Root-shoot growth responses during interspecific competition quantified using allometric modelling

Abstract

Background and aims: Plant competition studies are restricted by the difficulty of quantifying root systems of competitors. Analyses are usually limited to above-ground traits. Here, a new approach to address this issue is reported.

Methods: Root system weights of competing plants can be estimated from: shoot weights of competitors; combined root weights of competitors; and slopes (scaling exponents, α) and intercepts (allometric coefficients, β) of ln-regressions of root weight on shoot weight of isolated plants. If competition induces no change in root : shoot growth, α and β values of competing and isolated plants will be equal. Measured combined root weight of competitors will equal that estimated allometrically from measured shoot weights of each competing plant. Combined root weights can be partitioned directly among competitors. If, as will be more usual, competition changes relative root and shoot growth, the competitors' combined root weight will not equal that estimated allometrically and cannot be partitioned directly. However, if the isolated-plant α and β values are adjusted until the estimated combined root weight of competitors matches the measured combined root weight, the latter can be partitioned among competitors using their new α and β values. The approach is illustrated using two herbaceous species, Dactylis glomerata and Plantago lanceolata.

Key results: Allometric modelling revealed a large and continuous increase in the root : shoot ratio by Dactylis, but not Plantago, during competition. This was associated with a superior whole-plant dry weight increase in Dactylis, which was ultimately 2·5-fold greater than that of Plantago. Whole-plant growth dominance of Dactylis over Plantago, as deduced from allometric modelling, occurred 14-24 d earlier than suggested by shoot data alone.

Conclusion: Given reasonable assumptions, allometric modelling can analyse competitive interactions in any species mixture, and overcomes a long-standing problem in studies of competition.

Fig. 1.

Combined root dry weights (R_C) of competing Dactylis glomerata and Plantago lanceolata estimated from the allometries obtained for isolated plants (Table 1) and the measured combined root weights of the two species when growing together (R_M). Each symbol represents a separate harvest and is the mean of three replicates. The bold line is the linear regression of ln-transformed data (cf. eqn 4): slope = 0·935 (0·874–0·996, 95 % CL), intercept 1·26 (0·875–1·82), R² = 0·982, n = 20. The grey line is the 1 : 1 relationship (slope = 1, intercept = 0).

Fig. 2.

Root and shoot dry weights of (A) Dactylis glomerata and (B) Plantago lanceolata grown in isolation or competing with each other, as indicated. Root weights of competitors were derived from the allometric modelling of combined root weights measured in the experiment, assuming that both species responded allometrically to competition (Table 2). Each symbol represents a separate harvest and is the mean of three replicates. α and β values for isolated plants are given in Table 1; those for competitors, in Table 2. Diagonal grey lines show constant root : shoot (R:S) ratios from R:S = 0·2 (the lowermost) to R:S = 1, in 0·1 increments.

Fig. 3.

Total dry weights (i.e. the sum of shoot and root weights) of Dactylis glomerata and Plantago lanceolata when competing with one another. Shoot weights were measured. Root weights were derived from the allometric modelling of combined root weights measured in the experiment (Fig. 2). Each symbol represents a separate harvest and is the mean of three replicates. The bold line is the RMA regression of ln-transformed data: slope = 0·778 (0·724–0·832, 95 % CL), intercept 3·93 (2·79–5·53), R² = 0·977, n = 20. The grey line is the 1 : 1 relationship (slope = 1, intercept = 0).