Aims Factors limiting distributions of species are fundamental to ecology and evolution but have rarely been addressed experimentally for multiple species. The conspicuous linear distribution patterns of plant species confined to river corridors in the Central European lowlands constitute an especially long-standing distribution puzzle. We experimentally tested our novel hypothesis that the tolerance of species to river corridor conditions is independent of the degree of confinement to river corridor habitats, but that species not confined to river corridors are better able to take advantage of the more benign non-river corridor conditions.
Methods We grew 42 herbaceous species differing in their confinement to river corridors in a common garden experiment on loamy soil typical for river corridor areas and sandy soil typical for non-river corridor areas, and with and without a flooding period. For a subset of species, we grew plants of both river corridor and non-river corridor origin to test for adaptation to river corridor conditions.
Important findings Species more confined to river corridor areas benefited less from the more benign non-flooded and non-river corridor soil conditions than species of wider distributional range did. For subsets of 7 and 12 widespread species, the response to flooding and soil origin, respectively, did not differ between plants from river corridor sites and plants from other sites, suggesting that the habitat tolerance of widespread species is due to phenotypic plasticity rather than to local adaptation. Overall, we found clear support for our novel hypothesis that species not confined to river corridors are more able to take advantage of the more benign non-river corridor conditions. Our study provides a general hypothesis on differences between species confined to stressful habitats and widespread species out for test in further multispecies comparative experiments.

Aims In order to better understand how tornados structure the ecotone between eastern deciduous forest and tallgrass prairie, we sampled trees in a recent tornado blowdown area and in the adjacent undisturbed forest. Both are part of the Cross Timbers ecotone of Osage County, Northeastern Oklahoma, USA.
Methods We set up a 1-ha square plot in each area and sampled all tree stems at least 50 cm tall where stems were identified to species and measured for size and spatial location in both plots. For the stems in the blowdown plot, we also scored damage and resprouting. We then used the spatial location data to compute the spatial heterogeneity and degree of clumping in both areas.
Important findings Significant clumping was observed in the control forest at the small spatial scale of 1–8 m and at the medium spatial scale of 30–36 m. The blowdown area: showed domination by post oak (Quercus stellata) with small stems, branch damage and stem resprouting above 1 m most common; had no trees that either lost their leaves without branch or stem damage or lived without some sort of resprouting, had a significant negative correlation between damage and resprouting and clumped only at small spatial scales for stems of medium size, for stems with branch damage and for stems that resprouted at or below 1 m height. We conclude that in addition to largely eliminating a forest's spatial heterogeneity, tornados may also structure this ecotone by increasing tree persistence through resprouting.

Aims For assisting faster restoration of damaged or severely disturbed coastal ecosystems, selected mangrove species have been planted on previously mangrove-inhabited sites of the tropical and subtropical coasts of southern China. The objective of this study was to understand the stand dynamics of the planted mangroves and their functional traits in comparison with natural mangrove forests under similar site conditions.
Methods Species composition, stand density, tree size distribution, and aboveground production were investigated along three transects in a 50-year-old planted mangrove stand and three transects in an adjacent natural mangrove stand in Shenzhen Bay, South China. Measurements were made on tree distribution by species, stand structure, and aboveground biomass (AGB) distribution. Analyses were performed on the spatial patterns of tree size distribution and species association.
Important findings We found that the planted and natural mangrove stands did not differ in stand density, average diameter at breast height (DBH), species composition, and AGB. Spatial distribution of AGB and frequency at species level were also similar between the planted and natural stands. However, the traits in stand structure were more variable in the planted stand than in the natural stand, indicating higher spatiotemporal heterogeneity in the development and succession of planted mangroves. Geostatistical analyses show that both DBH and AGB were spatially auto-correlated within a specific range in the direction perpendicular to coastline. More than 60% of the variance in these attributes was due to spatial autocorrelation. The Ripley's K -function analysis shows that the two dominant species, Kandelia obovata and Avicennia marina, clumped in broader scales in the natural stand than in the planted stand and displayed significant interspecific competition across the whole transect. It is suggested that interspecific competition interacts with spatial autocorrelation as the underlying mechanism shaping the mangrove structure. This study demonstrates that at age 50, mangrove plantations can perform similarly in stand structure, spatial arrangement of selected stand characteristics and species associations to the natural mangrove forests.

