Did you ever wonder how it happens that spruce trees typically have such nice, conical tops? The uppermost shoot, called the leader, produces a particular hormone that suppresses growth in the branches below, most effectively in the branches nearest the leader. The effect dwindles to negligible on the lowermost branches. Voilá! –a conical top to the tree.
If the leader is damaged—chewed by a porcupine, or invaded by an insect, or cut off, the next-lower branches rapidly start to grow, and the damaged tree-top may now display two or three new leaders, until one of them might eventually take over. The dominance of the leader at the apex is disrupted, allowing lower branches to grow more. A similar but usually smaller effect may occur on the end of branches.
The effect of that suppressive hormone is countered by another hormone, one that encourages growth. The balance between the two hormones differs among species of plants, so not all plants grow with conical tops. But one can see the same phenomenon in other trees: for example, look at the cottonwood trees that have been decapitated to prevent them from growing up into the power lines. The remaining branches grow well and begin to reach upward more than before the top was removed.
The strength of apical dominance and the effects of release from that dominance vary among species, although very few general patterns have been discerned (such as effects of habitat, geography, climate, or life history). And the evolutionary pressures that govern both the strength of apical dominance and the effects of its release have been little studied. So I’ll just present a few examples here, to illustrate some of the variation and complexities.
Herbivory, by grazing and browsing animals, often crops off the tops of a plant, ending the dominance effect and leading to branching, which frequently increases the production of flowers and fruits (this is why gardeners commonly pinch off the tops and ends of branches, making a plant bushier and potentially more fruitful). But this begs the obvious question: if branchi-ness is good for the reproductive output of the plant, why was apical dominance so strong, suppressing the branches? Presumably, there are other advantages associated with apical dominance. For example, if the plant allocates resources not to branches but to good vertical growth, this can be advantageous by reaching more light and increasing the survival of the plant, which would eventually lead to greater lifetime reproduction. That option might over-ride the advantages of short-term increased fruit production in most circumstances. Nevertheless, the ability to respond to release, if herbivory occurs, is itself a useful trait. In other words, the evolutionary fitness of a plant may be better with strong apical dominance in the absence of herbivory, but better with release when herbivory occurs.
The club moss (Lycopodium) grows along the ground, sending up short vertical shoots. If the main upright shoot dies, dominance is reduced, and lateral spread increases. The horizontal growth then eventually reaches a new location and a new upright shoot forms there. A living, dominating shoot is successfully exploiting a good site, but when it fails and releases lateral growth, the individual plant as a whole may survive by reaching a new site. In short, the dominance is advantageous in certain circumstances but not in all.
In a species of fireweed (Epilobium in some taxonomies), strong apical dominance leads to good seed production and good seed dispersal (by wind) in areas where competition for light is severe, giving an advantage to good vertical growth and reduced branching. However, this only works where soil nutrients are sufficient to support such growth; in poor sites, which cannot support good growth, the potential advantages of apical dominance may be negated. Again, the advantage of apical dominance is seen in certain circumstances, in this case related to resources.
The relationship between resource availability and the effects of apical dominance are seen also in bearberry (Arctostaphylos uva-ursi). Here (in contrast to fireweed), apical dominance is weak in rich habitats, the plants are much-branched, and the plant is exploiting its present location to the fullest possible extent. In resource-poor habitats, apical dominance is strong, as the plant allocates its limited nutrients to growing in situ: release from apical dominance, perhaps by herbivory, leads to increased branching, increasing the chances of the plant stretching out to find a better site.
These few examples serve to emphasize that multiple factors interact to determine the strength of apical dominance and the consequences of release from that dominance. No wonder that researchers have yet to find general patterns in the ecology of apical dominance. The physiology is well understood; now to learn the whys and wherefores!