Dynamics of temporal wetlands under changing weather conditions

Wetlands are dynamic ecosystems that render mankind a great deal of direct and indirect services. Their resilience and structure  are mostly dependent on their hydrology, which is affected by both landscape variables and regional climatic conditions. Most of the use of direct or indirect benefits from wetlands alter the hydrology or stimulate other factors that could lead to an altered hydrology. This direct or indirect alteration of wetland hydrology has led to losses of ‘water-logged capital’ worldwide (Maltby 2013).

This thesis presents the dynamics of temporal wetlands where hydrological cycles are dependent on seasonal weather patterns. The following quote elegantly captures this theme. “Wetlands often are transient features in landscape development, and can be regarded in many cases as the authors of their own destruction. The inherent processes of natural change, such as sedimentation, peat growth and soil development lead to hydrologic changes that may be less favorable for existing plants and animals than competitors. So too, a plant assemblage may alter conditions in ways that make the habitats less favorable for survival of its component species, and more favorable for the development of a different community. Wetland ecosystems can pass through many such ‘seral’ stages, emphasizing their dynamic yet temporary character.” (Maltby 2009).

Dune slacks are temporal wetlands where hydrology plays a crucial role in determining the types of plant species that occupy them. The hydrology in dune slacks is dependent on weather patterns. This means that it is mainly governed by meteorological conditions of precipitation and evapotranspiration. Changes in the global climate may imply that the pattern of weather conditions, both at local and global levels, will become more and more erratic. Extreme events of torrential summer rains could lead to events of flooding, for instance in parts of Western Europe. On the other hand, winters could become wetter while summers could become drier. Besides, temporal wetlands are not limited to dune slacks, but they also occur from mountain tops to desert valleys all across the globe. For instance, dune slacks on the Wadden Sea islands of the Netherlands have suffered severe desiccation due to water abstractions, expansion of agriculture, tourism, and pine plantations. In order to restore the natural dynamics of dune slacks, several restoration projects were initiated. These restoration projects often involved abandonment of water abstraction facilities as part of the hydrological restoration and removal of top soil from the aging dune slacks, in order to facilitate restart of vegetation succession. In such restoration activities in which millions of euros are invested, an assessment of long-term success is paramount. In order to investigate whether restoration of dune slack vegetation is successful over the long-run (Chapter 2), vegetation records of long-term permanent plots were analyzed using multivariate statistics and studied changes in environmental variables such as soil organic matter accumulation and changes in soil acidity. Geomorphological positions (isolated and non-isolated) and hydrological conditions (fed by surface water vs. groundwater) affected the pattern and rates of species colonization in dune slacks. In the long run, these factors also determined the success of the restoration project. Isolated dune slacks generally accumulated a third of organic matter accumulated in non-isolated dune slacks and, in terms of changes in soil acidity, soil pH was lower in non-isolated dune slacks, which is expected to be basically due to humic acid in soil organic matter. In terms of species colonization patterns, there seemed to be no difference between isolated and non-isolated dune slacks but both the cumulative and actual species richness (numbers of species) were higher in non-isolated dune slacks. In general, hydrology plays a key role in keeping soil organic matter lower and soil pH higher, which was observed in isolated dune slacks where there is exfiltration of groundwater into the dune slacks. A high species richness in dune slacks is determined by occurrence of flooding by surface water, the proximity of a local species pool and the way the seeds are being dispersed. 

