Vegetation dynamics and erosion resistance of sea dyke grassland

In the Netherlands, in addition to the width and height of the dyke body itself, renewed measures for reconstruction and maintenance of dykes have stressed the importance to the safeguarding of the dyke, of the grass cover's protection of the clay-layer against erosion.From an inventory of different aspects of dyke grassland erodibility and the construction of the grass-clay complex (Chapter 1), it became clear that roots and the configuration of the root system hold the key to the specific erosion resistance properties of grasslands on clay dykes. Roots have a considerable influence on soil stability as well as soil porosity. They contribute to the formation of small aggregates in clay soils and supply adhesive substances which bind these particles, resulting in a fine structured and stable soil. Conduits remain where roots die off, increasing the total pore volume and thus the permeability of the soil, which enables rapid draining of incoming water from wave attack or overtopping. A high porosity and fine structure of cemented particles ensures that only relatively small amounts of soil material will be washed away, while a dense root network prevents larger soil particles to flush away.Management of the grass sward has a great impact on the quality of erosion resistance, because of its influence on vegetation cover and root density. Sheep grazing or frequent cutting usually results in a closed sward but also reduces the amount of root growth. Haymaking on the other hand, leads to deeper root growth at the cost of shoot density. When fertilizer is applied on pastures or when cuttings are not removed, root densities are low, and overgrazing or suffocation results in large open spots. Previous studies on dyke grasslands revealed that, in contrast with species-poor, heavily fertilized and intensively grazed pastures or fertilized hayfields, the species-rich vegetation types (unfertilized pasture or hay meadow) have a dense root system and a relatively high erosion resistance.In Chapter 1 the relation between management, vegetation type, root density and erodibility is reviewed in detail, with the emphasis on the increase of root density by lowering the nutrient level in soils. This review brings us to the main question of this thesis: is it possible to enhance root density and erosion resistance of dyke grassland by cessation of fertilization combined with a less intensive grazing or cutting regime? To answer this question, an experiment was set up on dykes along the Dutch coast (Chapter 2) to analyse the effects of cessation of fertilization and different management types on: botanical composition and vegetation cover (Chapter 3); above- and below ground biomass and the amount of nutrients in plant and soil (Chapter 4); and erosion resistance (Chapter 6). Seasonal variability and spatial patterns in vegetation cover and root density have also been analysed (Chapter 5), and practical implications of the results discussed (Chapter 7).ExperimentIn January 1991 14 locations were selected on grazed dykes for four treatments on each location: continuing grazing with the use of fertilizer; two periods grazing without fertilizer application; haymaking followed by grazing without fertilizer use; and haymaking (two cuts) without fertilizer application. On mown dykes 9 locations were selected for two treatments: haymaking with and without fertilizer application. Species-rich hay meadows that had not been fertilized for at least 20 years were treated as references.In March 1994, data on vegetation cover and root density were collected from plots in the different treatments at all sites, and erosion spray experiments were carried out. Samples were taken at the same time for erosion centrifuge experiments. In the summer of 1994 the botanical composition in the plots was described and soil samples were taken together with data of the above and belowground biomass. The aboveground biomass was also determined for the preceding years. Seasonal variability in above- and belowground production was analysed on the basis of six-weekly measurements in 1992 in fertilized pastures and on reference dykes. Data on spatial variation were collected in 1993. The experiment continued until the summer of 1997, and data were collected in 1995 and 1997.Botanical composition and vegetation cover.The vegetation of the investigated dyke grasslands (Chapter 3) can be described as relatively species poor (8 to18 species per 25 m 2 ), although some sporadic species-rich grasslands were located (25 to 34 species per 25 m 2 ). Nine plant communities were distinguished. Three communities were assigned to variants of the Lolio-Cynosuretum cristati and a related trunk community, and one to the Poo-Lolietum, while five communities were described as variants and subassociations of the Arrhenatheretum elatioris and trunk communities characterized by species of the Arrhenatheretalia. Although the variance in species composition was primarily determined by the difference in study sites and linked habitat factors (soil texture, exposure to sunlight and salt spray), multivariate analysis of species cover and environmental factors revealed a small but significant effect of cessation of fertilizer application in combination with grazing or haymaking.Cessation of fertilizer application produced a slight increase of species numbers (from 13 to 16 species per 25 m 