Large seasonal variation in fine root economics, ectomycorrhizal and hydraulic strategies in Pinus koraiensis —Partly dependent on tree life stages and root orders

Plant plastic responses to environmental changes are multidimensional and vary over time. Below‐ground, the magnitude and direction of seasonal changes in fine root traits have been largely overlooked, despite their potential importance in matching plant resource acquisition and transport strategies to fluctuating environmental conditions. Furthermore, we know virtually nothing on whether plants of contrasting life stages or different root orders show different degrees of seasonal plasticity. We measured 17 fine root traits on a dominant tree species, Pinus koraiensis , covering aspects of root economics, ectomycorrhizal and hydraulic strategies at four timepoints along its growing season. We performed these measurements on P. koraiensis individuals at four life stages (seedling, sapling, mature and over‐mature tree) and on roots of first to fifth order. Overall, absorptive and transport roots had higher root tissue density at the start and the end of the growing season. The intensity of absorptive root ectomycorrhization coincided negatively with the two peaks of fine root growth, while root branching intensity increased at these two peaks. The axial and radial transport capacity of absorptive roots steadily decreased from the start to the end of the growing season, while the axial transport capacity of transport roots was highest in Summer. However, when distinguishing between tree life stages and the five most distal root orders, the seasonal trends and plasticity of fine root traits differed partly across both tree life stages and first to fifth orders. Synthesis . By capturing the intraspecific responses of fine root traits to seasonal fluctuations in situ, our results bring preliminary evidence of a temporal shift in carbon allocation strategy along the growing season, whereby increased carbon allocation to root growth may reduce investment in ectomycorrhizal symbionts. Seasonal adjustments of fine root traits differed partly across tree life stages, highlighting ontogenetic shifts in below‐ground resource acquisition strategy, and root orders, confirming the functional heterogeneity of fine root systems. Further studies are needed to test whether such seasonal patterns hold globally across multiple species or differ to such an extent that they should be considered as part of species ecological strategies.