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Highlights • Large amounts of seedlings emerged from the natural soil seedbank at the start. • Seven tree and two shrub species of natural soil seedbank survived in plantations. • Soil nitrogen of natural forest and plantation significantly affect seedling growth. • Soil seedbank translocation can be a useful approach for plantation regeneration. Abstract The enhancement of biodiversity is an important strategy to improve the structure and functioning of monoculture plantations. However, compared with seedling transplanting, there is limited information about the utilization of soil seedbanks in regenerating plantation forests. In the present study, to investigate the potential application of soil seedbanks in plantation forest regeneration, the topsoil (0–10 cm) of natural forests was translocated to different monoculture plantations, namely, Acacia crassicarpa , Castanopsis hystrix and Eucalyptus urophylla monocultures. Seed germination, seedling growth and related environmental conditions of the plantation forests were monitored in the experiment. Although, at the very start, large numbers of seedlings emerged from the soil seedbank, only nine species survived after 2.5 years, including seven tree species and two shrub species. Notably, Machilus chekiangensis , a tree species with high economic value, was established in the plantation forests. Except for the common species that emerged in the three plantations, there were some special species that were only associated with a specific plantation. Redundancy analysis revealed that nitrogen from both the natural forest and plantation forest soil had significant effects on seedlings growth. However, environmental conditions such as light (reflected by leaf area index), soil temperature, moisture and pH showed no obvious influence on seedling growth. Our study suggests that soil seedbank translocation from a natural forest to a plantation forest can be a useful approach for plantation forest regeneration because it could introduce valuable plant species and improve the plant biodiversity. [ABSTRACT FROM AUTHOR]

Soil erosion control and water retention is a major ecological function of plantations and may be affected by both plantation type and age. In this study, we investigated both surface runoff and sediment load under different plantations in southern China for six years (2008–2013) in order to illustrate the influence of plantation type and age on water retention and soil erosion. Our results showed that mature plantations functioned better than young plantations in terms of soil erosion control and runoff reduction, which reduced runoff and soil erosion by 64.5 ± 6.2% and 72.2 ± 7.9% more than young plantations, respectively. Our study indicated that plantations play a beneficial role in controlling soil erosion and reducing water loss, and this effect is more pronounced with plantation age. Both runoff coefficient and sediment load were least under Eucalyptus spp . either in young or mature plantations, suggesting that Eucalyptus spp . is better adapted for water retention and soil conservation compared to other plant species. Correlation analysis showed that herb species and herb Margalef index were the top two factors affecting the surface runoff and sediment load in young plantations but litter storage and tree Shannon index were the top two affecting factors in mature plantations. This shift was consistent with the decreasing trend of herbs and increasing trend of litter storage with the time of vegetation restoration. [ABSTRACT FROM AUTHOR]

Transforming plantation monocultures into sustainable, mixed-species forests that contain native species is an important goal in South China. The establishment of native species in Eucalyptus plantations is generally considered difficult, however, because of the potential allelopathic effects of Eucalyptus . In this study, a field trial with completely randomized block was conducted to determine the degree to which Eucalyptus litter and Eucalyptus roots limit the establishment of native trees in Eucalyptus plantations. Growth of seedlings of four native tree species ( Schima superba , Michelia macclurei , Cinnamomum burmannii , Cinnamomum camphora ) that were tested was inhibited by Eucalyptus roots. Seedling emergence was less sensitive than subsequent seedling growth to inhibition by Eucalyptus roots. In contrast, litter enhanced the emergence and growth of seedlings of most of the tested species ( Castanopsis chinensis , Elaeocarpus sylvestris , S. superba , Liquidambar formosana , C. burmannii , and C. camphora ), suggesting that retaining litter on the forest floor may promote the establishment and growth of native tree species in Eucalyptus plantations. We propose the following strategies for establishing native tree species in Eucalyptus plantations: (1) direct sowing of the native tree species C. chinensis , Castanea henryi , Erythrophleum fordii , and C. camphora ; (2) transplanting of E. sylvestris , S. superba , and Tsoongiodendron odorum , which had low germination rates but high seedling establishment rates; and (3) the exclusion of Eucalyptus roots via inter-row trenching to enhance the establishment of S. superba , L. formosana , C. burmannii , and C. camphora . [ABSTRACT FROM AUTHOR]

