Role of plasticity and local adaptation in the colonization of lead-zinc soils by Thlaspi montanum

High concentrations of heavy metals in the soil are a classic example of a strong selective force that can lead to local adaptation and parapatric speciation.  However, not all studies examining this have demonstrated local adaptation of plant populations to metal-contaminated soils.  Here I have performed a reciprocal transplant experiment to examine whether Thlaspi montanum var montanum (Brassicaceae) populations that have colonized the edges of mine tailings are adapted to the elevated levels of Pb, Zn, and Cd in the soil.  Replicated plots have been established at four abandoned mine sites near the Rocky Mountain Biological Laboratory, on the edges of the tailings (mine plots) and approximately 100m away from the tailings (non-mine plots).  Only non-reproductive individuals have been used, and the height and number of leaves have been measured over one growing season.  These data show that plants are locally adapted at one of the four sites (significant population by environment interaction: F_1,180 = 5.56, p = 0.0194).  There is no evidence of local adaptation at the other three sites.  Plants from the mine tailings at these sites are significantly larger than the non-mine plants, either in leaf number, height, or both.  However, both mine and non-mine plants exhibit little change in performance between environments, indicating that the difference in size did not affect their response to the treatments.  These results suggest that at three of the four mine sites, T. montanum populations exhibit phenotypic plasticity for metal tolerance.  Limitations on this plasticity may be preventing the colonization of plants onto the more heavily contaminated center of the tailings, which remain bare at these three sites.