Arnold G. van der Valk

The Vegetation of Restored and Natural Prairie Wetlands

Thousands of wetland restorations have been done in the glaciated mid-continent of the United States. Wetlands in this region revegetate by natural recolonization after hydrology is restored. The floristic composition of the vegetation and seed banks of 10 restored wetlands in northern Iowa were compared to those of 10 adjacent natural wetlands to test the hypothesis that communities rapidly develop through natural recolonization. Restoration programs in the prairie pothole region assume that the efficient-community hypothesis is true: all plant species that can become established and survive under the environmental conditions found at a site will eventually be found growing there and/or will be found in its seed bank. Three years after restoration, natural wetlands had a mean of 46 species compared to 27 species for restored wetlands. Some guilds of species have significantly fewer (e.g., sedge meadow) or more (e.g., submersed aquatics) species in restored than natural wetlands. The distribution and abundance of most species at different elevations were significantly different in natural and restored wetlands. The seed banks of restored wetlands contained fewer species and fewer seeds than those of natural wetlands. There were, however, some similarities between the vegetation of restored and natural wetlands. Emergent species richness in restored wetlands was generally similar to that in natural wetlands, although there were fewer shallow emergent species in restored wetlands. The seed banks of restored wetlands, however, were not similar to those of natural wetlands in composition, mean species richness, or mean total seed density. Submersed aquatic, wet prairie, and sedge meadow species were not present in the seed banks of restored wetlands. These patterns of recolonization seem related to dispersal ability, indicating the efficient-community hypothesis cannot be completely accepted as a basis for restorations in the prairie pothole region.

Recruitment from the seed bank and the development of zonation of emergent vegetation during a drawdown in a Prairie Wetland

(1) Patterns of recruitment of five emergent species from the seed-bank along a height gradient in an experimental wetland complex during a period with no standing water (drawdown), are described in relation to the development of zonation. The drawdown was preceded by two years in which water levels were maintained at 1 m higher than normal which destroyed most of the emergent vegetation. (2) Zonation of established, adult emergents was shown by the separation of peak frequencies of different species along the height gradient and the occurrence only one species in 64% of quadrats sampled. (3) For four emergent species, the distribution of seedlings along the height gradient during drawdown were similar to the pre-flooding distributions of adult plants. Scirpus lacustris and Typha spp. both had peak abundances of seedlings and adults at lower heights than those of Scolochloafestucacea and Carex atherodes. The seedlings of the fifth species, Phragmites australis, reached their maximum density at a height well below that at which adult plants were most frequently encountered prior to deep-flooding. (4) Seedlings of Scirpus lacustris, Typha spp. and Phragmites australis all had maximum densities at the same height. Seedlings of Scolochloafestucacea and Carex atherodes also reached maximum densities at about the same height. Seedlings of more than one species were found in 81 % of the permanent quadrats sampled. (5) For a given species, the difference in heights between permanent quadrats where maximum densities of seedlings occurred and seed-bank samples with maximum densities of germinable seeds was generally no more than 20 cm; most distributions were unimodal. Along the height gradient, differences in environmental conditions seem to have had less impact than the distribution of seeds on the distribution of seedlings. (6) These differences in distributions along the height gradient and frequencies of monodominant stands between seedlings and adult plants imply that post-recruitment processes play a major role in the development of zonation patterns in the wetland studied.

Effects of sediment load on seedling emergence from wetland seed banks

We examined the effects of sediment depth on emergence of seedlings from wetland seed banks, with the goal of understanding potential effects on wetlands of sediment runoff from agricultural fields. Seed germination was studied in the greenhouse using seed bank samples taken from natural wetlands in central Iowa, U.S.A. Sediment loads as low as 0.25 cm significantly reduced the number of species and total number of individuals recruited from seed bank samples. Addition of sediment decreased the number of individuals appearing for most, but not all, species. The change in number of seedlings that occurred in treatments with 1 cm of sediment cover was related to seed mass, with larger-seeded species showing the least effect of burial by sediment.

