Diverse root trait syndromes underlie species' nutrient acquisition strategies in a phosphorus‐limited forest

Abstract Plants allocate finite carbon‐based photosynthates into root construction, mycorrhizal symbiosis, and exudate production for nutrient acquisition. Global syntheses suggest these belowground allocation constraints shape species' root trait expression and trade‐offs, yet whether similar root trait relationships occur in phosphorus (P)‐limited Southeast (SE) Asian tropical forests remains uncertain. Here, we examine whether the root trait trade‐offs that define the global root economic space (RES) are applicable to arbuscular mycorrhizal (AM) and ectomycorrhizal (EcM) species co‐occurring across forest types in SE Asia. We examined 33 species colonized by AM and EcM fungi in an old‐growth and secondary forest in Singapore to quantify root functional traits that enable P uptake such as phosphomonoesterase (PME) activity, mycorrhizal colonization rate, and root morphological and chemical traits. Our study revealed a RES defined by two main axes of root trait variation that relate to (1) soil explorative capacity and (2) root resource conservatism and nutrient use. However, in contrast to other studies, mycorrhizal colonization was consistently low in most species and varied independently of the soil exploration root trait gradient. Furthermore, root traits differed strikingly between mycorrhizal types. SE Asian EcM species consistently expressed thin roots with a high specific root length and root PME activity, whereas AM species exhibited diverse trait combinations, but with notably thicker roots containing greater potassium and magnesium content. Finally, root traits varied little among common species across forest types, indicating that differences in root trait expression between the old‐growth and secondary forest were caused by species turnover. Our findings highlight the need to incorporate root enzyme activity and mycorrhizal colonization when assessing root functional traits in order to generalize P acquisition trade‐offs among tropical species from contrasting phylogenetic lineages, mycorrhizal and forest types. Such information will be crucial to refine how global vegetation models capture belowground function across species‐diverse tropical forest ecosystems.