Aims Eurasian watermilfoil (Myriophyllum spicatum L.) is a common invasive plant in American lakes and has many negative impacts on invaded ecosystems. Drastic decline of this plant at the northern end of Cayuga Lake in the New York State has occurred since the 1980s, with a much smaller magnitude of decline of Eurasian watermilfoil at the southern end (Johnson et al. 2000) During the same period, increases in the abundance of native plants, particularly water stargrass (Heteranthera dubia (Jacq.) MacMill.), have been observed (Johnson et al. (1998)) We aimed to investigate the mechanisms responsible for the decline of Eurasian watermilfoil and evaluate the responses of co-occurring plants at the two ends of Cayuga Lake over time. We hypothesized that plant interactions might have contributed to the drastic decline of Eurasian watermilfoil, particularly allelopathy by native water stargrass.
Methods A lake survey was conducted to assess distribution and abundance of plant communities at the northern end and the southern end of Cayuga Lake. Additionally, two sets of greenhouse experiments were conducted to investigate the interactions between invasive Eurasian watermilfoil and native water stargrass. A competition experiment evaluated intra- versus inter-specific competition among plants grown together; an allelopathy experiment examined the responses of plants to each other's extracts.
Important findings The lake survey showed that water stargrass was extremely abundant at the northern end, whereas Eurasian watermilfoil was sparse at the northern end but one of the most common species at the southern end. The survey also revealed that water stargrass was more abundant than Eurasian watermilfoil at sites where the two species coexisted in the lake. Results from greenhouse experiments revealed no effects of Eurasian watermilfoil on water stargrass growth. However, Eurasian watermilfoil biomass was reduced by 46% when treated with high concentration of water stargrass extracts. This is likely due to osmotic effects rather than allelopathic effects of water stargrass. We proposed several possible reasons for the drastic decline of Eurasian watermilfoil and the increase in water stargrass abundance at the northern end of Cayuga Lake, including space competition, nutrients, substrates, wind exposure and water clarity in addition to insect herbivory and mechanic harvesting.

Aims and Methods Disturbance is supposed to play an important role for biodiversity and ecosystem stability as described by the intermediate disturbance hypothesis (IDH), which predicts highest species richness at intermediate levels of disturbances. In this study, we tested the effects of artificial soil disturbances on diversity of annual and perennial vascular plants and bryophytes in a field experiment in 86 agricultural grasslands differing in land use in two regions of Germany. On each grassland, we implemented four treatments: three treatments differing in application time of soil disturbances and one control. One year after experimental disturbance, we recorded vegetation and measured biomass productivity and bare ground. We analysed the disturbance response taking effects of region and land-use-accompanied disturbance regimes into account.
Important findings Region and land-use type strongly determined plant species richness. Experimental disturbances had small positive effects on the species richness of annuals, but none on perennials or bryophytes. Bare ground was positively related to species richness of bryophytes. However, exceeding the creation of 12% bare ground further disturbance had a detrimental effect on bryophyte species richness, which corresponds to the IDH. As biomass productivity was unaffected by disturbance our results indicate that the disturbance effect on species richness of annuals was not due to decreased overall productivity, but rather due to short-term lowered inter- and intraspecific competition at the newly created microsites. Generally, our results highlight the importance of soil disturbances for species richness of annual plants and bryophytes in agricultural grasslands. However, most grasslands were disturbed naturally or by land-use practices and our additional experimental soil disturbances only had a small short-term effect. Overall, total plant diversity in grasslands seemed to be more limited by the availability of propagules rather than by suitable microsites for germination. Thus, nature conservation efforts to increase grassland diversity should focus on overcoming propagule limitation, for instance by additional sowing of seeds, while the creation of additional open patches by disturbance might only be appropriate where natural disturbances are scarce.

Aims Studies that investigate the space-filling heterogeneity of biological structures in plant communities remain scarce. The main objective of this study was to evaluate the relationship between newly developed photographic measures of structural heterogeneity in digital images and plant species composition in the context of a long-term grassland experiment.
Methods We tested a close-range photographic protocol using measures of structural heterogeneity in gray-tone images, namely mean information gain (MIG) and spatial anisotropy, to assess differences in the compositional (species richness) and functional characteristics (plant height and flowering) of 78 managed grassland communities. We also implemented a random placement model of community assembly to explore the links between our measures of structural complexity and the geometric pattern of plant communities.
Important findings MIG and spatial anisotropy correlated with the growth and species richness of grassland communities. Simulations showed that structural heterogeneity in gray-tone digital images is a function of the size distribution and orientation pattern of plant modules. This easy, fast and non-destructive methodological approach could eventually serve to monitor the diversity and integrity of various ecosystems at different resolutions across space and time.

