diff --git a/isometric-park-fna/Simulation.cs b/isometric-park-fna/Simulation.cs
--- a/isometric-park-fna/Simulation.cs
+++ b/isometric-park-fna/Simulation.cs
@@ -266,6 +266,14 @@
             this.PreserveCounts = new int[this.map.MapWidth, this.map.MapHeight];
         }
 
+        private double interpolate(double percent, double start, double end) {
+            var low = Math.Min(start, end);
+            var high = Math.Max(start, end);
+            var range = high - low;
+
+            return range * percent + low;
+        }
+
         private void advanceSimulation()
         {
             var oldSeason = this.Season;
@@ -282,7 +290,8 @@
                 for(int j = 0; j < this.map.MapHeight; j++)
                 {
                     var cell = this.map.cells[i][j];
-                    if (random.NextDouble() > (this.PreserveCounts[i, j] > 0 ? PRESERVE_SPONTANEOUS_NEW_TREE_CHANCE : SPONTANEOUS_NEW_TREE_CHANCE ))
+                    var rate = interpolate((8 - (double)this.PreserveCounts[i, j] / 8.0d, PRESERVE_SPONTANEOUS_NEW_TREE_CHANCE, SPONTANEOUS_NEW_TREE_CHANCE);
+                    if (random.NextDouble() > rate)
                     {
                         var random_type = random.Next(0, 4);
                         cell.AddTree(this.DateTime, (TreeType)random_type);