Root productivity in nutrient-rich soil patches by invasive and native Northeastern shrubs

Background/Question/Methods

It is often under elevated resource conditions that invasive plants have performance advantages over co-occurring natives.  When a resource is spatially heterogeneous, such as nutrients in forest soils, growth rate advantages may come from higher root system productivity in general and/or from greater responsiveness to nutrient patches (foraging precision) specifically.  To determine if shrubs invasive in understory communities of northeastern North America respond to nutrient patches more strongly than do natives, we conducted a manipulative experiment with three invasive and three native species.  In a non-competitive field setting, we grew eight young plants of each species (aged 2-3 yr initially) for one year, placing six ingrowth bags with sieved soil (controls, n=3) or soil amended with chopped grass (simulated patches, n=3) around each plant.  We measured fine root growth and morphological characteristics in all ingrowth bags, as well as aboveground growth, at the conclusion of the year.

Results/Conclusions

We found that invasive Lonicera maackii produced more root length than most other species, in keeping with its rapid aboveground growth.  Native Viburnum dentatum produced more root mass than any other species, though it and Lindera benzoin (also native) had the greatest root diameters, suggesting that they were investing in root tissue for longevity rather than to maximize soil exploration or nutrient extraction.  Invasive Rubus phoenicolasius  and native R. allegheniensis had the highest root tissue density, also indicative of root longevity.  At the other extreme, invasive Berberis thunbergii had higher specific root length than all species but L. maackii.  While almost all species had significant foraging precision (60-70% of root length was in amended bags), interspecific differences were not detectable.  Our results confirm that root system productivity parallels aboveground productivity in understory shrubs, but also demonstrate that tissue longevity is an important tradeoff to growth rate in these species.  Further, the strategy of investment in current versus future growth partially, but not completely, separates the invasive from the native shrubs that we studied.  A suite of additional factors, including mycorrhizal association and nutrient uptake rate, likely contributes to observed performance advantages for invasive plants under elevated nutrient conditions.