Root trait syndromes are driven by phylogeny, native-invasive status and fungal colonization

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Alexander Ebert, Jason Fridley and Douglas A. Frank, Biology, Syracuse University, Syracuse, NY

Background/Question/Methods
Non-native, invasive woody shrubs can alter the composition and functioning of many terrestrial ecosystems. While many studies have linked aboveground traits such as higher assimilation and growth rates to the greater competitive abilities of invaders, less is known about how belowground root traits may contribute to invader growth advantages. In this study, we asked how a variety of root traits differed between native and invasive shrubs and the degree to which the traits were influenced by nutrient treatments and colonization by arbuscular mycorrhizal fungi (AMF). Using softwood cuttings from a common garden in Syracuse, NY, we grew 12 species of phylogenetically paired native and invasive shrubs from three families in a fully factorial design with mycorrhizal and nonmycorrhizal conditions under high and low nutrient regimes. We performed a principal components analysis and used linear mixed-effects models to analyze individual nutrient and morphological root traits, including branching intensity, total fine root length, specific root length, first-order root diameter, mean first-order root length, root tissue density, root phosphorus content and root nitrogen content.
Results/Conclusions
Invaders had lower root nitrogen content, lower root tissue density, smaller fine root diameter and greater specific root length. Other traits varied more with respect to AMF colonization with mycorrhizal plants exhibiting large increases in root phosphorus content, lower carbon:nitrogen ratios, and shorter average first-order root lengths. Branching intensity (defined as the number of first order roots per length of second order roots) differed significantly according to nativity as well as the interaction between nativity and AMF colonization. Thus, invaders had greater branching intensity than natives, but colonization by AMF increased branching even more among invaders. This is consistent with our previous findings that native and invasive shrub growth rates responded to mycorrhizas differently. The PCA showed that the shrub species were distinctly grouped by phylogeny in trait space. However, within each phylogenetic group, invaders showed a directional shift along PC1, which corresponded to morphological root traits such as smaller root diameter and greater specific root length. Also, within the phylogenetic groups, mycorrhizal plants shifted more along PC2 which corresponded to nutrient traits such as greater phosphorus concentration and a lower carbon:nitrogen ratio. Using these metrics, we were able to create a framework describing the root trait syndromes exhibited by native and invasive shrubs, which further illustrates how invaders can be more adaptive to resource uptake through both fixed morphologies and plastic trait responses.