Aims Facilitation by nurse plants is a common interaction in harsh environments and this positive plant–plant interaction may promote vegetation recovery in ecosystems affected by human activities. Determining the relevance of this process, however, requires assessing how nurse plants influence the establishment of other species, as well as the proportion of species in the regional species pool that would benefit from the presence of nurse plants in human-disturbed areas. Further, since vegetation recovery is a time-dependent process, the community-level consequences of facilitation are likely to vary among landscapes with different disturbance history. Thus, an integrative perspective of the relevance of nurse plants for vegetation recovery could be obtained by measuring their effects across different human-disturbed landscapes of the target region. This study focuses on these issues and uses a regional-scale approach to assess the community-level effects of a widespread nurse plant of American deserts, the creosotebush (Larrea tridentata).
Methods This study was conducted in the southernmost portion of Chihuahuan Desert because most floodplain valleys of this region have been affected by human activities during the past centuries. For this study, we selected 10 floodplain valleys differing in their age (i.e. the time elapsed after human activities were ceased). At each landscape, we measured the cover of creosotebushes and the proportion of plant species positively associated with them, as well as the density of seeds in the soil beneath creosotebush canopies. All these data were regressed against the age of the landscapes. Further, to assess whether positive association patterns were due to facilitation or other processes, we conducted field experiments and measured the ecophysiological performance of plant species established beneath and outside creosotebush canopies.
Important findings Most plant species from the target region were positively associated to creosotebushes, and our field experiments and ecophysiological measures indicated that these distribution patterns can be attributed to facilitative interactions. In most landscapes, the density of seeds was higher beneath creosotebushes than in the surrounding habitats, suggesting that these shrubs may also act as seed traps. The community-level effects of creosotebushes increased with landscape age and creosotebush cover, indicating that magnitude of these effects depends on the disturbance history of each site. These results highlight the relevance of performing large-scale assessments for identifying the consequences of facilitation on vegetation recovery across space and time. We then propose that this kind of large-scale approach should be taken into account in the development of conservation programs aimed at the recovery and preservation of plant biodiversity in harsh environments.

Aims Forest fragmentation and the associated augmentation of forest edge zones are increasing worldwide. Forest edges are characterized by altered plant species richness and community composition. As the tree layer and its species composition has been shown to influence herb layer composition, changes in tree species composition or richness may weaken or strengthen edge effects in forest ecosystems. We studied effects of the edge–center transition, tree species composition and their potential interaction on the understory vegetation in the Hainich National Park, Germany's largest connected deciduous forest, allowing to cover large edge–center transects.
Methods We established 12 transects in an area of 75 km 2 of continuous forest, 6 beech-dominated and 6 in multispecies forest stands. Each transect reached from the forest edge up to 500 m into the forest interior. Vegetation relevés were conducted in regular, logarithmic distances along each transect.
Important findings Herb species richness was influenced by an interaction of edge effects and tree diversity level. With increasing distance from the forest edge, herb species richness remained constant in multispecies forest stands but rapidly decreased in beech-dominated forest stands. Further, herb richness was higher in the interior of multispecies forest stands. Percent forest specialists increased and percent generalists decreased with distance from the edge and this contrasting pattern was much more pronounced in beech-dominated transects. By using structural equation modeling, we identified litter depth mediated by tree species composition as the most important driver of herb layer plant species richness.

