Intra-plant regulation of growth and plant-herbivore interactions

Plant modularity and sink/source regulation imply that plants consist of pa rts which may be functionally semiautonomous. but whose functioning is loca lly and globally regulated by a large number of competing, physiological si nks and sources. As a consequence, plant functions occur at various hierarc hical levels of organization. This has potential repercussions for the ecol ogical and evolutionary responses of plants to environmental challenges. Fo r instance, the functional responses of plants to external disturbances, su ch as herbivory, may be based on shoot- or module-level reactions, and may nor occur at the whole-plant level. While modularity and sink/source regula tion are well recognized and included In general plant/herbivore hypotheses , their roles as determinants for plant responses have received less attent ion. We discuss the ways in which plant modularity and sink/source regulati on as internal constraints of growth may modify the effects of herbivory. T he discussion consists of four steps. rme first review factors which at a v ery general level promote and constrain tree design and function and partic ularly growth, the latter being an important determinant of plant quality f or herbivores. Next, on the basis of modularity and sink/source regulation, we introduce a null hypothesis of plant functioning to predict the respons es of growth to an external challenge. Third, we describe the growth charac teristics of deciduous birches (Betula) and evergreen pines (Pinus) as exam ples of the importance of recognizing inter-specific differences in the int ernal design and function of trees for herbivory studies. Fourth, we discus s the evolutionary implications of our null model. On the basis of the mode l, it is obvious that many of the spatial and temporal variations in the su itability of trees to herbivores may result directly from attributes of pla nt design and function per se. This could explain the occurrence of herbivo re-induced susceptibility as well as same forms of induced resistance in pl ants. Accordingly, some tree traits which lead to herbivore avoidance of tr ees may not require explanations emphasizing selection for defence.