Aims Elevated nitrogen (N) deposition in tropical regions may accelerate ecosystem phosphorus (P) limitation. However, it is not explicitly addressed that how changes in soil N and P availability affect foliar nutrients and photosynthesis of plants in tropical forests. In this study, we examined the effects of N and P additions on foliar nutrients and net photosynthesis of two dominant understory species, Randia canthioides (R. canthioides) and Cryptocarya concinna (C. concinna) in an N-saturated old-growth tropical forest (>400-year-old) in southern China.
Methods A full factorial NP addition experiment (2×2) was established in 2007 and continued through August 2010. Four treatments, including control, N addition (150kg N ha^-1 year^-1), P addition (150kg P ha^-1 year^-1) and NP addition (150kg N ha^-1 year^-1 plus 150kg P ha^-1 year^-1) were set up in this experiment. Photosynthetic traits (maximum photosynthetic CO₂ assimilation (A max), stomatal conductance (g s), leaf transpiration (E), light saturating point, concentrations of chlorophyll a/b and foliar nutrients (N and P) of the two species were measured with standard methods.
Important findings Three years of N addition had no significant effects on any measured photosynthetic parameter of either species. However, N addition significantly elevated foliar N and P concentrations of one species (R. canthioides), resulting in lower photosynthetic nitrogen use efficiency (PNUE). N treatments decreased foliar P concentration of the other (C. concinna), resulting in increased photosynthetic phosphorus use efficiency, which was potentially related to N-induced P shortage. In contrast, positive effects of P treatments on g s of R. canthioides, A max and chlorophyll a+b of C. concinna were observed. P treatments also elevated foliar P and PNUE of both species, implying P induced more efficient use of N. Our results suggested a more important role of P than N on influencing photosynthetic traits of these two understory species. Alleviation of P shortage through P addition may enhance photosynthetic performances of some understory species in N-rich tropical forests.

Aims Some shade-tolerant understory tree species such as mountain maple (Acer spicatum L.) exhibit light-foraging growth habits. Changes in environmental conditions, such as the rise of carbon dioxide concentration ([CO₂]) in the atmosphere and soil warming, may affect the performance of these species under different light environments. We investigated how elevated [CO₂] and soil warming influence the growth and biomass responses of mountain maple seedlings to light availability.
Methods The treatments were two levels of light (100% and 30% of the ambient light in the greenhouse), two [CO₂] (392 μmol mol^-1 (ambient) and 784 μmol mol^-1 (elevated)) and two soil temperatures (T soil) (17 and 22°C). After one growing season, we measured seedling height, root collar diameter, leaf biomass, stem biomass and root biomass.
Important findings We found that under the ambient [CO₂], the high-light level increased seedlings height by 70% and 56% at the low T soil and high T soil, respectively. Under the elevated [CO₂], however, the high-light level increased seedling height by 52% and 13% at the low T soil and high T soil, respectively. The responses of biomasses to light generally followed the response patterns of height growth under both [CO₂] and T soil and the magnitude of biomass response to light was the lowest under the elevated [CO₂] and warmer T soil. The results suggest that the elevated [CO₂] and warmer T soil under the projected future climate may have negative impact on the colonization of open sites and forest canopy gaps by mountain maple.

