Plant root systems must navigate a heterogeneous soil environment. Many modern plants have the ability to preferentially place their roots into nutrient-enriched soil patches, while reducing root growth outside of the patch. This response has been termed root foraging 'precision’. It was previously shown that this is a phylogenetically conserved trait. However, the evolution of this trait is not completely resolved. To explore this, we obtained data from a previous phylogenetic analysis of 120 species (Kemble and Cahill, 2005). Additionally, we searched different databases for “root AND nutrient AND patch AND heterogeneity”, which resulted in 220 papers for meta-analysis. We screened for papers that measured the proportion of roots in both the nutrient rich and poor zones. Our literature search allowed us to calculate precision as the log ratio of root traits in the nutrient rich to nutrient poor zone for 159 species from 338 observations (some species were studied multiple times). A precision of zero would represent indifference to soil patches, and a positive number indicates high precision. We grafted the information of our taxa from the dated mega tree of Zanne et al. (2004). We made hypotheses about the phylogenetic distribution of foraging ability, drew species precision on an evolutionary scale and used statistical methods to make ancestral trait reconstructions.
Results/Conclusions
Precision was a phylogenetically conserved trait, with eudicots exhibiting the highest precision on average (0.67), gymnosperms exhibiting the lowest precision (0.08) and monocots were intermediate (0.37). We used historical trait reconstructions to calculate that the historical ancestor of all modern eudicots had the highest precision (0.4), the common ancestor of all modern monocots had the lowest precision (0.29) and the common ancestor of all modern gymnosperms was intermediate (0.31). The common ancestor of gymnosperms and angiosperms was calculated to have a precision of 0.34, suggesting that the low precision in modern gymnosperms, and the high precision in modern eudicots are both derived traits. Interestingly, the monocots seem to have maintained the ancestral trait on average. This summary represents averages, and individual species show a large amount of variability within each clade. Evolutionary history thus provides a new perspective on root foraging strategies suggesting they can evolve in relatively short times, frequently reversing evolutionary trajectories. Further, the trait reconstructions suggest that low root foraging precision is a relatively recent innovation, even in old clades such as gymnosperms whose ancestors were predicted to have higher root foraging precision.