Aims Mesic grasslands have a long evolutionary history of grazing by large herbivores and as a consequence, grassland species have numerous adaptations allowing them to respond favourably to grazing. Although empirical evidence has been equivocal, theory predicts that such adaptations combined with alterations in resources can lead to grazing-induced overcompensation in aboveground net primary production (ANPP; grazed ANPP> ungrazed ANPP) under certain conditions. We tested two specific predictions from theory. First, overcompensation is more likely to occur in annually burned grasslands because limiting nutrients that would be lost with frequent fires are recycled through grazers and stimulate ANPP. Second, overcompensation of biomass lost to grazers is more likely to occur in unburned sites where grazing has the greatest effect on increasing light availability through alterations in canopy structure.
Methods We tested these nutrient versus light-based predictions in grazed grasslands that had been annually burned or protected from fire for>20 years. We assessed responses in ANPP to grazing by large ungulates using both permanent and moveable grazing exclosures (252 exclosures from which biomass was harvested from 3192 quadrats) in a 2-year study. Study sites were located at the Konza Prairie Biological Station (KPBS) in North America and at Kruger National Park (KNP) in South Africa. At KPBS, sites were grazed by North American bison whereas in KNP sites were grazed either by a diverse suite of herbivores (e.g. blue wildebeest, Burchell's zebra, African buffalo) or by a single large ungulate (African buffalo).
Important findings We found no evidence for overcompensation in either burned or unburned sites, regardless of grazer type. Thus, there was no support for either mechanism leading to overcompensation. Instead, complete compensation of total biomass lost to grazers was the most common response characterizing grazing–ANPP relationships with, in some cases, undercompensation of grass ANPP being offset by increased ANPP of forbs likely due to competitive release. The capability of these very different grass-dominated systems to maintain ANPP while being grazed has important implications for energy flow, ecosystem function and the trophic dynamics of grasslands.

Aims Rising atmospheric CO₂ has been shown to increase aboveground net primary productivity (ANPP) in water-limited perennial grasslands, in part by reducing stomatal conductance and transpiration, thereby reducing depletion of soil moisture. However, the benefits of CO₂ enrichment for ANPP will vary with soil type and may be reduced if water limitation is low. Little is known about CO₂ effects on ANPP of Panicum virgatum, a perennial C₄ tallgrass and potential bioenergy crop. We hypothesized that if water limitation is minimized, (i) CO₂ enrichment would not increase P. virgatum ANPP because photosynthetic rates of this C₄ grass would not increase and because decreased transpiration at elevated CO₂ would provide little additional benefit in increased soil moisture and (ii) soil type will have little effect on P. virgatum CO₂ responses because of high overall soil moisture.
Methods Growth and leaf physiology of P. virgatum cv. 'Alamo' were studied as plants established for 4 years on silty clay and clay soils along a 250 to 500 μl l -1 gradient in atmospheric CO₂ located in central Texas, USA. Plants were watered to replace evapotranspiration, fertilized with NO 3 NH 4 and P 2 O 5 and clipped to standard height during mid-season.
Important findings ANPP increased through the third year of growth. Soil moisture (0–20 cm), ANPP, tiller numbers and leaf area index were 8–18% higher on the clay than on the silty clay soil. ANPP did not increase with CO₂ except in the planting year. However, biomass removed with clipping strongly increased with CO₂ in years 2 and 3, suggesting that CO₂ enrichment increased the early- to mid-season growth of establishing P. virgatum but not later regrowth or that of fully established plants. Furthermore, CO₂ enrichment differentially affected two components of ANPP in years 2 and 3, increasing tiller mass and reducing tiller numbers. This reallocation of resources in clipped P. virgatum suggested increased meristem limitation of productivity with CO₂ enrichment. CO₂ enrichment had little effect on photosynthesis but increasingly reduced stomatal conductance and transpiration as the plants established. As a result, water use efficiency became increasingly coupled to CO₂ as leaf area increased during establishment. These results suggest that for well-watered and clipped P. virgatum, ANPP differed between soil types, was not affected by CO₂ enrichment when fully established but interacted with clipping to alter allocation patterns during establishment. Soil type effects on ANPP-CO₂ responses will likely become more apparent when water is more limiting.

