Aims A key idea in plant community ecology is that the identity of the limiting resource shifts from soil nutrients in low productivity sites to light in high productivity sites. This idea, and its implications for plant community structure, has been tested many times in artificial productivity gradients (fertilization studies), but whether it applies to natural productivity gradients is unclear.
Methods To test whether seedling light limitation would increase across a natural productivity gradient, I conducted a cross-site field experiment in southwest Michigan, USA. At each of six old fields naturally varying in productivity, I exposed transplanted seedlings of big bluestem (Andropogon gerardii) to a light addition (tie-back) treatment that increased light availability and measured their biomass after one and two growing seasons.
Important findings Seedlings responded positively to the tie-back treatment, but positive responses did not increase across the natural productivity gradient. These results suggest that although light does limit seedling establishment, the strength of light limitation does not depend on variation in productivity in natural systems. Instead, light limitation interacted with a variety of site differences to determine establishment. Although the general principle that light limitation increases with productivity is well established, these results indicate that it may not always occur in natural systems.

Aims The clustering of plants with similar leaf traits along environmental gradients may arise from adaptation as well as acclimation to heterogeneous habitat conditions. Determining the forces that shape plant leaf traits requires both linking variation in trait morphology with abiotic gradients and linking that trait variation with plant performance under varying abiotic conditions. Across the spectrum of plant types, shade-tolerant evergreen herbs are relatively low in trait plasticity, compared to deciduous and sun-adapted species. These plants employ stress-tolerant strategies for survival, which coincide with relatively static trait morphologies, slow growth and hence a lower ability to adjust to changing environmental conditions.
Methods We investigate how the survival of two ecologically similar understory evergreen species, Asarum arifolium and Hepatica nobilis, corresponds with variation in six commonly measured functional traits (leaf area, specific leaf area, plant height, leaf number, leaf length and shoot mass) along natural and experimental abiotic gradients. We examine temporal (the period 2007–9) and spatial (100 km) variations in these traits after (i) translocating 576 plants across a span from the southern Appalachian Mountains in NC, USA, to the Piedmont, GA, USA, which includes north- and south-facing slope habitats and (ii) the experimental manipulation of diffuse light and soil moisture.
Important findings We find that when translocated into a novel habitats, with novel environmental conditions that often are more extreme than the source habitat, both species appear capable of considerable morphological acclimation and generally converge to similar trait values. Hepatica nobilis does not exhibit mean trait values particularly different from those of A. arifolium, but it demonstrates much greater phenotypic plasticity. These results indicate that relatively conservative plant species nonetheless acclimate and survive across heterogeneous environmental conditions.

Aims Root systems play an essential role in grassland functioning in both acquisition and storage of resources. Nevertheless, root functional traits have not received as much attention as those measured on above-ground organs, and little is known about their relations. Our objectives were to test whether morphological and root system traits allowed identification of grass species' functional strategies and to determine whether a relation exists between above- and below-ground traits.
Methods Functional traits of root tissues (specific root length, diameter, tissue density and nitrogen concentration), whole root systems (root mass, root length density, root mass percentage below a depth of 20cm and fine root %) and two major leaf traits (specific leaf area and leaf dry matter content) were determined under field conditions and their relations were analysed in eleven perennial temperate Poaceae species.
Important findings Canonical correspondence analysis along Axis 1 revealed a gradient of species, from those with deep, dense and coarse root systems with a large root mass to those with shallow root systems, thin roots and high specific root length; this suggests strong correlations among root traits. Correlations between specific root length and specific leaf area reveal two groups of species, which probably indicates different drought-tolerance capacities. Root trait syndromes enable ranking grasses along a gradient from conservative-strategy species (from stressful habitats), which display a deep and coarse root system, to acquisitive species (from rich and moist meadows), which display a shallow and thin root system. Although both types display similar above-ground strategies, drought-tolerant species have lower specific root lengths than drought-sensitive species, revealing more conservative root strategies.

Aims Understanding the drivers for leaf traits is critical to improving our predictions on ecosystems' responses to global changes. Geographic patterns of leaf traits are shaped by phylogenetic, biological and environmental factors simultaneously. However, till now few studies have examined how these factors influenced leaf traits together, and how their effects differed at the within- and among-site levels.
Methods We sampled leaf traits from a 1100 km shrub-biome transect across central Inner-Mongolia, including leaf mass per area (LMA), mass-based photosynthetic rate, nitrogen (N) and phosphorus (P) concentrations. We examined the effects of phylogenetic, biological (height and growth rate) and environmental (climate and soil) factors on leaf traits with mixed-model analyses of variance. Variation partitioning method was used to separate the joint and independent effects of these three types of factors.
Important findings (i) Climate and soil fertility (total or available nutrient concentrations) together explained 11.4–41.4% of among-site variations, with remarkable difference among traits. (ii) Height and height growth rate together explained 0.4–31.9% of trait variations (mostly among-site variations). Our results could only weakly support the ability of leaf traits as predictors for whole-plant growth. (iii) LMA was negatively related to height, which was consistent with the resource-use strategy hypothesis but inconsistent with the hypotheses proposed for coexisting trees, suggesting that the LMA–height relationship is shaped by rather different mechanisms between the within- and among-communities levels. (iv) The variation partitioning analysis showed that, the relationships between leaf traits and biological characters largely reflected the differences in both leaf traits and biological characters among species that occupying different sites. The relative importance of phylogenetic, biological and environmental factors differed remarkably among leaf traits, between the within- and among-communities levels, and between different biomes. (v) Our results strongly suggest the necessity of examining the three types of factors simultaneously, and at both the within- and among-communities levels, for a better understanding of the drivers for leaf traits patterns.

