Does Root Tensile Strength Exhibit Seasonal Variation? Evidence from Two Herbaceous Species

Root tensile strength (<i>T_r</i>) is a fundamental root mechanical trait and serves as a key parameter for assessing the contribution of vegetation to slope stability. <i>T_r</i> is known to exhibit high intraspecific variability, but whether <i>T_r</i> varies with season remains unclear. Here, we investigated the seasonal variation in <i>T_r</i> in two commonly seen herbaceous species, i.e., <i>Artemisia argyi</i> and <i>Cirsium setosum</i>, both of which can be future candidates for revegetating species along roadsides in temperate and subtropical regions. We examined the <i>T_r</i> of their first- (closest to the stem base) and third-order lateral roots sampled in the southwest of Henan, China, in two distinct periods: September (late growing season) and December (dormant season). We found that the <i>T_r</i> of the thicker, first-order roots in September was significantly greater than that in December. However, such seasonal variation was not found for the thinner third-order roots. When fitting the relationship between <i>T_r</i> and root diameter using a two-parameter power law equation, the calibrated equation using the data collected in September led to a marked predictive bias to the data collected in December. All the above patterns were consistent for both species. Soil moisture, which exhibited strong seasonal variation in the study area, might be the key cause of variation in <i>T_r</i>. Our study is among the first to demonstrate seasonal variation in root mechanical traits, indicating that season potentially plays a non-negligible role in impacting soil reinforcement and slope stability by modifying roots' mechanical quality.