Ruili Wang, Qiufeng Wang, Ning Zhao, Zhiwei Xu, Xianjin Zhu, Cuicui Jiao, Guirui Yu and Nianpeng He
Fine roots are the smallest terminal parts of the root system and act as the primary belowground organs in acquiring limiting nutrients and water from the soil. However, inconsistency in definitions of fine roots and different protocols among studies have meant that knowledge of root system traits has, to date, lagged far behind our understanding of above-ground traits. In particular, it is still hotly debated whether fine root traits among plant species vary along a single root economics spectrum (RES), i.e. a fundamental trade-off between traits related to rapid resource capture and traits related to high resource conservation. Here we supposed that rather than varying along a single RES, root morphological and nutrient traits are driven by different selection pressures and vary along several dimensions.
In this study, we sampled first-order roots (i.e. the most distal root) using a standardized protocol, and measured six important root traits related to resource use strategies, from 181 plant species from subtropical to boreal forests. Base on this large dataset, we concluded that different evolutionary and environmental factors affect species-level (i.e. average trait values) values of root thickness and nutrient concentration, resulting in a decoupled pattern between them. Specifically, morphological traits, including root diameter and specific root length (i.e. root length divided by root dry mass), were mainly constrained by evolutionary relationships and showed little plasticity to changing environments, whereas the large-scale variation in root nutrient concentration was significantly influenced by soil variables. For community-level traits (i.e. community-weighted means), morphological root traits were mainly driven by mean annual temperature via shifting species composition, whereas nutrient traits were strongly influenced by soil phosphorus availability.
For both species and community levels, our study confirms the multidimensionality of root traits, with morphological and nutrient traits driven by different selection pressures. In addition, strong relationships between community-level root traits and environmental variables, due to environmental filters, indicate that in contrast to individual species-level traits, community-aggregated root traits could be used to improve our ability to predict how the distribution of vegetation will change in response to a changing climate.
Image caption: Ecologists at work, root sampling in the forest ecosystem.