Hydrology of dune slacks plays a key role (Chapter 2), but this is dependent on weather conditions. How hydrology affects species dynamics was studied in Chapter 3. Using bi-weekly measurements of water levels recorded for six years in the dune slack Koegelwieck on the Wadden Sea Island of Terschelling, long-term waterlevel fluctuation patterns were studied, using hydrological modelling software  (Menyanthes). Long-term fluctuation patterns influenced by meteorology, site heights, and coastal expansion variables were simulated. It was found that at Koegelwieck, hydrology is governed by meteorology i.e. precipitation and evaporation. These two meteorological variables explained on average about 85 % of the variance. With the computer model Menyanthes the mean summer groundwater level MSGL was calculated, which was then used as an explanatory variable in the vegetation analysis. In addition, measured environmental variables such as organic matter, soil pH and age of the vegetation were used as variables that determined changes in the vegetation. These results indicated that MSGL is a significant variable that affects the vegetation composition on most chronosequence sites at Koegelwieck. Soil organic matter, soil pH and age of the slack also  affected the vegetation composition. However, the effect of dry years became pronounced on the recently sod-cut site of 1995 where the occurrence of two very dry years (1996/97) led to a shift in vegetation composition to an older successional stage with many shrub species. Prolonged wetness at  the site of 1990 resulted in little accumulation of organic matter, and in a relatively high soil pH. The small dune slack species Littorella uniflora started to dominate and kept the vegetation in a pioneer stage. Chapter 4 and Chapter 5 present studies on the stability of pioneer dune-slack vegetation and the conditions that are necessary to keep the pioneer stages stable in the long run. For this follow-up study on the possible existence of alternation stable states (ASS) in dune slacks (Chapter 4), combinations of techniques such as multivariate analysis, calculations of similarity indices and statistical analysis were used on both permanent plot vegetation records and measurements of environmental variables. In general, despite the possible operation of positive feed-back mechanisms in dune slacks, the results indicate convergence of vegetation toward a homogenous community type within 12-14 years. Thus, it may be concluded that successional stages in dune slacks, including the pioneer stages, are states in transition. In addition, it was found that changes in vegetation preceded changes in soil conditions such as acidity and organic matter accumulation, which means that, when coupled with small perturbations of hydrology or meteorology, plant species can trigger a sudden shift in stages of succession. The  study in Chapter 5 led to the conclusion that the rate of soil organic matter accumulation is primarily determined by plant productivity. The results indicated that when dune slacks were flooded for the most part of the year, above-ground plant productivity and soil organic matter accumulation could remain very low since only species with the adaptive trait of radial oxygen loss were able to survive the flooding. Such species are often low-productivity species, such as L. uniflora, which are capable of maintaining vegetation succession at a pioneer stage. It was concluded that the rate of soil organic matter accumulation in wet dune slacks is primarily controlled by above-ground biomass of the vegetation. Both above-ground biomass and SOM accumulation can remain very low over a long period of time when dune slacks are flooded during most of the year and plants with adaptive traits are able to maintain vegetation succession at a pioneer stage. Radial oxygen loss (ROL) is an adaptation of several wetland species occurring in water-logged conditions. Using this adaptation, the plants put atmospheric oxygen into the soil surrounding their roots so that the roots can function under very harsh conditions. In dune slacks it was observed that species such as L. uniflora and S. nigricans use this adaptation to survive the conditions of high-water levels in which species without this capability cannot survive. In Chapter 6 a comparative study on a cushion plant ( Eriocaulon schimperi) occurring in groundwater fed mires on the Bale Mountains in Ethiopia was presented. E. schimperi has a root-bound strategy where it invest about 90% of its biomass underground and pumps oxygen into the soil in order to oxidize the anaerobic condition that surrounds its rooting zone. This root-bound strategy of E. schimperi also resulted in complete oxygenation of the peat layer well beyond its rooting zone reaching to about 60 cm below the surface. In an earlier study on the peatlands of the Patagonia region in Southern Argentina, it was found that another cushion plant, Astelia pumlia, performs a similar functional role preventing methane from deeper layer to reach the surface and escape to the atmosphere. What is unique about this species from the Bale Mountains is the fact that this species performs an identical functional role in a meso-eutrophic environment, unlike Astelia pumlia, which thrives in a very nutrient-poor bogs, mainly in competition with Sphagnum species, which are known for their efficiency in nutrient uptake.