Abstract:
Although fine root traits are crucially important for plants and ecosystems, little is known about the mechanisms driving their variation. According to the theory of the plant economics spectrum, plants have to adopt a consistent strategy for all organs on a one-dimensional axis from slow to fast nutrient acquisition. The different strategies are supposed to be reflected in fine root traits, and to vary with resource availability. This master’s thesis aimed to investigate the variation of tree fine root traits along a
topographical gradient in southern Ecuador, and their coordination with aboveground traits, in order to find evidence for a plant economics spectrum.
In a tropical montane forest near Loja, fine root systems (diameter < 2 mm) of 179 adult trees were sampled from 18 plots at different slope positions. The steep slopes in this megadiverse ecosystem are characterized by decreasing fertility from the lower to the upper slope, and a high turnover of tree species on small scales along the slopes. The traits root diameter, specific root length (SRL), root tissue density (RTD), root branching intensity (RBI) and nitrogen content (N root ) were measured on each root system. The root traits were tested for being correlated with a topography index, edaphic factors, and aboveground traits. These relationships were analyzed both on the level of plot means and individual trees. Moreover, functional dispersion of the root traits was calculated for each plot and related to the topographical gradient. Pagel’s ? was calculated as an indicator of phylogenetic signal for each root trait. Furthermore, mixed effect models and principal component analyses were calculated in order to better understand the complex relationships between environment, plant traits, and phylogeny.
Fine roots tended to be thicker and less intensely branched, contain less nitrogen and have lower SRL with increasing distance from the lower slope. Functional dispersion decreased from lower to upper slope. Especially SRL and N root were closely linked to aboveground traits. Trees with high SRL and N root tended to have softer wood and leaves, higher specific leaf area and higher leaf N and P concentrations. These trends were also apparent when comparing plot means instead of data from tree individuals. This indicates that trees evolved consistent resource economic strategies that manifest
in their fine roots, wood, and leaves. Species with a slow, conservative strategy are typical for the upper slopes, whereas acquisitive species assembled at the lower slopes. However, RTD was independent from the topographical gradient and most aboveground traits. SRL, N root and root diameter had strong phylogenetic signals. Rubiaceae and Moraceae were typical families with acquisitive traits growing at the lower slope, whereas
the early-diverged Lauraceae occurred predominantly at the upper slope and had very thick roots of the conservative type.
These findings provide valuable new insights into patterns of fine root trait variation. They confirm the hypothesis of a plant economics spectrum where root traits are integrated, and demonstrate that tree species in tropical montane forests assemble along topographical gradients according to their resource economics strategies and the associated trait combinations.
