Establishment, growth and degeneration of Ammophila arenaria in coastal sand dunes

Introduction This study deals with the establishment, growth, and degeneration of Ammophila arenaria (marram grass), a grass species that dominates the vegetation in coastal foredunes. Following natural establishment from rhizomes on high parts of the beach A.arenaria reduces wind velocity, which results in the accretion of windblown sand and the formation of dunes. A. arenaria grows vigorously in mobile dunes where fresh sand is deposited by wind, but it disappears from the vegetation when these dunes become stabilized. Because of its ability to stabilize the sand, A. arenaria is often used in dunes to control erosion. It is planted according to a long practicised manual technique, using culms that are collected from stands at the foredune. When foredunes are reconstructed, however, (e.g. after storms) the establishment of A. arenaria is often unsuccessful, which means that erosion control can become very costly. This study was carried out to develop new methods of establishing A. arenaria and to investigate the cause of the replant failures on reconstructed dunes. In addition, experiments were carried out to eludicate the relationship between the colonization of windblown sand and the vigour of A. arenaria . Establishment Two alternative methods were developed: (1) the sowing of seeds, which is rarely practiced and (2) the disk-harrowing of rhizomes, which has not previously been applied. Experiments showed that seeds hardly germinate at constant temperatures. A high rate of germination can be achieved with fluctuating high temperatures, but at low temperatures the seeds required stratification in order to germinate. In the field the seeds germinated well if they had been sown during the winter, which was probably because of natural stratification. Seeds can be sown mechanically while afterwards the sand surface needs to be stabilized. For this purpose straw proved to be more effective for seedbed stabilization than spraid compost. Rhizomes (i.e. vertical underground stems) were collected from the foredune by mechanical sieving of the sand. Experiments proved that the disk-harrowing of rhizomes was a useful method of establishing A. arenaria provided that the sand surface was temporarily stabilized with straw or planted bundles of reed (chapter 2). Growth Field experiments showed that higher sowing rates and higher planting densities of rhizomes resulted in higher numbers of seedlings and primary shoots. However, after one growing season production of biomass and numbers of tillers appeared to be independent of the initial density. Application of slow-release NPK fertilizer (Osmocote, 12 to 14 months active at 21 °C) increased dry matter yield and numbers of tillers (chapter 2). In a large-scale field experiment, rhizomes and a combination of planted culms and rhizomes produced more biomass and percentage cover than a sown stand during the first growing season. The lowest dry matter yield and percentage cover were obtained with traditionally planted culms. All plantings had been supplied with the same amount of slow- release NPK fertilizer. In the second year, however, the highest production was recorded for planted culms and for seeds in combination with compost. During these two years less than 5 per cent of the total area had to be replanted. All methods, therefore, were satisfactory in terms of sand stabilization. The influence of uncontrolled factors was demonstrated by a 100 per cent higher production from a one-year-old stand in 1986 compared to 1987. The origin of the culms and rhizomes also influenced growth. Culms or rhizomes that had been collected from a stable dune with degenerated A.arenaria produced less dry matter and percentage cover than when the plant material was obtained from a mobile dune which was covered by vigorous plants (chapter 3). Degeneration Growth of seedlings of A.arenaria was strongly reduced in sand from the root zone of a foredune, when compared to growth in fresh (sea) sand. However, no differences occurred when both sand samples were sterilized prior to planting of the seedlings. In sea sand, growth was equal to that in sterilized sand. It was concluded, therefore, that the rhizosphere of A. arenaria contained harmful soil organisms (chapter 4). In order to trace the nature of these organisms, biocides (bactericides, a nematicide, and fungicides) were applied to rhizosphere sand, which was planted with seedlings of A.arenaria. Bacteria were not supposed to be involved in the degeneration of A. arenaria , as bactericides did not affect plant growth. The nematicides effectively eliminated endoparasitic nematodes ( Heterodera avenae group, Meloidogyne maritima , and Pratylenchus sp.) and the application led to increased plant growth. Fungicides also enhanced growth, however, they also eliminated the nematodes H. avenae and M. maritima. It was concluded that nematodes were involved in the growth reduction and degeneration of A. arenaria, but the involvement of soil fungi could not be established unequivocally. Results of a preliminary inoculation experiment suggested that a complex of soil fungi and nematodes is responsible for the degeneration of A. arenaria (chapters 5 and 8). The harmful soil organisms from a certain location reduced growth of local, as well as of foreign populations of A. arenaria. The growth of Calammophila baltica (purple, or hybrid marram grass, a sterile bastard of A. arenaria x Calamagrostis epigejos ) was also reduced by harmful soil organisms, but less than A. arenaria (chapters 4 and 6). In three Dutch coastal dune systems harmful soil organisms were detected in the root zones of stable, as well as of mobile foredunes (degenerating and vigorous A. arenaria , respectively), but not in beach sand. The relation between sand deposition by wind and vigorous growth of A. arenaria was explained by supposing that windblown sand, originating from the beach, enables A. arenaria to escape harmful soil organisms (chapter 6). However, within one year after plants had produced new roots in fresh windblown sand, the root system became colonized by harmful soil organisms (chapter 7). If harmful organisms were present in the sand prior to root growth, root hair formation was reduced severely and the branching of the roots was stimulated (chapter 7). This deformation of the root system by harmful soil organisms is assumed to be related to the degeneration of A. arenaria. A reduced uptake function and a shallow placement of the root system due to attack by harmful soil organisms in stable dunes increases the susceptibility of the plants to stress of drought, high soil temperatures, and shortage of nutrients. It is concluded that the degeneration of A. arenaria in stable dunes is caused by a combination of harmful biotic factors and abiotic stress.