Morphological responses of shoots and fine roots to weed control in <i>Abies sachalinensis</i> and <i>Picea jezoensis</i> seedlings

A mechanistic understanding of how morphological traits of seedlings are relevant to their functions against competition is the basis for managing vegetation in plantations. This study evaluated the growth responses and functional traits of shoots and fine roots in seedlings of boreal silvicultural species to manual weed control. Aboveground morphological traits reflecting competitive ability, including the total number of shoots per individual, length and weight per current shoot, area and weight per current leaf, and their ratio were evaluated. The fine roots were classified as pioneer roots or fibrous roots and were evaluated for diameter and specific root length (SRL). Compared with <i>Picea jezoensis</i>, <i>Abies sachalinensis</i> has superior competitive ability by reducing the total number of shoots by approximately 40%, preserving shoot length, and increasing specific leaf area by approximately 30% under competition. This finding is partly inconsistent with conventional strategies against shade, and <i>A. sachalinensis</i> is tolerant and partly avoidant of competing vegetation compared with <i>P. jezoensis</i>. The lack of interspecies differences in growth responses to weed control indicates that individual growth cannot be explained by aboveground responses alone. Thinner and higher SRL of fibrous roots in competing conditions were observed only in <i>A. sachalinensis</i>. However, thinner and higher SRL of pioneer roots in competing conditions were observed in both species. Altogether, these results highlighted that the fine root system widely adopts an avoidance strategy against competition regardless of aboveground competitive strategies. Because soil resources can be depleted, the preemptive strategy to access soil space against competition is responsible.