Aims Grazing and water availability are the primary drivers of vegetation dynamics in grazing-dominated regions of Mongolia with a semi-arid climate and frequent droughts. Nomadic animal husbandry still plays a large part in the economy of Mongolia, but more variable precipitation regime and increase in livestock number have severely affected grassland ecosystems through overgrazing, leading to pasture degradation. This study aimed to examine the effects of grazing exclusion, interannual variation of plant-available precipitation (PAP) and their interaction on the aboveground biomass (AGB) of each dominant species, the AGB of annual species and the total AGB in a Mongolian dry steppe, using long-term field data.
Methods To detect the effect of grazing on vegetation dynamics, vegetation surveys were conducted in a non-grazed exclosure zone and a fully grazed area outside the exclosure. We assessed the effects of grazing, PAP and their interaction on AGB parameters using a generalized linear model. A detrended correspondence analysis (DCA) was used to visualize the effects of grazing and PAP on the AGB of each species.
Important findings Grazing, PAP and their interaction had significant effects on AGB. The effect of grazing on AGB was larger with higher precipitation and higher amounts of AGB (i.e. forage) while AGB was strongly limited in drought years, which resulted in a smaller grazing effect. The current year PAP had the highest impact (r = 0.88, P < 0.01) on AGB. The dominance of annual species was characterized by the amount of PAP in the current and preceding years: annuals dominated in wet years that followed consecutive dry years. The DCA Axis 1 reflected the variation of AGB with interannual variation of PAP while the DCA Axis 2 differentiated the grazing effect. The DCA scatter diagram based on species score illustrated that Artemisia adamsii (an unpalatable herb) was clearly linked to grazing disturbance whereas palatable perennials such as Agropyron cristatum, Stipa krylovii and Cleistogenes squarrosa were related to grazing abandonment and wetter conditions. In brief, number of livestock, hence the grazing impacts on vegetation dynamic in this region could have driven by forage availability, which is mainly controlled by current-year PAP.

Aims Boreal forests play an important role in the global carbon cycle. Compared with the boreal forests in North America and Europe, relatively few research studies have been conducted in Siberian boreal forests. Knowledge related to the role of Siberian forests in the global carbon balance is thus essential for a full understanding of global carbon cycle.
Methods This study investigated the net ecosystem exchange (NEE) during growing season (May–September) in an eastern Siberian boreal larch forest for a 3-year period in 2004–2006 with contrasting meteorological conditions.
Important findings The study found that the forest served as a carbon sink during all of the 3 studied years; in addition, the meteorological conditions essentially influenced the specific annual value of the strength of the carbon sinks in each year. Although 2005 was the warmest year and much wetter than 2004, 2005 also featured the greatest amount of ecosystem respiration, which resulted in a minimum value of NEE. The study also found that the phenological changes observed during the three study years had a relatively small effect on annual NEE. Leaf expansion was 26 days earlier in 2005 than in the other 2 years, which resulted in a longer growing season in 2005. However, the NEE in 2005 was counterbalanced by the large rate of ecosystem respiration that was caused by the higher temperatures in the year. This study showed that meteorological variables had larger influences on the interannual variations in NEE for a Siberian boreal larch forest, as compared with phenological changes. The overall results of this study will improve our understanding of the carbon balance of Siberian boreal larch forests and thus can help to forecast the response of these forests to future climate change.

Aims Climate change largely impacts ecosystem carbon and water cycles by changing plant gas exchange, which may further cause positive or negative feedback to global climate change. However, long-term global change manipulative experiments are seldom conducted to reveal plant ecophysiological responses to climatic warming and altered precipitation regimes.
Methods An 8-year field experiment with both warming and increased precipitation was conducted in a temperate grassland in northern China. We measured leaf gas exchange rates (including plant photosynthesis, transpiration and instantaneous water use efficiency [WUE]) of two dominant plant species (Stipa sareptana var. krylovii and Agropyron cristatum) from 2005 to 2012 (except 2006 and 2010) and those of other six species from 2011 to 2012.
Important findings Increased precipitation significantly stimulated plant photosynthetic rates (A) by 29.5% and 19.9% and transpiration rates (E) by 42.2% and 51.2% for both dominant species S. sareptana var. krylovii and A. cristatum, respectively, across the 8 years. Similarly, A and E of the six plant functional types were all stimulated by increased precipitation in 2011 and 2012. As the balance of A and E, the instantaneous WUEs of different plant species had species-specific responses to increased precipitation. In contrast, neither warming nor its interaction with increased precipitation significantly affected plant leaf gas exchange rates. Furthermore, A and E of the two dominant species and their response magnitudes to water treatments positively correlated with rainfall amount in July across years. We did not find any significant difference between the short-term versus long-term responses of plant photosynthesis, suggesting the flexibility of leaf gas exchange under climate change. The results suggest that changing precipitation rather than global warming plays a prominent role in determining production of this grassland in the context of climate change.

