Recruitment of trees at tropical alpine treelines: Erica in Africa versus Polylepis in South America

Abstract

Background: Constraints on tree regeneration have been described from several temperate treeline sites, however data from tropical mountains are few. Aim: We test the hypothesis that high altitude treelines are caused by constraints on tree regeneration. We report the results of recent studies on climatic variables and on the reproduction of Erica spp. in east Africa, and compare those with similar studies on treeline stands of Polylepis spp. in Ecuador and Bolivia. Methods: We established altitudinal and horizontal transects comprising sampling plots from forests to surrounding grasslands. Data on microclimate, vegetation structure and reproductive activity were collected in the field, and supplemented by tests on germination requirements in the field in permanent exclosures and in the laboratory. Results: Our measurements demonstrated that the number and severity of frost events and potential water stress were higher near the soil surface (0.5 m) than in free air (2.0 m). Seeds of Erica trimera did not germinate, as the necessary climatic requirements (high diurnal temperature range, but no frost) were rarely fulfilled in the field. Most studied species were capable of clonal growth, and the relative importance of vegetative persistence tended to increase with altitude. These effects were independent of the intensity of land use. Conclusions: The harsh microclimate near the soil surface at tropical treelines may be one reason why recruitment from seed is limited for treeline-forming species. This is important for restoration or conservation schemes as well as predictions of treeline dynamics under global change.