The Virtual Eelgrass Meadow (VEM) was developed to link the population dynamics of eelgrass ramets with a mechanistic, physiological model of eelgrass growth that simulates rhizome elongation rates and new lateral shoot branching in two dimensions. The programming structure relies on an Individual Based Modeling approach emphasizing the botanical details of eelgrass ramet production and utilization of space. Growth rates depend on forced values of nitrogen loading and sediment sulfide concentrations, as well as the dynamic variables of light and temperature that also determine allocation of new production to leaves, rhizomes, or new lateral shoots. The pattern of lateral shoot colonization along the sediment surface is controlled by clonal growth rules specifying branching angle and the influence of neighbor shoot density on the local light environment. This virtual eelgrass meadow provides a platform that links several modeling techniques, previously used separately in other models of clonal plants, to examine how plant physiology is related to landscape level processes.


Over the course of formulating the VEM, existing models that simulate specific growth rates were examined to reveal limitations in the selection of maximum growth rates followed in traditional formulations. A model is presented that simulates a maximum specific growth rate explicitly in terms of light and temperature conditions. An adaptive partitioning coefficient, dependent on temperature, is introduced to allocate new growth to leaves or the growing rhizome. In an effort to explore the implications of documented increased canopy height in response to nitrogen loading, an elongation factor is included that increases the length at which mature leaves are shed. Because nodes located on the rhizome mark the former point of attachment of a sloughed leaf, the length or biomass at which mature leaves are shed triggers the appearance of new nodes within the VEM. Lateral shoot production is timed to occur after a given number of nodes appear, thereby suggesting leaf production as a mechanistic timing device that regulates population growth rate.

The utilization of space by clonal plants is generally considered a function of the rhizome spacers between ramets, the branching angle of new lateral shoots, and the rate at which these branches are produced. By addressing these parameters in the VEM, it was possible to evaluate how the well documented “self-thinning” law might occur in clonal plants. Model output from simulations at varying initial shoot densities indicates that the structure of the VEM adequately reproduces density-dependent branching rates that were previously measured in lagoon mesocosm experiments. This result suggests that a pre-programmed set of architectural rules permit clonal plants to use space effectively while minimizing crowding and potential mortality of connected, neighbouring ramets.


Further testing of the VEM revealed model sensitivity to temperatures above 21ºC, as well as significant effects of the searching distance parameter (used to calculate local densities) on model output. The effect of elevated nutrient levels on shoots produced decreased lateral branching rates as well as lower rhizome production. The VEM presents a novel perspective of eelgrass autoecology that suggests canopy height may be a key morphological characteristic useful in monitoring sensitive eelgrass meadows under changing environmental conditions related to light availability and nitrogen loading.