PLANT COMPENSATORY GROWTH - HERBIVORY OR COMPETITION

The presence of an overcompensatory response to damage in some plant s pecies has recently created a debate concerning whether this trait is an adaptation to herbivory, or simply a physiological consequence of a daptations to competition for light. According to the latter hypothesi s, competition for light favors fast vertical growth and strong apical dominance. The removal of apical dominance by damaging the primary sh oot allows the growth of secondary shoots and hence increases producti vity. We compare predictions of these two hypotheses in a model-system where plants are exposed to both a risk of damage and a risk of compe tition. Compensatory seed production is assumed to depend on the numbe r of dormant buds that can be activated by damage, and on the seed pro duction of surviving shoots. In accordance with earlier theoretical an alyses, we expect that intensive herbivory can favor overcompensatory seed production. In contrast, competition for light should at best lea d to exact compensation when the competitive environment remains uncha nged. Competition acts against overcompensation for two reasons. First , competitive plants should have poor resource reserves to support com pensatory growth. Second, competition for light is assumed to favor un branched architecture and thus, activation of many secondary shoots sh ould not increase the seed yield. However, we cannot exclude the possi bility that plants adapted to competition may overcompensate when grow n singly. In spite of this caveat, it is likely that overcompensation requires damage related adaptations that may evolve only under intensi ve and relatively predictable risk of damage.