Response of foliar functional traits to experimental N and P addition among overstory and understory species in a tropical secondary forest

Pervasive soil phosphorus losses in terrestrial ecosystems in China

Future phosphorus (P) shortages could seriously affect terrestrial productivity and food security. We investigated the changes in topsoil available P (AP) and total P (TP) in China's forests, grasslands, paddy fields, and upland croplands during the 1980s-2010s based on substantial repeated soil P measurements (63,220 samples in the 1980s, 2000s, and 2010s) and machine learning techniques. Between the 1980s and 2010s, total soil AP stock increased with a small but significant rate of 0.13 kg P ha-1  year-1 , but total soil TP stock declined substantially (4.5 kg P ha-1  year-1 ) in the four ecosystems. We quantified the P budgets of soil-plant systems by harmonizing P fluxes from various sources for this period. Matching trends of soil contents over the decades with P budgets and fluxes, we found that the P-surplus in cultivated soils (especially in upland croplands) might be overestimated due to the great soil TP pool compared to fertilization and the substantial soil P losses through plant uptake and water erosion that offset the P additions. Our findings of P-deficit in China raise the alarm on the sustainability of future biomass production (especially in forests), highlight the urgency of P recycling in croplands, and emphasize the critical role of country-level basic data in guiding sound policies to tackle the global P crises.

Stability in the leaf functional traits of understory herbaceous species after 12-yr of nitrogen addition in temperate larch plantations

Leaf functional traits play critical roles in plant functioning. Although the functional traits of overstory trees have been extensively studied, minimal research has been conducted regarding understory species, despite the understory layer is an important component of temperate forests. Such insufficiency limit the broader understanding of processes and functions in forest ecosystems, particularly when under the increasing atmospheric nitrogen (N) deposition. Here, we investigated the responses of 18 leaf functional traits in six understory herbaceous species within young and mature stands (three species per stand) in larch (Larix principis-rupprechtii) plantations that subjected to 12 years of anthropogenic N addition. We found that N addition did not significantly impact the photosynthetic traits of understory herbaceous species in either stand; it only led to increased chlorophyll content in Geum aleppicum Jacq. Similarly, with the exception of decreases in the predawn leaf water potential of Sanguisorba officinalis L., N addition did not significantly affect leaf hydraulic traits. With the exception of changes to adaxial epidermis thickness in Potentilla chinensis Ser. (decreased) and G. aleppicum (increased), N addition had negligible effects on leaf anatomical traits and specific leaf area, however, interspecific variations in the plasticity of leaf anatomical traits were observed. Stable responses to N addition were also observed for nonstructural carbohydrates (NSC) and their components (soluble sugars and starch), with the exception of Polygonum divaricatum L., which exhibited increases in NSC. Overall, our results suggest that the functional traits of understory herbaceous species exhibit stability under conditions of long-term N enrichment in temperate plantations.

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

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.

Functional Traits of Quercus aliena var. acuteserrata in Qinling Huangguan Forest Dynamics Plot: The Relative Importance of Plant Size and Habitat

Variation in intraspecific functional traits is one of the important components of community variation, and has drawn the attention of researchers. Studying the variation of traits under different plant sizes and habitats helps to reveal the adaptation mechanism of plants. We explored intraspecific trait variations by focusing on the widespread species Quercus aliena var. acuteserrata in a 25 ha warm, temperate, deciduous broadleaved forest plot in the Qinling Mountains. We measured nine morphological and chemical traits for 90 individuals from different plant sizes and habitats. In addition, we evaluated the relative impact of plant size and environment on Q. aliena var. acuteserrata with multiple regression models. We found that plant size explained the most variance of traits. As plant size increased, the trees tended to have lower leaf nitrogen concentrations, lower leaf phosphorus concentrations, higher leaf carbon concentrations, higher leaf dry matter content (LDMC), and thinner leaves, indicating the transformation from rapid resource acquisition strategy to conservative resource-use strategy. Habitats could only explain the changes in chemical traits. Leaf carbon concentration was principally affected by topographical factors and was significant different among habitats. Leaf nitrogen concentration and LPC were significantly limited by soil N and P. In conclusion, shifts in size-dependent traits met the growth requirements of Q. aliena var. acutiserrata; the high tolerance traits associated with this tree species might elucidate important mechanisms for coping with changing environments.

Experimental Approach Alters N and P Addition Effects on Leaf Traits and Growth Rate of Subtropical Schima superba (Reinw. ex Blume) Seedlings

Nitrogen (N) and/or phosphorus (P) addition has controversial effects on tree functional traits and growth; however, this experimental approach may clarify these controversial results. In this study, field and pot experiments were designed with +N (100 kg N ha−1 yr−1), +P (50 kg P ha−1 yr−1), +NP (100 kg N plus 50 kg P ha−1 yr−1), and a control (no N or P addition) to comparatively investigate the effects of N and P addition on 24 leaf traits and the growth rate of Schima superba (Reinw. ex Blume ) seedlings in subtropical China. We found that the experimental approach alters N and P addition effects on leaf traits and tree growth. Nitrogen addition strongly altered leaf biochemical and physiological traits and limited tree growth compared to P addition in the pot experiment, while the effects of N and P addition on leaf traits and tree growth were weaker in the field, since the seedlings might be mainly limited by light availability rather than nutrient supplies. The inference from the pot experiment might amplify the impact of N deposition on forest plants in complicated natural systems. These findings will help guide refining pot fertilization experiments to simulate trees in the field under environmental change. Future directions should consider reducing the confounding effects of biotic and abiotic factors on fertilization in the field, and refinement of the control seedlings’ genetic diversity, mycorrhizal symbiont, and root competition for long-term fertilization experiments are required.