1.2. Effects of herbivory and competition on plant architecture

Defoliation does not have the same impact on all plants. Plants with low growth forms or rosettes recover better than those with erect growth forms (Parr & Way 1988, Mitchley & Willems 1995). In addition, plants that have tough, narrow, finely lobate or scale-like leaves recover relatively better from herbivory compared to plants with broad and mesomorphic leaves, which are often prone to desiccation and contamination (Oksanen & Virtanen 1995). The regrowth after herbivory takes place in two zones of intense growth: apical and leaf meristems. In grasses, the leaf meristem is situated at the base of the lamina, while the apical meristem in enwrapped by the leaves. Further, when the apical meristem is pressed close to the ground, the consequent twofold protection makes grasses resistant against herbivores (Prins et al. 1980). In contrast, typical grassland herbs have an exposed apical meristem and leaf meristems placed quite diffusely next to the leaf (Prins et al. 1980), and they are thus less protected against grazing. There are some exceptions to this rule. In Plantago maritima, 80% of the growth takes place at the base of the leaves, and the old leaves envelop the apical meristem. This enables partially consumed leaves to regrow (Prins et al. 1980). Other species with an extremely good compensatory ability include Ipomopsis aggregata (Paige & Whitham 1987) and Gentianella campestris (Lennartsson et al. 1997, 1998), which owe their good capacity for regeneration to their “bud bank“ situated at the base and nodes of the stalk (Lehtilä 1999).

Injury of a plant by a herbivore usually leads to the loss of apical dominance, i.e. the interruption in correlative inhibition, whereupon the auxin production of the leading shoot tips ceases. This, in turn, enhances the production of new lateral branches and inflorescence (Cline 1991). Repeated herbivore attacks result in highly branched, bush-like growth forms (McNaughton 1984). Apical dominance is generally considered an adaptation to competition for light, because rapidly growing, unbranched individuals are more likely to be successful in the competition for light compared to ones that allocate resources away from vertical growth to branches close to the ground (Irwin & Aarssen 1991, 1996, Bonser & Aarssen 1996). If the phenotype before damage is already very branched, indicating weak apical dominance, the removal of apical dominance may have only marginal effects on branching intensity (Järemo et al. 1996).

Even though the area of photosynthetically active tissue decreases due to herbivory, this may lead to increased light availability for the remaining, previously shaded leaves. This, in turn, will increase their photosynthetic capacity (McNaughton 1979). The five main traits associated with high tolerance to herbivory after damage are 1) an increased net photosynthetic rate, 2) a high relative growth rate, 3) increased branching or tillering after release from apical dominance, 4) mobilization of the pre-existing high levels of carbon storage in roots for allocation to above-ground reproduction, and 5) an ability to move the carbon stores from roots to shoots (Strauss & Agrawal 1999). However, there is usually a breakdown point, after which the plant is unable to fully recover from damage. When the decrease of leaf area or the loss of nutrients exceeds this point, the potential positive responses cannot compensate for the negative impacts of herbivory (Crawley 1997).