Mediterranean grasslands stand out as biodiversity hotspots due to their inclusion of diverse habitats that foster a remarkable range of biological diversity, characterized by inter-annual variability of floristic composition. This study aims to assess the inter-annual dynamics of the floristic composition in a grassland within a typical silvopastoral system in the Mediterranean basin, within the framework of rainfall variability. The study was performed on a farm representative of Mediterranean silvopastoral systems in Central-Western Sardinia (Italy). Vegetation surveys started in the spring of 2018 and concluded in the spring of 2022. They were conducted annually in the spring (S) and winter (W) seasons, using the point quadrat method along eight permanent transects. The dynamics of floristic composition were compared within and between years in terms of variability of specific percentage contribution (CSP) of each plant species. The results highlighted substantial differences in the composition of plant assemblages between the two seasons (W vs S) and between surveys within each season for all a posteriori comparisons. Anthemis arvensis, Lolium rigidum, Festuca ligustica and Medicago polymorpha were the main discriminating plant species between seasons. Considering the discriminant species between the four winter surveys, we found a positive effect of August rain on Cynodon dactylon CSP and a negative one on Trifolium subterraneum CSP. Considering the spring surveys, we found a positive effect of January rain on L. rigidum and a negative effect on Plantago lanceolata. The rain affected the inter-annual dynamics of floristic composition in the Mediterranean grasslands. Other factors, such as temperature, remain to be investigated. Moreover, a more extended data series may allow us to strengthen our results.

Climate change, false breaks, inter-annual variability, Mediterranean grasslands, seed bank, vegetation dynamics

Mediterranean grasslands stand out as biodiversity hotspots due to their inclusion of diverse habitats that foster a remarkable range of biological diversity, including many plant and animal species (Gigante et al. 2018; Giallonardo et al. 2019). They are secondary formations and holds cultural significance owing to their rich and ancient human history (Catorci et al. 2021). They provide several ecosystem services, including nutrient cycling, carbon sequestration, and water cycle regulation (Ribeiro et al. 2014; Seddaiu et al. 2018; Grenke et al. 2022) and agricultural goods (Unger and Jogen 2014). However, they are facing significant challenges, including intensive grazing management, marginal grassland abandonment (Klimek et al. 2007), and climate change (Dibari et al. 2021). These factors have led to declining surface area and grassland quality, presenting a severe conservation problem (Klimek et al. 2007).

The floristic composition and vegetation structure in grasslands result from complex interactions involving various biological and environmental factors (Puerto and Rico 1992). Livestock grazing is a crucial factor that affects vegetation structure in grasslands. The intensity and timing of grazing can significantly impact plant diversity and abundance (Ribeiro et al. 2014).

Inter-annual variability of floristic composition is an intrinsic feature of Mediterranean grasslands. Understanding this variability is crucial for the sustainable management of pastures and ensuring they provide nutritious resources for livestock effectively. The most prominent climatic factors shaping grasslands are the presence of intense summer droughts, unpredictable autumn rains and significant intra- and inter-annual fluctuations in precipitation (Fernández-Moya et al. 2011), particularly during the spring and summer months (Catorci et al. 2021). These conditions result in substantial differences in productivity, species composition, and overall diversity of these ecosystems from one year to the next (Carmona et al. 2015). The timing and variability of rainfall within a year influence the growth and performance of plant species (Suttle et al. 2007). Annual species, which evolved specific adaptations to survive under such conditions, dominate these communities. These species spend the summer months in seed form, effectively avoiding the harsh conditions of high temperatures and water scarcity during this period. Autumn is a critical period for the renewal of plant life in these ecosystems: the first rains initiate germination and regrowth of many species.

Some models suggest that regions experiencing a significant increase in temperature accompanied by a simultaneous decrease in precipitation are particularly vulnerable to biodiversity loss. In such areas, there is a likelihood of an increase in less palatable drought-resistant species (Tardella et al. 2016), and drought is considered the main factor controlling forage quality and palatability (Dibari et al. 2021). Additionally, research has demonstrated that grasslands with a high level of biodiversity are less susceptible to the adverse effects of drought events, showing a more rapid recovery following periods of drought. Indeed, biodiversity plays a crucial role in enhancing the resilience of plant communities to extreme weather events and contributes to the overall stability of ecosystems (Tardella et al. 2016).

Climate conditions in the Mediterranean basin have been experiencing a trend toward increased aridity in recent decades (Kafle and Bruins 2009). This shift towards drier conditions is expected to persist, with projections indicating a potential 20% decrease in total annual precipitation by 2050. Furthermore, inter-annual variation in rainfall will increase, leading to years of exceptionally wet conditions alternating with years of drought in the Mediterranean region. These fluctuations in precipitation can pose significant challenges for various sectors, including agriculture, water supply, and the stability of ecosystems. As a result, it is becoming increasingly crucial to develop adaptation strategies and comprehensive plans to address the impacts of these changing climatic conditions (Golodets et al. 2013).

This study aims to assess the inter-annual dynamics of the floristic composition in a grassland within a typical silvopastoral system in the Mediterranean basin within the framework of rainfall variability.

Surveys were carried out over five years twice each year: the first in winter, following the annual vegetative growth before the winter vegetative dormancy period, and the second in spring, at the peak of the vegetation season. This seasonal sampling approach allows for comprehensively assessing inter and intra-annual variability in floristic composition, capturing vegetation dynamics at two critical points in the year and providing valuable data for understanding fluctuations in plant communities within the Mediterranean silvopastoral system.

