Abstract
Nitrogen (N) deposition and drought are two major stressors that influence tree growth and propagation. However, few studies have investigated their interactions. In this study, saplings of the two co-occurring species Ormosia pinnata (leguminous) and Schima superba (non-leguminous) were cultivated under two N addition rates (0 and 80 kg N ha–1 year–1) with well-watered (WW, 80% of field capacity), moderate drought (MD, 60% of field capacity), and severe drought conditions (SD, 40% of field capacity). We examined their growth, as well as multiple anatomical and non-structural carbohydrate (NSC) responses, after 2 years. Results revealed that N addition significantly promoted the growth of MD-stressed S. superba, whereas no significant effect was detected in O. pinnata. Decreased leaf water potential (both Ψmd and Ψpd) was also observed with N addition for both species under MD, but not under SD. Furthermore, the application of N positively impacted drought adaptive responses in the stem xylem of S. superba, showing decreased stem xylem vessel diameter (DH), theoretical hydraulic conductivity (Kth), and increased vessel frequency (VF) upon drought under N addition; such impacts were not observed in O. pinnata. Regarding leaf anatomy, N addition also caused drought-stressed S. superba to generate leaves with a lower density of veins (VD) and stomata (SD), which potentially contributed to an enhanced acclimation to drought. However, the same factors led to a decrease in the palisade mesophyll thickness (PMT) of SD-stressed O. pinnata. Moreover, N addition increased the xylem soluble sugar and starch of MD-stressed O. pinnata, and decreased the xylem soluble sugar under SD for both species. The results suggest that N addition does not consistently modify tree growth and anatomical traits under variable water availability. S. superba appeared to have a greater capacity to be more adaptable under the future interactive effects of N addition and drought due to major modifications in its anatomical traits.
Highlights
Atmospheric nitrogen (N) deposition and drought are two major stressors that influence the structures, functionalities, and services of forest ecosystems (Wamelink et al, 2009; Ma et al, 2012; Garcia-Valdes et al, 2021)
We found that the application of N had little interaction with drought in the growth of leguminous O. pinnata saplings
It led to enhanced growth in non-leguminous S. superba saplings under moderated drought conditions
Summary
Atmospheric nitrogen (N) deposition and drought are two major stressors that influence the structures, functionalities, and services of forest ecosystems (Wamelink et al, 2009; Ma et al, 2012; Garcia-Valdes et al, 2021). Given the assumption that a drier and N-enriched environment is likely to occur in the future, an improved elucidation of how tree species cope with combined effects of N deposition and drought will be essential for improving predictions regarding the functioning of forest ecosystems. Other studies have suggested that N addition has a “fertilization effect” on plant growth, which increases evaporative consumption and water demands, thereby increasing drought sensitivity (Meyer-Grünefeldt et al, 2015; Dziedek et al, 2016). These conflicting results reflect the need to improve the mechanistic understandings of the authors on how N addition and drought would interactively affect the physiological performance of trees. Little is known in regard to the combined effects of drought and N deposition on the variations in the NSC of trees
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