With the increasing trend of converting monocultures into mixed forests, more and more studies have been carried out to investigate the admixing effects on tree growth and aboveground carbon storage. However, few studies have considered the impact of mixed forests on belowground carbon sequestration, particularly changes in soil carbon and nitrogen stocks as a forest grows. In this study, paired pure Pinus massoniana plantations, Cinnamomum camphora plantations and mixed Pinus massoniana–Cinnamomum camphora plantations at ages of 10, 24 and 45 years were selected to test whether the mixed plantations sequestrate more organic carbon (OC) and nitrogen (N) in soils and whether this admixing effect becomes more pronounced with stand ages. The results showed that tree species identification, composition and stand age significantly affected soil OC and N stocks. The soil OC and N stocks were the highest in mixed Pinus–Cinnamomum stands compared to those in counterpart monocultures with the same age in the whole soil profile or specific soil depth layers (0–10, 10–20 and 20–30 cm) for most cases, followed by Cinnamomum stands and Pinus stands with the lowest. These positive admixing effects were mostly nonadditive. Along the chronosequence, the soil OC stock peaked in the 24-year-old stand and was maintained as relatively stable thereafter. The admixing effects were also the highest at this stage. However, in the topsoil layer, the admixing effects increased with stand ages in terms of soil OC stocks. When comparing mixed Pinus–Cinnamomum plantations with corresponding monocultures within the same age, the soil N stock in mixed stands was 8.30, 11.17 and 31.45 % higher than the predicted mean value estimated from counterpart pure species plantations in 10-, 24- and 45-year-old stands, respectively. This suggests that these admixing effects were more pronounced along the chronosequence.

Despite the great importance of fine roots, which are referred to as roots smaller than 2 mm in diameter, in terms of carbon and nutrient cycling in terrestrial ecosystems, how fine root biomass, production, and turnover rate change with stand development remains poorly understood. Here we assessed the variations of fine root biomass, production, and morphology of trees and understory vegetation in Chinese fir (Cunninghamia lanceolata (Lamb.) Hook) plantations at the ages of 7 years old, 17 years old and 25 years old in southern China, representing the sapling, pole and mature stage, respectively. Fine roots of trees and understory vegetation were sampled with sequential coring method to a depth of 60 cm and sliced into 4 layers (0–15, 15–30, 30–45 and 45–60 cm). Fine root biomass and necromass were highest in the pole stages among these three different aged Chinese fir plantations, although the significant differences were only detected for fine root necromass between 25-year-old and 7-year-old plantations. Fine root biomass of Chinese fir was heterogeneous in both temporal and spatial dimensions. Seasonal variation of fine root biomass in three age groups showed a similar pattern that the standing fine root biomass reached a peak in January and fell to the lowest in July. Vertically, the fine root biomass decreased with the increase of soil depth, but this extinction rate decreased with stand development. The effects of stand age on either total fine root length and surface area, or specific root length were not significant. However, the root tissue density increased significantly with Chinese fir stand ages, which suggested that the fine roots on Chinese fir may resort more to the mycorrhizal associations for the nutrient and water acquisition in the later stage of Chinese fir plantations. In addition to the stand age effect, the fine roots exhibited highly spatial and temporal variations in Chinese plantations, indicating different root foraging strategies for soil nutrient and water acquisition. Therefore, the fine root research not only helps to understand its role in carbon sequestration in terrestrial ecosystem under global climate change, but can also improve our understanding of nutrient management in forest ecosystem. At the same time, the research on the productivity of the Chinese fir growth stage provides guiding significance for the construction and management of Chinese fir.

Mixed forests are believed to enhance ecosystem functioning and sustainability due to complementary resource use, environmental benefits and improved soil properties. The facilitation between different species may induce overyielding. Meanwhile, the species-specific fine root foraging strategies and tradeoffs would determine the structure and dynamics of plant communities. Here the aim was to investigate the admixing effects of fine-root biomass, vertical distribution and morphology in Pinus massoniana–Cinnamomum camphora mixed plantations and corresponding monocultures at 10-, 24- and 45-year old stands. The fine root biomass in the Pinus–Cinnamomum mixed forest exerted a certain degree of overyielding effect. These positive admixing effects, however, did not enhance with forest stand development. The overall relative yield total ranged from 1.83 and 1.51 to 1.33 in 10-, 24- and 45-year-old stand, respectively. The overyielding was mainly attributed to the over-performance of late successional species, Cinnamomum, in mixed stands. The vertical fine root biomass distribution model showed fine roots of pioneer species, Pinus, shifted to the superficial layer when mixed with Cinnamomum. Furthermore, the specific root length (SRL) of Pinus was significantly higher in Pinus–Cinnamomum mixed stands than that in monocultures, and the magnitude of differences increased over time. However, the vertical fine-root distribution and SRL for Cinnamomum did not show significant differences between monoculture and mixtures. Our results indicated that the magnitude of fine root overyielding in mixed forests showed a high degree of consistency with the total amount of fine root biomass itself, suggesting the overyielding effects in mixed forests were correlated with the degree of belowground interaction and competition degree involved. The late successional species, Cinnamomum, invested more carbon to belowground by increasing the fine root biomass in mixtures. While the pioneer species, Pinus, adapted to the presence of the species Cinnamomum by modification of vertical distribution and root morphological plasticity in the mixtures. These species-specific fine root foraging strategies might imply the differences of forest growth strategies of co-occurring species and contribute to the success and failure of particular species during the succession over time.