The biology of freshwater wetlands

Preface 1. Introduction 2. Water and soil 3. Microorganisms and invertebrates 4. Wetland plants and animals 5. Spatial and temporal patterns 6. Wetland functions 7. Invasive species 8. Restoration and creation 9. Global climate change 10. The value and future of wetlands Glossary Bibliography Index

Maintaining tree islands in the Florida Everglades: nutrient redistribution is the key

The Florida Everglades is an oligotrophic wetland system with tree islands as one of its most prominent landscape features. Total soil phosphorus concentrations on tree islands can be 6 to 100 times greater than phosphorus levels in the surrounding marshes and sloughs, making tree islands nutrient hotspots. Several mechanisms are believed to redistribute phosphorus to tree islands: subsurface water flows generated by evapotranspiration of trees, higher deposition rates of dry fallout, deposition of guano by birds and other animals, groundwater upwelling, and bedrock mineralization by tree exudates. A conceptual model is proposed, in which the focused redistribution of limiting nutrients, especially phosphorus, onto tree islands controls their maintenance and expansion. Because of increased primary production and peat accretion rates, the redistribution of phosphorus can result in an increase in both tree island elevation and size. Human changes to hydrology have greatly decreased the number and size of tr...

Plant diversity, composition, and invasion of restored and natural prairie pothole wetlands: Implications for restoration

Hundreds of wetlands comprising thousands of hectares have been restored in the Midwestern United States. In nearly all cases, restoration consisted of simply restoring wetland hydrology. For this reason, the success of these restorations relies on natural colonization. We compared the structure and composition of the vegetation in two types of wetlands: 10 natural wetlands and 17 five-to-seven-year-old restored wetlands. The overall vegetative composition of restored wetlands was different from that of natural wetlands. Restored wetland flora was formed from a subset of species found in natural wetlands. The species restricted to natural wetlands tended to be native perennials and were evenly represented along the elevational gradient. The few species that were restricted to restored wetlands were largely mudflat annuals whose presence is more indicative of the presence of more unvegetated habitat in restored wetlands than of the presence of a distinctive restored wetlands flora. In addition, restored wetlands had lower vegetative cover and species richness than natural wetlands. Both wetland types had similar numbers of exotic species at the whole wetland (4.4 species per wetland) and quadrat scale (1.5 species m2), and dominance of exotics increased with elevation. The lower species richness, greater compositional variability, and lack of a distinctive flora support the hypothesis that dispersal limitation is the primary cause of the differences between the vegetation in restored and natural wetlands.

The role of water depth and soil temperature in determining initial composition of prairie wetland coenoclines

In this study, we examined the effects of water depth and temperature on seedling recruitment from a prairie wetland seed bank. We collected seed-bank samples from natural and restored prairie pothole wetlands in northwestern Iowa and combined them into a single sample. We examined seedling recruitment from this seed-bank sample in an experimental study using a factorial design of 4 temperature treatments (5° night and 15° day to 20° night and 30° day) and 3 water-depth treatments (0, 2, and 7 cm).Principal Components Analysis showed that both water depth and temperature had significant effects on the composition of the seedling community as measured by changes in relative stem density and biomass. Water depth had its strongest effects on stem density while temperature had its strongest effects on biomass.For the 22 most common species, stem density varied with water depth for 95% of the species and with temperature for 50% of the species. Most species with water depth responses had lower stem counts as water depth increased, and for the majority of species with temperature responses stem density increased with temperature.Total, annual, and perennial species richness was negatively correlated with water depth. Total and annual species richness was positively correlated to temperature, while perennial species richness was unresponsive to temperature. In addition, species found at low elevations as adults emerged at higher rates in the deep water treatments while species that occurred at higher elevations as adults had their highest emergence rates in the low water treatments.Our results suggest that differences in environmental conditions along coenoclines can affect the initial distribution of species emerging from the soil seed bank. Water depth sorted seedlings according to their adult water-depth tolerances, and temperature determined the proportion of annuals in the seedling community.

