Evidence for competition and cooperation among climbing plants - PubMed (original) (raw)

Evidence for competition and cooperation among climbing plants

Jay M Biernaskie. Proc Biol Sci. 2011.

Abstract

A plant's best strategy for acquiring resources may often depend on the identity of neighbours. Here, I ask whether plants adjust their strategy to local relatedness: individuals may cooperate (reduce competitiveness) with kin but compete relatively intensely with non-kin. In a greenhouse experiment with Ipomoea hederacea, neighbouring siblings from the same inbred line were relatively uniform in height; groups of mixed lines, however, were increasingly variable as their mean height increased. The reproductive yield of mixed and sibling groups was similar overall, but when adjusted to a common mean height and height inequality, the yield of mixed groups was significantly less. Where this difference in yield was most pronounced (among groups that varied most in height), mixed groups tended to allocate more mass to roots than comparable sibling groups, and overall, mixed groups produced significantly fewer seeds per unit mass of roots. These results suggest that, from the group perspective, non-kin may have wasted resources in below-ground competition at the expense of reproduction; kin groups, on the other hand, displayed the relative efficiency that is expected of reduced competitiveness.

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Figures

Figure 1.

Figure 1.

Leverage plots showing the interacting effects of relatedness and mean height on the height inequality (coefficient of variation, CV) of plants in a group (corresponding to the model in table 1c). Filled circles are sibling groups (where colour corresponds to genotype), and open circles are mixed groups. Height inequality increased significantly in mixed groups (solid line; p < 0.0001; estimated slope: 0.0047 (0.003, 0.007)) but not in sibling groups (dashed line; p = 0.69; estimated slope: −0.0003 (−0.0002, 0.01)).

Figure 2.

Figure 2.

The average yield of mixed and sibling groups as overall means (horizontal lines in (a)) and as a function of mean height and the height inequality (CV) of plants in a group ((b), corresponding to the model in table 2a). In (a), the vertical span of each diamond is a 95% CI, and coloured lines (one for each of the three genotypes) illustrate the average change in yield at the level of individual plants. In (b), predicted values come from separate multiple regression models (one for siblings, one for mixed groups) that included ‘table’ (block), CV in height and mean height. The points shown (sibling groups as filled circles, mixed groups as open circles) fall upon the least-squares regression plane predicted by the corresponding model. Partial regression coefficients and associated _p_-values: sibling groups: 19.0 (p = 0.021) for CV in height, 0.50 (p < 0.0001) for mean height; mixed groups: −2.8 (p = 0.64) for CV in height, 0.36 (p < 0.0001) for mean height.

Figure 3.

Figure 3.

Leverage plots from a multiple regression of the mean yield of subordinate plants on their mean height and the height inequality (CV) of plants in their group (corresponding to the model in table 2b). While accounting for a common positive relationship between the mean height of subordinates and their mean yield (a), a marginal increase in height inequality (b) had a significantly positive effect on the mean yield of subordinates in sibling groups (p = 0.005; estimated slope: 21.9 (6.6, 37.2) but not in mixed groups (p = 0.85; estimated slope: −0.80 (−8.9, 7.3)).

Figure 4.

Figure 4.

(a) Leverage plot of the difference in root mass between paired mixed and sibling groups (paired to match in mean height; colours indicate the genotype of the sibling group) regressed on the mean height inequality (CV) of the pair, showing a significant positive slope (_F_1,18 = 27.4; p < 0.0001). (b) Mixed groups also yielded significantly fewer seeds per unit of root mass, on average, than their paired sibling group (paired _t_-test; _t_20 = 2.74; p = 0.013). Mean values are block-centred (where ‘pair’ is the blocking factor), and the vertical span of each diamond is a 95% CI.

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