Anatomical trait variation across root orders shapes the distinct root economics space of herbaceous monocots and dicots

The trait-based root economics space provides a useful framework for understanding the nutrient acquisition strategies across species. However, it remains poorly understood how differences in anatomical features across root branching orders shape the root economics space. We measured order-based resource economics and anatomical traits of absorptive roots of 32 perennial herbaceous monocots and dicots in a temperate steppe and examined the trait coordination across root orders to gain insights into the nutrient acquisition strategies of herbaceous grassland plants. We found that differences in proportion of cortex in root cross-section and secondary growth drive variation in the root economics space across root orders of herbaceous species. In monocots, the absence of secondary growth resulted in comparably consistent trait coordination across root orders. Specifically, the gradient of root collaboration with arbuscular mycorrhizal (AM) fungal constituted the main dimension of root trait variation, mediated by the proportion of cortex in the root cross-section. In contrast, we found substantial variation in the trait distribution of dicots across root orders due to secondary growth. In lower-order roots of dicots, mycorrhizal colonization rates varied independently of specific root length, but were negatively associated with root nitrogen (N) content along a principal component axis. These relations seemed to be due to the alternative root symbiosis with nitrogen-fixing bacteria weakening the classic collaboration gradient within the root economics space. Importantly, our results demonstrate that the mycorrhizal colonization rates were more strongly correlated with the proportion of cortex than with root diameter in dicots. Consequently, secondary growth in higher-order roots of dicots turned the widely recognized trade-off between specific root length and mycorrhizal colonization rates into a positive correlation. Our study reveals the contrasting resource acquisition strategies of monocots and dicots across root orders, highlighting the fundamental role of root anatomical traits in shaping the root economics space. These findings provide valuable insights into how root anatomical traits predict plant strategies and set the foundation for exploring the potential influence of root anatomy on plant and ecosystem functions.