Tree Competition Constrains Aboveground Growth in Association with Reduced Root Length

Tree competition strongly shapes forest productivity, yet how it alters root growth dynamics and thereby influences tree performance remains poorly understood. Here, using 50 custom rhizoboxes containing 130 one-year-old seedlings, we conducted long-term, high-frequency, non-destructive monitoring of visible living root length at the observation panel in Cyclobalanopsis glauca and Cunninghamia lanceolata subjected to control, intraspecific, and interspecific competition. Each competition-treatment rhizobox contained one target seedling with either one or three neighboring seedlings. We found that competition significantly reduced visible living root length, altered temporal variation in visible living root length, and advanced the onset of summer root decline. The reduction was stronger under interspecific than intraspecific competition, with mean visible living root lengths of 802 mm (95% CI: 709–894 mm) and 2147 mm (95% CI: 1976–2317 mm), respectively. Although competition also altered root phenology, these phenological shifts showed little relationship with aboveground growth. Instead, root length consistently explained variation in stem radial growth across treatments. Our findings demonstrate that competition-related reductions in aboveground growth were more closely associated with reduced root length than shifts in root phenology, highlighting the potential importance of root length dynamics in plant competitive interactions.