Print PageApical dominance is important to plant architecture and resource partitioning. Initial preferential investment in apical regions, and suppression of axillary buds, may constrain plant fitness through limits on the number or size of developing flowers. If apical regions are damaged through herbivory or other causes, plants increase investment in previously suppressed buds. However, in the field, the disruption of apical dominance does not necessarily increase plant fitness (e.g. higher seed production). Models examining the consequences of apical dominance for plant-herbivore interactions suggest that apical damage may induce herbivory tolerance through bud release. These models primarily focus on situations where apical damage may be predictable and subsequent herbivory infrequent, such as with ungulate herbivory. In contrast, insect herbivory is less predictable and is a repeated damage risk during a growing season. Thus, the consequences of apical dominance for plant performance may differ when considering insect herbivores compared to ungulate grazers. We developed a stochastic dynamic programming model evaluating under what ecological scenarios apical dominance is an optimal life history decision. Our motivating example is Cirsium canscens, a well-studied thistle with widespread, variable, insect floral herbivory. We calculated the optimal investment between flower heads, and flowering size, at a range of mortalities.
Results/Conclusions
Our model predicts initial investment in a single flower head (apical dominance) even though heads have identical vital rates (survival, growth, and fecundity). This decision was driven by size differences between flower heads and time of year, though investment patterns varied with mortality risk. In early season, plants invested all resources in the largest flower head (A) until A reached a particular size. If mortality was high, A flowered at this size, and plants allocated all resources to the other head (B). In contrast, if mortality was low, A grew larger initially, but plants began allocating to B relatively earlier, at the expense of A’s growth, until both heads were equal in size. Then the plant invested in both flower heads until both reached an optimal flowering size. The optimal flowering size also depended on mortality risk, and flowering size was larger with low mortality. Flowering sizes of A and B were similar with high mortality, but at low mortalities B flowered at larger sizes compared to A. Therefore, in our model, apical dominance provides an optimal strategy, though the overall consequences of investment do not necessarily favor the apical head.
