Quantification of the change in stemflow velocity and evaluation of its potential washing effects in a pine forest as affected by the wildfire in southwest China

Purpose Substantial wildfires combust tree stem surface while does not alter the tree canopy. This process likely alters stemflow velocity and thus its washing effects on the stem surface and the soil around the base of trunks. We aimed to quantify the stemflow velocity and evaluate its potential washing effects. Methods Burned and unburned trees were selected in a pine forest in Guiyang, southwest China. The stemflow velocity was quantified using the dyeing method under different stemflow rates and stem heights. Bark fissure characteristics were measured to explain the variance of stemflow velocity. Meanwhile, forty-eight soil samples were collected to analyze the soil's cohesion and internal friction angle using a typical strain-controlled direct shear apparatus under four normal stresses. Results Stemflow velocity increased by 3040% after wildfire. Stemflow velocity was positively related to stemflow rate. The maximum stemflow velocity among pathways for each tree tended to increase with stem height. A negative linear dependence was detected between the stemflow velocity and pine bark fissure depth and bark fissure width. Compared to the unburn soil, the wildfire reduced the internal friction angle and soil cohesion by up to 32% and 62%, respectively. The enhanced stemflow velocity probably provided energy beyond the critical values for the stemflow to detach and transport soil particles. Conclusion The enhanced stemflow energy can increase soil erosion around the base of the trunk, particularly on steep hillslopes.