Effects of nutrient manipulation (NUMEX) on the performance of tropical montane trees and interactions with climate variation and ENSO-related weather extremes

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Tropical mountain ecosystems and their vegetation are considered to be especially sensitive to global environmental change. Due to the complexity of these ecosystems and the interaction of different drivers (land use change, climate change, and increasing nutrient deposition), understanding current responses and predicting future changes in mountain ecosystems is difficult but urgently needed. The effects of higher rates of atmospheric N and P deposition on the biological diversity and ecosystem functioning of tropical ecosystems are still poorly understood and our knowledge of N and P limitation in tropical forest ecosystems is still limited, but it will be of critical importance to know the effects of increased nutrient availability to conserve these ecosystems. The effects of continued nutrient addition are investigated along an elevational transect of Andean forests in southern Ecuador within the ongoing Ecuadorian NUtrient Manipulation EXperiment (NUMEX). NUMEX started in January 2008 adding moderate amounts of N (50 kg ha-1 yr-1), P (10 kg ha-1 yr-1), and N and P to representative stands at 1000, 2000 and 3000 m. The project of 12 months duration will synthesize 9 years of data retrieved on the NUMEX plots and includes the collection of a final data set on aboveground productivity and tree recruitment. In addition, the current El Niño event (2015-2016) and the expected La Niña event (2016-2017) offer the unique opportunity to study the effects of extreme weather events on nutrient-manipulated forest plots. With the proposed project, the influences of climate variation and weather seasonality on tree growth and the potential interactions or feedbacks between climate and increasing N and P availability will be analysed. For a better understanding of the ecosystem response to a changing environment and the contribution of tree species composition to this response, it is essential to disentangle the effects of climate-related drivers and nutrient availability. The results will allow predicting future changes in tree species composition and forest structure as a consequence of increasing nutrient deposition and should be incorporated in monitoring schemes for Andean forests and the ecosystem services provided by these forests. The project will continue to be responsible for the coordination and maintenance of NUMEX. Further objectives of the prolongation phase are to summarize above- and belowground vegetation effects on stand and species level for the 9 years of NUMEX and to lead the final joint synthesis of the aggregated data of the participating projects.