3.1 Mortality
After a recovery growing season that followed the drought duration treatment in combination with N-fertilization, the mortality of beech was higher than oak, and the mortality rate seemed to show a tendency to increase with drought duration for the two species (Fig. 1). Especially, the N-fertilized beech saplings had much higher mortality rate within each drought duration treatment category (Fig. 1b).
3.2 Leaf water potential andphotosynthesis during drought duration treatment (2018)
Drought duration significantly decreased leaf water potential (Table 1, Fig. 2a,b) in both species. A significant S x D interaction (P < 0.001; Table 1) indicated that the leaf water potential of the two species responded to drought duration significantly differently, showing a ψleaf order of D0 = D1 > D2 = D3 for oak (Fig. 2a), and of D0 > D1 > D2 = D3 for beech (Fig. 2b). Neither N-fertilization nor any interaction of N with other factors affected leaf water potential (Table 1). In beech, N-fertilization significantly decreased ψleaf in D0 plants (Fig. 2b).
Species, drought duration, N-fertilization, as well as their two-way and three-way interactions significantly affected the Aleafof plants two months after the onset of the drought treatment on 6 August 2018 (except for non-effects of N and S x N interaction) and after four months on 28 September 2018 (Table 1). Oak’s Aleaf significantly decreased with drought duration for the two measurement dates (Fig. 2c,e), while in beech Aleaf significantly increased from D0 to D1, and then decreased in D2 and D3 (Fig. 2d,f) on both dates. N-fertilization seemed to decrease Aleafof oak under D1 and D2 (Fig. 2c,e), whereas it significantly increased Aleaf of beech under D1 (Fig. 2d,f). Photosynthesis almost ceased in D3 plants after four-month of treatment (Fig. 2e,f), especially in beech (Fig. 2f).
During the drought duration treatment, the relative height increment (RHI) differed significantly only between species (Table 1), showing higher RHI in oak than in beech (Fig. 2g,h). Neither N and D, nor their interactions affected RHI (Table 1, Fig. 2g,h).