Now that approximate trends for dune growth, vegetation growth, and forest growth have been established, you need to introduce one of the following variables to the model. Here is an example. Salt spray will affect the area behind the dune and is harmful to vegetation growth. Vegetation growth will increase as salt spray decreases.
Salt spray is inversely proportional to dune height.
When the dune height equals 20 meters, assume that salt spray no longer reaches the area behind the dune.