Читать книгу Ecology - Michael Begon - Страница 96

For many rhizomatous and stoloniferous species, this changing age structure is in turn associated with a changing level to which the connections between individual ramets remain intact. A young ramet may benefit from the nutrients flowing from an older ramet to which it is attached and from which it grew. But the pros and cons of attachment will have changed markedly by the time the daughter is fully established in its own right and the parent has entered a postreproductive phase of senescence – a comment equally applicable to unitary organisms with parental care, like ourselves (Caraco & Kelly, 1991).

The changing benefits and costs of integration have been studied experimentally in the pasture grass Holcus lanatus, by comparing the growth of: (i) ramets that were left with a physiological connection to their parent plant, and in the same pot, so that parent and daughter might compete (competing, connected: CC); (ii) ramets that were left in the same pot so competition was still possible but had their connection severed (competing, not connected: CN); and (iii) ramets that had their connection severed and were repotted in their parent’s soil after the parent had been removed, so no competition was possible (independent plants, neither competing nor connected: NN) (Figure 4.4). These treatments were applied to daughter ramets of various ages, which were then examined after a further eight weeks’ growth. For the youngest daughters, just one week old (Figure 4.4a), connection to the parent significantly enhanced growth (CC > CN), but competition with the parent had no apparent effect (CN ≈ NN). For slightly older daughters, two weeks old (Figure 4.4b), competition with the parent did depress growth (NN > CN), but physiological connection with the parent effectively negated this (CC > CN; CC ≈ NN). For even older daughters, however, the balance shifted further still. Competition with the parent again depressed growth of the daughter (NN > CN), but this time physiological connection to the parent was either not enough to fully overcome this (at four weeks, Figure 4.4c; NN > CC > CN) or eventually appeared to represent a further drain on the daughter’s resources (after eight weeks, Figure 4.4d; NN > CN > CC).

Figure 4.4 Integration within a plant leads to a shifting balance of positive and negative effects between parent and daughter modules as modules age. The growth of daughter ramets of the grass Holcus lanatus, which were initially (a) one week, (b) two weeks, (c) four weeks and (d) eight weeks old, and were then grown on for a further eight weeks. LSD, least significant difference, needs to be exceeded for two means to be significantly different from each other. For further discussion, see text. CC, competing, connected; CN, competing, not connected; NN, independent plants, neither competing nor connected.

Source: After Bullock et al. (1994).