A. G. van der Valk

Seed germination traits of annuals and emergents recruited during drawdowns in the Delta Marsh, Manitoba, Canada☆

Seed germination traits of the most abundant mudflat annuals and perennial emergents that become established during drawdowns (periods when all or part of a wetland is free of standing water) in the Delta Marsh, Manitoba, Canada, were examined. Seeds of both annuals (Aster laurentianus Fern., Atriplex patula L. and Chenopodium rubrum L.) and emergent perennials (Hordeum jubatum L., Scolochloa festucacea (Wild.) Link, Phragmites australis (Cav.) Trin. ex Steud. and Typha glauca Godr.) from this prairie, lacustrine marsh germinated best in the light. Stratification also improved the seed germination percentages of all species except for Scolochloa. Germination percentages were highest in alternating 15/25°C and 20/30°C temperature regimes and lowest in a 5/15°C regime, Seed germination percentages for all species were highest when the seeds were on the surface and declined sharply when seeds were covered by as little as 1 cm of sand. Seedlings of species with large seeds (e.g. Hordeum and Scolochloa) could reach the surface from soil depths of down to 5 cm wheras seedlings of the smallest seeded species (Typha) could reach the surface from no more than 1 cm depth. Seed germination of Hordeum and Phragmites was unaffected by salinities as high as 5000 mg l−1 of NaCl, whereas seed germination of Typha and Scolochloa was reduced significantly by 1000 mg l−1 of NaCl. Of the three annuals, seed germination of Aster was least affected by increasing salinity levels (no effect until 4000 mg l−1) while that of Chenopodium was the most sensitive (reduced at 2000 mg l−1). Because the germination of seeds of each species is affected differently by temperature, salinity, light conditions and depth of burial, microenvironmental variations from site to site during a drawdown could result in different species becoming established at sites with identical seed banks.

A reconstruction of the recent vegetational history of a prairie marsh, eagle lake, Iowa, from its seed bank

Abstract Substrate samples were collected at four depths (0–5, 10–15, 20–25, and 30–35 cm) from six vegetation types at Eagle Lake, Iowa. The number of viable seeds in a sample was estimated by placing samples in environments appropriate for seed germination and counting the number of seedlings. Quantitative and qualitative differences in the composition of the seed bank at different depths and locations revealed that the marsh has two distinct vegetation areas. In the northern area, the vegetation changes cyclically. These cycles involve a rotation of three vegetation types: submerged, mudflat, and emergent ( Scirpus validus Vahl./ Typha glauca Godr.). In the southern (shallower) area, the composition of the seed bank indicated that there have been no cyclical changes in vegetation. The presence, however, of Typha seed in 0–5 cm samples and in some 10–15 cm samples (in numbers exceeding those normally found in contemporary Typha communities) in vegetation types presently dominated by Sparganium eurycarpum Engelm., Scirpus fluviatilis (Torr.) Gray, and Carex atherodes Spreng. suggest that, in recent years, Typha has declined significantly in the southern area. Because of the failure of their seeds to germinate under assay conditions or because the dominant species annually produces very little seed, most shallow-water emergent community types could not be detected in the seed bank.

The impact of litter and annual plants on recruitment from the seed bank of a lacustrine wetland

In an experimental marsh complex, 2 years of high water (1 m above normal) killed most of the emergent vegetation. During 1983, the first year of a 2-year drawdown, immediately following this period of high water, field experiments involving the removal of fallen emergent, filamentous algal and mixed (algal and emergent) litter were established at 6, 4 and 7 sites, respectively. All three indicated that the removal of litter significantly increased the number of species and the number of individuals of a species recruited from the seed bank. During 1984, the number of species and number of individuals in the removal treatment at emergent- and mixed-litter sites was again significantly higher than in the controls, but there was no longer a difference at algal litter sites. The addition of a mat of Typha litter in June 1983, at five sites that were free of litter, reduced seedling recruitment from the seed bank almost completely in both 1983 and 1984. The removal of 1982-standing litter, from sites at higher elevations that were invaded by Typha or Phragmites during the high water period, also increased the number of species and the number of individuals of a species compared with the controls in 1983. Removal of seedlings of the mudflat annual, Atriplex patula L., increased the number of grass shoots significantly in both 1983 and 1984. Grasses in the seedling removal treatment were also taller and flowered in 1983, the first year of the drawdown.