2 in unfertilized treatments on grazed dykes and 14 to 17 species on mown dykes), and a strong decline in cover /abundance of Lolium perenne and an increased abundance of species indicative of less nutrient-rich conditions like Festuca rubra and Trifolium dubium. Furthermore, the proportion of herbs in the grass sward was observed to have increased. The question remains as to whether this rather rapid response to the change in regime, compared to other experiments, is an effect of the poor moisture conditions of dyke grasslands, resulting in a direct, drastic lowering of nutrient availability when fertilization has stopped.The observed vegetation change after fertilization cessation combined with haymaking, which also on sea dykes can be considered as the result of the interaction between the changed nutrient availability and vegetation structure, did not adversely affect the sward density. The somewhat lower cover of the vegetation in the species-rich hay meadows is compensated by a high shoot density, indicating a diffuse distribution of shoots over the surface and the presence of small open spots. This means that apart from vegetation cover (cut back to 2 cm above ground level) shoot density must also be taken into account when judging the sward quality from a civil engineering point of view.Above and belowground biomassThe rather rapid decline observed in aboveground biomass production 3-4 years after stopping fertilization (from 9-10 tonnes dry matter to 7 t ha -1y -1) followed by a further decline up to 5-6 t ha -1y -1after 7 years, complies with the results of a restoration experiment of species-rich meadows on clay and peat soils. Shifting from grazing to haymaking, without the use of fertilizer, results in the most rapid fall in productivity from 9 to 5 t ha -1y -1after 7 years, reaching a level that is favourable for the development of species-rich grassland. The peak biomass (July) is higher on sun exposed slopes, whereas regrowth (July - October)on these slopes is lower compared to slopes that receive a smaller amount of sunlight. In agreement with the decrease in production a decline in availability of mineral nitrogen was measured. The total soil N pool in unfertilized treatments, however, did not change when compared to the fertilized plots, although there seems to be a small net N off-take according to our calculations, especially in the mown treatments. In spite of low P concentrations in shoots of the species-rich old meadows, the low N/P ratio indicates that productivity is N limited in these grasslands.Cessation of fertilization resulted in an increase of root length and root weight in the mown treatments on both pastures and hay meadows. This extension of the root system at the cost of aboveground plant parts was expressed by a lower shoot/root ratio in the unfertilized mown treatment on former pastures. The species-rich old hay meadows were characterized by a relatively high root length and root weight and a low shoot/root ratio; the maximum difference between these meadows and the species-poor fertilized pastures was detected at a depth of 3-15 cm. The latter demonstrated a sharp decline of root length density compared with a less steep decline in the species-rich meadows. In the unfertilized mown treatments the vertical distribution of roots shows similarities to that of the old meadows. Pastures exhibited relatively high values for specific root length (SRL, thin roots) and low total root weights, whereas low SRI, values and high root weights (thick roots) were found in hayfields. The species-rich hay meadows, however, had high SRL values and high total root weights. The reactions of grassland plants and root growth in response to the decline in N availability and the different management types (grazing or haymaking) together with their morphological consequences are discussed.Temporal and spatial variationSeasonal variation in root characteristics, which can affect the erodibility of dyke grasslands, was investigated in fertilized pastures and unfertilized old hayfields (Chapter 5). Maximum aboveground biomass was reached in May-June in the fertilized pasture and in July in the hay meadow; both achieving maximum regrowth was reached in September. In agreement with the results given in Chapter 4 the investment in belowground biomass was much larger in the unfertilized hayfield than in the fertilized pasture. No significant fluctuation in root mass was measured during the year in both grassland types, so that variation in shoot/root ratio is similar to the variation in aboveground biomass. In the fertilized pasture the seasonal fluctuation in root length took place in the top 3 cm of the soil, where the increase of the predominantly fine roots paralleled the aboveground growth, with peaks in July and September. In the unfertilized hay meadow, where we measured higher root lengths at depths of 3-20 cm than in the fertilized pasture, the variation in root length occurred below this upper layer (from 3 to 40 cm).The spatial variation in floristic diversity, vegetation cover and root weight density was studied (Chapter 5) in three grasslands belonging to the Arrhenatheretum elatioris brizetosum (old unfertilized hayfield), Lolio-Cynosuretum cristati (lightly fertilized pasture) and Poo-Lolietum (heavily fertilized pasture). The first two types exhibited a more mosaic pattern of floristic composition compared to the spatial floristic homogeneity of the