Abstract: Although the removal or addition of understory vegetation has been an important forest management practice in forest plantations, the effects of this management practice on soil respiration are unclear. The overall objective of this study was to measure and model soil respiration and its components in a mixed forest plantation with native species in south China and to assess the effects of understory species management on soil respiration and on the contribution of root respiration (R r) to total soil respiration (R s). An experiment was conducted in a plantation containing a mixture of 30 native tree species and in which understory plants had been removed or replaced by Cassia alata Linn. The four treatments were the control (Control), C. alata addition (CA), understory removal (UR) and understory removal with C. alata addition (UR+CA). Trenched subplots were used to quantify R r by comparing R s outside the 1-m2 trenched subplots (plants and roots present) and inside the trenched subplots (plants and roots absent) in each treatment. Annual soil respiration were modeled using the values measured for R s, soil temperature and soil moisture. Our results indicate that understory removal reduced R s rate and soil moisture but increased soil temperature. Regression models revealed that soil temperature was the main factor and soil moisture was secondary. Understory manipulations and trenching increased the temperature sensitivity of R s. Annual R s for the Control, CA, UR and UR+CA treatments averaged 594, 718, 557 and 608gCm−2 yr−1, respectively. UR decreased annual R s by 6%, but CA increased R s by about 21%. Our results also indicate that management of understory species increased the contribution of R r to R s. [Copyright &y& Elsevier]

Abstract: In this study, the stand level root respiration was estimated for two monoculture plantations: Acacia crassicarpa and Eucalyptus urophylla, based on in situ measurement of specific root respiration using simplified root chamber method. The respiration rates of fine roots (<5mm) were significantly higher than those of coarse roots (>5mm) for both A. crassicarpa and E. urophylla species. The root respiration of A. crassicarpa showed a clear seasonal pattern with a higher value in the wet season. For E. urophylla, the seasonal pattern was observed for fine roots but not for coarse roots. After determining the biomass of fine roots and coarse roots and their specific rates of respiration at different time points, root respiration at the stand level (R a) was estimated using a direct up-scaling model. We found that the R a accounted for 14% and 19% of total soil respiration (R s) for A. crassicarpa and E. urophylla, respectively. The fine (R Tf) and coarse (R Tc) root respiration at the stand level accounted for about 47% and 53% of the R a for A. crassicarpa, and accounted for 58% and 42% for E. urophylla. This suggests that coarse root respiration cannot be ignored when estimating the root respiration at the stand level. Our results showed that the Q 10 values were more accurate in representing the temperature dependence when the confounding effect of soil moisture was considered. This study introduces an alternative approach to estimate stand level root respiration, but its reliability is largely dependent on the accuracy of root biomass quantification. [Copyright &y& Elsevier]

Abstract: In this paper, we describe a simplified system that we designed to measure rhizosphere respiration of fine roots in situ. The system consists of three major components: an air-flushing unit, a root-chamber and a CO2 sampling apparatus. We tested and measured the reliability and efficiency of the air-flushing unit while it was attached to root-chamber. The results showed that the flushing efficiencies were at least 98% when 4l or more volume of water was used to flush the air out of both the small and medium root-chamber systems. Because the measured values of CO2-C sampled by syringe in gaseous form and measured by gas chromatograph (GC) was comparable to those sampled by NaOH solution trapping apparatus and measured by total organic C (TOC) analyzer, for simplicity we recommend the former approach. Overall, our results demonstrate that the system was reliable for measuring rhizosphere respiration of fine roots. [Copyright &y& Elsevier]

