Root Porosity Contributes to Root Trait Space of Wetland Monocotyledons Independently of Economics Traits.

Root aerenchyma, a key adaptive trait to anoxic soils has rarely been integrated into trait-based plant ecology. This study aims to evaluate the relationship between root porosity and root economics-related traits among wetland plants, focusing on the effect of aerenchyma on root tissue density, a central trait in plant economics spectrum. Root porosity, root tissue density with air-space included (RTD) or excluded (RTDA), and other root traits such as root dry matter content (root dry mass to fresh mass ratio) were measured separately for adventitious and lateral roots of 16 garden-grown Ontario wetland monocots with contrasting root longevities. Principal component analysis showed that in the root adaptive trait space of wetland plants, the first dimension is defined by economics-related traits, the second dimension by lateral root porosity and the ratio of lateral to adventitious root length, and the third dimension by adventitious root porosity. Interspecific variation in root porosity was unrelated to root economics traits. Consequently, root tissue density excluding air space (RTDA) better differentiated between species with contrasting root longevities than RTD did, consistently both for adventitious and lateral roots. Root dry matter content accurately predicted RTDA. Interspecific variation in root porosity is independent of root economics: Wetland plants can construct economically conservative or acquisitive roots of any porosity. Consequently, to consistently express root functional relationships among wetland plant species, root tissue density should be expressed with excluding the air space (i.e., RTDA), or with the more easily measured root dry matter content.