Aims Forest canopy openings modify the natural environment, producing changes in light quality and intensity, precipitation and temperature. In turn, these changes promote the acclimation of understory species. However, little work has been done on underground responses to those environmental changes. The objective of this work was to determine how Osmorhiza depauperata, Phleum alpinum and Poa pratensis change its root length density and root colonization by mycorrhiza as a function of light availability in a Nothofagus pumilio (i.e. lenga) forest harvested following the variable retention prescription.
Methods We selected three microenvironments in an old growth forest harvested by the variable retention prescription: aggregated retention, dispersed retention with influence of aggregated retention and dispersed retention. A non-harvested primary forest (PF), similar to the harvested one, was used as a control. Every 2 months, from October 2008 to April 2009, we took soil cores from randomly selected plants. From these soil cores, root length density and colonization percentage (CP) by arbuscular mycorrhizae were estimated.
Important findings Light availability changed significantly among the microenvironments. In general, root length density was significantly greater in P. pratensis than in P. alpinum and both species greater than in O. depauperata. Light availability increased root length density in all species, although the magnitude of these increases difference among species. Root length density was 187% greater in P. pratensis, 101% in P. alpinum and 94% in O. depauperata in the disperse retention system than in the PF. Mycorrhiza CP was higher in O. depauperata than in P. alpinum and P. pratensis. Also, it was lower in the PF than in the harvested microenvironments. CPs were very low.

Aims The importance of quantifying carbon stocks in terrestrial ecosystems is crucial for determining climate change dynamics. However, the present regional assessments of carbon stocks in tropical grasslands are extrapolated to unsampled areas with a high degree of uncertainty and without considering the carbon and nitrogen composition of vegetation and soil along altitudinal ranges. This study aims to assess carbon and nitrogen concentrations in soil and vegetation, aboveground carbon stocks distribution and soil organic carbon stocks along an altitudinal range in the páramo region in the Ecuadorian Andes.
Methods The vegetation inventory was conducted using 15×15 m sampling plots distributed in three altitudinal ranges. Based on the patterns exhibited by the dominant vegetation growth forms, biomass and soil were sampled to quantify the corresponding carbon and nitrogen concentrations. Subsequently, the aboveground live biomass along the páramo altitudinal range was estimated using allometric equations. Finally, soil and vegetation carbon stocks were estimated for the entire basin.
Important findings Altitudinal analysis supported a potential distribution of carbon and nitrogen concentrations in soil, litter and live tissues, where higher concentrations were found in the low altitudinal range mainly for tussocks and acaulescent rosettes. Cellulose in litter showed higher concentrations at low altitudinal ranges for acaulescent rosettes and cushions only. For the same growth forms, lignin patterns in litter were higher in high altitudinal ranges. Soil texture provided complementary information: high percentage of silt was highly correlated to high soil nitrogen and carbon concentration. Tussocks were found to be responsive to altitude with their, highest aboveground carbon stocks occurring at the low altitudinal range, but cushions and acaulescent rosettes responded differently. The established relationships among soil, vegetation and altitude shown in this study must be taken into account to estimate both aboveground and soil organic carbon stocks in páramo regions—such estimates will be considerably inaccurate if these relationships are ignored.

Aims Release of carbon from plant roots initiates a chain of reactions involving the soil microbial community and microbial predators, eventually leading to nutrient enrichment, a process known as the 'microbial loop'. However, root exudation has also been shown to stimulate nutrient immobilization, thereby reducing plant growth. Both mechanisms depend on carbon exudation, but generate two opposite soil nutrient dynamics. We suggest here that this difference might arise from temporal variation in soil carbon inputs.
Methods We examined how continuous and pulsed carbon inputs affect the performance of wheat (Triticum aestivum), a fast-growing annual, while competing with sage (Salvia officinalis), a slow-growing perennial. We manipulated the temporal mode of soil carbon inputs under different soil organic matter (SOM) and nitrogen availabilities. Carbon treatment included the following two carbon input modes: (i) Continuous: a daily release of minute amounts of glucose, and (ii) Pulsed: once every 3 days, a short release of high amounts of glucose. The two carbon input modes differed only in the temporal dynamic of glucose, but not in total amount of glucose added. We predicted that pulsed carbon inputs should result in nutrient enrichment, creating favorable conditions for the wheat plants.
Important findings Carbon addition caused a reduction in the sage total biomass, while increasing the total wheat biomass. In SOM-poor soil without nitrogen and in SOM-rich soil with nitrogen, wheat root allocation was higher under continuous than under pulsed carbon input. Such an allocation shift is a common response of plants to reduced nutrient availability. We thus suggest that the continuous carbon supply stimulated the proliferation of soil microorganisms, which in turn competed with the plants over available soil nutrients. The fact that bacterial abundance was at its peak under this carbon input mode support this assertion. Multivariate analyses indicated that besides the above described changes in plant biomasses and bacterial abundances, carbon supply led to an accumulation of organic matter, reduction in NO 3 levels and increased levels of NH 4 in the soil. The overall difference between the two carbon input modes resulted primarily from the lower total wheat biomass, and lower levels of NO 3 and soil PH characterizing pots submitted to carbon pulses, compared to those subjected to continuous carbon supply. Carbon supply, in general, and carbon input mode, in particular, can lead to belowground chain reactions cascading up to affect plant performance.

