, characterised by fast growth rates, high tissue nutrient content and high decomposability, to conservative plants with the opposite traits. If such trade-offs and spectra are universal, then a whole suite of traits becomes predictable from one or few key indicator traits.
Trait-based ecology has made tremendous progress in all three areas, as well as their synthesis (Shipley et al. 2016), and is taking great strides towards applications in restoration and management (Laughlin 2014). The theory, however, has been largely developed with a focus on dry upland systems and may, therefore, be incomplete. There is an urgent need to extend the proof of concept to a wider range of ecosystems.
Functional traits for functional wetlands
Wetlands differ from terrestrial systems in their conditions, plant adaptations and prevalent processes. They represent an ideal testing ground for established theory and can offer novel insights. Because of their important roles in the regulation of regional water flow and quality as well as the global climate, their study should be a high priority.
To start building a functional trait-based ecology of wetlands, our essay review in Journal of Ecology assembled the available knowledge of i) which functional traits of wetland plants make them thrive in these environments, and how these might differ from terrestrial plants, as well as ii) how these traits in turn influence the complex web of processes that combine to generate ecosystem services.
Commonly measured traits were found to be subject to environmental filtering as predicted by theory, but additional traits and patterns of trait co-variation can differ in wetland plants. Wetness represents a complex gradient that comprises not only water availability but also changes in soil biochemistry (redox conditions), nutrient availability, and in some cases irregular mechanical disturbance. If and how wetland-specific plant adaptations trade off with other functional traits is largely unresolved.
Encouragingly, interest in trait-based approaches to wetland functioning has been stirring for some time. We referenced exciting studies that uncover trade-offs and patterns of traits in mosses and their effects on carbon cycling (Laing et al. 2014); that study plant trait effects on microbial processes in wetland soils (Sutton-Grier et al. 2013); or that integrate plant traits into hydrological models of river flow (Nepf 2012).
Much remains to be done. Our summary graph provides an immediate overview of the complex ecology that underlies wetland ecosystem services. It can be explored interactively and thus may be useful as a pedagogical tool. Above all, the graph is meant to serve as a baseline for discussion. The relationships shown – and, more importantly, those missing – outline a road map for future research. Experts from around the world are invited to add to it in a collaborative effort to complete the picture and together build a trait-based ecology of wetland vegetation.
Read about more BES wetland research on the Methods in Ecology and Evolution blog; Biomonitoring Pollution in Wetlands: A New Method for More Reliable Interpretation of Chemical Data