A1 Effects of nutrient manipulation (NUMEX) on the performance of tree species and the suitability of tree species as indicators for changing ecosystem properties

PI(s) for this project:

Dr. Jürgen Homeier


Elevated N and P inputs affect virtually all components and processes of terrestrial and aquatic ecosystems since the productivity of plant communities is limited by N or P, or both in the vast majority of ecosystems on the globe. Human activities will globally affect the cycles of N and P during the coming decades and tropical forests are likely to respond sensitively to these changes. But the size and direction of these responses are unclear.
The project aims to contribute to a better understanding of the effects of nutrient addition on tropical montane forests. Based on earlier research work on the structure and productivity of Andean forests, their tree diversity and their short-term response to nutrient addition, in the proposed project the role of N and P supply (and of continued addition of these nutrients) is tackled on three levels: The ecosystem level, the tree species level and the functional trait level.
The combination of these levels is essential to understand (A) the net ecosystem response (e.g. carbon storage and other services provided), (B) the contribution of species composition and diversity to this response and (C) the underlying functional mechanisms. The effects of continued nutrient addition is investigated along an elevational transect (1000 to 4000 m asl) of Andean forests within the ongoing N and P addition experiment (NUMEX = Ecuadorian NUtriewnt Manipulation EXperiment), that 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).
Since the study area harbours the best-studied tropical montane forests worldwide, the study offers the unique opportunity to bridge the existing knowledge gap between tree diversity and ecosystem functioning of these species-rich forests. The expected results will allow predicting future changes in tree species composition and forest structure as a result of increasing nutrient deposition and should be incorporated in monitoring schemes for Andean forests and the ecosystem services provided by them.



WP1 – Effects of nutrient addition on tree growth and carbon allocation
Long-term effects of N and P addition on carbon allocation will be examined by monitoring the key stand parameters tree diameter growth, fine litter production, fine litter quality, leaf morphology, and foliar nutrient concentrations in the existing NUMEX plots at 1000, 2000 and 3000m. A special focus will be on belowground carbon allocation.

WP 2 – Effects of nutrient addition on tree nutrient allocation at the tree line
The NUMEX design will be implemented in Polylepis forest at 4000m asl and the same key stand parameters as in the established NUMEX gradient will be studied to determine the expected changes in carbon allocation in response to N and P addition at stand- and tree species-level.

WP 3 – Effects of continued nutrient addition on tree species composition

Since trees are long-lived organisms, fast changes in species composition after few years of nutrient manipulation cannot be expected in the studied forest types. Therefore, the proposed project will use the following proxy parameters to characterize tree species’ competitive fitness: Mortality of mature trees, leaf area loss by herbivores, leaf traits (see WP4). 

Comparing these parameters and monitoring of tree regeneration dynamics will show if the currently common species are likely being replaced by other species under changed nutrient availability.

 WP 4 – Identification of nutrient-sensitive tree taxa and the determination of functional traits responsive to nutrient addition

This work package combines trait data and information on tree species’ site preferences along natural nutrient gradients with results from NUMEX on tree species' responses to changes in nutrient availability with the objective to identify “nutrient sensitive” indicator traits and species.
A wide range of growth-related traits are already available for the most common montane forest tree species between 1000 and 3000 m elevation. Leaf traits (e.g. SLA, foliar nutrient concentrations), wood traits (e.g. wood specific gravity, xylem vessel density and mean vessel diameter) and growth-related traits (stem diameter growth, growth and recruitment responses to nutrient addition) have been gathered for more than 150 tree species.