Facilitation as a driver of plant assemblages in Caatinga
Introduction
Biological interactions such as facilitation, competition, herbivory and mutualism, are key ecological processes that influence community composition (i.e.), assembly rules, (Kraft et al., 2015). Facilitation is a positive interaction in which the presence of a nurse plant reduces the environmental severity experienced by neighboring plants (Jankju, 2013), increasing growth rate, recruitment, and survival as well as enhancing the reproductive success of these facilitated plants (Hierro and Cock, 2013). In contrast to competition, facilitation diversifies niches and increases the species richness of communities (Valiente-Banuet and Verdú, 2013, Valiente-Banuet and Verdú, 2007). Therefore, facilitation is a key factor in ecosystem resilience and resistance (Pugnaire and Lázaro, 2000) that has further been recently recognized as a ubiquitous phenomenon in community assembly, especially in arid and semi-arid regions (Filazzola and Lortie, 2014).
Facilitation tends to group these plants into clumps or patches of vegetation dominated by the nurse plant, thus forming a characteristic mosaic of aggregated plants (Eccles et al., 1999), while plants in competition-dominated communities are likely to exhibit spatial segregation (Maestre et al., 2009). The benefits of nurse plants to facilitated plants may include the creation of favorable microclimates through shading or increases in nutrient content, organic matter, moisture and oxygenation of the soil (Filazzola and Lortie, 2014, McIntire and Fajardo, 2014). Additional facilitation mechanisms include protection from herbivores, increased pollination rates or seed recruitment and improved soil microbiota (Filazzola and Lortie, 2014).
To increase the species richness of communities and avoid niche overlap between nurse and facilitated plants, facilitation should promote the coexistence of ecologically dissimilar species (Castillo et al., 2010). By investigating this phenomenon, the analysis of phylogenetic community structure can offer valuable insights (Gastauer and Meira-Neto, 2013, Webb et al., 2002). If phylogenetically related species are ecologically more similar, i.e., their ecological traits are conserved within evolutionary niches, facilitation should promote phylogenetic overdispersion (Fig. 1A). Although facilitation would result in phylogenetic overdispersion or phylogenetic evenness if ecological traits are predominantly convergent (Cavender-Bares et al., 2009, Valiente-Banuet and Verdú, 2007), it would never cause phylogenetic clustering (Fig. 1D).
In the case of conserved ecological niches within evolutionary lineages, competition would cause phylogenetic overdispersion (i.e., competitive exclusion of more related lineages due to competition for limited resources such as water or nutrients, Fig. 1B), while environmental filtering would cause phylogenetic clustering (i.e., filtering out of lineages lacking adaptative traits, for instance, against herbivore pressure, Fig. 1C, Webb et al., 2002). If traits are convergent, competition and environmental filtering would cause phylogenetic evenness, as with facilitation (Fig. 1E and F); however, competition and environmental filtering should decrease species richness (Fig. 1, Cavender-Bares et al., 2009, Gastauer and Meira-Neto, 2013).
The objective of this study is to analyze facilitation as a driver of plant assemblages in the semiarid savannas of the Caatinga in Brazil, where the plant community is structured in two main vegetation types: the patchy Caatinga and the Caatinga with segregated plants. Although other ecological and evolutionary processes may influence the phylogenetic structure of plant communities (Cavender-Bares et al., 2009, Sargent and Ackerly, 2008) we expect that coexisting species in patches of the patchy Caatinga founded by nurse plants will not be phylogenetically clustered and will show higher species richness than Caatinga with segregated plants. Therefore, the aims of this study are as follows: (i) we describe and quantify the functional traits of nurse plants in Caatinga patches to highlight their main functions and possible benefits to facilitated species. Next, (ii) we determine if the species coexisting within patches with nurse plants (i.e., under facilitation) show phylogenetic evenness or overdispersion, indicating a low niche overlap. Furthermore, (iii) we test whether patchy Caatinga is phylogenetically overdispersed, which would indicate niche diversification promoted by facilitation. Finally, (iv) we investigate whether patchy Caatinga shows higher species richness than Caatinga with segregated plants; nurse plants should ameliorate stressful environmental conditions, balance competitive exclusion and diversify niches.
Section snippets
Vegetation and study area
The Caatinga is a type of vegetation distributed in the biogeographical region that expands from 3° S to 17° S and from 35° W to 45° W with a tropical savanna-like, semi-arid vegetation that covers more than 800,000 km2 in northeast Brazil (Veloso et al., 1991). The Brazilian Caatinga occurs on a wide range of soil types in the depressions between plateaus and on hills composed of sandstones, granite, gneiss and other metamorphic rocks (Menezes et al., 2012), and it represents the largest
Results
During the fieldwork, we identified 35 nurse species, five of which occurred with high frequency. The most common nurse species, i.e., species facilitating more than 10 patches each, were Poincianella microphylla, Aspidosperma pyrifolium, Poincianella pyramidalis, Croton echioides and Tacinga palmadora (Table 2).
Of the identified nurse species, 33 were phanerophytes; one was a camephyte; and one was a hemicryptophyte. Eighteen species were resprouters; nine had spines and/or thorns; five were
Discussion
The results from this study suggest plant-plant facilitation may be a driver of community assembly in Caatinga, and this is supported by the higher herbaceous richness in patchy Caatinga than in Caatinga with segregated plants. The phylogenetic diversity of the herbaceous layer in the patchy Caatinga is higher than Caatinga with segregated plants as an effect of the increased richness. Additionally, phylogenetic overdispersion is found in patches of patchy Caatinga. Those results are consistent
Acknowledgements
The authors thank the DNIT (Departamento Nacional de Infraestrutura de Transportes) (50600.056799/2013-21), Funarbe (6991) and BR235/UFV Team (1071/2013-DPP) for grants and financial support. The authors also thank CNPq (301913/2012-9), CAPES and FAPEMIG (APQ-01309-16) for providing grants and scholarships; Dr. Luciano Paganucci de Queiroz and UEFS Herbarium for help in plants identification, Biodiversitas Foundation, Caboclo and Marcos Vinicius Varjão Romão for fieldwork help. JAAMN holds a
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