Aims Leaf traits of trees exposed to elevated [CO₂] in association with other environmental factors are poorly understood in tropical and subtropical regions. Our goal was to investigate the impacts of elevated [CO₂] and N fertilization on leaf traits in southern China.
Methods Four tree species, Schima superba Gardn. et Champ. (S. superba), Ormosia pinnata (Lour.) Merr (O. pinnata), Castanopsis hystrix AC. DC. (C. hystrix) and Acmena acuminatissima (Blume) Merr. et Perry (A. acuminatissima) were studied in a factorial combination of atmospheric [CO₂] (ambient at ~390 μmol mol ? 1 and elevated [CO₂] at ~700 μmol mol^-1) and N fertilization (ambient and ambient + 100 kg N ha^-1 yr^-1) in open-top chambers in southern China for 5 years. Leaf mass per unit leaf area (LMA), leaf nutrient concentration and photosynthesis (A sat) were measured.
Important findings Results indicated that leaf traits and photosynthesis were affected differently by elevated [CO₂] and N fertilization among species. Elevated [CO₂] decreased LMA in all species, while N fertilization did not affect LMA. Leaf mass-based N concentration (N M) was significantly greater in O. pinnata and C. hystrix grown in elevated [CO₂] but was lower in S. superba. Leaf mass-based P concentration (P M) was significantly greater in C. hystrix and A. acuminatissima exposed to elevated [CO₂] but was lower in S. superba. N fertilization significantly increased P M in O. pinnata but decreased P M in S. superba. Photosynthetic stimulation in O. pinnata, C. hystrix and A. acuminatissima was sustained after 5 years of CO₂ fumigation. N fertilization did not modify the effects of elevated [CO₂] on photosynthesis. Leaf traits (N M, N A, P M, P A) and light-saturated photosynthesis were decreased from the upper to lower canopy. Canopy position did not alter the responses of leaf traits and photosynthesis to elevated [CO₂]. Results suggest that photosynthetic stimulation by elevated [CO₂] in native species in subtropical regions may be sustained in the long term.

Aim Due to the important role of lianas in the functioning of forest ecosystem, knowledge of the factors that affect them are important in the management of forests. Currently, there are conflicting reports on the response of liana communities to disturbance, calling for more research in the area. The present study was carried out to investigate the response of liana diversity and structure to human disturbance within two major forests in the Penang National Park, Malaysia. The study also looked at the implication of the findings for conservation.
Methods A total of 15 40 × 40-m 2 (or 40-m × 40-m) plots each were randomly located across a range of habitats in a primary forest and disturbed secondary forest. Trees with diameter at breast height ≥10 cm were examined for lianas with diameter ≥2 cm. Both lianas and trees were enumerated and compared between the two forests. Diversity and structural variables of lianas were compared between the two forests using the t -test analysis. Tree abundance was also compared between the two forests with t -test, while linear regression analysis was run to determine the effects of tree abundance on liana abundance.
Important findings A total of 46 liana species belonging to 27 genera and 15 families were identified in the study. Human disturbance significantly reduced liana species richness and species diversity in the secondary forest. Liana abundance remained the same in both forests whereas liana basal area was significantly higher in the primary forest. Twiners and hook climbers were significantly more abundant in the primary and secondary forest, respectively. Large diameter lianas were more abundant in the primary forest compared with the secondary forest. The diameter distribution of most families in the primary forest followed the inverted J-shaped curve whereas only a few of the families in the secondary forest did so. Tree abundance was significantly higher in the primary forest. The abundance of lianas significantly depended on tree abundance in all the forests. The study has provided evidence of negative effects of human disturbance on liana diversity and structure that does not auger well for biodiversity in the forest. In view of the critical role of lianas in maintaining biodiversity in the forest ecosystem, lianas in the national park should be protected from further exploitation.

