Combined Effects of Scarification, Phytohormones, Stratification, and Soil Type on the Germination and/or Seedling Performance of Three Tamaulipan Thornscrub Forest Species.

Tamaulipan thornforests in south Texas and northeast Mexico are a conservation hotspot. Shortages of native seedlings limit regional restoration and are largely driven by knowledge gaps regarding propagation of the 75+ thornforest species planted during restorations. We previously investigated three thornforest species with low or inconsistent germination or seedling survival: Ebenopsis ebano (Fabaceae), Cordia boissieri (Boraginaceae), and Zanthoxylum fagara (Rutaceae), and identified the types and dosages of chemical seed treatments that maximized germination. However, chemical treatments were performed in isolation and combinational treatments may be required to break dormancy or maximize germination. This study builds on prior work by investigating the effects of all possible combinations of sulfuric acid (SA), gibberellic acid (GA), and indole-3-butyric acid (IBA) treatments on germination of the same focal species, and further quantified the combined effects of five chemical treatments, three stratification treatments, and six soil mixture types on the germination and seedling performance of the focal species. Ebenopsis ebano germination peaked with SA and was not improved with additional chemical treatments. Cordia boissieri germination was highest with GA only in our indoor experiment but peaked with GA + IBA + SA in our outdoor experiment. Zanthoxylum fagara germination was near zero in all treatments. Stratification treatments marginally reduced E. ebano germination and reduced C. boissieri seedling height. Soil type had significant impacts on E. ebano germination and leaf abundance (residual differences up to 40% or 4 leaves, respectively) and influenced some of the effects of chemical treatments. These results enhance our understanding of thornforest seed ecology and best practices for nursery propagation of seedlings.

Effects of Scarification, Phytohormones, Soil Type, and Warming on the Germination and/or Seedling Performance of Three Tamaulipan Thornscrub Forest Species.

The Tamaulipan thornforests of south Texas and northeast Mexico are an ecologically and economically important conservation hotspot. Thornforest restoration is limited by native tree and shrub seedling availability for planting. Seedling shortages arise from low seed availability and knowledge gaps regarding best practices for germinating and growing the 70+ thornforest species desired for restoration plantings. To fill key knowledge gaps, we investigated three ecologically important thornforest species with low or highly variable germination or seedling survival rates: Ebenopsis ebano, Cordia boissieri, and Zanthoxylum fagara. For each, we quantified the effects of different dosages of chemical seed treatments used to promote germination (sulfuric acid, SA; gibberellic acid, GA; indole-3-butyric acid, IBA) on germination likelihood and timing. We also quantified the effects that these chemical seed treatments, soil media mixture type, and soil warming had on seedling survival, growth, and root morphology. Ebenopsis germination peaked (>90%) with 40-60 min SA treatment. Cordia germination peaked (40%) with 100 mg/L GA treatment. Zanthoxylum germination was negligible across all treatments. Seed molding was rare but stirring during SA treatment reduced Ebenopsis molding by 4%. Ebenopsis seedling survival, height, leaf count, and root morphology were minimally affected by seed treatments, generally reduced by warming, and influenced by soil mix, which also mediated responses to warming. These results suggest improvements to existing practices that could increase Ebenopsis germination by 10-20% and potentially double Cordia germination.