Aims We aimed to quantify the variation of leaf δ 13 C along an arid and semi-arid grassland transect in northern China. We also evaluated the effects of environmental factors (i.e. precipitation, temperature and altitude) on the spatial variation of leaf δ 13 C in northern grasslands and Tibetan Plateau, China.Method We sampled leaves of plant species belonging to three herb genera (Stipa spp., Leymus spp. and Cleistogenes spp.) and three shrub genera (Caragana spp., Reaumuria spp. and Nitraria spp.) for carbon isotope analysis from 50 locations along a 3200-km arid and semi-arid grassland transect in northern China. Leaf δ 13 C data in Tibetan Plateau and northern grasslands in China were also compiled from studies in literature.
Important findings Along the transect, leaf δ 13 C for C₃ plants ranged from ?28.0‰ to ?23.3‰, and from^-16.3‰ to^-13.8‰ for C₄ plant Cleistogenes spp.. The change in leaf δ 13 C ranged from ?0.26‰ to ?3.51‰ with every 100mm increase of annual precipitation, and leaf δ 13 C of shrubs (Nitraria spp., Reaumuria spp. and Caragana spp.) responded more markedly to climatic factors (precipitation and temperature) than that of herbs (Stipa spp., Leymus spp. and Cleistogenes spp.), indicating higher sensitivity of shrub δ 13 C to climatic changes. The most important factor regulating spatial variations of leaf δ 13 C in Tibetan Plateau was altitude, while it was precipitation in northern grasslands. Our results suggested that shrubs are more adapted to increasing drought in arid and semi-arid grassland. Controls of environmental factors on leaf δ 13 C depended on the most limiting factors in arid grassland (precipitation) and Tibetan grasslands (atmospheric CO₂ concentration).

Aims Cyclopia and Aspalathus are legumes harvested for production of Honeybush and Rooibos tea, respectively. Farmers grow these species from either seeds or cuttings over several years with continuous annual harvesting. The aims of this study were to assess the effect of plant age, plant species, toposequence, planting material and farmer practice on nitrogen (N) nutrition and water-use efficiency of two Cyclopia and Aspalathus species in the Cape fynbos.
Methods The study was conducted using plants from Koksrivier farm located near Gansbaai (33° S 18° E, 39 m.a.s.l), and at Kanetberg farm near Barrydale (33° S 21° E, 830 m.a.s.l). The 15 N natural abundance technique was used to determine N 2 fixation, carbon (C) assimilation and δ 13 C in shoot of Cyclopia and Aspalathus species.
Important findings Older tea plantations of C. genistoides and C. subternata derived more N from fixation and exhibited greater water-use efficiency than younger plants. At Koksrivier, Aspalathus caledonensis and A. aspalathoides showed greater water-use efficiency and derived more N from fixation than Cyclopia genistoides. Annual harvesting of C. genistoides decreased N 2 fixation. At Kanetberg, C. subternata plants on the upper and middle slopes derived more N from atmospheric fixation than those on the lower slope. C. subternata plants grown from seedlings recorded greater %Ndfa than cuttings. N 2 fixation and water-use efficiency of Cyclopia was affected by age, slope and planting material. Further, symbiotic N nutrition and water-use efficiency of Cyclopia and Aspalathus were related.

