Linking root traits to potential growth rate in six temperate tree species

There is an extremely limited understanding of how plants of different potential growth rate vary in root traits, especially in woody species. We contrasted fine root morphology, physiology, and elemental construction between a fast- and a slow-growing species in each of three families: Aceraceae (maple), Fagaceae (oak), and Pinaceae (pine). Measurements were primarily made on 1-year-old seedlings growing in a growth chamber. Across all three families, first- and second-order roots of fast-growing species had greater specific root length, thinner diameters, and faster respiration rates than those of slow-growing species. These fine roots of fast-growing species in Aceraceae and Fagaceae also had faster phosphorus (P) uptake on a surface area basis than those of slow-growing species, whereas little difference in P uptake was found between Pinaceae species. On a dry weight basis, roots of fast-growing species in Aceraceae and Fagaceae had higher nitrogen concentrations, lower carbon:nitrogen ratios and higher tissue construction costs than roots of slow-growing species (data were unavailable for Pinaceae). Tissue density did not vary in a consistent pattern between fast- and slow-growing species across all three families (P=0.169). To better understand the ecological significance of differences in these root characteristics, a root efficiency model was used to compare P uptake and root carbon (C) cost of each species in simulated field situations in two soils, one with low P buffering capacity (loamy sand) and another with relatively high P buffering capacity (silt loam). For the soil conditions modeled, fast-growing species of Aceraceae and Fagaceae were 17-70% more efficient (defined as cumulative P gain divided by cumulative C cost) at nutrient capture than slow-growing species while the fast-growing Pinaceae species was 20-24% less efficient than the slow-growing species. However, among all three families, roots of fast-growing species reached maximum lifetime efficiency 5-120 days sooner, depending on soil type. Thus, modeling results indicated that root traits of fast- and slow-growing species affected P acquisition in simulated field soil although soil type also had a strong impact.