Aims Natural secondary forest (NSF) and larch plantation are two of the predominant forest types in Northeast China. However, how the two types of forests compare in sustaining soil quality is not well understood. This study was conducted to determine how natural secondary forest and larch plantation would differ in soil microbial biomass and soil organic matter quality.
Methods Microbial biomass carbon (MBC), microbial biomass nitrogen (MBN), soil organic carbon (SOC) and total nitrogen (TN) in the 0- to 15-cm and 15- to 30-cm soil layers were investigated by making chemical and biological measurements in the montane region of eastern Liaoning Province, Northeast China, during the growing season of 2008 in stands of NSF and Larix olgensis plantation (LOP).
Important findings We found that soil MBC and MBN were significantly lower in the LOP than in the NSF. Both MBC and MBN declined significantly with increasing soil depth in the two types of stands. The ratios of MBC to SOC (MBC/SOC) and MBN to TN (MBN/TN) were also significantly lower in the LOP than in the NSF. Moreover, the values of MBC, MBC/SOC, and MBN/TN significantly varied with time and followed a similar pattern during the growing season, all with an apparent peak in summer. Our results indicate that NSF is better in sustaining soil microbial biomass and nutrients than larch plantation in the temperate Northeast China. This calls for cautions in large-scale conversions of the native forests to coniferous plantations as a forest management practice on concerns of sustaining soil productivity.

Aims Evaluation of the interaction intensity between Masson pine (Pinus massoniana) and broadleaved trees will facilitate selecting tree combinations suitable for reforestation in abandoned sites in subtropical areas.
Methods Pinus massoniana and seven broadleaved trees species were grown either in monoculture or in two-species mixture. Biomass of tree species was measured and inter-specific interactions were estimated using log response ratio. Test of homogeneity of variances was performed to compare the stability of biomass in the monoculture of the broadleaved trees with that in the mixture.
Important findings Our results showed that the direction and intensity of interactions between P. massoniana and the broadleaved trees varied from year to year and the identity of the broadleaved species. Facilitative interactions were found between Camellia oleifera, Rhus chinensis and P. massoniana. Pinus massoniana had competitive effects on Lithocarpus glaber, Cyclobalanopsis glauca and Elaeocarpus japonicus. Significantly negative relations were found between biomass of P. massoniana and the broadleaved trees in the third year of the experiment, and the compensatory effects between P. massoniana and the broadleaved trees may be involved in stability maintenance in the multi-species forests in the subtropical area. The results of homogeneity test of variances also showed that the biomass per pot in the mixture had significantly lower variances than that in the monoculture, suggesting that total biomass is more stable in the mixture than the monoculture.

Aims Root interactions between neighbour plants represent a fundamental aspect of the competitive dynamics in pure stand and mixed cropping systems. The comprehension of such phenomena places big methodological challenges, and still needs clarification. The objectives of this work were (i) to test if a species with coloured roots can be used to examine the interaction in a legume-non-legume intercropping system; (ii) to verify the importance of initial root growth on the successive root development of mixture component plants; (iii) to test if the root interaction in the shallow layers has consequences for deep root growth and (iv) to compare the effect of intraspecific and interspecific competition on root development and biomass growth.
Methods A detailed study on root growth and interaction was carried out using rhizotron tubes where two legume species were grown in pure stands or were intercropped with red beet, a variety of Beta vulgaris L. with clear red roots. Within the rhizotrons, the three species were grown either without competitors, with two plants of the same species to measure intraspecific competition or with one legume and one red beet plant to study interspecific competition. The use of mixtures where one component has clearly coloured roots, together with several scalar measurements of root depth and proliferation, allowed the measurement of the root system of each species when grown in the mixtures.
Important findings The use of rhizotron tubes coupled with species with coloured roots represented a valuable method to study the belowground interaction in mixed cropping systems. The initial root growth was a very important feature for the subsequent dominance of a species and it was not related to seed dimension. Initial root growth was also important because the root interactions in the shallower soil layers were found to influence the root growth in deeper soil. The root system of the red beet showed much faster and deeper growth than that of the legumes, and made red beet the dominant component in the mixtures while the legume root system was confined to the shallower soil layer. Intraspecific competition was well tolerated by the legumes, but it was limiting for the highly competitive red beet. The outcome of root interaction between neighbour plants was confirmed to be species-specific as it changed according to the intensity of the competitive effect/response of each species of the mixture: both legumes were slightly affected by the intraspecific and highly affected by interspecific competition while red beet was more affected by intraspecific competition but strongly dominant when intercropped with legumes.