Wetland classification and inventory: A summary

Regional, national and local wetland classifications have been developed and successfully applied. These have invariably been orientated towards conservation and management goals, and the information used to assess wetland loss or to assign management priorities. Existing national and regional classification systems have not only been useful, but they provide an essential base for developing an international system. At the international level, differences among existing systems in the definition of a wetland and how wetland types are defined assume great importance and need to be resolved. Classification is an essential prerequisite for wetland inventory. A number of international inventories have been undertaken, although these have not generally utilized the available high technology and data storage systems available through remote sensing and geographic information systems. More extensive international inventories will require standardization of techniques for data collection, storage and dissemination. A minimum data set needs to be defined with standards for data accuracy. An international committee under the auspices of an international agency (e.g. IWRB, Ramsar Bureau, IUCN) needs to be established to develop an international classification system and guidelines for carrying out a complete inventory of the worlds wetlands.

Uptake and release of nutrients by living and decomposing Typha glauca godr. tissues at Eagle Lake, Iowa

Abstract Uptake and release of nitrogen, phosphorus, potassium, calcium, and sodium from living above-ground and below-ground tissues and from decomposing litter of Typha glauca Godr. were studied at Eagle Lake, IA, during 1976. All nutrients were accumulated rapidly by shoots in the spring. Some of the nitrogen and phosphorus came from belowground storage; but potassium, calcium, and sodium were extracted entirely from the soil. Nutrients were immobilized in shoot tissues for different periods of time. Potassium content declined as rapidly as it had accumulated, and there was no evidence of belowground storage. Nitrogen and phosphorus content also declined, though not as rapidly. Approximately 45% of the nitrogen and phosphorus lost from the shoots was translocated to the rhizomes and stored. Calcium and sodium were conserved in shoot tissues until the shoots died. In the decomposing litter, potassium and sodium content declined, phosphorus and calcium content remained relatively constant, but nitrogen content increased. Over the full year of production and decomposition, this Typha glauca stand accumulated calcium and nitrogen, maintained phosphorus levels, and lost potassium and sodium.

The impact of a natural drawdown on the growth of four emergent species in a prairie glacial marsh

Abstract Changes in total, vegetative and flowering shoot densities, weights, heights and standing crops of four emergent species before, during, and after a drought indicate that the growth of three of these species ( Typha glauca Godr., Scirpus fluviatilis (Torr.) Gray and Sparganium eurycarpum Engelm.) was adversely affected by the drought. The drought, however, temporarily reversed the decline in vigor, which had started before the drought, in the fourth species, Scirpus validus Vahl, and enabled this species to persist for two more years in the marsh. The data suggest that periodic drawdowns enable several emergent species to coexist in a community because of their diverse responses to disturbance.

Effects of prolonged flooding on the distribution and biomass of emergent species along a freshwater wetland coenocline

Water levels were raised 1 m for two years in 10 cells of an experimental wetland complex located in the Delta Marsh, Manitoba, Canada. The mean area covered by each of the 5 dominant emergent species in these cells declined significantly during the first year of flooding. There was no significant difference in the total acreage of emergents between flooding years. Three species were completely (Carex atherodes Spreng, Scolochloa festucacea (Willd.) Link) or almost (Scirpus lacustris L. spp. glaucus (Sm.) Hartm.) eliminated from the cells. Typha glauca Godr. and Phragmites australis (Cav.) Trin. Still covered in the second year of flooding about 40 and 25%, respectively, of the area that they had occupied in the cells in the preflooding year. The mean elevation at which the two surviving species were found both increased 0.1 and 0.2 m over preflooding elevations during the first and second year of flooding, respectively. There is no evidence that during the flooding years any species migrated upslope toward more optimal water depths. Aboveground standing crop for all five species declined significantly during the first year of flooding, but was not significantly different between the two flooding years. Total belowground biomass did not differ between spring and fall, but declined significantly (about 20%) during the first year of flooding and remained about at that level during the second year. In the spring following the two years of flooding, mean total belowground biomass was only 40 g m-2, less than 10% of the belowground biomass during the preflooding year.

