Dear Dr. Marion:
I just finished reading your paper, "Assessing and Understanding Trail Degradation: Results from Big South Fork National River and Recreation Area". It was advertized by IMBA (unjustifiably) as support for mountain biking. You may find the attached paper of interest. Please pay particular attention to the analysis of Wilson & Seney and Thurston and Reader. Their conclusions don't follow from their data, so they should not be taken at face value.
Your paper is useful for drawing attention to the way trails are harmed. But it is not the best design for drawing some of the distinctions you make. For example, to compare types of use, you need to create an experimental design, not a survey design. And you need to control for all major confounding factors. For example, to compare hiking with mountain biking, you need to apply both treatments to identical soils on either the same trail, or very similar trails, and measure before and after. You compared hikers and mountain bikers on completely different trails! So the differences found could be due to the differences in the trails or soils, the weather during use, or the past amount of traffic. After applying typical hiking and mountain biking treatments to identical soils under identical conditions, you would be able to compare their impacts. You also made the same error that all other researchers have made: ignoring distance travelled. If (let's suppose) hikers and mountain bikers caused the same erosion PER FOOT (which is what you measured), you have to multiply by typical distances travelled, to get the total per-user impact. Mountain bikers travel several times as far as hikers, and thus have several times the impact.
You also threw away data (cross sectional area of erosion), converting measured area to binary (on/off) data. I understand that you had limited time & resources, but I think it would be better to use all the data you can get.
You also accept without question that all forms of recreation are acceptable. That decision is up to the politicians, and is not supported -- or supportable -- by science. All options should remain on the table, including banning mountain bikes and other ORVs, as well as horses. Mountain bikes and other ORVs are machines, and have no right to be on trails (for the court case, see http://mjvande.nfshost.com/mtb10.htm). They greatly interfere with both the wildlife and the other trail users. Horses are exotic species, but arguably have a right to be here because their genus evolved in North America. However, when they are being used as vehicles, they undoubtedly have much greater impacts. The best way to ensure equal access (the essence of our democracy) is to ban all vehicles from the trails, including animals used as vehicles.
I understand that you had time- and resources constraints, but I would like to encourage you to do an experimental study, if you want to draw defensible conclusions.