Belowground differentiation among trees in a degraded tropical dry forest landscape: no evidence of a collaboration gradient

Fine roots are specialized in nutrient and water acquisition and are critical for species performance and ecosystem functioning. Recent evidence has shown a broad root economic space determined by the orthogonal collaboration and conservation gradients related to resource acquisition and resource conservation, respectively. However, whether these gradients exist among tree species growing in degraded ecosystems where root growth is limited by soil conditions is much an open question. We measured six fine root traits (root diameter, specific root length, root dry matter content, root tissue density, branching intensity, and percentage of arbuscular mycorrhizal colonization) in 11 young tree species growing in sympatry for 9 years in degraded pastures in a tropical dry forest (TDF) in Colombia to determine (1) the covariation between fine root traits and (2) the patterns of belowground niche differentiation among 11 species coexisting under the same soil conditions. The covariation between fine root traits resembled the acquisitive-conservative, but not the collaboration gradient for this degraded habitat. The percentage of mycorrhizal colonization, a critical trait associated with the collaboration gradient, was unrelated to any fine root trait. Furthermore, we found a strong belowground differentiation among species, mainly across root diameter and branching intensity. Our results suggest that compacted degraded soils in TDF landscapes may affect the collaborative association with mycorrhizae, mostly allowing species differentiation along the do-it-yourself gradient. This finding suggests a hypothesis that needs to be tested with more species and sites. We discuss the importance of using root traits to aid species selection for restoration purposes.