Responses in Growth and Anatomical Traits of Two Subtropical Tree Species to Nitrogen Addition, Drought, and Their Interactions

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.

Different-Sized Vessels of Quercus variabilis Blume Respond Diversely to Six-Year Canopy and Understory N Addition in a Warm-Temperate Transitional Zone

“…Annual mean vessel is another morphological anatomical indicator reflecting environmental changes, especially water availability [7,25,44,58]. Although some studies have reported that hydraulic conductivity of xylem could be changed by N addition [16,59], this might not be reflected in the annual mean vessel area in our study, as no obvious or significant responses were seen for either CN or UN treatment. Annual mean vessel area did not show any obvious variation between control and N treatments; however, compare to control, annual KH and percentage of annual total vessel area of CN-trees and UN-trees had a tendency to increase during 2013-2018, although the increase was not statistically significant (Figures 3 and 4, Table 2).…”

“…Annual mean vessel area did not show any obvious variation between control and N treatments; however, compare to control, annual KH and percentage of annual total vessel area of CN-trees and UN-trees had a tendency to increase during 2013-2018, although the increase was not statistically significant (Figures 3 and 4, Table 2). Previous studies have shown that N addition could increase plant water conductivity by increasing vessel area or the number of vessels [16,41]. Therefore, we speculated that percentage of total vessel area and KH might be increased by N addition due to increase in the number of vessels rather than increase in vessel size (Figure 4, Supplementary Figure S3).…”

“…It was reported that these large vessels at the start of tree-ring are formed before initiation of photosynthesis of the current year and mainly use the NSC stored from the previous year [35,36]. Moreover, previous work at the same experimental site has shown that CN increased leaf N content and fine root biomass [16], which might improve photosynthesis and, consequently, the amount of carbon stored in previous year. By contrast, trees in UN treatment showed a decrease in fine root biomass and soil pH [27,28], which might inhibit carbon production for the initiation of the large vessels in the next year.…”

“…These changes might result in a reduction in carbon allocation to tree-ring formation. Furthermore, increase in foliar N content was also greater in CN-trees than that in UN-trees [16], and this might enhance leaf photosynthesis [55,56], thereby promoting a greater allocation of photosynthates to tree-ring growth in CN-trees [57]. All these above changes might result in less tree-ring growth under UN than CN treatment.…”

“…Many studies have reported the effects of simulated N deposition on tree growth, but most N treatments were applied directly to the soil or understory of the canopy, ignoring N retention by the canopy in a natural atmospheric deposition situation [15,16]. In the field or in a natural atmospheric N deposition situation, N would first contact the canopy, where it could be intercepted and absorbed by leaves or fixed by canopy biota before reaching the forest floor [17][18][19].…”