A desired condition is not a picture. It’s a movie.
This is a map of four seral stages for the Pagosa Springs district of the San Juan National Forest. Young stands of trees (class 1) are very rare. So are the purple areas representing the oldest stands of trees (class 4). Most of the map shows middle-aged stands (red and green). Think about how this information might be used in forest planning. For instance, the purple areas might be important habitat for late-seral stage wildlife species, they might be mapped as ecological reserves, or they might have some unique social values we want to protect.
Here is a simulation of what could happen to these stands of trees over time due to fire, insects and disease. Each interval in the movie is a 10-year increment. It is based on work by Kevin McGarigal of the University of Massachusetts and Bill Romme now at CSU, for the San Juan Forest Plan Revision using a GIS-based simulator called RMLANDS. It formed an understanding of the historical range of variability of vegetation for the DEIS.
The stand size and distribution is most dependent upon fire interval and fire size, randomly simulated based on historical data. Over time, the tree conditions seem to float across the landscape like shifting sand. There are some places where topography seems to influence the disturbances to allow persistence of older trees, but even these areas are eventually affected by the random events.
The smaller the scale, the larger the variation. If you look at a particular place, there is more change over time in the color of the place. The larger the scale, there is more likelihood that you’ll find the color you are looking for somewhere.
When planning for forests influenced by disturbance, landscape ecologists advise us that it’s important to think of time and space. It calls for a discussion beyond static desired conditions. Instead, a discussion is needed on the disturbance processes, if anything should be or can be done to shape those processes, and what we should do with the conditions that might result. This is a very different type of forest plan than we have done in the past.