The pastures in the study area exhibited typical Mediterranean pasture characteristics. They were indeed very species-rich pastures, as already observed in similar areas (Tárrega et al. 2009; Farris et al. 2010; Bagella et al. 2013), which is related to the traditional land use (Bagella et al. 2016). The dominance of Therophytes is another aspect typical of Mediterranean grasslands (Pitt and Heady 1978; Díaz-Villa et al. 2003; Tárrega et al. 2009; Fernández-Moya et al. 2011) because this life form is particularly well suited to disturbance tolerance (Grime 1977) and soil disturbance by trampling favoured seedlings recruitment (Noy-Meir et al. 1989). Fabaceae and Poaceae played a fundamental role in these grasslands. Fabaceae, including some excellent forage species such as T. subterraneum and M. polymorpha, ensure the supply of proteins to the grazing animal during the entire grazing season, improve soil fertility with natural N fixation (Lucas et al. 2010) and support C cycling and the productivity and persistence of these secondary grasslands (Bagella et al. 2020). Poaceae support the forage production during the coldest months, the excellent forage species L. rigidum with the contribution of A. barbata and F. ligustica. Moreover, they contribute to limiting soil erosion (Bagella et al. 2020).

As expected in the two seasons under comparison, the plant assemblage composition was different with a higher variability in winter than in spring.

The spring vegetative cover was generally characterized by higher floral diversity and a more significant presence of forage grasses and legumes compared to winter. The bulk of primary production and biodiversity occurs in this season (Moreno and Pulido 2009), extending through late spring and early summer, during which plants undergo seed formation and eventual senescence. Specifically, during the spring season, L. rigidum, M. polymorpha, and F. ligustica, with an average coverage percentage close to 10% for L. rigidum and slightly lower for M. polymorpha and F. ligustica, represent a significant portion of the vegetation cover. L. rigidum and M. polymorpha are considered excellent and good forage plants, respectively, and F. ligustica is considered fair (Bagella et al. 2013). Therefore, the presence and substantial coverage of these species should constitute the primary support for pasture quality during the spring season.

On the other hand, the winter vegetation cover was dominated by A. arvensis, which is not consumed by grazing animals (Bagella et al. 2013). However, even in this season, though in limited quantities, other plant species are available.

Plants grow in response to the initial autumn rain events (Pitt and Heady 1978). In the case of annual species, the rainfall that induces seed germination is termed the break of season. If this event occurs too soon, it can induce germination of pasture species that do not survive and are termed false breaks (Chapman and Asseng 2001; Turner et al. 2001). In our study, we observed an effect of the August rainfall on the development of two species that most characterize the floristic composition during the winter season, which is more related to the rainfall in this month than in the following months. However, the two species on which these effects are most evident, C. dactylon and T. subterraneum, show an opposite response, positive in the former case and negative in the latter, which should be related to their different adaptation strategies. C. dactylon is a perennial stolonifera C4 plant which grows mainly in summer under maximal light availability and high temperatures. The C4 strategy in the Mediterranean grassland species is unusual, typically dominated by annual C3 grasses and legumes that are active during the wet period of the year (Aires et al. 2008). Using this adaptation C. dactylon avoids competition for light, water and nutrients with other annual species whose seeds germinate later. The observed positive effect of August rainfall on the CSP of this species in the winter could be explained by the efficient use of water by a species with photosynthetic efficiency at high temperatures and radiation intensities (Galiano 1985). T. subterraneum is an annual early-season species that starts growing slowly in autumn or winter if the weather is mild, grows rapidly in the spring and sets seed by the end of the spring (Papanastasis 1981). It was instead negatively affected by break season rainfall (Turner et al. 2001). The negative effect could probably be related to a false break due to a combination of a germination-inducing rainfall event followed by a period of drought determining a widespread death of seedlings, as already observed in Australian grasslands (Chappman and Asseng 2001). In the study area, competition for water with C. dactylon, which has higher evapotranspiration enhancing soil water depletion, exacerbates the situation (Aires et al. 2008).

Concerning the floristic composition in spring, we found a direct effect of the January rainfall on the cover of two species: P. lanceolata and L. rigidum, which CSP was positively and negatively affected, respectively. P. lanceolata is a perennial species which produces rosettes and reproduces by seed production and clonal propagation (Cavers et al. 1980). It consistently improved under high temperatures and drought conditions (Rasmussen et al. 2020). L. rigidum is an annual species whose seeds germinate during autumn and winter (Steadman et al. 2003). Seed dormancy prevents false breaks before this time due to sporadic summer rains but allows germination when rainfall is sufficient to sustain plant growth and development (Steadman et al. 2004). This should be why abundant rains in January favoured seed germination and seedling growth. The less demanding P. lanceolata was favoured in the year with less rainy January. These observations confirm that in an environment where water is scarce, the species abundance is regulated by competition-driven water depletion (Tsialtas et al. 2001).

Rainfall is a crucial determinant of production and composition in arid and semi-arid systems (Dudney et al. 2017), and extreme events, such as extended autumn droughts, are forecasted to increase in Mediterranean regions under climate change scenarios (Nogueira et al. 2017). Such changes, particularly those responsible for false breaks, may also adversely affect the soil seed bank, which is strategic for conserving and regenerating Mediterranean grasslands.

In this paper, we explored the inter-annual dynamics of floristic composition in the Mediterranean grasslands, considering the rain as a key factor for this dynamic. Other factors, such as temperature, remain to be investigated. Moreover, a more extended data series may allow us to strengthen our results.

This paper was funded by the Fondazione di Sardegna (Annualità 2022-2023): “Verso una ricerca floristica di nuova generazione: dalla gap analysis alla valutazione della biodiversità con il supporto della Citizen Science” (Scientific Coordinator: Simonetta Bagella).

This research was carried out within the "Life Regenerate – Revitalizing multifunctional Mediterranean agrosilvopastoral systems using dynamic and profitable operational practices (LIFE16 ENV/ES/000276)" project.