Effects of woody vegetation patches on species composition in Stipa tenacissima steppes
Introduction
Drylands cover around 45% of the world's land area (Prăvălie, 2016). Climate models suggest that dryland cover will increase by 23% by the end of this century (Huang et al., 2016). In drylands, vegetation is frequently patchy, and this structure is closely linked to ecosystem function (Berdugo et al., 2020). Vegetation patches modulate the flow of water, nutrients and sediments, acting as resource sinks (Ludwig and Tongway, 1995; Aguiar and Sala, 1999; Merino-Martín et al., 2015). They also affect community composition and species richness (Ludwig et al., 2004; Zhang et al., 2016).
Steppes dominated by the tussock grass Stipa tenacissima L. cover 70,000 km2 in the western Mediterranean basin. They frequently form mosaics of woody vegetation patches immersed in a matrix of S. tenacissima tussocks, small sub-shrubs and bare soil. In S. tenacissima steppes, patches of woody species act as keystone components of the community (sensu Hurlbert, 1997), their cover is often low, but they affect ecosystem functioning, vascular plant richness, and plant and soil macroinvertebrate communities (Maestre and Cortina, 2004, Maestre and Cortina, 2005; Doblas-Miranda et al., 2009; Rolo et al., 2016). Woody patches in S. tenacissima steppes modify climate and soil properties favoring the establishment of shade tolerant species at their understory (Maestre and Cortina, 2005). These species benefit from the decrease in evaporative demand and temperature range, and the increase in soil fertility to the point that they may withstand the reduction in soil water availability (Amat et al., 2015).
In S. tenacissima steppes, woody vegetation was removed in the past to reduce competition and promote forage and fiber production (Servicio del Esparto, 1953; Fernández-Palazón, 1974, Gasque and García-Fayos, 2004). In the early stages of succession, patches of woody species are usually associated with the margins of abandoned agricultural terraces and rock outcrops unsuitable for agriculture (Cortina et al., 2009; Rolo et al., 2016). Steppe colonization by patch-forming species may be initiated in these patches, and progress at rates that depend on climatic conditions –particularly water availability, topography, soil fertility, and species dispersal and establishment ability (Rolo et al., 2016; Tormo et al., 2012, Castillo-Escrivà et al., 2019). Furthermore, establishment may be driven by the dynamic nature of positive and negative plant-plant and plant-animal interactions (Maestre and Cortina, 2004, Soliveres et al., 2015; Amat et al., 2015, Castillo-Escrivà et al., 2019). However, studies on woody patch dynamics have paid scarce attention to their multi-specific nature by considering them as homogeneous entities or focusing solely on patch-forming species. No study has described so far the composition of patch-forming species and the drivers controlling patch composition.
Patch capacity to modify the environment depends on their composition and size. For example, contrasting rooting habits, water and nutrient demand, litter accumulation, and the amount and quality of radiation in the understory in different patch-forming species generate different microhabitats and colonization opportunities (Aguiar et al., 1992; Archer et al., 2002; Blank and Carmel, 2012; Arroyo et al., 2015). Furthermore, as patches increase in size, they provide further opportunities for colonization because of the intensification of the environmental effects, the time elapsed since the establishment of the first patch-forming species, and the increase in habitat heterogeneity. Heterogeneity in environmental conditions within the patches has been well characterized in drylands – including S. tenacissima steppes, as well as its effects on community composition and plant functional traits (Hochstrasser and Peters, 2004, Holmgren et al., 2010; Pescador et al., 2014; Amat et al., 2015; Soliveres et al., 2015). Yet, our knowledge on the heterogeneity of woody patches in S. tenacissima steppes, its drivers and its role in community assemblage is still scarce.
To better understand patch features and the effect of woody patches on community composition, we studied the cover of vascular plants in 450 woody patches distributed in 15 small catchments dominated by S. tenacissima steppes along a climatic gradient in southeastern Spain, and generated explanatory models of overstory and understory composition and the recruitment of patch-forming species based on patch and catchment characteristics.
Section snippets
Material and methods
We selected 15 catchments along a 60 km-transect in a semiarid area in Alicante, southeastern Spain (Fig. 1, Table 1). Catchment slopes are covered with S. tenacissima steppes, whereas catchment bottoms are frequently occupied by abandoned rainfed agricultural terraces. We intentionally excluded catchments with a significant presence of Pinus halepensis Mill., because this species has often been planted, and planting techniques alter soils and vegetation. In each catchment, we sampled variables
Patch-forming species in slopes and abandoned terraces
The number of woody patches in the slopes of the studied catchments ranged from 69 to 365, which corresponds to a range of densities of 31–155 patches/ha (Table 1). We identified six patch-forming species: Pistacia lentiscus L., Quercus coccifera L., Rhamnus lycioides L., Juniperus oxycedrus L., Ephedra fragilis Desf. and Osyris lanceolata Hochst. & Steud. Patches dominated by R. lycioides (i.e, patches where canopy projection area of R. lycioides was bigger than the projected area of any other
Drivers of patch-forming species dominance and recruitment
Climate determined the number of patches dominated by different patch-forming species in S. tenacissima steppes, and their relative cover. Patches dominated by Q. coccifera and J. oxycedrus showed their maximum abundance at low mean annual temperatures and low mean annual precipitations. Relative cover of patch-forming species within the patches showed a similar trend: as temperature increased, cover of Q. coccifera and J. oxycedrus decreased. This result was unexpected as Q. coccifera forms
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This research has been financially supported by the Spanish Ministry of Science, Education and Universities and European Regional Development Funds (FEDER), (projects UNCROACH, CGL2011-30581-C02-01 and COSTERA, RTI2018-095954-B-I00). JT was supported by a Juan de la Cierva Contract and BA was supported by an FPU fellowship, both from the Spanish Ministry of Science and Innovation. We also thank María Joao Ribeiro Da Silva, Lucía de Soto, Lorena Guixot, Núria Jover, Adela Blasco, Alejandro
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