Nutrient foraging strategies are associated with productivity and population growth in forest shrubs

Abstract

Background and aims: Temperate deciduous forest understoreys are experiencing widespread changes in community composition, concurrent with increases in rates of nitrogen supply. These shifts in plant abundance may be driven by interspecific differences in nutrient foraging (i.e. conservative vs. acquisitive strategies) and, thus, adaptation to contemporary nutrient loading conditions. This study sought to determine if interspecific differences in nutrient foraging could help explain patterns of shrub success and decline in eastern North American forests.

Methods: Using plants grown in a common garden, fine root traits associated with nutrient foraging were measured for six shrub species. Traits included the mean and skewness of the root diameter distribution, specific root length (SRL), C:N ratio, root tissue density, arbuscular mycorrhizal colonization and foraging precision. Above- and below-ground productivity were also determined for the same plants, and population growth rates were estimated using data from a long-term study of community dynamics. Root traits were compared among species and associations among root traits, measures of productivity and rates of population growth were evaluated.

Key results: Species fell into groups having thick or thin root forms, which correspond to conservative vs. acquisitive nutrient foraging strategies. Interspecific variation in root morphology and tissue construction correlated with measures of productivity and rates of cover expansion. Of the four species with acquisitive traits, three were introduced species that have become invasive in recent decades, and the fourth was a weedy native. In contrast, the two species with conservative traits were historically dominant shrubs that have declined in abundance in eastern North American forests.

Conclusions: In forest understoreys of eastern North America, elevated nutrient availability may impose a filter on species success in addition to above-ground processes such as herbivory and overstorey canopy conditions. Shrubs that have root traits associated with rapid uptake of soil nutrients may be more likely to increase in abundance, while species without such traits may be less likely to keep pace with more productive species.

Keywords: Below-ground dynamics; deciduous forest understorey; foraging strategies; functional traits; nutrient acquisition; rhizosphere; root economics spectrum; root morphology; woody shrubs.

Fig. 1.

Root functional traits and metrics of productivity (mean ± s.e.) for the six species included in the study. All root traits are measured on fine roots (<2 mm diameter) except RTD. Within each panel, bars that do not share a letter have statistically separable means based on Tukey’s HSD tests. Metrics with two sets of bars (I–L) differed by enrichment status or the species × enrichment interaction (Table 2); bar heights depict sums for the three ingrowth bags of each type. C:N, carbon to nitrogen ratio; Diam_mean, mean parameter of diameter distribution; Diam_skew, skewness parameter of diameter distribution; Length_rt, root length production; Mass_rt, root mass production; Mycor, mycorrhizal coverage; Precision_lngth, length-based foraging precision; Precision_mass, mass-based foraging precision; RGR_sht, relative growth rate of shoot biomass; RTD, root tissue density; SRL, specific root length; Surf Area_rt, root surface area production.

Fig. 2.

Distributions of fine root diameter, computed on the basis of length, for the species included in the study. Curves depict log-normal distributions whose parameters are the mean (± s.e.) of the fitted parameter estimates across individuals of the species.

Fig. 3.

Principal component (PC) biplot of (A) root functional traits and (B) productivity metrics for the six species included in the study. Points represent individual plants (n = 46). Abbreviations are defined in the legend of Fig. 1.

Fig. 4.

Associations among the primary variables measured in the study: the principal component (PC) axes representing root structure (PC1_root) and productivity (PC1_prod), as well as the rate of cover increase (Spread) in the Buell–Small Succession Study. Filled symbols depict mean species-level data (mean ± s.e. for PC scores) while open symbols depict plant-level data. Abbreviations are defined in the legend of Fig. 1.