Integration

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) (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 (unsevered,

>9 8-8.9 7-7.9 6-6.9 5-5.9 4-4.9 3-3.9 2-2.9 1-1.9 0-0.9

January 1976

Control

Fertilized Ï

Mature phase

>9 8-8.9 7-7.9 6-6.9 5-5.9 4-4.9 3-3.9 2-2.9 1-1.9 0-0.9

>9 8-8.9 7-7.9 6-6.9 5-5.9 4-4.9 3-3.9 2-2.9 1-1.9 0-0.9

Mature phase

July 1976

Control Fertilized

Figure 4.3 The age structure of shoots in clones of the sand sedge Carex arenaria growing on sand dunes in North Wales, UK. Clones are composed of shoots of different ages. The effect of applying fertilizer is to change this age structure. The clones become dominated by young shoots and the older shoots die. (After Noble et al., 1979.)

2.0 1.6 1.2 0.8 0.4 0.0

-

1

-

LSD =

0.154 g

-

-

-

1

1

UU SU SM

UU SU SM

UU SU SM

UU SU SM

Figure 4.4 The growth of daughter ramets of the grass Holcus lanatus, which were initially (a) 1 week, (b) 2 weeks, (c) 4 weeks and (d) 8 weeks old, and were then grown on for a further 8 weeks. LSD, least significant difference, which needs to be exceeded for two means to be significantly different from each other. For further discussion, see text. (After Bullock et al., 1994a.)

unmoved: UU); (ii) ramets that had their connection severed but were left in the same pot so competition was possible (severed, unmoved: SU); and (iii) ramets that had their connection severed and were repotted in their parent's soil, but after the parent had been removed, so no competition was possible (SM) (Figure 4.4). These treatments were applied to daughter ramets of various ages, which were then examined after a further 8 weeks' growth. For the youngest daughters (Figure 4.4a) attachment to the parent significantly enhanced growth (UU > SU), but competition with the parent had no apparent effect (SU ~ SM). For slightly older daughters (Figure 4.4b), growth could be depressed by the parent (SU < SM), but physiological connection effectively negated this (UU > SU; UU ~ SM). For even older daughters, the balance shifted further still: physiological connection to the parent was either not enough to fully overcome the adverse effects of the parent's presence (Figure 4.4c; SM > UU > SU) or eventually appeared to represent a drain on the resources of the daughter (Figure 4.4d; SM > SU > UU).

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