Aims We tested whether—in addition to weather conditions—the concentrations of nitrogen and phosphorus in the substrate have an effect on the radial stem increment of Nothofagus dombeyi trees in old-growth forest stands on volcanic soil at the western slopes of the Andes in South-Central Chile.
Methods We took soil samples and tree increment cores from five proximate sites (1000–1300 m a.s.l.) that are located in the volcanic region of the Conguillío National Park and differ in the age of the substrate (Miocene—3500 years B.P.) and in its concentrations of nitrogen (N) and phosphorus (P). The soil samples were also analysed on their concentrations of other plant mineral nutrients, carbon (C) and nitrogen isotope ratios (δ 15 N). Tree-ring widths and the stem basal area increment (BAI) were related to climate parameters. In selected tree rings, the stable isotope ratios of carbon (δ 13 C) and oxygen (δ 18 O) were determined and related to growth and climate parameters.
Important findings Consistent with theory, the soils on the oldest substrate showed the highest (least negative) δ 15 N values, but mineral N was the only nutrient whose concentration exhibited a straightforward (increasing) relationship with increasing substrate age. The BAI was largest on the soil with the highest concentration of plant-available P. In contrast to BAI, tree-ring chronologies did not differ among the study sites. However, tree-ring chronologies and BAI exhibited significantly positive correlations with summer precipitation, and negative correlations, with summer (December) temperature. A negative correlation was found between δ 13 C and precipitation anomalies in the growing season (November–March). We interpret the negative correlations between growth and temperature, and between δ 13 C and δ 18 O in the tree rings, as an impairment of net carbon assimilation by anomalously warm weather conditions during the growing season. We conclude that the growth of N. dombeyi is mainly affected by high temperature and low precipitation in spring and summer irrespective of the substrate's age, and enhanced by higher concentrations of plant-available P in the soil. Our results may be representative of N. dombeyi stands on volcanic substrate within their principal distribution range along the Andes of South America.

Aims Soil heterogeneity is ubiquitous in many ecosystems. We hypothesized that plant communities with higher species richness might be better adapted to soil heterogeneity and produce more biomass than those with lower richness. This is because there is niche differentiation among species and different species can complement each other and occupy a broader range of niches when plant species richness is high. However, no study has tested how soil particle heterogeneity affects the yield of plant communities, and whether such effects depend on the spatial scale of the heterogeneity and the species richness within the communities.
Methods In a greenhouse experiment, we sowed seeds of four-species or eight-species mixtures in three heterogeneous treatments consisting of 32, 8 or 2 patches of both small (1.5mm) and large quartz (3.0mm) particles arranged in a chessboard manner and one homogeneous treatment with an even mixture of small and large quartz particles.
Important findings Biomass production was significantly greater in the communities with high species richness than those with low species richness. However, soil particle heterogeneity or its interactions with patch scale or species richness did not significantly affect biomass production of the experimental communities. This work indicates that plant species richness may have a bigger impact on plant productivity than soil particle heterogeneity. Further studies should consider multiple sets of plant species during longer time periods to unravel the potential mechanisms of soil heterogeneity and its interactions with the impacts of species richness on community yield and species coexistence.

Aims The two coexisting Leymus chinensis ecotypes exhibit remarkable divergences in adaptive strategies under drought and salinity in semi-humid meadows and semi-arid steppes. In order to detect the major genetic and environmental factors dominating the intraspecific phenotype variations and ecotype formation, the questions regarding the two distinct phenotypic forms (ecotypes) in L. chinensis were addressed: (i) did environments drive the L. chinensis ecotype formation? (ii) was there a molecular basis for the morphological divergence between the two ecotypes? (iii) which driving force dominated the intraspecies divergence, divergent natural selection, genetic drift or stabilizing selection?
Methods We applied a series experiments on demographical, morphological and physiological traits of two Leymus chinensis ecotypes with gray green (GG) and yellow green (YG) leaf color in nine wild sites along a longitudinal gradient from 114° to 124°E in northeast China. The environmental data including mean annual precipitation, mean annual temperature, elevation and soil properties were collected. We compared the differences of morphological, physiological and genetic differentiations between the two ecotypes.
Important findings The GG type exhibited stronger fitness than YG type from the population densities, morphological traits (e.g. shoot height, leaf area, leaf and seed weights et al.), leaf mass per area (LMA) and physiological traits [relative water content (RWC), proline, soluble sugar contents]. Most of above phenotypes (e.g. total shoot densities, spike length et al.) were significantly correlated with mean annual precipitation, mean annual temperature and soil water content (SWC), rarely a correlated with soil pH and soil nutrient. Transplanted populations showed convergence trend by their leaf chlorophyll contents and osmotic adjustments (proline and soluble sugar contents) in the greenhouse, but still exhibited their divergences between two ecotypes in the outdoor transplantation, suggesting that whether L. chinensis ecotype differentiated could be largely affected by the environmental conditions. Furthermore, by the comparison result of quantitative genetic variation (Q ST) values from phenotypes with theoretical neutral genetic differentiation (F ST), differentiation in phenotypic traits greatly surpassed neutral predictions, implying that directional natural selection played a crucial role in L. chinensis ecotype differentiation. In addition, microsatellite analysis from Neighbor-joining tree and Bayesian assignment generated into two groups according to ecotypes, indicating molecular genetic differentiation also propelled the two ecotypes divergence. We conclude that L. chinensis population variations were driven by combing divergent natural selection (precipitation, temperature and SWCs) along the large-scale gradient and significantly intrinsic genetic differentiation.

