Nurse plants as ecosystem engineers in changing environments of the high tropical andes: Impacts on microhabitats and plant diversity

Background/Question/Methods: Recent conceptual developments in community ecology suggest that non-trophic positive interactions among plants need to be incorporated into predictive models of the impacts of climate change, particularly in extreme environments. However, empirical research on the role of facilitation in modulating the response of alpine communities to land use and climate change is scarce, especially in tropical Andean ecosystems. These ecosystems comprise more than 90% of the tropical alpine biome and exhibit outstanding levels of species and functional diversity and unique environmental conditions, including a lack of temperature seasonality. This complicates extrapolating conclusions derived from alpine areas located at higher latitudes. Here we review the available research in the Venezuelan Andes on the role of ecosystem engineers (EE) of contrasting growth-forms (shrubs, giant rosettes and cushions) on: a) the colonization of woody plants in disturbed and non-disturbed areas at the upper treeline; and b) biogenic habitat modification and community structuring in the more extreme conditions of high sub-nival environments. On the light of the available evidence we discuss the importance of considering the effects of EE for predicting the impacts of global change on tropical alpine communities, which have been shown to be specially exposed to rapid environmental change.

Results/Conclusions: Research at the tropical upper forest-line indicates that shrubs that dominate old-fields and undisturbed areas facilitate the establishment of woody species from closed forests. The available evidence indicates that these effects could be linked with dampening effects of shrubs on incident radiation and daily temperature oscillations. In more extreme sub-nival environments, shrubs, giant rosettes and cushions have been shown to modify microhabitat conditions, with effects being growth-form specific and species-specific: while all studied EE strongly reduced maximum temperatures and increased soil organic matter in their neighborhood, their effects on minimum temperatures and soil water differed. In turn, even though EE generally increased local plant cover and diversity, community effects were also growth-form and species-specific. In the case of cushions, the intensity and frequency of facilitation increased with increasing elevation and nutrient stress (but dropped at extreme elevations). In the northern tropical Andes, future temperature increases are expected to be accompanied by reduced precipitation. Hence, we propose that predicting the role of EE in modulating climate change impacts on these diverse alpine communities will require knowledge of growth-form and species-specific effects on temperature and water availability, and how they interact with changes in productivity and nutrient induced stress.