Variation of first-order root traits across climatic gradients and evolutionary trends in geological time

Background/Question/Methods

Global inter-specific variation observed in plant leaf traits has been shown to represent the leaf evolution and adaptation to a wide range of environmental conditions. Similar patterns in functional trait variation and evolution has been assumed to exist in plant roots. However, our understandings of large scale patterns and the underlying mechanisms of root trait variation and evolution are very limited, especially when compared with leaf traits. Here we collected the first-order roots (stream-based ordering system) of 65 tree species in six forests sites from subtropical to temperate zones in China. Root diameter, specific root length (length to dry mass ratio) and branching ratio (number of first order roots per second order root) were measured. We examined the degree of variation in key functional traits in morphology and architecture for the first-order roots across large geographical scales. In addition, we conducted regression analysis at the family level to assess root trait variation in an evolutionary context.

Results/Conclusions

We found strong climatic and phylogenetic patterns in root traits. Root morphology in the wet, subtropical zone was much more variable among species, where trees with thick and thin roots coexisted. In contrast, the variation was narrower among species in temperate, more arid zone, where most tree species tended to grow thin roots, usually with a higher branching ratio. These thin roots are putatively better equipped to cope with the generally lower and temporally more variable soil moisture in the temperate zone. In an evolutionary context, we found average root diameter decreased and branching ratio increased markedly with family divergence time from mid to late Cretaceous. During this geological time the atmospheric CO₂ concentration progressively decreased, so plants might have increased their stomatal conductance at the expense of increased water loss. Producing root systems with smaller diameter and higher branching ratio may have helped plants cope with periodic water stress. Future study is required to resolve whether the patterns identified here are applicable to a wider range of species.