Testing the epiphyte enemy escape hypothesis

In tropical rainforests, much of the plant world unfolds high above the ground. Orchids, bromeliads and many ferns grow on branches and tree trunks rather than in the soil. These so-called epiphytes – plants that grow on other plants – make up almost 10 percent of all vascular plant species worldwide. The advantages of this unusual lifestyle are obvious: in the forest canopy plants can reach more light and do not need to invest energy in building a massive trunk like trees do. But life high above the ground also comes with challenges. Without roots in the soil, obtaining water and nutrients can be difficult. Despite these constraints, this lifestyle has evolved in several plant families. At the same time, close relatives of many epiphytes still grow on the forest floor and often look remarkably similar. Why does one species live exclusively on trees while its relatives remain rooted in the ground? An international research project led by the Institute of Botany at BOKU University in Vienna is now investigating a little-tested explanation: plants may have moved into the tree canopy to escape their enemies in the soil. Particularly dangerous are pathogenic fungi that attack seeds and young seedlings. This idea is known as the “Epiphyte Enemy Escape” hypothesis. Over the course of evolution, plants develop genetic defenses against such pathogens. But if a species no longer encounters certain pathogens for multiple generations, these defenses may be lost – following the evolutionary principle of “use it or lose it.” Epiphytes might therefore become highly susceptible to soil pathogens and be unable to survive on the forest floor. The FWF-funded project “EpiEneScape” will test this hypothesis using simple but revealing experiments. Researchers will germinate seeds of epiphytic species and their ground-dwelling relatives on different substrates – such as forest soil and materials from tree branches. This will show where young plants are able to grow successfully and where they fail. In addition, the team will measure functional traits such as water storage capacity to better understand how these plants are adapted to the very different conditions in the canopy and on the forest floor. The study is a collaboration between BOKU University Vienna and the Universidad Veracruzana in Mexico. Fieldwork and experiments will take place both in greenhouses and in the tropical lowland and montane rainforests of Mexico. The researchers hope their findings will help explain why so many plant species have evolved to live in the forest canopy – and what role natural enemies in the soil may have played in shaping this remarkable lifestyle.