Plant co-existence mechanisms related to stress and disturbance intensities in sub-Mediterranean and sub-desertic grassland systems

Plant species co-existence may depend upon a complex chain of events involving competitive ability, complementarity and facilitation between plants and numerous interactions with herbivores, pathogens, soil features, fauna and micro-organisms. The co-existence of species in herbaceous vegetation is possible by the presence of factors that limit the expression of the dominance of plants. This limitation may operate through stress or disturbance or by a combination of the two, and its effect is usually to debilitate the potential dominants and to allow plants of smaller stature to regenerate and to co-exist with them. Species co-existence can reflect niche partitioning at several spatial and temporal scales. The functional strategies avoiding the temporal and/or spatial overlapping of the neighbouring individuals are key factors in the niche partitioning inside the plant community, so that species do not exclude each other by competition. Stress and disturbance are the two external factors affecting the vegetation. Their effects on the system change according their intensity. Stress includes factors which restrict the photosynthetic activity. In fact, plant biomass production in vegetation is subjected to a variety of environmental constraints, the most frequent of which are related to shortages and excesses in the supply of solar energy, water, and mineral nutrients. Disturbance includes mechanisms which limit the plant biomass by causing its partial or total destruction (grazing, mowing, forest cutting, soil erosion, and fire). Vegetation composition and species distribution may be influenced by negative processes (competition) and positive interactions (facilitation). Competition shows a maximal intensity in habitats with high productivity and low disturbance, while this phenomenon disappears under conditions of low productivity or intense disturbance. In order to obtain a more complete assessment of the role of these factors upon species density it is necessary to consider the rather different circumstances which arise when the intensities of stress and/or disturbance become severe. In harsh habitats plants are subjected to extreme stress or disturbance conditions, thus seedling establishment is promoted under the canopies of established vegetation. In this positive interaction, known as facilitation, some plants benefit from closely associated neighbours. The intensities of stress and/or disturbance experienced by herbaceous vegetation are sufficient, not only to eliminate potential dominants but also to produce local environments which are inhospitable to all, except a few specialised plants. Facilitation occurs widely but is more frequently documented in unproductive habitats and dominates under harsh conditions. Stress and disturbance involve functional responses by plants; thus, the analysis of plant functional traits (biological characteristics of plants responding to the dominant processes in an ecosystem) is a useful tool for the understanding of plant community assemblage. Grazing is a major disturbance which often causes extensive changes in plant richness and composition. However, pasture plants do not undergo herbivory passively, but react to defend themselves against grazers by means avoidance and tolerance strategies that enable them to survive and grow in grazed systems. Grazing avoidance involves mechanisms that reduce the probability and severity of grazing (mechanical and chemical defence or escape strategy), while grazing tolerance consists of mechanisms that promote growth following defoliation. The research aim was to assess how different types and intensity of disturbance affect species and functional composition of sub-Mediterranean and sub-desertic grassland systems, also in relation with environmental stress. Consequently, the specific research goals were to: i) identify changes in species and functional trait composition and in diversity of plant communities, under different disturbance types (grazing vs. mowing; mown vs. unmown meadows; grazed vs. ungrazed pastures; sheep grazing vs. horse grazing); ii) identify coenological and functional responses of grassland to intensity of stress and intensity and timing of disturbance, by analysing the plant functional traits, that are expression of competitive ability, facilitation, co-existence, avoidance and tolerance strategies; iii) identify plant-plant spatial interactions, i.e. whether species have species-specific spatial associations, especially in harsh environments, and understand how disturbance intensity affect species co-occurrence. Two study areas were considered: sub-Mediterranean grasslands of Umbria-Marches Apennine (central Italy) and the sub-desertic rangelands of southern Peruvian Andes (Peru). In the studied sub-Mediterranean grasslands the competition for light and soil resources may be extremely intense among species. In fact, competition plays a key role, especially in dense and more productive grasslands, where the co-existence of plants is mainly linked to the competitive exclusion. For this reason, in these plant communities the species co-existence is driven by a specific pattern of functional plant traits that ensures foliage expansion affording a competitive advantage. Furthermore, clonal growth forms and vegetative propagation