• Canopy cover best indicated biomass in high understory layer. • Niche complementarity drove biomass in low understory layer. • Soil fertility and mass ratio hypotheses significantly influenced understory biomass. • Effects of ecological variables on understory biomass depended on plantation type. Understory vegetation plays a very important role in maintaining biodiversity, soil nutrient cycling and carbon stocks, yet biomass productivity at the understory layer has been largely overlooked in previous studies. Our study provides one of the most comprehensive tests of how niche attributes, soil fertility and canopy attributes shape understory biomass in a forest restoration context. We investigated the relative effects on understory biomass of five categories of ecological variables related to five hypotheses – i) niche complementarity: functional diversity indices of understory plants, ii) mass ratio: community weighted mean (CWM) traits of understory plants, iii) overstory attributes: planted tree diameter, basal area, height, canopy cover and stem density, iv) soil nutrients: soil pH and soil C, N, P, and v) plantation type (an exotic broad-leaved monoculture, a native broad-leaved monoculture and a native conifer mix). Here, understory biomass was studied at two understory layers: high (2 to 7 m above ground) and low (less than 2 m above ground) in the plantations of subtropical area of Southern China. We surveyed vegetation, measured soil nutrients and determined five important leaf functional traits for each species. Multiple linear regression models were performed to select the best models and to assess the statistical significance of the effects of each variable. Our results showed that the best models for predicting biomass in the high (mean = 603.29 g m−2) and low (mean = 596.81 g m−2) layers were, respectively, the models for canopy cover and functional richness (FRic) interacting with plantation type. Model analyses showed a positive relationship between canopy cover and high-layer biomass (P < 0.001 and P < 0.05 respectively in native mixed-conifer and broad-leaved plantations) and between FRic and low-layer biomass (P < 0.001 in both native mixed-conifer and broad-leaved plantations) in the two native plantations. A positive relationship was also found between the CWM of leaf dry matter content and biomass in the high understory layer in the native mixed-conifer plantation (P < 0.001). Meanwhile, soil N was negatively correlated with low layer biomass (P < 0.01). Our study highlighted the dominant role of overstory attributes and niche complementarity in explaining forest understory productivity compared to other hypotheses. The effects of ecological factors on understory biomass varied in direction and magnitude, depending on understory stratum and plantation type. Indeed, the mechanisms behind biomass production in the high and low understory layers appeared to be decoupled. The typical biodiversity-ecosystem functioning relationship only drove low-layer biomass, while high-layer biomass was influenced by canopy cover. [ABSTRACT FROM AUTHOR]

• Positive diversity-stability relationships were found in mixed species plantations. • Community stability across all plantations augmented during restoration. • Species dominance instead of species asynchrony maintained forest ecosystem function. Understanding biodiversity and its relationship with ecosystem functioning along forest succession is extremely important to assessing dynamics of community stability and ecosystem integrity. Previous studies have revealed that increases in species richness, phylogenetic diversity (PD), species asynchrony and dominance can stabilize the temporal stability of community biomass in grasslands. However, how these determinants influence the stabilizing effect of biodiversity on forest productivity remains poorly understood. We analyzed the relationships between woody plant diversity and productivity stability along 30 years of restoration (1985–2015) in four types of subtropical forest: a mixed Eucalyptus plantation (EE), an Acacia mangium monoculture (AM), a mixed native species plantation (NS), and a mixed coniferous plantation (MC). Our results showed that community stability in three mixed species plantations (EE, NS, and MC) rather than the AM monoculture augmented remarkably as restoration proceeded. Both species richness and phylogenetic diversity significantly stabilized community productivity in the mixed species plantations instead of the monoculture during the 30-year restoration period. Species asynchrony was not a significant factor contributing to the forest productivity stability. Instead, community stability was mainly driven by that of the dominant tree species, and to a lesser extent, by the species richness. We demonstrated the more important role of species dominance in maintaining the temporal stability of forest productivity, which differs from that (species asynchrony) for grassland communities. [ABSTRACT FROM AUTHOR]