Coupling N Cycling and N Productivity in Relation to Seasonal Stress in Quercus Pyrenaica Willd. Saplings

Abstract

The N economy of saplings of the Mediterranean Quercus pyrenaica Willd. species was analysed at whole-plant level. The amount and timing of N cycling were determined by strong seasonality in N productivity, which peaked in late spring. Most leaf N was allocated to a main first flush of leaves in early spring, and a minor second flush during early summer or autumn. After peak of N productivity, the allocation of biomass production and N resources was mainly switched to lignotuber and coarse roots. Q. pyrenaica displayed a non-conservative pattern of N use as compared with other co-occurring Quercus species. Potential leaf N-resorption efficiency was moderate, but N proficiency was low, which contributed to high losses of N throughout leaf litter and to short mean residence times of N (MRTN). Additionally, summer water stress reduced foliar area, producing leaf litter with a high N concentration due to almost negligible resorption efficiency, and in some cases, the loss of the whole above-ground fraction. A large investment of plant resources to the root system appeared as a key trait in the N economy and plant fitness of young Q. pyrenaica saplings, allowing them: (1) to counteract N losses and to maintain a positive N balance, (2) to access deep soil resources (water and nutrients), and (3) to ‘safe’-store N for coupling supply and demand processes during spring growth or sprouting after a strong summer drought.