Nutrient and productivity relations of the dune grasses Ammophila arenaria and Elymus mollis

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Abstract
The purpose of this research was to investigate the influence of nitrogen availability on growth and the patterns of dry matter and nitrogen allocation of the dune grasses Ammophila arenaria and Elymus mollis. Plants were grown from rhizome segments under near-ambient coastal conditions and received either high or low supplies of nitrogen. Sequential harvests, and the separation of plant material into live blade, dead blade, tiller, rhizome, and root fractions were made during the exponential phase of growth. The concentrations of total nitrogen in these fractions were ascertained by the micro-Kjeldahl method. Despite similar relative rates of dry matter accumulation over the entire period of growth, Ammophila rates eventually surpassed those of Elymus due to a higher rate of blade area development under both nitrogen regimes. This was the result of allocating a larger proportion of daily assimilate to the production of new blades and a lower rate of blade area senescence, rather than a greater net assimilation rate. Low nitrogen availability decreased mean relative growth rate, mean leaf area ratio, mean leaf area expansion rate, and the leaf area partitioning coefficient but tended to increase mean net assimilation rate in both species. Studies of dry matter allocation revealed a “shoot-oriented” pattern of Ammophila growth, with an emphasis on the live blade fraction and a correspondingly diminished root fraction. Conversely, Elymus allocated more dry matter to roots and less to live blades. Allometric plots showed that twice as much live blade area was supported per gram of root by Ammophila than by Elymus. These differences were maintained despite changes in the availability of nitrogen. Although the patterns of dry matter and nitrogen partitioning were somewhat independent within a species (i.e. they were not tightly coupled), a general correspondence between the allocation of these resources was observed when the two species were compared. Ammophila allocated more nitrogen to live blades and less to tillers and roots than did Elymus. These characteristics of growth and resource allocation of Ammophila are thought to contribute to its greater above-ground productivity along the Pacific coast of North America compared to the native Elymus.