Aims Soil moisture content (SMC) influences establishment, survival and development of plant species and is considered as the most important limiting factor in tropical dry forest (TDF). In this study we attempt to establish the relationship between leaf attributes and of tree saplings in TDF and address the following questions: (i) how are the functional attributes of dominant tree saplings of TDF affected by seasonal changes in SMC at different habitats?, (ii) what is the relationship of functional attributes with each other?, (iii) how are the functional attributes and their plasticity affected by habitat conditions? and (iv) can the functional attributes in single or in combination predict the growth rate of tree saplings of TDF? The study was conducted on four sites (Hathinala, Gaighat, Harnakachar and Ranitali, listed in order of decreasing SMC) within the tropical dry deciduous forest in northern India.
Methods We analysed eight leaf attributes, specific leaf area (SLA); leaf dry matter content (LDMC); leaf nitrogen concentration (leaf N); leaf phosphorus concentration (leaf P); chlorophyll concentration (Chl); mass-based photosynthetic rate (A mass); mass-based stomatal conductance (Gs mass); intrinsic water use efficiency (WUEi) and three growth attributes, relative diameter increment (RDI); relative height increment (RHI); relative growth rate (RGR) of the 10 dominant tree saplings (viz., Acacia catechu, Anogeissus latifolia, Boswellia serrata, Buchanania lanzan, Diospyros melanoxylon, Hardwickia binata, Lagerstroemia parviflora, Lannea coromandelica, Shorea robusta and Terminalia tomentosa) of a TDF and observed the effects of site, season and species for a period of 2 years. Saplings were selected in gradients of deciduousness. Step-wise multiple regression was performed to predict RDI, RHI and RGR from mean values of SMC and leaf attributes.
Important findings All the 11 attributes were interrelated and differed significantly among the 10 saplings. Species response varied across sites and seasons. Across the SMC gradient, the attributes showed variable plasticity that differed across species. Among the 10 saplings, the highly deciduous Boswellia serrata showed the maximum plasticity in seven functional attributes. According to the step-wise multiple regressions, 65% variability in RDI and 67% variability in RGR were due to Gs mass, and for RHI, 61% variability was due to A mass. SMC and the other attributes, viz ., SLA, Chl, WUEi and LDMC in combination could contribute only for ~2–6% of the variability in RDI, RHI and RGR, which indicates that other traits/factors, not accounted in this study are also important in modulating the growth of tree saplings in TDFs. In conclusion, growth of the tree saplings in the tropical dry environment is determined by soil moisture, whereas the response of saplings of different tree species is modulated by alterations in key functional attributes such as SLA, Chl, WUEi and LDMC.

Aims An open-field warming experiment enables us to test the effects of projected temperature increase on change in plant phenology with fewer confounding factors and to study phenological response to temperature ranges beyond natural variability. This study aims to (i) examine the effect of temperature increase on leaf unfolding and senescence of oriental oak (Quercus variabilis Blume) under experimental warming and (ii) measure temperature-related parameters used in estimating phenological response to temperature elevation.
Methods Using an open-field warming system with infrared heaters, we increased the air temperature by ~3°C in the warmed plots compared with that of the control plots consistently for 2 years. Leaf unfolding and senescence dates of Q. variabilis seedlings were recorded and temperature-related phenological parameters were analysed.
Important findings The timing of leaf unfolding was advanced by 3–8 days (1.1–3.0 days/°C) and the date of leaf senescence was delayed by 14–19 days (5.0–7.3 days/°C) under elevated air temperatures. However, the cumulative degree days (CDD) of leaf unfolding were not significantly changed by experimental warming, which suggest the applicability of a constant CDD value to estimate the change in spring leaf phenology under 3°C warming. Consistent ranges of advancement and temperature sensitivity in spring phenology and delayed autumn phenology and proposed temperature parameters from this study might be applied to predict future phenological change.

Aims Recent work has identified a worldwide 'economics' spectrum of correlated leaf traits that mainly reflects the compromises between maximizing leaf longevity and short-term productivity. However, during the early stages of tree growth different species tend to exhibit a common strategy, because competition for soil water and nutrients forces the maximization of short-term productivity owing to the need for rapid growth during the most vulnerable part of the tree's life cycle. Accordingly, our aim here was to compare the variations that occur during ontogeny in the different leaf traits (morphology and leaf chemical composition) of several coexisting Mediterranean woody species differing in their leaf life spans and to test our hypothesis that tree species with a long leaf life span should exhibit larger shifts in leaf characteristics along ontogeny.
Methods Six Mediterranean tree species differing in leaf life span, selected from three plots located in central-western Spain, were studied during three growth stages: seedlings, juveniles and mature trees. Leaf life span, leaf morphology (leaf area, dry weight, thickness and mass per unit area) and chemical composition (N and fibre concentrations) were measured in all six species. The magnitude of the ontogenetic changes in the different traits was estimated and related to the mean leaf longevity of the different species.
Important findings Along ontogeny, strong changes were observed in all variables analysed. The early growth stages showed lower leaf thickness, leaf thickness and mass per unit area and N, cellulose and hemicellulose concentrations than mature trees, but a higher lignin content. However, these changes were especially marked in species with a longer leaf life span at maturity. Interspecific differences in leaf life span, leaf morphology and chemical composition were stronger at the mature stage than at the seedling stage. We conclude that greater plasticity and more intense strategy shifts along ontogeny are necessarily associated with long leaf life span. Our results thus provide a new aspect that should be incorporated into the analysis of the costs and benefits associated with the different strategies related to leaf persistence displayed by the different species. Accordingly, the intensity of the alterations in leaf traits among different growth stages should be added to the suite of traits that change along the leaf economics spectrum.