Effects of N and P additions on twig traits of wild apple (Malus sieversii) saplings

Abstract

Background: Wild apple (Malus sieversii) is under second-class national protection in China and one of the lineal ancestors of cultivated apples worldwide. In recent decades, the natural habitation area of wild apple trees has been seriously declining, resulting in a lack of saplings and difficulty in population regeneration. Artificial near-natural breeding is crucial for protecting and restoring wild apple populations, and adding nitrogen (N) and phosphorous (P) is one of the important measures to improve the growth performance of saplings. In this study, field experiments using N (CK, N1, N2, and N3: 0, 10, 20, and 40 g m^{- 2} yr^{- 1}, respectively), P (CK, P1, P2, and P3: 0, 2, 4, and 8 g m^{- 2} yr^{- 1}, respectively), N20Px (CK, N2P1, N2P2, and N2P3: N20P2, N20P4 and N20P8 g m^{- 2} yr^{- 1}, respectively), and NxP4 (CK, N1P2, N2P2, and N3P2: N10P4, N20P4, and N40P4 g m^{- 2} yr^{- 1}, respectively) treatments (totaling 12 levels, including one CK) were conducted in four consecutive years. The twig traits (including four current-year stem, 10 leaf, and three ratio traits) and comprehensive growth performance of wild apple saplings were analyzed under different nutrient treatments.

Results: N addition had a significantly positive effect on stem length, basal diameter, leaf area, and leaf dry mass, whereas P addition had a significantly positive effect on stem length and basal diameter only. The combination of N and P (NxP4 and N20Px) treatments evidently promoted stem growth at moderate concentrations; however, the N20Px treatment showed a markedly negative effect at low concentrations and a positive effect at moderate and high concentrations. The ratio traits (leaf intensity, leaf area ratio, and leaf to stem mass ratio) decreased with the increase in nutrient concentration under each treatment. In the plant trait network, basal diameter, stem mass, and twig mass were tightly connected to other traits after nutrient treatments, indicating that stem traits play an important role in twig growth. The membership function revealed that the greatest comprehensive growth performance of saplings was achieved after N addition alone, followed by that under the NxP4 treatment (except for N40P4).

Conclusions: Consequently, artificial nutrient treatments for four years significantly but differentially altered the growth status of wild apple saplings, and the use of appropriate N fertilizer promoted sapling growth. These results can provide scientific basis for the conservation and management of wild apple populations.

Keywords: Growth performance; Malus sieversii; Nutrient addition effect; Plant trait network; Population conservation; Twig trait.

Fig. 1

The effects of Malus sieversii stem traits for treatment with N, P, and N + P. The effects of treatment with only N (without P addition), only P (without N addition), N20Px, NxP4, P with the addition of N, and N with the addition of P on stem length (L), basal diameter (BD), stem mass (SM), twig mass (TM), leaf intensity (LI), leaf area ratio (LAR), and leaf area-to-stem mass ratio (LAMR) of Malus sieversii. The values represent the mean with 95% confidence intervals (CIs). The effect was considered to be significant (*) when the 95% CI did not overlap with zero. Low: N10, P2, N20P2, and N10P4; Medium: N20, P4, and N20P4; High: N40, P8, N20P8, and N40P4. N20P4 is common to both N20Px and NxP4, therefore, it is only shown in the former. (a): N effect, (b): P effect, (c): N20Px effect, (d): NxP4 effect, (e): P effect with N addition, (f): N effect with P addition. the same below

Fig. 2

The effects of Malus sieversii leaf traits for treatment with N, P, and N + P. The effects of only N (without P addition), only P (without N addition), N20Px, NxP4, P with the addition of N, and N with the addition of P on leaf area (LA), leaf number (LN), total blade mass (TBM), total petiole mass (TPM), total leaf area (TLA), total leaf mass (TLM), single petiole mass (SPM), single blade mass (SBM), single leaf mass (SLM), and specific leaf area (SLA) of Malus sieversii. The values represent the mean with 95% confidence intervals (CIs). The effect was considered to be significant (*) when the 95% CI did not overlap with zero. Low: N10, P2, N20P2, and N10P4; Medium: N20, P4, and N20P4; High: N40, P8, N20P8, and N40P4. N20P4 is common to both N20Px and NxP4, therefore, it is only shown in the former

Fig. 3

Plant trait networks (PTNs) of Malus sieversii for treatment with N, P, and N + P. Nodes indicate different twig traits, and node size indicates the degree. Red and blue lines indicate positive and negative correlations, respectively. Circles with black borders represent the key trait. The width of the line indicates the strength of the association. L: Length; BD: basal diameter; LA: leaf area; LN: leaf number; TM: twig mass; TBM: total blade mass; TPM: total petiole mass; SM: stem mass; TLA: total leaf area; TLM: total leaf mass; SPM: single petiole mass; SBM: single blade mass; SLM: single leaf mass; SLA: specific leaf area; LI: leaf intensity; LAR: leaf area ratio; LAMR: leaf area-to-stem mass ratio. The topological properties of the network and the characteristics of the node (trait) are shown in Tables S1 and S2. N20Px: N20P2, N20P4, and N20p8; NxP4: N10P4, N20P4, and N40P4

Fig. 4

The membership function values of twig traits of wild apple saplings treated with N, P, N20Px, and NxP4. Low: N10, P2, N20P2, and N10P4; Medium: N20, P4, and N20P4; High: N40, P8, N20P8, and N40P4