Soil exploration by invasive and non-invasive forest shrubs: Inferences from root architecture

Background/Question/Methods

Many forest understories of northeastern North America are heavily invaded by shrubs.  Given that nutrients are both limiting and distributed heterogeneously in the soil in these ecosystems, advantages for invasive species may come, in part, from greater efficiency in soil exploration.  We tested this hypothesis by comparing root architectural traits associated with nutrient foraging in six species common in the Northeast (invasives:  Berberis thunbergii, Lonicera maackii, Rubus phoenicolasius; natives:  Viburnum dentatum, Lindera benzoin, Rubus allegeniensis).  To characterize root architecture, we excavated and generated 3D renderings of complete, coarse root systems of 36 field-grown plants (six individuals per species).  Topological and morphological parameters were derived from the resulting virtual plants, and supplemented with information on composition (C:N) and 2D structure of fine roots.

Results/Conclusions

Our preliminary results suggest that some characteristics of nutrient foraging are shared among the shrub species investigated, others are associated with life form, while still others vary among species.  For several species, most notably B. thunbergii and V. dentatum, root systems took on an increasingly dichotomously branched (as opposed to herringbone) topology as plant size increased and root axes developed.  This suggests that, after reaching a certain age or size, they prioritized soil exploration over transport efficiency, which is a necessary tradeoff to dichotomous branching.  Fully developed Rubus plants were less topologically complex than other species, which we attribute to roots generating additional ramets rather than exploring soil as individuals.  The distribution of roots with depth was highly skewed for most species, with the vast majority of a plant’s root volume remaining near the soil surface.  However, roots of L. maackii  were distributed more uniformly with depth, and occasional roots penetrated vertically.  B. thunbergii had the highest specific root length of fine roots among the species studied, corroborating the idea that it invested in exploration capacity, while sacrificing root longevity.  Although no single trait or suite of traits separated invasive from non-invasive plants, root architectural characteristics are clearly associated with interspecific differences in foraging among understory shrubs.