Summary1. Plant litter is a key component in terrestrial ecosystems. It plays a major role in nutrient cycles and community organization. Land use and climate change may change the accumulation of litter in herbaceous ecosystems and affect plant community dynamics. Additionally, the transfer of seeds containing plant material (i.e. litter) is a widespread technique in grassland restoration. 2. Ecosystem responses to litter represent the outcome of interactions, whose sign and strength will depend on many variables (e.g. litter amount, seed size). A previous meta-analysis (from 1999) reported that litter had an overall negative effect on seed germination and seedling establishment in different ecosystems. However, recent studies indicated that this might not be the case in grassland ecosystems. 3. We used 914 data from 46 independent studies to analyse the effects of litter on seedling (i) emergence, (ii) survival and (iii) biomass, employing meta-analytical techniques. Each data set was stratified according to methodology, grassland type, irrigation conditions, litter amount and seed size. 4. We found an overall neutral effect of litter presence on seedling emergence and survival and a positive effect on seedling biomass. However, whereas for field experiments the response remained neutral, it was positive for common garden studies. In glasshouse experiments, litter effects were negative for emergence and positive for biomass. 5. Litter may have a positive effect on seedling recruitment in dry grasslands or under water-limited conditions, or in the presence of low to medium litter amounts (< 500 g m À2 ). However, high litter amounts (> 500 g m À2 ) will inhibit seedling recruitment. Large seeds showed a more positive response to litter presence with respect to seedling emergence and survival, but not concerning biomass. 6. Synthesis. Under dry conditions (e.g. dry grasslands or dry periods) or with low to medium litter amounts, litter presence has a positive effect on seedling establishment. However, climate and land use change may promote litter accumulation and reduce seedling establishment, affecting grasslands composition and ecosystem functions.

No abstract

Non-native plant species may contain allelopathic substances that might help to out-compete native vegetation. These allelochemicals may be released from live or dead plant tissues and be accumulated in the soil. We tested whether non-native species leaf litter and their leachates reduced seedling establishment and growth of native species. We subjected seeds of six native species to the effect of litter leachates of three of the most important invasive plants in Europe and to mannitol solutions with similar osmotic potential in germination chamber experiments. Additionally, we measured the effect of the same litter on emergence and growth of the native species in an outdoor pot experiment. Litter leachates delayed and reduced germination and affected initial root growth of all native species. The effects of leachates were significantly higher than those of mannitol, indicating the action of toxic, most probably allelochemical substances. Emergence of seedlings in pots was also reduced, but total biomass per pot was not affected and biomass per seedling increased. Allelochemicals may affect germination and early stages of seedling recruitment. However, these negative effects seem to cease shortly after germination, when other mechanisms such as competition may be more important. Consequently, litter-borne allelochemicals are unlikely to drive the invasion of the studied non-native species, but they may contribute to maintain mono-dominant stands reinforcing invasion success.

Living plant neighbours, but also their dead aboveground remains (i.e. litter), may individually exert negative or positive effects on plant recruitment. Although living plants and litter co-occur in most ecosystems, few studies have addressed their combined effects, and conclusions are ambivalent. Therefore, we examined the response in terms of seedling emergence and growth of herbaceous grassland and forest species to different litter types and amounts and the presence of competitors. We conducted a pot experiment testing the effects of litter type (grass, oak), litter amount (low, medium, high) and interspecific competition (presence or absence of four Festuca arundinacea individuals) on seedling emergence and biomass of four congeneric pairs of hemicryptophytes from two habitat types (woodland, grassland). Interactions between litter and competition were weak. Litter presence increased competitor biomass. It also had positive effects on seedling emergence at low litter amounts and negative effects at high litter amounts, while competition had no effect on seedling emergence. Seedling biomass was negatively affected by the presence of competitors, and this effect was stronger in combination with high amounts of litter. Litter affected seedling emergence while competition determined the biomass of the emerged individuals, both affecting early stages of seedling recruitment. High litter accumulation also reduced seedling biomass, but this effect seemed to be additive to competitor effects. This suggests that live and dead plant mass can affect species recruitment in natural systems, but the mechanisms by which they operate and their timing differ.

Small scale disturbances could act as patches that provide sites for the colonization of competitively inferior species, promoting the establishment of non-native species in some cases. We analyzed the vegetation associated with feral horse dung piles in montane pampas grasslands in Mid-East Argentina and described the changes following their abandonment, evaluating whether dung piles act as invasion windows, allowing the entrance of alien plant species. We estimated the portion of the study area directly covered by horse manure and dung height was used to estimate the time elapsed after the abandonment of each pile. Vegetation replacement on dung piles of different ages was assessed and compared with grassland controls using discriminant analysis. We used regression analysis to look for changes in vegetation cover, species richness, species diversity and evenness in response to height (age) of the dung piles, and principal component analyses (PCA) to identify groups of plants associated with different successional stages. We compared cover of alien plant species on dung piles with grassland controls using one-way ANOVA. On average, 2.5% of the study area was covered by horse dung. Total vegetation cover, species richness, diversity and evenness increased after the piles were abandoned. Characteristic plant groups were associated with initial, middle and last phases of the studied succession. Vegetation on the dung piles significantly differed from that in grassland controls and two species were consistently associated with dung piles: the invasive Red Star Thistle, Centaurea calcitrapa, and a native grazing-intolerant grass, Nassella clarazii. Non-native species cover was also higher in dung piles than in control plots. Dung piles cover a significant portion of grassland area in our study site, produce significant changes in the vegetation and are associated with some invasive alien plants that could eventually colonize more pristine areas in the vicinity. On the other hand, they might represent refuges for palatable species, since horses seem to avoid them for grazing.

No abstract

No abstract

Biological soil crusts are widely distributed in arid and semiarid regions. They have an important ecological role, especially by modifying physical and chemical properties of soils. Biological crusts may also modify seed germination and seedling establishment. The effects vary widely according to the type of crust and the vascular plant species. The objective of this study was to determine the effect of moss-dominated biological soil crusts on the emergence, biomass and survival of Poa ligularis Nees ex Steud. under different irrigation regimes. We collected seeds of P. ligularis and biological soil crusts composed of two species of mosses: Syntrichia princeps (De Not.) Mitt and Ceratodon purpureus (Hedw.) Brid. from an area in the Monte of Argentina. The result showed that seedling emergence of P. ligularis was higher in treatments with bare soil than in soil covered by crusts, and also in those with watering to field capacity. Mean emergence time was higher in treatments with bare soil and watering to field capacity. Seedling biomass also showed significant differences between treatments. These results suggest that biological soil crusts dominated by mosses do not promote P. ligularis emergence, although they would not affect its survival.