Ecologically distinct pine species show differential root development after outplanting in response to nursery nutrient cultivation

The capacity to rapidly expand root systems to increase soil foraging is key for the survival of tree seedlings in arid regions. Plants alter root growth dynamics and architecture in response to environmental changes and this plasticity in root development allows plants to maximize soil resource uptake. However, how variation in plant physiological status, such as nutrient content, affects root growth dynamics and architecture is poorly understood. We assessed how variation in seedling nutrient status affects root growth dynamics and architecture in three closely related, yet ecologically distinct pine species: Pinus nigra J.F. Arnold, Pinus pinaster Ait., and Pinus pinea L. Seedlings were cultivated under contrasting fertilization regimes to promote different levels of mineral nutrient content. Seedlings were then transplanted into rhizotrons to evaluate root growth dynamics and architecture over 10 weeks. Overall, increased seedling nutrient content enhanced root development, but species showed distinct root growth patterns and architecture. Pinus pinaster, which inhabits moderate climatic stress environments, showed the greatest root growth plasticity with variation in seedling nutrient content. In contrast, root development was less plastic in the pines adapted to more stressful environments, P. pinea, which occurs in sites with long and strong summer drought and P. nigra, which inhabits cold winter areas. High nutrient-content seedlings developed larger root systems by maintaining a greater number of growing roots rather than by increasing the elongation rate of individual roots, reflecting a strategy that likely improves soil foraging capacity. Nitrogen use efficiency for root growth (new root growth per unit of plant nitrogen content at planting) increased with increasing nutrient content in P. pinaster, yet the opposite effect occurred in the other two species. New roots of high nutrient-content seedlings had lower specific root length than low content seedlings in all species, suggesting that internal seedling nutrient status determines specific root length. The means by which increased seedling nutrient content affects root development after transplanting in pine species seems to be related to species ecology and stress resistance physiology. With a surplus of mineral nutrients during nursery cultivation, pine species from high stressful environments have low flexibility in modifying root development, while species from moderate climatic stress environments prioritize root development.