Aims Invasive species alter the composition and function of communities, threatening the conservation of important ecosystems worldwide. In savanna ecosystems, fire plays a key role by modifying biogeochemical cycles and shaping the composition and structure of vegetation communities. Although many studies have examined the long-term effects of fire frequency on grassland communities, few studies have examined the relationship between fire regime and woody species invasions.
Methods The Serengeti ecosystem is an ecologically and economically valuable natural resource in East Africa whose conservation is currently threatened by a variety of factors, including invasive species. We determined the abundance of Maerua subcordata, a noxious woody shrub, in three different regions of the Serengeti National Park, Tanzania. We then examined the influence of fire frequency, developed through the use of 7 years of Moderate Resolution Imaging Spectroradiometer (MODIS) imagery, on M. subcordata abundance.
Important findings In all regions, burned transects had significantly greater abundances of M. subcordata compared to unburned transects. Furthermore, the number of times each transect burned from 2000 to 2006 had a significant positive effect on the abundance of M. subcordata and the number of years since a transect last burned had a significant negative relationship with abundance. These results are particularly important as this species provides little forage value and is potentially toxic to wildlife and cattle. Additional studies are needed to determine the ecological consequences of increasing M. subcordata abundance, as the potential expansion of noxious shrubs into protected areas of important conservation status is of serious concern.

Aims Woody plant encroachments in arid and semiarid ecosystems are widely reported but the physiological mechanisms still need to be further revealed. In the current study, we aim to determine whether differences in leaf physiological traits help explain grassland susceptibility to woody plant encroachment and whether distinctive physiological adaptations allow some shrub species to invade grasslands.
Methods We compared physiological traits (photosynthesis, leaf water status, pigment compositions and leaf antioxidant capacities) of six species representing three functional groups: woody encroachers (Prosopis velutina, Larrea tridentata), woody non-encroachers (Acacia greggii, Lycium fremontii) and C4 grasses (Bouteloua curtipendula, Bouteloua barbata) which are naturally growing in a botanical garden in University of Arizona, USA.
Important findings We infer that P. velutina (encroacher) but not A. greggii or L. fremontii (non-encroachers) is encroaching in grasslands because the former species has higher water and light utilization efficiencies (instantaneous water use efficiency, instantaneous light use efficiency, and Fv/Fm). The extremely high carotenoid and total antioxidant capacity in its leaves appears to help the shrub L. tridentata (encroacher) survive high ambient oxidative damage caused by both drought and high light stresses in this grassland. The two C₄ grass species, B. curtipendula and B. barbata, grow well in the arid ecosystem but may be susceptible to disturbances.

Aims The process of facilitation, where a species increases the survival, growth, and fitness of another species, is becoming increasingly recognized as a critical factor in shaping the structure of plant communities. This process is particularly important in stressful environments. Yet few studies have attempted to incorporate positive interactions into community ecological theories such as the neutral theory of biodiversity. Here, we use an equalizing trade-off model as a foundation to study the potential impact of facilitation on species richness and community temporal turnover.
Methods Based on a spatially explicit birth–death trade-off model, we assume that the occurrence of facilitation is dependent on the presence of interspecific neighbours. We further propose that the realized birth rate for a given individual subject to facilitation is proportional to the number of interspecific neighbours within its neighbourhood. Thus, in our model, the individuals of rare species will benefit more from the existence of heterospecific individuals than common species.
Important findings As the facilitative coefficient increased, the species richness for simulated communities at the dynamically stochastic equilibrium was also increasing. Simulations also demonstrated that facilitation could increase the replacement of species through time: communities with facilitation become more dissimilar (i.e. have smaller Bray–Curtis similarity values) than communities without or with a lower degree of facilitation after the same time interval. Facilitation from interspecific neighbours on rare species increased their population sizes and consequently made them less prone to extinction, thus enhancing species richness. Meanwhile, in a saturated community, with the increase of species richness, mean population size of entire communities decreased, making species more prone to extinction on average, and thus increased the community temporal turnover. Our results suggest that future experimental work on the effect of facilitation on community-level properties should focus not just on species richness but also on other descriptors of community dynamics such as the temporal species turnover.