Aims Natural 15 N abundance provides integrated information about nitrogen (N) input, transformation and output, indirectly reflecting N cycling traits within terrestrial ecosystems. However, relationships between natural 15 N abundance and N cycling processes are poorly understood in China. Here, our primary objectives were to (i) examine the effects of grazing at varying levels of intensity on δ 15 N of soils and plants in a semi-arid grassland; (ii) detect the relationships between δ 15 N of soils and four major N cycling processes (i.e. mineralization, nitrification, denitrification and ammonia volatilization); and (iii) determine whether δ 15 N of soils can be used as an indicator of N cycling in this semi-arid grassland.
Methods The field experiment was conducted within the long-term (17-year) grazing enclosures in a semi-arid grassland in Inner Mongolia. Five grazing intensities (0.00, 1.33, 2.67, 4.00 and 5.33 sheep ha^-1) were designed. δ 15 N values of topsoils (0–10 cm), surface soils (0–2 cm) and plants were measured in 2006. Differences in δ 15 N of soils and plants between the five grazing intensities were examined. Rates of four soil N cycling processes were measured periodically during the 2005 and 2006 growing seasons. The δ 15 N values of topsoils were linked to the four N cycling processes to investigate their relationships.
Important findings The δ 15 N values of topsoils (5.20–5.96‰) were substantially higher than the δ 15 N values of plants (2.51–2.93‰) and surface soils (1.44–2.92‰) regardless of grazing intensities. The 15 N-depleted N losses during microbial decomposition of organic matter in concert with the downward movement of residual substrate over time are the possible causes of higher δ 15 N values in topsoils than in surface soils. In addition, the δ 15 N values of topsoils were positively correlated with the δ 15 N values of both plants and surface soils. Grazing, especially the high-intensity grazing (5.33 sheep ha^-1), resulted in a significant decrease in δ 15 N of surface soils. However, no statistically significant variations in δ 15 N of topsoils and plants were found in response to grazing. The δ 15 N values of topsoils exhibited significant dependence on the cumulative rates of NH 3 volatilization, net nitrification and denitrification in 2005 but not in 2006.

Aims Elevated atmospheric CO₂ has the potential to enhance the net primary productivity of terrestrial ecosystems. However, the role of soil microorganisms on soil C cycling following this increased available C remains ambiguous. This study was conducted to determine how quality and quantity of plant litter inputs would affect soil microorganisms and consequently C turnover.
Methods Soil microbial biomass and community structure, bacterial community-level physiological profile, and CO₂ emission caused by different substrate C decomposition were investigated using techniques of biological measurements, chemical and stable C isotope analysis, and BIOLOG-ECO microplates in a semiarid grassland ecosystem of northern China in 2006 and 2007 by mixing three contrasting types of plant materials, C₃ shoot litter (SC 3), C₃ root litter (RC 3), and C₄ shoot litter (SC 4), into the 10- to 20-cm soil layer at rates equivalent to 0 (C 0), 60 (C 60), 120 (C 120) and 240 g C m ?2 (C 240).
Important findings Litter addition significantly enriched soil microbial biomass C and N and resulted in changes in microbial structure. Principal component analysis of microbial structure clearly differentiated among zero addition, C₃ -plant-derived litter, and C₄ -plant-derived litter and among shoot- and root-derived litter of C₃ plants; soil microorganisms mainly utilized carbohydrates without litter addition, carboxylic acids with C₃ -plant-derived litter addition and amino acids with C₄ -plant-derived litter addition. We also detected stimulated decomposition of older substrate with C₄ -plant-derived litter inputs. Our results show that both quality and quantity of belowground litter are involved in affecting soil microbial community structure in semiarid grassland ecosystem.

Aims Rhodiola dumulosa is a perennial herb growing in a naturally fragmented habitat of high-mountain rocks. This research aims to (i) investigate the mating system characteristics and pollination biology of R. dumulosa, (ii) study the effects of ecological factors on the mating system and pollination biology of R. dumulosa and (iii) assess the relationship between its pollination and mating system.
Methods Mating system parameters were analysed using allozyme markers. Growth of pollen tubes from artificial self- and cross-pollination was also examined. Field investigations were conducted on insect flower visitation during the blooming period of R. dumulosa. Relationships among mating system, insect flower visitation frequency and environmental factors were assessed.
Important findings The results showed that (i) R. dumulosa has a mixed-mating system: it is self-compatible and primarily outbreeding, with the multilocus outcrossing rate (t m) ranging from 0.589 ± 0.078 (± standard deviation) to 0.846 ± 0.077; inbreeding coefficients of maternal parents (F) were zero, indicating that inbreeding depression was intense. (ii) Light intensity and temperature impacted the visitation of the principal pollinators, the syrphid Eristalis tenax and the bumblebee Bombus pyrosoma. (iii) Population size plays an important role in the mating system and pollination. Populations received more frequent visits by the principal pollinators usually had higher outcrossing rates. Pollination to some extent affects the mating system of R. dumulosa and it is important to maintain large populations to prevent further inbreeding and to maintain pollinators to facilitate outcrossing.