Characteristics of recently restored wetlands in the prairie pothole region

Between 1987 and 1991, 1892 prairie potholes were restored in northern Iowa, southern Minnesota, and southeastern South Dakota by state and federal agencies, most as part of the Conservation Reserve Program. The total area covered by these restored wetlands is approximately 2714 ha. Most restorations are small (less than 4 ha) wetlands with a seasonal hydrologic regime. Wetlands with an ephemeral/ temporary water regime are under-represented compared to their pre-drainage extent. Information on basin morphometry, hydrology, and vegetation-zone development was collected on 62 wetlands restored in 1988. Earthen dams are installed on most (73%) restorations in the region, increasing the full pool volume but not the mean depth of the basin. Overall, restored wetlands have basin morphometries that are comparable to those of similarly sized natural wetlands. About 60% of the basins had their predicted hydrology or held water longer than predicted. Nevertheless, about 20% of the projects that we examined were hydrologic failures and either never flooded or had significant structural problems. Most restored wetlands had developed emergent and submersed aquatic vegetation zones, but only a few, had developed wet prairie and sedge meadow vegetation zones.

The restoration of sedge meadows: seed viability, seed germination requirements, and seedling growth ofCarex species

To better understand how to establishCarex species from seed in created and restored wetlands, a series of experimental studies was conducted onCarex seed and seedlings. These studies included (1) the effect of seed age (1 to 18 months after collection) on viability and germination, (2) storage conditions (+4,-4 or +4/-4°C; wet or dry) on seed germination, (3) soil moisture on seed germination, and (4), soil amendments (fertilizer, topsoil, and compost) on seedling recruitment and growth. Seeds ofCarex species of several provenances would not germinate to any appreciable extent once they were more than six months old. ForCarex aquatilis, germination could be increased by storing its seed at either 4°C or—4°C. ForCarex lacustris andCarex stricta, seed germination decreased significantly, by about 50 to 100% in all 8 storage treatments. Germination ofCarex stipata seed was highest in 1 cm of standing water, and its seed did not germinate in the driest soil moisture treatment.Carex stricta seed germination was not affected very much by soil moistures. NoCarex spp. were recruited from seed in any of the field soil-amendment treatment plots. In both the greenhouse and field, the addition of compost, topsoil, and fertilizer increased the growth ofCarex stricta. In a greenhouse study, above ground, below ground, and total dry mass increased linearly with the percent compost added. Our results suggest that the probability of establishingCarex spp. from seed in created and restored wetlands in the Upper Midwest would be maximized by using fresh seed, preferably seed produced earlier in the same growing season; by keeping soil moisture levels as high as possible; and by raising the soil’s organic matter content, if need by, through the use of suitable soil amendments to levels found in natural sedge meadows.

The potential role of ducks in wetland seed dispersal

A controlled feeding study and field collections of duck feces were used to examine the potential of ducks for dispersing wetland plant species in the prairie pothole region of North America. To determine how long seeds of wetland species remain in their digestive system, captive mallards were fed known quantities of seed of either Atriplex patula, Chenopodium album, Chenopodium rubrum, Echinochloa crusgalli, Polygonum spp., Scirpus acutus, Scirpus validus, or Sparganium eurycarpum. Fecal samples were collected hourly from ducks for 24 to 30 hours after seeds were ingested. On average, 23% of the seeds ingested were recovered intact, and 36% of these were still viable. In other words, only 7% of the seeds ingested were still viable when they were evacuated, with a range from 1% for Echinochloa crusgalli to 16% for Scirpus validus. The mean passage time of seeds was 7.6 h, with a range of 5.1 h for Echinochloa crusgalli to 11.1 h for Chenopodium rubrum. Nearly all seeds were evacuated after 24 to 30 h. Seed size and seed coat strength (lignin content) were negatively correlated with the probability of a viable passage but not correlated with passage rates. After ingestion, mallards are estimated to be able to transport internally viable seeds up to 1,400 km but more typically 20 to 30 km. Field collections of duck feces at five locations in the prairie pothole region confirmed that ducks are dispersing intact seeds of common wetland species (Carex sp., Echinochloa sp., Panicum sp., Polygonum spp., Potamogeton spp., Salix sp., and Scirpus spp.). There were between 0.3 to 5.2 intact seeds per duck. Only 1.6% of this intact seed of four taxa (Carex sp., Potamogeton spp., Salix sp., and Scirpus spp.) germinated under greenhouse conditions. Although any given duck is carrying only a few viable seeds, the millions of ducks moving among wetlands, especially during spring and fall migrations, collectively are effective dispersal agents for many wetland plant species.