Herbivory and growth in terrestrial and aquatic populations of amphibious stream plants

1. Many amphibious plant species grow in the transition between terrestrial and submerged vegetation in small lowland streams. We determined biomass development, leaf turnover rate and invertebrate herbivory during summer in terrestrial and aquatic populations of three amphibious species to evaluate advantages and disadvantages of aerial and submerged life.
2. Terrestrial populations had higher area shoot density, biomass and leaf production than aquatic populations, while leaf turnover rate and longevity were the same. Terrestrial populations experienced lower percentage grazing loss of leaf production (average 1.2–5.1%) than aquatic populations (2.9–17.3%), while the same plant dry mass was consumed per unit ground area.
3. Grazing loss increased linearly with leaf age apart from the youngest leaf stages. Grazing loss during the lifetime of leaves was therefore 2.4–3.1 times higher than mean apparent loss to standing leaves of all ages. The results imply that variation in density of grazers relative to plant production can account for differences in grazing impact between terrestrial and aquatic populations, and that fast leaf turnover keeps apparent grazing damage down.
4. We conclude that the ability of amphibious plants to grow submerged permits them to expand their niche and escape intense competition on land, but the stream does not provide a refugium against grazing and constrains plant production compared with the terrestrial habitat.