modes that imply horizontal space occupation, can be interpreted as a strategy aimed at maximizing the species competitive ability when there is higher exploitation of soil resources, allowing individuals to explore the neighbouring areas and find unexploited soil niches. This helps to avoid competition for soil resources with the dominant species. Accordingly, the plant functional composition also affects the competitive ability in relation to the stress and the disturbance intensity. In terms of biodiversity conservation, the research outputs represent an advance in the understanding of the ecological processes involved at plant community level, as well as at landscape scale. Some plant traits may reflect selection by herbivores, while others are likely to be by-products of selection for other ecological functions. Moreover, several plant traits may have a dual role. The study findings allow to highlight a general scheme, in which the stress intensity filters the pool of traits at landscape scale, while the disturbance intensity leads to the distribution, occurrence and abundance of single traits at plant community level. Stress intensity (drought stress) selects the trait composition of the plant community at a landscape scale. Thus, traits and associated plant species are distributed at landscape scale depending on stress intensity. In xeric grassland, stress tolerance and avoidance strategies by hairs and leaf texture, determining low evapotranspiration and low palatability, are widespread. Therophyte and chamaephyte life forms are fostered. In semimesic condition either avoidance mechanisms (rosette form, chemical substances, spines and prostrate form) or tolerance strategies (re-growth capacity) occur, as well as late flowering and tall species (upright forbs, tall tussock grasses). More productive environments develop numerous functional types and a higher floristic richness. Disturbance intensity (number of herbivores), type (mowing or grazing, and the different livestock type) and timing act as driving forces in promoting or suppressing the plant functional traits expression. The pool of traits is filtered in different ways by the system. High intensity of disturbance favours avoidance strategies (i.e. prostrate form, rosette forbs, hairs) and vegetative reproduction (clonal ability). The intermediate disturbance condition leads to the largest pool of traits that allows maximum floristic richness. The co-existence of species is promoted by the co-existence of avoidance and tolerance strategies, due to the possibility to use the maximum number of spatial (micro-scale) and temporal niches (change in sward structure during the growing season). In low intensity of disturbance and abandonment conditions, tolerance strategies and dominant species are promoted; on the contrary, the low statured (rosette and prostrate form), accidental and subordinate species are disadvantaged. Large herbivores like horses cause the increase of short grasses, sedges, rosette forbs and annuals (growth forms with poor root systems). These plants do not ensure the maintenance of soil on steep slopes. Upright forbs, dominant unpalatable tall grasses (Brachypodium rupestre) and chamaephyte species are promoted by selective defoliation of small herbivores like sheep. Facilitative interactions between palatable and unpalatable species were observed. Grazing in springtime negatively affects the early spring flowering species of mid/tall dimensions and relevant species such as orchids. This is an issue for biodiversity conservation. The research findings highlight the strong importance of plant–plant spatial interactions in the Peruvian dry Puna. In this harsh environment many species need some kind of facilitative interaction with nurses (mainly with tall grasses, shrubs or cushion plants). Tall grasses (mainly Festuca orthophylla) are the most important nurse species because they have the highest number of spatially associated plants. Thus, the management of this species should be viewed as a key factor for dry Puna biodiversity conservation. In fact, as camelids prefer the fresh, regrown leaves of F. orthophylla that resprout after burning throughout the Peruvian dry Puna, there is the widespread practice of burning these plant communities to renew forage for livestock. Subsequent to burning, shelter and regenerative niches are probably few and small for some years. Because of this, species with strong spatial relationship with F. orthophylla could be threatened with local extinction, especially in conditions of overgrazing. The effects of combined overgrazing and fire lead to the decrease of tall species F. orthophylla and to an increase of the dwarf and spiny shrub Tetraglochin cristatum. The reduction of nurse cover value leads to the decrease of facilitative interactions and consequently to a reduction of species richness. Thus, inappropriate land use practices (e.g. excessive grazing and uncontrolled fire) are the fundamental causes of land degradation. The abundance of tall grasses and shrubs counters soil erosion by wind, and reduction of their cover value could lead to increased soil loss. This could contribute to desertification more than climatic change, or contribute to a cumulative process that amplifies the impact of climatic change. Key words: Avoidance and tolerance strategies, Competition, Facilitation, Plant functional traits, Plant-plant spatial interactions