Aims The present study aims (i) to examine if recently reported interspecific shoot-level biomass allocational trade-offs, i.e. isometric trade-offs between leaf mass (LM) and stem mass (SM) and between leaf size and leaf number, hold intraspecifically and (ii) to explore whether those scaling relationships are independent of shoot type (i.e. long vs. short shoots).
Methods In order to address our questions, we used Fagus sylvatica saplings growing under a broad light range that were sampled in the Western Carpathians Mountains (Slovakia).
Important findings We found that: (i) intraspecific shoot-level biomass allocational trade-offs differ from those reported interspecifically and that (ii) long and short shoots differ in biomass allocation scaling coefficients. Allometric relationships with slopes statistically smaller than 1.0 or higher than^-1.0, were found between SM and LM and between mean leafing intensity and individual leaf mass, respectively, in long shoots. In contrast, isometric scaling was found in short shoots. This suggests that leaf mass in short shoots is unaffected by shoot stem mass, in contrast to long shoots. Short shoots also had a larger fraction of biomass allocated to leaves. Beech shoots, as has been observed in other shoot dimorphic species, are specialized, with short shoots specializing in carbon gain and long shoots in space acquisition. A greater shift in LM than in SM among species during speciation shifting from allometric intraspecific relationships to an isometric interspecific scaling relationship between those traits could explain the discrepancies between the outputs of the present intraspecific study and others similar studies. This study draws attention to the importance of considering shoot types in future studies dealing with allocation rules in species with dimorphic shoots.

Aims Human alterations of the environment are combining in unprecedented ways, making predictions of alterations to natural communities a difficult and pressing challenge. Estuarine systems have been subject to a high degree of modification, including increased nitrogen (N) inputs and altered salinity, factors important in shaping estuarine plant communities. As human populations increase and the climate changes, both N and salinity levels are likely to increase in these coastal marshes. Our objective was to evaluate the interactive effects of N and salinity on US West Coast salt marsh species; in particular, the performance of the dominant species Sarcocornia pacifica (pickleweed) alone and in mixed species assemblages. We expected increased salinity to favor S. pacifica but that N enrichment could help maintain greater species richness through use of N in salinity tolerance mechanisms.
Methods We crossed treatments of N (added or not) and salinity (salt added or not) in a field experiment at a salt marsh in the San Francisco Estuary, California, USA, in each of three habitats: (i) monotypic pickleweed on the marsh plain, (ii) monotypic pickleweed along channels and (iii) mixed assemblages along channels. In a greenhouse experiment, we crossed treatments of N (added or not) and salinity (at three levels to simulate brackish to saline conditions) in (i) pots of pickleweed only and (ii) the same species mix as in the field.
Important findings N addition doubled S. pacifica biomass and branching in both channel and marsh plain habitats regardless of salinity and greatly increased its dominance over Distichlis spicata and Jaumea carnosa in mixed assemblages along channels. In the greenhouse, S. pacifica biomass increased 6- to 10-fold with N addition over the range of salinities, while D. spicata and J. carnosa biomass increased with N addition only at lower salinity levels. Thus, while localized management could influence outcomes, expected overall increases in both N and salinity with human population growth and climate change are likely to enhance the production of S. pacifica in US West Coast marshes while reducing the diversity of mixed species assemblages. This decline in diversity may have implications for the resilience of marshes already subject to multiple stressors as the climate changes.