Despite the accumulating evidence of the beneficial effects of diverse mixed species forests on ecosystem functioning and services, foresters in subtropical forest cultivation in China still prefer easily managed monocultures, which is also due to the complexity of mixed forests and the unknown underlying mechanisms related to relationships between biodiversity and forest growth. In a designed pot experiment, we selected two early-successional tree species (Pinus massoniana Lamb., Liquidambar formosana Hance.) and two late-successional tree species (Schima superba Champ., Elaeocarpus decipiens Hemsl.) and planted four saplings in one pot with regard to tree species diversity (monoculture, two species and four species mixtures), each combination replicated four times. In this three-year duration experiment, the effect of tree species diversity, tree identity, and functional traits on sapling growth (tree height, ground diameter, crown projection area), were analyzed. The results showed that the increments of ground diameter and crown projection area increased with tree species richness, whereas the mean tree height increment showed the opposite effect. This growth variation was species specific and related to functional traits (early or late succession), as the increments of the early successional tree species (P. massoniana Lamb. and L. formosana Hance.) had a positive correlation with tree species richness, while the late successional tree species (E. decipiens Hemsl. and S. superba Champ.) showed negative effects. In addition, our study provided evidence for the allometric differences between mixtures and monocultures, which have an important reference value on mixed-species forests.

Salt stress is common but detrimental to plant growth, even in mangroves that live in saline areas. Boron (B) is an essential micronutrient that performs an important role in many functions in plants; however, its protective role under salt stress is poorly understood, especially in long-lived woody plants. In this study, we conducted an indoor experiment under simulated tidal conditions with four treatments (10‰ salinity, 40‰ salinity, 40‰ salinity + 100 μM B, and 40‰ salinity + 500 μM B) and three mangrove species (Avicennia marina, Aegiceras corniculatum, and Bruguiera gymnorrhiza) to investigate the effects of exogenous B on salt tolerance in plant growth, morphology, physiology, and leaf anatomy. The results showed that exogenous low-concentration B treatment (100 μM B) improved the performance of mangrove species under high salinity stress, especially in terms of physiology and leaf anatomy, while high-concentration B treatment (500 μM B) had adverse effects. Additionally, we found that the response to exogenous B varied among species in physiology and leaf anatomy, such as proline, malondialdehyde, activity of antioxidant enzymes, palisade tissue, and spongy tissue, which may be related to the salt tolerance of different species. This study may provide useful insights into the alleviation of salt stress by B in mangrove growth and development, which may facilitate mangrove cultivation and afforestation in a saline environment.

With the increasing trend of converting monocultures into mixed forests, more and more studies were carried out to investigate the admixing effects on the tree growth and aboveground carbon storage. However, few studies have considered the impact of mixed forests on the belowground carbon sequestration, and changes of soil carbon and nitrogen stock as forest grows in particular. In this study, paired pure Pinus massoniana plantations, Cinnamomum camphora plantations and mixed Pinus massoniana-Cinnamomum camphora plantations at ages of 10, 24 and 45 years were selected to test whether the mixed plantations sequestrate more organic carbon (OC) and nitrogen (N) in soils and whether this admixing effect becomes more pronounced with stand ages. The results showed that tree species identity and composition as well as stand age significantly affect soil OC and N stock. The soil OC and N stocks were the highest in mixed Pinus-Cinnamomum stands compared to those in counterpart monocultures within the same age in the whole soil profiles or specific soil depth layers (0–10 cm, 10–20 cm and 20–30 cm) for most cases, followed by Cinnamomum stands, and the Pinus stands the lowest. And these positive admixing effects were mostly non-additive. Along chronosequence, the soil OC stock capacity peaked at 24-yr-old stand and maintained relatively stable thereafter. The admixing effects were also the highest at this stage. However, at topsoil layer, the admixing effects increased with stand ages in terms of soil OC stocks. When comparing mixed Pinus-Cinnamomum plantations with corresponding monocultures within the same age, the soil N stock in mixed stands was 8.30 %, 11.17 % and 31.45 % higher than the predicted mean value estimated from counterpart pure species plantations in 10-yr-old, 24-yr-old and 45-yr-old stands, respectively, suggesting these admixing effects were more pronounced along chronosequence.