The elevations at which tropical treelines occur are believed to represent the point where low mean temperatures limit the growth of upright woody trees. Consequently, tropical treelines are predicted to shift to higher elevations with global warming. However, treelines throughout the tropics have remained stationary despite increasing global mean temperatures. The goal of the study reported here was to build a more comprehensive understanding of the effects of mean temperature, low-temperature extremes, shading, and their interactions on seedling survival at tropical treelines. We conducted a seedling transplant study using three dominant canopy-forming treeline species in the southern tropical Andes. We found species-specific differences and contrasting responses in seedling survival to changes in mean temperature. The most abundant naturally occurring species at the seedling stage outside the treeline, Weinmannia fagaroides, showed a negative relationship between the survival of transplanted seedlings and mean temperature, the opposite of a priori expectations. Conversely, Clethra cuneata showed increased survival at higher mean temperatures, but survival also increased with higher absolute low temperatures and the presence of shade. Finally, the survival of Gynoxys nitida seedlings was insensitive to temperature but increased under shade. These findings show that multiple factors can determine the upper distributional limit of species forming the current tropical treeline. As such, predictions of future local and regional tropical treeline shifts may need to consider several factors beyond changes in mean temperature. If the treeline remains stationary and cloud forests are unable to expand into higher elevations, there may be severe species loss in this biodiversity hotspot.
Timberline Freezing tolerance Ecotone Climate change Species migration
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We would like to thank A. Ccahuana, N. Ccahuana, C. Gutierrez, and C. J. Pardo for help in the field. We also thank N.P. Lemoine and D.R. Anderson for statistical advice. We thank Peru’s SERNANP (Servicio Nacional de Áreas Naturales Protegidas por el Estado) and the personnel of Manu National Park for their ongoing support of research and conservation. This work was funded by the Fairchild Tropical Botanic Garden, NSF Grants DEB-1350125 and DEB-1257655, the ICTB Kenan Tropical Botany Research Award, the Tinker Field Grant, and the Dissertation Evidence Acquisition and Dissertation Year Fellowships awarded to E. Rehm by Florida International University.
Author contribution statement
ER and KF conceived and designed the project. ER analyzed the data. ER and KF wrote the manuscript.
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