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

Abstract

<p>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.</p><p>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. <sup>13</sup>C and <sup>15</sup>N 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 <sup>13</sup>C and <sup>15</sup>N allocation to root, stems and leaves were assessed in the two growing seasons.</p><p>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.</p><p>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.</p>