Improving the scale and precision of hypotheses to explain root foraging ability

Abstract

Background: Numerous hypotheses have been proposed to explain the wide variation in the ability of plants to forage for resources by proliferating roots in soil nutrient patches. Comparative analyses have found little evidence to support many of these hypotheses, raising the question of what role resource-foraging ability plays in determining plant fitness and community structure.

Scope: In the present viewpoint, we respond to Grime's (2007; Annals of Botany 99: 1017-1021) suggestion that we misinterpreted the scope of the scale-precision trade-off hypothesis, which states that there is a trade-off between the spatial scale over which plant species forage and the precision with which they are able to proliferate roots in resource patches. We use a meta-analysis of published foraging scale-precision correlations to demonstrate that there is no empirical support for the scale-precision trade-off hypothesis. Based on correlations between foraging precision and various plant morphological and ecophysiological traits, we found that foraging precision forms part of the 'fast' suite of plant traits related to rapid growth rates and resource uptake rates.

Conclusions: We suggest there is a need not only to examine correlations between foraging precision and other plant traits, but to expand our notion of what traits might be important in determining the resource-foraging ability of plants. By placing foraging ability in the broader context of plant traits and resource economy strategies, it will be possible to develop a new and empirically supported framework to understand how plasticity in resource uptake and allocation affect plant fitness and community structure.

Fig. 1.

Funnel graph of scale–precision correlations (r) versus sample size (number of species) for 13 data points collected from eight data sources (1, Bliss et al., 2002; 2, Campbell et al., 1991; 3, Einsmann et al., 1999; 4, Farley and Fitter, 1999; 5, Grime and Mackey, 2002; 6, Johnson and Biondini, 2001; 7, Rajaniemi and Reynolds, 2004; 8, Wijesinghe et al., 2001). The dashed line indicates the 95 % confidence interval for the correlation coefficient at a given sample size; points outside the dashed funnel represent correlations that would be considered statistically significant (α = 0·05). Individual data sources were included more than once if they used multiple measures of foraging scale; Table 1 describes the data sources in more detail.

Fig. 2.

Funnel graph of root foraging scale–precision correlations (r) versus sample size (number of species) for random subsamples of the Great Plains Flora dataset (Johnson and Biondini, 2001). Subsets of the full 59-species data set ranged in size from six to 58 species, with 50 random subsets drawn at each sample size. The dashed line indicates the 95 % confidence interval for the correlation coefficient at a given sample size; points outside the dashed funnel represent correlations that would be considered statistically significant (α = 0·05). Scale–precision correlations were calculated based on a log₁₀-transformed root foraging scale (measured as root system biomass) and root foraging precision for each random subset of species.

Fig. 3.

Trait factor loadings from a principal components analysis (PCA) based on correlations among leaf and root ecophysiological traits (Tjoelker et al., 2005) and root foraging traits (Johnson and Biondini, 2001) for North American grassland species. Correlations were calculated for all pairwise combinations of log₁₀(x + 1)-transformed traits, and the resulting correlation matrix was used as the basis for the PCA analysis. Trait captions and sample sizes are explained in Table 2.