Serpentine and Nonserpentine Achillea millefolium Accessions Differ in Serpentine Substrate Tolerance and Response to Organic and Inorganic Amendments

Subgrade serpentine substrates are exceptionally difficult to revegetate due to multiple limitations including low N, P, and K, low Ca:Mg molar ratios, high levels of heavy metals including Ni, Cr, and Co, low organic matter, low CEC, and low water holding capacity. To examine the influence of plant origin on the success of the revegetation of serpentine substrates, granite and serpentine accessions of Achillea millefolium were grown on subgrade serpentine substrate amended with yard waste compost, slow-release NPK fertilizer, and/or CaSO₄ · 2H₂O (gypsum). The goals of this study were to: (1) identify the substrate amendment combination that maximized establishment of A. millefolium on serpentine substrate, (2) compare seedling establishment, survival, and growth of the serpentine and granite A. millefolium accessions in order to determine if a serpentine edaphic ecotype of A. millefolium exists and if this ecotype is superior to the granite accession for the establishment of vegetation on serpentine substrate and (3) if a serpentine edaphic ecotype of A. millefolium does exist, what physiological features with respect to mineral nutrition convey a higher tolerance of serpentine for this ecotype than the nonserpentine ecotype. Seedling establishment, survival, and growth were greatest for A. millefolium when the subgrade serpentine substrate was amended with 30% (v/v) compost and 220 mg kg substrate⁻¹ each of N, P, and K. The serpentine A. millefolium accession displayed a greater tolerance of the subgrade serpentine substrate, serpentine topsoil, and the amended subgrade serpentine substrate than the granite accession. Higher capacity of the serpentine A. millefolium accession for selective Ca translocation from roots to the shoot resulted in a significantly higher shoot Ca:Mg molar ratio than the granite accession and appeared to be the most important physiological feature conveying greater tolerance of the serpentine accession for serpentine substrates.