Understanding how biotic interactions and environmental filtering mediated by soil properties shape plant community assembly is a major challenge in ecology, especially when studying complex and hyperdiverse ecosystems like tropical forests. To shed light on the influence of both factors, we examined how the edaphic optimum of species (their niche position) related to their edaphic range (their niche breadth) along different environmental gradients and how this translates into functional strategies. Here we tested four scenarios describing the shape of the niche breadth—niche position relationship, including one neutral scenario and three scenarios proposing different relative influences of abiotic and biotic factors on community assembly along a soil resource gradient. To do so, we used soil concentration data for five key nutrients (N, P, Ca, Mg, and K), along with accurate measurements of 14 leaf, stem, and root traits for 246 tree species inventoried in 101 plots located across Eastern (French Guiana) and Western (Peru) Amazonia. We found that species niche breadth increased linearly with species niche position along each soil nutrient gradient. This increase was associated with more resource acquisitive traits in the leaves and the roots for soil N, Ca, Mg, and K concentration, while it was negatively associated with wood density for soil P concentration. These observations agreed with one of our hypothetical scenarios in which species with resource conservation traits are confined to the most nutrient‐depleted soils (abiotic filter), but they are outperformed by faster‐growing species in more fertile conditions (biotic filter). Our results refine and strengthen support for niche theories of species assembly while providing an integrated approach to improving forest management policies.

<p>In early stages of forest succession plants have a high nutrient demand, but it is still a matter of debate if regrowth success of pioneer species is related to plant functional traits favoring fast soil colonization and nutrient acquisition. In general, we would expect trade-offs between plant growth performance and fine root morphological properties in association with different plant life-history strategies. Hence, we hypothesized that fast growing plants should have a more efficient root system that allows them to outcompete slow-growing neighbors in a resource-limited environment.</p> <p>To test our hypothesis we monitored plant successional growth dynamics in a tropical lowland rainforest reforestation experiment conducted in southwest Costa Rica. We collected absorptive roots (<2mm diameter) from plant individuals (comprising 20 tree species and 11 plant families) with different growth dynamics (as indicated by measurements of stem diameter and height). For these samples we assessed a suite of fine root morphological traits, such as legume nodulation status, and furthermore quantified fine root nutrient concentration and phosphatase activities, as well as microbial biomass and phosphatase activity in soils in the close vicinity of fine roots.</p> <p>We found stark differences in fine root characteristics between the tree species investigated in this study, such that fast growing species exhibited relatively larger specific root length and higher turnover, whereas slow growing species tend to rely on mechanical resistance by increasing root tissue density and root life span. Our results suggest that the identified differences in the root trait spectrum between fast and slow growing species reflect plant functional adaptions to resource limitation, edaphic properties and soil microbial symbioses. Our findings further highlight the crucial need to foster our understanding of belowground root morphological and physiological traits during forest succession, especially so when aiming to restore forest ecosystem functioning in formerly intensified land-use systems.</p>

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