Piecewise regression for Semidesert Sandy Loam (Fourwing Saltbush)

I have been working with Mark's data as I try to build the STM and think about the first ecosite (Semidesert SaL). I ran a multiple regression investigating factors that influence soil aggregate stability. "Good" BSC points showed the strongest influence on soil stability. I then ran a piecewise regression, which yielded the following pattern (breakpoint of 22 ± 5). Red points are "grazed", open points are "ungrazed". A similar pattern is shown in the presentation Mark put together that was presented at the SLC workshop (slide 9 of SemidesertSandyLoam.ppt).