Fine root traits of tropical montane trees in Ecuador - Effects of elevation, variation between tree functional types, and above-belowground linkage

Abstract

Tropical montane forests in the Andes are one of the most species-rich ecosystems of the world, and are therefore both valuable from a conservation perspective and interesting for testing ecological hypotheses. Particularly the belowground component of tropical ecosystems is so far understudied, which due to small-scale heterogeneity of soil properties in tropical montane forests allows addressing a multitude of open questions about associations between roots and their environment. Functional traits of fine roots are indicators of belowground resource economics strategies. However, the leading dimensions of root trait variation, their covariation with aboveground plant functional traits, and how they vary along environmental gradients are not fully understood. Furthermore, very little is known about their functional diversity and its dependency on abiotic factors. This thesis comprises results from three descriptive field studies on root functional traits in tropical montane forests of the southern Ecuadorian Andes. In all of these studies, morphological and chemical traits of fine roots were measured on the level of individual trees, with the combined datasets encompassing more than 200 tree species. The first and second of these main chapters deal with the community-level root trait variation along a small-scale topographic gradient in lower montane forests, and a 2000m elevational gradient respectively. The third one focuses on fewer tree species on three elevational levels, and examines the relationships between their microhabitat preferences, and their aboveground and belowground functional trait syndromes. Functional traits of fine roots were coordinated with each other, forming two separate axes of root trait variation. The first axis involved root tissue density and nutrient concentrations and reflects a growth–survival trade-off, with low tissue densities and high nutrient concentrations indicating an acquisitive strategy, and high tissue densities and low nutrient concentrations indicating a conservative strategy. The second axis reflects a gradient of root diameter that might, according to recent theories, be related to the degree of reliance on mycorrhiza. While the acquisition– conservation axis was highly coordinated with leaf and wood traits reflecting the same trade-off for aboveground plant organs, indicating selective pressures towards a consistent whole-plant economics strategy, the diameter-related axis was independent from aboveground functional traits. Most root functional traits varied both along the smaller-scale topographic and the larger-scale elevational gradient. Tree fine roots tended to display increasingly conservative trait syndromes and higher root diameters towards upper slope positions and higher elevations. Furthermore, root functional traits associated with the acquisition–conservation axis were dependent on both large-scale nitrogen and phosphorus availability, with root trait syndromes at sites with higher nutrient availability being more acquisitive. Diameter-related root traits, however, were independent from large-scale nutrient availability. The trends described above explained only relatively small fractions of total trait variation because many species with different root trait syndromes coexisted within plots. Decreasing functional diversity along both the topographic and elevational gradient indicated the coexistence of species with diverse belowground strategies under more beneficial environmental conditions, and the convergence towards more conservative fine root strategies under the less beneficial conditions of upper slopes and higher elevations. These patterns of coexistence were further explained by measuring soil nutrient availability on the spatial scale of tree individuals instead of plots. Within communities, tree species with conservative trait syndromes tended to grow at local microhabitats that were less rich in nitrogen. Thus, the high small-scale edaphic heterogeneity lead to the coexistence of species with diverse strategies within the conservation–acquisition trade-off in close proximity. A similar trend could neither be found for soil phosphorus, nor for diameter-related root traits. Intraspecific trait variation played only a minor role. All root functional traits displayed high degrees of phylogenetic signal. While previous results, stating that early-diverged angiosperms from the magnoliid clade tend to have thick fine roots, could be confirmed, the data in this work reveal that the later-diverged angiosperm tree families Meliaceae and Clusiaceae are similarly thick-rooted in spite of their high phylogenetic distance to the magnoliid clade. In conclusion, this work provides comprehensive evidence that root functional traits and root functional diversity in tropical montane forests vary along gradients of soil fertility, as environmental filtering promotes trait convergence towards conservative resource use strategies under nutrient-poor conditions. Hereby, the root trait– environment associations were surprisingly most pronounced at the smallest spatial scale, indicating the relevance of small-scale heterogeneity for community assembly processes in tropical forests.