In vegetation-rich areas, tree-line flash faults pose challenges for protection systems due to their high-impedance characteristics, including low fault current amplitude and high harmonic content. To address detection difficulties, this study employs experiments and modeling to explore the relationship between arc characteristics and environmental factors. A controllable test platform was built to analyze the effects of branch moisture content, discharge gap, and tree species on arc electrical parameters. Based on experimental observations of a significant arc resistance transition near 100 Ω, a mechanism-guided, piecewise adaptive Mayr arc model is proposed, where the time constant varies with conductance. Simulation results show that the model accurately reproduces zero-crossing features, amplitude variations, and nonlinear resistance, with an overall error below 8%. This physically clear and environmentally adaptive model provides a foundation for developing high-sensitivity protection technologies for tree-line flash faults under diverse global conditions.