Changes in the composition, structure, and production of plant communities along a perturbed wetland coenocline

Comparisons of a wetland coenocline before and after a drought-caused drawdown and the refilling of the basin to a higher-than-normal water level revealed that there was little difference in the above ground standing crop or in the floristic composition of the submersed, emergent or meadow zones along the coenocline. However, the position of many species along the coenocline shifted in response to these perturbations.Sagittaria latifolia, Carex spp.,Patamogeton sp. aff.pusillus all were found 2 to 4 m closer to shore.Glyceria grandis is normally found only in the meadow zone; but after the drawdown, it was found along the whole lenght of the coenocline. The drawdown also nearly eliminatedCeratophyllum demersum from the coenocline. Species richness as a result of these perturbations increased in the submersed (3.3 to 5.9) and emergent (3.6 to 4.3) zones, but decreased in the meadow zone (7.6 to 6.0). The Simpsons index in all three zones was not appreciably influenced by the perturbations. The average Simpsons indices for the meadow, emergent and submersed zones were 0.4, 0.7 and 0.5 respectively.

The biomass of an Indian monsoonal wetland before and after being overgrown with Paspalum distichum L.

Paspalum distichum L. has been the dominant species in the monsoonal wetlands of the Keoladeo National Park in northcentral India since 1982 when grazing by water buffalo and domestic cattle was halted. Maximum water levels in these wetlands occur immediately after the end of the summer monsoon in late September of early October and then decline until the next summer monsoon the following June. After the normal 1985 monsoon, maximum water depths were around 140 cm. After the poor 1986 monsoon, maximum water depths were only around 60 cm. Paspalum distichum maximum aboveground biomass at four sites ranged from 850 g m-2 at the shallowest site to 3400 g m−2 at a deep water site. The maximum biomass of other vegetation types, which had dominated this wetland prior to 1982, ranged from 1400 g m-2 at a deep water site (Ipomoea aquatica Forsk.) to only 240 g m-2 to 400 g m-2 at a deep-water submersed site (Hydrilla verticillata (L. f.) Royle/Cyperus alopecuroides Rottb.) and at a shallow emergent site (Scirpus tuberosus Desf./Sporobolus helvolus (Trin.) Dur. et Schinz). For all vegetation types, biomass changed seasonally in response to changing water levels and temperatures. After the 1986 monsoon, above-ground biomass for all vegetation types was much lower than it had been after the 1985 monsoon. Mean below-ground biomass was very low in all vegetation types (1 to 47 g m-2). Paspalum distichum had a higher aboveground biomass at nearly all water depths in all seasons than that of the pre-1982 vegetation types. Paspalum distichum belowground biomass, however, is comparable to, or less than, that of the pre-1982 vegetation types. During years with an average monsoon, the overall primary production of these wetlands is estimated to have increased 2.5 to 3.5-fold since they were overgrown with Paspalum distichum.

Litter decomposition in an Indian monsoonal wetland overgrown withPaspalum distichum

After the removal of domestic cattle and water buffalo in 1982, the monsoonal wetlands of the Keoladeo National Park, India were rapidly overgrown by the grassPaspalum distichum L. We studied the decomposition of above-ground litter ofPaspalum distichum and that of ten submersed, floating-leaved, free-floating and emergent species, including the dominants in these wetlands prior to 1982. The only species that had decomposition rates as low as those ofPaspalum were the emergents,Pseudoraphis spinescens (R. Br.) Vickery andTypha angustata Bory & Chaub, which had very restricted distributions both before and after 1982. All the previous dominants had much higher decomposition rates. The turnover time forPaspalum litter was estimated to be greater than one year, while litter of the previous dominants all had turnover times of less than one year. Thus, a permanent litter layer could develop in these wetlands that had not been present previously.