Environmental heterogeneity in the tropics is thought to lead to specialization in plants and thereby contribute to the diversity of the tropical flora. We examine this idea with data on the habitat specificity of 35 western Amazonian species from the genera Protium, Crepidospermum, and Tetragastris in the monophyletic tribe Protieae (Burseraceae) mapped on a molecular-based phylogeny. We surveyed three edaphic habitats that occur throughout terra firme Amazonia: white-sand, clay, and terrace soils in eight forests across more than 2000 km in the western Amazon. Twenty-six of the 35 species were found to be associated with only one of three soil types, and no species was associated with all three habitats; this pattern of edaphic specialization was consistent across the entire region. Habitat association mapped onto the phylogenetic tree shows association with terrace soils to be the probable ancestral state in the group, with subsequent speciation events onto clay and white-sand soils. The repeated gain of clay association within the clade likely coincides with the emergence of large areas of clay soils in the Miocene deposited during the Andean uplift. Character optimizations revealed that soil association was not phylogenetically clustered for white-sand and clay specialists, suggesting repeated independent evolution of soil specificity is common within the Protieae. This phylogenetic analysis also showed that multiple cases of putative sister taxa with parapatric distributions differ in their edaphic associations, suggesting that edaphic heterogeneity was an important driver of speciation in the Protieae in the Amazon basin.

A well-studied group of plants can serve as a model for addressing issues in conservation, evolution, and biogeography, making it possible to assign conservation status with confidence and detecting not only those taxa that are most threatened but also those that represent basal, unique, and/or relictual members of entire lineages. Clarified higher-level phylogenetic relationships open the door to more refined systematics of clades without having to worry if they are para- or polyphyletic. A well-studied group can also be an excellent testing ground for new or under-utilized tools and independent data sets. Research on the Burseraceae, with over 100 taxa in the Amazon, is rapidly arriving at the point where the family can be used effectively both as a tool for conservation and as a model for studying the processes influencing the origin and maintenance of high diversity in the Amazonian flora. First, we are resolving higher-level phylogenies as well as species-level taxonomy in various clades, allowing comparative approaches. Second, the family occurs throughout Amazonia and is well-represented in most habitats overall, but most of the taxa are restricted in their distributions and/or habitats; this makes it possible to test the relative importance of geographic barriers vs. habitat diversity in the speciation process. The family is sufficiently large to provide adequate statistical power for hypothesis testing and yet small enough to achieve the necessary sampling intensity, allowing us to assess the relative impacts of morphological innovation, ecological opportunity, and biogeographic events on the diversification of Burseraceae and related groups.

RESUMENEste estudio es una investigación comprensiva de la especialización por hábitat en plantas de los bosques tropicales y del papel de los herbívoros. Pusimos a prueba la hipótesis de que la alta presión de herbivorismo en los bosques tropicales intensifica las diferencias a lo largo de gradientes edáficos y causa una especialización por hábitat de escala más fina en las plantas. El sistema de estudio fue el mosaico de suelos sobre arena blanca pobres en nutrientes y de suelos de arcilla ricos en nutrientes en la Amazonía peruana. Planteamos la hipótesis de que el suelo empobrecido de arena blanca crea una fuerte presión selectiva hacia defensas de las plantas. Una alta inversión en defensas influye negativamente en el crecimiento, y de este modo las especies de arena blanca muy bien defendidas deberían ser superadas competitivamente en tipos de suelos más fértiles. Al mismo tiempo, plantas sin una inversión alta en defensas deberían estar excluidas por los herbívoros de los bosques sobre arena blanca. Inventarios en diferentes hábitats revelaron que la especialización edáfica es un patrón común en muchos grupos de árboles tropicales. Más aún, la especialización edáfica ha jugado un papel importante en la diversificación de las Burseraceas en la Amazonía, habiéndose producido especializaciones para cada tipo de suelo, muchas veces e independientemente. En relación con el papel de los herbívoros, descubrimos que los especialistas de arena blanca tienen una mayor resistencia a los herbívoros, y una tasa de crecimiento más baja. De este modo, los herbívoros acentúan las diferencias de los hábitats y, por tanto, incrementan las probabilidades de que la heterogeneidad edáfica produzca especialización por hábitat.Palabras clave: diversidad beta, gradiente ecológico, balances, especialización de hábitat, herbivorismo, experimento de trasplante recíproco, especiación ecológica, arena blanca. ABSTRACTThis study is a comprehensive investigation of habitat specialization by plants in tropical forests and the role of herbivores. We tested the hypothesis that high herbivore pressure in tropical forests intensifies the differences along edaphic gradients and causes finer-scale habitat specialization in plants. The study system was the mosaic of nutrient-poor white-sand and nutrient-rich clay soils in the Peruvian Amazon. Impoverished white-sand soil is hypothesized to create strong selective pressure for plant defenses. A high defense investment trades off against growth and thus highly defended white-sand species should be out-competed in more fertile soil types. At the same time, plants without a high defense allocation should be excluded from white-sand forests by herbivores. Inventories of different habitats revealed that edaphic specialization is a common pattern in many groups of tropical trees. Moreover, edaphic specialization has played an important role in the diversification of the Burseraceae in the Amazon, with specialization to each soil type arising multiple times independently. Concerning the role of herbiv...

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