In our last post Kerstin Pierick, postdoc at the University of Göttingen, discusses her last PhD thesis chapter ‘Above- and belowground strategies of tropical montane tree species are coordinated and driven by small-scale nitrogen availability’. She presents her astonishing study system, discusses the connection between environment and traits and remembers the importance of changing the point of view to understand nature.
A Spanish translation of this blog post can be found here.
About the Paper and the Research
The tropical montane forests along the eastern slopes of the Andes are one of the places with the highest tree species diversity on earth. From the Amazonian lowland forests to the timberline, environmental conditions, first and foremost temperature, change gradually. Along with that, the forests change visually: The lush pre-montane forests with canopy heights of up to 40 m and typical lowland rainforest features like lianas and buttress roots develop gradually into stunted forests with canopies below 10 m close to the timberline, overgrown with mosses, lichens and other epiphytes. Most tree species in the area are restricted to the specific conditions of narrow altitudinal belts, which is one plausible explanation for the outstanding tree diversity of the region.
In this extraordinary study system, which is fascinating and challenging to work in at the same time, we had already in previous studies tackled the question whether we can use functional traits to describe strategies of tree species, and whether these strategies determine which tree species grow under which conditions. We focused on morphological and chemical traits of leaves and fine roots that are linked to the conservation-acquisition trade-off. Plants that prioritize conservation optimize tissue longevity at the cost of fast growth and resource uptake by investing into robust tissues and defense. Plants that prioritize acquisition, on the other hand, produce fast-growing tissues with high resource uptake rates at the cost of longevity. Leaf and fine root traits, for example tissue densities, express this trade-off. In theory, a conservative strategy should be more appropriate when resources are scarce (the plant has to be stingy with its resources and protect them at all cost), whereas an acquisitive strategy should be more appropriate when nutrients are plentiful (where it pays off to be faster than your neighbors to win competition). In tropical montane forest, the key resource in question is nitrogen. As temperature decreases towards higher altitudes, decomposition processes in the soil are slowed down, and it takes longer for the nitrogen to become available to the plants again, which leads to less fertile soils at higher elevations. As a consequence, in theory, tree species in the lower elevational belts should have more acquisitive traits, while species at higher elevations should have more conservative traits. This is, in principle, was what we found in previous studies – but only on average. We were always baffled by the exceptionally wide range of traits we found in co-occurring species in our study plots, where species with very conservative and very acquisitive strategies co-exist next to each other.
Impression from the forests at around 2000 m a.s.l. (credit: 2a Kerstin Pierick, 2b Jürgen Homeier)
This is why we decided to zoom in and see if we can disentangle this within-plot trait variation. In this study, instead of measuring environmental conditions on the plot level as usual, we took a soil sample directly next to the stem base of each of more than 400 trees and measured the nitrogen availability. We then analyzed how well these tree-level soil fertility data predicted the functional traits of the trees, and the results were astonishing: Not only was there a lot of soil heterogeneity within our plots and a huge overlap in soil fertility between elevational levels, this small-scale heterogeneity also was closely associated with the tree species’ strategies. It explained a much larger proportion of variation in the tree functional traits than elevation alone. Within our 1 ha plots, tree species with conservative strategies grew in less fertile, and tree species with acquisitive strategies in more fertile spots.
These results let us draw two main conclusions: First, local soil heterogeneity – probably resulting from the rugged topography of the steep mountain slopes – is potentially another explaining factor to why so many tree species with different strategies can co-exist in tropical montane forests. Second, and this is applicable not only to tropical montane forests, sometimes the key to understanding ecosystems lies not on the spatial scale we usually work on, and zooming in to small spatial scales pays off.
About the Author
Even though one could have already seen it coming that I would become a plant ecologist when my nature-loving dad taught me to recognize plant species when I was little, I only decided I want to be a scientist when I had way too much fun digging out roots and teaching myself R for my Master’s thesis. This paper is the last (and my favorite) chapter of my PhD thesis. Currently, I’m a postdoc at the forestry faculty of the University of Göttingen in Germany, working on the relationships between forest structure, microclimate, forest biodiversity and ecosystem multifunctionality.
A huge ‘Thank You!’ to Selene Báez for the Spanish translation!
