Modelling responses of western Amazonian palms to soil nutrients

Abstract

Summary Understanding the responses of individual plant species along different edaphic gradients is a key question in ecology, with implications to community assembly, functioning of forest ecosystems, niche theory and conservation planning. In tropical rain forests, responses to soil nutrients have been described only for a handful of species. Even abundant and conspicuous components of the forest, such as the palms (Arecaceae), remain largely unknown in this respect. We inventoried all palm species and analysed soil chemistry in 96 sites across western Amazonia to (i) assess the relative importance of some micronutrients vs. macronutrients as drivers of patterns in palm species composition, (ii) model the response shapes of 61 palm taxa and of canopy vs. understorey palms along gradients of selected soil nutrients, (iii) determine if the response shapes of conspecific palm varieties differ in relation to the most important observed soil nutrient and (iv) assess if the rank order of the most abundant canopy species changes along edaphic gradients. Patterns in palm species composition were best explained by Mehlich‐III extractable exchangeable bases (Ca, K, Mg) and phosphorus (P), with the different palm species clearly separating along the soil cation concentration gradient. All 61 palm taxa exhibited statistical responses along soil nutrient gradients. Response shapes in relation to exchangeable bases varied among bimodal skewed (51% of species), unimodal skewed (47%) and monotonic (2%). There were no significant differences between canopy vs. understorey species in their mean response shapes to the exchangeable bases, but canopy species had significantly higher mean optimum value for soil P and greater mean niche width along the Ca, K, Mg, P and boron (B) gradients. Varieties of the same species of Bactris, Desmoncus and Geonoma generally had different response shapes and different optima for the exchangeable bases. Synthesis. Among the soil variables we studied, macronutrients (especially exchangeable bases and P) emerged as more important than micronutrients in predicting species abundances in palm communities of western Amazonian non‐inundated forests. Non‐Gaussian responses were predominant, conspecific palm varieties exhibited different response types, and the rank order of dominance of canopy palms varied along the gradients of exchangeable bases and P. Together, these findings advance niche theory about palms and may be used for generating better predictive models of palm species distributions and for experimental studies that search for the physiological mechanisms underlying inter‐ and infra‐specific trade‐offs along edaphic gradients.