A worldview of root traits: functional trait variation in fine roots of seed plants
Fine roots (acquisition roots usually < 1 mm in diameter) play a substantial role in plant functions and root traits are increasingly seen as main drivers of ecosystems processes. Nonetheless, the lack of belowground comparative analyses has limited a full description of the underlying mechanisms explaining root trait variation at global scales. A common assumption is that fine root trait assemblages must show an economical continuum (i.e. root economic spectrum, RES). Hence, trait variation across species should form an ecological gradient ranging from species with high metabolic activity and low tissue protection to those with low metabolic activity and highly protected. Under this hypothesis, it is expected that 1) traits associated with surface exposure such as root diameter (D) or specific root length (SRL) should be closely associated with traits reflecting metabolic activity, like N content and root tissue density (RTD); 2) analogous traits such as SRL and specific leaf area (SLA) or N content in leaf and roots should correlate positively; 3) Ecological and evolutionary characteristics such as growth habit, climate, mycorrhizal affiliation or relatedness should have relatively low impact on the association between traits.
To test how closely root trait syndromes follow the RES hypothesis, we compiled an unprecedented dataset including information about D, SRL, RTD and root N for 600+ species of seed plants. Additionally, we compiled climatic information for each species based on their geographical distribution. Our results indicated 1) that root traits are much less integrated than predicted from the RES hypothesis. Only D and SRL showed an expected negative relationship, others traits show weak or no relationship. 2) SRL did not correlate with SLA, but aboveground and belowground N content was correlated, although this integration varied between woody and non-woody plants. 3) Phylogenetic relatedness was the most important factor explaining root trait variation for N content (47%), D (42%) and SRL (33%), whereas climate (17%) was the most important factor for RTD. Phylogenetic structuring was stronger among woody than non-woody plants. In summary, our study indicates that root trait integration is largely independent from the expected RES hypothesis. Instead, phylogenetic associations and growth habits seem to play a dominant role in the distribution and integration of root traits at global scales. Future steps, such as a better representation of biomes and the inclusion of root traits from understudied phylogenetic groups are needed to confirm some of the trends described in this study.