Poo-Lolietum. The last-mentioned had a large spatial variation in the size of open spots, and in root growth. The high root weights in the Arrhenatheretum showed less spatial variation. Also, the Lolio-Cynosuretum had a more homogeneous spatial distribution of roots. Apparently, aboveground floristic homogeneity is not necessarily correlated with a small variation in root density. Within each grassland type, patterns in horizontal distribution of roots were not detected, and there was little variation in the vertical distribution of roots. However, in agreement with previous results (Chapter 4), in the unfertilized hay meadow, root mass declined less steeply with depth than in the other communities.ErodibilityThe erosion resistance of the dyke grassland sites was investigated by measuring three different parameters (Chapter 6): the shear strength at different depths of the rooted soil layer, the soil loss in erosion spray experiments (wear erosion), and the percentage weight loss of samples tested in an erosion centrifuge device (internal erosion).Our observations revealed that the shear strength of soils with high root densities was low, while the shear strength of soils with a high percentage of loam, low vegetation cover, low root length densities or few but relatively thick roots, was high. These results suggest that shear strength does not reflect soil cohesion due to a higher root density. Moreover, no correlation was found between erosion parameters and shear strength. We concluded that shear strength can not be used as a reliable parameter for erosion resistance of grass swards on clay.The erosion spray experiment did not reveal any significant trend in soil decline or washed out material between the different treatments. Only at higher flow rates did the species-rich hay meadows appear to be more resistant against wear erosion than species~ poor pastures. It would seem that differences in wear erosion between differently managed dyke grasslands were difficult to demonstrate, because of the relatively high vegetation cover and the small range in soil particle size.Most samples demonstrated a sudden increase in weight loss rate during the course of the centrifuge experiment. This was probably due to the roots in the soil which prevents erosion of soil particles until they are damaged to such an extent that the erosion rate is determined by soil properties only. Both erosion rates were lower in the 0-5 em samples, whereas in the 5-10 em samples erosion proceeded more rapidly reaching a sample collapse in most cases. Multiple regression of erosion parameters and soil and root characteristics revealed that at a depth of 0-5 em, with the largest amount of roots, the erosion rate in the first half of the experiment (before the sudden change) was determined mainly by root density, whereas at a depth of 5-10 em both root density and sand content influenced the rate of weight loss. In the second half of the experiment the erosion rate in both layers was influenced mainly by sand and loam content. Resistance against internal erosion is determined by the combined effects of root density and soil particle size, probably because of the influence of roots on aggregate stability and porosity. Regardingthe root length - root weight ratio, this type of soil reinforcement seems to result more from the root system as a whole, including many thin roots, than the tensile strength of the roots.Species-rich hay meadows show higher resistance values than fertilized grasslands, which can be attributed to their higher root densities. Cessation of fertilization led to a slight increase in erosion resistance after three years. This agrees with the increase in root density and the change in botanical composition of plots were fertilization has stopped.To judge the erodibility of grass swards on clay dykes, it is proposed to use botanical composition (reflecting root density), and shoot density (reflecting surface quality) as reliable criteria rather than clay quality of the upper layer.ManagementBoth rooting pattern and spatial heterogeneity of the vegetation, being the main factors determining erosion resistance of dyke grassland, can be influenced by the type of management. The main prerequisites for a high sod quality appears to be: the avoidance of fertilizer application and continuity of management and maintenance over longer periods The unfertilized old hay meadows proved to have the highest erosion resistance and root densities. It is argued that unfertilized meadows maintained by sheep-grazing, will also have a high erosion resistance, while the agricultural earnings do not seem to differ much from those of fertilized pastures. However, fertilizer can be applied at a maximum of 50 kg N ha -1y -1in combination with grazing, without substantial loss of sod quality, but complete stopping of fertilization gives even better results.To improve and maintain grassland quality in terms of erosion resistance, management rules consist of. regular mowing and dragging when grazing; allowing a maximum vegetation height of 10 em at the onset of winter; removal of cuttings after one week; active control of mole activity; and the continuity of management. It is concluded that for judging the erodibility of dyke grasslands more emphasis should be put on botanical composition, vegetation cover and root density rather than on clay quality in the sod, which is an important factor in present erodibility tests.