Fertilization effects on drought responses in sessile oak and Scots pine seedlings

Severe drought acutely impairs plant hydraulic functioning and impedes processes of carbon (C) and nutrient as well as their allocation. However, how fertilization would modify the allocation of C and nutrient between sink and source organs during drought stress remains largely unknown.We used three-year old potted seedlings of sessile oak and Scots pine in a greenhouse experiment where they were subjected to three different fertilization treatments (non-fertilized, moderate and high fertilization) and two water regimes (well-watered and severe drought) across two consecutive growing seasons. 13C and 15N labeling were labeled to trace the C and nitrogen (N) allocation. Leaf gas exchanges and predawn water potential, biomass of the different plant organs and NSC concentration, as well as relative 13C and 15N allocation to root, stems and leaves were assessed in the two growing seasons.Our results showed that sessile oak grown under fertilization suffered faster from drought and showed earlier death than unfertilized seedlings. Fertilization significantly improved aboveground growth, increased shoot: root ratio and reduced NSC storage in sessile oak. This leads to drought-induced C depletion and increased mortality under severe drought. Progressing drought altered C and N translocation strategies in sessile oak by prioritizing C allocation to and N retention in the roots under moderate fertilization, but not in high fertilization. In sessile oak, seasonal dynamics of C and N allocation is coupled and independent of drought and fertilization. By contrast, fertilization and drought, both had only minor impacts on Scots pine C allocation and the tradeoff of C allocation between growth and reserves, as well as the uptake of added N by root. Severe drought strongly decreased NSC in stem and root of Scots pine, while NSC concentrations in leaf and fine root kept stable and high and at the status of mortality.We conclude that sessile oak shows a more plastic response to environment changes than Scots pine by adjusting its C and N relations on the whole plant level. The impact of fertilization on tree seedlings drought responses seems to be species-specific and is also modified by the degree of drought and fertilization.