Aims Foliar herbivory and water stress may affect floral traits attractive to pollinators. Plant genotypes may differ in their responses to the interplay between these factors, and evolution of phenotypic plasticity could be expected, particularly in heterogeneous environments. We aimed at evaluating the effects of simulated herbivory and experimental drought on floral traits attractive to pollinators in genetic families of the annual tarweed Madia sativa, which inhabits heterogeneous environments in terms of water availability, herbivore abundance and pollinator abundance.
Methods In a greenhouse experiment with 15 inbred lines from a M. sativa population located in central Chile (Mediterranean-type climate), we measured the effects of apical bud damage and reduced water availability on: number of ray florets per flower head, length of ray florets, flower head diameter, number of open flower heads per plant, flowering plant height and flowering time.
Important findings Apical damage and water shortage reduced phenotypic expression of floral traits attractive to pollinators via additive and non-additive effects. Plants in low water showed decreased height and had fewer and shorter ray florets, and fewer and smaller flower heads. Damaged plants showed delayed flowering, were less tall, and showed shorter ray florets and smaller flower heads. The number of ray florets was reduced by damage only in the low water treatment. Plant height, flowering time and number of flower heads showed among-family variation. These traits also showed genetic variation for plasticity to water availability. Ray floret length, flower head size and time to flowering showed genetic variation for plastic responses to apical damage. Plasticity in flowering time may allow M. sativa to adjust to the increased aridity foreseen for its habitat. Because genetic variation for plastic responses was detected, conditions are given for evolutionary responses to selective forces acting on plastic traits. We suggest that the evolution of adaptive floral plasticity in M. sativa in this ecological scenario (heterogeneous environments) would result from selective forces that include not only pollinators but also resource availability and herbivore damage.

Aims We test the hypothesis that invasive plant species at their range edges experience lower herbivory and allocate less to defense at the edge of an expanding range edge than from more central parts of their distribution, during secondary invasion in a new range. Invasive plants are often able to spread rapidly through new areas. The success of invasive species in new ranges is frequently attributed to enemy release in these new areas and associated evolutionary changes minimizing allocation to defense in favor of growth and reproduction. Enemy release could also explain rapid advances of invasive species upon arriving in new habitats. If invasive species accumulate enemies over time in a new location, then these species may experience a release from their enemies at expanding range fronts. Enemy release at these range fronts may accelerate range expansion.
Methods We used populations of four woody invasive species within the invaded range, and four native control species. We quantified leaf herbivory and leaf physical defense traits at both range central and range edge locations, over two 1-month sampling periods, sampled 7 months apart.
Important findings Herbivory at the range edge did not differ to the range center but patterns were not consistent across species. There was a trend for lower herbivory at the range edge for Lantana camara, which was reflected in lower leaf toughness. Overall, leaf toughness was greater at the range edge location across invasive and control species. Physical defenses were different among range locations in a few species, though most species show the same trend, suggesting higher herbivory pressures at the range edge location or differences may be due to climatic factors. Leaves of L. camara were significantly less tough at range edges, suggesting that some species can potentially escape their enemies at range edges. However, our results overall do not support the hypothesis that plants at the edge of their ranges experience reduced impact from their enemies.