Aims Melampyrum pratense and M. subalpinum are two myrmecochorous species, which possess similar habitat requirements and frequently occur together. Despite this, their population sizes differ markedly. Melampyrum pratense populations are usually very large, whereas M. subalpinum has rather small and isolated populations. We suggest that such an imbalance might be partially influenced by the difference in ant-mediated seed-removal rates. Genus Melampyrum is considered to be exclusively myrmecochorous, though to achieve the recent distribution of some Melampyrum species during the Holocene myrmecochory would be highly insufficient. We suggest that endozoochory takes place in the long-distance migration, whereas myrmecochory is important for the removal of seeds on a local scale.
Methods For seed-preference analysis, M. pratense and M. subalpinum mixed seed samples were placed around Formica polyctena anthills. After a period of time, the remaining seeds of both species were counted for each sample. The results were analysed by analysis of variance and generalized linear mixed-effect model. To test myrmecochorous removal distances, M. pratense seeds were covered with fluorescent dactyloscopic powder and placed in the vicinity of a large ant trail. The area around the starting plot was searched in the dark using UV LED torchlight 7h after the beginning. The distance from the starting plot was measured for each seed found. Birds, rodents, leporine and a ruminant were fed with M. pratense seeds and fresh plants to test the possibility of endozoochorous dispersal of the species. Animal droppings were searched for intact seeds.
Important findings Our field studies show that from mixed seed samples, containing both species, ants significantly preferred the seeds of M. pratense. This may be one of factors that has positive influence on M. pratense success in seed dispersal on mixed stands and consequently in the colonization of favourable sites. Experiments focusing on ant-mediated dispersal distance revealed that F. polyctena ants are able to move seeds over a distance of 36 m in only 7h. This distance is among the furthest known myrmecochorous removals of forest plant seeds. A new Melampyrum seed disperser Oligolophus tridens (Opiliones) was observed repeatedly. Our pilot study documented that Melampyrum seeds are able to pass through the digestive tract of a cow intact. This suggests that large ruminants such as deer, bison or forest-grazing livestock may function as important long-distance dispersers of Melampyrum species.

Aims Although many studies have reported net gains of soil organic carbon (SOC) after afforestation on croplands, this is uncertain for Chinese paddy rice croplands. Here, we aimed to evaluate the effects of afforestation of paddy rice croplands on SOC sequestration and soil respiration (R s). Such knowledge would improve our understanding of the effectiveness of various land use options on greenhouse gas mitigation in China.
Methods The investigation was conducted on the Chongming Island, north subtropical China. Field sites were reclaimed from coastal salt marshes in the 1960s, and soils were homogeneous with simple land use histories. SOC stocks and R s levels were monitored over one year in a paddy rice cropland, an evergreen and a deciduous broad-leaved plantation established on previous paddy fields and a reference fallow land site never cultivated. Laboratory incubation of soil under fast-changing temperatures was used to compare the temperature sensitivity (Q 10) of SOC decomposition across land uses.
Important findings After 15–20 years of afforestation on paddy fields, SOC concentration only slightly increased at the depth of 0–5cm but decreased in deeper layers, which resulted in a net loss of SOC stock in the top 40cm. Seasonal increase of SOC was observed during the rice-growing period in croplands but not in afforested soils, suggesting a stronger SOC sequestration by paddy rice cropping. However, SOC sequestered under cropping was more labile, as indicated by its higher contents of dissolved organic carbon and microbial biomass. Also, paddy soils had higher annual R s than afforested soils; R s abruptly increased after paddy fields were drained and plowed and remained distinctively high throughout the dry farming period. Laboratory incubation revealed that paddy soils had a much higher Q 10 of SOC decomposition than afforested soils. Given that temperature was the primary controller of R s in this region, it was concluded that despite the stronger SOC sequestration by paddy rice cropping, its SOC was less stable than in afforested systems and might be more easily released into the atmosphere under global warming.

Aims Why invasive plants are more competitive in their introduced range than native range is still an unanswered question in plant invasion ecology. Here, we used the model invasive plant Solidago canadensis to test a hypothesis that enhanced production of allelopathic compounds results in greater competitive ability of invasive plants in the invaded range rather than in the native range. We also examined the degree to which the allelopathy contributes increased competitive ability of S. canadensis in the invaded range.
Methods We compared allelochemical production by S. canadensis growing in its native area (the USA) and invaded area (China) and also by populations that were collected from the two countries and grown together in a 'common garden' greenhouse experiment. We also tested the allelopathic effects of S. canadensis collected from either the USA or China on the germination of Kummerowia striata (a native plant in China). Finally, we conducted a common garden, greenhouse experiment in which K. striata was grown in monoculture or with S. canadensis from the USA or China to test the effects of allelopathy on plant–plant competition with suitable controls such as adding activated carbon to the soil to absorb the allelochemicals and thereby eliminating any corresponding allopathic effects.
Important findings Allelochemical contents (total phenolics, total flavones and total saponins) and allelopathic effects were greater in S. canadensis sampled from China than those from the USA as demonstrated in a field survey and a common garden experiment. Inhibition of K. striata germination using S. canadensis extracts or previously grown in soil was greater using samples from China than from the USA. The competitive ability of S. canadensis against K. striata was also greater for plants originating from China than those from the USA. Allelopathy could explain about 46% of the difference. These findings demonstrated that S. canadensis has evolved to be more allelopathic and competitive in the introduced range and that allelopathy significantly contributes to increased competitiveness for this invasive species.