Asymmetric Above- and Below-Ground Adaptation Strategies to Drought Stress in Cunninghamia lanceolata and Phoebe zhennan Seedlings

Frequent droughts, an increasingly common challenge under global climate change, severely threaten forest health and productivity. While most previous studies have focused on single-trait responses to drought, the interrelationships between stomatal regulation and hydraulic architecture remain unclear. In particular, substantial uncertainties persist regarding whether trees can physiologically adapt to drought through coordinated aboveground (i.e., stomatal regulation-hydraulic architecture) and belowground (i.e., root morphology) traits. With coniferous Cunninghamia lanceolata and broadleaved Phoebe zhennan seedlings from southern China plantations, we conducted a pot experiment to examine drought effects on photosynthetic physiology, hydraulic architecture, root plasticity, growth, and their coordination. Under drought, C. lanceolata reduced its stomatal conductance (gs) by 56.3–79.2% via early stomatal closure, exhibiting a conservative strategy. It maintained high Huber value, ensuring hydraulic safety, and adopted an acquisitive root strategy with high specific root length and fine roots (diameter < 0.05 mm, accounting for 55.6% of total root length), forming a conservative-aboveground–acquisitive-belowground strategy. In contrast, P. zhennan displayed an acquisitive stomatal strategy (gs decreased by 43.1%–77.8%) to sustain photosynthesis, supported by high hydraulic conductivity and high sapwood-specific conductivity; it developed a conservative root strategy with high root tissue density and high specific root area, realizing an acquisitive-aboveground–conservative-belowground strategy. These results reveal distinct multi-trait coordinated drought responses and inform the prediction of seedling growth stability under drought conditions.